New data and analysis show that a long-standing discrepancy in the measurement of the current expansion rate of the universe is real — even as the reason for this discrepancy remains a mystery.

Hubble galaxy thumbnails
The new study used Hubble Space Telescope images of galaxies that have hosted Type Ia supernovae.

There is a crisis unfolding in the field of cosmology.

Most measurements of the current expansion of the universe (called the Hubble constant, or H0) based on stars and other objects relatively close to Earth give a rate of 73 km/s/Mpc. These are referred to as “late-time” measurements. On the other hand, early-time measurements, which are based on the cosmic microwave background emitted just 380,000 years after the Big Bang, give a smaller rate of 68 km/s/Mpc.

They can’t both be right. Either something is wrong with the standard cosmological model for our universe’s evolution, upon which the early-time measurements rest, or something is wrong with the way scientists are working with late-time observations.

Climbing the Distance Ladder

Most of the late-time measurements of H0 use “distance ladders” to measure cosmic distances further and further outward into the universe. One of the most prolific distance-ladder collaborations is SH0ES (Supernovae and H0 for the Equation of State of dark energy), which Adam Riess (Johns Hopkins University and the Space Telescope Science Institute) has led for nearly two decades.

The first rung in the SH0ES method uses geometric parallax to double-check the distance to Cepheid variable stars in our galaxy, for which astronomers can also measure distance using their brightness variations. The second rung then compares Cepheids against Type Ia supernovae, another “standard candle” like Cepheids that astronomers can see to greater distances. The third rung compares distances based on supernovae and redshift measurements.

In a Zoom webinar on December 9th, Dan Scolnic (Duke University) announced, on behalf of a collaboration between SH0ES and another group, Pantheon+, that the teams had obtained a new late-time H0 measurement with the smallest uncertainty yet. The result is posted on the arXiv preprint server. After much data collection and analysis, the teams still find the universe’s expansion is expanding at a high present-day rate between 72 and 74 km/s/Mpc — a much smaller range than obtained from their previous late-time measurements.

The Hubble Tension is Real

The Pantheon team complemented the SH0ES team’s work by performing a meta-analysis of supernovae surveys, correcting for the inconsistencies that can crop up during the use of different instruments, baselines, and calibration methods. The SH0ES team then used this updated information, along with new Cepheid sightings from the Hubble Space Telescope, to take a closer look at their previously established distance ladder methods.

While the additional data helped reduce the range of possible H0 values from the team’s calculations, the systematic study of the methods involved is what really sets this study apart from previous ones.

“They've done a more complete and thorough cross correlation of terms between the different aspects of the distance ladder,” says Suhail Dhawan (Cambridge University, UK), who was not on either team.

The researchers set up about 70 different scenarios in which they changed the way they added things up along the distance ladder in order to measure systematic error. Small uncertainties can add up in big calculations in a way similar to the “butterfly effect.” Many have postulated that distance-ladder measurements are prone to systematic errors but understanding those systematics has been difficult. Using their dozens of scenarios, the SH0ES team determined the effect any particular error or combination of errors might have on the final Hubble constant calculation.

Scenarios tested in Hubble constant analysis
The researchers tested about 70 different scenarios to understand systematic uncertainties.
Riess et al. /

Thanks to the additional data and analysis, the results breach the “five-sigma threshold,” meaning there is only a 1 in 1 million chance that the discrepancy between late-time and early-time measurements arise from systematics.

In short, the so-called “Hubble tension” seems to be real. And it is looking more and more like something missing or wrong in the standard model of cosmology is causing the difference between early- and late-time measurements. The search is on for such new physics: the discovery of some as yet unknown law, particle, or property that’s causing these disparate measurements of the universe’s current expansion.

Read an in-depth take on the Hubble constant mismatch and what it could mean in the March 2022 issue of Sky & Telescope, coming to newsstands soon.


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December 14, 2021 at 5:19 pm

And do not forget that the standard model of cosmology was created by adjusting 5 parameters to match the observational data. This curve fitting lead to the "invention" of both dark matter and dark energy neither of which have ever been detected directly after 40 years of searching. It is time to scrap this model and establish a real theory.

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December 14, 2021 at 10:52 pm

We can't forget what never was. Dark matter and dark energy were independent discoveries (as was inflation) before the standard model was developed, and is observed by many independent means - the article describes that for dark energy.

The inflationary Hot Big Bang cosmology, the "standard model" of the last two decades, were a *consequence* of those discoveries. Look up the history if you don't know this, Wikipedia has articles on it.

