Cosmology

We have great certainty on how gravity works around our solar system: General Relativity (GR) has been found to be very accurate at these small scales. On large scales though, we still have a considerable lack of understanding about the evolution of the universe, and its constituents. While the LCDM model is in good agreement with cosmological data, this might change in the future. For this reason, we need to test GR on these scales.

There are a number of proposals on how to characterize deviations from GR on larges scales, by parametrizing different cosmological evolutions of the universe - the PPF, EFT and EA approaches. The objective is to use experimental data to constrain these parameters, and thus identify the most accurate cosmological model. In this talk I will show an alternative, systematic and general, parametrization method, in which we construct the most general quadratic action for linear cosmological perturbations, given some field content and gauge symmetries. I will show the example of linearly diffeomorphism-invariant scalar-tensor theories, in which case the parametrization encompasses well-known theories such as Horndeski and Beyond Horndeski. The method can straightforwardly be applied to any gravity theory with any fields and gauge symmetries.

I will discuss how we constrain properties of the universe using two
tracers of large scale structure measured by the BOSS (Baryon
Oscillations Spectroscopic Experiment): galaxies and Lyman-alpha
forest. I will show recent results from baryonic acoustic oscillations
measured in both tracers and discuss cosmological implications. I
will briefly mention other measurements and consider forecasts for
quasar-forest bispectrum to constrain primordial non-Gaussianity.

We are currently in an era of precision cosmology, but is it an era of accurate cosmology? By measuring cosmological parameters with many independent probes we can convince ourselves that our measurements of the parameters are indeed correct. Thanks to recent progress, strong gravitational lensing is now a powerful probe of cosmology. In this talk I'll report on a measurement of H0 at 5% precision using two strongly lensed quasars and a 20% measurement on the equation of state of dark energy using a double source plane lens. I'll also present new results from a z=1 strong lensing cluster where the dark matter profile deviates significantly from the NFW profile that is predicted by cold dark matter simulations, but it is consistent with simulations of self-interacting dark matter. I'll finish by discussing how upcoming wide area surveys can provide hundreds of exotic strong lenses that can be used for precise and accurate cosmology over the next decade.

Spectral distortions of the CMB provide a powerful new probe of early Universe processes. Even if so far no average spectral distortion has been seen, LCDM does predict several signals that are within reach of current technology. In this talk, I will give a broad brush overview of our most recent understanding of the formation and evolution of distortions in the early Universe, highlighting guaranteed LCDM signals and what we hope to learn from them about the Universe we live in.

Gravitational lensing by matter clumps can magnify various transient bursts in the sky, making them more detectable from the high redshift Universe. For one example, chirping gravitational waves from stellar-mass black hole binary mergers, as first detected by LIGO recently, can appear louder due to intervening galaxies. In the absence of electromagnetic counterpart, as I will discuss, lensing magnification can bias the determination of binary mass and redshift, which needs to be corrected for when testing source evolution and formation models through event statistics. As another example, compact dark matter of masses 10-100 solar masses can gravitationally lens fast radio bursts, creating double-peaked time-domain signature with a resolvable time delay on the order of milliseconds. I discuss that forthcoming fast radio burst surveys can directly probing this interesting mass range for compact dark matter by detecting 10^4 bursts per year.