Cosmology and Gravitation

Understanding the physics of galaxy formation is arguably among the greatest problems in modern astrophysics. Recent cosmological simulations have demonstrated that "feedback" by star formation, supernovae and active galactic nuclei appears to be critical in obtaining realistic disk galaxies, to slow down star formation to the small observed rates, to move gas and metals out of galaxies into the intergalactic medium, and to balance radiative cooling of the low-entropy gas at the centers of galaxy clusters. However the particular physical processes underlying this "feedback" still remain elusive. In particular, these simulations neglected cosmic rays and magnetic fields, which provide a comparable pressure support in comparison to turbulence in our Galaxy, and are known to couple dynamically and thermally to the gas. Using hydrodynamic simulations of galaxy formation, I will show how cosmic rays are able to drive powerful galactic winds in low-mass galaxies. This reduces the available amount of gas for star formation and implies a shallower slope of the faint-end of the galaxy luminosity function as required by observations. In the second part of the talk I demonstrate that cosmic-ray heating can balance radiative cooling of the low-entropy gas at the centers of galaxy clusters and helps in mitigating the star formation of the brightest cluster galaxies. New data on the low-frequency radio and gamma-ray emission of M87, the closest active galaxy interacting with the cooling cluster plasma, enable us to put forward a comprehensive, physics-based model of feedback by active galactic nuclei.

Weak gravitational lensing is a highly valued tool for inferring the structure of the spacetime metric between an observer and a cosmologically distant “wallpaper,” most commonly either the CMB or faint background galaxies. The best-measured quantities are the second derivatives of the projected scalar potential(s), which are manifested as apparent shearing and magnification of the wallpaper.  Given a collection of faint-galaxy images, what information can we extract about the shear and magnification that these images have undergone?  I will describe a new method of lensing inference that, unlike predecessors, is rigorously correct in the presence of noise and other observational realities, nearly optimal, and computationally feasible at the scale of current/future surveys like the Dark Energy Survey and the Large Synoptic Survey Telescope.

Photometric surveys are often larger and extend to fainter magnitudes than spectroscopic samples, and can therefore yield more precise cosmological measurements. However, photometric data are significantly contaminated by multiple sources of systematics, either intrinsic, observational, or instrumental. These systematics affect the properties of the raw images in complex ways, propagate into the final catalogues, and create spurious spatial correlations. Some of these correlations may also be imprinted in spectroscopic catalogues, since the latter rely on targets selected from imaging data. Therefore, not just precise — but also accurate — cosmological inferences from imaging surveys require careful mitigation of spatially-varying systematics. I will present a new framework of extended mode projection to robustly mitigate the impact of such systematics on power spectrum measurements. I will demonstrate the effectiveness of the technique, showing constraints on primordial non-Gaussianity using the clustering of 800,000 photometric quasars from the Sloan Digital Sky Survey in the redshift range 0.5 < z < 3.5. Finally, I will present a framework to map the observing conditions of the Science Verification data in the Dark Energy Survey (DES) and incorporate them into end-to-end simulations of the DES transfer function. The considerations presented here are relevant to all multi-epoch surveys, and will be essential for exploiting future high-cadence surveys such as the Large Synoptic Survey Telescope (LSST), which will require detailed null-tests and realistic end-to-end image simulations for correct cosmological inferences.

More than a decade after its discovery, cosmic acceleration still 
poses a puzzle for modern cosmology and a plethora of models of dark energy 
or modified gravity, able to reproduce the observed expansion history, have 
been proposed as alternatives to the cosmological standard model. In recent 
years it has become increasingly evident that probes of the expansion his- 
tory are not sufficient to distinguish among the candidate models, and that 
it is necessary to combine those with observations that probe the dynamics 
of inhomogeneities. Future cosmological surveys will map the evolution of 
inhomogeneities to high accuracy, allowing us to test the relationships be- 
tween matter overdensities, local curvature, and the Newtonian potential on 
cosmological scales.

I will discuss theoretical issues involved in finding an optimal framework to 
study deviations from General Relativity on cosmological scales, giving an 
overview of recent progress, with a focus on model-independent, parametrized 
approaches. I will summarize where we stand and what are the next steps 
we should take.

