arXiv subjects

Leandros Perivolaropoulos

Publications and source records attributed to Leandros Perivolaropoulos.

At least 73 records · Page 4Linked to original sources

arXiv2014

Large Scale Cosmological Anomalies and Inhomogeneous Dark Energy

A wide range of large scale observations hint towards possible modifications on the standard cosmological model which is based on a homogeneous and isotropic universe with a small cosmological constant and matter. These observations, also known as "cosmic anomalies" include unexpected Cosmic Microwave Background perturbations on large angular scales, large dipolar peculiar velocity flows of galaxies ("bulk flows"), the measurement of inhomogenous values of the fine structure constant on cosmological scales ("alpha dipole") and other effects. The presence of the observational anomalies could either be a large statistical fluctuation in the context of {\lcdm} or it could indicate a non-trivial departure from the cosmological principle on Hubble scales. Such a departure is very much constrained by cosmological observations for matter. For dark energy however there are no significant observational constraints for Hubble scale inhomogeneities. In this brief review I discuss some of the theoretical models that can naturally lead to inhomogeneous dark energy, their observational constraints and their potential to explain the large scale cosmic anomalies.

astro-ph.CO↗

arXiv2013

Stabilizing the Semilocal String with a Dilatonic Coupling

We demonstrate that the stability of the semilocal vortex can be significantly improved by the presence of a dilatonic coupling of the form e^\frac{q | Φ|^2}{η^2} F_{μν}F^{μν} with q>0 where ηis the scale of symmetry breaking that gives rise to the vortex. For q=0 we obtain the usual embedded (semilocal) Nielsen-Olesen vortex. We find the stability region of the parameter β= (\frac{m_Φ}{m_A})^2 (m_Φand m_A are the masses of the scalar and gauge fields respectively). We show that the stability region of βis 0<β<β_{max}(q) where β_{max}(q=0)=1 (as expected) and β_{max}(q) is an increasing function of q. This result may have significant implications for the stability of the electroweak vortex in the presence of a dilatonic coupling (dilatonic electroweak vortex).

hep-ph↗

arXiv2013

Observational constraints on viable f(R) parametrizations with geometrical and dynamical probes

We demonstrate that a wide range of viable f(R) parameterizations (including the Hu & Sawicki and the Starobinsky models) can be expressed as perturbations deviating from the LCDM Lagrangian. We constrain the deviation parameter b using a combination of geometrical and dynamical observational probes. In particular, we perform a joint likelihood analysis of the recent Supernovae Type Ia data, the Cosmic Microwave Background shift parameters, the Baryonic Acoustic Oscillations and the growth rate data provided by the various galaxy surveys. This analysis provides constraints for the following parameters: the matter density Omega_{m0}, the deviation from LCDM parameter b and the growth index gamma(z). We parametrize the growth index gamma(z) in three manners (constant, Taylor expansion around z=0, and Taylor expansion around the scale factor). We point out the numerical difficulty for solving the generalized f(R) Friedman equation at high redshifts due to stiffness of the resulting ordinary differential equation. We resolve this problem by constructing an efficient analytical perturbative method in the deviation parameter b. We demonstrate that this method is highly accurate, by comparing the resulting analytical expressions for the Hubble parameter, with the numerical solutions at low and intermediate redshifts. Surprisingly, despite of its perturbative nature, the accuracy of the method persists even for values of b that are of O(1).

astro-ph.CO↗

arXiv2012

CMB Maximum Temperature Asymmetry Axis: Alignment with Other Cosmic Asymmetries

We use a global pixel based estimator to identify the axis of the residual Maximum Temperature Asymmetry (MTA) (after the dipole subtraction) of the WMAP 7 year Internal Linear Combination (ILC) CMB temperature sky map. The estimator is based on considering the temperature differences between opposite pixels in the sky at various angular resolutions (4 degrees-15 degrees and selecting the axis that maximizes this difference. We consider three large scale Healpix resolutions (N_{side}=16 (3.7 degrees), N_{side}=8 (7.3 degrees) and N_{side}=4 (14.7 degrees)). We compare the direction and magnitude of this asymmetry with three other cosmic asymmetry axes (αdipole, Dark Energy Dipole and Dark Flow) and find that the four asymmetry axes are abnormally close to each other. We compare the observed MTA axis with the corresponding MTA axes of 10^4 Gaussian isotropic simulated ILC maps (based on LCDM). The fraction of simulated ILC maps that reproduces the observed magnitude of the MTA asymmetry and alignment with the observed αdipole is in the range of 0.1%-0.5%$ (depending on the resolution chosen for the CMB map). The corresponding magnitude+alignment probabilities with the other two asymmetry axes (Dark Energy Dipole and Dark Flow) are at the level of about 1%. We propose Extended Topological Quintessence as a physical model qualitatively consistent with this coincidence of directions.

astro-ph.CO↗

arXiv2012

Is there Correlation between Fine Structure and Dark Energy Cosmic Dipoles?

