Keir tells us about constraints on the fraction of dark matter that could be “ultralight” at various masses.

It seems probable now that one single mass of ultralight dark matter (ULDM) can’t be responsible for all the dark matter, but this doesn’t mean it can’t be a sub-component of the dark matter. The structure suppressing properties of ULDM could also have implications for the S8 tension, i.e. maybe a small sub-component of ULDM is what is causing the low S8 in local measurements?

The observational data Keir considers are the Planck CMB data and BOSS clustering data.

Keir: https://keirkwame.github.io/

Paper: https://arxiv.org/abs/2301.08361

Richard Easther talks about the era immediately after inflation. If reheating is delayed then the small perturbations in the density of the universe grow, just as they do during matter domination in the late universe. The growth of non-linear structures during this “early dark age” has not been explored in depth until recently and Richard gives an overview of recent work on the topic. Continue reading →

Johannes Røsok Eskilt and Patricia Diego-Palazuelo update us on the state of measurements of birefringence in the cosmic microwave background in 2022. The current most popular video on the channel was about a tentative hint of CMB birefringence back in 2020 and much can change in two years.

The tentative hint is very much still present. Johannes and Patricia are both members of Planck itself and have spent the last few years checking for systematics, taking the effects of EB correlations in the dust into account, adding WMAP data, adding additional frequency maps from Planck, and checking for a frequency dependence of the signal. Continue reading →

Tessa Baker talks about how the gravitational wave detector LISA will be able to measure the speed of gravitational waves. Knowing this speed will help us understand if we need to go beyond Einstein’s theory of gravity (general relativity). Continue reading →

Jiamin Hou, Zachary (Zack) Slepian, and Robert (Bob) Cahn talk about their very interesting, recent paper looking for parity violation in the large scale structure. Cutting to the chase, they find evidence for it at ~7σ. They also do various systematic tests and none reduce the significance below 4σ.

Tantalisingly, parity violation in the LSS could be related to birefringence in the cosmic microwave background, with many models that would produce one, also producing the other. Avid channel viewers will remember a talk from two years ago with evidence for exactly that. Continue reading →

Andrej Dvornik and Constance Mahony tell us about the Kilo-Degree Survey (KiDS) and their progress at probing the non-linear scales with data from the survey.

Constance talks about her recent paper showing that non-linear halo bias is absolutely necessary if you want to avoid errors larger than 5σ on the small, non-linear scales.

Andrej takes this and does the analysis on KiDS data to obtain cosmological constraints.

Constance: INSPIRE author page

Constance’s paper: arxiv.org/abs/2202.01790

Andrej: Personal Website

Andrej’s paper: arxiv.org/abs/2210.03110

Silvia tells us about her recent use of Planck satellite data to search for dark matter. She doesn’t use the CMB though, instead she is looking for microwave emission from the galactic centre. For her, the CMB is noise!

If dark matter decay/annihilation leads to electron positron emission then those electrons and positrons would emit microwave light through synchrotron radiation as they travel through the magnetic fields at the galactic centre.

She found that the polarisation signal is a more sensitive probe of this effect than the intensity. The main theory for why this is is that the observed signal in the polarisation has more features (coherent hot and cold spots), whereas the expected dark matter signal should be more smooth. Whereas both the intensity measurement and signal should be more smooth. Therefore, the observed polarisation “cold spots” can best constrain the dark matter signal, especially close to the galactic centre, where the dark matter signal should still be strong. Continue reading →

Colin Hill, Adrien La Posta and Even McDonough tell us about the state of play with early dark energy (EDE). They cover both observation, primarily the CMB; and fundamental physics models that might generate EDE phenomenology. Continue reading →

Alex Amon and Naomi Robertson talk about their recent work analysing all of the Dark Energy Survey (DES), the Hyper Suprime Cam survey (HSC), and the Kilo Degree Survey (KiDS) and the Baryon Oscillation Spectroscopic Survey (BOSS). In particular they look to see whether the three lensing surveys (DES, HSC and KiDS) are consistent with the clustering of galaxies in BOSS. Continue reading →

Dillon Brout tells us about the recent Pantheon+ supernovae catalogue and cosmology results. The cosmology results are excellent, including a factor of two improvement in the “figure of merit” compared to the previous Pantheon result (essentially a halving of error bars). Continue reading →