Connecting Early Universe Physics to Modern Advances in Observational Astronomy

We are living in a golden age for cosmology. Thanks in large part to our observations of the tiny fluctuations in the afterglow of the Big Bang, the cosmic microwave background, we now have a concordance model of cosmology which provides a precise inventory of the amount and type of matter and energy that makes up the Universe and a timeline of the Universe's development from a fraction of a second on. However, a few nagging uncertainties remain. It has become largely accepted that at early times the Universe underwent a rapid expansion known as inflation. However, in the nearly three decades since inflation was proposed, we are still unable to answer some basic questions about it, such as how it started, how it ended, and what kept the expansion going. There are countless models attempting to describe inflation, but we still don't have a basic underlying theory to ground it in. String theory, our current best hope for a 'theory of everything,' is notoriously hostile to inflation, though attempts to unite the two are ongoing. In order to make progress, therefore, we need to find ways to distinguish among the inflationary models being proposed, all of which, by design, produce a universe similar to what we see today . My approach is to seek out the most distinctive features of inflationary models so as to confirm or rule out as much as possible. One of the most enticing possibilities is the production of relics from the Big Bang -- actual physical objects produced in the early universe that may still exist today, or whose signatures we might still see. Although relics are speculative, their presence would be dramatic enough that the pay-off for either discovering or ruling them out would be huge, allowing us to distinguish among entire classes of models. I have been looking in particular at three kinds of Big Bang relics. First are primordial black holes. These tiny black holes, which could have been produced in the dense environment of the early universe, would show up either by pulling in and heating up the matter around them, which puts out x-ray radiation, or by 'evaporating' -- radiating themselves away as gamma rays. Second are cosmic strings: strings of high energy stretching across the universe would bend the light of distant stars and galaxies around them in a phenomenon known as gravitational lensing. Cosmic strings are particularly exciting to search for because they could be the very strings of string theory, and are therefore our best hope for directly observing something that would confirm the theory. A third type of relic is a particle known as an axion. If axions existed during inflation, they could make up the elusive dark matter that holds galaxies and clusters of galaxies together. However, my work has shown that axions are difficult to fit in with current observations if both string theory and inflation are correct. One of the most valuable tools we have for learning about the early universe is the study of the cosmological Dark Ages, a period after the Big Bang, but before stars and galaxies started to turn on, when the Universe was mostly neutral hydrogen gas. It was dark for two reasons: first, nothing much was shining; and second, neutral hydrogen gas is particularly good at absorbing radiation. Several ambitious radio telescope arrays are currently being built in order to try to get closer to observations of the neutral hydrogen in the Dark Ages so we can fill in the gap between the background radiation we see from the Big Bang and the stars and galaxies we can observe today. Cosmology, which was once the work of philosophers and theologians, is now a precision science. However, that precision needs to be balanced by a clear conceptual picture of what it is we're measuring. I hope to bring us closer to that understanding through my work of connecting early universe physics to the observations we can make today.