SEARCH · Searcharxiv

Results for “astro-ph”

Search indexed arXiv papers on artificial intelligence, large language models, computer vision and robotics. Read source abstracts and follow links to arXiv.

Quote a phrase for an exact phrase match. Source license links do not imply unrestricted reuse.

At least 901 records · Page 50Linked to original sources

arXiv1997

Relativistic Zel'dovich approximation in spherically symmetric model

We compare relativistic approximation methods, which describe gravitational instability in the expanding universe, in a spherically symmetric model. Linear perturbation theory, second-order perturbation theory, relativistic Zel'dovich approximation, and relativistic post-Zel'dovich approximation are considered and compared with the Lema\^ıtre-Tolman-Bondi solution in order to examine the accuracy of these approximations. We consider some cases of inhomogeneous matter distribution while the homogeneous top-hat model has been usually taken in the previous Newtonian works. It is found that the Zel'dovich-type approximations are generally more accurate than the conventional perturbation theories in the weakly nonlinear regime. Applicable range of the Zel'dovich-type approximations is also discussed.

astro-ph↗

arXiv2000

Atmospheric Muon Flux at Sea Level, Underground, and Underwater

The vertical sea-level muon spectrum at energies above 1 GeV and the underground/underwater muon intensities at depths up to 18 km w.e. are calculated. The results are particularly collated with a great body of the ground-level, underground, and underwater muon data. In the hadron-cascade calculations, the growth with energy of inelastic cross sections and pion, kaon, and nucleon generation in pion-nucleus collisions are taken into account. For evaluating the prompt muon contribution to the muon flux, we apply two phenomenological approaches to the charm production problem: the recombination quark-parton model and the quark-gluon string model. To solve the muon transport equation at large depths of homogeneous medium, a semi-analytical method is used. The simple fitting formulas describing our numerical results are given. Our analysis shows that, at depths up to 6-7 km w. e., essentially all underground data on the muon intensity correlate with each other and with predicted depth-intensity relation for conventional muons to within 10%. However, the high-energy sea-level data as well as the data at large depths are contradictory and cannot be quantitatively decribed by a single nuclear-cascade model.

hep-ph↗

arXiv2001

The Role of Wino Content in Neutralino Dark Matter

We investigate the dark matter prospects of supersymmetric models with nonuniversal gaugino masses. We find that for very particular values of the ratio of soft supersymmetry-breaking gaugino masses, M2/M1, an enhanced coannihilation efficiency between the lightest chargino and the lightest neutralino occurs, allowing for scalars with masses well above the normally accepted limits for viable dark matter in the universal case. As a specific example, we investigate models of hidden sector gaugino condensation. These models exhibit high scalar masses, previously thought dangerous, and the requisite freedom in the ratio of gaugino masses. The cosmologically viable regions of parameter space are investigated, allowing very specific statements to be made about the content of the supersymmetry-breaking hidden sector.

hep-ph↗

arXiv2002

Effective Neutrino Photon Interaction in a Magnetized Medium

Neutrino-photon processes, forbidden in vacuum, can take place in presence of a thermal medium or an external electro-magnetic field, mediated by the corresponding charged leptons (real or virtual). The effect of a medium or an electromagnetic field is two-fold - to induce an effective $ν-γ$ vertex and to modify the dispersion relations of all the particles involved to render the processes kinematically viable. It has already been noted that in presence of a thermal medium such an electromagnetic interaction translates into the neutrino acquiring a small effective charge. In this work, we extend this concept to the case of a thermal medium in presence of an external magnetic field and calculate the effective charge of a neutrino in the limit of a weak magnetic field. We find that the effective charge is direction dependent which is a direct effect of magnetic field breaking the isotropy of the space.

hep-ph↗

arXiv2006

Observational Constraints on Cosmic String Production During Brane Inflation

Overall, brane inflation is compatible with the recent analysis of the WMAP data. Here we explore the constraints of WMAP and 2dFGRS data on the various brane inflationary scenarios. Brane inflation naturally ends with the production of cosmic strings, which may provide a way to distinguish these models observationally. We argue that currently available data cannot exclude a non-negligible contribution from cosmic strings definitively. We perform a partial statistical analysis of mixed models that include a sub-dominant contribution from cosmic strings. Although the data favor models without cosmic strings, we conclude that they cannot definitively rule out a cosmic-string-induced contribution of $\sim 10 %$ to the observed temperature, polarization and galaxy density fluctuations. These results imply that $Gμ\lesssim 3.5\times 10^{-7}(λ/0.25)\sqrt{B/0.1}$, where $λ$ is a dimensionless parameter related to the interstring distance, and $B$ measures the importance of perturbations induced by cosmic strings. We argue that, conservatively, the data available currently still permit $B\lesssim 0.1$. Precision measurements sensitive to the B-mode polarization produced by vector density perturbation modes driven by the string network could provide evidence for these models. Accurate determinations of $n_s(k)$, the scalar fluctuation index, could also distinguish among various brane inflation models.

