arXiv subjects

L. Perivolaropoulos

Publications and source records attributed to L. Perivolaropoulos.

At least 73 records · Page 4Linked to original sources

arXiv2003

Equation of State of Oscillating Brans-Dicke Scalar and Extra Dimensions

We consider a Brans-Dicke scalar field stabilized by a general power law potential with power index $n$ at a finite equilibrium value. Redshifting matter induces oscillations of the scalar field around its equilibrium due to the scalar field coupling to the trace of the energy momentum tensor. If the stabilizing potential is sufficiently steep these high frequency oscillations are consistent with observational and experimental constraints for arbitrary value of the Brans-Dicke parameter $ω$. We study analytically and numerically the equation of state of these high frequency oscillations in terms of the parameters $ω$ and $n$ and find the corresponding evolution of the universe scale factor. We find that the equation of state parameter can be negative and less than -1 but it is not related to the evolution of the scale factor in the usual way. Nevertheless, accelerating expansion is found for a certain parameter range. Our analysis applies also to oscillations of the size of extra dimensions (the radion field) around an equilibrium value. This duality between self-coupled Brans-Dicke and radion dynamics is applicable for $ω= -1 + 1/D$ where D is the number of extra dimensions.

hep-ph↗

arXiv2002

Cosmological Effects of Radion Oscillations

We show that the redshift of pressureless matter density due to the expansion of the universe generically induces small oscillations in the stabilized radius of extra dimensions (the radion field). The frequency of these oscillations is proportional to the mass of the radion and can have interesting cosmological consequences. For very low radion masses $m_b$ ($m_b\sim10-100 H_0\simeq10^{-32} eV$) these low frequency oscillations lead to oscillations in the expansion rate of the universe. The occurrence of acceleration periods could naturally lead to a resolution of the coincidence problem, without need of dark energy. Even though this scenario for low radion mass is consistent with several observational tests it has difficulty to meet fifth force constraints. If viewed as an effective Brans-Dicke theory it predicts $ω=-1+\frac{1}{D}$ ($D$ is the number of extra dimensions), while experiments on scales larger than $1mm$ imply $ω>2500$. By deriving the generalized Newtonian potential corresponding to a massive toroidally compact radion we demonstrate that Newtonian gravity is modified only on scales smaller than $m_b^{-1}$. Thus, these constraints do not apply for $m_b>10^{-3} eV$ (high frequency oscillations) corresponding to scales less than the current experiments ($0.3mm$). Even though these high frequency oscillations can not resolve the coincidence problem they provide a natural mechanism for dark matter generation. This type of dark matter has many similarities with the axion.

hep-ph↗

arXiv2002

Rotating Toroidal Branes in Supermembrane and Matrix Theory

In the lightcone frame, where the supermembrane theory and the Matrix model are strikingly similar, the equations of motion admit an elegant complexification in even dimensional spaces. Although the explicit rotational symmetry of the target space is lost, the remaining unitary symmetries apart from providing a simple and unifying description of all known solutions suggest new ones for rotating spherical and toroidal membranes. In this framework the angular momentum is represented by U(1) charges which balance the nonlinear attractive forces of the membrane. We examine in detail a six dimensional rotating toroidal membrane solution which lives in a 3-torus, $T^3$ and admits stable radial modes. In Matrix Theory it corresponds to a toroidal N-$D_{0}$ brane bound state. We demonstrate its existence and discuss its radial stability.

hep-th↗

arXiv2002

Dark Energy and the quietness of the Local Hubble Flow

The linearity and quietness of the Local ($< 10 Mpc$) Hubble Flow (LHF) in view of the very clumpy local universe is a long standing puzzle in standard and in open CDM cosmogony. The question addressed in this paper is whether the antigravity component of the recently discovered dark energy can cool the velocity flow enough to provide a solution to this puzzle. We calculate the growth of matter fluctuations in a flat universe containing a fraction $Ω_X(t_0)$ of dark energy obeying the time independent equation of state $p_X = w ρ_X$. We find that dark energy can indeed cool the LHF. However the dark energy parameter values required to make the predicted velocity dispersion consistent with the observed value $v_{rms}\simeq 40km/sec$ have been ruled out by other observational tests constraining the dark energy parameters $w$ and $Ω_X$. Therefore despite the claims of recent qualitative studies dark energy with time independent equation of state can not by itself explain the quietness and linearity of the Local Hubble Flow.

