Link to the Conference Program page

Youtube recorded videos (~ 2-½ hours each day): Day 1 | Day 2 | Day 3 | Day 4 | Day 5 |

There are at least 2 people who have created an index of each day's talks with links to key sections of each talk, along with some commentary.

One was created by me and is a thread at the cosmology subreddit. Links to each day: Day 1 | Day 2 | Day 3 | Day 4 | Day 5 |

The other was created by conference participant and panelist Behnam Javamardi, available on his blog: Day 1 | Day 2 | Day 3 | Day 4 | Day 5 |

There is also a twitter #H02020 hashtag, which has some additional coverage.

If I come across additional reference materials on this conference, I will add them to this post.

Tags: #Cosmology #H0 #Talks

I'd like to point out an excellent overview talk from this program by Raphael Flauger on Feb. 28 that reviews Hubble measurement physics and results from each of the major determination methods and discusses current status on Hubble tension. The video for Flauger's talk is here. That page does not have the talk slides (at least not yet anyway) but the slides are available in PDF format here. Update: the slides are also now available on the KITP video page. Here is a PDF of the slides.

In the cosmology subreddit that I'm a member of, there are frequent questions from those who have a grasp of the basics and are keenly interested in learning more about cosmology but do not work in the field themselves. I especially want to give a shout-out about this talk to that group. It's an excellent and reasonably accessible review of a key trending problem-area needing resolution for there to be a better understanding of whether adjustments are needed to the ΛCDM standard model of cosmology. This overview talk very clearly summarizes what would otherwise take hours and hours of digging to get. The rest of the program has very good material but some of it is rather specialized and deep.

A recent discussion in this reddit thread contrasted differing takes within the community on whether inflation preceded or followed the big bang. There are various reasons for this, some of it having to do with the history and evolution of what is meant by the term 'big bang', which was also explored in this earlier thread.

Here are comments from the overview page for this KITP program on this: “inflation not only explains the large-scale homogeneity and isotropy of the Universe, but also provides a causal mechanism that results in the seeds for the subsequent growth of structure. However, what happens after inflation remains poorly understood. The end of inflation must provide a hot Big Bang, also known as reheating, which eventually must lead to a thermal bath of Standard Model particles, dark matter, and any additional Beyond the Standard Model (BSM) sectors, at least by the time of nucleosynthesis.”

Tags: #Cosmology #H0 #Talks

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

January 2020: In new paper arXiv: 2001.09213, the MCP reports an updated $H_0$ value of 73.9 ± 3.0 based on improved distance measurements for 4 of the 6 systems they previously reported on. MCP researchers Braatz, Pesce, Condon, Reid, et al teamed up with SH0ES team members Scolnic and Riess for this paper. Here is a clip of Figure 1 from the paper. Additional details in this reddit thread.

July 2019: video and slides of a presentation by Mark Reid at the KITP-UCSB conference Tensions between the Early and the Late Universe on July 16, 2019: H0: NGC 4258 and the Megamaser Cosmology Project -—

This post is about the paper Science with the ngVLA: H2O Megamaser Cosmology, which was posted on the arXiv preprint server in Oct. 2018.

What's This About? A recent comment by one of the people I regularly follow (who writes excellent posts on astronomy/cosmology topics) mentioned this NRAO key project as providing direct geometric measurements of the Hubble Constant (H_0) independent of the Cosmic Microwave Background (CMB) and standard candle methods. The megamaser method gives angular diameter distances to water megamasers¹ in the nuclear regions of host active galaxies within ~0.1 pc of the SMBH. The galaxies are typically well into the Hubble flow at between 50 – 200 Mpc distance. This slide compares the distances to various studied megamasers to those for Cepheids. As indicated in the slide, these megamaser distances can be used to calibrate other distance methods.

Not being familiar with this MCP project but finding it of interest, I've tried to integrate and summarize some reading on it into this mini-overview, non-expert's report.

What Are They? These H₂O megamasers being studied are a special type of astrophysical maser originating in a flat accretion disk, with warm (~400 K) dense gas clouds, in Keplerian motion orbiting the SMBHs of host active galaxies. They produce stimulated coherent emission at 22 GHz from collisionally excited transitions between rotational energy levels of ortho-states of water². Typical rotation velocities are ~500 km s^-1. Megamasers have a large isotropic luminosity, typically 10³ solar luminosities. A radio continuum background source is needed to provide the radiation that's amplified by the maser.

