Quantum Uncertainty Explains Fading Cosmic Expansion, Study Shows

Einstein’s Biggest Blunder Might Have Been Even Bigger! — Sabine Hossenfelder

Sabine Hossenfelder

Astronomical data showing a decreasing expansion rate may provide the first empirical evidence of quantum gravity, as Heisenberg uncertainty between cosmic size and expansion speed systematically suppresses dark energy.

AI-assisted summary of Einstein’s Biggest Blunder Might Have Been Even Bigger! — original source. Check the original for full context.

Source published:

Key ideas

  • Recent cosmological data from surveys including DESI and DES indicate that the cosmic expansion rate is not accelerating at a constant pace, but is instead decreasing over time. (0:00)
  • A new theoretical physics paper proposes that dark energy's decline stems from Heisenberg's uncertainty principle applied to the cosmos, where the universe's size and expansion rate act as complementary variables. (1:46)
  • If validated, the cosmological uncertainty model would constitute the first empirical evidence of gravity exhibiting quantum mechanical behavior. (0:00)
  • The proposal suffers from significant limitations: it relies on an unconstrained free parameter and lacks derivation from formal quantum gravity frameworks like string theory or loop quantum gravity. (4:02)

Detailed notes

## Background This video was published on YouTube by Sabine Hossenfelder on September 15, 2026. According to the description, it covers recent astronomical observations suggesting the expansion of the universe is not accelerating at a steady rate, and explores a new paper proposing that quantum mechanics could explain this phenomenon as gravity's first observed quantum behavior.

## TL;DR Recent observational data from surveys like DESI and DES indicate that cosmic acceleration is declining over time rather than remaining constant, hinting that dark energy is dynamic. A newly published physics paper suggests this shift can be explained by applying Heisenberg's uncertainty principle to the cosmos as complementary variables of universe size and expansion rate. The resulting quantum variance mathematically reduces the observed acceleration compared to classical predictions. While conceptually clever and minimal, the model relies on a free parameter and lacks a derivation from an underlying theory of quantum gravity, giving it a 4 out of 10 on Hossenfelder's 'bullshit meter.'

## Atmosphere & style The presentation carries Sabine Hossenfelder’s signature dry, analytical wit and skepticism, blending genuine scientific intrigue with sharp, grounded reality checks. Complex cosmological mathematics and historical anecdotes about Einstein are delivered with brisk, no-nonsense pacing and deadpan humor. Watching it feels like an accessible yet intellectually rigorous briefing from an insider who appreciates elegant theoretical ideas but refuses to let hype outrun empirical evidence.

## Bulletin - [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) Recent cosmological data from surveys including DESI and DES indicate that the cosmic expansion rate is not accelerating at a constant pace, but is instead decreasing over time. - [01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) A new theoretical physics paper proposes that dark energy's decline stems from Heisenberg's uncertainty principle applied to the cosmos, where the universe's size and expansion rate act as complementary variables. - [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) If validated, the cosmological uncertainty model would constitute the first empirical evidence of gravity exhibiting quantum mechanical behavior. - [04:02](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=242s) The proposal suffers from significant limitations: it relies on an unconstrained free parameter and lacks derivation from formal quantum gravity frameworks like string theory or loop quantum gravity.