Given a large landscape of vacua that statistically favors large values of the neutrino mass sum, $m_\nu$, I will present the probability distribution over $m_\nu$ obtained by weighting this prior by the amount of galaxies that are produced. Using Boltzmann codes to compute the smoothed density contrast on Mpc scales, we find that large dark matter halos form abundantly for $m_\nu \gtrsim 10$\,eV. However, in this regime structure forms late and is dominated by cluster scales, as in a top-down scenario. I will argue that this change of regime is catastrophic: baryonic gas will cool too slowly to form stars in an abundance comparable to our universe. Upon implementing this cooling boundary, the anthropic prediction for $m_\nu$ is consistent at better than $2\sigma$ with the entire range of values allowed by current experimental bounds, $58$\,meV $\leq m_\nu \lesssim 0.23$\,eV. A degenerate hierarchy is mildly preferred. Without a catastrophic boundary at or below $10$\,eV, the theoretical expectation would conflict strongly with the observed mass range. Thus the asserted cooling failure can be regarded as a prediction of the anthropic solution to the neutrino mass problem.

Visible matter consists mostly of hydrogen and helium, only a small fraction
of which is in stars. Until recently, the bulk of the gas in the local
universe was in fact not seen. In the largest structures, massive galaxy
clusters, the gas is seen via its x-ray emission, but in the much more
numerous groups and isolated galaxies, it has not been possible to detect
it. I will describe how, in the last year or so, the situation has changed,
with the detection of a cross-correlation between the thermal SZ effect and
lensing maps, and through the stacking of SZ images at the locations of
galaxies. These and similar techniques will tell us about the physics by
which stars and massive black holes heat and move gas in and around the
halos of galaxies and cluster, and will allow for tighter constraints on the
value of the primordial power spectrum (sigma_8).

The cosmic microwave background contains a wealth of
information about cosmology as well as high energy physics. It tells
us about the composition and geometry of the universe, the properties
of neutrinos, dark matter, and even about the conditions in our
universe long before the cosmic microwave background was emitted.
After a brief review of what we may hope to learn from studies of the
cosmic microwave background about the early universe, I will review
measurements of the angular power spectrum of temperature
perturbations from the first 15.5 months of Planck data by the Planck
collaboration and by Renee Hlozek, David Spergel and myself. I will
then discuss the implications for the early universe of the recently
released Planck full mission data as well as the joint analysis
between BICEP/KeckArray and Planck.

An exciting and largely unexplored frontier in observational and theoretical cosmology is to understand the properties of the universe between 400,000 years and one billion years after the big bang. Notably, the first galaxies formed in this time period, perhaps a few hundred million years after the big bang.  These galaxies strongly influenced the gas in their surroundings as well as the formation of subsequent generations of galaxies. The early galaxies emitted ultraviolet light and ionized "bubbles" of hydrogen gas around them.  These ionized bubbles grew, merged, and eventually filled the entire volume of the universe with ionized hydrogen in a process known as reionization. Understanding this process will constrain the properties of the first luminous sources, and fill in a significant gap in our story of structure formation, whereby the universe transitions from simple initial conditions to its present day complexity. I will briefly summarize current observational constraints and describe some new ideas for better determining when the reionization process completed using existing Lyman-alpha forest data.

It is well known that S-matrix Analyticity, Lorentz invariance and Unitarity place strong constraints on whether Effective Field Theories can be UV completed. A large class of gravitational field theories such as Massive Gravity and DGP inspired braneworld models contain as limits Galileon theories which in the past have been argued to violate the conditions necessary for a UV completion. I will present arguments that imply quite the opposite, although Galileons do not admit a standard Wilsonian UV completion, I will use the recently discovered Galileon duality to argue that these theories can exhibit a non-Wilsonian completion consistent with the `Classicalization’ proposal. These theories have quintessentially gravitational properties such as non-polynomially bounded scattering amplitudes, absence of local off-shell observables and an exponentially soft 2-2 scattering amplitude. These properties show up in a violation of the standard Wightman axioms and are consistent with many of the known properties of UV completions of gravity. I will argue that the superluminal solutions found in the low energy EFT are absent in the UV completion.

The last years have seen a renewed interest in theories of massive gravity. They represent an infra-red modification of gravity where the gravitational force weakens at very large scales. Heuristically, they provide the playground to understand a possible modification of GR which could potentially provide a dynamical solution to the cosmological constant problem. In this talk I will discuss a number of theoretical aspects of massive gravity theories, focusing on the relevance of the so-called Vainstein mechanism, both at the classical and the quantum level. I will also discuss how we can use cosmology from the early and late universe to constrain these theories. For example, what does the Cosmic Microwave Background Radiation know about the graviton mass?

This talk will cover a number of fun topics related to studying and understanding the Cosmic Microwave Background, including where it came from, how it is like an inside-out star, CMB numerology, how much information it contains and how it is evolving in time.