We present a detailed analysis (including redshift tomography) of the cosmic dipoles in the Keck+VLT quasar absorber and in the Union2 SnIa samples. We show that the fine structure constant cosmic dipole obtained through the Keck+VLT quasar absorber sample at $4.1σ$ level is anomalously aligned with the corresponding dark energy dipole obtained through the Union2 sample at $2σ$ level. The angular separation between the two dipole directions is $11.3^\circ \pm 11.8^\circ$. We use Monte Carlo simulations to find the probability of obtaining the observed dipole magnitudes with the observed alignment, in the context of an isotropic cosmological model with no correlation between dark energy and fine structure constant $α$. We find that this probability is less than one part in $10^6$. We propose a simple physical model (extended topological quintessence) which naturally predicts a spherical inhomogeneous distribution for both dark energy density and fine structure constant values. The model is based on the existence of a recently formed giant global monopole with Hubble scale core which also couples non-minimally to electromagnetism. Aligned dipole anisotropies would naturally emerge for an off-centre observer for both the fine structure constant and for dark energy density. This model smoothly reduces to \lcdm for proper limits of its parameters. Two predictions of this model are (a) a correlation between the existence of strong cosmic electromagnetic fields and the value of $α$ and (b) the existence of a dark flow on Hubble scales due to the repulsive gravity of the global defect core (`Great Repulser') aligned with the dark energy and $α$ dipoles. The direction of the dark flow is predicted to be towards the spatial region of lower accelerating expansion. Existing data about the dark flow are consistent with this prediction.

astro-ph.CO↗

arXiv2011

Topological Quintessence

A global monopole (or other topological defect) formed during a recent phase transition with core size comparable to the present Hubble scale, could induce the observed accelerating expansion of the universe. In such a model, topological considerations trap the scalar field close to a local maximum of its potential in a cosmologically large region of space. We perform detailed numerical simulations of such an inhomogeneous dark energy system (topological quintessence) minimally coupled to gravity, in a flat background of initially homogeneous matter. We find that when the energy density of the field in the monopole core starts dominating the background density, the spacetime in the core starts to accelerate its expansion in accordance to a ΛCDM model with an effective inhomogeneous spherical dark energy density parameter Ω_Λ(r). The matter density profile is found to respond to the global monopole profile via an anti-correlation (matter underdensity in the monopole core). Away from the monopole core, the spacetime is effectively Einstein-deSitter (Ω_Λ(r_{out}) -> 0) while at the center Ω_Λ(r ~ 0) is maximum. We fit the numerically obtained expansion rate at the monopole core to the Union2 data and show that the quality of fit is almost identical to that of ΛCDM. Finally, we discuss potential observational signatures of this class of inhomogeneous dark energy models.

astro-ph.CO↗

arXiv2010

Parametrization for the Scale Dependent Growth in Modified Gravity

We propose a scale dependent analytic approximation to the exact linear growth of density perturbations in Scalar-Tensor (ST) cosmologies. In particular, we show that on large subhorizon scales, in the Newtonian gauge, the usual scale independent subhorizon growth equation does not describe the growth of perturbations accurately, as a result of scale-dependent relativistic corrections to the Poisson equation. A comparison with exact linear numerical analysis indicates that our approximation is a significant improvement over the standard subhorizon scale independent result on large subhorizon scales. A comparison with the corresponding results in the Synchronous gauge demonstrates the validity and consistency of our analysis.

astro-ph.CO↗

arXiv2009

The spherical collapse model and cluster formation beyond the $Λ$ cosmology: Indications for a clustered dark energy?