## Key takeaways - [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) Astronomical observations suggest that cosmic acceleration is weakening rather than remaining steady, raising the possibility of a future cosmic contraction. - [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) Albert Einstein originally introduced a negative cosmological constant in 1917 to maintain a static universe, later abandoning it once cosmic expansion was discovered. - [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) The cosmological constant was revived with a positive sign in the late 1990s following distant supernova measurements proving accelerated cosmic expansion. - [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) Modern surveys like DESI and DES suggest dark energy is dynamical rather than uniform, challenging standard cosmological models. - [01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) The new paper treats the overall size of the universe and its rate of expansion as quantum complementary observables that cannot be simultaneously measured with arbitrary precision. - [01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) The observed cosmic acceleration is reduced by quantum variance—the mathematical difference between the mean of the square of the expansion rate and the square of its mean. - [01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) Because cosmic uncertainty scales alongside the volume of the universe, the correction factor naturally changes over cosmic history. - [01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) Theoretical physicists such as Lawrence Krauss remain cautious, questioning whether the empirical evidence for dynamic dark energy will endure under future scrutiny. - [04:02](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=242s) A major weakness of the hypothesis is its inability to calculate the absolute magnitude of the quantum effect from first principles, requiring a free parameter. - [04:02](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=242s) The cosmic uncertainty relation lacks a rigorous derivation from established quantum gravity candidates, earning the model a 4 out of 10 on the channel's evaluation scale. - [04:02](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=242s) Despite these flaws, the model provides an inventive, minimalist attempt to connect cosmological observational anomalies with quantum foundations.

## Detailed walkthrough ### [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) The Fading Acceleration and Einstein's Constant

Recent astronomical observations have revealed a startling twist in cosmology: the universe is not expanding the way physicists long assumed. Instead of accelerating at a steady, unchanging rate, the acceleration appears to be decreasing. If that downward trend continues, the cosmic expansion could eventually halt and reverse, causing the universe to shrink. A new paper published in a top physics journal proposes a radical explanation: the universe as a whole is subject to the quantum uncertainty principle. If correct, this would mark the very first empirical evidence of gravity behaving according to quantum mechanics.

