CosmoPhys

Link to paper: [2003.07355] Early Dark Energy Does Not Restore Cosmological Concordance, by J. Colin Hill, Evan McDonough, Michael W. Toomey, Stephon Alexander

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].

Colossus¹ is a recently available open-source, pure-python (nothing to compile) calculations toolkit developed by computational astrophysicist Benedikt Diemer during his PhD thesis work at University of Chicago.²

There are 3 separate python modules for cosmology, LSS, and DM Halos. You can choose to work with any of 20 built-in cosmology models based on results from Planck18 (with or w/o BAO), Planck15, Planck13, WMAP, etc. You can also create your own cosmology model by specifying values for a minimum of 6 parameters.

A paper on it is available at https://arxiv.org/abs/1712.04512. The code is available at this BitBucket repository. You can clone the repo, or download a zip file, or you can install it using pip. Documentation with many examples in html format is available here and the docs on the 3 modules are also available as interactive, live-code Jupyter notebooks here. The html tutorials doc files are just exports from the Jupyter notebooks.

I had in mind to create a list of recent papers I found of interest that also had an author's talk available and in some cases a review. But after starting this post, I realized it's really so much better to have this data input into the recently initiated and excellent ResearchSeminars.org site, which has great listing and filtering capabilities and is becoming widely used. So I volunteered to the organizers of two cosmology talk series to input their data: Cosmology Talks on youtube hosted by Shaun Hotchkiss and CosmoConβ on youtube, aka Cosmology from Home. The target audience for these are researchers in the field. Now at the Research Seminars site, both Cosmology Talks and CosmoConβ are listed with all their current talks. Also, the Cosmology Talks series includes indexed links to the times of major sections of each talk as a convenience and helpful reference feature.

New developments since this post was originally created: videos of presentations at the KITP-UCSB conference Tensions between the Early and the Late Universe in mid-July 2019: (1) Tomasso Treu presentation: Time delay cosmography and the Hubble constant tension, and (2) Anowar J. Shajib presentation: Towards a 1% Hubble constant measurement with time delay cosmography. ___

This paper [arxiv: 1902.03261] is a current status review on improving systematics¹ in SNe Ia distance measurements by combining optical with near-infrared (NIR) photometry. Though initially being done at low redshifts (z <= 0.04), there are plans to extend this work to higher redshifts². The paper is authored by some of the more noted researchers in this area, e.g., Arturo Avelino and Robert Kirshner³. My goal here is a top-level summary, so that if you don't read the paper you'll hopefully get the gist of it, while also including helpful supplemental info. In a few cases, I added some font bolding for emphasis.

“This is significant for supernova cosmology because, along with photometric-calibration uncertainties, uncertain dust [extinction](https://en.wikipedia.org/wiki/Extinction_(astronomy) estimates and the intrinsic variability of SN Ia colors present challenging and important systematic problems for dark energy measurements. (pg2)”

“Recent work has demonstrated that SN Ia in the NIR are more nearly standard candles, even before correction for light curve (LC) shape or host galaxy dust reddening... Overall, a substantial body of evidence indicates that rest-frame LCs of SN Ia in NIR are both better standard candles than at optical wavelengths and less sensitive to the confounding effects of dust. When NIR data are combined with UBV RI photometry, this yields accurate and precise distance estimates. (pg2)”

Huterer and Shafer authored this excellent review paper: Dark energy two decades after: Observables, probes, consistency tests [arxiv:1709.01091]. They list 5 primary and 6 other probes for DE in Table 1, with details in section 5 and section 6 of the paper. An image clip of Table 1 is shown below.

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.”

This plot is Figure 1 from Di Valentino [2011.00246] A combined analysis of the $H_0$ late time direct measurements and the impact on the Dark Energy sector:

Slide from Adam Riess talk on 22-Feb-2021 using the above Di Valentino plot:

Below is a plot of the H₀ measurement data as of July 2019 and is taken from the paper Tensions between the Early and the Late Universe. This paper is a summary review of a KITP-UCSB workshop convened to bring together both experimental and theoretical researchers in the field to review and assess the current state of affairs and identify promising next steps at resolution of this issue.

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.”

Subject paper: Measuring the Duration of Last Scattering, by Boryana Hadzhiyska and David N. Spergel

General Background: A very common simplification one hears is that the cosmic microwave background (CMB) photons began freely streaming ~380k yrs after the big bang at redshift ~1090 when the universe cooled sufficiently for neutral hydrogen atoms to form and Thomson scattering no longer impeded the free flow of photons. What's rarely mentioned in science media articles is that what's called the surface of last scatter, when individual CMB photons decoupled and began freely traveling, has a thickness (or depth) layer to it. This thickness has been compared to the skin on an apple¹. Though the term 'last scattering surface' is commonly used, it's more accurately characterized as a 'last scattering layer'² due to its width. The layer is opaque and has been compared to a fog bank on Earth.³