内容来源：https://www.quantamagazine.org/how-fast-is-the-universe-really-expanding-20260723/

内容总结：

宇宙膨胀之谜：观测数据与理论模型出现“哈勃张力”

（据科学媒体Quanta Magazine报道）宇宙膨胀的速度究竟有多快？这一困扰天文学界数十年的谜题，如今正演变为一场激烈的科学辩论。两位主要测量方法得出的结果存在显著差异，这一矛盾被科学家称为“哈勃张力”。

从“宇宙加速”到“哈勃张力”

1930年代以来，天文学家已知宇宙在不断膨胀。1990年代，科学家发现宇宙膨胀非但没有减速，反而在加速——这一颠覆性发现直接催生了“暗能量”假说，并荣获2011年诺贝尔物理学奖。然而，故事并未就此终结。

诺贝尔奖得主、约翰霍普金斯大学天体物理学家亚当·里斯（Adam Riess）指出，目前测量当前宇宙膨胀速率（即哈勃常数）的两种主要方法给出了不同的答案。一种方法利用Ia型超新星作为“标准烛光”进行本地测量；另一种则基于宇宙微波背景辐射（宇宙大爆炸的“婴儿照”）结合标准宇宙学模型（ΛCDM模型）进行推算。

8%的差距：模型遭遇挑战

里斯团队利用哈勃和韦伯太空望远镜的观测数据，测得哈勃常数约为73（单位：公里/秒/百万秒差距）。而根据欧洲航天局“普朗克”卫星对宇宙微波背景辐射的观测，结合ΛCDM模型推算出的哈勃常数则约为67.5。两者相差约8%。尽管数字看似微小，但测量精度均已达到1%以内，这一差距在统计学上超过了5西格玛（物理学家公认的“发现”标准），意味着存在系统性偏差的可能性极低。

是“海王星”还是“广义相对论”？

里斯将这一困境比喻为19世纪天文学的两段历史：当时水星轨道的异常最终导致了爱因斯坦的广义相对论；而天王星的异常则直接导致了海王星的发现。目前的“哈勃张力”可能是ΛCDM模型需要“小修小补”（如调整暗能量性质），也可能预示着需要全新的物理理论。

未来展望

尽管面临挑战，ΛCDM模型仍然是描述宇宙最成功的理论框架。为破解这一谜题，新一代望远镜即将投入观测：美国的地面望远镜“维拉·鲁宾”、欧洲航天局的“欧几里得”卫星，以及计划今年9月发射的NASA“南希·格雷斯·罗曼”太空望远镜（视野是哈勃的100倍）。这些望远镜有望提供海量数据，帮助科学家判断暗能量是否在随时间变化，并最终揭示宇宙膨胀的真实速率。

中文翻译：

宇宙到底膨胀得有多快？
引言
宇宙学中最大的谜团之一似乎越来越扑朔迷离。天文学家自20世纪30年代起便知宇宙在膨胀，但到了20世纪90年代，人们发现膨胀非但没有减速，反而在加速——这一发现让整个领域震惊不已。某种力量必然在推动这种加速，科学家将其归因于暗能量。这场最初在宇宙学领域引发的地震级冲击，最终促成了一次诺贝尔奖的颁发。

但这个故事还有续集，且伴随另一个重大剧情转折。估算当前宇宙膨胀速度的两种主要方法，得出了截然不同的答案。因此，宇宙究竟膨胀得有多快，成了一个争议颇大的问题——伴随着不少焦虑——这一分歧被称为“哈勃张力”。或许最令人惊讶的是，最早发出警示声音的人之一，正是天体物理学家亚当·里斯，他凭借引发加速争论的诺贝尔奖成果，率先点燃了这场辩论。

里斯与联合主持人史蒂夫·斯特罗加茨共同来到《求索之乐》节目，解释了这一切是如何演变的、哈勃张力可能告诉我们什么，以及接下来会发生什么。

您可以在苹果播客、Spotify、TuneIn或您最喜爱的播客应用中收听，也可从《量子》杂志官网流媒体播放。

以下是节目文字实录：

[音乐播放]

史蒂夫·斯特罗加茨：我是史蒂夫·斯特罗加茨。
詹娜·莱文：我是詹娜·莱文。
斯特罗加茨：欢迎收听《求索之乐》。
莱文：这是《量子》杂志的一档播客，我们将探讨当今数学和科学领域一些最大的未解之谜。
斯特罗加茨：见到你很高兴，詹娜。最近怎么样？
莱文：嗨，史蒂夫。见到你也很高兴。我很期待听你今天要聊的话题。
斯特罗加茨：没错，我简直迫不及待了。我和亚当·里斯进行了一次特别有趣的对话，我想你在这个圈子里应该认识他。
莱文：当然认识。不久前我还和他聊过。
斯特罗加茨：嗯，是的。我想在天体物理学界你们肯定有过交集，所以听到这个我并不意外。但实际上，我自己已经几十年没和亚当说过话或见过面了。原来我刚开始当教授时，他还是个学生。
莱文：哦，真的吗？是你的学生？
斯特罗加茨：他是我在麻省理工学院任教时第一份工作教过的学生之一。
莱文：哇，真不可思议，想想你班上的那些年轻人，其中一位后来成了诺贝尔奖得主。
斯特罗加茨：确实如此。他是个令人难忘的学生，是个非常出色的人。不过，我想向您请教一下天体物理学方面的问题，因为我和亚当讨论的内容之一就是宇宙膨胀以及膨胀加速的问题。
莱文：对。
斯特罗加茨：你知道，1998年我们得知宇宙加速膨胀的消息时，你也在场。你还记得当时的情况吗？比如你的圈子里是什么感受？
莱文：哦，当然记得。我当时在伯克利，亚当也在那里。还有索尔·珀尔马特，另一个研究小组的成员，对吧？这两个小组当时是独立工作的。我当然记得，我当时在天体粒子物理中心，索尔在伯克利山上，他带着这些研究结果下山来，我当时就想，“不会吧，老兄。”说实话，当时有很多讨论，主要是关于超新星温度的巨大敏感性，你知道，影响非常大。
斯特罗加茨：所以你提到能量和敏感性的原因是，结果太令人震惊了，每个人都想变得非常非常谨慎。
莱文：非常非常谨慎。这其实不是我的专业领域，这是观测性的工作。我偏理论一些。但我清清楚楚地记得，当时我就坐在房间里，听观测者们讨论，而理论家们则对他们进行严格质询，这些理论家对数据非常感兴趣。你知道，那些理论家正处在接触数据的最前沿。那确实是令人激动的时代，毫无疑问。
斯特罗加茨：这正是我们要聊的内容。所以，对于那些可能不了解亚亚当的听众来说，他除了曾是麻省理工学院的学生外，不久后还因发现宇宙加速膨胀的研究成果与他人共同获得了诺贝尔奖。那是在2011年。但自那以后，正如我们将听到的，这个故事出现了不少剧情转折。随着新数据的出现，他不得不重新审视自己以及该领域其他人的一些发现。那么，你准备好了吗，詹娜？我们是不是该开启宇宙之旅了？
莱文：开始吧。这绝对是宇宙学中一个非常有趣的时刻。
斯特罗加茨：太好了。好的，那我们开始吧。

[音乐播放]

