Cosmology and Gravitation

The cosmological constant problem is arguably the deepest gap in our understanding of modern physics. The discovery of cosmic acceleration in the past decade and its surprising coincidence with cosmic structure formation has added an extra layer of complexity to the problem. I will describe how revisiting/revising some standard assumptions in the theory of gravity can decouple the quantum vacuum from geometry, which can potentially solve the cosmological constant problem. I will then argue that a possible fascinating outcome of such a theory is to relate black hole formation to cosmic acceleration, providing a possible solution to the cosmic coincidence. A diverse range of experimental/observational probes over the next decade will tell us whether we are close to the end of this century-old mystery, which in turn could shed light on the nature of quantum gravity and black holes.

The standard cosmological model features two periods of accelerated expansion: an inflationary epoch at early times, and a dark energy dominated epoch at late times. These periods of accelerated expansion can lead to surprisingly strong constraints on models with extra dimensions. I will describe new mathematical results which enable one to reconstruct features of a higher-dimensional theory based on the behaviour of the accelerating four-dimensional cosmology. When applied to inflation, these results pose several interesting questions for the construction of concrete models. When applied to dark energy, they provide a new technique to combine measurements of dark energy parameters and constraints on variation of Newton's constant. This technique can transform near-future dark energy surveys into become powerful probes of extra dimensional physics.

Weak lensing has emerged as a powerful probe of fundamental physics such as dark energy and dark matter. After briefly reviewing the standard argument for the power of lensing, I present a variety of surprises: some quantities that are supposedly simple measures of cosmic shear are actually polluted by other effects and some quantities apparently unrelated to lensing are contaminated by lensing. These effects may lead to opportunities to strengthen the constraints lensing will place on dark energy.

If Dark Energy is dynamical, it would indicate the existence of new physics beyond the standard model coupled to gravity. I will argue that the best motivated models of this new physics are all tied to whatever resolves the cosmological constant problem, and discuss the cosmological implications of several proposals that have been put forward in this vein.

I will present three ideas about black holes and cosmology. First, I will discuss a way of understanding the simple patterns which emerge from the notoriously thorny numerical simulations of binary black hole merger, and some of the directions where this understanding may lead. Second, I will suggest a sequence of practical bootstrap tests designed to give sharp observational confirmation of the essential idea underlying the inflationary paradigm: that the universe underwent a period of accelerated expansion followed by a long period of decelerated expansion. Third, I will investigate a way that one might try to detect the strong bending of light rays in the vicinity of a black hole.

We consider a probe codimension-2 brane inflation scenario in a warped six-dimensional flux compactification. First, we stabilise the modulus of the model by means of a cap regularisation of the codimension-2 singularities of the background solution. Then, we discuss the cosmological evolution of the world-volume of a probe codimension-2 brane when it moves along the radial direction of the internal space. In order to have slow-roll inflation, one needs the warping of the internal space to be weak, in contrast to the recent string inflation constructions with strong warping. We discuss the parameter range that the inflation is in agreement with the observationally inferred parameters and which furthermore is consistent with the probe brane approximation. We show that this scenario is falsifiable if substantial tensor modes are detected.

I will compute the probability distribution for bubble collisions in an inflating false vacuum which decays by bubble nucleation. The number of collisions in our backward lightcone can be large in realistic models without tuning. In addition, we calculate the angular position and size distribution of the collisions on the cosmic microwave background sky.

I will report results from simulations of galaxy-scale dark halos of unprecedented numerical resolution. Convergence tests demonstrate detailed convergence for (sub)structures for over six decades in mass, enabling detailed forecasts of the expected dark matter signal both in Earth-bound direct-detection experiments as well as in indirect detection experiments which attempt to image dark matter annihilation radiation in gamma rays.

Varied experimental results have recently sparked theoretical interest in the dark matter sector. I will review some of these results and the basic ideas in particle physics that might explain them, as well as some requirements for those models to work. Then I'll discuss a new model dark matter sector that can better explain many of the experimental results. I'll also mention the interesting cosmological history required in this type of model. Finally, if there's time, I'll discuss ongoing efforts at McGill to develop basic physics shared by many of the new dark matter models.

The standard cosmological framework explains an impressive range of large-scale astrophysical phenomena, but an agreement between its predictions and the properties of the dark matter halos of nearby galaxies has not been established. In this talk, I will highlight some key observables that constrain galaxy structure and some key differences between cosmological predictions and halo properties inferred from these measurements. I will also discuss proposed 'observational' solutions to some of these discrepancies, such as the role of coherent non-circular motions in spiral galaxies and the measured abundance of gas-rich, starless halos in the nearby Universe.

An astrophysical black hole is completely described with just two parameters: its mass and its dimensionless spin. A few dozen black holes have mass estimates, but until recently none had a reliable spin estimate. The first spins have now been measured for black holes in X-ray binaries.

I will discuss the possibility that a 'Wilson line' degree of freedom can play the role of an inflaton in a warped flux compactification, in the context of the DBI inflationary scenario. I will show how warped DBI Wilson line inflation offers an attractive alternative to ordinary (position field) DBI inflation, inasmuch as observational and theoretical constraints get considerably relaxed. Thus, besides the large non-Gaussianities produced in DBI scenarios, Wilson lines allow for an observable amount of gravitational waves, within consistent approximations.