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 →

Gerrit Farren and Alex Krolewski tell us about their work cross correlating the ACT lensing maps with unWISE galaxies. Their method is an update of earlier analyses using Planck lensing. Those earlier analyses were in tension with Planck primary CMB measurements, which was similar to e.g. cosmic shear, and other clustering measurements, at smaller scales. Continue reading →

Gerrit Farren and Alex Krolewski tell us about their work cross correlating the ACT lensing maps with unWISE galaxies. Their method is an update of earlier analyses using Planck lensing. Those earlier analyses were in tension with Planck primary CMB measurements, which was similar to e.g. cosmic shear, and other clustering measurements, at smaller scales.

Curiously, this new analysis reveals no tension with primary CMB. A re-analysis of Planck lensing cross unWISE, with updated assumptions and modelling, shows that Planck lensing is actually *also* not in tension with primary CMB (a combination of effects were responsible for removing the tension).

This analysis covers the large(ish) scale low(ish) redshift quadrant of the (k,z) space, suggesting that the origin of the S8 tension is likely to either be a very low redshift phenomenon, or scale dependent.

However Alex and Gerrit do note that a recent cross-correlation of Planck with DESI LRGs does seem to still indicate some tension at the same redshift and scales, so the dust hasn’t 100% settled on the large scale, low redshift quadrant.

Gerrit: https://www.maths.cam.ac.uk/person/gsf29

Alex: https://perimeterinstitute.ca/people/alex-krolewski

Paper: https://arxiv.org/abs/2309.05659

Cyril tells us about recent work on Chern-Simons gravity and how if it is the correct general model of gravity then parity violation can be generated during inflation in such a way that correlation functions of galaxies measured today could see that parity violation. Given the recent evidence for parity violation in galaxy correlation functions, that is intriguing.

The model also requires parameter values that could produce baryogenesis, so the same physics would end up producing the (observed?) parity violation and the baryon asymmetry in the universe.

Even if the current hints of parity violation turn out to be wrong, future experiments would be sensitive to smaller magnitudes of this effect, meaning the model will be well constrained by the next wave of large scale structure measurements

Cyril: cyril-creque.github.io

Paper: arxiv.org/abs/2303.04815