Planck and SH0ES Split Over Universe Expansion Rate at 3.7 Sigma

PBS Space Time

The irreconcilable discrepancy between early-universe CMB predictions and local supernova measurements indicates that either standard distance calibrations contain systematic errors or cosmology requires new physics such as dynamical dark energy.

AI-assisted summary of The Crisis in Cosmology — original source. Check the original for full context.

Source published: 2019-01-24

Key ideas

Modern measurements of the local Hubble constant from the SH0ES project pin the current expansion rate at 73.5 ± 1.7 km/s/Mpc, reducing distance-ladder uncertainty to approximately 2 percent. (5:16)
Forward-modeling the Cosmic Microwave Background observed by the Planck satellite yields a conflicting modern expansion rate of 66.9 ± 0.6 km/s/Mpc under standard Lambda-CDM cosmology. (9:44)
The 3.7-sigma discrepancy between early- and late-universe measurements carries less than a 1-in-7,000 chance of being statistical noise, creating the modern 'crisis in cosmology.' (9:44)
Proposed explanations for the tension range from unmodeled systematic errors in standard candles to revolutionary physics, including sterile neutrinos, interacting dark matter, or evolving dark energy. (10:20)

Detailed notes

## Background Published on YouTube by PBS Space Time on January 24, 2019, this video is written and hosted by astrophysicist Matt O'Dowd. According to the description, the search for the Hubble constant—the rate of cosmic expansion—has consumed astronomers for generations, but now two independent, highly precise methods have yielded conflicting values, challenging our fundamental assumptions about the universe.

## TL;DR The episode examines the 'Hubble tension,' an irreconcilable 3.7-sigma discrepancy between two premier methods of measuring the universe's modern expansion rate ($H_0$). On one side, the SH0ES collaboration uses the cosmic distance ladder of Cepheid variables and Type Ia supernovae to calculate a local rate of roughly 73.5 km/s/Mpc; on the other, the Planck satellite measures primordial acoustic oscillations in the Cosmic Microwave Background to project a rate of approximately 66.9 km/s/Mpc via the standard $\Lambda\text{CDM}$ model. Host Matt O'Dowd explores whether this rift stems from hidden observational systematic errors or heralds groundbreaking physics such as sterile neutrinos, interacting dark matter, or evolving dark energy. The program wraps up with a community Q&A tackling negative-mass fluids, CPT symmetry inversion, and white holes.

## Atmosphere & style The presentation is intellectually sharp, energetic, and pedagogically rigorous, characterized by Matt O'Dowd's signature blend of clear-eyed technical precision and dry wit. Rather than treating the cosmological discrepancy as a discouraging impasse, the narrative builds a sense of eager anticipation, framing the crisis as an exhilarating threshold where established models might break open to reveal new physical laws. The closing mailbag segment softens the cosmic gravitas into an approachable, playful dialogue with viewers, seamlessly transitioning from rigorous mathematical paradoxes to lighthearted physics humor.

## Bulletin - [05:16](https://www.youtube.com/watch?v=72cM_E6bsOs&t=316s) Modern measurements of the local Hubble constant from the SH0ES project pin the current expansion rate at 73.5 ± 1.7 km/s/Mpc, reducing distance-ladder uncertainty to approximately 2 percent. - [09:44](https://www.youtube.com/watch?v=72cM_E6bsOs&t=584s) Forward-modeling the Cosmic Microwave Background observed by the Planck satellite yields a conflicting modern expansion rate of 66.9 ± 0.6 km/s/Mpc under standard Lambda-CDM cosmology. - [09:44](https://www.youtube.com/watch?v=72cM_E6bsOs&t=584s) The 3.7-sigma discrepancy between early- and late-universe measurements carries less than a 1-in-7,000 chance of being statistical noise, creating the modern 'crisis in cosmology.' - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) Proposed explanations for the tension range from unmodeled systematic errors in standard candles to revolutionary physics, including sterile neutrinos, interacting dark matter, or evolving dark energy.

## Key takeaways - [00:02](https://www.youtube.com/watch?v=72cM_E6bsOs&t=2s) Edwin Hubble paired spiral nebulae distance measurements with Vesto Slipher's redshift observations to prove galaxies recede at speeds proportional to their distance. - [00:02](https://www.youtube.com/watch?v=72cM_E6bsOs&t=2s) The Hubble constant (H0) quantifies the current rate of cosmic expansion, acting as the foundation for reconstructing the cosmic timeline from the Big Bang to the universe's ultimate fate. - [02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) Cosmic distance measurements traditionally suffered from uncertainties up to a factor of two because errors compound across each rung of the cosmic distance ladder. - [02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) The distance ladder operates by calibrating geometric stellar parallax to Cepheid variable stars, which in turn calibrate distant Type Ia supernovae standard candles. - [02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) Efforts to constrain the Hubble constant using distant Type Ia supernovae in the late 1990s revealed that cosmic expansion is accelerating, proving the existence of dark energy. - [05:16](https://www.youtube.com/watch?v=72cM_E6bsOs&t=316s) The SH0ES project tightened the local distance ladder by using the Hubble Space Telescope to re-observe supernova host galaxies with well-calibrated Cepheid variables. - [06:47](https://www.youtube.com/watch?v=72cM_E6bsOs&t=407s) Baryon Acoustic Oscillations froze into the Cosmic Microwave Background roughly 400,000 years after the Big Bang when photons decoupled from matter. - [09:44](https://www.youtube.com/watch?v=72cM_E6bsOs&t=584s) The Planck satellite extracted the infant universe's matter-energy ratios from the CMB power spectrum and projected them forward 13.8 billion years to predict a modern expansion rate of 66.9 km/s/Mpc. - [09:44](https://www.youtube.com/watch?v=72cM_E6bsOs&t=584s) The 3.7-sigma divergence between Planck's early-universe prediction and SH0ES's direct local measurement cannot easily be dismissed as random observational scatter. - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) Systematic error explanations focus on possible miscalibrations in Cepheid physics or unmodeled gravitational lensing distortion of the CMB. - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) New physics explanations propose sterile neutrinos altering the early energy balance, dark matter shifting the acoustic sound horizon, or dynamical dark energy that strengthens over time. - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) Independent cosmological probes, including gravitational wave standard sirens and strong gravitational lensing time delays, are required to break the tie between Planck and SH0ES. - [13:19](https://www.youtube.com/watch?v=72cM_E6bsOs&t=799s) Negative mass fluid models attempting to unify dark matter and dark energy conflict with observed gravitational lensing and introduce runaway perpetual motion paradoxes. - [15:08](https://www.youtube.com/watch?v=72cM_E6bsOs&t=908s) Time reversal under fundamental CPT invariance corresponds to inverting physical momentum vectors rather than rewinding clocks, preserving physical laws across simultaneous charge, parity, and time inversions.

## Detailed walkthrough ### [00:02](https://www.youtube.com/watch?v=72cM_E6bsOs&t=2s) Hubble's Discovery and the Expansion Rate of the Universe

In 1929, Edwin Hubble revolutionized our understanding of reality by establishing that the cosmos extends far beyond our own galaxy. By measuring the distances to spiral nebulae, Hubble produced incontrovertible proof that these structures were located millions of light-years away, firmly establishing them as independent galaxies rather than clouds of gas within the Milky Way. When he paired these distance measurements with the Doppler-shift velocity observations collected by Vesto Slipher, Hubble noticed a startling pattern: distant galaxies are receding from Earth at speeds directly proportional to their distance. The galaxies are not hurtling through static space on their own power; rather, the intervening fabric of space itself is stretching and expanding.

This modern rate of expansion is encapsulated by a single cosmological parameter: the Hubble constant, written as $H_0$ (pronounced "H-naught"). More than just a measure of how fast neighboring galaxies appear to retreat, $H_0$ dictates the expansion rate of the universe in our current era. When combined with the known gravitational influences of matter and energy, this single number allows cosmologists to reconstruct the entire cosmic timeline from the Big Bang all the way to the ultimate fate of the universe. As host Matt O'Dowd explains: “The search for a single number, the Hubble constant, the rate of expansion of our universe, has consumed astronomers for generations.” Because all observations of distant light traveling through expanding space depend upon this value, resolving $H_0$ has been an obsession for thousands of researchers across decades.

- **Vesto Slipher’s Contribution**: Pioneered redshift measurements, showing that the light from distant nebulae was stretched to longer wavelengths, indicating rapid recession. - **Cosmic Scale**: Hubble’s distance measurements proved that spiral nebulae were millions of light-years away, vastly expanding the known scale of reality. - **Physical Meaning of $H_0$**: Represents the current, modern-era expansion rate of the universe, serving as the anchor point for modeling cosmic history.

---

[02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) The Cosmic Distance Ladder: From Parallax to Supernovae

Throughout the 20th century, measuring the Hubble constant was notoriously imprecise; until the turn of the millennium, astronomers could only pin down $H_0$ to within a factor of two, somewhere between 50 and 100 kilometers per second per megaparsec. The units themselves describe recession speed relative to distance: one megaparsec equals roughly 3.3 million light-years. If $H_0$ were 75 km/s/Mpc, an observer would expect a galaxy one megaparsec away to retreat at 75 kilometers per second, while a galaxy two megaparsecs away would recede at 150 kilometers per second. While measuring the velocity component of this equation is straightforward—simply observing the redshifted light of a galaxy—determining cosmic distances with high accuracy is extraordinarily difficult.

