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Cosmology Talks: Cosmological Constraints from BAO

Posted in The Universe and Stuff, YouTube with tags astronomy, Cosmology, Cosmology Talks, Dark Energy, Dark Energy Spectroscopic Instrument, DESI, Physics, Science, Shaun Hotchkiss, Universe on April 5, 2024 by telescoper

Here’s another video in the Cosmology Talks series curated by Shaun Hotchkiss. This one very timely after yesterday’s announcement. Here is the description on the YouTube page:

The Dark Energy Spectroscopic Instrument (DESI) has produced cosmological constraints! And it is living up to its name. Two researchers from DESI, Seshadri Nadathur and Andreu Font-Ribera, tell us about DESI’s measurements of the Baryon Acoustic Oscillations (BAO) released today. These results use one full year of DESI data and are the first cosmological constraints from the telescope that have been released. Mostly, it is what you might expect: tighter constraints. However, in the realm of the equation of state of dark energy, they find, even with BAO alone, that there is a hint of evidence for evolving dark energy. When they combine their data with CMB and Supernovae, who both also find small hints of evolving dark energy on their own, the evidence for dark energy not being a cosmological constant jumps as high as 3.9σ with one combination of the datasets. It seems there still is “concordance cosmology”, it’s just not ΛCDM for these datasets. The fact that all three probes are tentatively favouring this is intriguing, as it makes it unlikely to be due to systematic errors in one measurement pipeline.

My own take is that the results are very interesting but I think we need to know a lot more about possible systematics before jumping to conclusions about time-varying dark energy. Am I getting conservative in my old age? These results from DESI do of course further underline the motivation for Euclid (another Stage IV survey), which may have an even better capability to identify departures from the standard model.

P.S. Here’s a nice graphic showing the cosmic web showing revealed by the DESI survey:

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Cosmology Talks: Intrinsic Alignments – A Guide for All Cosmologists

Posted in OJAp Papers, The Universe and Stuff, YouTube with tags Claire Lamman, Cosmology Talks, Elisa Legnani, intrinsic alignments, Jingjing Shi, Niko Šarčević, Shaun Hotchkiss, Susan Pyne, Tassia Ferreira, weak gravitational lensing on February 24, 2024 by telescoper

I was just thinking this afternoon that I haven’t posted recently any of the Cosmology Talks curated by Shaun Hotchkiss, then I looked and found that I had the perfect excuse for doing so. This particular talk is actually about one of the two new OJAp papers I announced in my previous post, i.e. “The IA Guide: A Breakdown of Intrinsic Alignment Formalisms” and the authors are: Claire Lamman (Harvard, USA); Eleni Tsaprazi (Stockholm, Sweden); Jingjing Shi (Tokyo, Japan); Nikolina Niko Šarčević (Newcastle, UK); Susan Pyne (UCL, UK); Elisa Legnani (Barcelona, Spain); and Tassia Ferreira (Oxford, UK).

Here is Shaun’s description of the video:

Claire Lamman, Jingjing Shi, Niko Šarčević, Susan Pyne, Elisa Legnani and Tassia Ferreira tell us about the intrinsic alignments guide they wrote (along with Eleni Tsaprazi, who couldn’t make the video recording).

They wanted to write something that wasn’t quite a review, but also wasn’t quite a set of lecture notes. Instead they aimed for what might be best framed as a “cheat sheet” for intrinsic alignments. Everything you need to know about the topic, compressed into one article. However, there’s still a lot about the topic, so the compression is still 33 pages and 10 figures big.

To construct the guide they broke the topic of intrinsic alignments into sub-fields and then asked questions like “what are the key equations for this sub-field?”, “what are the different notations people use?”, “what might be confusing to a newcomer?” They then wrote the guide to answer those questions, even including subsections with quick definitions of each common term, and short lists of common alternative notations.

And here is the video!

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Cosmology Talks: James Alvey on Big Bang Nucleosynthesis in 2021

Posted in The Universe and Stuff with tags Big Bang Nucleosynthesis, Cosmology, Cosmology Talks, James Alvey, nucleosynthesis, Shaun Hotchkiss on October 15, 2021 by telescoper

It’s been a while since I last 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. I found this one particular interesting as it is a field that I lost track of quite a long time ago and it was great to see what has been going on!

Here James Alvey gives a pedagofical overview of the general state of the field of Big Bang Nucleosynthesis (BBN) in 2021, including the basic physics that goes into BBN calculations through each of the relevant epochs (neutrino decoupling, the deuterium bottleneck, etc). He gives particular emphasis to the recent LUNA measurements of the D + p →γ + 3He reaction (or deuterium + proton goes to photon and 3-Helium). This was previously the source of greatest uncertainty in predicting the final deuterium abundance of BBN. He ends by talking about the implications of the LUNA measurements on possible new physics beyond the standard model, in particular possible thermal relics.

There is a Nature paper about the LUNA results here and two other papers on this topic by James can be found here and here.

