Cosmology and Gravitation

How did inflation actually happen? Precision measurements of statistical properties of primordial fluctuations generated during inflation offer a direct probe of the physics of inflation. When we calculate statistical properties of primordial fluctuations generated during inflation, we usually assume that the initial state of quantum fluctuations is in a preferred vacuum state called Bunch-Davies vacuum. While there is some motivation for choosing such a state, this is an assumption, and thus needs to be tested by observations. In this talk I will present new probes of initial state of quantum fluctuations during inflation: the 3-point function of the cosmic microwave background anisotropy, the 2-point function of galaxies, and a spectral distortion of the thermal spectrum of the cosmic microwave background.   

In this talk I provide a model where the late time acceleration of the universe emerges from a BCS-like condensation of sterile neutrinos. This scenario can be naturally accommodated by general relativity covariantly coupled to sterile neutrinos, where the neutrinos act like an "aether" field.  We show that when active neutrinos couple to the neutrino condensate, they oscillate at a rate proportional to the dark energy density.  As a result, the oscillation of neutrinos and dark energy are tied in with the same mechanism.  I also show that neutrinos could oscillate even if dark energy is not a neutrino condensate.  I end with a discussion of stability of this model and predictions of CPT violating oscillations of such models.

With the emergence of the dark sector in cosmology,  a variety of modified theories of gravity have come to the fore. I will discuss a framework which can be used to test gravity on large scales and the observational programmes that might lead to the tightest constraints.

I will discuss string cosmology and the dynamics of multiple scalar fields in potentials that can become negative, and their features as (Early) Dark Energy models. The point of departure is the ``String Axiverse'', a scenario that motivates the existence of cosmologically light axion fields as a generic consequence of string theory. These fields can constitute part of the Dark Matter, suppressing structure formation in a manner similar to massive neutrinos. Future observations will constrain their existence to percent level accuracy. I couple such an axion to its corresponding modulus and give a detailed presentation of the rich cosmology of such a model, ranging from the setting of initial conditions on the fields during inflation, to the asymptotic future. A dynamical systems analysis reveals the existence of many fixed point attractors, repellers and saddle points, which I analyse in detail, and provide a geometric interpretation of. These fixed points can be used to bound the couplings in the model. A systematic scan of certain regions of parameter space reveals that the future evolution of the universe in this model can be rich, containing multiple epochs of accelerated expansion.

(Given time) I will also discuss the relevance of isocurvature perturbations in these models, and the motivation to study negative (terminal) vauca in the context of eternal inflation and the string landscape, as recently discussed by Susskind.

(based primarily on Marsh, Tarrant, Copeland and Ferreira (to appear) but also arXiv:1102.4851  arXiv:1110.0502)

We review the notion of a quantum state of the universe and its role in fundamental cosmology. Then we discuss recent work which points towards a profound connection, at the level of the quantum state, between (asymptotic) Euclidean AdS spaces and Lorentzian de Sitter spaces. This gives a new framework in which (a mild generalization of) AdS/CFT can be applied to inflationary cosmology.   For the specific case of the Hartle-Hawking no-boundary quantum state the ADS/ de Sitter connection yields a natural proposal for a more precise `dual' formulation of the wave function, in terms of field theories on the future de Sitter boundary that are certain relevant deformations of the CFTs that occur in AdS/CFT.

The curvaton scenario provides a simple explanation for the generation of the cosmological perturbations, however most works have focused on cases with rather trivial curvaton energy potentials, e.g. quadratic ones. In this talk I will present the rich phenomenology of curvatons by showing that non-quadratic curvatons exhibit new behaviors, leading to interesting signals in the resulting density perturbations. A string theory realization of the curvaton scenario will also be discussed, where D-branes located in a warped throat region of the internal space play the role of curvatons.

In this talk I will present evidence that accounting for the presence of hierarchies in string compactifications naturally leads to a UV sensitivity of dark matter in contrast to what is usually assumed. In particular, we will see that the existence of cosmological moduli may lead to a non-thermal history for the early universe and modifications in the primordial production of dark matter. If such a history were realized it would not only require probing new regions in dark matter searches, but also imply that a detection of dark matter would provide a direct probe on the early universe and the UV -- contrary to the thermal WIMP case. Regardless of the history of the early universe I will argue that if current string constructions are representative of more general models then all weak-scale dark matter would indeed be UV sensitive and would be a new prediction of string theory - falsifiable by experiment.

Dwarf galaxies are the most know dark matter dominated luminous objects in Universe. Observing the line of sight velocity and position of stars in Milky way satellites, and assuming the dark matter potential and a specific configuration of stellar orbits, one can obtain the mass profile of dark matter in galaxies. In this talk I will show that by considering a generic case of  phase-space density as a function of energy and angular momentum and by relaxing the specific choice of orbital configuration (which is assumed in literature), we can find  the dark matter potential and mass profile in this general case and restudy the challenges of Cold Dark Matter(CDM) paradigm like core-cusp or missing satellite problem.

I will discuss a wide class of models which realise a bounce in a spatially flat Friedmann universe in standard General Relativity. The key ingredient is a noncanonical, minimally coupled scalar field
belonging to the class of theories with Kinetic Gravity Braiding/Galileon-like self-couplings. In these models, the universe smoothly volves from contraction to expansion, suffering neither from ghosts
nor gradient instabilities around the turning point. The end-point of he evolution can be a standard radiation-domination era or an nflationary phase.
The talk is based on arXiv:1109.1047v2 [hep-th], JCAP11(2011)021.

Recent progress in massive gravity has made it possible to construct consistent theories of interacting spin-2 fields.  In this talk I'll describe these developments, focusing on the resolution of the Boulware-Deser ghost problem and the promotion of massive gravity to a bimetric theory of gravity with two dynamical, interacting spin-2 fields.  I'll then discuss the generalization of these bimetric theories to theories of multiple interacting spin-2 fields.