To cross these immense chasms of space, astronomers rely on the "cosmic distance ladder," where each rung calibrates the next. Edwin Hubble utilized Cepheid variable stars, pulsating giant stars whose intrinsic luminosity directly correlates with their pulsation periods—a relationship discovered by Henrietta Leavitt. By comparing a Cepheid's true brightness with its observed brightness from Earth, astronomers treat it as a "standard candle" to deduce its host galaxy's distance. However, these stars must first be calibrated against closer stars whose distances are directly measured via stellar parallax (the subtle shift in apparent position caused by Earth orbiting the Sun). Because Cepheids can only be observed out to limited distances, astronomers also use Type Ia supernovae—cataclysmic thermonuclear explosions that occur when a white dwarf star siphons too much material from a binary companion and detonates at a predictable mass threshold. As Matt O'Dowd points out: “With each step on the ladder, uncertainties compound.”

```mermaid flowchart LR accTitle: The Cosmic Distance Ladder accDescr: Stepwise calibration process from nearby stellar parallax to distant Type Ia supernovae. subgraph Ladder["Cosmic Distance Ladder"] A["Stellar Parallax (Earth orbit baseline)"] -->|"Calibrates"| B["Cepheid Variables (Period-Luminosity)"] B -->|"Calibrates"| C["Type Ia Supernovae (Standard Candles)"] end C --> D["Recession Velocities (Redshift via Slipher)"] D --> E["Local H0 Measurement"] ```

- **Distance Ladder Breakdown**: - *Stellar Parallax*: Measures geometric shifts of nearby stars as Earth circles the Sun; calibrates the base rung. - *Cepheid Variables*: Pulsating supergiant stars where pulsation period dictates absolute luminosity. - *Type Ia Supernovae*: White dwarf detonations with uniform peak brightness, visible across billions of light-years. - **Dark Energy Discovery**: During the late 1990s, teams tracking distant Type Ia supernovae to constrain $H_0$ unexpectedly discovered that cosmic expansion is accelerating, proving the existence of dark energy.

---

[05:16](https://www.youtube.com/watch?v=72cM_E6bsOs&t=316s) Refining the Local Expansion: The SH0ES Project

In the wake of discovering dark energy, Nobel laureate Adam Riess set out to refine the measurement of $H_0$ with unprecedented accuracy. Riess spearheaded the SH0ES project—short for Supernovae, $H_0$, for the Equation of State of Dark Energy. Operating with the Hubble Space Telescope, the SH0ES team focused on reinforcing the weakest links in the distance ladder by re-observing host galaxies of past historical supernovae and cross-calibrating them with newly discovered, highly reliable Cepheid variables. By tightening the calibration between Cepheid periods and supernova peak brightnesses, every past supernova distance calculation across the cosmos was systematically improved.

Through this meticulous calibration, the SH0ES team successfully narrowed down the local value of the Hubble constant to $73.5 \pm 1.7\text{ km/s/Mpc}$, squeezing the margin of error down to roughly 2 percent. As Matt O'Dowd reflects on this achievement: “That 2%-ish uncertainty is a hell of a lot better than the old factor of 2 uncertainty.” Because the SH0ES project measures galaxies within roughly 2 billion light-years of Earth, it provides an empirical, direct measurement of the universe's expansion rate in the contemporary epoch. However, confirming such a critical number demands testing it against completely independent methods that do not rely on standard candles or the distance ladder.

- **SH0ES Objective**: Eliminate systematic errors on the cosmic distance ladder by anchoring Type Ia supernovae directly to Hubble Space Telescope Cepheid observations. - **Current Local Result**: $73.5 \pm 1.7\text{ km/s/Mpc}$. - **Observational Reach**: Samples galaxies out to approximately 2 billion light-years, capturing modern cosmic expansion.

---

[06:47](https://www.youtube.com/watch?v=72cM_E6bsOs&t=407s) Reading the Early Universe: The CMB and BAO

Instead of measuring nearby receding galaxies, astronomers can take the opposite approach: observe the newborn universe and use cosmological physics to predict what the modern expansion rate *ought* to be. The prime data source for this method is the Cosmic Microwave Background (CMB), the relic thermal glow released approximately 400,000 years after the Big Bang when the universe cooled sufficiently for neutral hydrogen to form, allowing photons to decouple from matter and stream freely. Over nearly 14 billion years of cosmic expansion, this radiation has stretched by a factor of 1,100 into the microwave spectrum. High-resolution maps created by the European Space Agency's Planck satellite display tiny temperature fluctuations across the sky—minute temperature differences on the order of 1 part in 100,000 that trace subtle density variations in the primordial plasma.

These temperature speckles are the frozen signatures of Baryon Acoustic Oscillations (BAO). Before recombination, the universe was an opaque soup where gravity pulled matter inward toward over-dense regions while outward radiation pressure pushed back, generating colossal sound waves rippling across the early cosmos. When the universe decoupled, these acoustic oscillations abruptly froze in place. By taking the power spectrum of the CMB—measuring the physical size and angular distribution of these hot and cold speckles—cosmologists can pinpoint which oscillation modes were at their peaks when the radiation escaped. As Matt O'Dowd explains: “In the era just before the release of the CMB, matter and light were trapped together.”

```mermaid flowchart TD accTitle: Early Universe Forward-Modeling of H0 accDescr: How the Planck satellite uses early universe physics to predict the modern Hubble constant. subgraph EarlyUniverse["Early Universe Physics (z ≈ 1100)"] A["Primordial Density Perturbations"] --> B["Baryon Acoustic Oscillations (Gravity vs. Radiation Pressure)"] B --> C["CMB Decoupling (Frozen at ~400,000 Years)"] end subgraph Analysis["Cosmological Modeling"] C --> D["Planck Power Spectrum Analysis (Angular scale of acoustic peaks)"] D --> E["Extract Parameters: Baryons, Dark Matter, Dark Energy"] E -->|"Evolve forward 13.8 Gyr via Lambda-CDM"| F["Predicted Modern H0"] end ```

- **Decoupling Era**: Occurred ~400,000 years after the Big Bang; universe transitioned from opaque plasma to transparent neutral gas. - **Wavelength Stretching**: Cosmic expansion has stretched the CMB photons by a factor of 1,100 since their emission. - **Sound Waves in Space**: Baryon acoustic oscillations sloshed through plasma on all scales until recombination halted the photon-baryon drag. - **Parameter Extraction**: The peaks in the CMB angular power spectrum reveal the exact ratios of normal matter, cold dark matter, and radiation present in the infant universe.

---

[09:44](https://www.youtube.com/watch?v=72cM_E6bsOs&t=584s) The Cosmological Discrepancy: A 3.7-Sigma Clash

Using the pristine data collected by the Planck satellite, astrophysicists establish the baseline recipe of the early universe—including the initial amounts of baryonic matter, cold dark matter, and radiation—and then evolve those conditions forward across 13.8 billion years of cosmic time using the standard $\Lambda\text{CDM}$ model of cosmology. Because the Planck power spectrum contains such an extraordinary density of information, the Planck team calculated the modern expansion rate with even tighter statistical precision than the SH0ES collaboration. The crisis is that the two methodologies do not agree. As Matt O'Dowd puts it bluntly: “The problem is, the results don't agree.”

Planck’s early-universe forward model yields a modern Hubble constant of $66.9 \pm 0.6\text{ km/s/Mpc}$, whereas the SH0ES distance-ladder measurement yields $73.5 \pm 1.7\text{ km/s/Mpc}$. While these numbers might seem close at first glance, their narrow uncertainty margins make them mathematically irreconcilable. The discrepancy sits at a 3.7-sigma statistical significance, representing less than a 1-in-7,000 chance that the gulf is a mere fluke of random measurement noise. Because the early-universe measurements are extraordinarily robust and the modern distance ladder has been cross-checked repeatedly, this tension suggests that either the supernova distance ladder harbors unrecognized systematic errors, or our standard model of cosmology is fundamentally missing something about how the universe evolved.

| Measurement Method | Team / Instrument | Target Observed | Derived $H_0$ (km/s/Mpc) | Uncertainty | |:--- |:--- |:--- |:--- |:--- | | **Local Distance Ladder** | SH0ES / Hubble Space Telescope | Cepheids & Type Ia Supernovae | **73.5** | $\pm 1.7$ (~2.3%) | | **Early Universe CMB** | Planck Collaboration / Planck Satellite | Relic acoustic peaks in CMB | **66.9** | $\pm 0.6$ (~0.9%) |

- **Statistical Tension**: The divergence between 66.9 and 73.5 km/s/Mpc constitutes a $3.7\sigma$ discrepancy. - **Random Error Odds**: Only a 1-in-7,000 probability that the difference arises from random observational scatter. - **The Core Dilemma**: Either an unknown physical process operated between the early universe and the present day to alter the cosmic expansion rate, or astronomical standard candles are plagued by subtle, unaccounted-for physics.

