Cosmology Talks – Colin Hill on Early Dark Energy

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:

 

2 Responses to “Cosmology Talks – Colin Hill on Early Dark Energy”

  1. Phillip Helbig Says:

    “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.”

    But how believable would an ad-hoc explanation such as early dark energy be even if it did work?

    Another fix, possibly less ad-hoc, is not to assume a flat universe (although data does of course indicate that it is nearly flat:

    https://arxiv.org/abs/1911.02087

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