astro-ph↗

arXiv2001

Scale Dependence of Dark Energy Antigravity

We investigate the effects of negative pressure induced by dark energy (cosmological constant or quintessence) on the dynamics at various astrophysical scales. Negative pressure induces a repulsive term (antigravity) in Newton's law which dominates on large scales. Assuming a value of the cosmological constant consistent with the recent SnIa data we determine the critical scale $r_c$ beyond which antigravity dominates the dynamics ($r_c \sim 1Mpc $) and discuss some of the dynamical effects implied. We show that dynamically induced mass estimates on the scale of the Local Group and beyond are significantly modified due to negative pressure. We also briefly discuss possible dynamical tests (eg effects on local Hubble flow) that can be applied on relatively small scales (a few $Mpc$) to determine the density and equation of state of dark energy.

astro-ph↗

arXiv2001

Quadrupole Instabilities of Relativistic Rotating Membranes

We generalize recent study of the stability of isotropic (spherical) rotating membranes to the anisotropic ellipsoidal membrane. We find that while the stability persists for deformations of spin $l=1$, the quadrupole and higher spin deformations ($l\geq 2$) lead to instabilities. We find the relevant instability modes and the corresponding eigenvalues. These indicate that the ellipsoidal rotating membranes generically decay into finger-like configurations.

hep-th↗

arXiv2001

Q Rings

We show the existence of new stable ring-like localized scalar field configurations whose stability is due to a combination of topological and nontopological charges. In that sense these defects may be called semitopological. These rings are Noether charged and also carry Noether current (they are superconducting). They are local minima of the energy in scalar field theories with an unbroken U(1) global symmetry. We obtain numerical solutions of the field configuration corresponding to large rings and derive virial theorems demonstrating their stability. We also derive the minimum energy field configurations in 3D and simulate the evolution of a finite size Q ring on a three dimensional lattice thus generalizing our demonstration of stability.

hep-ph↗

arXiv2000

Metastability of Spherical Membranes in Supermembrane and Matrix Theory

Motivated by recent work we study rotating ellipsoidal membranes in the framework of the light-cone supermembrane theory. We investigate stability properties of these classical solutions which are important for the quantization of super membranes. We find the stability modes for all sectors of small multipole deformations. We exhibit an isomorphism of the linearized membrane equation with that of the SU(N) matrix model for every value of $N$. The boundaries of the linearized stability region are at a finite distance and they appear for finite size perturbations.

hep-th↗

arXiv2000

Construction of Cosmic String Induced Temperature Anisotropy Maps with CMBFAST and Statistical Analysis

We use the publicly available code CMBFAST as modified by L. Pogosian and T. Vachaspati to simulate the effects of wiggly cosmic strings on the Cosmic Microwave Background (CMB). Using the modified CMBFAST code which takes into account vector modes and models wiggly cosmic strings by the one scale model, we go beyond the angular power spectrum to construct CMB temperature maps with resolution of a few degrees. The statistics of these maps are then studied using conventional and recently proposed statistical tests optimized for the detection of hidden temperature discontinuities induced by the Gott-Kaiser-Stebbins effect. We show however, that these realistic maps can not be distinguished in a statistically significant way from purely Gaussian maps with identical power spectrum.

astro-ph↗

arXiv2000

Some Dynamical Effects of the Cosmological Constant

Newton's law gets modified in the presence of a cosmological constant by a small repulsive term (antigarvity) that is proportional to the distance. Assuming a value of the cosmological constant consistent with the recent SnIa data ($Λ\simeq 10^{-52} m^{-2}$) we investigate the significance of this term on various astrophysical scales. We find that on galactic scales or smaller (less than a few tens of kpc) the dynamical effects of the vacuum energy are negligible by several orders of magnitude. On scales of 1Mpc or larger however we find that vacuum energy can significantly affect the dynamics. For example we show that the velocity data in the Local Group of galaxies correspond to galactic masses increased by 35% in the presence of vacuum energy. The effect is even more important on larger low density systems like clusters of galaxies or superclusters.

