Jeongin Moon, David Valcin and Christoph Saulder tell us about the first cosmologically relevant results from DESI (The Dark Energy Spectroscopic Instrument). Specifically, they are presenting the first detection of the BAO (Baryon Acoustic Oscillations) from DESI.
With a “first detection” one would expect something mediocre, but even this result, using only two months of DESI data, has similar precision to the final BOSS data after years of operation. This is a very exciting time for cosmology as we wait for the full one year DESI results and cosmological constraints. Continue reading →
Giovanni Arico tells us about his recent work adding baryons to the modelling of Dark Energy Survey cosmic shear analysis. By adding baryonic modelling he and collaborators are able to use all of the DES data. Continue reading →
Minh Nguyen tells us about his recent work looking for evidence of modified gravity in cosmological observations. Rather than look at a specific model, Minh and his collaborators look for evidence of a deviation in the “growth index” γ.
In general relativity and ΛCDM γ≃0.55 but in a more general theory of gravity it could deviate. Minh looks at measurements of peculiar velocity, clustering and Planck CMB data (temperature, polarisation and lensing) and finds 3.7σ evidence for γ greater than 0.55. Continue reading →
Stuart tells us about his recent work exploring how to use peculiar velocity measurements to constrain modified gravity (specifically f(R) and DGP models). He finds that even using just linear scales we would be able to detect or rule out model parameter regions that would be entirely consist with current measurements of the background expansion.
He does this by predicting the auto and cross spectra between galaxy overdensity and peculiar velocity, using just linear theory – and then analysing those observables in modified gravity simulations to measure the parameters used in the simulations.
At this point it is just a proof of concept, as for each modified gravity scenario the method is only constraining the linear galaxy bias and one model parameter (other cosmological parameters are known) and also not taking into account additional observational uncertainties involved in measuring peculiar velocities in the real world.
Still, the method does work, at least when the effect produces a large enough deviation from LCDM so the proof of concept works. Future careful work with 6DF and SDSS data (and, one day soon, DESI) should allow the method to bring about real constraints on these models.
Johannes Lange tells us about his recent work with collaborators constraining cosmological parameters, in particular S8, using the non-linear scales in the Dark Energy Survey (DES), the Kilo Degree Survey (KiDS) and the Baryon Oscillation Spectroscopic Survey (BOSS).
They generate predictions for the non-linear scales via the Aemulus suite of simulations, which ran with a variety of cosmological parameters. To connect the simulations’ halos with galaxy observations they use a halo occupation distribution (HOD) model to generate a large number of sets of mock galaxies. The halo model parameters are then marginalised over when comparing the mocks to observations, ultimately giving the final cosmology constraints.
As with more or less all other large scale structure probes nowadays they find a mild tension between their constraints and predictions for S8 coming from CMB observations and the LCDM model.
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.
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 →
Cosmology Talks 