[10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) The Discrepancy: Systematics or Brand New Physics

The cosmological tension first emerged into clear view in 2016, when Adam Riess and his team released a new calibration of the Hubble constant ($H_0$) derived from Type Ia supernovae, placing it in stark conflict with the European Space Agency's Planck satellite results from a few years earlier. Rather than vanishing with scrutiny, the gap has only hardened: independent calibration methods have been brought in, calculations have been meticulously audited, and the error bars continue to shrink. Faced with this stubborn mismatch, cosmologists divide the possible culprit into two broad categories: mundane systematic errors or revolutionary new physics.

```mermaid flowchart TD accTitle: Resolving the Hubble Tension accDescr: Two potential paths to resolve the Hubble constant discrepancy between early and late universe measurements.

subgraph Systematics["Systematic Errors"] S1["Cepheid variable miscalibration"] S2["Miscalibrated lensing of CMB speckles"] end

subgraph NewPhysics["Unknown Physics"] P1["Fast-moving particle: Sterile neutrino"] P2["Interacting dark matter shifting acoustic peaks"] P3["Dynamical dark energy increasing over cosmic time"] end

H0["Hubble Tension: Planck vs SH0ES"] --> Systematics H0 --> NewPhysics ```

Under the systematics hypothesis, unmodeled observational biases are skewing one or both methods. Astronomers might not understand Cepheid variable stars as thoroughly as assumed, or gravitational lensing could alter the characteristic speckles of the cosmic microwave background (CMB) in ways not yet accounted for. Ongoing checks continue to rule out candidate errors one by one, yet the possibility of an overlooked bias lingers. The far more tantalizing scenario is that the standard cosmological model lacks key physics required to interpret the CMB. A swarm of fast-moving, non-interacting particles beyond the Standard Model—such as sterile neutrinos—could tilt the early universe's energy balance. Alternatively, dark matter might interact more actively with ordinary matter and radiation, shifting the physical scale of CMB acoustic peaks. A third alternative is that dark energy is not a rigid cosmological constant, but dynamically increases over time, forcing the modern universe to expand faster than early-universe extrapolations anticipate.

Resolving whether Planck or SH0ES has the correct measurement will rely on independent arbiters such as gravitational wave standard sirens and strong gravitational lensing time delays. Matt O'Dowd reflects on the two possible outcomes with equal relish: if the numbers eventually converge, a century-long quest to measure the universe's expansion rate will come to a satisfying close; but if the rift endures, physics gets an extraordinary gift: “We'll have a new tool to investigate the mysterious physics of dark energy, dark matter, or of unknown particles beyond the standard model.”

[13:19](https://www.youtube.com/watch?v=72cM_E6bsOs&t=799s) Viewer Feedback: Negative Mass Fluids and Perpetual Motion

Moving to viewer comments, O'Dowd circles back to the journal club discussion of Dr. Jamie Farnes' controversial paper proposing a negative-mass dark fluid to unify dark matter and dark energy. Addressing viewer Leo Staley—who reported via a friend of Farnes that the author himself did not necessarily fully believe the paper's claims, but intended primarily to provoke debate—O'Dowd expresses full respect for floating fringe concepts to ignite scientific discussion: “I totally respect that motivation to publish even quite fringe ideas, and he certainly sparked a conversation. I mean, look, I'm still talking about it.”

The scientific pushback against the negative-mass fluid hypothesis remains formidable. Commenter Andrew Paulfreyman noted that gravitational lensing should produce the opposite geometric effect if dark matter possessed negative mass rather than positive mass. O'Dowd confirms that gravitational lensing relies on mass curving spacetime, and observations of warped background galaxies consistently demand an attractive, positive-mass distribution; negative mass would likely fail to produce the strong lensing signatures observed throughout the cosmos. Viewer Marik Zilberman highlighted another fatal flaw: negative masses inherently generate runaway motion, perpetual motion machines, and logical paradoxes. O'Dowd notes that when a model generates such pathological physical consequences, it serves as an immediate red flag, promising an upcoming challenge question episode dedicated to exploring these negative-mass paradoxes.

[15:08](https://www.youtube.com/watch?v=72cM_E6bsOs&t=908s) Viewer Feedback: CPT Reversal, White Holes, and the "Speed of Dark"

The conversation shifts to the episode on fundamental spacetime symmetries, beginning with a vital clarification regarding the "T" in CPT (Charge, Parity, Time) invariance. Reversing time in CPT is not a matter of magically running a cosmic clock backward; instead, it represents the exact reversal of all physical motion, including both linear and angular momentum. If nature obeyed pure time-reversal symmetry, flipping every momentum vector would cause the cosmos to trace its history backward to the initial state. However, pure T symmetry is fundamentally broken in nature, as proved by differing forward and backward reaction rates in certain weak-force quantum interactions. The universe is invariant only under the simultaneous inversion of all three components: “Now, a CPT inverted universe is not the same as this universe, but the laws of Physics are the same. The point is that you can't tell which of the two you're in.”

Responding to viewer questions about how these inversions alter matter and gravity: - **Mass inversion (TinyFox Tom):** Under standard CPT motion reversal, mass sign is unchanged. However, in mathematical formulations of true time reversal—such as Richard Feynman’s path integral approach—a negative-mass particle traveling backward in time is mathematically indistinguishable from a positive-mass particle traveling forward in time, though its real-world physical validity remains unproven. - **Black holes under time inversion (Rishit Vora):** An idealized time reversal across a black hole's interior yields a white hole, where everything that ever crossed the event horizon is ejected in reverse: “Everything that ever fell in would come rushing out, and presumably reassemble itself into the stars, spaceships, monkeys, that originally fell in.” Because T symmetry is broken, however, a rewound real-world black hole would not assemble cleanly back into its starting objects, though quantum information remains conserved in an altered form. - **A humorous unified field theory:** O'Dowd highlights an equation submitted by viewer Mr. Nation, proposing that dark energy equals dark matter multiplied by the speed of dark squared ($DE = dm \cdot c_d^2$). O'Dowd caps the episode by admiring the cheek of the submission: “Genius on so many levels. Not only scientific levels, but still, levels.”

## Notable quotes > [15:08](https://www.youtube.com/watch?v=72cM_E6bsOs&t=908s) A CPT inverted universe is not the same as this universe, but the laws of Physics are the same. The point is that you can't tell which of the two you're in. — Matt O'Dowd

> [00:02](https://www.youtube.com/watch?v=72cM_E6bsOs&t=2s) The search for a single number, the Hubble constant, the rate of expansion of our universe, has consumed astronomers for generations. — Matt O'Dowd

