Cosmology Talks is hosting a mini-workshop on Thursday 10 April at 18:00 UTC about DESI DR2 Cosmology.

The workshop will be purely discussion based, and framed around the recent DESI DR2 papers and the talks listed below. Most of the speakers from the talks will be present, as well as some invited experts on related fields.

Important! It is not expected that the talks and papers are fully understood by all participants, but the contents of either the relevant Cosmology Talks video or video 1 and video 2 from DESI’s YouTube channel will be assumed to be common knowledge of all participants before the discussion begins. This provides a common starting position for discussion among all attendees.

Questions/thoughts are, however, welcome on any of the recent DESI papers, or talks listed below.

Cosmology Talks: DESI 2025 – Cosmology Results from DR2 BAO (Garcia-Quintero & Martini)

DESI’s YouTube Channel:

1. DESI shakes up the dark Universe: An overview
2. DESI DR2 Results: Measurements of Baryon Acoustic Oscillations and Cosmological Constraints

3. Extended Dark Energy analysis using DESI DR2 BAO measurements
4. DESI DR2 Baryon Acoustic Oscillations from Galaxies and Quasars
5. DESI DR2: Validation of the Lyman alpha BAO analysis using synthetic datasets
6. Dark Energy Spectroscopic Instrument Survey Data Release 1
7. DESI DR2 results: Baryon Acoustic Oscillations from the Lyman-alpha forest
8. Combined Clustering and Weak Lensing from the DESI Year-1 Dataset and Stage III Lensing Surveys
9. Neutrino masses from the latest DESI results

The schedule is (starting at 18:00 UTC):

• 10 minutes (approx) introducing the workshop format.
• 50 minutes discussion in one Zoom room.
• 10 minutes (approx) break time.
• 50 minutes of multiple discussions, in breakout rooms, based on ideas that arose during the earlier discussion.

Pre and post-discussion will take place in the Cosmo Discussion Mattermost space, in the Cosmology Talks channel. The speakers and invited experts will also be present there to ask questions.

Participation at the workshop is open to all cosmologists and researchers from neighbouring fields.

Zoom: https://zoom.cosmologytalks.com/

Gerrit Farren and Alex Krolewski tell us about their work cross correlating the ACT lensing maps with unWISE galaxies. Their method is an update of earlier analyses using Planck lensing. Those earlier analyses were in tension with Planck primary CMB measurements, which was similar to e.g. cosmic shear, and other clustering measurements, at smaller scales.

Curiously, this new analysis reveals no tension with primary CMB. A re-analysis of Planck lensing cross unWISE, with updated assumptions and modelling, shows that Planck lensing is actually *also* not in tension with primary CMB (a combination of effects were responsible for removing the tension).

This analysis covers the large(ish) scale low(ish) redshift quadrant of the (k,z) space, suggesting that the origin of the S8 tension is likely to either be a very low redshift phenomenon, or scale dependent.

However Alex and Gerrit do note that a recent cross-correlation of Planck with DESI LRGs does seem to still indicate some tension at the same redshift and scales, so the dust hasn’t 100% settled on the large scale, low redshift quadrant.

Gerrit: https://www.maths.cam.ac.uk/person/gsf29

Alex: https://perimeterinstitute.ca/people/alex-krolewski

Paper: https://arxiv.org/abs/2309.05659

Members from the Dark Energy Survey and Kilo Degree Survey have teamed up to do a joint cosmic shear analysis, complete with final cosmology results. They tell us the final cosmology results, and go into details explaining what the technical difficulties were to combine the data sets.

Talk slides: docs.google.com/presentation…

Paper: https://arxiv.org/abs/2305.17173

Dillon Brout, Adam Riess and Dan Scolnic talk about the latest SH0ES measurement of the Hubble parameter, making use of the new Pantheon+ supernovae data set.

