Cosmology and Gravitation

The Cosmic Microwave Background Radiation is our most important source of information about the early universe. Many of its features are in good agreement with the predictions of the so-called standard model of cosmology -- the Lambda Cold Dark Matter Inflationary Big Bang. However, the large-angle correlations in the microwave background exhibit several statistically significant anomalies compared to the predictions of the standard model. On the one hand, the lowest multipoles seem to be correlated not just with each other but with the geometry of the solar system. On the other hand, when we look at the part of the sky that we most trust -- the part outside the galactic plane, there is a dramatic lack of large angle correlations. So much so that no choice of angular powerspectrum can explain it if the alms are Gaussian random statistically isotropic variables of zero mean.

The Dark Energy might constitute an observable fraction of the total energy density of our Universe as far back as the time of matter radiation equality or even big bang nucleosynthesis. In this talk, I will review the cosmological implications of such an \'Early Dark Energy\' component, and discuss how it might - or might not - be detected by observations. In particular, I will show how assuming the early dark energy to be negligible will bias the interpretation of cosmological data.

One of the most challenging problems in theoretical physics today is the so called cosmological constant problem. While current observational constraints are consistent with the predictions of GR with a tiny cosmological constant, often referred to as the dark energy, it remains possible that it\'s the deviation of the law of gravity at large distance from Einstein\'s theory that resolves the puzzle. In this talk, I will briefly review some of the theoretical attempts made along this line, including the simple massive gravity, large extra dimensions, Unimodular gravity, classically constrained gravity, as well as their difficulties. I will then focus on some most recent study on the theory of massive graviton in de Sitter space, which may be more closely related to the reality both today and during the inflationary epoch. In particular, I would describe a model, in which one is able to open up the forbidden mass range of the graviton on a de Sitter background discovered by Higuchi.

In this talk I will discuss gravitational wave production by early universe sources. I will focus on the gravitational waves produced by a network of cosmic strings and the bounds that can be placed on cosmic string model parameters using current and future experiments. I will also talk about recent work on gravitational waves produced by sources in the early universe when the expansion of the universe cannot be neglected. As an example of such a process I will consider the preheating epoch that may follow inflation.

I\'ll discuss three promising upcoming experimental measurements that will probe the expansion history of the universe: (1) growth bases tests of Dark Energy with the Sunyaev-Zeldovich Effect, including new results from the South Pole Telescope and APEX-SZ, (2) inflationary constraints that will be provided by the next generation of CMB-polarization experiments, with prospects from EBEX, and (3) standard ruler measurements from Baryon Acoustic oscillations, including an introduction to an ambitious new Canadian Hydrogen Intensity Mapping initiative called CHIME. The talk will build on the PI-presentation last week from SPT-collaborator Jeff McMahon. I\'ll focus on the experimental aspects of each measurement, it\'s interface to theory, and spend most of the time on (3) CHIME.

WMAP measurements of CMB temperature anisotropies reveal a power asymmetry: the average amplitude of temperature fluctuations in one hemisphere is larger than the average amplitude in the opposite hemisphere at the 99% confidence level. This power asymmetry may be generated during inflation by a large-amplitude superhorizon perturbation that causes the mean energy density to vary across the observable Universe. Such a superhorizon perturbation would also induce large-scale temperature anisotropies in the CMB; measurements of the CMB quadrupole and octupole (but not the dipole!) therefore constrain the perturbation\'s amplitude and wavelength. I will show how a superhorizon perturbation in a multi-field inflationary theory, the curvaton model, can produce the observed power asymmetry without generating unacceptable temperature fluctuations in the CMB. I will also discuss how this mechanism for generating the power asymmetry will be tested by forthcoming CMB experiments.

The South Pole Telescope (SPT) is a 10-meter submm-wave telescope optimized for large-field imaging of the cosmic microwave background (CMB) at arcminute resolution. The first key project of the SPT is a large area survey to find galaxy clusters using the Sunyaev-Zel\'dovich effect. Combined with optically determined redshifts, the survey yields will be used to place constraints on the nature of dark energy, via its effect on the growth of clusters and the geometry of the universe. Working toward this goal, the SPT has surveyed two 100 square degree fields at high sensitivity. This talk will review the status of the SPT, present some of the first SZ results, and discuss future science with this instrument.

I will discuss fine tuning in modified gravity models that can account for today’s dark energy. I will introduce some models where the underlying cosmological constant may be Planck scale but starts as a redundant coupling which can be eliminated by a field redefinition. The observed vacuum energy arises when the redundancy is explicitly broken. I’ll give a recipe for constructing models that realize this mechanism and satisfy all solar system constraints on gravity, including one based on Gauss-Bonnet gravity which provides a technically natural explanation for dark energy.

The QUaD experiment has recently released CMB polarization results at el>200 which are the most sensitive to date. The predicted series of peaks in the EE spectrum are shown to be present for the first time while BB remains undetectable. After briefly reviewing the motivation for polarization measurements I will move on to the experiment, observations, analysis technique and the final results. Finally I will mention on-going efforts to detect gravitational wave B modes.

The so-called cosmological backreaction arises when one directly averages the Einstein equations to recover cosmology. While usually applied to avoid employing dark energy models, strictly speaking any cosmological model should be built from such an averaging procedure rather than an assumed background. We apply the Buchert formalism to Einstein-de Sitter, Lambda CDM and quintessence cosmologies, and as a first approach to the full problem, evaluate numerically the discrepancies arising from linear perturbation theory between the averaged behaviour and the assumed behaviour. (References: J. Behrend, IB and G. Robbers, JCAP01(2008)013, aXiv:0710.4964; IB, G. Robbers and J. Behrend, in preperation)