Astrophysics

The standard model of cosmology has some puzzles/problems such as the cosmological constant problem and the horizon problem which according to many stem from our lack of understanding of the very early universe. This in turn means that almost none of the theories of quantum gravity are at a stage where anything substantial can be said about observational cosmology. In the past few years Causal Set theory has proved itself different in this case where a possible solution to the Cosmological constant problem was proposed. Now some work in progress has also shown that some models of Causal Set dynamics give exponential expansion in the early universe. I hope to discuss both of these exciting prospects but this talk will mainly focus on the first proposal.

We present the first year SDSS-II Supernova Survey results and their implications for cosmology and future supernova surveys. We then discuss challenges that face next-generation surveys, such as LSST, which will deliver of order a million supernovae without spectroscopic confirmation. As a way to address these challenges, we introduce BEAMS, a statistical method to do photometric supernova cosmology, and present a preliminary application to SDSS data. Finally we highlight the importance of future surveys such as LSST, given the surprising result that we may not detect dark energy dynamics for the next decade, if the dark energy scales during matter and radiation domination.

The reason cosmologists have a job is that the Universe as a whole -- the stuff between planets and stars and galaxies -- is, despite first appearances, a pretty interesting place. The strangest fact about it is that it's expanding, and always has been, as far as we know (and though Einstein's theory of gravity predicts this, Albert himself didn't much care for the idea, at least at first). After about seventy years -- it was discovered in 1929 -- this expansion was kind of old hat, but then new observations came around that shattered the old complacency. The old idea was that the Universe was expanding, but slowing down as it went -- since gravity, as far as anyone knew, could only cause attractive forces. What the new observations demonstrated is that the Universe's expansion is, in fact, accelerating -- getting faster with time. This is so shocking that most astronomers and cosmologists couldn't believe it at first, and some still don't. In this talk, I'll explain a bit about how we know this, why it's so shocking, and tell you something about the crazy ideas people at Perimeter have for what's going on.

The Baryon Acoustic Oscillations (BAO) are the latest weapon in the quest for precision cosmology and dark energy. Many presentations on BAO are complicated and unclear and I will therefore present BAO with particular emphasis on trying to give the simplest theoretical description, both at the linear and nonlinear level, and will describe some of the observational challenges to measuring BAO.

Recently there has been renewed interest in cosmological gravity, namely 3d gravity coupled to a Chern-Simons term with parameter $\mu$, following claims that at $\mu =1$ the theory becomes chiral and stable. In this talk we will investigate cosmological gravity by setting up a concrete holographic dictionary with a dual 2d field theory and we will demonstrate that the theory is in fact not chiral in the limit $\mu = 1$. Instead our holographic dictionary implies that at $\mu =1$ the dual field theory becomes a logarithmic CFT (LCFT). Our results resolve confusions in the earlier literature and the resulting new class of holographic dualities involving LCFTs may have interesting applications to strongly correlated condensed matter systems.

Cosmological bubble collisions arising from first order phase transitions are a generic consequence of the Eternal Inflation scenario. I will present our computational strategy for generating and evolving these bubbles in 3+1 dimensions and in a self-consistently expanding background. I will show the existence of classical field transitions--the classical nucleation of bubbles during collisions--which can dramatically alter the canonical description of eternal inflation.