Dark energy is the catch-all term for whatever mysterious force drives the universe to accelerate outwards. The simplest theoretical incarnation is a cosmological constant—an energy density that stays perfectly uniform over time. Albert Einstein originally introduced this constant into general relativity in 1917, ironically using a negative term to prevent the universe from expanding and force it into a static state. Once Edwin Hubble and others demonstrated that the universe was actively expanding, Einstein discarded the term, famously lamenting it as his "biggest blunder" (though historians still debate whether he ever actually uttered that specific phrase). In the late 1990s, observations of distant supernovae proved that cosmic expansion was not slowing down under gravity, but accelerating. The cosmological constant was promptly revived—this time with a positive sign.

```mermaid flowchart TD accTitle: Evolution of the Cosmological Constant accDescr: Diagram showing how Einstein's cosmological constant evolved from 1917 to modern observations. subgraph Historical["Historical Models"] A["1917: Einstein's Constant<br/>(Introduced to keep universe static)"] --> B["1930s: Term Discarded<br/>(Einstein's 'biggest blunder')"] B --> C["Late 1990s: Dark Energy Discovered<br/>(Constant resurrected with positive sign)"] end subgraph Modern["Modern Shift"] C --> D["Recent Data: DESI & Surveys<br/>(Dark energy value is decreasing)"] D --> E["New Proposal: Quantum Uncertainty<br/>(Variance reduces observed acceleration)"] end ```

- Einstein introduced the cosmological constant in 1917 to keep the universe static. - In the late 1990s, the constant returned with a positive sign to account for accelerated expansion. - Modern observational efforts—including the Dark Energy Spectroscopic Instrument (DESI), the Dark Energy Survey (DES), and large-scale cosmic structure analyses—indicate dark energy is not truly constant, but decreasing over time.

---

[01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) Applying Quantum Uncertainty to the Cosmos

The author of the new paper argues that dark energy's time-dependent decline stems directly from Heisenberg's uncertainty principle operating at the cosmic scale. In quantum mechanics, certain complementary pairs of observables cannot simultaneously possess arbitrarily sharp values. For instance, pinpointing a particle's exact position causes repeated measurements of its velocity to fluctuate wildly, producing a broad statistical spread. Applying this foundational logic to cosmology means one cannot simultaneously know both the precise physical size of the universe and the exact rate at which that size changes.

Because this quantum uncertainty scales with the size of the universe, it cannot remain constant over cosmic history. In astronomical observations, researchers extract the mean value of the square of the expansion rate, whereas theoretical general relativity predicts the square of the mean value. The difference between these two mathematical operations is precisely the variance—the quantum uncertainty:

$$\Delta = \langle \dot{a}^2 \rangle - \langle \dot{a} \rangle^2$$

Consequently, the observed cosmic acceleration is systematically smaller than classical equations predict, diminished by a correction factor derived from quantum variance that evolves alongside the universe's volume.

- Cosmic complementary variables: The overall scale/size of the universe versus its rate of expansion. - The mathematical discrepancy between the mean of the square and the square of the mean yields the quantum variance. - This variance predicts a specific time dependence for the cosmic expansion rate, though current observational datasets are not yet precise enough to confirm the exact curve. - Scientific consensus remains cautious; for instance, theoretical physicist Lawrence Krauss has expressed skepticism on joint news podcasts, doubting that evidence for dynamic dark energy will hold up over time.

---

[04:02](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=242s) Theoretical Shortcomings and Einstein’s Legacy

Evaluating the paper's overall merits, the core thesis is an appealing, minimalistic piece of reasoning grounded in fundamental physics. However, it suffers from two significant theoretical flaws: 1. **Unconstrained magnitude:** The model cannot calculate the absolute size of the quantum effect from first principles, requiring a free parameter. As the joke goes: "The constant is free. The predictive power costs extra." 2. **Missing quantum gravity derivation:** The author does not derive this cosmic uncertainty relation from any leading quantum gravity framework (such as string theory or loop quantum gravity).

Because of these loose ends, the paper rates a 4 out of 10 on the "bullshit meter"—promising and clever, but incomplete. Nonetheless, bridging the gap between cosmological observational anomalies and quantum gravity represents the kind of conceptual breakthrough foundational physics desperately needs. There is also a satisfying historical irony: Einstein would turn out to be wrong for introducing the constant to force a static cosmos, wrong for throwing it away, and wrong for ignoring quantum mechanics—yet his framework remains the bedrock that makes the entire debate possible.