斯特罗加茨：亚当·里斯是约翰·霍普金斯大学的天文学和物理学教授，同时也是太空望远镜科学研究所的高级科学人员。在他的众多荣誉中，亚当因参与发现宇宙加速膨胀，于2011年共同获得诺贝尔物理学奖。欢迎来到《求索之乐》，亚当。
亚当·里斯：谢谢你的邀请。
斯特罗加茨：哦，我真高兴见到你。我想，大概从我教你的那段时间算起，有35年了吧。那时你还在麻省理工学院数学系上我的一门课——那是复分析课程。不过，我想问一下，作为如今的两个老家伙，你还记得当时的情景吗？
里斯：当然记得。麻省理工学院对我影响深远。在那里当学生感觉像是一个巨大的飞跃，所以每门课都像是“约伯的试炼”。我记得你那门课是那些极其艰难课程中的一片宁静之海。
老实说，我觉得这是因为你。我清楚地记得坐在那间教室里。你当时是个年轻的教授。我记得有一天，我坐在那里满脸困惑，正准备举手提问，你说：“亚当，你看起来被这个概念困扰了。有什么地方让你感到困惑吗？”
我当时很惊讶。首先，从来没有教授叫过我的名字，更不用说注意到我脸上的表情，还关心我是不是遇到了困扰。然后我们就直接开始讨论了。那门课真的很棒。我真希望自己后来的职业生涯一直专注于那个领域，但我后来转向了物理学。
斯特罗加茨：谢谢你分享这段回忆。我得说，我确实有时候会观察学生的表情，能看出一些端倪。你的表情当时就很丰富，现在也是。
我想我现在教过几千个学生了，所以不是所有学生都记得，但我记得那些获诺贝尔奖的，而且你可能是唯一一个。我真的为你感到骄傲。现在能和你聊天，我真的很激动。总之，太好了。
你说你后来转向了物理学，还是说你当时对物理学的兴趣就已经非常浓厚了？
里斯：当然，是的。你知道，在麻省理工学院，所有东西都是用数字而不是名字来区分的。所以我当时是“课程八”，同时选修数学系的必修课，也就是“课程十八”。那门课是1804，不是年份，虽然有时我们觉得那么古老了，我知道。但，你知道，我的真正热情所在是物理学。是真正理解这个物理世界。
斯特罗加茨：所以很快，这份兴趣就把你引向了天体物理学的子领域或相关领域，去研究宇宙中一种非常壮观的物体——超新星。我得承认，我不是天体物理学家。你可能也意识到了，所以我需要你来解释一些基础知识。什么是超新星？为什么研究它们很有趣？
里斯：好的。我先回答第二个问题——为什么研究它们很有趣——然后回头说一下：问题是什么，或者说目的是什么？对我来说，当我从物理学转向天体物理学，第一次了解到我们对宇宙的认知时，我被宇宙正在膨胀这一发现深深吸引。我觉得这太神奇了。作为一个孩子，这是我完全没有预料到的。
我本以为，宇宙就是一直存在的东西。它就像基石，是永恒的、不变的。所以了解到这一点，以及后来发现我们对宇宙还有很多不了解的地方时，感觉非常奇妙。通过观察它的膨胀、测量它的膨胀、了解它的膨胀历史，我们可以解构宇宙的本质。
我们可以算出它的年龄，可以推断它最终的命运。我们可以弄清楚它由什么构成。对我来说，这些都是我希望能得到答案的问题。而实现这一目标的方法就是找到宇宙膨胀的可靠示踪物。也就是宇宙中那些我们可以观测、像试验粒子一样的天体。
碰巧的是，当我开始研究生学习时，爆炸的恒星——超新星——恰好成为了当时最好的示踪物之一。因为它们非常明亮，我们可以在很远的地方看到它们。所以我们可以看到很远的历史，也就是过去。但同样重要的是，我确信我们之后会讨论到，要测量宇宙膨胀，你需要测量示踪物的两个方面。你需要测量所谓的红移，这本质上是因为膨胀导致示踪物发出的光波长被拉长。
这是对宇宙膨胀的直接测量。但你需要测量的另一个量是那些超新星有多远，这告诉我们你看到的是多久以前的过去，这样你基本上就是在追踪宇宙的膨胀历史，就像你在门框上标记一个正在长高的孩子的身高一样。对吧，你会想标记他们的身高，也想标记他们达到那个身高的时间。这两个量就是我们描绘宇宙膨胀图景的方法。而这些在观测上极具挑战性。
斯特罗加茨：太迷人了。你已经把我们带入了非常基础的问题中。我刚才说“哦，你喜欢天体物理学”，可能我错误地强调了“天体”部分，好像你是对超新星本身好奇。它们只是你的工具。当然，它们是很酷的天体，但你使用它们是因为你想问那些真正宏大的问题：宇宙膨胀得有多快？我们怎么知道它在膨胀？诸如此类的问题。
里斯：没错。这有点像我们有时称之为天体物理学与宇宙学的分野。宇宙学，研究宇宙作为一个整体实体的结构、形状、未来和过去，对我来说真的非常迷人。因此，你必须研究所有的天体物理学、物理学、数学以及各种细节，因为你需要从中提炼出信息。
就我而言，我倾向于研究各种爆炸、脉动的恒星，把它们作为那些关键的示踪物。
斯特罗加茨：所以，关于宇宙膨胀的问题——以及你所说的示踪物，即爆炸的恒星——然后利用红移来估算它们以多快的速度远离我们。我得说，这真是绝妙的思维。
我想请你给我们做个小小的科普，讲讲人们是如何估算星系距离的，因为这并非显而易见，而且你们有很多不同的技术，无论是使用三角学（这让我倍感亲切），还是你提到的标准烛光概念。给我讲讲宇宙距离阶梯的基础知识吧。
里斯：这其实是我工作的核心。我应该说，当我们观察这些示踪物时，我们需要测量它们的红移和距离。红移是容易的部分。你只要拍个光谱，识别出某些颜色的谱线，这些谱线已经移动到了更红的位置。这很简单。
困难的是距离。许多人称之为整个宇宙学中最大的挑战，就是弄清楚天体有多远。这是一个非常根本的问题。这几乎可以追溯到你小时候。你仰望天空，一切看起来都是二维的，对吧？你没有深度的概念。深度知觉完全缺失。所以，我们感到敬畏，但如果我们理解了那些天体有多远，我们看到了多久以前的过去，我们会更加敬畏。
那么，如何攻克这个难题呢？正如你所说，我们从已知的东西开始，那就是几何学。所以，我们尽可能测量视差。就像我们有两只眼睛，每只眼睛都能从不同视角观察附近的物体，附近物体相对于远处物体的视角或角度变化，让我们的大脑和眼睛进行几何计算，估算出物体距离多远。
问题在于，太空中的物体非常非常遥远，所以那个角度要么变得非常小，要么你需要一个更大的基线，即两个视角之间的分离距离。我们能得到的最大的基线是地球绕太阳公转时产生的——我们可以在1月份（地球在太阳一侧）和7月份（地球在另一侧）观测一颗近星相对于一颗远星的位置。
如果幸运的话，你也许能分辨出那颗近星的微小角度变化。这样你就能测量出一些最近恒星的距离。我们通过太空卫星等设备来完成这项工作。问题是，我们想探索更深的宇宙。这只是一种我们能在银河系内应用的技术。
要测量星系的距离——它们比银河系内的任何东西都要远成千上万、数百万、数十亿倍——那种视差角会变得小到无法察觉。所以我们必须换一种完全不同的方法。
我们最常用的方法，就是船长们在夜间使用的技巧：灯塔的亮度，对吧？所以，如果你是一名夜间航行的船长，你想确保自己离岩石海岸足够远。你会看一座灯塔，观察它是否显得黯淡，从而判断它是否很远，因为光线会随着距离的平方而衰减或稀释。这当然也是几何学，但它需要你了解你所看到的是什么。那是一个真正明亮的物体，一座灯塔，而不是一个小手电筒。然后你就能估算距离。
天文学家将这种方法称为“标准烛光”，即那些灯塔。他们寻找太空中能充当这个角色的天体。当我开始研究生学习时，人们普遍认识到，一类被称为Ia型超新星的爆炸恒星可以成为出色的灯塔。它们非常均匀，非常一致。虽然有些小差异。但它们提供了一个相当标准的光源，在几周内的总能量输出大约相当于40到50亿个太阳的亮度。这让你能够看到非常遥远的宇宙深处。
斯特罗加茨：让我强调一下你最后说的话，因为你的整个主题如此令人震撼，以至于你随口就讲出了这些你已经习以为常的东西。如果听众没注意到，亚当刚刚说，这样一颗爆炸的恒星，其亮度大约相当于十亿颗像我们太阳那样正常发光的恒星。好吧。一颗星星抵得上十亿颗。那可是个巨大的爆炸。
里斯：是的，确实如此。你知道，这是因为恒星拥有大量燃料，它们会以太阳那样的正常速率消耗数十亿年。但它们储备着大量能量。在一次爆炸事件中，太阳总可用能量的相当一部分，不是在数十亿年里慢慢释放，而是在数小时、数天、数周内倾泻而出。
所以，这是一个巨大的爆炸。这有点像，你知道，区别在于把车开起来，一次只烧一点点油箱里的汽油，和直接点燃油箱。所有的燃料一下子就烧光了。
斯特罗加茨：哇，太神奇了。还有标准烛光这个概念。我喜欢解释这些Ia型超新星时所用到的基础物理学。无论它们何时爆炸，释放的能量都大致相同。你说不是完全一致，但也非常接近了。你能不能给我们讲讲，是什么理论让我们对此有信心？因为那也是一个非常精妙的东西。
里斯：我想说，在过去的整整一个世纪里，宇宙学家们一直在寻找他们能找到的最佳标准烛光。你观察一个遥远的星系，每个星系看起来都和其他星系不同，因为星系是一大群恒星，不存在一个“标准群体”，对吧？它们的数量各不相同。所以它们不会成为好的标准烛光。你需要的是那些同样的单个天体，只是位于不同星系的不同位置。恒星可以，但它们太暗了。
但有一种超新星，关于它仍有一些争论，但大体原理是：它是一颗被称为白矮星的老年恒星的核心，它依靠一种叫做电子简并压的量子力学压力来支撑自身，对抗其强大的引力。伟大的印度天体物理学家钱德拉塞卡首先证明，这种状态只有在达到某个特定的临界质量（即钱德拉塞卡极限）时才稳定。这个极限是太阳质量的1.4倍。
这意味着，如果一颗恒星超过了这个极限——如果它正好处于钱德拉塞卡极限，并且假设它有一个伴星（另一颗绕其运行的恒星），物质从一颗星转移到另一颗星——当它接近甚至超过钱德拉塞卡极限时，就会发生失控的热核爆炸，因为这种电子简并压不再足以抵抗引力。它会压缩并压碎恒星，创造聚变所需的条件，基本上是在所有剩余燃料中进行热核聚变。
就像我说的，关于具体细节还有一些争论，但这是大致的轮廓，它给出了一种相当均匀的爆炸，比我们知道的任何其他宏观尺度上的东西都要均匀得多。
斯特罗加茨：如果我没理解错的话，如果我说得过于粗浅，请纠正我——这几乎像是在说，与其说是标准烛光，不如说是一颗标准的氢弹。
里斯：没错。
斯特罗加茨：因为你刚才说的大概就是这个意思，我们知道恒星中的物质总量。它大约是1.4个太阳质量。它会通过这种热核反应（也就是氢弹的高级说法）完全爆炸。也许我说的有点偏差。
里斯：是的。只是把氢换成了碳和氧，但没错。
斯特罗加茨：好的，太好了。所以它不是氢弹，是碳氧弹，但仍然是聚变反应，对吧？
里斯：是聚变。尽管关于伴星如何提供物质，或者恒星自转速度有多快等一些细节，会稍微影响你获得的能量总量。事实上，那种微小的变化——我当年的博士论文课题——就是弄清楚如何解释这种微小变化。
如果你想象一种标准的灯泡，比如一个60瓦的灯泡——我不知道现在的孩子还知不知道。你可以想象有些是58瓦，有些是62瓦，因为工厂生产的不完全一样。这可能会让你在估算距离时产生一点偏差。因为你可能看到一个看起来稍暗的超新星，因为它实际上是58瓦的，但你会误以为它更远，其实不然。
考虑到存在一定的分布，你怎么判断哪些是本身明亮、哪些是本身暗淡的呢？20世纪90年代早期发现的一个现象，后来也成了我博士论文的一部分，那就是威力更大的超新星，它的光变曲线上升到峰值和下降的速度，都比那些更暗的超新星要慢。
另一个令人困扰的效应是，我们有时不得不透过含有尘埃的星系来观察超新星，这些尘埃会遮挡光线。回到我的灯塔比喻，这就好比在有雾的夜晚看灯塔，对吧？雾会使灯塔看起来更暗，让你误以为它比实际距离更远。
但星系中的尘埃也会改变超新星光的颜色。所以，如果你能同时测量光变曲线的形状（这告诉你它是亮灯泡还是暗灯泡）和颜色（这告诉你前方有多少尘埃），你就能区分这些相互竞争的影响，回到你最初的目标：弄清楚超新星有多远。这就是我20世纪90年代初搬到哈佛后博士论文的主题。
斯特罗加茨：非常感谢你细致的讲解，因为这就像在做福尔摩斯式的推理，你掌握的证据并不完整，必须进行大量的推理，比如利用不同颜色。你还没用到“光变曲线”这个说法，但你已经暗示了亮度这个概念——它并不是只有一次闪光，就像我说的“轰”的一声。也许你该给我们多讲讲，当你观察一颗超新星时，你真正测量的是什么？
里斯：我应该指出，像这样的超新星非常罕见。所以，并不是说随便哪天晚上你仰望星空就能看到一颗。
在像我们这样的星系中，大约一个世纪才发生一次。所以，如果你想找一颗超新星，你挑一个附近的星系盯着看，发现超新星的可能性微乎其微。
突破发生在20世纪90年代，当时天文学家开始建造广角望远镜，并配有能覆盖焦平面的探测器，这样你拍一张照片就能包含成千上万个星系。
然后，你可能一个月后再拍一张。通过计算，你可以确定其中一个星系在那个月内会发生超新星。因为你已经买了足够多的彩票，你一定会中奖。在20世纪90年代，我们学会了拍摄这样的图像，进行数字相减，然后发现一个新的光点。超新星有我们所说的光变曲线。它通常需要大约两到三周的时间，从爆炸（你完全看不到）到达到最大亮度，这可以算作一种标准烛光，也就是距离指示器。
然后它会持续几个月逐渐变暗。几个月后，它大约会暗100倍。这个领域另一个引人入胜的地方在于，它不仅令人震撼，而且带有一种“消防演习”的性质，一切工作都必须迅速及时地完成，因为这些天体正在逐渐消失。你不能说，“呃，我们把观测推迟几周，回来再想想要怎么做。”不行，你必须赶在超新星消失之前，想清楚要进行哪些测量。
斯特罗加茨：你能说说你刚开始做这类研究时的情景吗？比如，你是在看一串数字吗？我猜应该是这样，并不是什么视觉性的东西吧？
里斯：实际上非常视觉化。是的，你会得到那些精美的图像——如果你见过或能在脑海中想象出一个美丽的旋涡状星系。然后你会看到它上面有一个极其明亮的亮斑，亮度可能和整个星系相当。看起来就像有人在某个地方打开了一盏聚光灯，光芒覆盖了整个区域。
现在，当你把星系放得越来越远，就像我们讨论加速宇宙时那样，我们看得如此之远，以至于整个图像缩小到只有几个像素，所以你就会说，“好吧，那个像素看起来比那个像素亮一点。”但当它们离得近时，它们是用望远镜能看到的最美丽的天体之一。
斯特罗加茨：哈，真的吗？我很惊讶。
里斯：这是在天体物理学和宇宙学领域工作的一大乐趣。这个主题很迷人，图片也很酷。
斯特罗加茨：那么，跟我说说那种乐趣吧。你还记得你第一次遇到这种情况时的情景吗？比如，你的心跳加速了吗？你是否觉得这像是一种神圣的、近乎宗教般的体验，还是别的什么？
里斯：我不会说是宗教般的，但我会说，就像我们很多人刚开始做研究时一样，对吧？你坐下来和你的导师——在这里是鲍勃·基尔希纳——他好像无所不知，充满智慧。他说，“好吧，我们来研究这些Ia型超新星。我们需要找到一颗。”然后你就说，好的。这时你就像是在进行一次寻宝游戏。然后，一些信息进来了，你把望远镜对准那里，拍下这张照片，看到这个美丽的、像雪花一样的旋涡状天体。接着那个亮斑出现了，你拍下它的光谱，光谱揭示出那颗超新星的化学成分，告诉你，是的，它是一颗Ia型。
事实证明，Ia型超新星在爆炸中会产生大量的硅和硫。所以你要寻找硅和硫的特征信号。我记得我制作了一张华丽的彩色图像，贴在我的墙上，那是我观测到的第一颗超新星。是的，我永远不会忘记它的名字、编号、距离等等。但后来，我还观测了许多许多颗。
斯特罗加茨：哦，我喜欢这个说法。这就像你的初恋。你永远不会忘记你的初恋。
里斯：哦，当然。是的。
斯特罗加茨：好了，现在我们有了些关于如何测量距离的背景知识，带我们回到90年代末吧。我们知道宇宙在膨胀。在那时我们已经知道这一点很久了，但我们当时不知道的是什么？你最终发现了什么？
里斯：所以，如果你能理解我们能够测量天体有多远，以及它们的光被红移了多少，那么这实际上就能告诉我们宇宙在其历史不同阶段膨胀了多少。这使我们能够得出宇宙的膨胀率。这是一个历史性的数值，被称为哈勃常数，因为哈勃是最先测量它的人。那很棒。
但接下来你的问题可能是，这个速率是在加快？还是在减慢？宇宙的最终命运取决于这个故事如何变化。
宇宙学家可以用一个很妙的技巧来回答这个问题，弄清楚它是如何变化的。我们可以观察更远的宇宙，也就是回溯更久远的过去，进行基本相同的测量——不是测量今天宇宙膨胀得有多快，而是利用遥远的天体告诉我们它在过去膨胀得有多快，然后最好是看看这个膨胀率变化了多少。
现在，我认为直到20世纪90年代，主流观点一直是宇宙膨胀会减慢，因为大爆炸之后，宇宙中所有物体的引力相互吸引——宇宙自身的质量会像刹车一样——最终减缓膨胀。
就像我把一个球抛向空中，地球的引力会使其减速。如果我测量这种减速，我基本上可以“称出”地球的重量。我还可以，如果仔细测量，判断那个球是会落回地球，还是抛出的速度达到了逃逸速度，以至于没有足够的质量把它拉回来，从而离开地球。所以20世纪90年代的问题是：宇宙是否足够“重”到停止膨胀，还是足够“轻”以至于会永远膨胀下去？
回答这个问题的方法，就是观测当前膨胀减慢了多少。因此，在开发出利用这些超新星测量宇宙膨胀速度的技术之后，两个天文学家团队——我所在的“高红移超新星搜寻团队”和一个竞争团队“超新星宇宙学项目”——率先寻找并发现了极其遥远的Ia型超新星，它们在80到100亿年前爆炸。所以，我们现在是在回溯宇宙历史的一半甚至三分之二处，来测量膨胀率的变化。
正如我所说，我们原以为它正在减慢。大约在1997、1998年，当我们首次获得大量数据时，我非常幸运地被选中负责分析第一批大规模数据。完成分析并进行计算后，我得出了一个疯狂的结果，最终表明宇宙正在加速，而不是减速。
斯特罗加茨：这太疯狂了。我的意思是，你当时一定在想，等等，是加速吗？
里斯：当然。事实上，我确信我搞错了。因为上过你的数学课，我对犯错误很熟悉，你知道，作为学生，你尝试做事，然后一遍又一遍地得到错误的答案。然后你试图找到自己的错误。
所以，是的，有一段时间我非常焦虑。我做了大量的交叉验证——因为总是自己发现错误更好。最后我决定和团队分享，让他们来找错误。然后，你知道，每个人都用尽所有方法进行了交叉验证，但我们找不到错误。然后我们开始接受这种可能性：这是真的，这是来自天空的信号，而且背后有深刻的物理学原因。
原因在于，在爱因斯坦的广义相对论中，引力还有另一种选择。它有另一种“模式”。你知道，在牛顿的理论中，引力只有吸引力。它只会把东西拉在一起，对吧？在爱因斯坦的理论中，空旷空间的引力可以是排斥性的。他称之为宇宙学常数，甚至在某个时候引入它，试图在他认为宇宙不膨胀时让宇宙保持平衡。
这个常数一直存在。物理学家后来将其解释为空间真空的能量，或者我们现在称之为暗能量，它可以产生相反的效果。所以我们得出的解释是，显然爱因斯坦发现了一些东西，引入这个项可以很好地拟合数据。抛开它则数据拟合得很差。