## Mentioned Edwin Hubble, Vesto Slipher, Henrietta Leavitt, Adam Riess, Jamie Farnes, Richard Feynman, Matt O'Dowd, Hubble Space Telescope, Planck satellite, European Space Agency, SH0ES project

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The search for a single number: the hubble constant, which is the rate of expansion of our universe, has consumed astronomers for generations. Finally, two powerful and independent methods have refined its measurement to unprecedented precision. The only problem is that they don’t agree. This calls into question some of our most basic assumptions about the universe.

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Previous Episode: Why String Theory Is Wrong https://youtu.be/IhpGdumLRqs

Hosted by Matt O'Dowd Written by Matt O'Dowd Graphics by Luke Maroldi Directing by Andrew Kornhaber

In 1929, Edwin Hubble discovered the universe. He gave us our first incontrovertible proof that there are galaxies outside the Milky Way by measuring the distances to the spiral nebulae. They were many millions of light years from us –far outside the Milky Way and so must be galaxies in their own right. Combined with the Doppler-shift velocity measurements of Vesto Slipher, Hubble revealed that the galaxies are not only receding from us, but they are receding at a rate proportional to their distance.

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[Source](https://www.youtube.com/watch?v=72cM_E6bsOs)

中文简报

普朗克卫星与SH0ES在宇宙膨胀率测量上出现3.7个标准差分歧早期宇宙CMB预测与邻近超新星测量之间不可调和的差异表明，要么标准距离校准存在系统误差，要么宇宙学需要如动态暗能量等新物理学。本期节目探讨了“哈勃张力”，即测量宇宙现代膨胀率（$H_0$）的两种主要方法之间存在不可调和的3.7个标准差分歧。一方面，SH0ES合作项目利用造父变星和Ia型超新星组成的宇宙距离阶梯，计算得出局域膨胀率约为73.5 km/s/Mpc；另一方面，普朗克卫星测量了宇宙微波背景中的原始声学振荡，并通过标准$\Lambda\text{CDM}$模型推算出约为66.9 km/s/Mpc的膨胀率。主持人马特·奥多德（Matt O'Dowd）探讨了这种裂痕究竟是源于未发现的观测系统误差，还是预示着惰性中微子、相互作用暗物质或演化暗能量等突破性新物理学。节目最后以社群问答收尾，讨论了负质量流体、CPT对称性反演以及白洞等话题。 SH0ES项目对局域哈勃常数的现代测量将当前膨胀率确定为73.5 ± 1.7 km/s/Mpc，使距离阶梯的不确定度降低至约2%。 (5:16) 在标准Lambda-CDM宇宙学模型下，根据普朗克卫星观测到的宇宙微波背景进行正向建模，推导出的现代膨胀率却为存在冲突的66.9 ± 0.6 km/s/Mpc。 (9:44) 早期与晚期宇宙测量之间3.7个标准差的差异，其属于统计噪声的概率不足七千分之一，从而引发了现代“宇宙学危机”。 (9:44) 针对该张力提出的解释涵盖了从标准烛光中未建模的系统误差，到颠覆性的新物理学，包括惰性中微子、相互作用暗物质或演化暗能量等。 (10:20)

Detailed notes — zh

## Background 该视频由科普频道 PBS Space Time 于 2019 年 1 月 24 日在 YouTube 平台发布。视频简介指出，几代天文学家一直在追寻表征宇宙膨胀速率的哈勃常数，如今两套独立且精度前所未有的测量方法却得出了互不相容的结果，这一矛盾正在动摇人类对宇宙最基础假设的认知。 ## TL;DR 本期节目深入探讨了天体物理学界著名的“宇宙学危机”：通过造父变星与 Ia 型超新星构建的宇宙距离阶梯测得的现代宇宙局部膨胀速率（SH0ES 团队测得 73.5 km/s/Mpc），与普朗克卫星基于早期宇宙微波背景辐射（CMB）结合 ΛCDM 标准模型推导出的数值（66.9 km/s/Mpc）产生了高达 3.7 个标准差的严重分歧。这种统计上无法调和的差距表明，要么测量体系中隐藏着未被察觉的系统误差，要么早期宇宙演化模型中遗漏了关键的新物理学机制，例如暗能量动态变化、惰性中微子或异常的暗物质相互作用。节目最后还延伸解答了观众关于负质量暗流体假说及其引发的物理悖论，并厘清了 CPT 对称性中时间反演（T 反转）的本质物理内涵。 ## Atmosphere & style 全片充盈着严谨而振奋的理性探索氛围，主持人马特·奥多德（Matt O'Dowd）以标志性的沉稳语调与清晰逻辑，将极其硬核的宇宙学理论层层剖解。面对这一可能击碎现有理论框架的学术危机，叙事没有走向虚无或沮丧，反而洋溢着对“旧模型崩溃、新物理破土而出”的科学浪漫与智性狂欢。后半段的观众答疑环节则在严密的思维交锋中穿插着恰到好处的极客幽默，为高密度的理论推演增添了亲切生动的互动质感。 ## Bulletin - [02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) 基于Ia型超新星与造父变星构建的宇宙距离阶梯，SH0ES项目将近邻宇宙哈勃常数直接测量值收窄至73.5 ± 1.7 km/s/Mpc，不确定度约2%。 - [07:54](https://www.youtube.com/watch?v=72cM_E6bsOs&t=474s) 普朗克卫星基于早期宇宙的宇宙微波背景辐射（CMB）与ΛCDM标准模型推算出的哈勃常数为66.9 ± 0.6 km/s/Mpc。 - [07:54](https://www.youtube.com/watch?v=72cM_E6bsOs&t=474s) 两种独立测算结果的分歧高达3.7个标准差（3.7σ），统计学上因随机误差导致的概率仅为1/7000，引发了天体物理学界瞩目的“宇宙学危机”。 - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) 该测量分歧若非未知系统误差所致，则暗示现代物理学在早期宇宙模型中遗漏了关键机制，例如惰性中微子、暗物质异常相互作用或动态暗能量。 ## Key takeaways - [00:02](https://www.youtube.com/watch?v=72cM_E6bsOs&t=2s) 1929年埃德温·哈勃通过造父变星测定螺旋星云距离，打破了银河系即宇宙全部的旧认知，并确立了星系退行速度与距离成正比的哈勃定律。 - [00:02](https://www.youtube.com/watch?v=72cM_E6bsOs&t=2s) 哈勃常数（H0）代表现代纪元宇宙空间的膨胀速率，结合宇宙中的引力效应可推演出从大爆炸到终局的完整演化史。 - [02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) 历史上H0的估算范围曾存在2倍的不确定性（50至100 km/s/Mpc），其核心难点在于高精度的深空天体距离测定。 - [02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) 天文学界通过三角视差法校准造父变星周光关系，再由造父变星校准宿主星系中的Ia型超新星爆发光度，逐级建立了宇宙距离阶梯。 - [02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) 亚当·里斯领导的SH0ES项目利用哈勃空间望远镜交叉校准超新星与造父变星，将直接测定的局部宇宙H0锁定在73.5 ± 1.7 km/s/Mpc。 - [06:01](https://www.youtube.com/watch?v=72cM_E6bsOs&t=361s) 宇宙微波背景辐射（CMB）释放于大爆炸后约40万年的复合纪元，其冷热温差仅约为十万分之一，记录了早期宇宙的微小密度起伏。 - [07:54](https://www.youtube.com/watch?v=72cM_E6bsOs&t=474s) 重子声学振荡（BAO）是早期等离子体中引力坍缩与光子辐射压抗衡形成的声波，退耦时被冻结在CMB的角功率谱中。 - [07:54](https://www.youtube.com/watch?v=72cM_E6bsOs&t=474s) 普朗克团队基于CMB功率谱锁定原初宇宙学参数，并借助ΛCDM模型推算出当今宇宙的H0为66.9 ± 0.6 km/s/Mpc。 - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) 局部直接测量与早期宇宙间接推算的分歧达到3.7σ，随着校准技术的升级与误差范围的缩小，这一哈勃张力被证实真实存在。 - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) 分歧的潜在诱因之一是未知的系统误差，例如造父变星物理机制理解不全或引力透镜效应建模存在盲区。 - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) 分歧的另一可能诱因是CMB推算中缺失了新物理学，包括存在未知的极速轻粒子（如惰性中微子）或暗物质产生强于预期的相互作用。 - [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) 暗能量若并非宇宙学常数设定的恒定状态而是随时间增强，也会导致早期模型推算出的膨胀率显著低于当今实测值。 - [13:19](https://www.youtube.com/watch?v=72cM_E6bsOs&t=799s) 引力透镜实测数据明确显示暗物质具有正质量，负质量暗流体假说在透镜效应观测和理论预测（如永动机悖论）上存在严重缺陷。 - [13:19](https://www.youtube.com/watch?v=72cM_E6bsOs&t=799s) CPT对称性中的时间反演（T）本质上是全宇宙所有粒子动量与角动量的逆转，由于纯T对称性已被弱相互作用打破，纯粹倒流的时空无法无损还原黑洞等系统。 ## Detailed walkthrough ### [00:02](https://www.youtube.com/watch?v=72cM_E6bsOs&t=2s) 哈勃常数的发现与宇宙膨胀的意义寻找一个单一的数值——哈勃常数（Hubble constant, $H_0$），即我们宇宙的膨胀速率，已经耗费了几代天文学家的心血。如今，两套强大且完全独立的方法终于将这一常数的测量推向了前所未有的精度。然而，正如主持人 Matt O'Dowd 所说：“The only problem is that they don't agree, and it's causing to question some of the most basic assumptions about the universe.”（唯一的问题在于它们并不一致，这让人开始质疑某些关于宇宙的最基本假设。）回溯到 1929 年，埃德温·哈勃（Edwin Hubble）可以说是真正意义上“发现了宇宙”。通过测量到螺旋星云的距离，他给出了首个不可辩驳的证据，证明在银河系之外还存在其他星系。这些星云距离我们数百万光年，远在银河系之外，必然是独立的星系。结合维斯托·斯里弗（Vesto Slipher）测得的多普勒频移退行速度，哈勃揭示出这些星系不仅都在远离我们而去，而且其退行速率与它们到我们的距离成正比。一个原本被认为局限于银河系的宇宙骤然变得无比辽阔，同时它还在持续膨胀。星系之所以看起来在急速远离我们，是因为它们之间的空间本身正在拉伸。宇宙的这种膨胀被浓缩在哈勃常数 $H_0$ 之中。它不仅告诉我们星系远离的速度与其距离的关系，更根本的是，它给出了现代纪元（modern era）宇宙的膨胀速率。将宇宙的膨胀速率与其中所包含的物质与能量的引力效应结合起来，天文学家就能够推算出宇宙从大爆炸直至其最终宿命的完整膨胀历史。不仅如此，对于解读那些在膨胀宇宙中穿行了数十亿年才抵达我们的遥远天体发出的光线而言，$H_0$ 也是最不可或缺的基石。 - 1929 年埃德温·哈勃测定螺旋星云距离，打破了银河系即宇宙全部的旧观念。 - 结合维斯托·斯里弗的多普勒红移测速，确立了星系退行速度与距离成正比的哈勃定律。 - $H_0$ 代表现代宇宙的膨胀速率，结合物质和能量的引力作用可推演整个宇宙的演化史。 --- ### [02:15](https://www.youtube.com/watch?v=72cM_E6bsOs&t=135s) 宇宙距离阶梯与超新星测量法直到千禧年之前，天文学界对哈勃常数的最好估计也仅仅能限制在 2 倍的因子范围内，大约在 50 到 100 $\text{km/s/Mpc}$ 之间。这个单位由两部分构成：退行速度（千米每秒，$\text{km/s}$）除以距离（百万秒差距，$\text{Mpc}$，约等于 330 万光年）。如果哈勃常数是 75 $\text{km/s/Mpc}$，就意味着一个星系每距离我们 1 个百万秒差距，其远离的速度就会额外增加 75 千米每秒。在历史上，测量哈勃常数需要尽可能多地获取星系的退行速度和距离。退行速度相对简单，通过斯里弗的方法测量由于宇宙膨胀导致光波长被拉伸的红移即可；但测量距离却极为棘手。哈勃最初借助的是造父变星（Cepheid variables），这是一种处于生命末期的巨星。正如亨丽爱塔·勒维特（Henrietta Leavitt）所发现的，造父变星的脉动周期与其真实亮度（光度）存在对应关系。通过测量遥远星系中造父变星的周期，天文学家就能得知它们的固有真实亮度，进而与观测到的视亮度对比推算出距离。造父变星因此成为最早的“标准烛光”（standard candles）。然而，造父变星的周光关系必须先通过三角视差法测定附近的近邻造父变星来进行校准，这种层层递进的测距体系被称为“宇宙距离阶梯”（cosmic distance ladder）。随着台阶逐级向上，不确定性也在层层累加。 ```mermaid flowchart TD accTitle: Cosmic Distance Ladder Architecture accDescr: Step-by-step determination of astronomical distances from parallax to supernovae subgraph Step1["近邻校准"] A["恒星视差法 (Stellar Parallax)"] -->|"几何几何测距"| B["银河系及近邻造父变星"] end subgraph Step2["次级中继"] B -->|"校准周光关系"| C["遥远星系造父变星"] end subgraph Step3["深空巡天"] C -->|"校准宿主星系爆发"| D["Ia 型超新星 (Type Ia SN)"] D -->|"测定深空星系退行"| E["现代膨胀速率 H0 (SH0ES)"] end ``` 造父变星虽然优异，但观测距离有限。为了看得更远，天文学家引入了 Ia 型超新星。