And it is a real theory in the way that it describes observations better than any competitors. I'll cite from the Astro2020 US Astronomical and Astrophysics Decadal Survey appendix C Panel on Cosmology:

On the theory: "There is no doubt that the standard cosmological model is a triumph. By adopting simple versions of inflation, dark matter, and dark energy, the model can match observational results despite orders of magnitude of improvement in cosmological measurements over the past 20 years."

On the name: "The question of what process set the Hot Big Bang in motion and created the seeds of structure has been with us for many decades. Early theoretical developments, together with observations over the past two decades, have established the inflationary paradigm as the dominant picture in the field."

Note that the model adopted simple models out of many possible for the already observed phenomena. [Inflation is a bit different since the last BICEP3/Keck data on its own for the first time prefer a simple model, a Higgs like scalar quantum field which quantum fluctuations "created the seeds of structure ". In that way we may know more about inflation soon than we do of dark matter - dark energy is pretty much nailed by Weinberg's anthropic multiverse in the eBOSS cosmological paper but the Panel on Cosmology prefers to leave that open.]

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Deryk Houston

December 15, 2021 at 8:14 pm

I'm always left with the impression that cosmology is a pseudo science. I've been following it for over fifty years now and it fascinates me how the believers insist on hanging on to the Big bang theory no matter what comes up.
Even when it's clear that there is something terribly wrong after observations showing problems with the hubble constant....they still insist that theory matches observation.

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December 27, 2021 at 2:44 pm

Deryk, one of the nice things about, say, chemistry as opposed to cosmology is there is no theological implication inherent in the periodic table.

Unfortunately, we'll never know the truth, as all challengers and dissenters have been run out of the field as heretics and unbelievers -- for at least about the five decades you mention.

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December 17, 2021 at 5:35 pm

While I don’t think cosmology is pseudoscience, it’s a bit of an “appeal to the stone” to say the question of dark matter and dark energy have been unquestionably settled. Similar to your discussion, NASA’s explanation is as follows “ By fitting a theoretical model of the composition of the universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter.” That just means they’ve found assumptions that make their model work and they cause those dark energy and dark matter. Please cite, if you know, anyone who has directly sampled either. Also appealing to a model “Weinberg’s anthropic multiverse” that is not even wrong because it is not a scientific theory that can be tested as is required by science, seems a very hand wavy way to provide evidence which is to say it’s not much evidence.

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December 17, 2021 at 5:38 pm

I agree Deryk - we are seeing a LOT of this in the past 25 years: Groups with a theory and fervently trying to match data to it like a dog with a bone!

We see this in climate studies (no room for anthropogenic heat emissions in the model? I say there's MUCH more to THAT story (think Thermodynamics), we see this all the time in cosmological modelling - dark matter and energy, if THAT much of the equation, should have been detected by now. I am also not entirely clear why NO one really considers that a photo travelling THAT long (billions of years) may not have even more interactions and absorption/wavelength shifts that we cannot see. (think "tired photon" theory - sort of).

I think it best to setup an AI like they do with Alpha Zero and AlphaGo (AI learned both complex games in very short order and are currently outplaying ALL computer and human players!) - let it start with only basic rules and run amuck! I'd bet the theories these would generate would astound us!

Science has become way too commercially-driven and even "peer" reviews are very biased in general. I think it time to remind ourselves what Science is - learning by admitting we do NOT know, and coming up with a testable hypothesis to match the data, not interpret data to match our hypotheses!

Anyway - interesting times to be sure! Cheers and happy holidays all!

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Anthony Barreiro

December 14, 2021 at 5:41 pm

Could this most recent finding be evidence that the "Hubble constant" has not been constant over the life of the universe? Could the rate of expansion have been slower in the early universe and faster today? Wouldn't that be expected if dark energy is a real thing? I've read that some theorists prefer the term "Hubble parameter" rather than "Hubble constant". By the way, this is a sincere question. I don't know enough about this stuff to have a dog in the fight.

From a layperson's perspective, if there are reliable observations that don't jibe with current theory, this seems like a good thing -- the theorists have something to work on. If we ever figure out how everything works, the universe will be a boring place!

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December 14, 2021 at 10:38 pm

Yes, the expansion rate as measured by the normalized change in scale is the time dependent Hubble parameter, while the Hubble rate is its value now [ ].

"The relative expansion of the universe is parametrized by a dimensionless scale factor a."

"d(t)=a(t)d_0 where d(t) is the proper distance [between pairs of unbound objects] at epoch t, d_0 is the distance at the reference time t_0, usually also referred to as comoving distance, and a(t) is the scale factor."