---

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## Notable quotes > [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) Instead of accelerating at a steady, unchanging rate, the acceleration appears to be decreasing. — Sabine Hossenfelder

> [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) If that downward trend continues, the cosmic expansion could eventually halt and reverse, causing the universe to shrink. — Sabine Hossenfelder

> [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) If correct, this would mark the very first empirical evidence of gravity behaving according to quantum mechanics. — Sabine Hossenfelder

> [00:00](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=0s) Once Edwin Hubble and others demonstrated that the universe was actively expanding, Einstein discarded the term, famously lamenting it as his "biggest blunder" — Sabine Hossenfelder

> [01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) In quantum mechanics, certain complementary pairs of observables cannot simultaneously possess arbitrarily sharp values. — Sabine Hossenfelder

> [01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) Applying this foundational logic to cosmology means one cannot simultaneously know both the precise physical size of the universe and the exact rate at which that size changes. — Sabine Hossenfelder

> [01:46](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=106s) The difference between these two mathematical operations is precisely the variance—the quantum uncertainty — Sabine Hossenfelder

> [04:02](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=242s) The constant is free. The predictive power costs extra. — Sabine Hossenfelder

> [04:02](https://www.youtube.com/watch?v=Kln2KRqrZOk&t=242s) Einstein would turn out to be wrong for introducing the constant to force a static cosmos, wrong for throwing it away, and wrong for ignoring quantum mechanics—yet his framework remains the bedrock that makes the entire debate possible. — Sabine Hossenfelder

## Mentioned Albert Einstein, Dark Energy, Cosmological Constant, Edwin Hubble, Dark Energy Spectroscopic Instrument (DESI), Dark Energy Survey (DES), Heisenberg's Uncertainty Principle, Lawrence Krauss, General Relativity, String Theory, Loop Quantum Gravity, Incogni

## Video description Take back your personal data with Incogni! Use code Sabine at the link below and get 60% off annual plans: https://incogni.com/sabine

For years the expansion of the universe looked like it was speeding up at a steady rate. The latest observations say otherwise, and one physicist thinks the explanation has been hiding in quantum mechanics all along. And if that holds, we are looking at gravity revealing its quantum nature for the first time. Let's take a look.

Paper: https://doi.org/10.1103/zgnd-h2xv

#science #sciencenews #physics #darkenergy #quantumphysics

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[Source](https://www.youtube.com/watch?v=Kln2KRqrZOk)

中文简报

研究表明量子不确定性可解释宇宙加速膨胀的减弱 显示膨胀率下降的天文数据可能提供量子引力的首个经验证据，因为宇宙尺度与膨胀速度之间的海森堡不确定性系统性地抑制了暗能量。 来自DESI和DES等巡天项目的最新观测数据表明，宇宙加速膨胀并非恒定不变，而是随时间减弱，这暗示暗能量是动态的。一篇新发表的物理学论文提出，这一转变可以通过将海森堡不确定性原理应用于宇宙来解释，即把宇宙大小和膨胀率视为互补变量。由此产生的量子方差在数学上降低了相对于经典预测所观测到的加速度。尽管该模型在构思上巧妙且简洁，但它依赖于一个自由参数，并且缺乏基于量子引力基础理论的推导，因此在霍森菲尔德（Hossenfelder）的“扯淡计量器”上被评为4分（满分10分）。 来自DESI和DES等巡天项目的最新宇宙学数据表明，宇宙膨胀率并非以恒定速度加速，而是随时间推移而下降。 (0:00) 一篇新的理论物理学论文提出，暗能量的衰减源于应用于宇宙的海森堡不确定性原理，其中宇宙的大小和膨胀率充当互补变量。 (1:46) 如果得到证实，这一宇宙学不确定性模型将构成引力展现量子力学特性的首个经验证据。 (0:00) 该提案存在显著局限性：它依赖于一个不受约束的自由参数，并且缺乏弦理论或圈量子引力等正规量子引力框架的理论推导。 (4:02)

Reading summary

Recent astronomical data suggesting that cosmic acceleration is slowing has prompted a radical proposal: the universe as a whole may be subject to the quantum uncertainty principle. If confirmed, this model would offer the first observational signature of quantum gravity, though the theory currently lacks first-principles derivation and conclusive empirical backing. Decreasing Acceleration Challenges the Cosmological Constant (0:00) Recent data from major observational efforts—including the Dark Energy Spectroscopic Instrument (DESI), the Dark Energy Survey (DES), and large-scale cosmic structure analyses—indicate that the universe's acceleration is not fixed, but decreasing over time. If this downward trajectory persists, the expansion of the universe could eventually halt and reverse into contraction. These observations challenge the prevailing model of dark energy as an immutable cosmological constant. Historical Revisions of Einstein's Constant (0:00) The cosmological constant has undergone repeated reassessments since Albert Einstein introduced it in 1917. Einstein initially incorporated a negative term into general relativity to prevent cosmic expansion and force a static state, later abandoning it as