当白矮星（死亡恒星的致密遗骸）从双星系统的伴星那里汲取过多物质、失控的核聚变被引爆时，就会发生这种爆炸。由于其爆发光度高度可预测，它们成为了极佳的标准烛光。在 1990 年代，天文学家原本打算利用超新星更精确地钉死哈勃常数，却意外发现了宇宙正在加速膨胀，从而揭示了暗能量的存在。作为这项诺贝尔奖研究的领军人物之一，亚当·里斯（Adam Riess）带领的 SH0ES（Supernovae $H_0$ for the Equation of State）项目继续致力于提升测距精度。他们利用哈勃空间望远镜，将以往记录的超新星爆发与更精准的造父变星观测进行交叉校准，使整条宇宙距离阶梯的精度大为提高。根据该团队的最新测算，哈勃常数被收窄到了 $73.5 \pm 1.7\ \text{km/s/Mpc}$，不确定度缩减至约 2%，相比过去的 2 倍差距无疑是巨大的飞跃。 - $1\ \text{Mpc}$ 约为 330 万光年；速度除以距离即为宇宙空间膨胀率的度量。 - 视差法测定近处造父变星 -> 造父变星校准宿主星系距离 -> 宿主星系校准 Ia 型超新星爆发光度。 - 亚当·里斯等人在 1990 年代利用超新星测定 $H_0$ 时意外发现了暗能量驱动的宇宙加速膨胀。 - SH0ES 团队基于宇宙距离阶梯测得的局部直接膨胀率为 $73.5 \pm 1.7\ \text{km/s/Mpc}$。 --- ### [06:01](https://www.youtube.com/watch?v=72cM_E6bsOs&t=361s) 早期宇宙的指纹：宇宙微波背景辐射既然近邻宇宙的局部测量已经如此精确，危机又从何而来？科学要求对同一参数进行完全独立的交叉验证。SH0ES 项目测量的是距离我们约 20 亿光年以内的星系退行，这本质上是对现代膨胀速率的“直接测量”。但还有另一条截然不同的路径：如果能测定宇宙创生之初的膨胀速率，再结合我们对大爆炸以来物质、辐射和暗能量等全部引力影响的最佳演化模型，就能够推算出今天的膨胀速率应该是多少。正如 Matt O'Dowd 所说：“So, we'd better hope that it does give the same result, or there is a big problem, with either our supernova measurements or with our understanding of how the universe evolved.”（所以，我们最好期盼它能给出相同的结果，否则无论是在超新星测量方面，还是在对宇宙演化机制的理解上，都存在着巨大的问题。）这另一条路径依靠的是比造父变星和超新星更为可靠的观测：宇宙微波背景辐射（CMB）。这是宇宙早期炽热致密状态遗留下来的余热微光，释放于大爆炸后约 40 万年。当时宇宙冷却到足以让光子摆脱带电粒子的束缚，宇宙由此变得透明。历经近 140 亿年在膨胀宇宙中的穿行，这束光的波长被拉长了 1100 倍。普朗克卫星（Planck satellite）绘制的全天 CMB 地图展示了遍布天球的微小温度起伏，这些“斑点”（speckles）反映了当时等离子体极其微弱的密度涨落。图中蓝色区域的温度比红色区域低约十万分之一，但密度稍高，正是这些微小的过密区域在后来的引力坍缩中演化成了现代宇宙的巨大星系团。 - CMB 释放于大爆炸后约 40 万年的复合纪元，至今光子波长已被宇宙膨胀拉伸了 1100 倍。 - 普朗克卫星拍摄的 CMB 温度涨落幅度极小，冷热区域的温差仅约为十万分之一。 - 这些原始微弱的密度涨落，最终孕育了后来的大尺度星系团结构。 --- ### [07:54](https://www.youtube.com/watch?v=72cM_E6bsOs&t=474s) 重子声学振荡与 3.7 个标准差的宇宙学危机这些 CMB 斑点究竟如何能揭示哈勃常数？关键在于斑点的大小尺度。在 CMB 释放之前的早期宇宙中，物质与光子紧密耦合在一起。重子物质在自身引力作用下试图收缩坍塌，而光子辐射则产生极强的向外辐射压以抵抗坍缩。这两种相互对抗的作用力在宇宙原初等离子体中激发出震荡，形成了在全宇宙荡漾的巨大声波，即重子声学振荡（Baryon Acoustic Oscillations, BAO）。在长达 40 万年的时间里，等离子体在不同尺度上反复在致密与稀疏之间来回晃荡。随后，复合纪元降临，光与物质退耦，声波振荡戛然而止，振荡在最后一瞬间的状态被永久地冻结并印刻在 CMB 的斑点结构中。天文学家通常使用功率谱（power spectrum）来呈现不同角尺度上斑点的分布丰度。功率谱中峰值的位置精确指明了在 CMB 释放那一刻，哪些尺度的振荡模式刚好处于峰值状态。这取决于当时物质与辐射的能量密度，以及极早时期的宇宙膨胀率。普朗克团队首先求解出能够完美拟合这一功率谱的初始宇宙学参数组合（包括暗物质、重子物质、辐射密度及初始膨胀率），然后利用标准宇宙学模型（$\Lambda\text{CDM}$）模拟这一宇宙演化至今天，从而推算出当下的 $H_0$。 ```mermaid flowchart LR accTitle: CMB to Hubble Constant Inference accDescr: Process of deriving H0 from cosmic microwave background observations using the Lambda-CDM model subgraph Early["早期宇宙物理"] A["引力坍缩 vs 辐射压"] --> B["重子声学振荡 (BAO)"] B -->|"退耦冻结 (40万年)"| C["CMB 各向异性斑点"] end subgraph Model["模型拟合与推演"] C --> D["普朗克角度功率谱"] D --> E["锁定初始宇宙学参数"] E -->|"标准模型 (Lambda-CDM) 演化推导"| F["CMB 预测的 H0: 66.9"] end ``` 普朗克功率谱蕴含的信息极为丰富，使得该团队得出的 $H_0$ 甚至比超新星方法具有更高的统计精度。但致命的问题出现了：普朗克卫星基于早期宇宙推算出的哈勃常数是 $66.9 \pm 0.6\ \text{km/s/Mpc}$，而通过超新星与造父变星直接测量的结果却是 $73.5 \pm 1.7\ \text{km/s/Mpc}$。从直觉上看，利用处于时间两极的极端数据测得的这两个数值似乎已经相当接近，但在统计学上它们是完全无法调和的。两者的差异达到了惊人的 3.7 个标准差（$3.7\sigma$），这意味着这种分歧纯粹由随机误差导致的概率只有大约七千分之一（1/7000）。两个在各自领域做到极致的独立物理测量，得出了互不相容的两个常数，这正是天体物理学中备受瞩目的“宇宙学危机”（The Crisis in Cosmology）。 - 重子声学振荡（BAO）是早期宇宙中物质引力坍缩与光子辐射压抗衡产生的原初声波。 - 普朗克卫星基于 CMB 功率谱和标准模型推导出的常数为 $66.9 \pm 0.6\ \text{km/s/Mpc}$。 - 近邻宇宙的超新星直接测量结果为 $73.5 \pm 1.7\ \text{km/s/Mpc}$。 - 两者相差达 $3.7\sigma$，随机误差导致的概率仅为 1/7000，构成了深刻的科学冲突。 ### [10:20](https://www.youtube.com/watch?v=72cM_E6bsOs&t=620s) 哈勃张力与宇宙学危机的潜在根源测量宇宙膨胀速率（哈勃常数 $H_0$）的分歧最早爆发于 2016 年。当时亚当·里斯（Adam Riess）利用超新星完成的全新校准结果，与几年前普朗克卫星（Planck）基于宇宙微波背景辐射（CMB）得出的测算产生了严重冲突。自那以后，天文学界不断改进校准技术、反复核查计算过程，并引入多种独立方法对作为标准烛光的超新星进行重新校准。然而，这种差距不仅没有消失，测量的误差范围（error bars）反而在持续缩小，证实这一分歧是真实存在的客观现象。面对这种几乎要“击碎整个宇宙学”的僵局，主持人马特·奥多德（Matt O'Dowd）指出，可以从两个主要方向来审视该分歧的成因： 1. **未知的系统误差（Systematic Errors）：** 无论在超新星测距阶梯还是普朗克观测中，可能存在尚未被察觉的系统偏差，导致测量值被高估或低估。例如，我们对造父变星（Cepheid variables）物理机制的理解可能并不完全透彻，或者引力透镜效应扭曲普朗克斑图的方式与当前的理论预期存在出入。虽然科研人员正逐一排查并排除已知的系统误差，但依然无法完全排除存在未知盲区的可能。 2. **CMB 计算中缺失了全新物理学（New Physics）：** “这是最令人兴奋的可能性，”奥多德强调说。如果观测数据本身没有大问题，那么就意味着我们在利用早期宇宙数据推算当今宇宙时，遗漏了某些关键物理规律。 ```mermaid flowchart TD accTitle: 哈勃常数分歧的潜在解决路径 accDescr: 图表展示哈勃常数测量分歧的两种解释：测量系统误差与未知的新物理学机制。 Discrepancy["哈勃常数 $H_0$ 测量分歧"] subgraph ErrorTrack["路径一：观测与分析偏差"] Systematic["未知系统误差"] Systematic --> CepheidBias["造父变星理解不完全"] Systematic --> LensingBias["引力透镜建模存在偏差"] end subgraph PhysicsTrack["路径二：未知物理规律"] NewPhysics["早期宇宙新物理学"] NewPhysics --> SterileNeutrino["超高速新粒子 （如惰性中微子）"] NewPhysics --> DarkMatterInteract["暗物质与辐射/物质 发生超预期相互作用"] NewPhysics --> DynamicDarkEnergy["暗能量随时间动态增加 （非常数模型）"] end Discrepancy --> ErrorTrack Discrepancy --> PhysicsTrack ``` 在新物理学的假设下，目前有三个主要候选方向： - **超高速移动的新粒子：** 若早期宇宙存在数量充足的未知极速粒子（例如标准模型之外、不参与弱相互作用的假想惰性中微子 sterile neutrino），它们会打破早期宇宙的能量平衡，从而颠覆现有的推演模型。 - **暗物质粒子行为异常：** 暗物质与普通物质或辐射之间的相互作用可能强于既有认知，进而改变 CMB 斑纹的角度尺度。 - **暗能量并非恒定不变：** 当前宇宙学标准模型设定暗能量由宇宙学常数驱动（即密度恒定）。但如果暗能量随着时间推移在增强，就会导致早期宇宙推算出的膨胀速率明显低于今天实测的哈勃常数。最终的定论取决于真实、精确的哈勃常数究竟落在普朗克团队还是 SH0ES 团队一方。未来，引力波测量和强引力透镜等全新的独立测量工具将加入这场裁决。如果误差最终消除、两组数据收敛合一，那么人类历经近一个世纪探索宇宙膨胀率的壮丽征程将圆满画上句号；但如果分歧依然坚挺，“那会更酷，”因为这意味着人类获得了一把全新的钥匙，得以借此窥探暗能量、暗物质乃至超越粒子物理标准模型的新世界。 --- ### [13:19](https://www.youtube.com/watch?v=72cM_E6bsOs&t=799s) 观众互动：负质量暗流体与时间反演对称性在节目的评论互动环节，奥多德对近期关于“负质量暗流体”以及“CPT 对称性”两期节目的观众留言进行了解答： - **关于杰米·法恩斯博士（Dr. Jamie Farnes）的“负质量暗流体”论文：** 一位观众转述了法恩斯博士朋友的说法，称作者本人并不一定百分之百相信论文中的所有主张，其核心意图在于抛砖引玉、激发物理学界的脑力激荡。奥多德对此表示充分理解与尊重，认为发表非主流假说以引发学术探讨完全值得肯定。针对观众 Andrew Paulfreyman 提出的引力透镜质疑，奥多德从物理直觉上表示赞同。引力透镜是光线穿过引力场时发生的偏折，天文学家正是通过观测遥远天体成像的扭曲畸变来反推前景星系的质量分布。实测数据明确显示暗物质表现为正质量；如果暗物质由负质量流体构成，其透镜效果将完全相反，甚至根本无法形成当前观测到的强引力透镜效应。对于观众 Marik Zilberman 指出的负质量会导致永动机与各种荒谬悖论的观点，奥多德深表认同：“当一个理论导致这些病态预测时，这就是一个巨大的警示信号。”他透露节目组未来将专门推出一期挑战问答，深入探讨负质量引发的悖论。 - **关于 CPT 对称性与时间反演（T 反转）：** 针对观众对“时间反演”的困惑，奥多德澄清道：CPT 中的“T”并不是字面意义上让时钟倒流，更准确的理解是“反转所有运动”——包括物体的线动量和角动量，即令宇宙中一切事物的运动方向全面掉头。如果宇宙具备纯粹的 T 对称性，将运动逆转后，整个系统将分毫不差地退回初始状态。然而，某些量子相互作用正向与逆向反应速率的不一致，已经无可辩驳地证实纯粹的 T 对称性是被打破的；唯有在电荷（C）、宇称（P）与时间（T）同时反转的 CPT 联合变换下，物理定律才保持严格守恒。在回答观众 TinyFox Tom 关于质量反转的问题时，奥多德指出：在 CPT 的运动反演语境下，质量并不会反转；但如果是纯粹数学意义上的时间倒流，答案在本质上是肯定的。在费曼路径积分等数学表述中，一个负质量粒子在时间中逆流而行，在数学上完全等价于一个正质量粒子在时间中正向前进。针对观众 Rishit Vora 询问时间反演对黑洞的影响，奥多德解释道，若实施包含黑洞内部在内的完全时间反演，黑洞在理论上会演化为一个“白洞”，吞入的所有物质都将奔涌而出，重新拼装成最初坠入的恒星、飞船甚至是猴子。但由于黑洞内部的物质状态未知，且自然界的 T 对称性早已被打破，倒带的黑洞不可能完全无损地还原成原来的形态，只是其中的信息会以全新的形态重新呈现。 - **趣味收尾：** 节目最后引述了网友 Mr. Nation 的趣味留言，对方提出了一套戏谑的暗区大统一公式：$\text{DE} = \text{dm} \cdot c_d^2$（暗能量等于暗物质乘以“暗速”的平方）。奥多德调侃道，这简直是在“全方位、多层次”上闪耀着智慧的光芒——虽然不一定体现在科学层次上，但无论如何，确实极具层次感。 ## Notable quotes > [06:01](https://www.youtube.com/watch?v=72cM_E6bsOs&t=361s) So, we'd better hope that it does give the same result, or there is a big problem, with either our supernova measurements or with our understanding of how the universe evolved. — Matt O'Dowd ## Mentioned PBS Space Time, Matt O'Dowd, Edwin Hubble, Vesto Slipher, Henrietta Leavitt, Adam Riess, Jamie Farnes, Andrew Paulfreyman, Marik Zilberman, TinyFox Tom, Rishit Vora, Mr. Nation, SH0ES, 普朗克卫星, 哈勃空间望远镜 ## Video description PBS Member Stations rely on viewers like you. To support your local station, go to: http://to.pbs.org/DonateSPACE ↓ More info below ↓ Check out the new Space Time Merch Store! https://pbsspacetime.com/ Support Space Time on Patreon https://www.patreon.com/pbsspacetime The search for a single number: the hubble constant, which is the rate of expansion of our universe, has consumed astronomers for generations. Finally, two powerful and independent methods have refined its measurement to unprecedented precision. The only problem is that they don’t agree. This calls into question some of our most basic assumptions about the universe. Tweet at us! @pbsspacetime Facebook: facebook.com/pbsspacetime Email us! pbsspacetime [at] gmail [dot] com Comment on Reddit: http://www.reddit.com/r/pbsspacetime Help translate our videos! https://www.youtube.com/timedtext_cs Previous Episode: Why String Theory Is Wrong https://youtu.be/IhpGdumLRqs Hosted by Matt O'Dowd Written by Matt O'Dowd Graphics by Luke Maroldi Directing by Andrew Kornhaber In 1929, Edwin Hubble discovered the universe. He gave us our first incontrovertible proof that there are galaxies outside the Milky Way by measuring the distances to the spiral nebulae. They were many millions of light years from us –far outside the Milky Way and so must be galaxies in their own right. Combined with the Doppler-shift velocity measurements of Vesto Slipher, Hubble revealed that the galaxies are not only receding from us, but they are receding at a rate proportional to their distance. Special thanks to our Patreon Big Bang, Quasar and Hypernova Supporters: Big Bang Anton Lifshits CoolAsCats David Nicklas Fabrice Eap Juan Benet Justin Lloyd TIm Davis Quasar Dean Fuqua James Flowers Mark Rosenthal Tambe Barsbay Vinnie Falco Hypernova Chuck Zegar Danton Spivey Donal Botkin Edmund Fokschaner Jens Theisen John Hofmann Jordan Young Joseph Salomone Mark Heising Matthew O’Connor Thanks to our Patreon Gamma Ray Burst Supporters: Alexey Eromenko Antonio Ruiz Bradley Jenkins Brandon Labonte Buruk Aregawi Carlo Mogavero Daniel Lyons David Behtala David Crane David Schmidt Dustan Jones Geoffrey Short Greg Weiss Jack Frosch James Hughes James Quintero Jinal Doshi JJ Bagnell John Webber Jon Folks Jonah Joseph Emison Josh Thomas Kenneth F Leonard Kevin Warne Kyle Hofer Malte Ubl Mark Vasile Nathan Hitchings Nicholas Rose Nick Virtue Richard Broman Sammy J Scott Gossett Sigurd Ruud Frivik Tommy Mogensen سلطان الخليفي [Source](https://www.youtube.com/watch?v=72cM_E6bsOs)

Source quotes

[6:01] Matt O'Dowd: So, we'd better hope that it does give the same result, or there is a big problem, with either our supernova measurements or with our understanding of how the universe evolved.

Source quotes

[15:08] Matt O'Dowd: A CPT inverted universe is not the same as this universe, but the laws of Physics are the same. The point is that you can't tell which of the two you're in. [0:02] Matt O'Dowd: The search for a single number, the Hubble constant, the rate of expansion of our universe, has consumed astronomers for generations.

Original transcript

[0:02] The search for a single number, the Hubble constant,... [0:05] ...the rate of expansion of our universe,... [0:07] ...has consumed astronomers for generations. [0:10] Finally, two powerful and independent methods... [0:13] ...have refined its measurement to unprecedented precision. [0:16] The only problem... is that they don't agree,... [0:19] ...and it's causing to question... [0:21] ...some of the most basic assumptions about the universe. [0:29] In 1929, Edwin Hubble... discovered the universe. [0:33] He gave us our first incontrovertible proof... [0:36] ...that there are galaxies outside the Milky Way,... [0:39] ...by measuring the distances to the spiral nebulae [0:42] They were many millions of light years from us,... [0:45] ...far outside the Milky Way, and so must be galaxies in their own right. [0:49] Combined with the Doppler shift velocity measurements of Vesto Slipher,... [0:53] ...Hubble revealed that the galaxies are not only receding from us,... [0:56] ...but they are receding at a rate proportional to their distance. [0:59] An impossibly vast universe had been discovered beyond the Milky Way,... [1:03] ...and at the same time that universe was revealed to be expanding. [1:07] The galaxies appear to be racing away from us, because the intervening space is expanding. [1:12] We encapsulate the expansion of the universe with a single number, called the Hubble constant. [1:18] H-naught (H0) [1:19] It tells us how fast the galaxies appear to be retreating from us, dependent on their distance apart [1:25] But, more fundamentally, H0 tells us the rate of expansion of the universe... [1:29] ...in the modern era. [1:31] Ever since Hubble's great discovery,... [1:33] ...the search for H0 has been the all-consuming obsession... [1:37] ...of thousands of astronomers across the generations. [1:40] And, understandably,... [1:42] ...the rate of expansion of the universe, combined with the gravitational effect... [1:45] ...of the matter and energy it contains,... [1:47] ...can be used to determine its entire expansion history,... [1:51] ...from the Big Bang to its final fate. [1:54] And it's fundamental for interpreting our observations of the distant universe,... [1:58] ...whose light has traveled billions of years through this expanding cosmos. [2:02] You can imagine the alarm when the two most powerful methods... [2:05] ...used to measure this fundamental parameter, the Hubble constant,... [2:09] ...gave different results! [2:10] But before we get to that, let's talk about the great quest to measure the Hubble constant. [2:15] Until the new millennium, the best we could do was to estimate H0 within a factor of 2,... [2:21] ... somewhere between 50 and 100 kilometers per second per megaparsec. [2:26] These strange units warrant some explanation. [2:29] Km/s, that's for the recession speed of a given galaxy. [2:33] Megaparsecs is for its distance,... [2:35] ...with 1 megaparsec being around 3.3 million light-years. [2:39] If the Hubble constant were, say, 75 km per second per megaparsec,... [2:44] ...then for every 1 mega parsec distance,... [2:46] ...we'd expect the galaxy to be retreating from us at an additional 75 kilometers per second. [2:51] Historically, measurement of the Hubble constant... [2:54] ...meant measuring the recession velocity and distance for as many galaxies as possible. [2:59] The velocity part is relatively easy. [3:01] Just do as Vesto Slipher did, and measure redshift. [3:05] This is the lengthening of the wavelength of light from that galaxy,... [3:09] ...which was stretched as it travels to us through an expanding universe. [3:13] The distance... that's tricky. [3:16] Hubble used Cepheid variables,... [3:18] ...giant stars, during the last phases of their lives. [3:22] They pulsate with a period... [3:23] ...that's related to their true brightness, as discovered by Henrietta Leavitt. [3:27] Measuring Cepheid periods in other galaxies gave Hubble their true brightnesses,... [3:32] ...as though undimmed by distance. [3:34] Cepheids became what we call "standard candles", objects of known luminosity,... [3:39] ...whose observed brightness, therefore, tells us their distance. [3:42] But this calculation involves assumptions and uncertainties. [3:46] For one thing, the Cepheid period-luminosity relationship... [3:49] ...first had to be calibrated,... [3:51] ...based on nearby Cepheids,... [3:53] ...whose distances can be figured using stellar parallax [3:57] Tracking their tiny motions on the sky, as Earth orbits the Sun. [4:01] This stepwise determination of astronomical distances is called the cosmic distance ladder. [4:07] With each step on the ladder, uncertainties compound. [4:10] Add this to our uncertainties in the behavior and observation of Cepheids themselves,... [4:15] ...and the precise measurement of the Hubble constant... [4:17] ...has been a slow laborious process. [4:20] As larger telescopes and more expansive surveys were completed,... [4:23] ...we gradually whittled down the errors in H0. [4:26] An