We generalize the small scale dynamics of the universe by taking into account models with an equation of state which evolves with time, and provide a complete formulation of the cluster virialization attempting to address the nonlinear regime of structure formation. In the context of the current dark energy models, we find that galaxy clusters appear to form at z\sim 1-2, in agreement with previous studies. Also, we investigate thoroughly the evolution of spherical matter perturbations, as the latter decouple from the background expansion and start to ``turn around'' and finally collapse. Within this framework, we find that the concentration parameter depends on the choice of the considered dark energy (homogeneous or clustered). In particular, if the distribution of the dark energy is clustered then we produce more concentrated structures with respect to the homogeneous dark energy. Finally, comparing the predicted concentration parameter with the observed concentration parameter, measured for four massive galaxy clusters, we find that the scenario which contains a pure homogeneous dark energy is unable to reproduce the data. The situation becomes somewhat better in the case of an inhomogeneous (clustered) dark energy.

astro-ph.CO↗

arXiv2009

New Parametrization for the Scale Dependent Growth Function in General Relativity

We demonstrate the scale dependence of the growth function of cosmological perturbations in dark energy models based on General Relativity. This scale dependence is more prominent on cosmological scales of $100h^{-1}Mpc$ or larger. We derive a new scale dependent parametrization which generalizes the well known Newtonian approximation result $f_0(a)\equiv \frac{d\ln δ_0}{d\ln a}=\om(a)^γ$ ($γ={6/11}$ for \lcdm) which is a good approximation on scales less than $50h^{-1}Mpc$. Our generalized parametrization is of the form $f(a)=\frac{f_0(a)}{1+ξ(a,k)}$ where $ξ(a,k)=\frac{3 H_{0}^{2} \omm}{a k^2}$. We demonstrate that this parametrization fits the exact result of a full general relativistic evaluation of the growth function up to horizon scales for both \lcdm and dynamical dark energy. In contrast, the scale independent parametrization does not provide a good fit on scales beyond 5% of the horizon scale ($k\simeq 0.01 h^{-1}Mpc$).

astro-ph.CO↗

arXiv2008

Vacuum Energy, the Cosmological Constant and Compact Extra Dimensions: Constraints from Casimir Effect Experiments

We consider a universe with a compact extra dimension and a cosmological constant emerging from a suitable ultraviolet cutoff on the zero point energy of the vacuum. We derive the Casimir force between parallel conducting plates as a function of the following scales: plate separation, radius of the extra dimension and cutoff energy scale. We find that there are critical values of these scales where the Casimir force between the plates changes sign. For the cutoff energy scale required to reproduce the observed value of the cosmological constant, we find that the Casimir force changes sign and becomes repulsive for plate separations less than a critical separation $d_0=0.6mm$, assuming a zero radius of the extra dimension (no extra dimension). This prediction contradicts Casimir experiments which indicate an attractive force down to plate separations of $100nm$. For a non-zero extra dimension radius, the critical separation $d_0$ gets even larger than $0.6mm$ and remains inconsistent with Casimir force experiments. We conclude that with or without the presence of a compact extra dimension, vacuum energy with any suitable cutoff can not play the role of the cosmological constant.

astro-ph↗

arXiv2000

Dynamics of Nontopological Solitons - Q Balls

We use numerical simulations and semi-analytical methods to investigate the stability and the interactions of nontopological stationary qball solutions. In the context of a simple model we map the parameter sectors of stability for a single qball and verify the result using numerical simulations of time evolution. The system of two interacting qballs is also studied in one and two space dimensions. We find that the system generically performs breather type oscillations with frequency equal to the difference of the internal qball frequencies. This result is shown to be consistent with the form of the qball interaction potential. Finally we perform simulations of qball scattering and show that the right angle scattering effect observed in topological soliton scattering in two dimensions, persists also in the case of qballs where no topologically conserved quantities are present. For relativistic collision velocities the qball charge is split into a forward and a right angle scattering component. As the collision velocity increases, the forward component gets amplified at the expense of the right angle component. The animated simulations of this study may be found on-line at the address http://leandros.chem.demokritos.gr/qballs/index.html

hep-ph↗

arXiv1998

Phase Transitions in the Core of Global Embedded Defects

We demonstrate the existence of global monopole and vortex configurations whose core exhibits a phase structure. We determine the critical values of parameters for which the transition from the symmetric to the non-symmetric phase occurs and discuss the novel dynamics implied by the non-symmetric cores for defect interactions. We model phase transitions in the core of global embedded topological defects by identifying the relevant parameters with the vacuum expectation value of a dynamical scalar field. Finally, we argue that superheavy defects that undergo a core phase transition in the very early universe provide a novel realization for topological inflation.