hep-th↗

arXiv2003

Gravitational radiation from gamma-ray bursts as observational opportunities for LIGO and VIRGO

Gamma-ray bursts are believed to originate in core-collapse of massive stars. This produces an active nucleus containing a rapidly rotating Kerr black hole surrounded by a uniformly magnetized torus represented by two counter-oriented current rings. We quantify black hole spin-interactions with the torus and charged particles along open magnetic flux-tubes subtended by the event horizon. A major output of Egw=4e53 erg is radiated in gravitational waves of frequency fgw=500 Hz by a quadrupole mass-moment in the torus. Consistent with GRB-SNe, we find (i) Ts=90s (tens of s, Kouveliotou et al. 1993), (ii) aspherical SNe of kinetic energy Esn=2e51 erg (2e51 erg in SN1998bw, Hoeflich et al. 1999) and (iii) GRB-energies Egamma=2e50 erg (3e50erg in Frail et al. 2001). GRB-SNe occur perhaps about once a year within D=100Mpc. Correlating LIGO/Virgo detectors enables searches for nearby events and their spectral closure density 6e-9 around 250Hz in the stochastic background radiation in gravitational waves. At current sensitivity, LIGO-Hanford may place an upper bound around 150MSolar in GRB030329. Detection of Egw thus provides a method for identifying Kerr black holes by calorimetry.

gr-qc↗

arXiv2012

On the Quantum Resolution of Cosmological Singularities using AdS/CFT

The AdS/CFT correspondence allows us to map a dynamical cosmology to a dual quantum field theory living on the boundary of spacetime. Specifically, we study a five-dimensional model cosmology in type IIB supergravity, where the dual theory is an unstable deformation of $\N=4$ supersymmetric SU(N) gauge theory on $\Rbar\times S^3$. A one-loop computation shows that the coupling governing the instability is asymptotically free, so quantum corrections cannot turn the potential around. The big crunch singularity in the bulk occurs when a boundary scalar field runs to infinity, in finite time. Consistent quantum evolution requires that we impose boundary conditions at infinite scalar field, i.e. a self-adjoint extension of the system. We find that quantum spreading of the homogeneous mode of the boundary scalar leads to a natural UV cutoff in particle production as the wavefunction for the homogeneous mode bounces back from infinity. However a perturbative calculation indicates that despite this, the logarithmic running of the boundary coupling governing the instability generally leads to significant particle production across the bounce. This prevents the wave packet of the homogeneous boundary scalar to return close to its initial form. Translating back to the bulk theory, we conclude that a quantum transition from a big crunch to a big bang is an improbable outcome of cosmological evolution in this class of five-dimensional models.

hep-th↗

arXiv2008

Howes et al. Reply

Howes et al. Reply to Comment on "Kinetic Simulations of Magnetized Turbulence in Astrophysical Plasmas" arXiv:0711.4355

astro-ph↗

arXiv1996

Higgsino Cold Dark Matter Motivated by Collider Data

Motivated by the supersymmetric interpretation of the CDF eeγγplus missing energy event and the reported Z->bb excess at LEP, we analyze the Higgsino as a cold dark matter candidate. We examine the constraints as implied by the collider experiments, and then calculate its relic density. We find that this Higgsino-like lightest supersymmetric particle is a viable cold dark matter candidate (0.05 < Omega h^2 < 1), and we discuss its favorable prospects for laboratory detection.

hep-ph↗

arXiv2001

Cosmology from Maxima-1, Boomerang and COBE/DMR CMB Observations

Recent results from BOOMERANG-98 and MAXIMA-1, taken together with COBE-DMR, provide consistent and high signal-to-noise measurements of the CMB power spectrum at spherical harmonic multipole bands over $2<\ell\lta800$. Analysis of the combined data yields 68% (95%) confidence limits on the total density, $Ω_{\rm {tot}}\simeq 1.11 \pm 0.07 (^{+0.13}_{-0.12})$, the baryon density, $Ω_b h^2\simeq 0.032^{+0.005}_{-0.004} (^{+0.009}_{-0.008})$, and the scalar spectral tilt, $n_s\simeq1.01^{+0.09}_{-0.07} (^{+0.17}_{-0.14})$. These data are consistent with inflationary initial conditions for structure formation. Taken together with other cosmological observations, they imply the existence of both non-baryonic dark matter and dark energy in the universe.

