The new studies run counter to calculations based on observations of the early universe. Is this bad news for the standard cosmological model?

The Hubble constant expresses the universe’s present-day rate of expansion. There’s only one current expansion rate of the universe, but different studies are coming up with different answers for what it is. 

Calculations based on observations of the early universe — namely, the cosmic microwave background (CMB) that is a sort of afterglow of the Big Bang — produce one answer for the Hubble constant. Observations of the “late universe” instead compare the distances of astronomical objects, often standard candles with known distances, to the speed those objects are moving away from us. The two techniques provide different answers, a discrepancy that has become known as the Hubble-constant tension.

Hubble constants
The past decade or so has seen dozens of measurements of the Hubble constant, using sources near (in the box labeled "Late") and far (in the box labeled "Early"). There seems to be a discrepancy depending on whether the measurements are based on the early universe or the present-day universe, as seen in the box labeled "Early vs. Late," though the amount of discrepancy depends on which sources are used.
Vivien Bonvin / HOLiCOW Team

No one knows why the early and late methods give different answers. At first, people thought that more and better measurements would cause the numbers to converge. But instead, in study after study, the error bars have shrunk to the point where the difference has become statistically significant.

The first and most precise measurements of the current expansion rate were made using standard candles, sources with known luminosity. If we know how luminous a source is, then we can reckon its distance according to how bright it appears. The two most widely used standard candles are Cepheid variable stars and Type 1a supernovae, but there are many others.

New Results

Two independent groups using data from the Hubble Telescope have just published new studies measuring the Hubble constant in different ways, yet their results match. The results give further credence to the late-universe consensus of an expansion rate around ~73 km/s/Mpc. But it also serves to deepen the tension, as studies published in the last decade calculating the rate from the properties of the CMB give an answer around ~67 km/s/Mpc. This may seem like no big deal, but the difference is big enough that some astrophysicists are calling it a “crisis for cosmology.”

The first study, led by Adam Riess (Space Telescope Science Institute), uses Cepheid variables, which are typically nearby, as a stepping stone to Type Ia supernovae, which can be seen much farther away. Using Hubble to measure Cepheids in galaxies hosting Type Ia supernovae, they measure the Hubble constant more precisely than any late-universe method has done so far. 

The second study, led by John Blakeslee (NSF's NOIRLab), uses surface brightness fluctuations (SBF) of 63 elliptical galaxies to calculate the Hubble constant. These fluctuations were first suggested as a tool for measuring intergalactic distances in 1988, but this is the first time they’ve been used in this way. The astronomers selected the galaxies from the MASSIVE galaxy survey, a study of the 100 biggest galaxies within 300 million light-years.

Elliptical galaxy NGC 1453
NGC 1453, a giant elliptical galaxy situated in the constellation Eridanus, was one of 63 galaxies used to calculate the expansion rate of the local universe.
Carnegie-Irvine Galaxy Survey

The SBF method looks for the differences in brightness between pixels within an image of a galaxy. For nearby galaxies, there are relatively few stars per pixel, and the statistical fluctuations in the number of stars per pixel is higher. As a result, nearby galaxies appear somewhat "bumpy" in their light distributions. For more distant galaxies, there are many more stars per pixel and the pixel-to-pixel variations consequently are lower, making the galaxies smoother in appearance. The relative smoothness of one galaxy compared to another, which may appear similar in total brightness, is a good indication that the smooth one is farther away.

“Surface brightness fluctuation is an alternative to methods such as Type 1a supernovae,” Blakeslee says. “It can be calibrated independently, and it occurs in different kinds of galaxies.”

Resolving the Tension

Some astronomers, like George Efstathiou (University of Cambridge, UK) think that all the late-universe measurements have systematic problems. But the more studies that are published from different groups, using different independent techniques, and looking at different parts of the sky, the less likely it seems that they would all give the same wrong answer.

It doesn’t seem likely that there is a problem with the CMB calculation, either. The measurement from the European Space Agency’s Planck mission is considered to be one of the most elegant and well-supported pieces of physics ever. But if all the late-universe calculations have been done correctly as well, then this could mean something is wrong with the standard cosmological model itself.

“Cosmic microwave background radiation doesn't give a direct measurement of the Hubble constant today,” Blakeslee says. “It needs to be combined with a cosmological model, which can then predict the expansion history of the universe.” And it’s possible that something vital has been “lost in translation.”

Gravitational waves as standard sirens
Gravitational-wave sources can act as "standard sirens," enabling an independent measure of the Hubble constant.
Fermilab

Hsin-Yu Chen (MIT), a member of the LIGO collaboration, and the author of a few studies pioneering the use of gravitational waves to measure the current expansion rate, says that everyone disagrees about the nature of the disagreement between early and late universe measurements.

