Astrophysics

The large scale distribution of matter in the Universe contains the answers to many mysteries, such as the nature of dark matter, the reionization of the Universe, and the growth of galaxies. Cosmological simulations are the only way to understand these questions. I will talk about how modern current simulation models, work, discuss some new models and improvements in our latest simulation runs, especially our implementations of reionization and cosmology. I will then talk about some new work to dramatically expand the region of applicability of these simulations using machine learning. This can both to expand their dynamic range and combine different simulations to infer the physical parameters of the Universe.

Redshift space distortions and weak gravitational lensing have been used as a powerful combination of growth data to test gravity. In particular, combining these two probes where spectroscopic and photometric surveys overlap, can yield much stronger dark energy and growth constraints than a combination of independent measurements of the two. However, this approach is limited due to a fairly small overlapping area between spectroscopic and photometric surveys. In this talk, I will introduce a new method to optimally combine spectroscopic and photometric surveys, using the DMASS galaxy sample as gravitational lenses. The new approach can extract the full statistical power of photometric surveys beyond the overlapping area. I will illustrate how this approach with DMASS improves cosmological constraints in the frame of modified gravity and will show its application to future surveys having a limited overlap such as DESI and LSST.

In this talk, I will present recent results about gravitational wave cosmology. I will argue that the spatial clustering of gravitational wave sources provides a wealth of invaluable information concerning the origin of binary black holes and the propagation law of gravitational waves. The former can clarify whether the observed black hole binaries are of stellar or primordial origin. The latter is important for constraining deviations from General Relativity on cosmological scales because such deviations predict modified propagation of gravitational waves compared to General Relativity. I will then explore the possibility of observed black holes having primordial origin and present its consequences for expected merger rates of such black holes and neutron stars. Time permitting, I will also summarize our recent progress made in the field of primordial gravitational waves and the implications for the inflationary models.

Although cold dark matter (CDM) has been established, this is only the case for measurements at large scales, which are larger than galaxy-sized structures. Even though we need to understand the important role of baryonic components, matter distribution at small scales can be the key to distinguishing different particle dark matter candidates. In fact, warm dark matter, self-interacting dark matter, and fuzzy dark matter have been proposed, yielding different matter distributions at sub-galactic scales. These small-scale distributions have been studied with numerical simulations. Whereas very reliable, numerical simulations suffer certain issues. They are limited by both numerical resolution and shot noise. They tend to take a lot of computational time. These might be a bottleneck for surveying through multi-dimensional parameter space of various dark matter models. I present semi-analytic models of small-scale structure, especially dark matter subhalos, which are based on structure formation theory and the tidal evolution of subhalos. Our models for CDM have been well tested against the results of various numerical simulations. The semi-analytic models are free from all the problems of the numerical simulations mentioned above. Therefore, they might be essential ingredients for identifying the particle nature of dark matter through gravitational (lensing, pulsar timing), astrophysical (satellite counts, stellar streams), and astroparticle probes (gamma rays).

The epoch of cosmic reionization can be probed using the secondary anisotropies imprinted on the cosmic microwave background (CMB) temperature and polarization field. I will discuss the imprints of patchy reionization on the kSZ power spectrum and CMB B-mode polarization. I will introduce two new scaling relations to connect the kSZ and secondary B-mode power spectrum with the physics of reionization. I will discuss the advantage of a joint study of the kSZ signal and secondary B-mode polarization from the ongoing/upcoming CMB experiments to get a better understanding of the epoch of reionization.

Inflation generically predicts a background of primordial gravitational waves, which generate a primordial B-mode component in the polarization of the cosmic microwave background (CMB). The measurement of such a B-mode signature would lend significant support to the paradigm of inflation. Observed B modes also contain a component from the gravitational lensing of primordial E modes, which can obscure the measurement of the primordial B modes. We reduce the sample variance in the BB spectrum contributed from this lensing component by a process called 'delensing.' In this talk, I will show results from the first demonstration in an improved constraint on primordial gravitational waves with delensing using data from BICEP/Keck, the South Pole Telescope (SPT), and Planck. In addition, I will provide an outlook of joint-analysis efforts of the BICEP/Keck and the SPT collaborations (the South Pole Observatory) to constrain primordial gravitational waves. 

Ground-based cosmic microwave background (CMB) experiments are now pushing into discovery space where new insights on inflation, dark matter, dark energy and neutrino physics will be obtained by unraveling signatures buried beneath the primordial fluctuations. I will present new results from the Atacama Cosmology Telescope (ACT) that exemplify the power of high-resolution measurements of the microwave sky, including high-fidelity maps of dark matter through gravitational lensing. These maps set the stage for a robust measurement of the neutrino mass scale and hierarchy with upcoming ACT and Simons Observatory data. I will also discuss a new proposal for dramatically improving the sensitivity to primordial non-Gaussianity using measurements of the cosmic velocity field, and sketch a path towards using this technique to detect or rule out multi-field inflation using a combination of upcoming CMB data from the Simons Observatory and optical galaxy data from the Vera Rubin Observatory.

The growing gravitational wave dataset makes black hole population studies possible. In this talk I will demonstrate how such studies can be used to study particle and nuclear physics. The key insight is that a wide range of initial stellar masses leave no compact remnant, due to the physics of pair-instability; the unpopulated space in the stellar graveyard is known as the black hole mass gap (BHMG). New physics can dramatically alter the late stages of stellar evolution and shift the BHMG, when it acts as an additional source of energy (loss) or modifies the equation of state. I will demonstrate how these predictions can be tested with the gravitational wave observations by the LIGO/Virgo collaboration, and show what we can already infer from GWTC-2. 

Large surveys of the positions of galaxies in the Universe are becoming increasingly powerful to shed light on some of the unsolved problems of cosmology, including the question of what caused the early Universe to expand. The analysis of the data is challenging, however, because the signal is small, the data is difficult to model, and its probability distribution is not fully known. I will present some recent ideas to approach these challenges.

I will discuss what happens when a black hole captures a much larger in
size cosmic string loop. In some cosmological scenarios, such encounters
are not unlikely for supermassive black holes in galactic nuclei, and
for primordial black holes. The talk will feature some fun physics and
geometry: non-flat quadrilaterals, black-hole superradiance,
one-dimensional geometric flows, and persistent ultra-relativistic
gravitational-wave whips.

Within the next several years pulsar timing arrays (PTAs) are positioned to detect the stochastic gravitational-wave background (GWB) likely produced by the collection of inspiralling supermassive black holes binaries, and potentially constrain some exotic physics. Searches for a GWB in real PTA data rely on Markov-Chain Monte Carlo (MCMC) analyses, which are computationally demanding and not easily accessible to non-experts. In order to develop a more intuitive understanding of what PTAs may (or may not) be able to detect, we built a simple yet realistic Fisher formalism for GWB searches with PTAs. Our formalism is able to accommodate realistic noise properties of PTAs, and allows to forecast their sensitivity not only to an isotropic GWB, but also, looking ahead, to GWB anisotropies. It moreover provides a useful tool to guide and optimize real data analysis. In this talk, I will describe the basic physics behind PTAs, then the Fisher formalism, and illustrate some applications to a real-life PTA. This talk is based on arXiv:2006.14570 and 2010.13958.