astro-ph↗

arXiv2000

Stabilizing Textures in 3+1 Dimensions with Semilocality

It is shown that textures in 3+1 dimensions can be stabilized by partial gauging (semilocality) of the vacuum manifold such that topological unwinding by a gauge transformation is not possible. This introduction of gauge fields can be used to evade Derrick's theorem without higher powers of derivatives and prevent the usual collapse of textures by the effective `pressure' terms induced by the gauge fields. A virial theorem is derived and shown to be a manifestation of the stability of the configuration towards collapse.

hep-ph↗

arXiv1999

Superconducting String Texture

We present a detailed analytical and numerical study of a novel type of static, superconducting, classically stable string texture in a renormalizable topologically trivial massive U(1) gauge model with one charged and one neutral scalar. An upper bound on the mass of the charged scalar as well as on the current that the string can carry are established. A preliminary unsuccesful search for stable solutions corresponding to large superconducting loops is also reported.

hep-ph↗

arXiv1999

Superconducting Semilocal Stringy (Hopf) Textures

The dynamics of texture-like configurations are briefly reviewed. Emphasis is given to configurations in 2+1 dimensions which are constructed numerically. Confirming previous semi-analytical studies it is shown that they can be stabilized by partial gauging of the vacuum manifold (semilocality) in a finite range of parameter space. When these configurations are extended to 3+1 dimensions (stringy textures) it is shown that they can support persistent currents if a twist (Hopf charge) is introduced in the scalar field sector. The pressure induced by these persistent currents is also studied in closed loops. In the context of a simple model, twist induced pressure is shown to be insufficient to stabilize the loops against collapse due to tension

hep-ph↗

arXiv1998

Hints of Cosmic String Induced Discontinuities in the COBE Data?

We apply new statistical tests on the four year 53 GHz DMR data and show that there is significant probability for the existence of coherent temperature discontinuities hidden in the CMB maps around the Galactic Poles. Comparing Monte Carlo simulations with the DMR maps we find that the probability for the existence of a coherent discontinuity with amplitude $0.2 \times ({{δT} \over T})_{rms}$ superposed on the gaussian fluctuations is more than double than the corresponding probability when no discontinuity is present. This result is consistent with the existence of a horizon size long cosmic string with mass per unit length $μ$ given by $Gμv_s γ_s \simeq 10^{-6}/π$ where $v_s$ is the string velocity and $γ_s$ is the corresponding Lorentz factor.

astro-ph↗

arXiv1998

Searching for Long Strings in CMB Maps

Using analytical methods and Monte Carlo simulations, we analyze new statistics designed to detect isolated step-like discontinuities which are coherent over large areas of Cosmic Microwave Background (CMB) pixel maps. Such coherent temperature discontinuities are predicted by the Kaiser-Stebbins effect to form due to long cosmic strings present in our present horizon. The background of the coherent step-like seed is assumed to be a scale invariant Gaussian random field which could have been produced by a superposition of seeds on smaller scales and/or by inflationary quantum fluctuations. We find that the proposed statistics can detect the presense of a coherent discontinuity at a sensitivity level almost an order of magnitude better compared to more conventional statistics like the skewness or the kurtosis.

astro-ph↗

arXiv1998

Core Phase Structure of Cosmic Strings and Monopoles

Global and local symmetries may or may not be restored inside topological defects depending upon the values of the parameters of the model. A detailed study of this parameter dependence of the core structure of strings and monopoles is presented in the context of simple models.

hep-ph↗

arXiv1998

Core Phase Transitions for Embedded Topological Defects

Vortices in superfluid 3He-B have been observed to undergo a core transition. We discuss the analog phenomenon in relativistic field theories which admit embedded global domain walls, vortices and monopoles with a core phase structure. They are present in scalar field theories with approximate global symmetries which are broken both spontaneously and in parts explicitly. For a particular range of parameters their symmetric core exhibits an instability and decays into the nonsymmetric phase.

hep-ph↗

arXiv1994

Cosmic String Theory: The Current Status

This is a pedagogical introduction to the basics of the cosmic string theory and also a review of the recent progress made with respect to the macrophysical predictions of the theory. Topics covered include, string formation and evolution, large scale structure formation, generation of peculiar velocity flows, cosmic microwave background (CMB) fluctuations, lensing, gravitational waves and constraints from the msec pulsar. Particular emphasis is placed on the signatures predicted on the CMB and the corresponding non-gaussian features.

astro-ph↗