Inflation and dark energy arising from geometrical tachyons

We study the motion of a BPS D3-brane in the NS5-brane ring background. The radion field becomes tachyonic in this geometrical set up. We investigate the potential of this geometrical tachyon in the cosmological scenario for inflation as well as dark energy. We evaluate the spectra of scalar and tensor perturbations generated during tachyon inflation and show that this model is compatible with recent observations of Cosmic Microwave Background (CMB) due to an extra freedom of the number of NS5-branes. It is not possible to explain the origin of both inflation and dark energy by using a single tachyon field, since the energy density at the potential minimum is not negligibly small because of the amplitude of scalar perturbations set by CMB anisotropies. However geometrical tachyon can account for dark energy when the number of NS5-branes is large, provided that inflation is realized by another scalar field.Comment: 11 pages, 8 figure

Hot Nuclear Matter : A Variational Approach

We develop a nonperturbative technique in field theory to study properties of infinite nuclear matter at zero temperature as well as at finite temperatures. Here we dress the nuclear matter with off-mass shell pions. The techniques of thermofield dynamics are used for finite temperature calculations. Equation of state is derived from the dynamics of the interacting system in a self consistent manner. The transition temperature for nuclear matter appears to be around 15 MeV.Comment: 16 pages, IP/BBSR/91-3

Neutron matter - Quark matter phase transition and Quark star

We consider the neutron matter quark matter phase transition along with possible existence of hybrid quark stars. The equation of state for neutron matter is obtained using a nonperturbative method with pion dressing of the neutron matter and an analysis similar to that of symmetric nuclear matter. The quark matter sector is treated perturbatively in the small distance domain. For bag constant $B^{1/4}$=148 MeV, a first order phase transition is seen. In the context of neutron quark hybrid stars, Tolman-Oppenheimer-Volkoff equations are solved using the equations of state for quark matter and for neutron matter with a phase transition as noted earlier. Stable solutions for such stars are obtained with the Chandrasekhar limit as 1.58 $M_\odot$ and radius around 10 km. The bulk of the star is quark matter with a thin crust of neutron matter of less than a kilometer.Comment: 28 pages including 9 figures, Revtex, IP/BBSR/92-8