Cosmology

Recent years have seen the paradigm of effective field theory (EFT) successfully applied to an increasing number of classical systems that range from the gravitational inspiral of compact binaries to hydrodynamics. Many of these systems exhibit dissipation in one form or another, such as radiation reaction or viscous fluid flow, that naturally results from the system being open. This "openness" can manifest as energy leaving the dynamical variables of interest via radiation or heat transfer, for example. As the EFT approach typically utilizes the action, and hence Hamilton's Principle of Extremal Action, it is crucial to determine how generally and consistently to accommodate dissipative effects in a variational principle. In this talk, I discuss why Hamilton's Principle fails to incorporate dissipation. I then provide a reformulation that has been used successfully to confirm well-established results as well as to provide new predictions regarding dissipative systems. I show specific examples drawn from EFT applications. Finally, I show how this reformulation of Hamilton's Principle turns out to correspond to the classical limit of quantum theories based on the so-called "in-in" or "closed-time-path" approaches.

After the first landmark gravitational-wave (GW) detection, GW astronomy will turn to the study of detector data to identify the physical properties of GW sources. The science payoff of GW observations must therefore depend critically on the accurate knowledge of the shapes of waveforms as functions of the source parameters. Effective-field-theory techniques have advanced and continue to advance the state of the art for the modeling of inspiraling-binary dynamics. But how far do we have to push our calculations to satisfy the needs of observations? I review current attempts to answer this question for different detectors and sources, and for the delicate problem of matching post-Newtonian and numerical-relativity waveforms.

The study of the anisotropies in the cosmic microwave background radiation over the past two decades has provided us with important information about the early universe. In particular, there is strong evidence that these anisotropies were generated long before the cosmic microwave radiation was emitted. The most commonly studied idea is that they originated as quantum fluctuations during a period of inflation. In addition to a spectrum of scalar perturbations consistent with the one that has been observed, inflation also predicts the presence of gravitational waves. These might be observable in the polarization of the cosmic microwave background. An observation of this signal would indicate that the inflaton must have traversed a super-Planckian distance. Realizing this in string theory has been challenging. I will describe the basic ingredients for a string theoretic setup in which the inflaton can move over a super-Planckian distance, leading to an observable gravitational wave signal within string theory. In addition to an observable tensor signal, the model may also lead to other interesting signatures which I will discuss such as modulations in the power spectrum of scalar perturbations, interesting shapes of non-Gaussianities, and possibly the formation of oscillons at the end of inflation.

General relativity is a covariant theory of two transverse, traceless graviton degrees of freedom. According to a theorem of Hojman, Kuchar, and Teitelboim, modifications of general relativity must either introduce new degrees of freedom or violate the principle of general covariance. In my talk, I will discuss modifications of general relativity that retain the same number of gravitational degrees of freedom, and therefore explicitly break general covariance. Motivated by cosmology, the modifications of interest maintain spatial covariance. Demanding consistency of the theory forces the physical Hamiltonian density to obey an analogue of the renormalization group equation, which encodes the invariance of the theory under flow through the space of conformally equivalent spatial metrics.

Problematic growths of curvature and anisotropy are found in nonsingular bouncing cosmologies that include both an ekpyrotic phase and a bouncing phase. Classically, initial curvature and anisotropy that are suppressed during the ekpyrotic phase will grow back exponentially during the nonsingular bouncing phase. Besides, curvature and shear perturbations are generated by quantum fluctuations during the ekpyrotic phase. In the bouncing phase, an adiabatic curvature perturbation grows to dominate and gives rise to a blue spectrum that spoils the scale-invariance. Meanwhile, a scalar shear perturbation grows nonlinear and creates an overwhelming anisotropy that disrupts the nonsingular bounce altogether. We examine the common origin of these problems and discuss possible ways to avoid them.

The talk consists of two parts: (1) Quasi-single inflation, where the isocurvature direction has mass of order Hubble parameter. This part is based on 0911.3380 and new results about higher mass, and a sharp turn in trajectory. (2) Multi-stream inflation, where the inflationary trajectory bifurcates. This part is based on 0903.2123, 1006.5021 and a on-going project on calculating the bifurcation probability in a complicated landscape.

Cosmic strings are a generic prediction of Grand Unified Theories that can leave a sufficient imprint in the Cosmic Microwave Background anisotropies to open an observational window into an otherwise unreachable high energy domain. Being formed as topological defects of a Higgs field, they are also naturally coupled to various other fields, that can lead to superconducting-like currents, hence radically changing their structure and properties. After having summarized the standard string network behaviour and its cosmological effects, I will concentrate on the current-carrying properties and show how those can modify drastically the overall picture. In particular, I will exhibit the many current case, including the special non abelian situation that requires more care to be fully understood.

Several mechanisms can lead to production of particles during inflation. I discuss how this phenomenon can induce a contribution to the primordial spectrum of gravitational waves with unusual properties: the tensors produced this way can violate parity; can have a large three-point function; can have a relatively large tensor-to-scalar ratio even if inflation occurs at low energies; finally, their spectrum can display a feature that can be directly detected by second-generation gravitational interferometers such as advanced LIGO.

The time variation of physical constants has been much discussed in the literature, motivated by claims of fine structure constant variations together with several theoretical ideas. Although it is well understood (by most, but not all!) cosmologists that one must consider only dimensionless constants, most discussions of the strength of gravity involve "G", which is of course dimensional. I discuss some applications of variations of "G" on cosmological observables, stressing the need to stay dimensionless. "Constants" might also vary in space. An idea which is perhaps less crazy is that cosmological parameters might vary across the observable Universe. I show how this leads to dipole modulation, which corresponds to a correlation between neighbouring multipoles in maps of the cosmic microwave sky. Searches for such signals could lead to constraints on the variation of the cosmological model on the largest accessible scales.