Cosmology

The new gravitational-wave signal GW190521in LIGO and Virgo marks the first observational detection of the elusive intermediate-mass black holes. The detection also confirms there exist a new class of black holes in the mass gap predicted by the pair-instability supernovae theory. In this talk, I will discuss the process that went behind inferring the astrophysical properties of this historic discovery. I would briefly address the alternative scenarios we looked into for a possible exotic origin of this signal, including any violation of General Relativity. For the upcoming ESA/NASA space mission LISA, I would highlight how this discovery opens a unique epoch of multi-band, multi-messenger astronomy.

The outskirts of accreting dark matter haloes exhibit a sudden drop in density delimiting the virialized region. After briefly describing the physics shaping this feature and how it is measured, I will discuss its applications. I will examine its connection to structure formation and how it can constrain the screening mechanisms of beyond-GR models of gravity.

Studying the smallest self-bound dark matter structure in our Universe can yield important clues about the fundamental particle nature of dark matter, and galaxy-scale strong gravitational lensing provides a unique way to detect and characterize dark matter on small scales at cosmological distances from the Milky Way. Research in this field can be broadly separated into works that aim to directly detect individual perturbers and works that aim to statistically constrain the matter distribution by looking at collective perturbations caused by an unresolved population of perturbers. We present recent advances in both of these approaches. With respect to the former, we present advances in the analysis of gravitational lenses and identification of small-scale clumps using machine learning. With respect to the latter, we introduce the convergence power spectrum as a promising statistical observable that can be extracted from strong lens images and used to distinguish between different dark matter scenarios, showing how different properties of the dark matter get imprinted on different scales. We include a discussion on the different contribution of substructure and line-of-sight structure to perturbations in strong lens images.

Mapping of galaxy density fluctuations on large scales is one of the most important goals of observational cosmology in this decade. These observations can significantly improve our knowledge of the universe, its origins and composition. In this talk I will review some of the science goals of the ongoing and future spectroscopic galaxy surveys and explain how these goals can be met. In particular, I will focus on some recent progress in theoretical modelling of the nonlinear structure formation and show how it can be used to extract cosmology from observations of the cosmic web. As an example I will present several analyses from the publicly available Baryon Oscillation Spectroscopic Survey (BOSS) data and results for the standard LCDM model as well as several relevant extensions.

Current and forthcoming observing runs at ground-based laser interferometry detectors are starting to uncover gravitational waves from binary black hole (BBH) mergers at cosmological distances, and a fraction of them are expected to be gravitationally lensed by intervening galaxy or cluster lenses with multiple images. Such strongly lensed events, if discovered, may offer a precious opportunity to localize BBH host galaxies and probe global and small-scale property of the lens mass profile. We investigate multiple BBH events showing parameter coincidence in the LIGO/Virgo O2 run, and search for additional sub-threshold signals that may be fainter lensed images. For the first time, we factor in the effect of the Morse phase shift in the analysis, and demonstrate how to measure the relative Morse phase via joint parameter inference. We confirm curiously high level of intrinsic and extrinsic parameter coincidence between GW170814 and GW170104, and uncover a third sub-threshold candidate lensed image, GWC170620, in a single-template search, which amounts to an estimated 10^-4 overall chance of statistical fluke. The measured relative Morse phases among the three events, although consistent with ray-optics lensing, point toward a complicated and unexpected image topology with a magnified image at a local maximum of the Fermat potential, which however casts doubt on the lensing hypothesis. The long time delays on the order of months necessarily require a massive lens of galaxy cluster scale. If a genuine set of multiple lensed images, we localize the source to ~ 16 deg^2 on the sky and suggest a range 0.4 < z < 0.7 for its redshift. Optical follow-up observations are encouraged to collect any additional information that may further shed light on the case.