Arnaud de Mattia, Hector Gil-Marín and Pauline Zarrouk tell us about the latest DESI key papers. These ones are covering the “full shape” of the galaxy power spectrum. This is in contrast to their results from April, which were centred around the Baryon Acoustic Oscillation (BAO) feature.

The analysis is an absolute tour de force, explaining why the video is so long. DESI have done a lot of analysis and it takes a while to unpack. (The raw recording of this video was more than 200 minutes long!)

The results aren’t quite so eye-opening as the BAO ones were in April. The evidence for evolving dark energy is still there, but the significance of this evidence hasn’t changed much. They are however able to do a bunch of tests of modified gravity and find that, unlike ΛCDM, General Relativity appears to be in great shape (at least as far as DESI observations are concerned). Continue reading →

Daniele Sorini tells us about his research looking into how the value of dark energy affects star formation in the universe. He builds an analytic model of star formation in the universe, which allows him to follow the universe to very late times and ask “if Λ was different, how many total stars could form over the entire history of the universe?”

This is the crucial question one wants to ask when examining anthropic explanations of Λ scientifically. Previous studies have asked this question using simulations, forcing them to stop after a finite amount of time.

Fascinatingly, Daniele finds that there is a value of Λ where star formation peaks, which is about 0.1 times the value in our universe. Previous simulation-based studies maybe didn’t spot this because they simply didn’t do simulations with Λ less than our observed value, except for Λ=0, but also they didn’t go to late enough times. Continue reading →

Ana Alexandre and Valentin Thoss tell us about their recent papers examining the consequences of a potential “memory burden” effect on decaying primordial blackholes (PBHs).

This memory burden effect would be a consequence of quantum gravity and would substantially slow down the Hawking evaporation of blackholes. The consequences for primordial blackholes would be very important because it would open a whole window of PBH masses as dark matter candidates. (Masses that are normally ignored because PBHs of these masses are presumed to have decayed by today.)

All fundamental physicists would agree that Hawking radiation must breakdown at some point at/before a blackhole reaches the Planck mass. This proposed memory burden effect goes a step further and claims that the breakdown must happen no later than when a blackhole has decayed to half of its original mass. This is not universally agreed upon, but it’s still worth examining the consequences if it is true. Continue reading →

Abigail Lee tells us about how she has used the J-region Asymptotic Giant Branch (JAGB) to measure extragalactic distances, ultimately using them as a rung on the distance ladder to measure the expansion rate of the universe.

The JAGB method is new, at least compared to cepheids and “tip of the red giant” (TRGB) methods, but is also very promising. The stars it uses are bright, so they can be seen at large distances, and they’re numerous, which means they can be observed far from the centres of galaxies (minimising “crowding” from other stars in the same pixels). So, even ignoring Hubble tensions, this is a very exciting time to be involved in extragalactic distance measurements.

Abby used data from JWST to detect the JAGB mode brightness, which is then the distance indicator of the method. With more telescope time, and more local calibrators, JAGB will only go from strength to strength.

When the measured distances are compared to JWST measurements of TRGB and cepheids it has very little scatter with TRGB, but a little more from cepheids. The ultimate Hubble parameter is also on the smaller side, consistent with CMB + ΛCDM and on the border of being inconsistent with SH0ES cepheid+HST based measurements.

Abby: abiglee7.github.io

The paper: arXiv: 2408.03474

Wendy Freedman and Barry Madore give a status update on the Chicago-Carnegie Hubble Program (CCHP). There has been much progress!

In this status update there are three independent distance ladder measurements of the expansion rate (i.e. the Hubble parameter) all using JWST data. There is the very well-known Cepheid method, the now also well known Tip of the Red Giant (TRGB) method, and making its debut, the J-Region Asymptotic Giant Branch (JAGB) method.

The TRGB and JAGB methods agree strikingly well and appear to be consistent with the CMB + ΛCDM value for the expansion rate. The Cepheid method also agrees to within less than 5%, which is still striking, but there is some deviation.