[音乐播放]

莱文：是啊，这太神奇了。就好像爱因斯坦不会犯错一样，因为他曾著名地把宇宙学常数称为自己“最大的错误”，原因正如亚当所说。他最初把它加入方程，试图让宇宙不膨胀，对吧？但它的平衡非常不稳定。所以如果平衡不完美，它并不会让宇宙静止。实际上，它会加速膨胀。
斯特罗加茨：嗯，我在想，宇宙学常数可能是那些了解科学史的人记得的东西，但它并不是标准广义相对论课程的一部分，对吧？它在90年代是不是有点生僻，还是说它一直都在那里，需要我们考虑？
莱文：我认为这是个好问题。对于愿意尝试任何可能性的理论家来说，它可能一直存在。所以本质上，人们更多地是在早期宇宙暴涨等背景下考虑宇宙学常数，但当时想法是它会蒸发掉，衰变成组成原始汤的所有粒子，它没有理由在今天还存留。
斯特罗加茨：想到这些暴涨场景让人们重新开始思考宇宙学常数，真的很有趣。
莱文：是啊。我认为很有趣的是，诺贝尔奖是因为宇宙加速膨胀而颁发的。它没有提到暗能量或宇宙学常数。他们不是因为这个获得的诺贝尔奖。
他们获奖是因为对超新星的观测，以及发现宇宙加速膨胀。我认为这与诺贝尔奖的哲学是一致的，即奖项颁发给经过验证的结果。现在，我们对暗能量一无所知，所以没有人为此获得诺贝尔奖。我们最多只能给它起个代用名。除了称之为暗能量并猜测它可能是空旷空间的能量，也许不是。
所以，如果有一天有人真的发现了暗能量是什么，那才是一个有待颁发的奖项。
斯特罗加茨：这说得很好，关于诺贝尔奖……我的意思是，当我们谈到爱因斯坦和广义相对论时，众所周知他并没有因为狭义或广义相对论获得诺贝尔奖，对吧？
他的诺贝尔奖——我笑了，因为当时他确信自己会获奖，以至于可以把奖金许诺给妻子。你不知道这个故事吗？
莱文：真的，我不知道。
斯特罗加茨：他承诺把诺贝尔奖的奖金给他的第一任妻子，因为他知道自己离婚后妻子需要一些钱。
莱文：太惊人了。
斯特罗加茨：但他获奖不是因为相对论。他获奖是因为，你知道，他对光电效应的解释。
莱文：嗯哼，是啊，不可思议。
斯特罗加茨：但这很有意思，诺贝尔奖委员会使用的这种审美标准或准则，他们希望像你说的那样，是经过验证的、实验上或观测上坚实的物理学。
莱文：我认为这很恰当。本来就该如此。
斯特罗加茨：好的。说到我们故事中的剧情转折，你知道，如果宇宙学常数是第一个转折，那么事实证明，就像我们中场休息后要听到的那样，还有另一个剧情转折即将到来。
莱文：太好了。充满悬念。

[音乐播放]

