Keir and Vivian tell us about their recent work looking at the Lyman-α power spectrum from eBOSS quasars. Specifically, they look at how consistent this power spectrum is with the power spectrum one would expect from the relevant scales given Planck CMB observations within the ΛCDM model and find a whopping 4.9σ tension.

There have been hints of a tension between Lyman-α and CMB for a while, but not with this magnitude. Keir and Vivian focus their analysis on the scales and observables that the eBOSS Lyman-α are most sensitive to and see that the slope of the power spectrum of this range of scales is very different to what is seen on CMB scales. Within ΛCDM the slope of the power spectrum should be constant, hence the whopping tension. Continue reading →

Kate tells us about her recent work showing how domain walls in the dark sector could be trapped in a laboratory.

In many well motivated models, a scalar field in the dark sector can have a double well potential. In this case, the scalar field can form “domain walls” if in some region of space the field occupies one side of the well, and in other regions of space the field occupies the other side of the well. The wall occurs at the transition point between the two regions. Continue reading →

Melissa Diamond, joined by Chris Cappiello, tells us how dark matter interactions with one set of standard model particles might be first constrained via experiments looking for interactions with other particles.

As a first example they explore how loop diagrams that arise when dark matter interacts with quarks (and thus pions, neutrons and protons), necessarily introduce an interaction with electrons too. For light enough dark matter, this would mean that the first direct detection of these dark matter models would come via electron scattering experiments, not nucleon scattering experiments.

At the moment this just adds to constraints in somewhat less-interesting parameter space, but future experiments will push the constraints into well motivated parameter ranges for dark matter production mechanisms.

This loop interaction would also generate a dark matter “millicharge”, which could be combined with astrophysical observations to make even tighter constraints. Continue reading →

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. Continue reading →

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 →