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

Cyril tells us about recent work on Chern-Simons gravity and how if it is the correct general model of gravity then parity violation can be generated during inflation in such a way that correlation functions of galaxies measured today could see that parity violation. Given the recent evidence for parity violation in galaxy correlation functions, that is intriguing.

The model also requires parameter values that could produce baryogenesis, so the same physics would end up producing the (observed?) parity violation and the baryon asymmetry in the universe.

Even if the current hints of parity violation turn out to be wrong, future experiments would be sensitive to smaller magnitudes of this effect, meaning the model will be well constrained by the next wave of large scale structure measurements

Cyril: cyril-creque.github.io

Paper: arxiv.org/abs/2303.04815

Lilan Yang and Mike Boylan-Kolchin tell us about high redshift JWST galaxies. Lilan is one of the astronomers using JWST to look at high redshifts and find new galaxies. Mike is a cosmologist who has pioneered looking at these high redshift galaxies to think about cosmology.

Lilan: https://db.ipmu.jp/member/personal/6862en.html

Mike: https://mrbk.github.io/

Papers: 2207.13101, 2207.13527, 2208.01611, 2212.06666

Jenny Wagner gives us the observational case for and against the Cosmological Principle, based on a paper she and 22 co-authors wrote summarising this case.

She starts by clearly defining what the Cosmological Principle is and then talks us through the observations that she and her coauthors find to be the most compelling evidence against the assumptions of full statistical isotropy and homogeneity. In each case she gives us the story either “side” would need to tell to reconcile the observation (the standard, FLRW side and the inhomogeneous cosmology side). Continue reading →

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

Dongwon Han, Niall MacCrann, Mathew Madhavacheril, Frank Qu and Blake Sherwin tell us about the Atacama Cosmology Telescope’s latest measurement of CMB weak lensing (“Advanced ACT” to be precise).

It is quite the measurement. Despite ACT being on the ground, the measurement now rivals the best Planck lensing measurement. This means the additional complications of the atmosphere have been overcome, and the better technology of telescope possible from not having to put something in space are starting to win. Continue reading →

Jeongin Moon, David Valcin and Christoph Saulder tell us about the first cosmologically relevant results from DESI (The Dark Energy Spectroscopic Instrument). Specifically, they are presenting the first detection of the BAO (Baryon Acoustic Oscillations) from DESI.

With a “first detection” one would expect something mediocre, but even this result, using only two months of DESI data, has similar precision to the final BOSS data after years of operation. This is a very exciting time for cosmology as we wait for the full one year DESI results and cosmological constraints. Continue reading →

Giovanni Arico tells us about his recent work adding baryons to the modelling of Dark Energy Survey cosmic shear analysis. By adding baryonic modelling he and collaborators are able to use all of the DES data. Continue reading →

Minh Nguyen tells us about his recent work looking for evidence of modified gravity in cosmological observations. Rather than look at a specific model, Minh and his collaborators look for evidence of a deviation in the “growth index” γ.

In general relativity and ΛCDM γ≃0.55 but in a more general theory of gravity it could deviate. Minh looks at measurements of peculiar velocity, clustering and Planck CMB data (temperature, polarisation and lensing) and finds 3.7σ evidence for γ greater than 0.55. Continue reading →

Stuart tells us about his recent work exploring how to use peculiar velocity measurements to constrain modified gravity (specifically f(R) and DGP models). He finds that even using just linear scales we would be able to detect or rule out model parameter regions that would be entirely consist with current measurements of the background expansion.

He does this by predicting the auto and cross spectra between galaxy overdensity and peculiar velocity, using just linear theory – and then analysing those observables in modified gravity simulations to measure the parameters used in the simulations.

At this point it is just a proof of concept, as for each modified gravity scenario the method is only constraining the linear galaxy bias and one model parameter (other cosmological parameters are known) and also not taking into account additional observational uncertainties involved in measuring peculiar velocities in the real world.

Still, the method does work, at least when the effect produces a large enough deviation from LCDM so the proof of concept works. Future careful work with 6DF and SDSS data (and, one day soon, DESI) should allow the method to bring about real constraints on these models.

Paper: https://arxiv.org/abs/2211.07101