astro-ph↗

arXiv2001

Leptogenesis via Collisions: Leaking Lepton Number to the Hidden Sector

We propose a lepto-baryogenesis mechanism in which the non-zero B-L of the universe is produced in out-of-equilibrium, lepton number and CP violating scattering processes that convert ordinary particles into particles of some hidden sector. In particular, we consider the processes $l ϕ> l' ϕ', \bar l' \bar phi'$ mediated by the heavy Majorana neutrinos $N$ of the seesaw mechanism, where $l$ and $ϕ$ are ordinary lepton and Higgs doublets and $l'$, $ϕ'$ their hidden counterparts. Such a leptogenesis mechanism is effective even if the reheating temperature is much smaller than the heavy neutrino masses. In particular, it can be as low as $10^{9}$ GeV.

hep-ph↗

arXiv2005

Feedback from quasars in star-forming galaxies and the triggering of massive galactic winds

The shining of quasars is a likely trigger of massive galatic winds, able to remove most ISM from a star-forming spheroid. However, the mechanism responsible for the deposition of energy into the ISM is still unclear. Starting from a model for feedback in galaxy formation with a two-phase medium (Monaco 2004a), we propose that the perturbation induced by radiative heating from a quasar on the ISM triggers a critical change of feedback regime. In the feedback model, SNRs expanding in the hot and pressurized phase of a star-forming spheroid tipically become pressure-confined before the hot interior gas is able to cool. Whenever the evaporation flow due to radiative heating of the quasar is significant with respect to the star-formation rate, the SNRs reach the point where their interior gas cools before being confined, forming a thick cold shell. We show that in this conditions the shells percolate into a super-shell of cold gas that sweeps the whole galaxy. Radiation pressure then pushes the shell out of the galaxy. This self-limiting mechanism leads to a correlation between black hole and bulge masses. The insertion of a motivated wind trigger criterion in a hierarchical galaxy formation model shows however that winds are not necessary to obtain a good black hole--bulge correlation. In absence of winds, good results are obtained if the mechanism responsible for the creation of a reservoir of low-angular momentum gas (able to accrete onto the black hole) deposits mass at a rate proportional to the star-formation rate. Using a novel galaxy formation model, we show under which conditions black hole masses are self-limited by the wind mechanism described above, and outline the possible observational consequences of this self-limitation. [Abridged]

astro-ph↗

arXiv2005

Origin of E+A galaxies: I. Physical properties of E+A's formed from galaxy merging and interaction

We investigate the structural, kinematical, and spectrophotometric properties of ``E+A'' galaxies -- those with strong Balmer absorption lines but no significant [OII] emission -- using numerical simulations combined with stellar population synthesis codes. We particularly focus on the two-dimensional (2D) distributions of line-of-sight velocity, velocity dispersion, colors, line index in E+A galaxies formed via the interaction and merging of two gas-rich spirals. Our numerical simulations demonstrate that E+A elliptical galaxies formed by major galaxy merging have positive radial color gradients and negative radial H$δ$ gradients by virtue of their central poststarburst populations. Furthermore, we show that the projected kinematical and spectroscopic properties of the simulated E+A galaxies can be remarkably different for different major merger models. For example, the simulated E+A ellipticals with kinematically decoupled cores clearly show regions of strong H$δ$ absorption which are very flattened, with differences in rotation and velocity dispersion between the old and young stars. E+A ellipticals are highly likely to show more rapid rotation and a smaller central velocity dispersion in young stars than in old ones.We also provide specific predictions on the structural, kinematical, and spectrophotometric properties of young globular cluster systems in E+A's. Based on these results, we discuss the importance of spatially resolved, integral field unit spectroscopy on large (8-10m) ground-based telescopes in confirming the formation of kinematically distinct cores in elliptical galaxies produced via dissipative merging and determining the most probable physical mechanism(s) for E+A formation with disky and spheroidal morphologies.