斯特罗加茨：欢迎回到《求索之乐》。我们正在与约翰·霍普金斯大学的天体物理学家亚当·里斯讨论宇宙加速膨胀的问题。
你提到了宇宙学常数。它常常追溯到爱因斯坦本人。在他的符号中，他使用希腊字母拉姆达来引入一个被某些人轻蔑或嘲讽地称为“修正因子”的东西。但这是广义相对论方程所允许的。正如你所说，爱因斯坦最初想要它，是因为他当时无法相信宇宙在膨胀。
里斯：对，你得理解爱因斯坦，他询问当时的实验学家、天文学家，宇宙在做什么？他们告诉他，天体的运动非常非常微小。
他们之所以这么说，是因为当时甚至还没有发现星系在银河系之外。所以天文学家认为宇宙就是银河系本身。当他问银河系在做什么——是在膨胀还是收缩——他们回答说，运动相当温和，似乎没有在膨胀或收缩。这让他感到困惑，因为如果你有一堆质量体，它静态不动，然后你放手，对吧？它们会由于引力而聚集在一起。
那么是什么让它保持静态呢？所以他认为，正如你所说，在他的方程中，有一种选项可以抵消引力的吸引力。所以他采用了这个选项，并将其设置为他的边界条件（我们会这么说），即宇宙是静态的。
当然有两个问题：一个是宇宙在膨胀，所以那不是正确的边界条件。另一个是，这是一个不稳定的平衡。就像把一颗弹珠放在篮球顶上。是的，理论上它可以待在那里，但如果你推它一下，只要有一点点偏差，它就会朝一个方向或另一个方向滚跑。所以当他看到宇宙在膨胀时，当哈勃和其他人证明了这一点时，他著名地说这（引入这个概念）是他职业生涯中最大的错误。然而，却又无法摆脱它。
就像很多数学和物理学一样，方程允许它存在，除非你知道它不可能存在。所以，大约80年后，我们在这里说，实际上看起来宇宙在加速，就好像那种排斥性的引力现在赢了，战胜了物质的吸引性引力。
斯特罗加茨：我听过一句有趣的话，我不确定是谁说的：“未被禁止的，就是必须做的。”你知道，在这种情况下，拉姆达项，宇宙学常数，没有被方程禁止。方程允许它存在。而实际上，现在我们相信存在类似的东西。正如你所说，现代术语通常是暗能量。但是，我们可能会陷入一个巨大的迷宫，问自己：这个暗能量到底是什么？
但我想换个方向，谈谈ΛCDM模型，即冷暗物质模型。因为，我是说，我知道你肯定想告诉我们，它可能需要一些调整，或者可能不仅仅是调整。但我们不妨先说明一下，这是一个非常有趣、强大的标准模型。告诉我们这个模型是什么，它的吸引力在哪里？它有哪些伟大的成功之处？
里斯：当然。当我们构建一个所谓的宇宙模型时，首先，我们说的是宇宙由什么构成。如果你想制造一个宇宙，配方是什么？也就是东西是什么。然后我们通常还会把物理定律作为其中的一部分。
但在宇宙的情况下，人们熟悉的是元素周期表中的元素。那是小东西，只占宇宙的4%。其余的都是黑暗的。黑暗意味着它不发光。所以我们不能直接看到它。我们必须通过它的引力来推断它。所以从20世纪30年代到70年代及以后，天文学家逐渐了解到，宇宙中有大量的物质具有吸引性引力，但却是黑暗的，不发光，称为暗物质。
它使星系的旋转速度远远超过仅由发光物质所指示的速度。否则恒星就会从它们的星系中飞走。它使星系绕其他星系和星系团运行，否则，如果没有额外的引力胶水，它们也会飞走。还有光线弯曲，称为引力透镜。所有这些技术都让我们弄清楚，宇宙大约25%是暗物质，也许……26%……大约4%是普通物质。所以大约30%在那里。
然后，我们在20世纪90年代的研究结果，以及随后对宇宙微波背景辐射的观测，都表明另外70%是这种暗能量。所以我们得到了一个完整的宇宙图景，可以解释我们所看到的一切，但其中96%是看不见的。
需要明确的是，当我们谈论它时，我们谈论的是它的引力作用，但我们并没有真正谈论，或者说还无法谈论，它的微观物理学、细节。暗物质，我们认为它是一种粒子，但我们不知道粒子的本质。我们不知道它是否稳定，是否有其他相互作用。
对于暗能量，我们基本上只是笼统地说它是真空的能量。但是，做量子力学计算的人试图估算它应该有多少，他们得到的答案与我们实际测量的值相差了120个数量级。所以我们拥有的，我要说，是一个非常好的“恰好如此”的故事，一个建立在许多坚实物理学概念基础上的现象学模型。但它仍然是一个模型。它不是物理学本身。它不是对那些事物本身的描述。目前，我们以最普通的形式来对待暗物质和暗能量，因为我们还没有发现它们有任何更复杂的特征，但我们正在寻找。
斯特罗加茨：最普通的形式，我明白你的意思。你刚才描述了一些模型中的参数。也就是说，在不说明暗能量是什么的情况下，我们可以估算出它对宇宙能量预算的贡献。还有暗物质的量，发光物质的量。这个标准模型中还有几个其他参数。但它的支持者（我认为这已经形成共识）认为，它是最好的模型——尽管它是个模型，对吧——它能解释很多事情。
里斯：绝对可以。它解释了很多事情。它解释了为什么宇宙在膨胀。它解释了为什么我们有特定轻元素的化学丰度。它解释了我们看到的宇宙微波背景辐射，以及我们看到的它的涨落。我们看到的那种特定的涨落谱。然后它预言了宇宙的膨胀历史，包括早期由吸引性引力主导的减速阶段，这使得星系团、行星和所有那些好东西能够形成。然后是一个后来的加速阶段，当空间膨胀稀释了物质后，由这种暗能量主导。
所以它解释了我们看到的大部分东西，而且它也应该如此，因为它是我们观察宇宙后发展起来的，我们添加或修改它以匹配我们所见。所以我可以说，直到21世纪初，甚至可能是2010年，它完美地拟合了一切。
但因为我们有这些深刻的问题，我们不仅仅是在寻找一个模型，我们是在寻找背后的物理学。人们继续进行更精确的实验，因为科学的本质一直是，你有一个模型，它是现实的最佳近似。但不可避免地，它会有一些缺点，也许你还没有看到或理解。
对于我们所称的ΛCDM模型，缺点一部分是理论上的，即我们不理解这些黑暗部分。但在观测上，它非常出色。然后随着21世纪10年代的发展，人们确实尝试进行越来越精确的实验来检验这个模型，因为我们想厘清暗能量是一个常数，还是一个随时间变化的东西？
宇宙中有一些先例，在大爆炸后不久有一个我们称为暴涨的时期，那也可能是一种暗能量，一种会随时间出现和消失的不同暗能量。我们想知道，我们今天的暗能量是在消失还是在变强？我们想知道这一点。我们想知道更多关于暗物质本质的信息。因此，在我们进行理论研究的同时，我们也进行更好的实验，试图告诉我们更多信息。
斯特罗加茨：我认为同时欣赏它的成功和不足是非常有趣的。对吧，因为我们要讲述的其余故事是存在差异的故事，但只有当你理解了理论有多好时，这些差异才变得真正引人注目，至少对我来说是这样。到目前为止，如你所说，该模型通过调整你无法以其他方式估计的参数，精度达到了1%以内。但即使有了这种调整，它也能解释许多不同的事物，精度在1%以内。
里斯：人们应该理解这个模型非常成功。它解释了很多东西。它作为标准模型已经存在了大约25年。而且它经受住了实验精度许多次巨大飞跃的考验。但我想我们也会谈到，我们也开始看到一些模型的裂缝迹象，我想我们仍在努力解决这些问题。
斯特罗加茨：那我们现在就来聊聊这个。我发现这在你自己的生涯中非常引人注目，这正说明了你是多么真正的科学家。如果你不介意我在这里称赞你几句，你提到了2010年代，你和其他人开始注意到这个问题。你去瑞典领诺贝尔奖时，你还是个年轻人，只有41岁左右，但你不满足。你没有准备仅仅成为一个管理者。你想继续做科学。
里斯：是啊，科学很有趣，对吧？我那时41岁，你知道，很多人说，“哦，你现在打算做什么？”我看了看其他诺贝尔奖得主，不幸的是，平均年龄已经上升到了70岁左右。所以普通人会觉得，我的学术生涯已经结束了，我是荣誉退休人员了。所以这就像是我要跑一段胜利之圈，去做做演讲之类的。但我才41岁，这不仅太早了，而且对我来说，停止我正在做的事情完全没有吸引力。
我的意思是，我当时正在使用哈勃太空望远镜，那是多么酷的事情，用来观测遥远的爆炸恒星和脉动恒星，描绘宇宙的膨胀历史。我决定，在斯德哥尔摩之旅后，唯一的出路就是尽可能地推掉那些演讲之类的事情，真正专注于科学，因为你知道，一旦停下来就很难回去了。
斯特罗加茨：好吧，那么，这就是剧情变得复杂的地方。你开始进行这些改进的测量。然后，告诉我们你发现了什么，以及张力是如何进入我们的戏剧的。
里斯：我们有一个新模型，ΛCDM，它有这些组成部分——暗物质、暗能量——这都很好。问题变成了，这种暗能量的本质是什么？所以作为一个物理学家，我们说，“好吧，让我们找一个我们能测量的数字或属性，它会告诉我们一些相关信息。”对于暗能量产生引力的方式，重要的是所谓的状态方程，即其压力与能量密度的比值。这个数字叫做W。如果它是一个宇宙学常数，那么W在所有时间都等于-1。
如果它是某种空间中的场——想想你知道的场，电场、磁场——这将是一种不同的场，但它会有能量，并且它的能量会像暗能量一样起作用，那么它可能不是-1，可能是其他数字。为了研究这个W数，人们已经付出了很多努力，建造了空间任务。
我发现做到这一点的更好方法之一是测量哈勃常数，即宇宙当前的膨胀速率，并将其与基于ΛCDM模型以及来自WMAP和普朗克等NASA和ESA发射的新太空卫星的精美宇宙微波背景数据所应得的预测值进行比较。我启动了一个名为SHOES的新项目，利用哈勃太空望远镜，试图将测量精度从之前的10%提高到1%。
起初进展非常顺利[笑]。我们得到的测量结果不确定度越来越小，并且它们与宇宙微波背景的结果大体

