Cosmology

I will discuss distinctions between dark energy and modified gravity explanations of cosmic acceleration from the horizon scale to the deeply non-linear regime using the modified action f(R) and braneworld DGP models as worked toy examples.

Observations are opening new windows to test general relativity at cosmological scales. In this talk, I will discuss how gravity determines the expansion and structure formation of the universe, what smoking guns of gravity in the cosmos we are expecting, what difficulties we are facing to perform unambiguous tests of gravity and what are possible ways to overcome these difficulties.

Non-linear structures in the universe leave characteristic imprints in the cosmic microwave background. These include Compton scattering (Sunyaev-Zeldovich effects) and gravitational lensing. The South Pole Telescope now has a catalog of massive galaxy clusters that were discovered this way, along with a measure of the background fluctuations generated by smaller clusters, that can be used to chart the growth of structure in the universe.

We have announced the results from 7 years of observations of the Wilkinson Microwave Anisotropy Probe (WMAP) on January 26. In this talk we will present the cosmological interpretation of the WMAP 7-year data, including the detection of primordial helium, images of polarization of microwave background around temperature peaks, and new limits on inflation and properties of neutrinos. We also report a significant detection of the Sunyaev-Zel'dovich effect and discuss implications for the gas pressure in clusters of galaxies.

We present a holographic description of four-dimensional single-scalar inflationary universes in terms of a three-dimensional quantum field theory. The holographic description correctly reproduces standard inflationary predictions in their regime of applicability. In the opposite case, wherein gravity is strongly coupled at early times, we propose a holographic description in terms of perturbative QFT and present models capable of satisfying the current observational constraints while exhibiting a phenomenology distinct from standard inflation. This provides a qualitatively new method for generating a nearly scale-invariant spectrum of primordial cosmological perturbations.

The primordial density fluctuations that seeded large-scale structure are known to be nearly Gaussian, as predicted by most early universe models like slow-roll inflation. Many of these models predict a small (but nonzero!) amount of primordial non-gaussianity, which can subtly affect the statistics of CMB anisotropies. Surprisingly, even a small primordial non-gaussianity can produce enormous changes in the large-scale clustering of galaxies and quasars at late times. I will describe the origin of this effect, and review recent constraints on non-gaussianity using measurements of the clustering of galaxies and quasars in SDSS.

We discuss a candidate mechanism through which one might address the various cosmological constant problems. We observe that the renormalization of gravitational couplings manifests non-local modifications to Einstein's equations as quantum corrected equations of motion, and in doing so offers a complimentary realization of the degravitation paradigm-- a realization through which its non-linear completion and the corresponding modified Bianchi identities are readily understood. We proceed to consider theories whose coupling to gravity might a priori induce non-trivial RG flow for gravitational couplings in the IR, and arrive at a class of non-local effective actions which yield a suitably degravitating filter function for Newton's constant upon subsequently being integrated out.

After prodigious work over several decades, binary black hole mergers can now be simulated in fully nonlinear numerical relativity. However, these simulations are still restricted to mass ratios q = m2/m1 > 1/10, initial spins a/M < 0.9, and initial separations r/M < 10. Fortunately, analytical techniques like black-hole perturbation theory and the post-Newtonian approximation allow us to study much of this region in parameter space that remains inaccessible to numerical relativity. I will use black-hole perturbation theory to establish a fundamental upper limit to the final spin that can be attained through binary mergers, and show how this limit can be used to improve predictions of final spins for finite mass ratios as well. I will also show that post-Newtonian inspirals between 1000 M < r < 10 M can align or anti-align black hole spins with each other, dramatically changing the distributions of final spins and recoil velocities that would be expected in astrophysical black hole mergers.

The hot, gaseous atmospheres of galaxies and clusters of galaxies are repositories for the energy output from accreting, supermassive black holes located in the nuclei of galaxies. X-ray observations show that star formation fueled by gas condensing out of hot atmospheres is strongly suppressed by feedback from active galactic nuclei (AGN). This mechanism may solve several outstanding problems in astrophysics, including the numbers of luminous galaxies and their colors, and the excess number of hot baryons in the Universe. The most energetic AGN outbursts may be powered by rapidly-spinning, ultra-massive black holes.

Stellar evolution from a protostar to neutron star is of one of the best studied subjects in modern astrophysics. Yet, it appears that there is still a lot to learn about the extreme conditions where the fundamental particle physics meets strong gravity regime. After all of the thermonuclear fuel is spent, and after the supernova explosion, but before the remaining mass crosses its own Schwarzschild radius, the temperature of the central core of the star might become higher than the electroweak symmetry restoration temperature. The source of energy, which can at least temporarily balance gravity, are baryon number violating instanton processes which are basically unsuppressed at temperatures above the electroweak scale. We constructed a solution to the Oppenheimer-Volkoff equation which describes such a star. The energy release rate is enormous at the core, but gravitational redshift and the enhanced neutrino interaction cross section at these densities make the energy release rate moderate at the surface of the star. The lifetime of this new quasi-equilibrium can be more than ten million years, which is long enough to represent a new stage in the evolution of a star.