What facilities are currently involved? “The MCP uses the most sensitive suite of telescopes working today at 22 GHz, including the GBT for surveys and spectral monitoring observations, and the High Sensitivity Array (the VLBA, GBT, VLA, and 100-m Effelsberg telescope) to map maser disk systems.” [quote source]

What's Involved in Measuring the Distances? Multiple steps, e.g.: (1) survey with the GBT to identify the rare, edge-on disk megamasers needed for this method (this maximizes the gain length as described in footnote¹ references), (2) image the sub-parsec disks with the High Sensitivity Array, (3) with the GBT, monitor spectral drifts in maser lines (red vs blue shifting) from centripetal accelerations of the clouds as they orbit the central BH, (4) VLBI observations for rotation curve mapping, and (5) modeling the disk dynamics. Basically, the distance to the galaxy is measured by comparing the observed angular radius of the maser's orbit to its measured linear distance from the black hole (see this visual slide and further informational details in footnote¹).

Current Status: The most complete set of H_0 results I've seen the MCP report is 67.6 ±4.0, a 6% measurement. The individual 6 measurements are shown in this slide, which also shows the distances in Mpc. A comment indicates they expect to improve the measurement to < 4%. (I've also seen a ±3% goal in several of their papers). In the slide, two of the results are shown in prep, which may explain the difference from the 69.3 ±4.2 result from their latest paper 1810.06686 which reports only 4 results.

In Extragalactic maser surveys [1802.04727] they report: “With respect to 22 GHz H₂O masers, we note that the MCP is close to completion. We can expect a final Hubble constant deduced from this survey with an uncertainty of only a few percent during the next one or two years.” (page 9) Their latest paper says their final results will be based on distances to 9 megamasers. Surveys of 3000 galaxies were necessary to identify those 9 (AGNs are required with a maser disk suitable for measurement and with an edge-on view).

What's Their Future Plan? A recent 5-page paper details their plans involving the next-generation Very Large Array: H2O Megamaser Cosmology with the ngVLA [1810.06686]. Their ultimate goal is a 1% measurement, which will (1) require a survey to identify additional megamasers – their forecast is that ngVLA can discover ~30 times more sources than the GBT, and (2) the ngVLA must have certain design features as outlined in section 6 of the paper.

An overview document on the ngVLA shows it's timeline as initiating early science in 2028 and full operations in 2034.

Are There Other Study Results from MCP? Besides its primary mission for H_0 measurements, MCP results to date include the “gold standard” most accurate method for extragalactic BH masses, (e.g., see 1801.06332), and also precise BH masses in relatively low-mass systems where the BH masses have been difficult to measure – this has important implications for galaxy evolution (e.g., see 1007.2851).

What are some other MCP references? An interesting set of slides from an NRAO presentation is Water Megamasers in Galaxies, which I recommend. The Megamaser Cosmology Project has a wiki page here.

Footnotes: ¹ For a good reference on the distance measurement techniques, see Braatz et al, Measuring the Hubble constant with observations of water-vapor megamasers. For a nice summary graphic of this geometric distance technique, see this. Also the wikipedia page has a short summary here.

² See this for a graphic showing various rotational energy level transitions of water. Also see this graphic. The particular transition of interest for megamaser study is 6₁,₆ –> 5₂,₃. For more complete data on these transitions, see Table 6 in 1510.06182. Other transitions of ortho-H20 such as at 183 GHz are also candidates, but 22 GHz is the most studied currently, and is directed at ortho-H20 rather than para-H2O: “we consider only transitions between ortho-states because these are the states for which the most extensive collisional data have been calculated, and because in a hot plasma the abundance of ortho-water will exceed that of para-water by a factor 3.” Note: these two water states “are not connected by radiative or collisional transitions.” Neufeld and Melnick 1987 ApJ 322,266. Two other good sources on the transition details is Astrophysical masers and The Physics of Water Masers observable with ALMA and SOFIA [1510.06182].

