### Scalar Fields, Dark Energy and Inflation

On large scales the universe is remarkably flat and homogeneous, and measurements of the cosmic
microwave background (CMB) show an almost scale invariant set of temperature fluctuations.
These phenomena can be explained within the paradigm of inflation, in which the universe
experiences a period of rapid expansion early in its development. To model this, one can use
a scalar field (the inflaton) dominated by its potential energy: as the field evolves, quantum fluctuations
are stretched beyond the causal horizon and become important later in the history of the universe
causing the perturbations in the CMB.

Observations show that the expansion of the universe has started to accelerate very recently
(cosmologically speaking) and the scalar field mechanism can again be invoked to explain this.
These are known as Quintessence models.
This is an attractive dynamical alternative to models in which the dark energy
candidate (that
causes the acceleration) is the energy of the vacuum, called the cosmological constant. An
important issue that needs to be addressed is the coincidence problem, why is this happening now
and not earlier? Alternatively, why do we observe the energy
densities of dark matter, dark energy and baryonic (normal) matter to be so similar when, in the
model with a cosmological constant, they differed by many orders of magnitude for most
of the universe's evolution?

My previous work linked dark energy and inflation, focusing on a model in
which quintessence was present in the early universe
as a subdominant field non-minimally coupled to the inflaton. We were able to study the
modifications to slow-roll inflation and reheating, when the inflaton decays into radiation, using
the observational constraints on the coupling between
dark energy and dark matter.

### Scalar Fields with Non-Canonical Kinetic Terms

I am currently working on models that use scalar fields with non-canonical kinetic terms to model the acceleration of the universe. In the context of inflation, this leads to interesting results due to the fact that the speed of sound of the perturbations is less than one, so in these models the predicted spectrum of fluctuations in the CMB is not completely Gaussian. With data from the next generation of experiments, this could be used to discriminate between models of inflation. An interesting example of models of this type involves the Dirac-Born-Infeld (DBI) action, arising in string theory constructions. In this setup, the inflaton is affected by a speed limit that allows inflation to proceed, even with quite steep potentials.

My current work is focused on an inflationary model of this type with a non-minimal coupling to gravity but as well as this I am also interested in other aspects of the physics of scalar fields with non-canonical kinetic terms. I am working to understand of the behaviour of adiabatic and entropic (non-adiabatic) curvature modes in inflationary systems and I am also studying models with mixed kinetic terms in the context of dark energy. I am particularly interested in the mechanism of reheating in these models, which is not well understood at present.

Measurements of dark energy can provide a different perspective on the physics of scalar field models with non-canonical kinetic terms. In my previous work in this area, I have investigated two-field models of dark energy with a kinetic coupling, both in the case of two canonical fields and in the so-called quintom case in which the presence of a phantom field allows the dark energy equation of state (the ratio of the pressure and energy density) to cross the phantom divide and reach values less than -1.

### Publications

** Last updated 4 ^{th} December 2010. Click HERE
for a up to date list. **

*"Embedding DBI inflation in Scalar-tensor theory"*

Carsten van de Bruck, David F. Mota and Joel M. Weller

(arXiv:1012.1567 [astro-ph.CO])-
*"A Quintom scenario with mixed kinetic terms"*

Emmanuel N. Saridakis and Joel M. Weller

*Phys. Rev. D 81, 123523 (2010)*(arXiv:0912.5304 [hep-th]) -
*Quintessence dynamics with two scalar fields and mixed kinetic terms*

Carsten van de Bruck and Joel M. Weller

*Phys. Rev. D 80, 123014 (2009)*(arXiv:0910.1934 [astro-ph.CO]) -
*Slow-roll inflation in the presence of a dark energy coupling*

Philippe Brax, Carsten van de Bruck, Lisa M. H. Hall, and Joel M. Weller

*Phys. Rev. D 79, 103508 (2009)*(arXiv:0812.2843 [gr-qc])