important advance was the development of new standard candles. [4:30] Cepheids are good, but can only be seen out to a certain distance. [4:34] Supernovae can be seen much further,... [4:38] ...and type 1a supernovae are the key. [4:41] These result when white dwarfs, ancient remnants of dead stars,... [4:45] ...absorb too much material from a binary partner [4:48] Runaway fusion causes them to detonate. [4:51] The resulting explosion has highly predictable brightness,... [4:55] ...making them excellent standard candles. [4:57] In the 1990s,... [4:59] ...astronomers were using these supernovae to better nail down the Hubble constant. [5:03] They inadvertently discovered... [5:05] ...that the expansion of the universe is actually accelerating,... [5:08] ...revealing the existence of dark energy. [5:11] One of the Nobel Prize winning researchers behind this discovery is Adam Riess. [5:16] Riess has continued the quest... [5:18] ...to refine our measurement of H0 to ever greater precision. [5:21] A big part of his work is to improve the calibration of type 1a supernovae as standard candles. [5:27] Riess's Supernovae H0 for the Equation of State project, - SHOES -,... [5:32] ...uses the Hubble Space Telescope to match old supernovae observations... [5:36] ...with new, more reliable Cepheid variables. [5:38] By improving this run on the cosmic distance ladder,... [5:41] ...all past supernovae distances also improve. [5:44] Recent teams have now narrowed down the Hubble constant to 73.5 ± 1,7... [5:51] ...kilometers per second per megaparsec [5:53] That 2%-ish uncertainty... [5:55] ...is a hell of a lot better than the old factor of 2 uncertainty. [5:59] So, where's the crisis? [6:01] Well, in order to fully believe a measurement like this,... [6:04] ...we prefer it to be made through independent methods. [6:07] The SHOES project measures the recession of galaxies up to around 2 billion light years away. [6:12] So it's a more or less direct measurement of the CURRENT expansion rate. [6:17] But there's another way to go. [6:19] What if we could measure the expansion rate of the universe at the very beginning? [6:23] Then, we could figure out what its current expansion rate should be, given our best understanding... [6:28] ...of all the gravitational influences that affected that expansion since the Big Bang. [6:33] So, we'd better hope that it does give the same result,... [6:37] ...or there is a big problem, with either our supernova measurements... [6:41] ...or with our understanding of how the universe evolved. [6:44] Spoiler: ... there IS a problem. [6:47] There's another reason to try to calculate H0 from observation of the early universe [6:52] It's that that observation I'm referring to is far more reliable than Cepheids and supernovae. [6:58] I'm talking about the Cosmic Microwave Background radiation, the CMB. [7:03] This is a topic we've been over, so, for now, just the TLDR. [7:07] The Cosmic Microwave Background is the remnant heat glow of the universe's initial hot dense state. [7:12] Released around 400,000 years after the Big Bang,... [7:15] ...when the universe had finally cooled down enough to become transparent to light. [7:19] We still see it today,... [7:21] ...now stretched by a factor of 1,100 by its near 14 billion year journey... [7:26] ...through an expanding universe. [7:28] This is the map of the CMB across the entire sky,... [7:32] ...created by the Planck satellite. [7:34] The speckles are tiny differences in temperature,... [7:37] ...corresponding to tiny differences in density. [7:40] The blue regions are a factor of 100,000 cooler than the red regions,... [7:46] ...and also slightly more dense [7:48] These over-densities... [7:50] ...would go on to collapse into the vast clusters of galaxies of the modern universe. [7:54] So,... how can the CMB tell us the Hubble constant? [7:58] The key is the sizes of those speckles. [8:01] In the era just before the release of the CMB, matter and light were trapped together. [8:06] Matter wanted to collapse under its own gravity,... [8:09] ...while light generated a powerful pressure to resist that collapse. [8:13] These counteractive forces produced oscillations,... [8:16] ...really vast sound waves that rippled across the universe. [8:20] These are the baryon acoustic oscillations,... [8:23] ...and they occurred on all different sized scales,... [8:26] ...sloshing between high and low density, over those 400,000 years. [8:31] Then,... [8:32] ...the release of the CMB meant that light and matter were no longer coupled together. [8:36] And so those oscillations stopped. [8:38] The state of the oscillations at the moment of that release... [8:42] ...is imprinted on the CMB, in those speckles. [8:44] We usually show the distribution of speckle sizes... [8:47] ...with what we call a power spectrum,... [8:49] ...which basically shows the abundance of speckles of different sizes. [8:53] The location of these peaks... [8:55] ...tells us which oscillation modes... [8:57] ...just happened to be at their peaks... [8:59] ...at the moment the CMB was released. [9:02] This, in turn, depends on the density of matter and radiation,... [9:05] ...as well as the expansion rate of the universe in that early epoch. [9:09] So, how do you get the Hubble constant, i.e., the current expansion rate, from all of this? [9:15] Well, first you figure out what starting cosmological parameters... [9:18] ...could give the power spectrum observed by Planck. [9:21] Those parameters include the starting combination of both dark and light matter, and radiation,... [9:26] ...as well as the initial expansion rate. [9:28] And then,... [9:29] ...you figure out how the universe described by these parameters... [9:32] ...should evolve to the present day. [9:34] This sounds involved,... [9:35] ...but the Planck power spectrum is so rich with information, that the Planck team... [9:39] ...claim to have calculated H0 with even better precision than SHOES. [9:44] The problem is, the results don't agree. [9:47] The Planck H0 is 66.9 ± 0.6 kilometers per second per megaparsec,... [9:53] ...compared to the supernova result of 73.5 ± 1.7. [9:58] Now, they're actually remarkably close,... [10:00] ...given we figured them out from data at the opposite ends of time. [10:04] But they also seem irreconcilably different,... [10:08] ...3,7 sigma different in fact. [10:10] Which means a 1/7000 chance... [10:13] ...that that level of difference could have happened through random errors. [10:17] This is the crisis in cosmology. [10:20] This discrepancy first emerged in 2016, when Riess's new calibration of the supernova-derived H0... [10:26] ...revealed it to be in real conflict with the Planck result from a couple of years earlier. [10:30] Since then, calibrations have been improved, results have been rechecked,... [10:35] ...and independent methods have been used to calibrate the supernovae as standard candles. [10:40] The difference is real,... [10:42] ...and, in fact, the error bars are only getting smaller. [10:46] Okay, before we declare all cosmology broken,... [10:49] ...let's think about the two main possible sources of this discrepancy. [10:53] First: there are unknown systematic sources of uncertainty... [10:57] ...in either the supernova or Planck measurements. [11:00] Biases, that are driving one or the other to be too high or too low. [11:05] Perhaps we don't understand