Cosmology and Gravitation

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.

Motivated by the question of how inflation started, we propose a Euclidean preparation of an asymptotically AdS2 spacetime that contains an inflating dS2 bubble. The setup can be embedded in a four dimensional theory with a Minkowski vacuum and a false vacuum. AdS2 times 2-sphere approximate the near horizon geometry of a 4d near-extremal RN wormhole. Likewise, in the false vacuum the near-horizon geometry of a near-extremal black hole is approximately dS2 times 2-sphere. We interpret the Euclidean solution as describing the decay of an excitation inside the wormhole to a false vacuum bubble. The result is an inflating region inside a non-traversable asymptotically Minkowski wormhole.

The Hubble tension is conventionally viewed as that between the cosmic microwave background (CMB) and the SH0ES measurement. A prominent proposal for a resolution of this discrepancy is to introduce a new component in the early universe, which initially acts as "early dark energy" (EDE), thus decreasing the physical size of the sound horizon imprinted in the CMB and increasing the inferred H_0, bringing it into near agreement with SH0ES. However, this impacts cosmological observables beyond the CMB -- in particular, the large scale structure (LSS) of the universe across a range of redshift. The H_0 tension resolving EDE cosmologies produce scale-dependent changes to the matter power spectrum, including 10% more power at k=1 h/Mpc. Motivated by this, I will present the results of two analyses of LSS constraints on the EDE scenario. Weak lensing and galaxy clustering data (from, e.g., the Dark Energy Survey) significantly constrain the EDE model, and the resulting H_0 is in significant tension with SH0ES. Complementary to this, including data from the Baryon Oscillation Spectroscopic Survey (BOSS), analyzed using the effective field theory (EFT) of LSS,  yields an EDE H_0 value that is in significant (3.6\sigma) tension with SH0ES. These results indicate that current LSS data disfavours the EDE model as a resolution of the Hubble tension, and, more generally, that the EDE model fails to restore cosmological concordance. A sensitivity forecast for EUCLID suggests that future LSS surveys can close the remaining parameter space of the model.

Modeling galaxy-dark matter connection is essential in deriving unbiased cosmological constraints from galaxy clustering observations. We show that a more physically motivated galaxy-dark matter incorporating secondary (assembly) biases results in more accurate predictions of galaxy clustering, yields novel insights into effects such as baryonic feedback, and significantly reduces the tension in galaxy lensing. We further present progress and opportunities in building a multi-tracer galaxy-dark matter connection framework that is rich in features and highly efficient, enabling more robust cosmological analyses with upcoming DESI data.

Stellar streams are a powerful tool for mapping of the Galactic matter distribution. Thanks to Gaia we now have full 6D (complete phase-space) view of a quickly growing subset of the Milky Way streams. These stars are precious as they can shed light not only on the broad-brush structure of the Galaxy, but also reveal small time-evolving perturbations of the Galactic potential. I will use the Sagittarius and the Orphan streams to present the most recent measurements of the dark matter halo of the Milky Way and one of its sub-halos.

With much of the cosmological information in the primary CMB having already been mined, the next decade of CMB observations will revolve around the secondary CMB lensing effect, which will touch nearly all aspects of observation in some way. At the same time, the increasingly low noise levels of these future observations will render existing "quadratic estimator" methods for analyzing CMB lensing obsolete. This leaves us in an exciting place where new methods need to be developed to fully take advantage of the upcoming generation of CMB data just on our doorstep. I will describe my work developing such new lensing analysis tool, made possible by Bayesian methods, modern statistical techniques, and borrowing ideas from machine learning. I will present the recent first-ever application of such methods to data (from the South Pole Telescope; https://arxiv.org/abs/2012.01709) and discuss prospects for this analysis in the future with regards to not just lensing but also primordial B modes, reionization, and extragalactic foreground fields.