astro-ph↗

arXiv2013

No large-angle correlations on the non-Galactic microwave sky

We investigate the angular two-point correlation function of temperature in the WMAP maps. Updating and extending earlier results, we confirm the lack of correlations outside the Galaxy on angular scales greater than about 60 degrees at a level that would occur in 0.025 per cent of realizations of the concordance model. This represents a dramatic increase in significance from the original observations by the COBE-DMR and a marked increase in significance from the first-year WMAP maps. Given the rest of the reported angular power spectrum C_\ell, the lack of large-angle correlations that one infers outside the plane of the Galaxy requires covariance among the C_\ell up to \ell=5. Alternately, it requires both the unusually small (5 per cent of realizations) full-sky large-angle correlations, and an unusual coincidence of alignment of the Galaxy with the pattern of cosmological fluctuations (less than 2 per cent of those 5 per cent). We argue that unless there is some undiscovered systematic error in their collection or reduction, the data point towards a violation of statistical isotropy. The near-vanishing of the large-angle correlations in the cut-sky maps, together with their disagreement with results inferred from full-sky maps, remain open problems, and are very difficult to understand within the concordance model.

astro-ph↗

arXiv2010

Understanding the origin of CMB constraints on Dark Energy

We study the observational constraints of CMB temperature and polarization anisotropies on models of dark energy, with special focus on models with variation in properties of dark energy with time. We demonstrate that the key constraint from CMB observations arises from the location of acoustic peaks. An additional constraint arises from the limits on Omega_nr from the relative amplitudes of acoustic peaks. Further, we show that the distance to the last scattering surface is not how the CMB observations constrain the combination of parameters for models of dark energy. We also use constraints from Supernova observations and show that unlike the Gold and Silver samples, the SNLS sample prefers a region of parameter space that has a significant overlap with the region preferred by the CMB observations. This is a verification of a conjecture made by us in an earlier work. We discuss combined constraints from WMAP5 and SNLS observations. We find that models with w ~ -1 are preferred for models with a constant equation of state parameters. In case of models with a time varying dark energy, we show that constraints on evolution of dark energy density are almost independent of the type of variation assumed for the equation of state parameter. This makes it easy to get approximate constraints from CMB observations on arbitrary models of dark energy. Constraints on models with a time varying dark energy are predominantly due to CMB observations, with Supernova constraints playing only a marginal role.

astro-ph↗

arXiv2003

Astrophysical tau neutrinos and their detection by large neutrino telescopes

We present results of the detailed Monte Carlo calculation of the rates of double-bang events in 1 km$^3$ underwater neutrino telescope with taking into account the effects of $τ$-neutrino propagation through the Earth. As an input, the moderately optimistic theoretical predictions for diffuse neutrino spectra of AGN jets are used.

astro-ph↗

arXiv2010

Fomin's conception of quantum cosmogenesis

The main aim of this paper is to extend the early approach to quantum cosmogenesis provided by Fomin. His approach was developed independently to the well-known Tryon description of the creation of the closed universe as a process of quantum fluctuation of vacuum. We apply the Fomin concept to derive the cosmological observables. We argue that Fomin's idea from his 1973 work, in contrast to Tryon's one has impact on the current Universe models and the proposed extension of his theory now can be tested by distant supernovae SNIa. Fomin's idea of the creation of the Universe is based on the intersection of two fundamental theories: general relativity and quantum field theory with the contemporary cosmological models with dark energy. As a result of comparison with contemporary approaches concerning dark energy, we found out that Fomin's idea appears in the context of the present acceleration of the Universe explanation: cosmological models with decaying vacuum. Contemporary it appears in the form of Ricci scalar dark energy connected with the holographic principle. We show also that the Fomin model admits the bounce instead of the initial singularity. We demonstrate that the Fomin model of cosmogenesis can be falsified and using SNIa data the values of model parameters is in agreement with observations.

astro-ph↗

arXiv2001

Testing General Relativity with LAGEOS, LAGEOS II and Ajisai laser-ranged satellites

The accuracy reached in the past few years by Satellite Laser Ranging (SLR) allows for measuring even tiny features of the Earth's gravitational field predicted by Einstein's General Relativity by means of artificial satellites. The gravitomagnetic dragging of the orbit of a test body is currently under measurement by analyzing a suitable combination of the orbital residuals of LAGEOS and LAGEOS II. The lower bound of the error in this experiment amount to 12.92%. It is due to the mismodeling in the even zonal harmonics of the geopotential which are the most important sources of systematic error. A similar approach could be used in order to measure the relativistic gravitoelectric pericenter shift in the field of the Earth with a lower bound of the systematic relative error of 0.6% due to the even zonal harmonics as well. The inclusion of the ranging data to the Japanese passive geodetic satellite Ajisai would improve such limits to 10.78% and 0.08% respectively and would allow to improve the accuracy in the determination of the PPN parameters beta and gamma.

gr-qc↗