Tags: #cosmology #H0 #physics

So the following starter list is discontinued in favor of the above:

[2005.10384] Beware of commonly used approximations I: errors in forecasts, by Nicola Bellomo, José Luis Bernal, Giulio Scelfo, Alvise Raccanelli, Licia Verde [2005.09666] Beware of commonly used approximations II: estimating systematic biases in the best-fit parameters, by José Luis Bernal, Nicola Bellomo, Alvise Raccanelli, Licia Verde Jose Bernal – How to tell if your cosmological approximations are accurate, Cosmology Talk with Shaun Hotchkiss¹

[2005.10656] H0 tension or T0 tension?, by Mikhail M. Ivanov, Yacine Ali-Haïmoud, Julien Lesgourgues The Hubble-Penzias-Wilson tension – Yacine Ali-Haïmoud, Newton 1665 seminars⁴ Top arXiv papers from Week 21, 2020, review by Sunny Vagnozzi² Is the Hubble Tension actually a Temperature Tension?, astrobites article

[2005.02445] The Accuracy of the Hubble Constant Measurement Verified through Cepheid Amplitudes, by Adam G. Riess, Wenlong Yuan, Stefano Casertano, Lucas M. Macri, Dan Scolnic Adam Riess – Cepheid crowding is not the cause of the Hubble tension, Cosmology Talk with Shaun Hotchkiss | Twitter thread |

[2004.10207] Hubble constant tension between CMB lensing and BAO measurements, by W.L. Kimmy Wu, Pavel Motloch, Wayne Hu, Marco Raveri Kimmy Wu – Planck lensing and line of sight BAO in mild tension. A vital clue in the Hubble mystery?, Cosmology Talk with Shaun Hotchkiss | Twitter thread |

[2004.08404] Reheating and Post-inflationary Production of Dark Matter, by Marcos A. G. Garcia, Kunio Kaneta, Yann Mambrini, Keith A. Olive [2003.02846] A Model of Metastable EeV Dark Matter, by Emilian Dudas, Lucien Heurtier, Yann Mambrini, Keith A. Olive, Mathias Pierre [1911.02463] Inflation and Leptogenesis in High-Scale Supersymmetry, by Kunio Kaneta, Yann Mambrini, Keith A. Olive, Sarunas Verner Dark Matter physics in the Early Universe – Yann Mambrini, Newton 1665 seminars

[2004.05049] The $H_0$ tension: $ΔG_N$ vs. $ΔN_{\rm eff}$, by Guillermo Ballesteros, Alessio Notari, Fabrizio Rompineve Attempting to alleviate the H0 tension with physically motivated models – Alessio Notari, Newton 1665 seminars

[2004.01139] The Atacama Cosmology Telescope: A CMB lensing mass map over 2100 square degrees of sky and its cross-correlation with BOSS-CMASS galaxies, by Omar Darwish, et al. Omar Darwish – Lensing maps are great, but they're even better with the tSZ effect removed!, Cosmology Talks with Shaun Hotchkiss | Twitter thread |

[2003.09655] Intensity Mapping as a Probe of Axion Dark Matter, by Jurek B. Bauer, David J. E. Marsh, Renée Hložek, Hamsa Padmanabhan, Alex Laguë Jurek Bauer – Fuzzy dark matter arising from GUT scale physics should be ruled in/out by SKA, Cosmology Talk with Shaun Hotchkiss | Twitter thread |

[2003.07355] Early Dark Energy Does Not Restore Cosmological Concordance, by J. Colin Hill, Evan McDonough, Michael W. Toomey, Stephon Alexander Colin Hill – Early dark energy doesn't make cosmology concordant again, Cosmology Talk with Shaun Hotchkiss | Twitter thread | Top arXiv papers from Week 12, 2020, review by Sunny Vagnozzi

[2003.03354] What will it take to measure individual neutrino mass states using cosmology?, by Maria Archidiacono, Steen Hannestad, Julien Lesgourgues Julien Lesgourgues – Cosmology won't measure individual neutrino mass states, Cosmology Talk with Shaun Hotchkiss | Twitter thread | Top arXiv papers from Week 11, 2020, review by Sunny Vagnozzi