“Perhaps the discrepancy is from systematic errors,” she says. “Or maybe the standard cosmological model needs to be fixed. Maybe it is wrong. Everyone has their own opinion.”

With the LIGO and VIRGO collaborations restarting more advanced operations next year, there will be a wealth of new gravitational-wave detections to analyze. Chen predicts that enough events will be observed in the next few years to give a Hubble constant measurement with only a 2% margin of error.

And when the James Webb Space Telescope launches this year, it will provide improved data for all the late-universe techniques. Whether the future is bleak or bright for the standard cosmological model, it seems as though we will have clarity as to the true nature of the disagreement before long.


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Peter Wilson

March 18, 2021 at 8:43 pm

All the Standard Model charts show deceleration in the early universe--due to dark matter--then acceleration in later epochs--due to dark energy.

The universe's expansion is accelerating--as expected (https://youtu.be/4goInwbOix4)--and the expansion rate at later times is larger. This is a crisis? Shouldn't a larger rate of expansion at later times be exactly what we expect from accelerating expansion?

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robert stenton

March 18, 2021 at 10:59 pm

In order to understand expansion it is to best visualize it in 3 dimensions. If you want you can imagine it as raisins spreading apart in rising bread. The Hubble Constant is measured in Mega-parsecs. So think of a cube of raisin bread a Mega-Parsec on an edge. An early Mega-Parsec is now many Mega-Parsecs on an edge but the unit volume determining Hubble has remained the same. So yes, you are right if you used the original now enlarged cube but because the volume determining Hubble has remained the same, Hubble's value should have remained the same.

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Arjmaj

March 19, 2021 at 5:37 pm

Im not a cosmologist (and my english is bad, can read it or hear it, but speaking or writing is difficult). Call me ignorant, and please correct me, but from your example, yes, the unit in wich you measure is the same, ¿but this unit does account for expansion of space?. ¿It only serves for measuring distances?. I think of space as some sort of "auto-replicant self". The amount of matter doesnt change, it never grows. But space, that dark abism, where you have 1 "space" you suddenly have 2. Then 4. Then 8. But it never does exactly that, maybe you have 1, 2, 4, 5, 10, 11, etc. As Hubble's value is the same, the problem is that space doesn't. ¿Hubble's value can account for 3 dimensional meassurement? We can meassure the distance to a galaxy, or even between galaxies. But the problem is that galaxies are not in the same plane. You can have 2 at the same distance from earth, lets say 90 degrees at each other from our point of view. At x distant in between them. But if one galaxy is, say, 1 billion km (i dont know tecnisisms and is for example only) above the other, and we cant meassure that discrepancy because, again, we are meassuring in one axis only, ¿how we can expect the same results as if we meassure 2 different galaxies than the above mentioned? apart from the obvious, that ¿we can't say exactly the distance. At best we can bullpark that number?. If you account for the error in the distance maybe you can have a solution, ¿but how?.
Im not want to sound stupid or that i know everithing. Its my take on the matter. If im wrong please correct me.

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robert stenton

March 21, 2021 at 8:20 am

Dear Arjmaj,
You are incorrect in assuming that we do not know the distance to other galaxies. Astronomers use what they call "standard candles" to determine those distances. Hubble used cephied variables to determine the distance to the Andromeda galaxy. These are stars that pulsate in brightness and the frequency of those pulsations is directly related to their brightness. Parallax was used to determine the distance away these stars where in our own galaxy and then by using their apparent brightness how far away Andromeda was. A certain type of supernova is also used as a standard candle. I hope this helps.

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robert stenton

March 21, 2021 at 9:30 am

I incorrectly spelled Cepheid.

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Arjmaj

March 21, 2021 at 11:10 am

I think you misunderstood me. I will try to be more specific. ¿Does our system of meassurement account for 3 dimensional space instead of 2 dimensional?
As i wrote in another response, ¿can we account for non meassurable distances?. We can meassure distances based on our point of view. If a galaxy is more far away than another but we cant meassure it because is on the low end spectrum, that error will get different results from what expected. ¿Maybe?

