Astrophysics

We discuss bulk and holographic features of black hole solutions of 4D anti de Sitter Einstein-Maxwell-Dilaton gravity. At finite temperature the field theory holographically dual to these solutions has a rich and interesting phenomenology reminiscent of electron motion in metals: phase transitions triggered by nonvanishing VEV of scalar operators, non-monotonic behavior of the electric conductivities etc. Conversely, in the zero temperature limit the transport properties for these models show an universal behavior.

In recent years, a number of observations have highlighted anomalies that might be explained by invoking dark matter annihilation. The excess of high energy positrons in cosmic rays reported by the PAMELA experiment is only one of the most prominent examples of such anomalies. Models where dark matter annihilates offer an attractive possibility to explain these observations, provided that the annihilation rate is enhanced over the typical values given by conventional models of thermal relic dark matter annihilation. An elegant proposal to achieve this, is that of a Sommerfeld mechanism produced by a mutual interaction between the dark matter particles prior to their annihilation. However, this enhancement can not be arbitrarily large without violating a number of astrophysical measurements. In this talk, I will discuss the degree to which these measurements can constrain Sommerfeld-enhanced models. In particular, I will talk about constraints coming from the actual abundance of dark matter and the extragalactic background light measured at multiple wavelengths.

We use a field theoretic generalization of the Wigner-Weisskopf method to study the stability of the Bunch-Davies vacuum state for a massless, conformally coupled interacting test field in de Sitter space. A simple example of the impact of vacuum decay upon a non-gaussian correlation is discussed. Single particle excitations also decay into two particle states, leading to particle production that hastens the exiting of modes from the de Sitter horizon resulting in the production of \emph{entangled superhorizon pairs} with a population consistent with unitary evolution. We find a non-perturbative, self-consistent "screening" mechanism that shuts off vacuum decay asymptotically, leading to a stationary vacuum state in a manner not unlike the approach to a fixed point in the space of states.

After reviewing the basics of Coleman deLuccia tunneling, especially in the thin-wall limit, I discuss an (almost) exact tunneling solution in a piecewise linear and quadratic potential. A comparison with the exact solution for a piecewise linear potential demonstrates the dependence of the tunneling rate on the exact shape of the potential. Finally, I will mention applications when determining initial conditions for inflation in the landscape. Based on arXiv:1102.4742 [hep-th].

One of the great promises of the Advanced LIGO era is the prospect of integrating gravitational wave astronomy into the greater astronomical community. This will allow for measurements that cross spectral bands and provide new paths for insight into some of the most violent processes in the universe. In this talk I'll discuss past and present efforts with Initial and Enhanced LIGO to search for transients with both electromagnetic and gravitational wave signatures, with special focus on electromagnetic followups of inspiral events and an eye towards the advanced detector era. In addition, I'll discuss some work on detecting gravitational waves with pulsar timing experiments, which seeks to bridge the gap between gravitational wave and electromagnetic astronomers in a different way.

At the time of recombination, 400,000 years after the Big Bang, the structure of the dark matter distribution was extremely simple and can be inferred directly from observations of structure in the cosmic microwave background. At this time dark matter particles had small thermal velocities and their distribution deviated from uniformity only through a gaussian field of small density fluctuations with associated motions. Later evolution was driven purely by gravity and so obeyed the collisionless Boltzmann equation. This has immediate consequences for the present distribution of dark matter, even in extremely nonlinear regions such as the part of the Galaxy where the Sun resides. I will show how this structure can be followed in full generality by integrating the Geodesic Deviation Equation in tandem with the equations of motion in a high-resolution N-body simulation, enhancing its effective resolution by more than 10 orders of magnitude. I will discuss how the predicted distribution at the Sun's position impacts the expectations for laboratory experiments seeking to detect the dark matter directly, in particular, the possibility of extremely narrow line signals that may be visible in axion detectors.

I propose late-time moduli decay as the common origin of baryons and dark matter. The baryon asymmetry is produced from the decay of new TeV scale particles, while dark matter is created from the chain decay of R-parity odd particles. The baryon and dark matter abundances are mainly controlled by the dilution factor from moduli decay, which is typically in the range 10^{-9}-10^{-7}. The exact number densities are determined by simple branching fractions from modulus decay, which are expected to be of similar order in the absence of symmetries. This scenario can naturally lead to the observed baryon asymmetry and, for moderate suppression of the two-body decays of the modulus to R-parity odd particles, can also yield the correct dark matter abundance in the 5-500 GeV mass range. I will present an explicit model for late-time baryogenesis along this line and discuss some of its cosmological and phenomenological consequences.

If the universe is a quantum mechanical system it has a quantum state. This state supplies a probabilistic measure for alternative histories of the universe. During eternal inflation these histories typically develop large inhomogeneities that lead to a mosaic structure on superhorizon scales consisting of homogeneous patches separated by inflating regions. As observers we do not see this structure directly. Rather our observations are confined to a small, nearly homogeneous region within our past light cone. This talk will describe how the probabilities for these observations can be calculated from the probabilities supplied by the quantum state without introducing a further ad hoc measure. The talk will emphasize the principles behind this result --- a quantum state, quantum spacetime leading to an ensemble of classical histories, quantum observers, a focus in local observations, and the use of coarse-grainings adapted to these observations. The principles will be illustrated in simple models in particular using the no-boundary wave function as a model of the quantum state. Applied to a model landscape we obtain specific predictions for features of the CMB spectrum and improvements in the `anthropic' bounds on the cosmological constant.

In this talk I will discuss a new class of cosmological scalar fields. Similarly to gravity, these theories are described by actions linearly depending on second derivatives. The latter can not be excluded without breaking the generally covariant formulation of the action principle. Despite the presence of these second derivatives the equations of motion are of the second order. Hence there are no new pathological degrees of freedom. Because of this structure of the theory the scalar field kinetically mixes with the metric without direct non-minimal couplings to curvature - the phenomenon we have called Kinetic Gravity Braiding. These theories have rather unusual cosmological dynamics which is useful to model Dark Energy and Inflation. I will discuss an equivalent hydrodynamical formulation of these theories and cosmological applications.

Did you know you could fit the entire human race in the volume of a sugar cube? Or that, if the Sun were made of bananas, it wouldn't make much difference? Or that 98 per cent of the Universe is invisible? Award-winning science writer Marcus Chown invites you to come along and discover how the Universe we live in is far stranger than anything we could possibly have invented.