Cepheid variables like we thought,... [11:08] ...or perhaps gravitational lensing alters the Planck speckles differently to how we thought. [11:13] Ongoing efforts are ruling out systematic errors one by one,... [11:17] ...but it's possible there's still something we haven't thought of yet [11:21] Second: there's some unknown physics... [11:24] ...that needs to be taken into account for the CMB calculation. [11:28] This is the most exciting possibility [11:30] There are a few options. [11:32] So let's start a new list. [11:34] One: A new type of very fast-moving particle. [11:38] Insufficient numbers could skew the energy balance of the early universe, and mess up the calculation. [11:44] That particle could be the sterile neutrino,... [11:48] ...a hypothetical, non-interacting neutrino, that isn't part of the standard model. [11:53] Two: Dark matter particles behave differently to how we thought. [11:57] Perhaps dark matter interacts more strongly with matter and radiation,... [12:01] ...which would shift the sizes of those CMB speckles. [12:05] Three: Dark energy isn't constant. [12:08] The current calculations assume that dark energy is described by the cosmological constant,... [12:13] ...which, by definition, doesn't change. [12:16] But if dark energy increases,... [12:18] ...that could explain why we observe a higher H0 in the modern universe... [12:22] ...than is predicted by extrapolating from the early universe. [12:26] The answer will depend on whether the more correct measurement of the Hubble constant... [12:30] ...comes from Planck or SHOES. [12:33] New observations and new telescopes will refine these numbers even further. [12:37] Independent methods, like using gravitational lensing, or gravitational waves,... [12:42] ...will weigh in on one side or the other. [12:44] Perhaps the uncertainties will be refined, and the two results will converge. [12:49] That'd be cool. [12:50] The near centennial quest to measure the expansion rate of the universe will be concluded. [12:55] Or perhaps the discrepancy will persist. [12:59] That would be even cooler [13:00] We'll have a new tool... [13:02] ...to investigate the mysterious physics of dark energy, dark matter,... [13:05] ...or of unknown particles beyond the standard model. [13:08] For now, we continue our obsessive quest for H0... [13:12] ...and for what it'll tell us of the origin and fate of our expanding space-time. [13:19] In today's comment responses, we need to catch up on 2 episodes. [13:23] First, it's our journal club on Dr. Jamie Farnes' paper... [13:26] ...about negative mass dark fluid... [13:29] ...as a unifying explanation of both dark matter and dark energy. [13:32] Then we'll get to comments on our CPT symmetry episode. [13:36] So, a friend of a friend of Dr. Farnes' chimed in. [13:40] Leo Staley's friend says that Dr. Farnes doesn't necessarily believe the claims of his paper,... [13:46] ...but rather its purpose was to spark interesting ideas among physicists. [13:51] Well, okay. I totally respect that motivation to publish even quite fringe ideas,... [13:56] ...and he certainly sparked a conversation. [13:58] I mean, look, I'm still talking about it. [14:01] Andrew Paulfreyman points out that... [14:03] ...the gravitational lensing measurements of dark matter... [14:06] ...will give the exact opposite results if dark matter is due to this negative mass fluid... [14:11] ...than if it's actual, positive mass matter. [14:14] And my intuition tells me that this is right. [14:18] Gravitational lensing is the bending of light by a gravitational field. [14:22] We see it in the warping of images of distant objects,... [14:26] ...due to the gravitational fields of more nearby galaxies. [14:30] We can use that warping to measure masses. [14:32] And yeah, those measures tell us that dark matter has positive mass [14:37] I'd need to do the simulations, but I have a feeling... [14:40] ...that we wouldn't even see this sort of strong gravitational lensing... [14:44] ...if the effect of dark matter was due to this dark fluid. [14:48] Marik Zilberman's distaste for negative masses... [14:51] ...is that they produce perpetual motion machines and paradoxes left and right. [14:55] Exactly what I thought. [14:57] When a theory leads to these - sort of - pathological predictions, it's a big red flag. [15:02] And we're actually going to do a challenge question episode,... [15:05] ...to explore these paradoxes. [15:07] Stay tuned. [15:08] Okay, let's move on to our episode on the ultimate symmetry of nature,... [15:12] ...the simultaneous reversal of charge, parity, and time. [15:17] First up: a few of you asked questions about time reversal, so I want to clarify. [15:22] The T in CPT symmetry isn't a literal rewinding of the clock. [15:27] It's best thought of as a reversal of all motion,... [15:31] ...both linear and angular momentum. [15:34] Everything reverses direction. [15:36] If the universe has this sort of T symmetry,... [15:39] ...then, if you reverse all motion,... [15:41] ...the universe will evolve exactly backwards, to its initial state. [15:45] Turns out that's not the case,... [15:47] ...as demonstrated by the different forward/backward reaction rates in certain quantum interactions. [15:54] But the universe IS symmetric under full CPT inversion. [15:59] Now, a CPT inverted universe is not the same as this universe,... [16:03] ...but the laws of Physics are the same. [16:05] The point is that you can't tell which of the two you're in. [16:09] TinyFox Tom asks whether mass would be inverted under CPT symmetry. [16:14] And I guess you're referring to the idea that time-reversed energy has its sign flipped. [16:20] So, the simple answer is no, because the T in CPT isn't a true time reversal. [16:26] But in the case of a true time reversal, the answer is, essentially, yes. [16:31] And a negative mass particle, moving backwards in time,... [16:34] ...is mathematically the same as a positive mass particle moving forward in time [16:39] That notion makes sense in the math,... [16:41] ...and is used in, for example, Feynman's path integral formulation of quantum mechanics. [16:47] But it's not so obvious whether this idea corresponds to anything physical. [16:52] Rishit Vora asks how a T inversion would affect a black hole? [16:57] Well, a true time reversal that included the interior of a black hole... [17:01] ...should transform it into a white hole. [17:04] Everything that ever fell in would come rushing out,... [17:07] ...and presumably reassemble itself into the stars, spaceships, monkeys,... that originally fell in. [17:13] As to the motion reversal symmetry of the T in CPT,... [17:17] ...frankly, I'm not sure, because we don't know the state of matter in the black hole. [17:23] But, at any rate, remember that T symmetry is broken. [17:27] Both the T of CPT and true time reversal symmetry. [17:31] So a rewound black hole shouldn't revert exactly to whatever it formed from. [17:36] That doesn't mean information is lost. [17:38] Just that it ends up in a different form. [17:40] And back to dark fluid for a sec. [17:42] Mr. Nation [?] has his own unified theory of the Dark Sector. [17:46] He reveals to us that dark energy equals dark matter,... [17:50] ...times the speed of dark... [17:51] ...squared. [ DE = dm•(cd^2) ] [17:53] Genius on so many levels. [17:55] Not only scientific levels, but still, levels.