Early Dark Energy Can Resolve the Hubble Constant Tension

New developments since this post was originally created:

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.

Review of the Early Dark Energy proposal by Sunny Vagnozzi; suggesting a different context: [2001.02451] Is the Hubble tension a hint of AdS around recombination? “One of the most promising solutions was proposed by Marc Kamionkowski’s group in Poulin et al., and envisages a phase of early dark energy (EDE) behaving as a cosmological constant at early times, before redshifting away faster than radiation, before recombination. In this way you reduce the sound horizon, which results in you inferring a higher H0 to keep the angular scale of the CMB first peak fixed. However, EDE does not completely solve the H0 tension. Long story short, the local measurement gives H0 of about 74 (in appropriate units) whereas EDE gives you H0 of about 70-71ish at best. How can one improve this? Well, the name of the game is to have more energy injection by EDE (the parameter both Poulin and the authors call f_EDE), but this also means that you then have to dilute this energy much faster, or risk screwing up recombination. Ye and Piao found a clever way of doing this, by using a potential wherein the initially frozen scalar field rolls down to the bottom of its potential, which is a negative plateau, i.e. it enters an AdS phase. Shortly after recombination, the field still has enough energy to climb out of the AdS vacuum and enter the dS vacuum of LCDM...”

Tags: #cosmology #DE #H0