arXiv subjects

Leandros Perivolaropoulos

Publications and source records attributed to Leandros Perivolaropoulos.

86 records · Page 5Linked to original sources

arXiv1994

Cosmic Strings and Cosmic Variance

By using a simple analytical model based on counting random multiple impulses inflicted on photons by a network of cosmic strings we show how to construct the general q-point temperature correlation function of the Cosmic Microwave Background radiation. Our analysis is sensible specially for large angular scales where the Kaiser-Stebbins effect is dominant. Then we concentrate our study on the four-point function and in particular on its zero-lag limit, namely, the excess kurtosis parameter, for which we obtain a predicted value of $\sim 10^{-2}$. In addition, we estimate the cosmic variance for the kurtosis due to a Gaussian fluctuation field, showing its dependence on the primordial spectral index of density fluctuations $n$ and finding agreement with previous published results for the particular case of a flat Harrison-Zel'dovich spectrum. Our value for the kurtosis compares well with previous analyses but falls below the threshold imposed by the cosmic variance when commonly accepted parameters from string simulations are considered. In particular the non-Gaussian signal is found to be inversely proportional to the scaling number of defects, as could be expected by the central limit theorem.

astro-ph↗

arXiv1994

Power Spectrum of Cosmic String Perturbations on the Microwave Background

I review recent progress towards a detailed understanding of the power spectrum of cosmic string perturbations induced on the Microwave Background (CMB). The use of a simple analytic model allows to include important effects that have not been included in previous studies. These effects are potential fluctuations on the last scattering surface (the Sachs-Wolfe effect) and Doppler CMB perturbations. These previously neglected fluctuations are shown to dominate over post-recombination fluctuations (Gott-Kaiser-Stebbins effect) on angular scales less than about $2^\circ$ assuming that no reionization occurs. The effective power spectrum index $n_{eff}$ is calculated on COBE scales and is shown to be somewhat larger than 1 ($1.35 \lsim n_{eff} \lsim 1.5$). After normalizing the model on COBE data and string simulations, I derive the predicted values of $({{δT}\over T})_{rms}$ for several ongoing CMB experiments on medium and small angular scales. A comparison is also made with current observations and predictions of models based on inflation.

astro-ph↗

arXiv1994

Existence and Stability of Spinning Embedded Vortices

I show the existence of a new type of vortex solution which is non-static but stationary and carries angular momentum. This {\it spinning vortex} can be embedded in models with trivial vacuum topology like a model with $SU(2)_{global}\times U(1)_{local} \rightarrow U(1)_{global}$ symmetry breaking. The stability properties of the embedded spinning vortex are also studied in detail and it is shown that stability improves drastically as angular momentum increases. The implications of this result for vortices embedded in the standard electroweak model are under study.

hep-ph↗

arXiv1994

Stable, Spinning Embedded Vortices

The spinning vortex is a stationary generalization of the Nielsen-Olesen vortex involving a linear time dependence of the Goldstone boson. Here we show that this vortex can be embedded in models with $SU(2)_{global}\times U(1)_{local}$ symmetry. We also map the stability sector in parameter space and show that for sufficiently large spinning velocities the vortex is stable for {\it any} value of the Higgs self coupling parameter $β$. This is a significant improvement of stability compared to the semilocal vortex introduced by Vachaspati and Achucarro which is stable only for $β<1$. This result may have significant implications for electroweak vortices.

hep-ph↗

arXiv1994

Signatures of Cosmic Strings on the Microwave Background

ABSRACT: We review recent progress on testing the hypothesis of the existence of cosmic string perturbations in microwave background maps. Using an analytical model for the string network we show that the predicted amplitude and spectrum of MBR fluctuations are consistent with the COBE data for a reasonable value of the single free parameter of the string model (the mass per unit length of the string $μ$). To {\it distinguish} the predictions of cosmic strings from those of Gaussian models it is necessary to apply specific {\it statistical tests} which are sensitive to non-random phases of the MBR temperature maps. We discuss two such statistical tests: First, the probability distribution and the moments of fluctuations ${{δT}\over T}$ and second the corresponding statistical quantities for the {\it gradient} of fluctuations along a fixed direction. We show that the second statistic can detect cosmic string specific signatures on MBR temperature maps with resolution of a few arcminutes or smaller.

astro-ph↗

arXiv1994

Spectral Analysis of Microwave Background Perturbations Induced by Cosmic Strings

Using a simple analytic model based on seed superposition we obtain the spectrum of microwave background perturbations induced by cosmic strings on all angular scales larger than about 2 armin. We assume standard recombination in an Einstein de Sitter universe with $h=1/2$ and study the fluctuation spectrum along a great circle in the sky. Doppler and potential perturbations on the last scattering surface (Sachs-Wolfe effect) are shown to dominate over post-recombination perturbations on scales smaller than about 2 degrees. Using a filter function corresponding to the COBE experiment we obtain an effective power spectrum index $n_{eff}\simeq 1.35$ in good agreement with the recently announced second year COBE data showing $n_{eff}\simeq 1.5$. The only free parameter ($Gμ$) of the string model is fixed by normalizing on the COBE detection leading to $Gμ=1.6$. Other parameters (e.g. the {\it rms} string velocity) are fixed by comparing with string simulations. Using these parameter values we compare the {\it rms} temperature fluctuations $({{ΔT}\over T})_{\it rms}$ predicted for ongoing experiments (Tenerife, SP91, MAX, OVRO etc) with detections and with the corresponding predictions of inflationary models.