The measurement accuracy has reached ± 1.0 km s⁻¹ Mpc⁻¹, and they analysed the data in 67 different possible ways and every time reach a result that is in significant tension with Planck + ΛCDM. Their baseline analysis, the one with the best χ² with the fewest free parameters, is now in 5σ tension, on its own, with Planck.

It isn’t clear where the solution to the Hubble tension will come from, but the fact that all local measurements of H₀ come in above Planck, and that the most accurate measurement is now in 5σ tension is very interesting. It’s worth also noting that the prediction from the early universe + ΛCDM doesn’t rely uniquely on Planck. Other CMB experiments give the same small value, and even just Big Bang Nucleosynthesis and local Baryon Acoustic Oscillation measurements, combined with ΛCDM give a small value of H₀.

So, if this isn’t evidence of new physics in cosmology, it will be a very strange series of errors that is causing it.

Dillon: http://djbrout.github.io/
Adam: https://physics-astronomy.jhu.edu/directory/adam-riess/
Dan: https://scholars.duke.edu/person/dscolnic

The paper (SH0ES): https://arxiv.org/abs/2112.04510
The paper (Pantheon+ data): https://arxiv.org/abs/2112.03863

Hironao tells about how the Hyper Suprime Cam survey collaboration have taken their own data, and the BOSS data from SDSS to do a joint cosmology constraint.

Specifically, they take the autocorrelation function of the BOSS galaxies and the cross-correlation of HSC weak lensing data with the BOSS galaxies to break degeneracies between cosmology and galaxy bias, thus allowing the full information in the galaxy data to be usable (or at least, lots more of it).

They work hard to extract information from the small scales in these two probes, using a cosmology emulator and halo occupation model, and they include various consistency tests to show that their analysis is robust. (They also did the analysis blind, to avoid human bias as much as possible).

The final constraints are comparable to recent similar DES and KiDS constraints, and consistent with both, but this analysis have a somewhat different degeneracy in the Ωm and σ8 plane.

Personally, I’d now love to see some sort of combined analysis of all three weak lensing probes as they’re all consistent with each other and I expect the combined constraints would be in a lot of tension with Planck (especially because of this different degeneracy direction).

Hironao: https://sites.google.com/view/hironaomiyatake/home

The paper: https://arxiv.org/abs/2111.02419

Julien tells us about the cosmological effects of different neutrino mass states (i.e. the same sum of masses, but different masses for each individual neutrino – e.g. “normal” vs “inverted”).

There are effects, but they’re all very small and not even the best future experiments will distinguish them.

Non-standard model neutrinos would still have interesting effects, but it seems that cosmology’s insight on the SM ones will be limited to the sum of the masses.

Paper: https://arxiv.org/abs/2003.03354
Julien: https://lesgourg.github.io/

Sesh tells us how the void-galaxy cross correlation provides information about cosmology via redshift space distortions and (importantly) the Alcock Paczynski effect. The information is independent of Baryon Acoustic Oscillations (BAO) and improves error bars by up to a factor of four. The combination of voids and BAO have very interesting insights into the Hubble discrepancy and the late-time acceleration of the Universe.

Speaker: Seshadri Nadathur

(the most relevant) Papers: https://arxiv.org/abs/2001.11044​ and https://arxiv.org/abs/1904.01030

Reference for cosmological perturbation theory: https://arxiv.org/abs/0809.4944​

Reference for Post Newtonian Theory

Paper: https://arxiv.org/abs/2001.00394

Graham tells us how Big Bang Nucleosynthesis (BBN) and the Cosmic Microwave Background can be used to constrain the potential existence of particles beyond the Standard Model.

The focus is on light dark matter (masses less than 1GeV), which escapes direct detection bounds by being too light to kick nucleons hard enough and is most interesting to cosmology because it has precisely the masses relevant in the early universe processes we can “observe”, e.g. BBN.

Graham volunteered to give this talk on less than 36 hours notice, so huge thanks to him for pulling something together so quickly!!

Speaker: Graham White https://www.triumf.ca/theory/graham-white

Graham’s paper: https://arxiv.org/abs/2003.02273