Jurek tells us about the prospects for constraining axion (aka ultralight aka fuzzy) dark matter with future 21cm intensity mapping survey such as SKA and HIRAX.
Axion models arising from specific energy scales predict that an axion with a given mass will only provide a certain fraction of the total dark matter. It seems plausible that with SKA we will be able to detect ultralight dark matter even if it arises from a GUT scale axion model. An observational noise model for SKA was included to make this claim, but as of yet no theoretical uncertainty is included in the calculation.
Hamsa tells us about how baryonic gases arrange themselves inside galaxies, specifically in the context of the HI halo model (with some deviation to discuss other gases like molecular hydrogen and carbon monoxide).
This is a great talk in its own right, full of really useful information for cosmologists who want to know how intensity mapping, etc, will be used for cosmology – but, it also acts as a good companion talk to Jurek Bauer’s talk on constraining axion dark matter using intensity mapping (https://youtu.be/bMlrDOWw978). Hamsa was a coauthor on Jurek’s paper and the expert in that collaboration on the HI/intensity mapping part.
This video builds up to eventually being about this paper, https://arxiv.org/abs/2002.01489​, however in getting there it covers the whole background of modelling HI and other baryonic gases within galaxies in an information packed, but accessible way.
Colin tells us about how even though early dark energy can alleviate the Hubble tension, it does so at the expense of increasing other tension. Early dark energy can raise the predicted expansion rate inferred from the cosmic microwave background (CMB), by changing the sound horizon at the last scattering surface. However, the early dark energy also suppresses the growth of perturbations that are within the horizon while it is active. This mean that, to fit the CMB, the matter density must increase (and the spectral index becomes more blue tilted). The consequence is that the matter power spectrum should get bigger.
In their paper, Colin and his coauthors show that this affects the weak lensing measurements by DES, KiDS and HSC, and therefore including those experiments in a full data analysis makes things discordant again. The Hubble parameter is pulled back down, restoring most of the tension between local and CMB measurements of H0, and the tension in S_8 gets magnified by the increased mismatch in the predicted and measured matter power spectrum.
It is also worth noting that, if you exclude the local measurements of H0, there is no preference for early dark energy in the data.
There is hope, perhaps. If the sound horizon could be changed without altering the growth of perturbations that might still be a valid resolution, but it is unlikely to be caused by early dark energy (alone).
Adam tells us about what he and collaborators considered to be the leading candidate for a systematic error in the SHOES measurement of the expansion rate of the Universe. This is “Cepheid crowding”, the possibility that background sources change our interpretation of Cepheid brightness, ruining one step in the SHOES distance ladder.
They devise a nice way to test whether the crowding is correctly accounted for and find that it is. So crowding cannot be the “explanation” of an error in the distance ladder measurement of H0.
He also stresses that both the early and late universe measurements of H0 are now backed up by multiple different measurements. Therefore, if the resolution isn’t fundamental physics, then no single systematic can entirely solve the tension.
We also discuss a few topics around the H0 tension, including what resolution of the tension he would pick as most likely if forced to gamble (answer: a deviation from vanilla ΛCDM in the early universe).
Kimmy tells us about a subtle but very interesting tension between Planck lensing data and line of sight Baryon Acoustic Oscillation (BAO) data.
She and her coauthors discovered this via an intriguing mismatch between Planck and South Pole Telescope (SPT) lensing results. The Planck and SPT power spectrum amplitudes matched, but when combined with BAO and Big Bang Nucleosynthesis the inferred Hubble parameters were slightly different.
Like great data-detectives they tracked the source of this discrepancy down to the mismatch between Planck lensing and line of sight BAO. Why the line of sight BAO might be causing this is unclear. On the Planck lensing side, it has something to do with the shape of the lensing power spectrum, e.g. the location of the peak – because SPT only measures the power spectrum’s tail and so is only sensitive to the amplitude.
The result is definitely interesting, and unknown by the community until now (as far as I’m aware). Whether it is a red herring or a vital clue in the hunt to solve the Hubble mystery remains to be seen. But it should provide fuel for both model builders and hunters of systematic errors trying to solve this mystery.
Jurek tells us about the prospects for constraining axion (aka ultralight aka fuzzy) dark matter with future 21cm intensity mapping survey such as SKA and HIRAX.
Axion models arising from specific energy scales predict that an axion with a given mass will only provide a certain fraction of the total dark matter. It seems plausible that with SKA we will be able to detect ultralight dark matter even if it arises from a GUT scale axion model. An observational noise model for SKA was included to make this claim, but as of yet no theoretical uncertainty is included in the calculation.
Omar tells us about the excellent quality lensing map he’s produced with the Atacama Cosmology Telescope collaboration. You honestly won’t believe how well this lensing map correlates with the cosmic infrared background (sorry about the clickbait). The map will be incredibly useful to cross-correlate with any dataset of tracers inside the relatively large window of ACT observations.
Omar also explains how he and the collaboration, for the first time, removed the annoying thermal Sunyaev–Zeldovich contamination that ordinarily produces a ~10% bias in sigma8 (and thus any other cosmological parameter correlated with sigma8).
Deanna tells us about what we could learn from future measurements of the spectral distortions in the CMB, as well as how spectral distortions complement current and future measurements of CMB anisotropies. She also discusses CLASS (v3.0), the code you can use to calculate predictions for both.
There is a guaranteed spectral distortion signal to detect within ΛCDM and the possibility to constrain many possible deviations, including primordial black holes and decaying dark matter. In fact, we can detect the signal even if the PBHs and/or decaying dark matter only make up one part in a million of the total dark matter!
Julien tells us about the cosmological effects of different neutrino mass states (i.e. the same sum of masses, but different masses for each individual neutrino – e.g. “normal” vs “inverted”).
There are effects, but they’re all very small and not even the best future experiments will distinguish them.
Non-standard model neutrinos would still have interesting effects, but it seems that cosmology’s insight on the SM ones will be limited to the sum of the masses.
Sesh tells us how the void-galaxy cross correlation provides information about cosmology via redshift space distortions and (importantly) the Alcock Paczynski effect. The information is independent of Baryon Acoustic Oscillations (BAO) and improves error bars by up to a factor of four. The combination of voids and BAO have very interesting insights into the Hubble discrepancy and the late-time acceleration of the Universe.
Cosmology Talks 