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Cosmology Talks: Volker Springel on GADGET-4

Posted in The Universe and Stuff with tags arXiv:2010.03567, Cosmology, Cosmology Talks, galaxy formation, large-scale structure of the Universe, N-body calculations, Shaun Hotchkiss, Volker Springel on May 18, 2021 by telescoper

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.

In this talk from a couple of months ago Volker Springel discusses Gadget-4 which is a parallel computational code that combines cosmological N-body and SPH code and is intended for simulations of cosmic structure formation and calculations relevant for galaxy evolution and galactic dynamics.

The predecessor of GADGET-2 is probably the most used computational code in cosmology; this talk discusses what new ideas are implemented in GADGET-4 to improve on the earlier version and what new features it has. Volker also explains what happened to GADGET-3!

The paper describing Gadget-4 can be found here.

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Cosmology Talks about the Open Journal of Astrophysics

Posted in Open Access, The Universe and Stuff with tags Cosmology Talks, Shaun Hotchkiss, The Open Journal of Astrophysics on May 3, 2021 by telescoper

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!

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Cosmology Talks: Dan Thomas on the first model-independent cosmological simulations of modified gravity

Posted in The Universe and Stuff with tags arXiv:2004.13051, arXiv:2103.05051, Cosmology, Cosmology Talks, Dan Thomas, modified gravity, Poisson's equation, Shaun Hotchkiss on April 7, 2021 by telescoper

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.

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Cosmology Talks: Alvaro Pozo on Potential Evidence for Wave Dark Matter

Posted in The Universe and Stuff with tags Alvaro Pozo, arXiv:2010.10337, arXiv:2101.11735, Cosmology, Cosmology Talks, dark matter, dwarf galaxies, Fuzzy Dark Matter, Shaun Hotchkiss, Wave Dark Matter on February 19, 2021 by telescoper

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.

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Cosmology Talks: Marika Asgari on Kids 1000

Posted in Cardiff, Maynooth, The Universe and Stuff with tags Cosmology Talks, Kilo-Degree Survey, Kilodegree survey, Marika Asgari, Shaun Hotchkiss on January 18, 2021 by telescoper

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 σ₈. In the combined parameter “S₈“, 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.

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Cosmology Talks: Eiichiro Komatsu & Yuto Minami on Parity Violation in the Cosmic Microwave Background

Posted in Cardiff, Maynooth, The Universe and Stuff with tags birefringence, Cosmic Microwave Background, Cosmology, Eeichiro Komatsu, Planck Satellite, Polarization, quad, Shaun Hotchkiss, Yuto Minami on December 2, 2020 by telescoper

In this video, Eiichiro Komatsu and Yuto Minami talk about their recent work, first devising a way to extract a parity violating signature in the cosmic microwave background, as manifested by a form of birefringence. If the universe is birefringent then E-mode polarization would change into B-mode as electromagnetic radiation travels through space, so there would be a non-zero correlation between the two measured modes. They try to measure this correlation using the Planck 2018 data, getting a 2.4 sigma `hint’ of a result.

A problem with the measurement is that systematic errors, such as imperfectly calibrated detector angles, could mimic the signal. Yuto and Eiichiro’s idea was to measure the detector angle by looking at the E-B correlation in the foregrounds, where light hasn’t travelled far enough to be affected by any potential birefringence in the universe. They argue that this allows them to distinguish between the two types of measured E-B correlation. However, this is only the case if there is no intrinsic correlation between the E-mode and B-mode polarization in the foregrounds, which may not be the case, but which they are testing. The method can be applied to any of the plethora of CMB experiments currently underway so there will probably be more results soon that may shed further light on this issue.

Incidentally this reminds me of Cardiff days when work was going on about the same affect using the Quad instrument. I wasn’t involved with Quad but I do remember having interesting chats about the theory behind the measurement or upper limit as it was (which is reported here). Looking at the paper I realize that paper involved researchers from the Department of Experimental Physics at Maynooth University.

P. S. The paper that accompanies this talk can be found here.

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Cosmology Talks: Mateja Gosença & Bodo Schwabe on Simulating Mixed Fuzzy and Cold Dark Matter

Posted in The Universe and Stuff with tags axion, AxioNyx, Bodo Schwabe, Cosmology, Fuzzy Dark Matter, Mateja Gosenca, Shaun Hotchkiss on November 11, 2020 by telescoper

It’s been too long since I shared 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.

Anyway, although I’ve been too busy to check out the talks much recently I couldn’t resist sharing this one not only because it’s on a topic I find interesting (and have worked on) but also because one of the presenters (Mateja Gosença) is a former PhD student of mine from Sussex! So before I go fully into proud supervisor mode, I’ll just say that the talk is about AxioNyx, which is a new public code for simulating both ultralight (or “Fuzzy”, so called because its ~~Compton~~ de Broglie wavelength is large enough to be astrophysically relevant) dark matter (FDM) and Cold dark matter (CDM) simultaneously. The code simulates the FDM using adaptive mesh refinement and the CDM using N-body particles.