I have from time to time posted videos from the series of Cosmology Talks curated by Shaun Hotchkiss. These are usually technical talks at the level you might expect for a cosmology seminar, but this time it’s something different. Shaun asked me if I’d like to give a talk about the Open Journal of Astrophysics, so one night last week we recorded this. We ended up chatting about quite a lot of things so it turned out longer than most of the videos in the series, but it’s not a technical talk so I hope you’ll find it bearable!

It’s time I shared another one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting. This one was published just yesterday.

In the talk Dan Thomas discusses his recent work first creating a framework for describing modified gravity (i.e. extensions of general relativity) in a model-independent way on non-linear scales and then running N-body simulations in that framework. The framework involves finding a correspondence between large scale linear theory where everything is under control and small scale non-linear post-Newtonian dynamics. After a lot of care and rigour it boils down to a modified Poisson equation – on both large and small scales (in a particular gauge). The full generality of the modification to the Poisson equation allows, essentially, for a time and space dependent value for Newton’s constant. For most modified gravity models, the first level of deviation from general relativity can be parametrised in this way. This approach allows the method to use to constrain modified gravity using observations without needing to run a new simulation for every step of a Monte Carlo parameter fit.

P. S. A couple of papers to go with this talk can be found here and here.

It’s time I shared another one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting. This is quite a recent one, from about a week ago.

In the talk, Alvaro Pozo tells us about a recent paper where he an collaborators detect the transition between a core (flat density profile) and halo (power law density profile) in dwarf galaxies. The full core + halo profile matches very closely what is expected in simulations of wave dark matter (sometimes called “fuzzy” dark matter), by which is meant dark matter consisting of a particle so light that its de Broglie wavelength is long enough to be astrophysically relevant. That is, there is a very flat core, which then drops off suddenly and then flattens off to a decaying power-law profile. The core matches the soliton expected in wave dark matter and the halo matches an outer NFW profile expected outside the soliton. They also detect evidence for tidal stripping of the matter in the galaxies. The galaxies closer to the centre of the Milky Way have their transition point between core and halo happen at smaller densities (despite the core density itself not being systematically smaller). The transition also appears to happen closer to the centre of the galaxy, which matches simulations. Of course the core+halo pattern they have clearly observed might be due to something else, but the match between wave dark matter simulations and observations is impressive. An important caveat is that the mass for the dark matter that they use is very small and in significant tension with Lyman Alpha constraints for wave-like dark matter. This might indicate that the source of this universal core+halo pattern they’re observing comes from something else, or it might indicate that the wave dark matter is more complicated than represented in the simplest models.

P. S. The papers that accompany this talk can be found here.

P.P.S. If you’re interested in wave dark matter there is a nice recent review article by Lam Hui here.

It’s time I shared another one of those interesting cosmology talks on the Youtube channel curated by Shaun Hotchkiss. This channel features technical talks rather than popular expositions so it won’t be everyone’s cup of tea but for those seriously interested in cosmology at a research level they should prove interesting. Since I haven’t posted any of these for a while I’ve got a few to catch up on – this one is from September 2020.

In this talk Marika Asgari tells us about the recent Kilo-Degree Survey (KiDS) cosmological results. These are the first results from KiDS after they have reached a sky coverage of 1000 square degrees. Marika first explains how they know that the results are “statistics dominated” and not “systematics dominated”, meaning that the dominant uncertainty comes from statistical errors, not systematic ones. She then presents the cosmological results, which primarily constrain the clumpiness of matter in the universe, and which therefore constrain Ω_{m} and σ_{8}. In the combined parameter “S_{8}“, which is constrained almost independently from Ω_{m} by their data they see a more than 3σ tension with the equivalent parameter one would infer from Planck.

P. S. The papers that accompany this talk can be found here and here.

Today is another big day for cosmology as the last and final – why do people say that? – data release from the Extended Baryon Oscillation Spectroscopic Survey (eBOSS) is now available. That is the culmination of 20 years of effort with the Sloan Digital Sky Survey. Here is a pretty picture showing the enormous scales probed by the survey:

There is an overview paper on the cosmological implications of the survey on the arXiv here. Fortunately, the latest Cosmology Talk on the YouTube channel of Shaun Hotchkiss features a very interesting presentation by Eva-Maria Mueller who is the first author of that paper:

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