英文来源：

How Fast Is the Universe Really Expanding?
Introduction
One of the biggest mysteries in cosmology seems to keep getting bigger. Astronomers have known since the 1930s that the universe is expanding, but in the 1990s, the discovery that this expansion is accelerating rather than slowing down came as a huge shock to the field. Something had to be driving that acceleration, and dark energy was proposed as the cause. What began as a seismic shock in cosmology eventually led to a Nobel prize.
But this story has a sequel, and it comes with another major plot twist. The two main methods for estimating the universe’s present-day expansion rate are producing significantly different answers. As a result, how fast the universe is really expanding has become a matter of considerable debate — with no small amount of angst — and the discrepancy has become known as the Hubble tension. Perhaps most surprising of all is that one of the loudest voices raising concern is Adam Riess, the astrophysicist whose Nobel Prize-winning work helped ignite the acceleration debate in the first place.
Riess joined co-host Steven Strogatz on The Joy of Why to explain how we got to this point, what the Hubble tension might be telling us, and what may happen next.
Listen on Apple Podcasts, Spotify, TuneIn or your favorite podcasting app, or you can stream it from Quanta.
Transcript
[Music plays]
STEVE STROGATZ: I’m Steve Strogatz.
JANNA LEVIN: And I’m Janna Levin.
STROGATZ: And this is The Joy of Why.
LEVIN: A podcast from Quanta Magazine, where we explore some of the biggest unanswered questions in math and science today.
STROGATZ: Well, great to see you, Janna. How’s it going?
LEVIN: Hi, Steve. Yeah, great to see you. I’m excited to hear what you have to talk about today.
STROGATZ: Yeah, good, because I could barely contain myself. This was an especially fun conversation I had with Adam Riess, who I guess is someone you would know from the circuit.
LEVIN: Yeah. Definitely. I was speaking with him not too long ago.
STROGATZ: Uh-huh. Well, right, I would think that in the astrophysical world you would have crossed paths, so I’m not super-surprised to hear it. But actually, I myself had not spoken to Adam or seen him for many, many decades. It turns out he was a, uh, a student when I was just beginning as a professor.
LEVIN: Oh, wow. In your class?
STROGATZ: He was literally one of my students at MIT in my first job.
LEVIN: Wow, how amazing to think these young kids in your class, one of them’s gonna be a Nobel prize winner.
STROGATZ: Well, exactly. Yeah, very memorable student. Really a great guy. But, I would actually like to pick your astrophysics brain if we could for a second because one of the things that I talked to Adam about is the issue of the universe expanding and also its expansion accelerating.
LEVIN: Right.
STROGATZ: You know, like you were there back in 1998 when we got the news about the acceleration. Do you, um, remember anything about that time? Like what it felt like in your community?
LEVIN: Oh, yeah. Well, I was at Berkeley, and that’s where Adam was. And also Saul Perlmutter, the other group, right? These were two different groups working. So I absolutely remember, I was at the Center for Particle Astrophysics, and Saul was kind of up in the hill in Berkeley, and he was coming down with these results, and I was like, “Come on, man.” Then there was a lot of discussion, honestly, about how there’s huge sensitivity to the temperature of the supernova, you know, very high power.
STROGATZ: And so the reason you’re bringing up this thing about the power and the sensitivity is that the results were so shocking that everybody wanted to be very, very careful.
LEVIN: Very, very careful. And this is outside my field, this is very observational. I’m more on the theoretical side. But, I remember so well, though, just being in the room while those conversations were happening between the observers, and they were really being interrogated by the theorists who were very interested in data. You know, the theorists right on the cusp of data. It was really interesting times, no question.
STROGATZ: That’s exactly where we’re gonna go. So, for our listeners who may not know about Adam, in addition to having been a student at MIT, soon afterward, he shared a Nobel Prize for his work on the discovery of the accelerating expansion of the universe. That was in 2011. But since then, as we’re gonna hear, there have been quite a few plot twists in the story. As new data have come in, he’s had to revisit some of his findings and the findings of other people in the field. So if you’re ready, Janna, should we go cosmic?
LEVIN: Let’s do it. It’s a really interesting moment in cosmology, for sure.
STROGATZ: Fantastic. Okay. Well, here we go.
[Music plays]
STROGATZ: Adam Riess is a professor of astronomy and physics at the Johns Hopkins University and a senior member of the science staff at the Space Telescope Science Institute. Among his many honors, Adam is the co-winner of the 2011 Nobel Prize in physics for his part in the discovery of the accelerating expansion of the universe. Welcome to The Joy of Why, Adam.
ADAM RIESS: Thank you for having me.
STROGATZ: Oh, well I am so excited to see you I think it’s something like, I’m guessing about 35 years from the time. When you were an undergraduate in a class that I was teaching in the math department at MIT, it was a course in complex analysis, but I’m wondering, as a couple of old men now, can you tell me what you remember from that experience?
RIESS: Sure, of course. MIT was very impactful to me. You know, being a student there felt like a big jump, and so each of those courses felt like, you know, Job’s trials. And I remember that class was a sea of tranquility in the midst of other things that were super tough and rough.
I think it was because of you, to be honest. I remember vividly sitting in that class. You were a young professor. And I remember one day sitting there puzzled and getting ready to raise my hand, and you said, “Adam, you look bothered by this concept. Is there something about it that bothers you?”
And I was amazed. No professor, first of all had ever said my name, let alone actually gauged the expression on my face and cared that something was disturbing me. And, you know, we got right into it. And, so that course was great. I’d like to say that I spent the rest of my career focused on that material, but I moved over towards physics.
STROGATZ: Well, thank you for sharing that memory. I have to say, I do sometimes look at students’ expressions on their face and I can see something. You had a very animated face. You still do.
I mean, I think I’ve taught a few thousand students now so I don’t remember all my students, but I do remember the ones that win Nobel prizes and I think you might be the only one. So it’s really very—I’m very proud of you. I’m thrilled to be able to talk to you now. Anyway, it’s great.
So you say you moved into physics after, or I guess you were already probably pretty intensely interested in physics at that point?
RIESS: Sure, I was. You know, at MIT, everything is numbers instead of names. So I was course eight, and I was taking the required courses in math, which was course 18. That course was 1804, not the year. Although sometimes we feel that old, I know. But, but, you know, really my passion was physics. It was really understanding the physical world.
STROGATZ: And so very quickly that interest led you into subfield or adjacent field of astrophysics, and into the study of very spectacular objects in the universe, called supernovas or supernovae — if we want to be in Latin about it. So I have to admit, I’m not an astrophysicist. You probably realize that, and I need some of the basics explained here. So what are supernovas and then why are they interesting objects to study?
RIESS: Right. I’ll take the second — why they’re interesting objects to study — and I’ll jump back and say, what is the question or what is the point? And so to me, when I went from physics to astrophysics and first learned what it is that we know about the universe, I was fascinated by the discovery that the universe is expanding. I just think that’s amazing. It’s, as a kid, it’s not what I would’ve expected.
I would’ve expected, the universe is just the thing that’s always been there. It’s like the bedrock, you know, it’s eternal, it’s unchanging. And so that was amazing to learn and then to further learn that we still don’t understand that much about the universe. And so by watching it expand, by measuring it expand, by seeing its expansion history, we can deconstruct the nature of the universe.
We can figure out how old it is, we could figure out what its ultimate fate is. We could figure out what it’s composed of. And so those are all really, to me, the questions that I wanted to have answered. It just turns out the means to do that is to have reliable tracers of the expanding universe. So objects in the universe that act like test particles that we could watch.
And it just so happens when I started graduate school that exploding stars, supernovae, became some of the best tracers right at that time. Because they’re so luminous, you could see them far away. So you could see far into the history, the past. But also, and this is the most important point, which I’m sure we’ll talk about, to measure the expansion of the universe, you need to measure two aspects of your tracer. You need to measure what’s called the red shift, which is essentially, the stretching of the wavelengths of light emitted by that tracer because of expansion.
So it’s a direct measure of the expansion of the universe, but then the other that you have to measure is how far away those supernovae are, which is telling you how far back in time you are looking so that you’re essentially tracking the expansion history of the universe the same way you would, you know, if you were to mark the height of a growing child on a door frame. Right, you would wanna mark their height and you’d wanna mark the time when they were at that height. And so those two quantities are how we chart the expansion of the universe. And those are observationally challenging.
STROGATZ: Well, that’s fascinating. So you’ve already anchored us in really fundamental questions here that although I was pitching it as, “Oh, you like astrophysics.” Maybe I was mistakenly emphasizing astro, like, you’re curious about supernovae as things in themselves. Like they are a tool for you. Sure they’re cool objects, but you’re using them because you wanna ask really big questions: How fast is the universe expanding? How do we know it’s expanding? That kind of thing.
RIESS: Right. This is sort of the dichotomy that we sometimes call astrophysics versus cosmology. Cosmology, the study of the structure, the shape, future, the past of the universe, as a whole entity, is really fascinating to me. And, you know, as a result, you have to study all the astrophysics and the physics and the math and the nitty gritties because, you know, you need to tease out that information.
It so happens, in my case, I tended to study various classes of stars exploding, pulsating as those critical tracers.
STROGATZ: So this question about the universe expanding and the — what you refer to as tracers — the exploding stars, and then estimating how far away they are using the redshift to tell us about how fast they’re receding. It’s really gorgeous thinking, I have to say.
I would love you to give us a little tutorial about how it is that people can estimate how far away galaxies are because it’s not an obvious thing and you have a lot of different techniques, whether using trigonometry — very near and dear to my heart — or your concept of standard candles. Just give me a little basics of the cosmic ladder.
RIESS: This is actually the meat and potatoes of what I do. I should have said, when we look at these tracers, we need to measure their red shifts and distances. And the redshift is the easy part. You just take a spectrum and you recognize certain colors or lines which have moved to a redder location. And it’s trivial.
The hard part is the distances. This is many have called the biggest challenge in all of cosmology is to figure out how far away things are. And, it’s a really fundamental problem. It goes back to, you know, almost being a little kid. You look out at the sky and everything looks very two dimensional, right? And you have no sense of depth. The depth perception is completely absent. And so, we’re in awe, but we’d be in more awe if we just understood how far away things were, how far back in time.
So how do you lick that problem? And as you said, we start out with the things we know, which is geometry. And so we measure parallaxes when we can. And so just like we have two eyes and we get a vantage point from each of them on nearby objects, and the changing perspective or angle of something nearby, relative to far away, allows our brain and eye to do the geometry and estimate how far away something is.
The problem is that space objects become very, very far away so that angle either becomes very tiny or you need a much bigger baseline, a separation between the two perspectives. And the best one, the biggest one we get is when the Earth goes around the sun and we could view a nearby star relative to a distant star, let’s say in January when we’re on one side of the sun and in July when we’re on the other.
And if you’re lucky, you might be able to tease out the little change in angle of that nearby star. And so you can gauge the distance to some of the nearest stars. And we do this with space satellites and things like that. The problem is we’re after deeper waters. This is only a technique we can apply within the Milky Way galaxy.
To be able to measure the distance to galaxies — which are thousands, millions, billions of times further away than anything in the Milky Way — that parallax angle would become imperceptibly small. So we have to switch to a completely different method.
And the method we use most commonly is what ship captains know to use at night, which is the brightness of a lighthouse, right? So, you know, if you’re a ship captain at night, you wanna make sure you’re far enough away from a rocky shore. So you look at a lighthouse and you look for it to be faint, telling you that it’s far away that the light attenuates or dilutes as one over distance squared. That’s geometry too, of course, but it requires some understanding of what it is you’re looking at. That it’s a truly luminous object, a lighthouse, not a little pen light. And then you can gauge distances.
And so that method astronomers call “standard candles,” the lighthouses. And they look for objects in space that can serve that role. And as I started graduate school, there was great recognition that a certain class of exploding stars, called Type IA supernovae could serve as outstanding lighthouses. They were very homogeneous, very uniform. They had small differences. But they gave you what was a pretty standard light source that was maybe 4 or 5 billion solar luminosity in total output for a few weeks. So that allows you to see very deep into space.
STROGATZ: So let me just underscore that last thing you said, because your whole subject is so mind blowing that you rattle off these things that, you know, you’re so used to by now. If the listener didn’t catch that, Adam just said that this one star exploding is about equivalent in brightness to about a billion stars shining normally like our sun. Okay. One star is as bright as a billion. That is a big boom.
RIESS: Yes, it really is. You know, it’s really because stars have a lot of fuel and they’ll spend billions of years putting that output at a kind of normal sun-like rate. But they have a lot of energy in reserve. And during an explosion event a fair fraction of the total available energy of the sun, instead of trickling out over billions of years, trickles out over hours, days, weeks, all the rest of it.
So, it is a big boom. It’s sort of like, you know, the difference between driving a car with a gas tank, little bit of gas at a time, versus lighting the gas tank on fire. It’s just all of the fuel just goes up.
