Tilman tells us about his recent work combining KiDS cosmic shear measurements and Planck measurements of the thermal Sunyaev Zeldovich (tSZ) effect from the cosmic microwave background scattering off hot gas in galaxy clusters and galaxy groups. The long term goal is to use cross-correlation of shear and the tSZ effect to help constrain (or essentially measure) baryon feedback and thus push to smaller scales. Continue reading →

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

Song Huang and Alexie Leauthaud tell us about their new galaxy cluster finder, which uses the stellar mass in the outer region of a galaxy as a method to determine the mass of the galaxy’s cluster.

It feels a bit like magic (to me) that the stars in individual galaxies can be used to weigh the mass of the whole cluster, but like other mass proxies one can devise a scaling relationship between the proxy and the mass – and then the proof is in the empirical pudding. Continue reading →

James Alvey tells us about the general state of BBN in 2021.

He gives a really nice pedagogical overview of the physics that goes into BBN calculations relevant for 2021 observations, talking through each of the relevant epochs (neutrino decoupling, the deuterium bottleneck, etc). Continue reading →

Helen and Francisco (Paco) tell us about their recent work using neural networks to predict the masses of subhalos within simulations. They find that the neural network trained on a subset of the subhalos is very good at predicting subhalo masses for the rest of the data. Continue reading →

Francis-Yan Cyr-Racine and Lloyd Knox talk about their work with Fei Ge pointing out a symmetry present in most cosmological observables.

The symmetry involves rescaling (almost) *all* the densities and temperatures in the universe thus leaving any dimensionless observables unchanged. When exploited it might pave a way to solving the Hubble tension as it allows one to change H0 without changing predictions for other crucial cosmological measurements (most of which are e.g. temperature, density, etc *contrasts* not absolute measurements). Continue reading →

Harry Goodhew and Gordon Lee talk about their recent work on “cosmological correlators”.

Observationally these would be power spectra, bispectra, etc; however on the theory side they find it easier to work with pieces of the “wavefunction of the universe”, which are closely related to observational correlation functions.

They show constraints on the form these correlators can take that arise from imposing unitarity during inflation. Contrary to prior expectations these constraints apply not just in space-times that are exactly de Sitter, but in fact in any flat FLRW space-time. Continue reading →

This is a recording of a panel event run by the organisers of the Cosmology from Home conference series: https://cosmologyfromhome.com/

The topic was a comparison of the relative merits of “full shape” and “template” methods to analyse galaxy clustering data. Essentially the difference comes down to whether you consider the entire power spectrum as a whole and fit to it in all its glory, or break into separate pieces that encapsulate specific physics effects. Continue reading →

Asta Heinesen and Hayley Macpherson tell us about their recent papers developing a formalism for measuring local parameters without assuming local isotropy (and homogeneity) and predicting what we should expect for the parameters in this formalism when we go beyond the isotropic approximation of FRW.

Asta talks about her paper from last year which developed the formalism, and how a finite number of terms can capture all the expected behaviour in the anisotropic luminosity distance, at each order of redshift.

Hayley then talks about how, together, they applied Asta’s formalism to Hayley’s fully relativistic simulations of cosmology. Continue reading →

Eloisa tells us about her work from 2012 (and following years) constructing a model universe space-time out of lattices of blackholes.

The motivation for this is to take a very bottom up approach to cosmology. We know that around isolated objects the correct metric is close to the Schwarzschild metric, so in principle the full metric of the universe should be able to be written as a patching together of such metrics. On the other hand, the universe on large scales is statistically homogeneous and isotropic and the Friedmann-Robertson-Walker metric appears to fit the data well.

What Eloisa and colleagues wanted to know is how these two paradigms come together, and they more or less found the answer.

Eloisa is also employed not at a university, or any other institute we might normally expect to find a cosmologist. She is employed at IBM. However, she hasn’t stopped doing cosmology research, IBM pay her to do numerical relativity and cosmology. In the video she talks a lot about how this is possible and what IBM want from her as an employee and why this isn’t so unique. In fact, she’s not even IBM’s first numerical relativist!

Eloisa: https://researcher.watson.ibm.com/researcher/view.php?person=ibm-Eloisa.Bentivegna
1st paper: https://arxiv.org/abs/1204.3568
2018 review article on the topic: https://arxiv.org/abs/1801.01083