arXiv subjects

Tirthabir Biswas

Publications and source records attributed to Tirthabir Biswas.

54 records · Page 3Linked to original sources

arXiv2006

Non-perturbative Gravity, Hagedorn Bounce & CMB

In hep-th/0508194 it was shown how non-perturbative corrections to gravity can resolve the big bang singularity, leading to a bouncing universe. Depending on the scale of the non-perturbative corrections, the temperature at the bounce may be close to or higher than the Hagedorn temperature. If matter is made up of strings, then massive string states will be excited near the bounce, and the bounce will occur inside (or at the onset of) the Hagedorn phase for string matter. As we discuss in this paper, in this case cosmological fluctuations can be generated via the string gas mechanism recently proposed in hep-th/0511140. In fact, the model discussed here demonstrates explicitly that it is possible to realize the assumptions made in hep-th/0511140 in the context of a concrete set of dynamical background equations. We also calculate the spectral tilt of thermodynamic stringy fluctuations generated in the Hagedorn regime in this bouncing universe scenario. Generally we find a scale-invariant spectrum with a red tilt which is very small but does not vanish.

hep-th↗

arXiv2006

Nonlinear Structure Formation and "Apparent" Acceleration: an Investigation

We present an analytically solvable nonlinear model of structure formation in a Universe with only dust. The model is an LTB solution (of General Relativity) and structures are shells of different density. We show that the luminosity distance-redshift relation has significant corrections at low redshift when the density contrast becomes nonlinear. A minimal effect is a correction in apparent magnitudes of order 0.15. We discuss different possibilities that could further enhance this effect and mimick Dark Energy.

astro-ph↗

arXiv2006

Can Inflation solve the Hierarchy Problem?

Inflation with tunneling from a false to a true vacuum becomes viable in the presence of a scalar field that slows down the initial de Sitter phase. As a by-product this field also sets dynamically the value of the Newton constant observed today. This can be very large if the tunneling rate (which is exponentially sensitive to the barrier) is small enough. Therefore along with Inflation we also provide a natural dynamical explanation for why gravity is so weak today. Moreover we predict a spectrum of gravity waves peaked at around 0.1 mHz, that will be detectable by the planned space inteferometer LISA. Finally we discuss interesting predictions on cosmological scalar and tensor fluctuations in the light the WMAP 3-year data.

hep-ph↗

arXiv2005

Current Acceleration from Dilaton and Stringy Cold Dark Matter

We argue that string theory has all the ingredients to provide us with candidates for the cold dark matter and explain the current acceleration of our Universe. In any generic string compactification the dilaton plays an important role as it couples to the Standard Model and other heavy non-relativistic degrees of freedom such as the string winding modes and wrapped branes, we collectively call them stringy cold dark matter. These couplings are non-universal which results in an interesting dynamics for a rolling dilaton. Initially, its potential can track radiation and matter while beginning to dominate the dynamics recently, triggering a phase of acceleration. This scenario can be realized as long as the dilaton also couples strongly to some heavy modes. We furnish examples of such modes. We provide analytical and numerical results and compare them with the current supernovae result. This favors certain stringy candidates.

hep-th↗

arXiv2005

Bouncing Universes in String-inspired Gravity

We consider the effects on cosmology of higher-derivative modifications of (effective) gravity that make it asymptotically free without introducing ghosts. The weakening of gravity at short distances allows pressure to prevent the singularity, producing a solution with contraction preceding expansion.

hep-th↗

arXiv2005

Moduli Stabilization in Brane Gas Cosmology with Superpotentials

In the context of brane gas cosmology in superstring theory, we show why it is impossible to simultaneously stabilize the dilaton and the radion with a general gas of strings (including massless modes) and D-branes. Although this requires invoking a different mechanism to stabilize these moduli fields, we find that the brane gas can still play a crucial role in the early universe in assisting moduli stabilization. We show that a modest energy density of specific types of brane gas can solve the overshoot problem that typically afflicts potentials arising from gaugino condensation.

hep-th↗

arXiv2005

Dynamical CP Violation in the Early Universe and Leptogenesis

In a recent publication, we suggested a mechanism for obtaining dynamical CP violation in the early Universe based on the out-of-equilibrium evolution of complex scalar fields. In this paper, we suggest several ways of transferring the CP asymmetry from the scalar sector to the leptonic sector. In particular, we point out how a ``transient MNS(P) matrix'' can generate an asymmetry between fermions and anti-fermions directly.

hep-ph↗

arXiv2005

Coupled Inflation and Brane Gases

We study an effective four-dimensional theory with an action with two scalar fields minimally coupled to gravity, and with a matter action which couples to the two scalar fields via an overall field-dependent coefficient in the action. Such a theory could arise from a dimensional reduction of supergravity coupled to a gas of branes winding the compactified dimensions. We show the existence of solutions corresponding to power-law inflation. The graceful exit from inflation can be obtained by postulating the decay of the branes, as would occur if the branes are unstable in the vacuum and stabilized at high densities by plasma effects. This construction provides an avenue for connecting string gas cosmology and the late-time universe.

hep-th↗

arXiv2004

Cosmology from Moduli Dynamics

We investigate moduli field dynamics in supergravity/M-theory like set ups where we turn on fluxes along some or all of the extra dimensions. As has been argued in the context of string theory, we observe that the fluxes tend to stabilize the squashing (or shape) modes. Generically we find that at late times the shape is frozen while the radion evolves as a quintessence field. At earlier times we have a phase of radiation domination where both the volume and the shape moduli are slowly evolving. However, depending on the initial conditions and the parameters of the theory, like the value of the fluxes, curvature of the internal manifold and so on, the dynamics of the internal manifold can be richer with interesting cosmological consequences, including inflation.

hep-th↗

arXiv2004

Linear Regge trajectories from worldsheet lattice parton field theory

We show that unlike conventional field theory, the particle field theory of the string's constituents produces in the ladder approximation linear Regge trajectories, in accord with its string theory dual. In this theory propagators are Gaussian and this feature facilitates the perturbative evaluation of scattering amplitudes. We develop general techniques for studying their general asymptotic form. We consider radiative corrections to the ladder Regge trajectory and discover that linearity is lost; however, this may be due to certain approximations we have made.

hep-th↗

arXiv2004

Can we have a stringy origin behind $Ω_Λ(t)\propto Ω_m(t)$?