a major blunder after Edwin Hubble verified that the cosmos is expanding. In the late 1990s, measurements of distant supernovae revealed that expansion was accelerating, prompting physicists to revive the constant with a positive sign. The latest survey findings suggest that dark energy may once again need reinterpretation as a time-dependent phenomenon. Applying Quantum Uncertainty to Cosmic Expansion (1:46) To explain the apparent decline in dark energy, a new paper published in a prominent physics journal suggests that Heisenberg's uncertainty principle governs the universe at cosmic scales. Under this framework, the total size of the universe and its rate of expansion function as complementary quantum variables that cannot possess simultaneously definite values. Because quantum uncertainty scales with the total volume of the universe, its influence naturally evolves across cosmic history. The mathematical mechanism hinges on the discrepancy between what observers measure and what classical theory calculates. While astronomers extract the mean of the square of the expansion rate, classical general relativity computes the square of the mean. The mathematical difference between these operations is the quantum variance, expressed as Δ = ⟨ȧ²⟩ - ⟨ȧ⟩². This variance reduces the observed cosmic acceleration relative to classical predictions by introducing a dynamic, volume-dependent correction factor. Theoretical Shortcomings and Empirical Uncertainties (4:02) Despite its conceptual appeal, the cosmic uncertainty proposal exhibits notable theoretical deficiencies. The model cannot calculate the absolute magnitude of the quantum effect from first principles, forcing it to rely on an unconstrained free parameter. In addition, the author does not derive the cosmological uncertainty relation from established quantum gravity candidates such as string theory or loop quantum gravity, leading Sabine Hossenfelder to assess the paper as incomplete (rating it 4 out of 10 on her evaluative scale). Empirical validation also remains unresolved. Existing observational datasets are not yet precise enough to confirm the specific curve predicted by the model's quantum variance. Moreover, consensus within the scientific community remains cautious; theoretical physicist Lawrence Krauss, for example, has voiced skepticism that evidence for dynamic, time-varying dark energy will hold up as measurements improve. The hypothesis that quantum uncertainty dampens cosmic acceleration represents an imaginative bridge between cosmological anomalies and quantum mechanics, highlighting the irony that Einstein's contested concepts still structure modern physics. However, establishing whether dark energy is truly decaying—and whether quantum variance is the cause—requires both higher-precision astronomical surveys and a rigorous derivation within an established framework of quantum gravity.

Source quotes

[0:00] Sabine Hossenfelder: Instead of accelerating at a steady, unchanging rate, the acceleration appears to be decreasing. [0:00] Sabine Hossenfelder: If that downward trend continues, the cosmic expansion could eventually halt and reverse, causing the universe to shrink. [0:00] Sabine Hossenfelder: If correct, this would mark the very first empirical evidence of gravity behaving according to quantum mechanics. [0:00] Sabine Hossenfelder: Once Edwin Hubble and others demonstrated that the universe was actively expanding, Einstein discarded the term, famously lamenting it as his "biggest blunder" [1:46] Sabine Hossenfelder: In quantum mechanics, certain complementary pairs of observables cannot simultaneously possess arbitrarily sharp values. [1:46] Sabine Hossenfelder: Applying this foundational logic to cosmology means one cannot simultaneously know both the precise physical size of the universe and the exact rate at which that size changes. [1:46] Sabine Hossenfelder: The difference between these two mathematical operations is precisely the variance—the quantum uncertainty [4:02] Sabine Hossenfelder: The constant is free. The predictive power costs extra. [4:02] Sabine Hossenfelder: Einstein would turn out to be wrong for introducing the constant to force a static cosmos, wrong for throwing it away, and wrong for ignoring quantum mechanics—yet his framework remains the bedrock that makes the entire debate possible.

Original transcript