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arXiv1993

On the Asymptotics of Nielsen-Olesen Vortices

We investigate analytically and numerically the asymptotic behavior of the Nielsen-Olesen vortex solutions and show that they approach their asymptotic values exponentially but with exponents that differ from the ones quoted in the literature. In particular, it is shown that for values of the Higgs self-coupling that are larger than a critical value, both the Higgs field and the gauge field approach exponentially their asymptotic values with {\it equal} exponents that are {\it independent} of the Higgs self-coupling.

hep-ph↗

arXiv1993

The Fourier Space Statistics of Seedlike Cosmological Perturbations

We propose a new test for distinguishing observationally cosmological models based on seed-like primordial perturbations (like cosmic strings or textures), from models based on Gaussian fluctuations. We investigate analytically the {\it Fourier space} statistical properties of temperature or density fluctuation patterns generated by seed-like objects and compare these properties with those of Gaussian fluctuations generated during inflation. We show that the proposed statistical test can easily identify temperature fluctuations produced by a superposition of a small number of seeds per horizon scale for {\it any} observational angular resolution and {\it any} seed geometry. However, due to the Central Limit Theorem, the distinction becomes more difficult as the number of seeds in the fluctuation pattern increases.

astro-ph↗

arXiv1993

Existence of Double Vortex Solutions

We show analytically and numerically the existence of double vortex solutions in two-Higgs systems. These solutions are generalizations of the \no vortices and exist for all values of the parameters in the Lagrangians considered. We derive analytically the asymptotic behavior of the solutions and confirm it numerically by solving the field equations. Finally, we show that these solutions can be embedded in realistic theories like the two-doublet extension of the standard model.

hep-ph↗

arXiv1992

Detailed Stability Analysis of Electroweak Strings

We give a detailed stability analysis of the Z-string in the standard electroweak model. We identify the mode that determines the stability of the string and numerically map the region of parameter space where the string is stable. For $\sin^2 θ_W = 0.23$, we find that the strings are unstable for a Higgs mass larger than 23GeV. Given the latest constraints on the Higgs mass from LEP, this shows that, if the standard electroweak model is realized in Nature, the existing vortex solutions are unstable.

hep-ph↗

arXiv1992

On the Statistics of CMB Fluctuations Induced by Topological Defects

We use the analytical model recently introduced in Ref. \cite{lp92}, to investigate the statistics of temperature fluctuations on the cosmic microwave background (CMB), induced by topological defects. The cases of cosmic strings and textures are studied. We derive analytically the characteristic function of the probability distribution for ${δT}\over T$ and use it to obtain the lowest twelve moments including the skewness and the kurtosis. The distribution function is also obtained and it is compared with the Gaussian distribution thus identifying long non-Gaussian tails. We show that for both cosmic strings and textures all odd moments (including skewness) vanish while the relative deviation from the Gaussian for even moments increases with the order of the moment. The non-Gaussian signatures of textures, derived from the distribution function and the moments, are found to be much more prominent than the corresponding signatures for strings. We discuss the physical origin of this result.

hep-ph↗

arXiv1992

COBE vs Cosmic Strings: An Analytical Model

We construct a simple analytical model to study the effects of cosmic strings on the microwave background radiation. Our model is based on counting random multiple impulses inflicted on photon trajectories by the string network between the time of recombination and today. We construct the temperature auto-correlation function and use it to obtain the effective power spectrum index n, the rms-quadrupole-normalized amplitude $Q_{rms-PS}$ and the rms temperature variation smoothed on small angular scales. For the values of the scaling solution parameters obtained in Refs.\cite{bb90},\cite{as90} we obtain $n=1.14 \pm 0.5$, $Q_{rms-PS}=(4.5\pm 1.5) Gμ$ and $({{ΔT}\over T})_{rms}=5.5 Gμ$. Demanding consistency of these results with the COBE data leads to $Gμ=(1.7 \pm 0.7)\times 10^{-6}$ (where $μ$ is the string mass per unit length), in good agreement with direct normalizations of $μ$ from observations.

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arXiv1992

Nontopological Global Field Dynamics

We use arguments based on Derrick's theorem to show that the property of collapse which is the key feature of global texture appears in several field theory models with broken global O(N) symmetry. Such models do not necessarily have nontrivial third homotopy group of the vacuum manifold but may give rise to collapsing global field configurations with properties similar to textures. It is verified that configurations with planar and cylindrical geometries do not collapse. The existence of critical parameters for collapse of spherically symmetric global field configurations is verified both analytically and numerically.

hep-ph↗

arXiv1992

Large Scale Structure by Global Monopoles and Cold Dark Matter

A cosmological model in which the primordial perturbations are provided by global monopoles and in which the dark matter is cold has several interesting features. The model is normalized by choosing its single parameter within the bounds obtained from gravitational wave constraints and by demanding coherent velocity f1ows of about 600km/sec on scales of $50 h^{-1} Mpc$. Using this normalization, the model predicts the existence of dominant structures with mass $2\times 10^{16} M_\odot$ on a scale $35 h^{-1}Mpc$ i.e. larger than the horizon at $t_{eq}$. The magnitude of the predicted mass function in the galactic mass range is in good agreement with the observed Schechter function.

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