Mark Fromhold and Lucia Hackermuller tell us about how they are 3D printing atom traps that allow them to cool atoms to a few micro Kelvin. This is super interesting for cosmology because it would allow them, among many other things, to potentially trap dark domain walls. We learned in another recent cosmology talk about the physics behind these dark domain walls, now here is the physics behind the cold atom trap.

In principle these traps may one day measure the gravitational effects of quantum objects, ultimately testing whether space-time curvature can be in a quantum superposition or not. Continue reading →

Nhat-Minh Nguyen and Beatrice Tucci tell us about their recent work comparing the performance of field inference (FLI) and simulation based inference (SBI). In an apples to apples comparison, they find that FLI comfortably outperforms SBI, even in what is essentially the “best case scenario” for SBI.

Field level inference gives up on using “summary statistics” to construct a cosmological likelihood (e.g. the power spectrum, the bispectrum, the location of the BAO peak, voids, etc) and instead constructs the cosmological likelihood at the level of the field itself. In other words the likelihood step of a statistical analysis is done comparing the measured density field at each point in Fourier space to a model’s actual density field. This means the set of model “parameters” necessarily also includes the entire set of Fourier modes of the initial conditions. Then, for example, when one would then talk about the “maximum likelihood” parameters in a FLI inference, one is talking also about the maximum likelihood set of initial conditions.

One then does the rest of the statistical analysis more or less the same as if one is analysing a measured power spectrum, e.g. one has priors on the inferred parameters, one has the likelihood function, and one produces posterior probability distributions for all of the model parameters.

In this analysis they fixed all cosmological parameters except the overall amplitude of the initial density fluctuations, via σ8. This means they also restricted the set of initial density fluctuations to those with a certain spectral index, but varied over all sets of initial density fluctuations that do produce this spectral index. They then evolve the initial conditions forward in time using the LEFTfield framework and do the FLI analysis on the evolved field. Continue reading →

Russell Boey, along with his coauthors Richard Easther and Yourong Wang, tells us about his simulations of a supermassive black hole traveling through an ultralight dark matter soliton. In particular, he has studied the dynamical friction effect on the black hole within the soliton.

This is especially interesting in the context of the “final parsec” problem, where the orbits of supermassive black hole binary systems stall in their decay as they reach one parsec separation. Maybe a different background, in the form of ULDM instead of WIMP DM, could help?

An ultralight dark matter soliton is much more dense than expectations from “ordinary” WIMP-like dark matter, so it is also expected that the dynamical friction in such a soliton should be large. This is indeed what Russell, Richard and Yourong found (and other coauthor Emily Kendall who isn’t present in the video). However, curiously, they also found a secondary effect where the black hole perturbs the soliton, which in turn causes the soliton to backreact on the black hole and sometimes speed it back up. Continue reading →

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. Continue reading →

Sylvia is a philosopher of science. Her focus is probability and she has worked on a few theories that aim to extend and modify the standard axioms of probability in order to tackle paradoxes related to infinite spaces. In particular there is a paradox of the “infinite fair lottery” where within standard probability it seems impossible to write down a “fair” probability function on the integers. If you give the integers any non-zero probability, the total probability of all integers is unbounded, so the function is not normalisable. If you give the integers zero probability, the total probability of all integers is also zero. No other option seems viable for a fair distribution.

This paradox arises in a number of places within cosmology, especially in the context of eternal inflation and a possible multiverse of big bangs bubbling off. If every bubble is to be treated fairly, and there will ultimately be an unbounded number of them, how do we assign probability?

The proposed solutions involve hyper-real numbers, such as infinitesimals and infinities with different relative sizes, (reflecting how quickly things converge or diverge respectively).

The multiverse has other problems, and other areas of cosmology where this issue arises also have their own problems (e.g. the initial conditions of inflation); however this could very well be part of the way towards fixing the cosmological multiverse. Continue reading →

Pritha Paul and Chris Clarkson tell us about their work, along with Roy Maartens, delving very deeply into standard observational cosmology. Specifically, they have looked at relativistic effects in the four point function/trispectrum of galaxy positions.

This might sound crazy and masochistic, but there are big rewards. On large enough scales, the relativistic effects start to grow, and tantalisingly, once one takes into account both relativistic effects and the observational effects of observing in redshift space, a parity violating signal emerges in both the bispectrum and trispectrum on large scales. This is very interesting given the possible observations of parity violation in the four point function of galaxy positions (i.e. Fourier transform of the trispectrum).

There are reasons to suspect the effect Pritha, Chris and Roy have uncovered within standard cosmology couldn’t be the thing potentially observed in the four point function, however it is possible to at least tell a story about how the one effect might show up in the other observation. Time will tell whether they are indeed related.

Irrespective of that, the result is still interesting as it is likely that Euclid and/or SKA will be able to spot this signal, thus detecting the effects of relativity within the large scale structure. Continue reading →

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.

In this video they go over both the guide and the topic of intrinsic alignments. Continue reading →

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 →