Cosmology

Present treatments of eternal inflation regulate infinities by imposing a geometric cutoff. We point out that some matter systems reach the cutoff in finite time. This implies a nonzero probability for a novel type of catastrophe. According to the most successful measure proposals, our galaxy is likely to encounter the cutoff within the next 5 billion years.

By combining insights from black holes and string theory we argue for the existence of a hidden phase space associated with an underlying fast dynamical system, which is largely invisible from a macroscopic point of view. The dynamical system is influenced by slow macroscopic observables, such as positions of objects. This leads to a collection of reaction forces, whose leading order Born Oppenheimer force is determined by the general principle that the phase space volume of the underlying system is preserved. We propose that this adiabatic force is responsible for inertia and gravity. This fact allows us to calculate the hidden phase space volume from the known laws of inertia and gravity. We find that in a cosmological setting the appearance of dark energy is naturally explained by the finite temperature of the underlying system. The adiabatic approximation that leads to the usual laws of inertia and gravity breaks down in the neighborhood of horizons. In this regime the reaction force degenerates into an entropic force, and the laws of inertia and gravity receive corrections due to thermal effects. A simple estimate of these effects leads to the conclusion that they coincide with observed phenomena attributed to dark matter.

We make some remarks about the semiclassical wavefunction of the universe around de-Sitter space. In five dimensional gravity with a positive cosmological constant it is possible to compute the full semiclassical measure for arbitrary geometries at superhorizon scales. In four dimensions, the same computation can be reformulated as a problem in conformal gravity.

Six dimensional (1, 0) supergravity theories have received recent attention due to the fact that the strong constraints coming from anomalies severely restrict the theory. These constraints are restrictive enough that it is possible to get a rather good handle on the space of theories that do not exhibit any (known) inconsistencies. Many useful observations can be made by studying this space of theories. In particular, the process of comparing these theories with six-dimensional string vacua turns out to be fruitful in many aspects. I will be presenting the lessons learned from research in this direction.

We compute the partition function of quantum Einstein gravity in three dimensional de Sitter space. The Euclidean path integral is formulated as a sum over geometries, including both perturbative loop and non-perturbative instanton corrections coming from geometries with non-trivial topology. These non-trivial geometries have a natural physical interpretation and lead to deviations from the standard thermal behaviour of the de Sitter horizon; this is the de Sitter analog of the celebrated "black hole Farey tail." Perturbative quantum corrections are computed to all orders in perturbation theory and the vacuum partition function, including all instanton and perturbative corrections, is shown to diverge in a way which can not be regulated using standard field theory techniques. Thus the Hartle-Hawking state is not normalizable.

We explore the duality, conjectured in earlier work, between the wave function of the multiverse and a 3D Euclidean field theory on the future boundary of spacetime. We propose that a measure for the multiverse, which is needed in order to extract quantitative probabilistic predictions, can be derived in terms of the boundary theory by imposing a UV cutoff. In the inflationary bulk, this measure corresponds to a cutoff at surfaces of constant comoving apparent horizon.

Cosmic inflation has given us a remarkably successful cosmological phenomenology. But the original goal of explaining why the cosmos is *likely* to take the form we observe has proven very difficult to realize. I review the status of "eternal inflation" with an eye on the roles various infinities have (both helpful and unhelpful) in our current understanding. I then discuss attempts to construct an alternative cosmological framework that is truly finite, using ideas about equilibrium and dark energy. I report some recent results that point to observable signatures.

Though the observed CMB is at very low energy, it encodes ultra high-energy physics in spatial variations of the photon temperature and polarization fluctuations. This effect is believed to be dominated by the initial quantum state of the Universe. I will describe the first theoretical tools by which to construct such a state from fundamental physics. There are three specific observational effects this initial state will produce: a ringing signal in the power spectrum of quantum field fluctuations, an enfolded type of non-Gaussian fluctuations, and a calculable primordial gravitational wave background. We may soon be able to compare these predictions against experiment, allowing one to rule out classes of quantum gravity models. Now is the critical time to undertake such investigations, with a number of ongoing and planned experiments such as WMAP, Planck, and CMBPol poised to collect a wealth of precision data.