Universe younger and faster than thought

[BWV]

Thoughts on this finding? Does it really require new physics to explain?

Riess observed 70 Cepheid stars — stars that pulse at a well-observed rate — calculated their distance and rate, and then compared them with a certain type of supernovae that are used as measuring sticks. It took about two years for the Hubble telescope to make these measurements, but eventually Riess calculated an expansion rate of 74.

Using that 74 figure means the universe is somewhere between 12.5 billion and 13 billion years old. That’s much younger than the established estimates of 13.6 billion to 13.8 billion.

https://arxiv.org/pdf/1903.07603.pdf

https://apnews.com/fac50d45a19f4239848b1712cfd22c36

 

[Orodruin]

This just cements the discrepancy in $H_0$ between near-time measurements and the value inferred from CMB measurements. Figuring out the reason for this discrepancy is an active field of research.

 

[Grinkle]

@Orodruin Do you know if there are any leading conjectures for the discrepancy? That sounds interesting.

 

[Orodruin]

I have not seen something in particular. It is not my direct field so I don’t tend to read everything that comes out - but I see some papers in the listings occasionally.

 

[Tom.G]

https://www.sciencealert.com/new-me...universe-confirm-something-is-definitely-awry

 

[BWV]

The two numbers reflect a change in the constant from the early universe - an acceleration of the acceleration?

 

[PeterDonis]

the value inferred from CMB measurements

Is there a good brief reference for how a value for $H_0$ is inferred from CMB measurements?

 

[Orodruin]

Is there a good brief reference for how a value for $H_0$ is inferred from CMB measurements?

That depends on your definition of brief. It is basically assuming LambdaCDM and extrapolating based on how the early Universe looks.

 

[substitute materials]

Is there a good brief reference for how a value for $H_0$ is inferred from CMB measurements?

The method that was explained to me measures the sound horizon of the Baryon Acoustic Oscillations in the CMB. Basically, the sound horizon shows us the scale on which inhomogeneities formed in the early universe, which should correspond to the spatial separation of galaxies in the modern universe. So we can infer the evolution of the scale factor between then and now if we know the both the modern spatial separation and the early sound horizon. I'm not sure if there are other methods to get Ho that are based on CMB measurements as well.

 

[PeterDonis]

That depends on your definition of brief.

Ok, is there any non-brief reference?

It is basically assuming LambdaCDM and extrapolating based on how the early Universe looks.

I get that much, but that is too brief.

 

[kimbyd]

That depends on your definition of brief. It is basically assuming LambdaCDM and extrapolating based on how the early Universe looks.

I get that much, but that is too brief.

The tricky thing is, that's kinda how it's done. Tegmark has an in-depth description of the analysis that is done:
https://space.mit.edu/home/tegmark/cmb/pipeline.html

To get how each individual parameter influences the data, you have to dig into the calculations of how the power spectrum is changed by the various parameters, which isn't all that easy.

That said, the measurement of $H_0$ from the CMB data is mostly sensitive to how the universe has expanded since the emission of the CMB, so a discrepancy here likely points to one of the following issues:
1) Local variation (if our local region is less dense than the rest of the universe, it might explain this).
2) Spatial curvature isn't zero.
3) Dark energy varies with time.

 

[PeterDonis]

To get how each individual parameter influences the data, you have to dig into the calculations of how the power spectrum is changed by the various parameters, which isn't all that easy.

Ok, so the basic answer is "it's complicated".

the measurement of $H_0$ from the CMB data is mostly sensitive to how the universe has expanded since the emission of the CMB, so a discrepancy here likely points to one of the following issues:
1) Local variation (if our local region is less dense than the rest of the universe, it might explain this).
2) Spatial curvature isn't zero.
3) Dark energy varies with time

Would it be fair to say that CMB data gives an estimate of what $H$ was at the time of last scattering, and then we evolve that forward in time to find what $H$ is predicted to be now, i.e., an estimate of $H_0$? And that the "evolve forward in time" part is what is sensitive to the specific model of how the universe has expanded?

 

[andrew s 1905]

It would be interesting to see how much the other CMB parameters changed if Ho was fixed to the "local" value during the optimisation.

Regards Andrew

 

[Roberto Teso]

Ok, is there any non-brief reference?

Starting from NASA LAMBDA Hubble Constant H₀ page you can find a lot of long references.

Enjoy the reading.

 

[kimbyd]

Ok, so the basic answer is "it's complicated".

Indeed!

Would it be fair to say that CMB data gives an estimate of what $H$ was at the time of last scattering, and then we evolve that forward in time to find what $H$ is predicted to be now, i.e., an estimate of $H_0$? And that the "evolve forward in time" part is what is sensitive to the specific model of how the universe has expanded?

Not really. A better way to understand it is that the CMB provides accurate measurements of other values which are related to $H_0$. Most importantly, the CMB provides an accurate measurement of the matter density.

Using WMAP, just because their website presents data in an easy-to-browse fashion, the matter density is measured to within a few percent even if you don't use any nearby data or assume a flat universe. The WMAP-only estimate of $H_0$ without the assumption of a flat universe is between $38 km/s/Mpc$ and $84km/s/Mpc$ (95% CL).

You get an accurate estimate of $H_0$ only if you combine the CMB data with nearby data or an assumption of flatness.

 

[PeterDonis]

Most importantly, the CMB provides an accurate measurement of the matter density.

You get an accurate estimate of $H_0$ only if you combine the CMB data with nearby data or an assumption of flatness.

Got it, thanks!