"The Hubble parameter is defined:
H(t) = a_dot(t)/a(t)
where the dot represents a time derivative. The Hubble parameter varies with time, not with space, being the Hubble constant H_0 the current value."

As you can see from the Wikipedia article, the scale change with time depends on the inner state of the system (LCDM universe), with in turn radiation, matter and dark energy dominating, each with its own evolution imprint.

You can also see that in illustrations of the Hot Big Bang universe [,_Accelerated_Expansion_of_the_Universe,_Big_Bang-Inflation.jpg ].

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December 15, 2021 at 4:56 am

My response to you ended up as a standalone comment [see there for references]. It is also incomplete, I can see now: yes, there is a Hubble parameter and the Hubble rate is its value now. This time dependence is expected since it depends on the inner state of the universe (radiation, matter or dark energy dominance in the energy budget).

What I forgot to add is that it is also accounted for in the measurements (if they cover a larger z-range, say) and in the LCDM model.

For example of how ill this accounting fits mere words, we can often read that the universe expansion is accelerating: the diameter dependence goes towards an exponential increase set by dark energy. But the Hubble parameter is then approaching a lower Hubble rate (since dark energy sets a lowest energy density "floor") so the scale growth is decelerating.

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December 15, 2021 at 4:57 am

3d attempt: My earlier response to you ended up as a standalone comment thread [see there].

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January 1, 2022 at 10:24 pm

Mirroring my thoughts. Thank you Anthony.

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December 14, 2021 at 10:23 pm

It is of course problematic if two different precise methods give different results.

But that doesn't mean we can say that there is a difference between early- and late-time measurements since other late-time methods gives results agreeing with early-time measurements. We have the tip of the red giant brance method and eBOSS latest galaxy survey which are high precision, as well as the new and still weak black hole merger results from the LIGO/Virgo O3b data collection, that all prefer the early-time result or in between.

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December 14, 2021 at 10:55 pm

*the tip of the red giant branch.

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Peter Wilson

December 15, 2021 at 10:53 am

Why worry about a numbers mismatch?

The standard model, or ΛCDM, is causally incomplete (1,2). It is missing several “complicating factors” that contribute to expansion. The simplest explanation for dark energy (Λ) is that it is one of these unused, causal factors, divided by another, squared:

Λ = η/(R_i)^2

This equation yields the observed value of dark energy, 10^-52 /m^2, without mystery or fanfare. It does not violate energy conservation; it is derived from that principle.

Why don’t we fix the bigger problem? Start with a model of the cosmos that is causally complete?

1. Causality in Cosmology,
2. Dark Energy Dialogue,

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Andrew James

December 23, 2021 at 4:43 pm

Just silly. Self promotion of one's ideas is desperate if you ask me. Dark matter makes more sense.

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Andrew James

January 30, 2022 at 4:00 pm

Just bad science. Ghost researchers have no place here or anywhere.

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December 15, 2021 at 6:47 pm

Very interesting report. My observation. H0 = 68 km/s/Mpc = 2.20371E-18 cm/s/cm and 74 km/s/Mpc = 2.39816E-18 cm/s/cm. Very small differences but the cosmology calculators show real differences in the Hubble Time for the age of the universe as an example. You can adjust the universe age by more than one billion years younger in some cases or more. Using the cosmology calculators, plug in different values for H0, use defaults, and you can see the numbers change 🙂 I use these calculators,,,

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December 18, 2021 at 12:23 pm

This is a fascinating issue. And I appreciate S&T for keeping us abreast of the developments. For me, this is real science. We are looking for inconsistencies in our theories and that is the way we advance.

Nonetheless, I suppose I do not quite understand the problem. It seems that if the expansion of the universe is accelerating then the Hubble expansion parameter would be less for "early time" measurements than for "late time" measurements. We are measuring the value for the early universe which was expanding more slowly than now.

If you use the raisin bread analogy and the dough is expanding slowly at the beginning and then speeds up later, then the speed divided by the between two raisins will go up.

What am I missing?

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Monica Young

December 19, 2021 at 2:49 pm

This is a common confusion! The issue is that the Hubble constant (H0) is the *current* rate of expansion, and the universe cannot *currently* be expanding in more than one way.

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December 22, 2021 at 4:06 pm

So, are they adjusting the early measurement to get the current rate of expansion?

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December 23, 2021 at 3:55 am

Confusingly, 'early-type measurements' (CMB) are actually measuring properties of the present-day Universe rather than at 380,000y after BB. As if cosmology wasn't confusing enough... 🙂

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