Search Talks

\'Thermal history of the universe after big-bang nucleosynthesis (BBN) is well understood both theoretically and observationally, and recent cosmological observations also begin to reveal the inflationary dynamics. However, the epoch between inflation and BBN is scarcely known. In this work we show that the detection of the stochastic gravitational wave background around 1Hz provides useful information about thermal history well before BBN. In particular, the reheating temperature of the universe may be determined by future space-based laser interferometer experiments such as DECIGO and/or BBO if it is around $10^{6-9}$ GeV, depending on the tensor-to-scalar ratio $r$ and dilution factor $F$.\'

If the spontaneous breaking of Peccei-Quinn symmetry comes from soft supersymmetry breaking, the fermionic partners of the symmetry-breaking fields have mass of order the gravitino mass, and are called flatinos. The lightest flatino, called here the flaxino, is a CDM candidate if it is the lightest supersymmetric particle. We here explore flaxino dark matter assuming that the lightest ordinary supersymmetric particle is the stau, with gravity-mediated supersymmetry breaking. The decay of the stau to the flaxino is fast enough not to spoil the standard predictions of Big Bang Nucleosynthesis, and its track and decay can be seen in future colliders.

It is an important task to embed inflation in a fundamental microphysical theory such as string theory. Since string theory possesses a vast landscape of 4-dimensional theories, we would like to know which portions contain inflation and which do not. I prove a no-go theorem that inflation and de Sitter vacua are forbidden in an exponentially large number of infinite families of simple and well understood compactifications of type IIA string theory. I also mention more complicated and less well understood compactifications, which may have the ingredients for our cosmology.

Considering gravitino dark matter scenarios, cosmological constraints on the sparticle masses and on the reheating temperature of inflation will be discussed. These constraints are relevant for prospects of phenomenology at the LHC and for our understanding of inflation and the baryon asymmetry of the Universe.

A class of non-canonical inflationary models is identified, where the leading-order contribution to the non-Gaussianity of the curvature perturbation is determined by the sound speed of the fluctuations in the inflaton field. Included in this class of models is the effective action for multiple coincident branes in the finite n limit. The action for this configuration is determined using a powerful iterative technique, based upon the fundamental representation of SU(2). In principle the upper bounds on the tensor-scalar ratio that arise in the standard, single-brane DBI inflationary scenario can be relaxed in such multi-brane configurations if a large and detectable non-Gaussianity is generated. Moreover models with a small number of coincident branes could generate a gravitational wave background that will be observable to future experiments.

Directional detection of dark matter can provide unambiguous observation of dark matter (DM) interactions even in the presence of insidious backgrounds. The DM-TPC collaboration is developing a detector with the goal of measuring the direction and sense (\'\'head-tail\'\') of nuclear recoils produced in spin-dependent DM interactions. The detector consists of a low pressure TPC with optical readout filled with CF4 gas at low pressure. A collision between a WIMP with a gas molecule results in a nucleus recoil of 1-2 mm. The measurement of the energy loss along the recoil allows us to determine the sense and the direction of the recoil. Results from a prototype detector operated in a low-energy neutron beam clearly demonstrate the suitability of this approach to measure directionality. In particular, the first observation of the \'\'head-tail\'\' effect for low-energy neutrons had been recently published by our Collaboration. A full-scale (1m^3) module is now being designed. This detector, which will be operated underground in 2009, will allow us to set limits on spin-dependent Dark Matter interactions using a directional detector. The sensitivity of this experiment will be discussed in this talk.

We consider the gravity in the system consisting of the BPS D3-brane embedded in the flat background geometry, produced by the solutions of the supergravity. The effective action for this system is represented by the sum of the Hilbert-Einstein and DBI actions. We derive the Wheeler-De Witt equation for this system and obtain analytical solutions in some special cases. We also calculate tunneling probability from Planckian size of D3-brane to the classical regime. This paper appeared in Phys. Rev. D 77, 066017 (2008)

I point out that there is a curvature singularity problem appearing on non-linear level that generally plagues f(R) models that modify Einstein gravity in the infrared. It is caused by the fact that for the effective scalar degree of freedom, the curvatur

We consider the most general treatment of primordial non-Gaussianities, arising from modifying the initial state. Besides considering non-Gaussian effects due to subhorizon particle production, we parameterize the initial non-Gaussian features in terms of a Boundary Effective Field Theory (BEFT). Both effects contribute to the final result for the bispectrum, and we use this to put constraints on the initial state.

We derive a simple consistency relation from the running of the tensor-to-scalar ratio. This new relation is first order in the slow-roll approximation. While for single field models we can obtain what can be found by using other observables, multi-field cases in general give non-trivial contributions dependent on the geometry of the field space and the inflationary dynamics, which can be probed observationally from this relation.

I will present a model of inflation in string theory, where the inflaton field corresponds to a Wilson line in the worldvolume of a D-brane, and in the presence of magnetic flux. Inflation ends in a hybrid fashion, when the Wilson line achieves a critical value and an open string mode becomes tachyonic. This scenario predicts a nearly flat, or red tilted, spectrum of scalar perturbations, with negligible primordial gravitational waves. Interestingly, there is a simple compactification in which the eta-problem, appearing in models of brane inflation, is absent.

We construct a consistent supersymmetric action for chiral and vector multiplets living on codimension-two branes in a six-dimensional chiral gauged supergravity. A nonzero brane tension can be compatible with the bulk supersymmetry by introducing a brane-localized Fayet-Iliopoulos term proportional to the brane tension. Moreover, we show that a brane chiral multiplet with nonzero R charge has a nontrivial coupling to the extra component of the U(1)_R gauge field strength and a singular scalar self-interaction term. We find that a unwarped football solution with 4D Minkowski vacuum exists for arbitrary brane tension and it preserves 4D N=1 supersymmetry. Dimensionally reducing to 4D for the football solution, we obtain the low energy action for moduli and brane multiplets in 4D effective supergravity with gauged U(1)_R. By assuming the bulk gaugino condensates as well as nonzero brane F- and/or D-term for the uplifting potential, we have all the moduli stabilized with a vanishing cosmological constant. The brane scalar with nonzero R charge then gets a soft mass of order the gravitino mass, the sign of which depends on the sign of the R charge as well as whether the brane F- or D-term dominates.