Astrophysics

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.

I will present analytic solutions to a class of cosmological models described by a canonical scalar field minimally coupled to gravity and experiencing self interactions through a hyperbolic potential. Using models and methods of solution inspired by 2T-physics, I will show how analytic solutions can be obtained including radiation and spacial curvature. Among the analytic solutions, there are many interesting geodesically complete cyclic solutions, both singular and non-singular ones. Cyclic cosmological models provide an alternative to inflation for solving the horizon and flatness problems as well as generating scale-invariant perturbations. I will argue in favor of the geodesically complete solutions as being more attractive for constructing a more satisfactory model of cosmology. When geodesic completeness is imposed, it restricts models and their parameters to certain a parameter subspace, including some quantization conditions on parameters. I will explain the theoretical origin of our model from the point of view of 2T-gravity as well as from the point of view of the colliding branes scenario. If time permits, I will discuss how to associate solutions of the quantum Wheeler-deWitt equation with the classical analytic solutions, physical aspects of some of the cyclic solutions, and outline future directions.

Reducing a higher dimensional theory to a 4-dimensional effective theory results in a number of scalar fields describing, for instance, fluctuations of higher dimensional scalar fields (dilaton) or the volume of the compact space (volume modulus). But the fields in the effective theory must be constructed with care: artifacts from the higher dimensions, such as higher dimensional diffeomorphisms and constraint equations, can affect the identification of the degrees of freedom. The effective theory including these effects resembles in many ways cosmological perturbation theory. I will show how constraints and diffeomorphisms generically lead the dilaton and volume modulus to combine into a single degree of freedom in the effective theory, the "breathing mode". This has important implications for models of moduli stabilization and inflation with extra dimensions.

The existence of concentric low variance circles in the CMB sky, generated by black-hole encounters in an aeon preceding our big bang, is a prediction of the Conformal Cyclic Cosmology. Detection of three families of such circles in WMAP data was recently reported by Gurzadyan & Penrose (2010). We reassess the statistical significance of those circles by comparing with Monte Carlo simulations of the CMB sky with realistic modeling of the anisotropic noise in WMAP data. We find that the circles are not anomalous and that all three groups are consistent at 3sigma level with a Gaussian CMB sky as predicted by inflationary cosmology model.

The 4D rotating black hole described by the Kerr geometry possesses many of what was called by Chandrasekhar "miraculous" properties. Most of them are related to the existence of a fundamental hidden symmetry of a principal conformal Killing-Yano (PCKY) tensor. In my talk I shall demonstrate that hidden symmetry of the PCKY tensor plays exceptional role also in higher dimensions. Namely, I shall present the most general spacetime admitting the PCKY tensor and show that is possesses the following properties: 1) It is of the algebraic type D and admits the Kerr-Schild form 2) It allows a separation of variables for the Hamilton-Jacobi, Klein-Gordon, Dirac, and stationary string equations. 3) When the Einstein equations with the cosmological constant are imposed the metric describes the most general known (spherical) Kerr-NUT-AdS black hole spacetime. I will also discuss the generalization of Killing-Yano symmetries for spacetimes with natural "torsion 3-form", such as the black hole of D=5 minimal supergravity, or the Kerr-Sen solution of heterotic string theory, and comment on connection to special Riemannian manifolds admiting Killing spinors.

We show that, in a model of modified gravity based on the spectral action functional, there is a nontrivial coupling between cosmic topology and inflation, in the sense that the shape of the possible slow-roll inflation potentials obtained in the model from the nonperturbative form of the spectral action are sensitive not only to the geometry (flat or positively curved) of the universe, but also to the different possible non-simply connected topologies. We show this by explicitly computing the nonperturbative spectral action for some candidate cosmic topologies, spherical space forms and flat ones given by Bieberbach manifolds and showing that the resulting inflation potential differs from that of the sphere or flat torus by a multiplicative factor. We then show that, while the slow-roll parameters differ between the spherical and flat manifolds but do not distinguish different topologies within each class, the power spectra detect the different scalings of the slow-roll potential and therefore distinguish between the various topologies, both in the spherical and in the flat case. (Based on joint work with Elena Pierpaoli and Kevin Teh)

For nearly the past century, the nature of dark matter in the Universe has puzzled astronomers and physicists. During the next decade, experiments will determine if a substantial amount of the dark matter is in the form of non-baryonic, Weakly-Interacting Massive Particles (WIMPs). In this talk I will discuss and interpret modern limits on WIMP dark matter from a variety of complementary methods. I will show that we are just now obtaining sensitivity to probe the parameter space of cosmologically-predicted WIMPs created during the earliest epoch in the Universe. I will discuss the science to extract from a positive signal in different experiments, and the prospects for an era of dark matter astrophysics.

The availability of high precision observational data in cosmology means that it is possible to go beyond simple descriptions of cosmic inflation in which the expansion is driven by a single scalar field. One set of models of particular interest involve the Dirac-Born-Infeld (DBI) action, arising in string cosmology, in which the dynamics of the field are affected by a speed limit in a manner akin to special relativity. In this talk, I will introduce a scalar-tensor theory in which the matter component is a field with a DBI action. Transforming to the Einstein frame, I will explore the effect of the resulting coupling on the background dynamics of the fields and the first-order perturbations. The coupling forces the scalar field into the minimum of its effective potential, so the dynamics are determined by the DBI field, which has the interesting effect of increasing the number of efolds of inflation and decreasing the boost factor of the DBI field. Focusing on this case, I will show that the power spectrum of the primordial perturbations is determined by the behaviour of the perturbations of the modified DBI field and calculate the effect of varying the model parameters on the inflationary observables.

Primodial magnetic fields are a potentially interesting origin for cosmic magnetism. Such fields can leave an interesting signal not only in the CMB temperature and polarization, but in structure at low redshift, contributing to the matter power spectrum and SZ effect at small scales. I will talk about the reasons for considering primordial fields, their origin and evolution, and how their observational consequences constrain their nature.