Archive for Early Dark Energy

The Hubble Tension and Early Dark Energy

Posted in The Universe and Stuff with tags , , , , on November 14, 2022 by telescoper

In recent times I’ve posted quite a few times about the Hubble Tension and possible resolutions thereof. I also had polls to gauge the level of tension among my readers, like this one

and this one:

I’m not sure if these are still working, though, as I think I’ve reached the number of votes allowed on the basic free version of crowdsignal that comes with the free version of WordPress. I refuse to pay for the enhanced version. I’m nothing if not cheap. You can however still see the votes so far.

Anyway, there is a new(ish) paper on the arXiv by Mark Kamionkowski and Adam Riess that presents a nice readable introduction to this topic. I’m still not convinced that the Hubble Tension is anything more than an observational systematic, but I think this is a good discussion of what it might be if it is more than that.

Here is the abstract:

Over the past decade, the disparity between the value of the cosmic expansion rate directly determined from measurements of distance and redshift or instead from the standard ΛCDM cosmological model calibrated by measurements from the early Universe, has grown to a level of significance requiring a solution. Proposed systematic errors are not supported by the breadth of available data (and “unknown errors” untestable by lack of definition). Simple theoretical explanations for this “Hubble tension” that are consistent with the majority of the data have been surprisingly hard to come by, but in recent years, attention has focused increasingly on models that alter the early or pre-recombination physics of ΛCDM as the most feasible. Here, we describe the nature of this tension, emphasizing recent developments on the observational side. We then explain why early-Universe solutions are currently favored and the constraints that any such model must satisfy. We discuss one workable example, early dark energy, and describe how it can be tested with future measurements. Given an assortment of more extended recent reviews on specific aspects of the problem, the discussion is intended to be fairly general and understandable to a broad audience.

1 Comment »

Cosmology Talks – Colin Hill on Early Dark Energy

Posted in The Universe and Stuff with tags , , , , , on June 2, 2020 by telescoper

Here is another one of those Cosmology Talks curated on YouTube by Shaun Hotchkiss.

In the talk, Colin Hill explains how even though early dark energy can alleviate the Hubble tension, it does so at the expense of increasing other tension. Early dark energy can raise the predicted expansion rate inferred from the cosmic microwave background (CMB), by changing the sound horizon at the last scattering surface. However, the early dark energy also suppresses the growth of perturbations that are within the horizon while it is active. This mean that, in order to fit the CMB power spectrum the matter density must increase (and the spectral index becomes more blue tilted) and the amplitude of the matter power spectrum should get bigger. In their paper, Colin and his coauthors show that this affects the weak lensing measurements by DES, KiDS and HSC, so that including those experiments in a full data analysis makes things discordant again. The Hubble parameter is pulled back down, restoring most of the tension between local and CMB measurements of H0, and the tension in S_8 gets magnified by the increased mismatch in the predicted and measured matter power spectrum.

The overall moral of this story is the current cosmological models are so heavily constrained by the data that a relatively simple fix in one one part of the model space tends to cause problems elsewhere. It’s a bit like one of those puzzles in which you have to arrange all the pieces in a magic square but every time you move one bit you mess up the others.

The paper that accompanies this talk can be found here.

And here’s my long-running poll about the Hubble tension:

 

Leave a comment »