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robert stenton

March 23, 2021 at 1:28 pm

Dear Arjmaj,
I think I may have misunderstood your concerns. Your question is how can astronomers be so accurate since objects in space are also moving around. The answer is that they take many measurements so that the movements of objects moving sideways to us cancel out. While they can't accurately measure sideways movements at such great distances (New Horizon which went to Pluto traveled 3 times faster than the Hubble constant) they can measure the dropper change of the spectrum of the most distant star very accurately even down to the speed of a tortoise walking! So instead of measuring the distance of two objects at a great distance moving apart as you might expect, they instead use the radial change of distance of objects moving away from us to determine the Hubble Constant. And the small change in that constant from very early times to more recent times may indeed be due to some error. Time will tell.
Bob

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robert stenton

March 23, 2021 at 1:41 pm

I mispelled Doppler

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DSailing

March 19, 2021 at 9:00 am

The Hubble parameter does change with time. It is usually referred to as H(t). The Hubble constant (usually referred to as H0) is the value of H(t) today.

The Hubble constant (H(t) today aka H0) computed from observations of events when the universe was only 380,000 years old and using the LCDM model of the universe to compute what H0 should be today, does not match what H0 is when computing it by measuring the expansion of galaxies within 100 million light years or so.

If all the measurements are accurate, that means something is wrong with the LCDM model of the universe. That is the Hubble tension.

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Arjmaj

March 19, 2021 at 6:02 pm

Ok. I dont know much. But, the hubble constant, does change with time, but ¿it can account for 3 dimensional meassurement?
My problem is that meassuring distances is only in one axis. Point a to b. ¿We can account for errors with this system?. Then you have the biggest problem, you know point a to b, but you need to know pont a to b in correlation with c. Making a-b a-c b-c doesnt solve the problem, we cant change our point of view, so we can meassure like we meassure distance between points in a paper. Subtle changes in depth, or even altitude from one point to another will alwyas carry discrepancies. You wont get the same result from earth to g1, earth to g2 and g1 to g2 and then earth to g3, earth to g4 and g3 to g4. You cant account for 3 dimensional space. ¿Maybe? I dont know. Its only my mind going ape with this.

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Andrew James

March 19, 2021 at 7:42 pm

Well said. [Mr. Wilson's repeated historical response to cosmology again seems to attempt to sow doubt debunk dark matter/energy.]

i also note you the term "Hubble tension', which this article doesn't specifically use but does appear elsewhere.

There are new techniques in the works such as examining polarisation data and the matter power spectrum to explain the H0 discrepancy.

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Peter Wilson

March 22, 2021 at 12:08 pm

Thank you, DSailing. for the clear answer to my question.

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Deryk Houston

March 20, 2021 at 8:43 pm

I have been trying to wrap my brain around the Big Bang all my life. (Admitted... my brain is quite small)
What I don't understand is when we look through our telescopes and accept the idea that the light that arrives on our lense is ancient light. The star of galaxy probably is not even there anymore etc. We are looking back further and further into the past to what is supposed to be the early universe. But why do we assume that someone is not looking back at us at exactly the same distance, and if they are, then my suspicion is that they could have the exact same interpretation of the light that lands on their telescope lense and be assuming that they are looking at the early universe.
Something is wrong with the current model and I've been waiting all my life for this nonsense to be exposed.
Thank you for your time.

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Andrew James

March 21, 2021 at 3:57 am

Why is it nonsense?

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Peter Wilson

March 22, 2021 at 12:47 pm

Because until some realism is added to the model, the brouhaha over dark energy is nonsense.

This is explained, or "exposed,” in my thesis, linked to in the 1st post. Nor is my thesis hand-waving: it comes with an equation for dark energy, one that yields the observed value. This equation is in terms of complicating factors that have been omitted from the Standard Model for convenience’s sake...then forgotten. An equation is the gold-standard in physics. Until someone besides myself demands a more realistic model, one that includes the complicating factors described in my thesis, the nonsense will continue.

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Andrew James

March 23, 2021 at 2:12 am

Sorry. Your "thesis" makes little to no sense. Just because a model is incomplete doesn't equate to it being wrong.

Wanting science to be wrong is the flaw here. Science is based on evidence and testing theory on postulates not tearing down it on belief or incorrect assumptions. In saying "This equation is in terms of complicating factors that have been omitted from the Standard Model for convenience’s sake...then forgotten. An equation is the gold-standard in physics." immediately supports this flawed logic. It is opinion that isn't supported by actual evidence.

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Peter Wilson

March 24, 2021 at 10:14 am

You are correct: a model being incomplete does not make it wrong.

But look at the situation: the "mystery" is unaccounted for expansion. The factors left out--eta and R_i--contribute to expansion!

So why not include them? The simplest possible model, sans eta and R_i, failed to account for the expansion's acceleration. What is the objection to adding known factors, factors that are known to contribute to expansion?!?

