Updates since this was originally posted:

• [2109.04451] The Atacama Cosmology Telescope: Constraints on Pre-Recombination Early Dark Energy , J. Colin Hill, et. al. Colin presents these results in a colloquium talk on 09-Sep-2021.
• In [2009.10740] Early dark energy is not excluded by current large-scale structure data a rebuttal is made and the authors “suggest that EDE still provides a potential resolution to the Hubble tension and that it is worthwhile to test the predictions of EDE with future data-sets and further study its theoretical possibilities.”
• Lead author Colin Hill was interviewed in a Cosmology Talk episode by Shaun Hotchkiss: Early dark energy doesn't make cosmology concordant again. Links to key parts of the video are available at the talk page.
• A detailed review of this paper was posted by cosmologist Sunny Vagnozzi on his blog here
• Follow-up paper: [2006.11235] Constraining Early Dark Energy with Large-Scale Structure, by Mikhail M. Ivanov, Evan McDonough, J. Colin Hill, Marko Simonović, Michael W. Toomey, Stephon Alexander, Matias Zaldarriaga. An astrobites article about it.

Background

Going back at least several years [1], but increasingly since late-2018 [2-7], there has been growing theoretical interest for the Hubble tension issue that suggests new physics models may be needed for the early universe prior to recombination that do not cause changes to late time cosmology, since that is tightly-constrained [4, 8].

For example, papers [2, 5] propose models for a new form of early dark energy (EDE) present at z ≳ 3000 that then dilutes away, resulting in a reduced sound horizon at decoupling. This results in a larger inferred $H_0$ value from CMB data versus Planck results, thus reducing the disparity between early and late time $H_0$ results.

These EDE proposals for resolving $H_0$ tension were characterized as being somewhere on the spectrum between “most plausible” [3] to “least unlikely” [4].

Findings from this New Paper

This new paper by Colin Hill et al analyzed these EDE models in relation to a number of data set combinations, including large scale structure data. In particular, the EDE proposals run into problems with LSS data. If interested, you can read the details but here's some of their summary conclusions:

“We conclude that the EDE scenario is, at best, no more likely to be concordant with all current cosmological data sets than ΛCDM, and appears unlikely to resolve the $H_0$ tension.” (pg1)

“There is no sign of concordance amongst these data sets: the LSS data pull the parameters in the opposite direction to that required to simultaneously fit the CMB and SH0ES data.” (pg 26)

“Taken in conjunction, these results paint a bleak picture for the viability of the EDE scenario as a candidateto restore cosmological concordance. More generally, it is likely that any model that attempts to decrease the sound horizon by increasing H(z) through a new dark-energy-like component that is active at early times will encounter the problems identified here. All such models, insofar as they can accommodate a close fit to both the CMB and SH0ES measurement, will do so at the cost of a shift in ΛCDM parameters that is not compatible with LSS data.” (pg 27)

“In the absence of significant shifts with respect to current LSS data, it seems unlikely that these next-generation data sets (WFIRST, DESI, VRO, etc) will reverse the negative trajectory we have seen here in the evidence for EDE as LSS data are included in the analysis.” (pg 27) -—

[1] [1607.05617] The trouble with $H_0$ [2] [1811.04083] Early Dark Energy Can Resolve The Hubble Tension [3] [1907.10625] Tensions between the Early and the Late Universe [4] [1908.03663] The Hubble Hunter's Guide [5] [1908.06995] Oscillating scalar fields and the Hubble tension: a resolution with novel signatures [6] [1911.11760] Early dark energy from massive neutrinos — a natural resolution of the Hubble tension. Some reactions to this paper in this thread . [7] The subject paper lists additional EDE proposals, see page 26 for additional references. [8] “there is not much freedom in changing the expansion history from that of a standard ΛCDM model below z ∼ 2: the guardrails offered by SNe and BAO do not allow this.” [1, pg 7]

Tags: #Cosmology #DE #H0

From the wikipedia article on cosmic voids: “As the Sachs–Wolfe effect is only significant if the universe is dominated by radiation or dark energy, the existence of voids is significant in providing physical evidence for dark energy.”

Tags: #cosmology #DE

• Videos of two presentations at the KITP-UCSB conference Tensions between the Early and the Late Universe in mid-July 2019: (1) Lloyd Knox presentation: Sounds Discordant, and (2) Vivian Poulin presentation: Early Dark Energy.
• Also see Sunny Vagnozzi's comments at the end of this post.
• See this later post on the impact of EDE models on large-scale structure observables: [2003-07355] Early Dark Energy Does Not Restore Cosmological Concordance. This paper was rebutted in [2009.10740] Early dark energy is not excluded by current large-scale structure data and the authors of this new paper “suggest that EDE still provides a potential resolution to the Hubble tension and that it is worthwhile to test the predictions of EDE with future data-sets and further study its theoretical possibilities.” ___
  

This is about two recent papers with the premise that H₀ tension resolution could come from new physics at early times before recombination.

The first paper, Sounds Discordant: Classical Distance Ladder & ΛCDM-based Determinations of the Cosmological Sound Horizon [arxiv:1811.00537] is based on looking at the tension in terms of the sound horizon r_s. They cite several advantages of doing so: (1) “added insensitivity to extreme changes in the cosmology at z < 0.1, since one does not need to extrapolate to z = 0”, (2) “the ΛCDM predictions for the sound horizon are more robust than those for H₀”, (3) “as with the inverse distance ladder, this approach clarifies that reconciliation can not be delivered by altering cosmology at z < 1”, (4) “it serves to clarify that the reconciliation of distance ladder, BAO, and CMB observations via a cosmological solution is likely to include a change to the cosmological model in the two decades of scale factor evolution prior to recombination”, and (5) “σ(r_s)/r_s from CMB data, assuming ΛCDM, is four times smaller than the σ(H₀)/H₀ from the same data and assumed model.”

They conclude that “cast in terms of r_s, rather than in terms of H₀, it is clear – as the inverse distance ladder approach also suggests – that if the solution to the discrepancies lies in cosmology, we need modifications to cosmology at early times, not late times. We need a model that, given the CMB data, produces a smaller sound horizon...Our claim [is] that any viable cosmological solution is likely to include significant changes from ΛCDM in the epoch immediately prior to recombination.”

Figure 3 shown below is a key graphic in the paper. Marius Millea, one of the authors, wrote an interesting tweet thread on it.

The above paper does not propose any models for what could produce a smaller sound horizon before recombination. But the paper Early Dark Energy Can Resolve The Hubble Tension [1811.04083] does, authored by Vivian Poulin, Tristan L. Smith, Tanvi Karwal, and Marc Kamionkowski.

1811.04083 proposes that an early-time resolution “is an exotic early dark energy (EDE) that behaves like a cosmological constant before some critical redshift z_c (z ~ 3000) but whose energy density then decays faster than radiation. This addresses the Hubble tension by increasing the early expansion rate while leaving the later evolution of the Universe unchanged.” Their work indicates that “a field accounting for 7% of the total energy density around z ~ 5000 and diluting faster than radiation afterwards can solve the Hubble tension without upsetting the fit to other data sets. (bolding is mine). “The tension is reduced to ~ 1.5σ as long as the EDE dilution occurs like radiation or faster.”

They also present a second model – an oscillating field EDE. They found that both models resolve the Hubble tension.

There is a reddit discussion thread on this paper here.

Tags: #cosmology #DE #H0