Research

My research interests are in the intersection of theoretical high energy physics and cosmology, particularly topics in the very early universe such as inflation.

If you are a student interested in doing research in this exciting area of physics, please stop by my office to chat.

In the past, my research has focused on the following topics:

Warped Effective Theories

Many high energy physics constructions involve extra dimensions as part of their descriptions, most notably superstring theory which requires 6 additional spatial dimensions. However, in order for the extra dimensions to have escaped our notice they must be curled up and small. Such a "compactification" is described by a 4-dimensional effective theory, which includes fields and parameters describing the details of how the extra dimensions are compactified. The study of physics like cosmology and inflation in models with extra dimensions thus often reduces to the study of the physics of these effective theories.

However, these effective theories typically are constructed only in a limit where the effects from warping are ignored. Warping arises because the compactification of the extra dimensions requires additional fields and sources throughout the extra dimensions that generate competing forces preventing either expansion or contraction of the extra dimensions. These additional fields and sources have energy and so they also create a local gravitational potential in the extra dimensions, called warping. Many cosmological applications, such as brane inflation, work best in strongly warped backgrounds, so unwarped effective theories are not sufficient. It is crucial, then, to construct effective theories where warping is included in order to study interesting applications of extra dimensions to cosmology.

Related Publications

B.Underwood, "A Breathing Mode for Warped Compactifications," Classical and Quantum Gravity 28 195013 (2011), arXiv:1009.4200 [hep-th], 2010.

A.R.Frey, G.Torroba, B.Underwood and M.R.Douglas, "The Universal Kahler Modulus in Warped Compactifications," Journal of High Energy Physics, 0901, 036 (2009), arXiv:0810.5768 [hep-th].

G.Shiu, G.Torroba, B.Underwood and M.R.Douglas, "Dynamics of Warped Flux Compactifications," Journal of High Energy Physics, 0806, 024 (2008), arXiv:0803.3068 [hep-th].

Non-Canonical Kinetic Dynamics

A leading candidate for explaining the observed homogeneity and isotropy of the early Universe and the origin of the primordial perturbations is to propose a very early period of rapid expansion, called inflation. The expansion dilutes any inhomogeneities and anisotropies, and quantum fluctuations stretched by the expansion to large scales become the origin of the primordial perturbations.

The simplest models of inflation involve a new type of matter called the inflaton field, which behaves like a homogeneous energy density in the very early Universe driving the exponential expansion. These models have been very successful in predicting the features seen in the CMB, but typically suffer from the problem that starting with typical initial conditions will not lead to inflation. This makes inflation less attractive as a dynamical description of the early Universe, since we would have to assume some special set of initial conditions. In a series of papers, I have been exploring how a generalization of the inflationary dynamics away from the usual "canonical" dynamics typically assumed can relax this initial conditions problem of inflation.

Related Publications

J.Karouby, B.Underwood, A.Vincent, "Preheating with the Brakes On: The Effects of a Speed Limit," Physical Review D84, 043528 (2011), arXiv:1105.3982 [hep-th], 2011.

P.Franche, R.Gwyn, B.Underwood and A.Wissanji, "Initial Conditions for Non-Canonical Inflation," Physical Review D82, 063528 (2010), arXiv:1002.2639 [hep-th].

P.Franche, R.Gwyn, B.Underwood and A.Wissanji, "Attractive Lagrangians for Non-Canonical Inflation," Physical Review D81, 123526 (2010), arXiv:0912.1857 [hep-th].

B.Underwood, "Brane Inflation is Attractive," Physical Review D 78, 023509 (2008), arXiv:0802.2117 [hep-th].

Brane Inflation

Inflation is a successful paradigm for the early universe, but several nagging questions remain: what is the inflaton field? Why are the conditions "just right" for inflation? Where does the potential energy come from? How does inflation embed into a theory of quantum gravity?

One approach towards answering these questions is to search for models of inflation in string theory. String theory comes with many new ingredients, such as extra dimensions and D-branes (short for "Dirichlet membranes"). It is natural, then, to try and construct models of inflation with these objects. In a series of papers, I have investigated the inflationary behavior, and violent end, of D-branes. Some of the questions we answered were: What happens when D-branes and anti-D-branes collide? Does the ultra-relativistic motion of a D-brane in a warped space lead to interesting effects or signatures? Where do D-branes "like" to sit (what are their vacua)?

Related Publications

J.M.Cline, L.Hoi and B.Underwood, "Dynamical Fine Tuning in Brane Inflation," Journal of High Energy Physics, 0906, 078 (2009), arXiv:0902.0339 [hep-th].

B.Underwood, "Brane Inflation is Attractive," Physical Review D 78, 023509 (2008), arXiv:0802.2117 [hep-th].

M.Huang, G.Shiu, B.Underwood, "Multifield DBI Inflation and Non-Gaussianities," Physical Review D 77, 023511 (2008), arXiv:0709.3299 [hep-th].

O. DeWolfe, L. McAllister, G. Shiu, B. Underwood, "D3-brane Vacua in Stabilized Compactifications," Journal of High Energy Physics, 0709 121 (2007), arXiv:hep-th/0703088.

G. Shiu, B. Underwood, "Observing the Geometry of Warped Compactification via Cosmic Inflation," Physical Review Letters, 98, 051301 (2007), arXiv:hep-th/0610151.

S. Kecskemeti, J. Maiden, G. Shiu, B. Underwood, "DBI Inflation in the Tip Region of a Warped Throat," Journal of High Energy Physics, 0609, 076 (2006), arXiv:hep-th/0605189.

D. Chialva, G. Shiu, B. Underwood, "Warped Reheating in Multithroat Brane Inflation," Journal of High Energy Physics, 0601 014 (2006), arXiv:hep-th/0508229.