Session J5: AS2: Astrophysics, Space Physics, Astronomy and Cosmology

Colliding Branes and Formation of Spacetime Singularities

We construct a class of analytic solutions with two free parameters to the five-dimensional Einstein field equations, which represents the collision of two timelike 3-branes. We study the local and global properties of the spacetime, and find that spacelike singularities generically develop after the collision, due to the mutual focus of the two branes. Non-singular spacetime can be constructed only in the case where both of the two branes violate the energy conditions.   

Cosmology of Orbifold Branes in Superstring

In recent years, some attempts have been made to derive a late time accelerating universe from a fundamental theory of particle physics that incorporates gravity, lime M-theory or Superstring theory. We study the brane scenario in Superstring theory and see how the Orbifold branes collide.   

A Time-Symmetric Model of Cosmology

By considering the symmetry of time surrounding the creation event, a simplified model of cosmology can be constructed that is consistent with current observations of our universe without resorting to the inflationary hypothesis of conventional cosmology. This talk outlines the derivation of this Time-Symmetric Model and illustrates how its predictions are confirmed through numerous modern astronomical observations - including the recently discovered accelerating universe.   

Supernova Cosmology and Wavelet Decomposition

The acceleration of the universe is a very recent development in the field of cosmology. One of the main ways of probing the dark energy that is believed to be causing this accelerations is by studying the properties of type Ia supernovae, namely their redshift and distance. The goal of this work s to build on current supernovae template spectra using the \'{a} trous wavelet decomposition. After normalizing the results specific spectral features of several different supernovae are compared. Several of these features have a well defined evolution over the course of the explosion. Relationships between stretch and the spectral index can be found and then used to construct new supernovae template spectra.   

Reheating of the universe after inflation with f(phi)R gravity

We show that reheating of the universe occurs spontaneously in a broad class of inflation models with $f(\phi)R$ gravity ($\phi$ is inflaton). The model does not require explicit couplings between $\phi$ and bosonic or fermionic matter fields. The couplings arise spontaneously when $\phi$ settles in the vacuum expectation value (vev) and oscillates, with coupling constants given by derivatives of $f(\phi)$ at the vev and the mass of resulting bosonic or fermionic fields. This mechanism allows inflaton quanta to decay into any fields which are not conformally invariant in $f(\phi)R$ gravity theories.   

Emergence of Fractal Geometry in One-Dimensional Models of the Expanding Universe

Concentrations of matter in the universe, such as galaxies and galactic clusters, originated as very mall density fluctuations in the early universe. The primordial fluctuation spectrum is revealed by studies of the angular correlation of CMB across the sky with WMAP. The existence of super-clusters and voids suggests that a natural length scale for the matter distribution may not exist. A point of controversy is whether the distribution is fractal and, if so, over what range of scales. The source of fractal behavior is the lack of a length scale in the two body gravitational interaction. Even with new, larger, sample sizes from recent surveys, it is difficult to extract information concerning fractal properties with confidence. Similarly, simulations with a billion particles only provide a thousand particles per dimension, far too small for accurate conclusions. With one dimensional ``toy models'' we can overcome these limitations by carrying out simulations with on the order of a quarter of a million particles without compromising the computation of the gravitational field. Here we present the recent results of our ongoing investigation of the emergence of fractal geometry in one dimensional models of the expanding universe.   

Dark energy and cosmic curvature: Monte-Carlo Markov Chain approach

We use the Monte-Carlo Markov Chain method to explore the dark energy property and the cosmic curvature by fitting two popular dark energy parameterizations to the observational data. The new 182 gold supernova Ia data and the ESSENCE data both give good constraint on the DE parameters and the cosmic curvature for the dark energy model $\omega_0 +\omega_a z/(1+z)$ The cosmic curvature is found to be $\Omega_k \le 0.03$ For the dark energy model $\omega_0 +\omega_a z/(1+z)^2$ the ESSENCE data gives better constraint on the cosmic curvature and we get $\Omega_k \le 0.02$.   

Quasars with a Kick

Mergers of spinning black holes can result in the final black hole receiving a `kick' from gravitational radiation of up to several thousand km/s. A recoiling super-massive black hole in an AGN can retain the inner part of its accretion disk, providing fuel for continuing activity. A search for evidence of such kicks in AGN spectra from the Sloan Digital Sky Survey (SDSS) leads us to place upper limits on the incidence of high velocity recoils in AGN. Other observational signatures will be discussed, including brief flares in soft X-rays that may occur when marginally-bound material falls back onto the moving accretion disk.   

The Effectiveness of Using Type Ia Supernovae as Standard Candles in the Infrared

Using observations of 20 Type Ia supernovae obtained over the past 3 years by the Carnegie Supernova Project collaboration at the Las Campanas observatory in Chile, along with results from previously studied Type Ia supernovae, I will present Hubble diagrams and derived absolute magnitudes at maximum brightness for the objects that have well observed lightcurves in the near-infrared. Type Ia supernovae at maximum light appear to be standard candles in the near-infrared to a precision of +/-0.15mag or better.