STROGATZ: Wow. That’s amazing. And then also this idea of standard candles. I love the basic physics that goes into explaining these Type I supernovae. Whenever they blow up, they will blow up with about the same amount of energy. You said it’s not perfect, but it’s pretty darn close. So if you could just tell us a little about the theory that gives us confidence that’s true, ’cause that’s a really pretty thing too.
RIESS: So I would say throughout the last century of cosmology, cosmologists looked for the best standard candle they could find. So, you look at a distant galaxy and every galaxy looks different than every other galaxy because a galaxy is a crowd of stars and there’s no such thing as a standard crowd, right? Those are different numbers. So those are not gonna be good standard candles, right? So what you need are actually individual objects that are the same kind of object just located in different places in different galaxies. Stars work, but they’re too dim.
But this one kind of supernova, which there’s still some debate about this, but generally the general principle is it’s the center, the core of an old star called a white dwarf star, which is holding itself up against its crushing gravity by a kind of quantum mechanical pressure called electron degeneracy pressure. And this is something that the great Indian astrophysicist, Chandrasekhar first showed was stable only at a certain critical mass known as Chandrasekhar’s limit. So it’s 1.4 times the mass of our sun.
And so that means that if a star exceeds that limit — if it’s sitting there at Chandrasekhar’s limit, and let’s say it has a friend, another star orbiting it, and material gets transferred from one star onto the other — if it gets close to or even exceeds Chandrasekhar’s limit, then you will get runaway thermonuclear explosion because this electronic degeneracy pressure is no longer strong enough to hold back gravity. And it will compress and crush the star, giving you the property necessary for fusion and basically doing thermonuclear fusion over all remaining fuel.
So as I said, there’s some debate about exactly the details, but this is the general broad-brush picture that gives you a fairly uniform explosion, far more uniform than anything else we know on a kind of macroscopic scale.
STROGATZ: If I followed you correctly there, and correct me if this is too crude, it’s almost like saying rather than standard candle, it’s a standard hydrogen bomb.
RIESS: Correct.
STROGATZ: Because it sounds like that’s what you just said, that we know the amount of material in the star. It’s gonna be 1.4 solar masses approximately. And it’s gonna completely blow up through this thermonuclear, which is fancy talk for hydrogen bomb. Maybe I’m a little bit off.
RIESS: Yeah. With the one substitution of it’s carbon and oxygen instead of hydrogen, but yes.
STROGATZ: Okay, good. Great. So it’s not a hydrogen bomb, it’s a carbon oxygen bomb, but still it’s fusion, right?
RIESS: It’s fusion. And while some details about how the other star maybe donates the material or how fast the star is rotating can perturb a little bit the amount of energy you get out. And in fact that small variation, that was my thesis project, was to figure out how to account for the small variation.
That if you think of a kind of a standard light bulb, like a 60-watt light bulb — I don’t know if kids still know. But you could imagine some are 58 and some are 62, ’cause the factory doesn’t make them all the same. And that could fool you into misestimating the distance a little bit. ’Cause you may be seeing one that looks a little fainter, ’cause it was 58 watts, but instead you’re, you think, oh, it’s further away, but it isn’t.
How do you figure out from here which ones are intrinsically bright or intrinsically faint, given a little bit of distribution? And one of the things that was discovered in the early 1990s and became part of my thesis was the ones that are more powerful, rise more slowly and fall more slowly, to reach their peak than the ones that are more dim.
And another confounding effect, we sometimes have to look at a supernova through a galaxy that has dust in it and that dust can obscure the light. So going back to my analogy of a lighthouse, it would be like looking at a lighthouse on a foggy night, right? It could make the lighthouse look dimmer and fool you into thinking it was further away than it really was.
But the dust in galaxies also shifts the colors of the supernova light. And so if you could simultaneously measure the light curve shape that tells you whether it’s a bright or dim light bulb, and the colors that tell you how much dust is in the way, you could tease out these competing effects and get back to what you wanted, which was to figure out how far away the supernova was. And that was the subject of my doctoral thesis when I moved to Harvard, in the early 1990s.
STROGATZ: Well, I appreciate your going through that because this is such a bit of Sherlock Holmes work that you don’t have complete evidence. You have to do a lot of reasoning like they’re the use of the different colors. Or you didn’t use the phrase light curve yet, but you sort of hinted at this idea that the brightness — it’s not just one flash, like I said, a boom. But maybe you should tell us a little more about that, that when you’re looking at a supernova, what are you really measuring?
RIESS: I should point out that a supernova is incredibly rare like this. So it’s not like, you know, you could expect to see one on any given night when you look out at the sky.
There is one in a galaxy like ours about once a century. And so, you know, if you wanna find a supernova, you just pick a nearby galaxy and you stare at it, you’re very unlikely to find a supernova.
The breakthrough came in the 1990s, when astronomers began to build telescopes that had wide angles, and they had detectors that they could cover the focal plane with, so that you could take a single image that might contain hundreds of thousands of galaxies in one frame.
Then you take another image, maybe a month later. And by math, you’re sure that one of those galaxies will have had a supernova over that month. Because you’ve just bought so many lottery tickets that you’re bound to win the lottery. And in the 1990s we learned to collect such images, digitally subtract one from another and find a new point of light. And then the supernova, it’s what we call its light curve. It usually takes about two or three weeks to go from explosion, when you don’t see it at all, to reach its maximum output, which is the sort of standard candle, if you will. The distance indicator.
And then it will fade over the course of months. After a few months, it’ll be about 100 times fainter. And so what’s also so fascinating in this field is not only is it kind of mind blowing, but it has this kind of fire drill aspect to it where everything has to be done very fast and timely because these things are fading away. You can’t say, “Uh, we’ll put that observation off a couple of weeks and come back and figure out what we wanna do.” No, you have to be, like, figuring out exactly what measurements you wanna make before the supernova fades away.
STROGATZ: Can you give us what it was like when you were first doing this kind of thing? Like, are you just looking at a stream of numbers? I assume you are, you’re not, there’s nothing visual here?
RIESS: It is actually very visual. Yeah, you get these elegant images of, if you’ve seen or can picture in your mind’s eye, a beautiful spiral-looking galaxy. And then you will see this extremely bright knot on it that may be as bright as the entire galaxy. It just looks like somebody turned on a spotlight in one spot, and it’s just bloomed over the whole area.
Now, as you place the galaxy further and further away, and as we talk about the accelerating universe, we look so far back that this whole image shrinks to just a few pixels, so that you’re saying, “Okay, that pixel looks a little brighter than that pixel.” But when they’re nearby, they’re some of the most beautiful objects you could see with a telescope.
STROGATZ: Huh, really? I’m so surprised.
RIESS: It’s one of the great joys of working in astrophysics and cosmology. The subject’s fascinating and the pictures are pretty cool too.
STROGATZ: So tell me about that joy. Can you remember back when you first encountered this? Like, was your heart beating? Did you think this is like a holy experience, like religious almost, or what?
RIESS: I wouldn’t say religious, but I would say like a lot of us starting out in research, right? You sit down with your advisor and he — in this case, Bob Kirschner — he sort of knows everything and is wise and. He’s like, “All right, let’s study these Type IA supernovae. We need to find one.” And you’re like, okay. And you’re kind of on an Easter egg hunt at that point. And then, you know, some information comes in and you turn the telescope there and you take this picture and you see this beautiful spiral-looking-like snowflake. And then there’s that bright spot and you take a spectrum, and the spectrum reveals the chemical composition of that supernova that tells you, yes, it is a Type IA.
It turns out Type IA’s produce a lot of silicon and sulfur in their explosions. And so you look for the signature of silicon and sulfur and I remember making a gorgeous color image and putting it on my wall as the first supernova I ever observed. And, yeah, I’ll never forget its name and number and distance and everything. But then there were many more after that.
STROGATZ: Oh, I like it. This is like your first love. You never forget your first love.
RIESS: Oh, for sure. Yes.
STROGATZ: Well, now that we have a little background about how to measure distances, take us to the late 90s. We know that the universe is expanding. We knew that for a long time at that point, but what didn’t we know? What did you end up discovering?
RIESS: So, if you can understand that we could measure how far away objects are and how much the light has been redshifted, then that maps to really telling us how much the universe has expanded up to different points in its history. And so that allows us to tell the expansion rate of the universe. It’s a historic number called the Hubble Constant, since Hubble was the first to measure that. And so that’s great.
But then your question might be, you know, is that rate speeding up? Is it slowing down? What is the ultimate fate of the universe depends on how this story is changing.
And so there’s a great trick that cosmologists can use to answer that, to figure out how it’s changing. We can look further out to look further back in time and make essentially the same set of measurements, not measuring now how fast the universe is expanding today, but using distant objects to tell us how fast it was expanding in the past, and then ideally how much that expansion is changing.
Now, the prevailing wisdom, I would say, up and through the 1990s was that the expansion of the universe would be slowing down because after the Big Bang, you would have the attraction of the gravity of all the objects in the universe — the mass of the universe itself would act like a brake — and it would slow the expansion.
Just like if I toss a ball into the air, the gravitational pull to the Earth will cause it to decelerate. And if I measure that deceleration, I could essentially weigh the Earth. And I can also, if I carefully measure that, figure out whether that ball will land on the Earth or maybe it was thrown with escape velocity and that there isn’t enough mass to pull it back and it will leave. And so the question in the 1990s was: Is the universe heavy enough to stop its expansion or is it light enough for the universe to expand forever?
And the way to answer this was to see how much the expansion was now slowing down. And so after developing the techniques to measure how fast the universe is expanding with these supernovae, two teams of astronomers — a team I was on called the High Z supernova team, and a competing team called the Supernova Cosmology Project — were the first to look for and find ultra distant Type IA supernovae, which exploded eight to 10 billion years ago. So looking now halfway, two-thirds of the way, through the history of the universe to measure the change in the expansion rate.
And as I said, we thought it would be slowing down. And circa 1997, 1998 when we first got our large collection of data, and I was very fortunate to be picked to lead the analysis of the first large tranche of data, I ran the calculations after doing the analysis and I came out with a crazy results that ultimately showed the universe was accelerating, not decelerating.
STROGATZ: It’s insane. I mean, you must have thought, wait a second, it’s accelerating?
RIESS: Sure. And in fact, I was sure that I did something wrong because having taken your math class right, I was very familiar with making mistakes, you know, this is what you do as a student is you try to do things and you get the wrong answer over and over and over. And then you try to find your mistake.
And so yeah, it was very worrisome for a while. So I did a lot of cross checks — because it’s always better to find your own mistake. Then finally willing to share with the team and let them find the mistake. And then, you know, everybody cross-checked everything we could and we could not find the mistake. And then we started getting comfortable with the possibility that it was real, that this was the signal on the sky and that there was a deep physics reason why this was.
And that is that in Einstein’s theory of general relativity, gravity has another option. It has another kind of gear. You know, in Newton’s theory, it’s only attractive. It only pulls stuff together, right? In Einstein’s theory, the gravity of empty space can be repulsive. Something he called the cosmological constant, and even invoked at one point to try to keep the universe in balance when he thought it wasn’t expanding.
And it’s always been around. Physicists later interpreted this as the energy of the vacuum of space, or what we now call dark energy, and it can go the other way. And so the interpretation we came to was that apparently Einstein was onto something, and that introducing this term gave a good fit to the data. Leaving it out gave a bad fit to the data.
[Music plays]
LEVIN: Yeah, it’s quite amazing. It was like Einstein couldn’t make a mistake ’cause he famously called the cosmological constant his greatest blunder, for the reason Adam said. He originally put it into his equation to try to make the universe not expand, right? But it’s very precariously balanced. And so if the balance isn’t perfect, it doesn’t make the universe static. It actually accelerates the expansion.
STROGATZ: Well, I’m wondering, the cosmological constant was something maybe people who knew history of science would remember, but it wasn’t part of standard general relativity coursework or anything, right? Wasn’t it sort of obscure in the ‘90s, or was it always there as like we need to think about it?
LEVIN: I think that’s a good question. It was probably there for theorists who would try anything. So essentially, people were thinking about the cosmological constant more in the context of things like inflation in the early universe, but then the idea was it would evaporate away, and it would decay into all the particles that made up the primordial soup, and it had no business hanging around today.
STROGATZ: That’s really interesting to think about that, that these inflationary scenarios had sort of softened people up to starting to think again about the cosmological constant.
LEVIN: Yeah. And I think it’s really interesting the Nobel prize was for the accelerated expansion. It doesn’t mention dark energy or a cosmological constant. That’s not what they got the Nobel prize for.
They got it for the observations of the supernovae, which is really interesting, and the accelerated expansion. And I think that’s consistent with the Nobel philosophy that they award prizes for verified results. Now, we don’t know anything about dark energy, so there’s no Nobel prize for that. The best we can do is give it this proxy name. Other than calling it dark energy and musing that maybe it’s the energy of empty space, maybe it’s not.