[0:00] In the past years we have seen a most  interesting discovery: The universe doesn’t [0:06] expand as we thought it does. New observations  have shown that the acceleration is decreasing, [0:13] which means that eventually the universe might  actually begin to shrink in size. Today I have [0:19] a remarkable paper, published in a top journal,  by a physicist who says that this is because the [0:24] universe itself has an uncertainty principle. And  these recent observations are actually the first [0:30] evidence that gravity obeys quantum principles.  Well that is interesting, let’s have a look. [0:36] Dark energy is whatever makes the expansion  of the universe speed up. The simplest type [0:41] of dark energy is one that doesn’t change  in time, this is also called a cosmological [0:46] constant. Einstein himself already  introduced this in 1917. Ironically, [0:51] he did it with the opposite sign to  stop the universe from expanding. [0:55] When it turned out later that the universe  expands, the cosmological constant became [1:00] unnecessary and Einstein referred  to this as his “biggest blunder.” [1:05] Then, in the late 1990s, observations revealed  that the universe doesn’t just expand, [1:11] but that the expansion also accelerates.  This brought back the cosmological constant; [1:16] this time with a positive sign.  The biggest remaining controversy [1:20] was whether Einstein actually ever  referred to it as his biggest blunder. [1:25] But observations have gotten better and  in the past years it’s become clear that [1:29] dark energy is not actually constant.  Rather, its value seems to be decreasing. [1:35] This has been independently confirmed now  by a number of different observations, [1:40] such as DESI and the dark energy survey  and analyses of structures in the universe. [1:46] The author of the new paper now says he knows why:  it’s a consequence of the uncertainty principle in [1:53] quantum physics, applied to the entire universe. The uncertainty principle in quantum mechanics [2:00] tells us that some pairs of quantities can’t both  be measured to arbitrary precision. The best-known [2:06] example is the position of a particle and its  velocity. It’s not that you can’t measure both, [2:12] of course you can. But say you have made sure  that the particle stays in one place, then, [2:17] if you repeat the measurement of the velocity,  the results will be all over the place. This is [2:23] what it means that the uncertainty is large, it  has a large spread in the measurement outcomes. [2:29] The author of the new paper now says that the  uncertainty principle must also apply to the [2:34] universe as a whole: you can’t simultaneously  know how large the universe is and how fast its [2:41] size is changing. We have an uncertainty  contributing to the changing size of the [2:46] universe. And since the uncertainty depends  on the size itself, it’s not a constant. [2:52] Then he points out that the quantity that we  extract from observations is the mean value [2:57] of the square of the expansion. Whereas what  the maths gives us is actually the square of [3:02] the mean value. The difference is exactly the  variance, so, the uncertainty. He concludes [3:07] that the acceleration that we actually  observe is smaller by a contribution that [3:12] comes from the uncertainty, and that  depends on the size of the universe. [3:17] Okay, I know this was somewhat confusing, but  the brief summary is that he says the quantum [3:21] uncertainty of the universe fits to the most  recent observations. And the idea also gives [3:27] rise to a prediction. That is because this  formalism fixes just how the expansion of [3:33] the universe depends on time. At the moment the  data aren’t good enough to say decisively just [3:39] how the time-dependence looks like. But  it might become possible in the future. [3:44] Indeed, astrophysicists haven’t even yet  agreed that dark energy is not constant. [3:49] If you have been listening to my monthly science  news podcast with Lawrence Krauss, you might have [3:55] noticed that he thinks the recent observations  that dark energy changes in time won’t hold up. [4:02] So what are we to make of this? Personally, I  like this paper. It’s a neat, minimalistic and [4:09] straight-forward argument from basic physics,  I like this stuff. That said, it has two major [4:15] shortcomings. One is that he can’t actually  predict the absolute size of the effect, [4:20] so there’s a free constant. The constant  is free. The predictive power costs extra. [4:25] The other issue is that he hasn’t derived this  quantum uncertainty from any candidate approach [4:30] to quantum gravity. These two points make the  paper theoretically somewhat unsatisfactory, [4:36] this is why I give it a 4 out of 10 on the  bullshit meter. Good idea, some loose ends. [4:42] But leaving aside the specifics of this paper,  I think it is something like this that we are [4:48] missing in the foundations of physics. Some way to  connect the increasingly puzzling observations and [4:54] tensions in cosmology to quantum gravity.  That could really move the field forward. [4:59] It would also be historically a neat closure.  It would leave Einstein wrong about introducing [5:04] the cosmological constant, then  wrong about discarding it again, [5:08] then wrong again because it’s ignored quantum  physics, and yet he was still somehow responsible [5:14] for us understanding all of this. Next up we’ll  study the quantum uncertainty of peer review. [5:20] I used to receive a lot of spam calls  after my cell phone number leaked, [5:25] probably from some online shops that I used  to use. 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