Inspired by the current observations that the ratio of the abundance of dark energy $Ω_Λ$, and the matter density, $Ω_{m}$, is such that $Ω_{m}/Ω_Λ\sim 0.37$, we provide a string inspired phenomenological model where we explain this order one ratio, the smallness of the cosmological constant, and also the recent cosmic acceleration. We observe that any effective theory motivated by a higher dimensional physics provides radion/dilaton couplings to the standard model and the dark matter component with different strengths. Provided radion/dilaton is a dynamical field we show that $Ω_Λ(t)$ tracks $Ω_{m}(t)$ and dominates very recently.

hep-th↗

arXiv2004

Dynamical CP Violation in the Early Universe

Following earlier ideas of Dolgov, we show that the asymmetrical dynamical evolution of fields in the early Universe provides a new source for CP violation. This can lead to baryogenesis without any additional CP-violating interactions. The magnitude of this CP violation is time-dependent. In particular, it vanishes (or is very small) in the late Universe after the fields have relaxed (or are in their final approach) to their vacuum values. We provide an explicit example in which our mechanism is realized.

hep-ph↗

arXiv2004

Quintessence and Inflation from Symmetry Breaking Transition of the Internal Manifold

We show that even in the simple framework of pure Kaluza-Klein gravity the shape moduli can generate potentials supporting inflation and/or quintessence. Using the shape moduli as the inflaton or quintessence-field has the additional benefit of being able to explain symmetry breaking in a natural geometric way. A numerical analysis suggests that in these models it may be possible to obtain sufficient e-foldings during inflation as well as a small cosmological constant at the current epoch (without fine tuning), while preserving the constraint coming from the fine structure constant.

hep-th↗

arXiv2004

Cosmology with Branes Wrapping Curved Internal Manifolds

In this paper we first derive solutions which can be interpreted as branes wrapping nontrivial curved manifolds, and then study their cosmological implications. We find that at early times the branes tend to shrink the internal manifold, while allowing the ``unwrapped'' dimensions to expand in congruence with what has already been observed in the case when the internal manifold is flat (tori). However, at late times the internal curvature terms become important leading to potentially interesting differences.

hep-th↗

arXiv2003

Quintessential Transition of the Internal Manifold

In this paper, I study the dynamics of the Kaluza-Klein internal manifold using its ``shape'' and the ``size'' as collective coordinates. The essential motivation is to be able to explain symmetry breaking in gauge theories through a transition of the internal manifold from a symmetrical metric space to a less symmetric one. We find that this may be possible depending on the values of certain group theoretical parameters. Further, this transition resembles a ``quintessential inflation'' scenario, the feasibility of which however, needs to be further studied in details.

gr-qc↗

arXiv2003

Symmetry Breaking of Gauge Theories via Internal Space Dynamics

In this paper, I explain how gauge symmetry can be broken in a geometric way, à la Kaluza-Klein. In higher dimensional gravitational theories, one usually considers the extra dimensions to be ``frozen'' in time. However, the internal manifold is actually a dynamic entity. For example, its metric can change even if one expects its topological properties to be invariant. It is conceivable then, that at an earlier epoch the internal manifold made a geometric transition from say a maximally symmetric metric space to a less symmetric one. We know in a Kaluza-Klein reduction scheme, the massless gauge bosons are associated with the Killing vectors of the internal manifold. After the transition of the internal manifold, the gauge bosons associated with the broken Killing isometries will pick up a mass thereby breaking the gauge invariance partially. In this paper, I explore this idea, work out the mass of broken gauge bosons for some simple examples, and also point out how a mechanism similar to that of Higgs may be at work.

hep-th↗

arXiv2002

Radial Dimensional Reduction: (Anti) de Sitter Theories from Flat

We propose a new form of dimensional reduction that constrains dilatation instead of a component of momentum. It corresponds to replacing toroidal compactification in a Cartesian coordinate with that in the logarithm of the radius. Massive theories in de Sitter or anti de Sitter space are thus produced from massless (scale invariant) theories in one higher space or time dimension. As an example, we derive free massive actions for arbitrary representations of the (anti) de Sitter group in arbitrary dimensions. (Previous general results were restricted to symmetric tensors.) We also discuss generalizations to interacting theories.

hep-th↗

arXiv2001

N=2 Harmonic Superforms, Multiplets and Actions

In this paper we develop the formalism to study superforms in N=2 harmonic superspace. We perform a thorough (if not complete) analysis of the superforms starting from 0-form and moving all the way up to 6-form. Like the N=1 case we find that the lower superforms (0,1,2,3) describe the various important N=2 supermultiplets. Also, the forms form chains, the field strength of 0-form being related to the gauge form of the 1-form, and so on. However, an important difference with the N=1 case is that there is now more than one chain. Our main aim was to study the higher-forms (4,5,6) to obtain the various N=2 action formulas via the ectoplasmic approach. Indeed, we reproduce the three known action formulas involving a volume integral, a contour integral and no integral over the harmonic subspace from the 6, 5 and 4-form analysis respectively. The next aim is to generalize the analysis to curved space-time (supergravity).

hep-th↗