arXiv subjects

S. Paban

Publications and source records attributed to S. Paban.

11 recordsLinked to original sources

arXiv2008

Perturbation Growth in Anisotropic Cosmologies

We study the growth of perturbations in an expanding Bianchi type-I metric that evolves according to an energy density that includes dust and a cosmological constant. Assuming an epoch where the cosmological constant is subdominant, we find that, for a reasonably large set of initial conditions, the metric fluctuations grow fast enough to make the metric inhomogeneous before the cosmological constant becomes the dominant form of energy. We have examined values for the cosmological constant that are in the interval $10^{10} {GeV}$ to $10^{16} {GeV}$.

hep-th↗

arXiv2007

Leptogenesis from Pseudo-Scalar Driven Inflation

We examine recent claims for a considerable amount of leptogenesis, in some inflationary scenarios, through the gravitational anomaly in the lepton number current. We find that when the short distances contributions are properly included the amount of lepton number generated is actually much smaller.

hep-th↗

arXiv2005

Evolution of Gravitationally Unstable de Sitter Compactifications

We study the time evolution of unstable $dS_p$ \times $S^q$ configurations with flux in theories of gravity with a cosmological constant. For certain values of the flux, we identify a stable configuration to which these unstable solutions flow. For other values of the flux the sphere wants to decompactify, regardless of the sign of the initial perturbation.

hep-th↗

arXiv2004

The Energy Density of "Wound" Fields in a Toroidal Universe

The observational limits on the present energy density of the Universe allow for a component that redshifts like $1/a^2$ and can contribute significantly to the total. We show that a possible origin for such a contribution is that the universe has a toroidal topology with "wound" scalar fields around its cycles.

astro-ph↗

arXiv2003

The Entropy of the Microwave Background and the Acceleration of the Universe

If the present acceleration of the universe is due to a cosmological constant, λ, then the entropy of the microwave background is bounded. It cannot exceed λ^{-3/4} \sim 10^{91}, which is much less than the entropy of empty de Sitter space λ^{-1} \sim 10^{122}. This is due to the limited efficiency of storing entropy by local field theoretical degrees of freedom. The observed entropy of the microwave background is of O(10^{85}).

hep-th↗

arXiv2002

Recurrent Nightmares?: Measurement Theory in de Sitter Space

The idea that asymptotic de Sitter space can be described by a finite Hilbert Space implies that any quantum measurement has an irreducible innacuracy. We argue that this prevents any measurement from verifying the existence of the Poincare recurrences that occur in the mathematical formulation of quantum de Sitter (dS) space. It also implies that the mathematical quantum theory of dS space is not unique. There will be many different Hamiltonians, which give the same results, within the uncertainty in all possible measurements.

hep-th↗

arXiv2002

Entropy of the Stiffest Stars

We analyze the properties of stars whose interior is described by the stiffest equation of state consistent with causality. We note the remarkable fact that the entropy of such stars scales like the area.

hep-th↗

arXiv2001

The Acceleration of the Universe, a Challenge for String Theory

Recent astronomical observations indicate that the universe is accelerating. We argue that generic quintessence models that accommodate the present day acceleration tend to accelerate eternally. As a consequence the resulting spacetimes exhibit event horizons. Hence, quintessence poses the same problems for string theory as asymptotic de Sitter spaces.

hep-th↗

arXiv2000

Evidence for Winding States in Noncommutative Quantum Field Theory

We study noncommutative field theories at finite temperature to learn more about the degrees of freedom in the non-planar sector of these systems. We find evidence for winding states. At temperatures for which the thermal wavelength is smaller than the noncommutativity scale, there is a drastic reduction of the degrees of freedom in the non-planar sector. In this regime, the non-planar sector has thermodynamics resembling that of a 1+1 dimensional field theory.

hep-th↗

arXiv2000

The Interplay Between $θ$ and T

We extend a recent computation of the dependence of the free energy, F, on the noncommutative scale $θ$ to theories with very different UV sensitivity. The temperature dependence of $F$ strongly suggests that a reduced number of degrees of freedom contributes to the free energy in the non-planar sector, $F_{\rm np}$, at high temperature. This phenomenon seems generic, independent of the UV sensitivity, and can be traced to modes whose thermal wavelengths become smaller than the noncommutativity scale. The temperature dependence of $F_{\rm np}$ can then be calculated at high temperature using classical statistical mechanics, without encountering a UV catastrophe even in large number of dimensions. This result is a telltale sign of the low number of degrees of freedom contributing to $F$ in the non-planar sector at high temperature. Such behavior is in marked contrast to what would happen in a field theory with a random set of higher derivative interactions.

hep-th↗

arXiv1993

Is Induced QCD really QCD? The preservation of asymptotic freedom by matter interactions

The program of induced QCD requires that there exist self-interactions among the heavy matter fields (an adjoint scalar and a few fermions in the fundamental representation) which tend to spoil the asymptotic freedom of SU(N) gauge theory. We consider general interactions between such matter fields and show that, on the contrary, to two loops, they all tend to enhance asymptotic freedom in the perturbative regime. This result casts doubt on whether induced QCD is equivalent to real QCD.

hep-th↗