So, it remains to be an award to be doled out one day if somebody actually discovers what the dark energy is.
STROGATZ: That’s a really good point, that the Nobel… I mean, when we talk about Einstein and general relativity, famously he didn’t get a Nobel prize for either special or general relativity, right?
His Nobel prize — I’m just chuckling because was so sure it was coming that he could promise it to his wife. Do you not know this story?
LEVIN: Really, I do not know this.
STROGATZ: He promised the money from his Nobel prize to come to his first wife, knowing when he got divorced that she would need some money.
LEVIN: That’s amazing.
STROGATZ: But he didn’t get it for relativity. He got it for you know, his explanation of the photoelectric effect.
LEVIN: Mmm-hmm, yeah, incredible.
STROGATZ: But it’s an interesting, sort of aesthetic or criterion that the Nobel Commission uses that they want, as you say, verifiable experimental or observationally solid physics.
LEVIN: And I think that’s appropriate. That’s how it should be.
STROGATZ: Well, alright. In connection with plot twists in our story, you know, if the cosmological constant was the first twist, it turns out, as we’re gonna hear after the break, that there is another plot twist coming.
LEVIN: Great. Suspense.
[Music plays]
STROGATZ: Welcome back to The Joy of Why. We’re speaking with Johns Hopkins astrophysicist Adam Riess about the accelerating expansion of the universe.
So you mentioned the cosmological constant. It’s often going back to Einstein himself. In his notation, he used the Greek letter lambda to insert what some people, dismissively or sarcastically, will call a fudge factor. But it was something that was allowed by the equations of general relativity. As you say, Einstein wanted it at first because he couldn’t believe at the time that the universe was expanding.
RIESS: Right, you have to understand Einstein, he asked experimentalists, astronomers of the day, what is the universe doing? And they told him that the motions of things were very, very small.
And the reason they did was because at that time, it was before the discovery even that galaxies were outside the Milky Way. And so astronomers thought of the universe as the Milky Way itself. And when he asked about what the Milky Way was doing — was it expanding, contracting — they were like, the motions are pretty modest, doesn’t seem like it’s expanding or contracting. And this would’ve been confusing to him because, if you had a collection of masses and it was static and you let go. Right? They would fall together.
And so what was keeping it static? And so he saw, as you said in his equations, that there was an option for something to counterbalance the attractive gravity. And so he took it and set it to his, you know, as we would say, boundary condition that the universe be static.
There were two problems, of course: One was the universe is expanding, and so that’s not the right boundary condition. The other is it’s an unstable equilibrium. It’s like taking a marble and putting it on top of a basketball. Yes, in principle it could sit there, but if you push it off, if there’s a slight difference, it’ll run away in one direction or the other. And so when he saw the universe was expanding, when Hubble and others showed it, he famously said this was the biggest blunder of his career, was to introduce this concept. And yet there’s no getting rid of it.
Like a lot of mathematics and physics, it’s allowed to be in the equations unless you have some knowledge that it can’t exist. And so here we were some, oh, 80 years later saying, actually it looks like the universe is accelerating, which would be as if that repulsive type gravity actually was now winning over the attractive gravity of matter.
STROGATZ: There’s that funny saying I’ve heard, I’m not sure who to attribute it to, that what is not forbidden is compulsory. You know, like in something like this where the lambda term, the cosmological constant, was not forbidden by the equations. It was permitted by them. And in fact, now we think there is something like that. As you say, the modern term is often dark energy. But, we could go down a big rabbit hole of asking ourselves, what is this dark energy?
But I think I would like to go in a different direction to talk about the Lambda Cold Dark Matter model, the Lambda CDM model, because I mean, I realize that you’re gonna want to tell us, there may be some tweaks needed to it, or maybe more than tweaks, but why don’t we first make the case that this is a really interesting, powerful, standard model. Tell us what this model is and what makes it compelling. What are some of its great successes?
RIESS: Sure. When we build what we call a model of the universe to first order, we’re talking about what it’s made of. What’s the recipe, if you wanted to make one. And so what is the stuff. Then we also usually talk about the laws of physics as part of that.
But in the case of the universe, what people are familiar with the elements in the periodic table of elements. That’s the small stuff that’s like 4% of the universe. And the rest is dark. Dark meaning it doesn’t emit light. And so we can’t see it directly. We have to infer it by its gravity. And so from the 1930s to the 1970s and later astronomers came to understand that there was a lot of matter in the universe that had attractive gravity, but was dark, was not emitting light, called dark matter.
It makes galaxies spin much faster than the luminous matter would indicate otherwise. Stars would just zing away from their galaxies. It makes galaxies orbit other galaxies and clusters, which again, they would just fly away if there wasn’t the extra gravitational glue. There’s bending of light called lensing. All of these techniques allow us to figure out that the universe is about 25% dark matter and about… maybe 26%… and about 4% normal matter. So 30% there and then.
With our results in the 1990s and then quickly followed up by observations of the cosmic microwave background radiation showed the other 70% is this dark energy. So we get a sort of full universe that explains what we see, but 96% of it is dark.
And to be clear, when we talk about it, we talk about its sort of gravitational action, but what we don’t talk about really, or can yet, is the microphysics, the details. Dark matter, we think it’s a particle, but we don’t know the nature of the particle. We don’t know if it’s stable, if it has other interactions.
For dark energy, we sort of generally wave our hands and say it’s the energy of the vacuum. But, people who do quantum mechanical calculations try to estimate how much that should be, and they get an answer that’s 120 orders of magnitude off from what we get. So what we have is a very good, I’m gonna say just-so-story, you know, a phenomenological model grounded on lots of strong physics concepts. But it’s still a model. It’s not the physics itself. It’s not a description of those items themselves. We, for now, treat each of those — dark matter, dark energy — in their most vanilla form because we haven’t yet found any sprinkles on them, but we’re looking.
STROGATZ: Vanilla form, I see what you mean. You were describing some of the parameters that go into the model. That is, without saying what dark energy is, we can estimate what its contribution is to the budget, the energy budget of the universe. There’s the amount of dark matter, the amount of luminous matter. And there’s a few other parameters that go into this standard model. But the advocates for it, which I think is the consensus, that it’s our best model — though it is a model, right — that it explains a lot of things.
RIESS: It absolutely does. So it explains many things. It explains why the universe is expanding. It explains why we have the particular chemical abundances of light elements. It explains the cosmic microwave background radiation that we see, the fluctuations in it that we see. The particular spectrum of fluctuations that we see. And then it predicts an expansion history of the universe, including a phase early on when it is decelerating and dominated by attractive gravity, which allows structures to form like clusters of galaxies and planets and all of that good stuff. And then a later phase when it would accelerate, when it’s dominated by this dark energy as the matter is diluted by the expansion of space.
So it explains a lot of what we see, and it should because it is developed as we look at the universe and we have added to it or changed it to match what we see. And so I would say up until the early 2000s, maybe even 2010, it fit everything really.
But because we have these deep questions, we’re not just looking for a model, we’re looking for the physics. People continue to do more precise experiments because the nature of science has always been that you have a model and it’s the best approximation for reality. But inevitably it has some shortcomings, which maybe you haven’t seen yet or understood.
In the case of Lambda-CDM as we call it, the shortcomings are partly theoretical that we don’t understand these dark parts. But observationally, it was excellent. And then as the 2010s went on, people did try to do more and more precise experiments to test this model, because we want to tease out is dark energy really a constant or is it something that varies over time?
There’s some precedent in the universe for there was an episode shortly after the Big Bang we call inflation that also would’ve been dark energy, a different dark energy that would’ve appeared and disappeared over time. We’re wondering, is our dark energy today disappearing or getting stronger? We would like to know that. We would like to know more about the nature of dark matter, and so therefore, while we do theory, we also do better experiments to try to tell us more.
STROGATZ: I think it’s super interesting to appreciate the successes and the gaps together. Right, because the rest of the story that we want to tell here is one of discrepancies but they only become really impressive, at least to me, when you understand, like, what you expect in terms of how good the theory is. Up until now, the model has been good to within 1%, as you say, by tuning parameters that you can’t otherwise estimate. But still with that tuning, you can account for lots of different things to within 1%.
RIESS: People should understand this model is very successful. It explains a lot. It’s been the standard model for about 25 years. And it has survived through many dramatic advances in the precision of experiments. But as I think we’ll talk about as well, we’ve begun to see some hints of cracks in the model as well which we’re still wrestling with I would say.
STROGATZ: So let’s go there now, because this is the, and I find this compelling in your own biography, like it’s what a true scientist you are. If you don’t mind me heaping some praise on you here that the year that you mentioned, you said 2010s, people started noticing this is you and other people started noticing and you’re going to Sweden, you’re getting the Nobel prize, you’re still a kid, you’re just 41 years old or something, and you’re not satisfied. You’re not ready to just become an administrator. You want to keep doing science.
RIESS: Yeah, well, science is fun, right? So I was 41 and, you know, a lot of people said, “Oh, what are you gonna do now, you know?” And I looked at the other Nobel laureates and, unfortunately the average age has evolved up to about your seventies. So the average person is like, well, I’ve already done my academic career, I’m emeritus. So this is like a victory lap I’m gonna do and go give talks and things like that. But I was 41 and it was too, not just too soon, it was completely unappealing to me to do that and to stop what I was actually doing.
I mean, I was in the middle of using the Hubble Space Telescope, how cool is that, to look at for distant, exploding stars and pulsating stars and mapping the expansion history of the universe. I decided that the only way forward after that trip to Stockholm was to try to push out as much as I could all those speaking things and try to really focus on the science because, you know, once you stop it’s hard to go back.
STROGATZ: Well, okay, so this is where the plot thickens. You start making these improved measurements. And then, so, tell us what you found and where tension enters our drama here.
RIESS: So, we have a new model, Lambda-CDM, and it has these parts — dark matter, dark energy — that’s all great. And the question became, what is the nature of this dark energy? And so as a physicist, we say, “Okay, let’s find a number or a property of it that we can measure and it will tell us something about it.” And so in the way that dark energy produces gravity, what matters is what’s called its equation of state, the ratio of its pressure to its energy density. And this number is called W. And so if it’s a cosmological constant, then W will be minus one for all time.
And if it’s some kind of field in space — think of a field you know of, the electric field, the magnetic field — this would be a different field, but it would’ve energy and its energy would act like dark energy, then it might not be minus one, it might be some other number. And there’s been a lot of effort in space missions built to try to study this W number.
And I saw one of the better ways to do this would be to measure the Hubble Constant, the actual present expansion rate of the universe, and compare that to the predicted value it should have following both this model Lambda-CDM and this exquisite cosmic microwave background data, that was coming from new space satellites like WMAP and Planck, flown by NASA and ESA. And I began a new project, called SHOES to leverage the Hubble Space Telescope and try to improve the measurement from 10% precision, which had been before to try to reach 1% precision.
And at first it was going really well [laughs]. We were getting measurements that were smaller and smaller uncertainties, and they were matching the cosmic microwave background to the most part. And then, a funny thing happened. It was about a little more than 10 years ago, it was 2013, and Planck, the new cosmic microwave background experiment from the European Space Agency, very state-of-the-art, came out with a pretty large adjustment of the predicted value of the Hubble Constant.
Our measurements typically had been in the low 70s in these arcane units that astronomers use, kilometers per second per mega parsec, which is a very strange number, but it’s the inverse of time. And so if you inverted it, it actually tells you the approximate age of the universe.
And so the classic number from local measurements had always been in the low 70s — 70 to 75. And we were honing in on around 73 plus or minus two. And that all looked okay with the previous cosmic microwave background data. But when better cosmic microwave background data came out, it was suddenly, “Oh no, it’s gotta be more like 67, plus or minus 0.5.”
Now, this might sound like a small difference. Again, back to your statement earlier, this is a good model, right? We’re trying to thread a needle from the other side of the universe. You know, we’re starting at the moment of the Big Bang and asking how fast the universe will be expanding some 13 to 14 billion years later. And we’re like, we’re off by 8 or 9%. That’s pretty good predictive work, I would say in almost any other field.
But we’re rigorous about this. And our error bars are small and their error bars are small. And the only thing connecting them is the theory, the model, tells us how to explain the trajectory of the universe from the Big Bang to the present time.
And something has to give either the cosmic microwave background measurement is wrong or the local measurement is wrong, or the story that connects them is not quite right. We’ve all been doing deep dives over the last decade, redoing the measurements very carefully. I’ve been using the James Webb Space Telescope now instead of the Hubble Space Telescope, but getting the same answer. People have been doing a lot of work on the cosmic microwave background side, getting the same answer.