[2003.02273] Cosmological Bounds on sub-GeV Dark Vector Bosons from Electromagnetic Energy Injection, by John Coffey, Lindsay Forestell, David E. Morrissey, Graham White Graham White – Light dark matter is an ideal mix of particle and cosmology, Cosmology Talk with Shaun Hotchkiss | Twitter thread |

[2002.01550] Calibration of the Tip of the Red Giant Branch (TRGB), by Wendy L. Freedman, et al. [1907.05922] The Carnegie-Chicago Hubble Program. VIII. An Independent Determination of the Hubble Constant Based on the Tip of the Red Giant Branch, by Wendy L. Freedman, et al. KITP workshop talk by Wendy Freedman: Tip of the Red Giant Branch Calibration of the Hubble Constant³ For a rebuttal, see [1908.00993] Consistent Calibration of the Tip of the Red Giant Branch in the Large Magellanic Cloud on the Hubble Space Telescope Photometric System and a Re-determination of the Hubble Constant

[2002.01489] New empirical constraints on the cosmological evolution of gas and stars in galaxies, by Hamsa Padmanabhan, Abraham Loeb Hamsa Padmanabhan – The overlap between HI halo modelling and cosmology, Cosmology Talks with Shaun Hotchkiss | Twitter thread |

[2001.11044] Testing Low-Redshift Cosmic Acceleration with Large-Scale Structure, by Seshadri Nadathur, Will J. Percival, Florian Beutler, Hans Winther [1904.01030] Beyond BAO: improving cosmological constraints from BOSS with measurement of the void-galaxy cross-correlation, by Seshadri Nadathur, Paul M. Carter, Will J. Percival, Hans A. Winther, Julian Bautista Seshadri Nadathur – Voids are powerful, free and have tantalising insights on H0, Cosmology Talk with Shaun Hotchkiss | Twitter thread | Twitter thread by Sesh Nadathur

[2001.00394] Viable Gauge Choices in Cosmologies with Non-Linear Structures, by Timothy Clifton, Christopher S. Gallagher, Sophia Goldberg, Karim A. Malik Kit Gallagher – Viable Gauge Choices in Cosmologies with Non-Linear Structures, Cosmology Talk with Shaun Hotchkiss | Twitter thread |

[1910.04619] The synergy between CMB spectral distortions and anisotropies, by Matteo Lucca, Nils Schöneberg, Deanna C. Hooper, Julien Lesgourgues, Jens Chluba Deanna Hooper – CMB spectral distortions are a prime untapped resource, Cosmology Talk with Shaun Hotchkiss | Twitter thread | Deanna's 20-tweet thread |

[1909.02821] Probing Diffuse Gas with Fast Radio Bursts, by Anthony Walters, Yin-Zhe Ma, Jonathan Sievers, Amanda Weltman [1905.07132] Fast Radio Burst Cosmology and HIRAX, by Amanda Weltman, Anthony Walters [1810.05836] A Living Theory Catalogue for Fast Radio Bursts, by E. Platts, A. Weltman, A. Walters, S. P. Tendulkar, J.E.B. Gordin, S. Kandhai Amanda Weltman – Fast radio bursts and cosmology, Cosmology Talk with Shaun Hotchkiss | Twitter thread |

[1908.05625] An Improved Distance to NGC 4258 and its Implications for the Hubble Constant, by M. J. Reid, D. W. Pesce, A. G. Riess [2001.09213] The Megamaser Cosmology Project. XIII. Combined Hubble constant constraints KITP workshop talk by Mark Reid: NGC 4258 and the Megamaser Cosmology Project

[1908.03663] The Hubble Hunter's Guide, by Lloyd Knox, Marius Millea The Hubble Hunter's Guide – Marius Millea, Newton 1665 seminars

[1812.08244] Experiment to detect dark energy forces using atom interferometry, by Dylan Sabulsky, Indranil Dutta, E. A. Hinds, Benjamin Elder, Clare Burrage, Edmund J. Copeland Clare Burrage – Atomic lab experiments rule out almost all of chameleon dark energy model-space, Cosmology Talk with Shaun Hotchkiss | Twitter thread |

[1811.04083] Early Dark Energy Can Resolve The Hubble Tension, by Vivian Poulin, Tristan L. Smith, Tanvi Karwal, Marc Kamionkowski KITP workshop talk by Vivian Poulin: Early Dark Energy Resolution of the Hubble Crisis For a contra argument, see the items related to 2003.07355 above.