The deviation pushes the Cepheid value for H0 up, making it consistent with the SH0ES value, although the error bars on this CCHP result are large enough that their Cepheid value is also consistent with the lower TRGB and JAGB methods. Most curiously (to this relative outsider) the JAGB value does not appear to be consistent with SH0ES, even when considering statistical and systematic errors at once. The striking consistency between TRGB and JAGB, means that one’s naive guess might be that there is some unknown systematic in the Cepheid method (in both the CCHP and SH0ES measurements). However, naive guesses are naive so, time will tell. Continue reading →

Ryan White and Tamara Davis from the Dark Energy Survey tell us about how they have measured time dilation in distant supernovae. This time dilation is precisely what one would expect in an expanding universe. “Precisely” is the right word too, as they have measured this effect to 0.5% precision and they get exactly the number predicted by an expanding universe. Continue reading →

Paul Shah and Tamara Davis tell us about how they have used this wonderful supernova catalogue from the Dark Energy Survey to detect the weak lensing magnification signal for the first time. There has been evidence of this signal in earlier catalogues, but at no bigger than 1.4σ. They’ve got it at 6σ!

They do this by correlating the scatter in the magnitude of the supernovae with the over-under density in galaxy catalogues along the same lines of sight of the supernovae. Where there is more matter, the light from the supernova should be magnified, and where there is less matter it should be de-magnified. And they do indeed see that along overdense lines of sight the supernovae are, on average, ever so slightly brighter, and on underdense lines of sight the are ever so slightly dimmer.

I can’t wait to see how this observable is used in the future to constrain all sorts of bits of cosmology. Nice work everyone!

Paper: arXiv: 2406.05047

Paul: paulshah.github.io

Tamara: smp.uq.edu.au/profile/186/tamara-davis

Ryan Camilleri and Tamara Davis tell us about how they have examined models beyond ΛCDM using the Dark Energy Survey’s wonderful supernova catalogue. Tantalisingly, they find that a number of models are “moderately preferred” over ΛCDM (in model comparison speak).

They also, very admirably, check whether crucial aspects of the DES pipeline are model dependent or not. They find that, so long as the reference model is close-ish to the true model then the pipeline is accurate. “Close-ish” is very generous here as well, as they even found in simulations that when one processed the data with models 10σ from the truth, the subsequent parameter constraints were still within 1σ of the truth. The moral of this is that, even though the supernovae were processed assuming ΛCDM, so long as the true cosmology isn’t too far from ΛCDM then this doesn’t matter.

This means, if you have your own model that they haven’t tested, you don’t need to simulate the entire DES analysis pipeline to analysis your model, you can do your model comparison at the level of the Hubble diagram. Nice! Continue reading →

Ryan Camilleri and Tamara Davis tell us about how they have used the Dark Energy Survey’s Year 5 supernovae catalogue, anchored to the Dark Energy Spectroscopic Instrument’s Baryon Acoustic Oscillations, to create an “inverse distance ladder”.

With this they are able to determine the Hubble Parameter, at redshift zero with a high accuracy, without needing to assume ΛCDM. The results still match Planck, meaning that the high redshift to low redshift matching appears to still not work out, even outside of ΛCDM.

The implications are large for any attempts to go beyond ΛCDM to solve the Hubble tension as it appears the z=2 to z=0.05 window is not the right window for finding the solution.

Paper: arXiv: 2406.05049

Ryan: smp.uq.edu.au/profile/13102/ryan-camilleri

Tamara: smp.uq.edu.au/profile/186/tamara-davis

Tamara Davis, Maria Vincenzi and Dillon Brout tell us about the Dark Energy Survey’s (DES) new supernova catalogue. The catalogue has more than 1500 new supernovae, and will allow a vast range of new cosmology constraints. It is a factor of around five larger than the next largest high redshift supernovae catalogue.

Very curiously, DES’ supernovae see hints of evolving dark energy. This is especially curious given that a few months after DES released this data, the Dark Energy Spectroscopic Instrument (DESI) also released data with similar hints. Continue reading →