hep-ph↗

arXiv1997

A New Statistic for the Detection of Long Strings in Microwave Background Maps

A new statistic is briefly reviewed, designed to detect isolated coherent step-like discontinuities produced by cosmic strings present at late times. As a background I superpose a scale invariant Gaussian random field which could have been produced by a superposition of seeds on all scales and/or by inflationary quantum fluctuations. The statistical variable considered is the Sample Mean Difference (SMD) between large neighbouring sectors of CMB maps, separated by lines in two dimensional maps and points in one dimensional maps. I find that the SMD statistics can detect at the $1σ$ level the presense of a long string with $Gμ(v_s γ_s)={1\over {8π}}({{δT}\over T})_{rms} \simeq 0.5 \times 10^{-7}$ while more conventional statistics like the skewness or the kurtosis require a value of $Gμ$ almost an order of magnitude larger for detectability at a comparable level.

astro-ph↗

arXiv1997

Domain Walls and Vortices with Non-Symmetric Core

We review recent work on a new class of topological defects which possess a nonsymmetric core. They arise in scalar field theories with global symmetries, U(1) for domain walls and SU(2) for vortices, which are explicitly broken to $Z_2$ and U(1) respectively. Both of the latter symmetries are spontaneously broken. For a particular range of parameters both types of defect solutions are shown to become unstable and decay to the well known stable walls and vortices with symmetric cores.

hep-ph↗

arXiv1997

A Statistic for the Detection of Long Strings in Microwave Background Maps

Using analytical methods and Monte Carlo simulations, we analyze a new statistic 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. The effects of uncorrelated Gaussian random noise are also studied. The statistical variable considered is the Sample Mean Difference (SMD) between large neighbouring sectors of CMB maps, separated by a straight line in two dimensional maps and a point in one dimensional maps. We find that including noise, the SMD statistics can detect at the $1 σ$ to $2 σ$ level the presense of a long string with $Gμ(v_s γ_s)= 1/(8π) ((δT)/T)_{rms} \simeq 0.5 \times 10^{-7}$ while more conventional statistics like the skewness or the kurtosis require a value of $Gμ$ almost an order of magnitude larger for detectability at a comparable level.

astro-ph↗

arXiv1997

Topological Defects with Non-Symmetric Core

We demonstrate that field theories involving explicit breaking of continous symmetries, incorporate two generic classes of topological defects each of which is stable for a particular range of parameters. The first class includes defects of the usual type where the symmetry gets restored in the core and vacuum energy gets trapped there. We show however that these defect solutions become unstable for certain ranges of parameters and decay not to the vacuum but to another type of stable defect where the symmetry in not restored in the core. In the wall case, initially spherical, bubble-like configurations are simulated numerically and shown to evolve generically towards a planar collapse. In the string case, the decay of the symmetric core vortex resembles the decay of a semilocal string to a skyrmion with the important difference that while the skyrmion is unstable and decays to the vacuum, the resulting non-symmetric vortex is topologically stable.

hep-ph↗

arXiv1995

Spectra and Statistics of Cosmic String Perturbations on the Microwave Background: A Monte Carlo Approach

Using Monte Carlo simulations of perturbations induced by cosmic strings on the microwave background, we demonstrate the scale invariance of string fluctuation patterns. By comparing string-induced fluctuation patterns with gaussian random phase ones, we show that the non-gaussian signatures of the string patterns are detectable by tests based on the moments of the distributions only for angular scales smaller than a few arcminutes and for maps based on the gradient of temperature fluctuations. However, we find that tests of the gaussianity of the moments fail when we include a reasonable amount of instrumental noise in a pattern. Signal to noise ratios of $3.3$ or greater completely suppress a string pattern's non-gaussian features even at the highest resolutions.

astro-ph↗

arXiv1995

Formation of Vortices in First Order Phase Transitions

Using a toy model Lagrangian we investigate the formation of vortices in first order phase transitions. The evolution and interactions of vacuum bubbles are also studied using both analytical approximations and a numerical simulation of scalar field dynamics. A long lived bubble wall bound state is discovered and its existence is justified by using a simplified potential for the bubble wall interaction. The conditions that need to be satisfied for vortex formation by bubble collisions are also studied with particular emphasis placed on geometrical considerations. These conditions are then implemented in a Monte Carlo simulation for the study of the probability of defect formation. It is shown that the probability of vortex formation by collision of relativistically expanding bubbles gets reduced by about 10% due to the above mentioned geometric effects.

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