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Andrew James

March 26, 2021 at 3:29 am

If your ideas were plausible I'd think the cosmologists brighter than you or me might have cottoned on to it, but instead I just see denigration of 'scientific authorities' just to enhance an unproven idea without peer review or any proof. Thank you.

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Peter Wilson

March 26, 2021 at 12:26 pm

God bless it! I was hoping no one would notice...
Authorities are infallible. The Titanic must not have needed a full complement of life-boats, and that's the reason my equation must be wrong: authorities never are.

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Andrew James

March 26, 2021 at 6:49 pm

Sorry. People have bend over backwards to be polite, but frankly these ideas on the cosmological constant are just nonsense. I immediately question your motive as a red flag. Your silly equation makes little to no sense, and it is neither peer reviewed, cited nor has observational evidence to support it. Using variational principles of classical mechanics to derive a end-all equation in this instance is nuts. e.g. What about the vacuum energy density? Teleological axioms are seldom useful with scientific theories, and are easily dismissed - especially when one reverts as evidence is to belittle many cosmologists who actually know better. Authorities might be fallible, bur novice scientists are likely dead wrong. Thanks.

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Andrew James

March 28, 2021 at 2:54 am

I was responding to Derek not your response. Really.

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Rod

March 19, 2021 at 7:12 am

I use c.g.s. units when looking at expanding space. H0 = 69 km/s/Mpc is 2.24 x 10^-18 cm/s/cm. Things get dicey when reviewing H0 values reported using c.g.s. units. Inflation for example, space could expand >= 3 x 10^30 cm/s (> 10^20 c velocity).

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Andrew James

March 19, 2021 at 8:37 pm

Why use non-standard c.g.s. instead of universal SI?
Link to usage s here https://www.iau.org/publications/proceedings_rules/units/

International System of Units continues to be avoided by some Americans. Considering the sciences in America mostly began endorses SI for over two decades now, Even the AAS says use "Use standard abbreviations for SI..." I just don't understand why there is persistence in using now antiquated intermediate pseudo units like c.g.s - the worse being Angstrom and 'dynes',

Note: Units are specifically expressed as km sec-1 Mpc-1

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Rod

March 21, 2021 at 7:59 am

Andrew James, I use CGS to compare the expansion rate changes quickly (and various papers published on the NASA ADS Abstract service use CGS too). The cosmology calculators, https://ned.ipac.caltech.edu/help/cosmology_calc.html take H0 back to the origin of the CMBR but nothing about the expansion rate of space back to Planck time and Planck length like inflation is dancing around or near. I can see quickly that space expansion rate(s) changed >= 10^48 order magnitude or more (e.g. 10^-18 cm/s/cm vs. 10^30 cm/s). Also there is the temperature >=10^27 Kelvin used too and vacuum energy density and the problem of the cosmological constant, but set this stuff aside. 3D space expansion rate changes >=10^48 magnitude between the *beginning of the universe* and when the CMBR formed, I think should be called out clearly to the public in BB cosmology.

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Andrew James

March 23, 2021 at 2:21 am

It's 2021 not 1980. None of the NED calculators use H0 in cm/s/cm but only by SI units. e.g. 1/Ho = (978 Gyr)/(Ho in km/sec/Mpc)

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Yaron Sheffer

March 24, 2021 at 5:13 pm

You guys are not dealing with cgs. The result of cm/s/cm is simply 1/s, inverse time, and it's the same in cgs and mks! So 2×10^-18/s is 5×10^17 s for the age of the universe = 1/H0.

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Yaron Sheffer

March 24, 2021 at 5:13 pm

You guys are not dealing with cgs. The result of cm/s/cm is simply 1/s, inverse time, and it's the same in cgs and mks! So 2×10^-18/s is 5×10^17 s for the age of the universe = 1/H0.

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Trevort

March 19, 2021 at 9:51 am

I venture a theory on why the rate of expansion of the Universe is accelerating in my book How Black Holes Make and Break Galaxies
https://www.amazon.com/Black-Holes-Break-Spiral-Galaxies-ebook/dp/B00R3QZL4O/ref=mp_s_a_1_1

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William-Zuna

March 20, 2021 at 4:37 pm

As a laymen, can I get some discussion about the possibility that the characteristics of the quantum foam of space over time actually affects the speed of light over time and consequently the conclusions we make using the speed of light as a constant over time. Thanks, kids

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Andrew James

March 23, 2021 at 2:29 am

Unlikely.

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Darrell Burgan

June 11, 2021 at 9:27 pm

Why is this such a crisis in cosmology? Seems nature is simply telling us the cosmological constant isn't a constant.

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