Footnotes: ¹ In addition to the Cosmology Talks series, Shaun Hotchkiss maintains an ongoing twitter thread on cosmologists tweeting about their papers. ² Each week Sunny Vagnozzi reviews three cosmology-related papers on his blog. ³ For other videos from this program, see KITP Conference: Tensions between the Early and the Late Universe ⁴ All Newton 1665 physics seminar videos ⁵ ESO Upcoming Conference, 22-Jun-2020 to 26-Jun-2020: H0: Assessing Uncertainties in Hubble’s Constant Across the Universe

Tags: #Cosmology #H0 #Talks

Subject paper: [1907.04869] H0LiCOW XIII. A 2.4% measurement of $H_0$ from lensed quasars: 5.3σ tension between early and late-Universe probes

This full report by the H0LiCOW collaboration¹ follows after their recent 5th (1905.09338) and 6th (1907.02533)² measurements based on a time-delay strong lensing technique. The 4th measurement was reported in September 2018 (1809.01274) and was the subject of a cosmology subreddit thread at that time.

From this paper's Summary section: “Our constraint on H_0 in flat ΛCDM is completely independent of and complementary to the latest results from the SH0ES collaboration, so these two measurements can be combined into a late-Universe constraint on H_0. Together, these are in tension with the best early-Universe (i.e., CMB) determination of H_0 from Planck at a significance of 5.3σ.”

Here are some examples of the enthusiastic reaction within the SH0ES team to these new results: Ex 1 and Ex 2.

For a summary overview of this technique, besides what's in the papers, see 'H_0 via standard clocks', Section 3, in 1903.05035. There's also an astrobites article on using strong gravitational lensing to measure H_0.

At a 3-day KITP-UCSB conference starting July 15, 2019, one of the topics presented was Tensions between the Early and the Late Universe by H0LiCOW team member Tommaso Treu: 'Time delay cosmography and the Hubble constant tension' [video of talk]. Also, the lead author of the 6th paper, Geoff Chi-Fan Chen ³ attended. Conference attendees tweeted highlights and visuals under the KITP_H0tTakes hashtag.

Here is a link to Figure 12 from this paper.

A list of papers on the arXiv with H0LiCOW in the title or abstract.

___ Footnotes: ¹ They have a web site here but at this time it hasn't been updated for these latest results. ² Also see this Astrobites article about the 6th measurement paper [1907.02533]. This is the one where they used adaptive-optics imaging from Keck and compared it with data from HST. ³ Geoff Chi-Fan Chen's personal website about his research work

Note: This note is based on my cosmology subreddit post on this paper.

Tags: #astrophysics #cosmology #H0

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.

Figure 1 from [1910.02978] Investigating the Hubble Constant Tension — Two Numbers in the Standard Cosmological Model

This is from the paper [1903.07603] Large Magellanic Cloud Cepheid Standards Provide a 1% Foundation for the Determination of the Hubble Constant and Stronger Evidence for Physics Beyond LambdaCDM, figure 4, page 22.

This next one is from a recent talk by Stefano Casertano (a co-author on the above paper). Note that he includes megamasers at the bottom of the graphic, which are not included in the graphic above. Photo is by Cora Dvorkin.

The graphic below is from an excellent Ethan Siegel article: This Is How Astronomers Will Resolve The Expanding Universe Controversy. It's a great overall review on this topic. His comments on it: “Modern measurement tensions from the distance ladder (red) with early signal data from the CMB and BAO (blue) shown for contrast. It is plausible that the early signal method is correct and there’s a fundamental flaw with the distance ladder; it’s plausible that there’s a small-scale error biasing the early signal method and the distance ladder is correct, or that both groups are right and some form of new physics (shown at top) is the culprit. But right now, we cannot be sure.(ADAM RIESS (PRIVATE COMMUNICATION))”

Other references: Slides from a talk by Stefano Casertano: Gaia DR2 and beyond: contributions to the local distance scale Additional materials from the Oct. 2018 KICP workshop: The Future of H₀: Crisis or Concordance Astrobites article May 2019 May 2019 article by astrophysicist Ethan Siegel

Tags: #cosmology #H0

• 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