The community has poured over this and scrutinized over it. And so it’s gotten a name called the Hubble tension, which is, as it sounds, it’s a tension between what you think the Hubble constant should be based on the model and the early universe and what it actually appears to be. And it has surpassed what we call five sigma, which, in physics talk means the discrepancy is five times larger than the error bar on the experiment, which is the point at which we say, “Hey, something’s going on.” And so it’s been a lot of fun, I would say, to try to figure out what that something is.
STROGATZ: Fantastic summary. That’s perfect. You’ve talked a lot about the cosmic microwave background, which we’ve been assuming is standard knowledge. I want to clarify. We’re taking a baby picture of the universe. Like you talked earlier about the door jam…
RIESS: Right, right.
STROGATZ: So I know at age 66, I’m 6’ 1”. Now imagine when I was in the hospital, and I’m not even one day old. I’m in the baby crib. If you had looked at me as that little baby and then you said, “Yeah, but I can tell how tall Steven is gonna be when he’s 66 because I have this great model and I’m gonna predict his height and I can predict his height to within 8 or 9% when he’s 66 from this one-day-old baby.” That’s what the cosmologists are trying to compare. The people doing the baby are the Lambda-CDM people. And you’re doing…
RIESS: Right. I’m measuring the door jam right now and the model said, in this case, “Hey, Steve was supposed to be 6 foot 1,” and we’re coming out with, he is more like seven foot tall and you’re, which, you know, might be fun, but you’re like, that doesn’t, that, that is really would be truly unexpected for you to be seven feet tall.
And you might say, “Hey, maybe somebody took that baby picture of you wrong or they were using the wrong tape measure or something.” And that’s the stuff we sort of know how to do, that we know how to double-check experiments, do it with different telescopes, have different groups of people do it, different tools, different tracers. So that work has gone on for a decade and nobody has found the problem with the measurements. And so then you start to wonder, is it the growth chart, the story that we tell the model.
And on the one hand you say, that should be pretty easy to play with. You told me this dark stuff is so vanilla that you don’t know what dark energy and dark matter are. Can’t you just turn some knobs, some features on those that change it, give yourself an extra couple growth spurts during your teenage years, and suddenly you’re seven feet tall? And the problem is it’s a fairly over-constrained problem that we do have a lot of data at other junctures along the way. And so it’s a wrestle both from the theory to come up with ideas that can do it, and from the observations to find out if there’s anything amiss with the observations. And so, that is one that I’ve been wrestling with a lot.
STROGATZ: I’m sure there are some listeners who are thinking they’ve heard about dark energy, dark matter. It seems like these are big question marks. Maybe the scientists don’t really know what they’re doing. You know, they’re using words, but it’s just to cover up their ignorance. And also these, as you pointed out, in many ways, there’s dust, there’s, these measurements are difficult, these objects are very far away. How — you know if I’m gonna be skeptical — how do you know that there aren’t errors in this whole story, systematic errors?
RIESS: The best way we know is that, first of all, we build in lots of redundancy. This is one of the things I love about science is I could sit there in my laboratory with my data and do some calculations and say, “Behold there’s dark energy,” right?
But I have to publish those results and other people check them. Meanwhile, somebody on the other side of the planet can analyze that same pile of data and they may come to a different conclusion. And if everybody’s coming to a different conclusion, we haven’t discovered anything. And so the important element of independent verification done by many people in all kinds of places over the planet with different backgrounds and whatnot. Then we come in with other telescopes. So I’ve talked about, we use ground-based telescopes. We see the same with the Hubble Space Telescope. We see the same with the James Webb Space Telescope. And so the sheer redundancy and cross-checking in this, when people get an alternative answer, we also have to run that down too and say, “Why did they get a different answer?”
STROGATZ: It’s a really tough problem. I think given how good the model is and, also given that we can’t just go around adjusting things without wrecking something else that already works. It’s not easy.
RIESS: Correct, that is the big challenge. However, having said that, if this is a hint of a crack in Lambda-CDM, there have been other hints as well, so this is not the only experiment.
So more recently, there are results from the DESI experiment, which is measuring the three-dimensional positions of galaxies over the whole universe and, compared to the cosmic microwave background, suggests that dark energy doesn’t look like the cosmological constant. Looks like there’s some change going on there. There is the way we measure the clumpiness of matter in the nearby universe that, again, based on the way the universe looked shortly after the Big Bang in the model, we thought it would look clumpier than it does. Instead, it looks smoother. So there are these hints which on the one hand could be cracks and if, you know, a really clever person eventually puts them all together might change the story.
These could be the loose thread on the sweater that you pull on and might unravel the sweater, or you just pluck it off and you’re like, “Okay, that was not that big a deal.” I think we don’t know, but I think one of the fun things in science is the adventure and the mystery of it, that this is the process. Every, everybody in the past who called something a standard model eventually had to revise it.
STROGATZ: Good point.
RIESS: I could give you historical examples of where both were the truth in the situation. The problem is that science is not history. You actually have to do the work, you have to do the experiments, you have to have the critical thoughts. That’s what makes science so much fun.
STROGATZ: It is really interesting, and I like your comment too, although I’d be tempted to push back. I mean, yes, science isn’t history, but we do have really interesting examples from history of science where sometimes a little tweak to a model was enough, and sometimes you needed a conceptual overhaul like Newton’s model for gravity to Einstein’s general relativity.
RIESS: You know, Steve, that is my favorite example of all time, is, and I’ll just tell for the listeners who may not be familiar, back in the 1800s, what astronomers did was they tracked the positions of planets religiously, and they worked out their orbits. And in the early part of the 1800s, they noticed that the outermost planet they knew at the time, Uranus, was misbehaving.
It was traveling too fast, and it was traveling too slow, and there was a speculation by scientists that there was another planet, which became Neptune, that was further out and it was pulling on Uranus, that they couldn’t see it. So they calculated with pure math where it should be and then looked for it with a telescope and found it, okay?
So that was great. So Newton’s theory was fine. It was just some missing stuff. And then later in the century, Mercury was the one that was misbehaving, the innermost planet. It travels in an elliptical orbit, and that orbit is not supposed to precess or rotate itself at the rate that it was, and people struggled with that. They came up with the same idea: “Oh, look, it’s gotta be a missing planet. We’ve seen this movie before.” And they imagined a planet called Vulcan that was between Mercury and the Sun. They looked for it. They couldn’t find it. Fifty years later, Einstein changes gravity and shows that Mercury will precess in general relativity, his theory of gravity.
And as you say, what looks like kind of a small little observational crack or, you know, loose thread on a sweater unravels the thing. And so that’s why it’s hard to tell. Is this a Neptune or is this a, you know, general relativity? I really don’t know.
STROGATZ: Perfect examples. Yeah. I’m glad you brought up the Uranus, Neptune thing because you see both types of things. Big conceptual overhaul needed or maybe just something’s missing and it’s not a big overhaul. So we don’t really know.
Okay. So as you say, it’s an adventure, this great enterprise we’re in — science — and always progressing. There’s something else coming along the Nancy Grace Roman Space Telescope.
RIESS: Yeah, the cavalry is coming. In this case, it’s many great observatories, which I’m very excited are coming online. There’s the Vera Rubin telescope, which is the biggest full view telescope that is on the ground, very powerful, just started observing a few months ago. There is the Euclid, the European Space Agency mission, which recently launched, and then there is the Nancy Grace Roman being launched by NASA in September of this year, which is kind of a Hubble Space Telescope on steroids.
It can collect in a single image about 100 times the field of view of Hubble, and it operates very quickly too. So it’s going to have a kind of collecting power that’s about 1,000 times faster than Hubble. And so with this large amount of data, we hope to tease out more information about the recent expansion history of the universe, if dark energy is changing and any other sort of hints we can get.
Yeah, so we’ll learn things about exoplanets and galaxies, all kinds of things, but this really should be a quantum leap in terms of our data capability and, as long as I’ve been in this field, data has really been crucial to making the kinds of breakthroughs that we have.
STROGATZ: If we could just close on an emotional point, our show is called The Joy of Why. Is there something that brings you particular joy? Is it what data can tell you or, what’s the fun in this for you being a physicist and an astrophysicist?
RIESS: To me it’s the mystery, and then it’s the potential to answer mystery. You know, when I was a kid, if I wanted to know, how old is the universe and what’s its fate, I would’ve thought you’re gonna ask a philosopher or rabbi, I don’t know. But these aren’t the kinds of things that mortal people answer.
And the fact that with science and with these capabilities that we could address really profound, big questions and do it in a kind of methodical way and get real answers. Yes, we end up with new questions. It doesn’t all come as one answer, like the famous number 42 in Hitchhiker’s Guide. It ends up being a kind of a riddle, but I like that.
STROGATZ: The other thing that occurs to me is an old Woody Allen movie where there’s a little boy, and he’s depressed but he says, “The universe is expanding.” And the mother says, “What is it to you? Brooklyn’s not expanding.” My question is something like, are your parents alive? Did they get to witness all of this stuff that happened to you with the expanding universe?
RIESS: They got to witness some of it. My mom is still around. She saw me win the Nobel prize. But, um, I would say that they thought these were profound things. When I was a kid, my dad would take me outside and we’d look up at the stars. And he was not a physicist, but he knew a few things, and he would say, “Isn’t it amazing when you look at the stars that the light has been traveling to us for millions of years, and so what you’re seeing is not actually the way they are now, it’s the way they were millions of years ago?”
This just kind of blew my mind. And he would say, “In fact, the stars might not even be there anymore, and we won’t know about that for millions of years.” And just this idea there’s information that’s traveling to us, the story’s changed along the way, we’re still seeing this old story. How could that be? Could I catch up to that light?
These are profound things that raise a feeling of awe in me and I think a lot of people. And so, you know, back to that kid in the Woody Allen movie, I mean, there’s kind of two perspectives there, right? The people who have awe and are curious, and more power to them. I’m that kind of person, too. And then the people who are like, “Hey, it’s not expanding in Brooklyn. Get back to work.” And I get that, too. I mean, we still need to eat, and we still need to do a lot of other things. But to those who have that awe gene or that curious gene, right, you know, once you look up and you know what’s out there, and you know the questions, you’re hooked.
And, you know, the fact that somehow nature’s put out just enough of a popcorn trail for us to follow, who doesn’t wanna follow that popcorn trail and see where it leads?
STROGATZ: Oh. I love it. Thank you so much for talking to us. This has been really great. A real pleasure to have you on The Joy of Why.
RIESS: My pleasure. And thanks for, uh, asking me what bothered me back in that class 35 years ago.
[Music plays]
LEVIN: Hmm. One thing in particular about Adam is that he hasn’t stopped. It’s not as though he won his Nobel prize, and now he’s satisfied, and he leaves it to the next generation. And I have seen this with many Nobel prize winners, actually. They continue for the rest of their lives to really have that childlike curiosity. It’s unyielding. And you hear it. It’s infectious.
STROGATZ: It’s a very uplifting story. I think it’s an almost paradigm example of how science works, in the case of Adam where we have new data coming in that’s challenging some of what we used to think. I have to say in this discussion with Adam, I hadn’t realized how good this Lambda-CDM model really is.
LEVIN: It’s very resilient, and it’s very hard to change it. It’s not like you can get away with messing with it. If that’s what you mean by a strong model, exactly right.
STROGATZ: That’s what I mean. I mean, it’s rigid. He calls it, I think at one point, overdetermined.
LEVIN: And that’s also so impressive. So a lot of times people will say, “Well, you, you know nothing about the universe ’cause 95% of it is dark. Some of it’s in the dark matter, and some of it’s in the dark energy, and you don’t know what either of those things are.” But I think what people don’t appreciate, it’s only because of how precise the observations are.
It’s only because we’re in an era of absolute precision cosmology that we’re able to look at the negative space and determine that actually we’re only 5%, you know? We’re just a little bit. We’re just a little residue. It’s amazing.
STROGATZ: It’s another humbling thought, right? So as one speck of residue to another, let me sign off and say thank you, Janna, for sharing your thoughts.
LEVIN: We’ll see you next time.
STROGATZ: Bye bye.
LEVIN: Bye.
[Music plays]
LEVIN: If you’re enjoying The Joy of Why and you’re not already subscribed, hit the subscribe or follow button wherever you’re listening. You can also leave a review for the show. It helps people find this podcast. Find articles, newsletters, videos and more at quantamagazine.org.
STROGATZ: The Joy of Why is a podcast from Quanta Magazine, an editorially independent publication supported by the Simons Foundation. Funding decisions by the Simons Foundation have no influence on the selection of topics, guests, or other editorial decisions in this podcast or in Quanta Magazine.
The Joy of Why is produced by PRX Productions. The production team is Caitlin Faulds, Jade Abdul-Malik, Genevieve Sponsler, and Merritt Jacob. The executive producer of PRX Productions is Jocelyn Gonzales. Edwin Ochoa is our project manager.
From Quanta Magazine, Simon Frantz and Samir Patel provided editorial guidance with support from Samuel Velasco, Simone Barr, and Michael Kanyongolo. Samir Patel is Quanta’s editor-in-chief. The episode art is by Chanelle Nibbelink, and our logo is by Jaki King and Kristina Armitage.
Special thanks to Garth Avery at the Cornell Broadcast Studio. I’m your host, Steve Strogatz. If you have any questions or comments, please email us at [email protected].
[Music fades]