arXiv subjects

Valeri Vardanyan

Publications and source records attributed to Valeri Vardanyan.

26 records · Page 2Linked to original sources

arXiv2018

Neutron star merger GW170817 strongly constrains doubly coupled bigravity

We study the implications of the recent detection of gravitational waves emitted by a pair of merging neutron stars and their electromagnetic counterpart, events GW170817 and GRB170817A, on the viability of the doubly coupled bimetric models of cosmic evolution, where the two metrics couple directly to matter through a composite, effective metric. We demonstrate that the bounds on the speed of gravitational waves place strong constraints on the doubly coupled models, forcing either the two metrics to be proportional at the background level or the models to become singly coupled. Proportional backgrounds are particularly interesting as they provide stable cosmological solutions with phenomenologies equivalent to that of $Λ$CDM at the background level as well as for linear perturbations, while nonlinearities are expected to show deviations from the standard model.

astro-ph.CO↗

arXiv2018

Dark energy, $α$-attractors, and large-scale structure surveys

Over the last few years, a large family of cosmological attractor models has been discovered, which can successfully match the latest inflation-related observational data. Many of these models can also describe a small cosmological constant $Λ$, which provides the most natural description of the present stage of the cosmological acceleration. In this paper, we study $α$-attractor models with dynamical dark energy, including the cosmological constant $Λ$ as a free parameter. Predominantly, the models with $Λ> 0$ converge to the asymptotic regime with the equation of state $w=-1$. However, there are some models with $w\neq -1$, which are compatible with the current observations. In the simplest models with $Λ= 0$, one has the tensor to scalar ratio $r=\frac{12α}{N^2}$ and the asymptotic equation of state $w=-1+\frac{2}{9α}$ (which in general differs from its present value). For example, in the seven disk M-theory related model with $α= 7/3$ one finds $r \sim 10^{-2}$ and the asymptotic equation of state is $w \sim -0.9$. Future observations, including large-scale structure surveys as well as B-mode detectors will test these, as well as more general models presented here. We also discuss gravitational reheating in models of quintessential inflation and argue that its investigation may be interesting from the point of view of inflationary cosmology. Such models require a much greater number of $e$-folds, and therefore predict a spectral index $n_{s}$ that can exceed the value in more conventional models by about $0.006$. This suggests a way to distinguish the conventional inflationary models from the models of quintessential inflation, even if they predict $w = -1$.

hep-th↗

arXiv2018

On nonlocally interacting metrics, and a simple proposal for cosmic acceleration

We propose a simple, nonlocal modification to general relativity (GR) on large scales, which provides a model of late-time cosmic acceleration in the absence of the cosmological constant and with the same number of free parameters as in standard cosmology. The model is motivated by adding to the gravity sector an extra spin-2 field interacting nonlocally with the physical metric coupled to matter. The form of the nonlocal interaction is inspired by the simplest form of the Deser-Woodard (DW) model, $αR\frac{1}{\Box}R$, with one of the Ricci scalars being replaced by a constant $m^{2}$, and gravity is therefore modified in the infrared by adding a simple term of the form $m^2\frac{1}{\Box}R$ to the Einstein-Hilbert term. We study cosmic expansion histories, and demonstrate that the new model can provide background expansions consistent with observations if $m$ is of the order of the Hubble expansion rate today, in contrast to the simple DW model with no viable cosmology. The model is best fit by $w_0\sim-1.075$ and $w_a\sim0.045$. We also compare the cosmology of the model to that of Maggiore and Mancarella (MM), $m^2R\frac{1}{\Box^2}R$, and demonstrate that the viable cosmic histories follow the standard-model evolution more closely compared to the MM model. We further demonstrate that the proposed model possesses the same number of physical degrees of freedom as in GR. Finally, we discuss the appearance of ghosts in the local formulation of the model, and argue that they are unphysical and harmless to the theory, keeping the physical degrees of freedom healthy.

gr-qc↗

arXiv2015

How can we tell whether dark energy is composed by multiple fields?

Dark energy is often assumed to be composed by a single scalar field. The background cosmic expansion is not sufficient to determine whether this is true or not. We study multi-field scalar-tensor models with a general dark matter source and write the observable modified gravity parameters (effective gravitational constant and anisotropic stress) in the form of a ratio of polynomials in the Fourier wavenumber k of order 2N, where N is the number of scalar fields. By comparing these observables to real data it is in principle possible to determine the number of dark energy scalar fields coupled to gravity. We also show that there are no realistic non-trivial cases in which the order of the polynomials is reduced.

gr-qc↗

arXiv2014

Seeking the Epoch of Maximum Luminosity for Dusty Quasars

Infrared luminosities vLv(7.8 um) arising from dust reradiation are determined for Sloan Digital Sky Survey (SDSS) quasars with 1.4 < z < 5 using detections at 22 um by the Wide-Field Infrared Survey Explorer. Infrared luminosity does not show a maximum at any redshift z < 5, reaching a plateau for z >~ 3 with maximum luminosity vLv(7.8 um) >~ 10^{47} erg per s; luminosity functions show one quasar per cubic Gpc having vLv(7.8 um) > 10^{46.6} erg per s for all 2 < z < 5. We conclude that the epoch when quasars first reached their maximum luminosity has not yet been identified at any redshift below 5. The most ultraviolet luminous quasars, defined by rest frame vLv(0.25 um), have the largest values of the ratio vLv(0.25 um)/vLv(7.8 um) with a maximum ratio at z = 2.9. From these results, we conclude that the quasars most luminous in the ultraviolet have the smallest dust content and appear luminous primarily because of lessened extinction. Observed ultraviolet/infrared luminosity ratios are used to define "obscured" quasars as those having > 5 magnitudes of ultraviolet extinction. We present a new summary of obscured quasars discovered with the Spitzer Infrared Spectrograph and determine the infrared luminosity function of these obscured quasars at z ~ 2.1. This is compared with infrared luminosity functions of optically discovered, unobscured quasars in the SDSS and in the AGN and Galaxy Evolution Survey. The comparison indicates comparable numbers of obscured and unobscured quasars at z ~ 2.1 with a possible excess of obscured quasars at fainter luminosities.

astro-ph.GA↗

arXiv2013

On the Possibility of Medium-Energy Compact X-ray Free-Electron Laser

The problem of X-ray Free-Electron Laser operating on self-amplified spontaneous emission in irregular microundulator is considered. The case when the spectrum width of spontaneous radiation is conditioned by the spatial distribution of sources creating the undulating field is considered. In this case gain function of the stimulated radiation is dozens of times higher than that of the conventional undulators. We propose a model of irregular microundulator, which can be used to construct a drastically cheap and compact X-ray free-electron laser operating on medium energy electron bunch.

physics.acc-ph↗

arXiv2012

Radiation of the electron bunch moving in non-regular fields

The problem of spontaneous radiation of the electron bunch grazing into a charged metallic surface with randomly distributed needle shaped asperities is considered. Distances between two neighboring asperities have been described by gamma distribution. Being repealed by highly charged asperities the electrons of the bunch move along non-regular periodical trajectories in the planes parallel to the metallic surface. The spatial periods of the trajectories are random quantities which are described by the same gamma distribution. The radiation characteristics of the bunch have been obtained. It is shown that the angular distributions of the number of photons radiated from the bunch and from a single electron are the same but the frequency distribution of the bunch is being drastically changed at the hard frequency region. It is proposed to develop a new non-destructive method for investigation of the metal surface roughness. The frequency distribution of the number of photons radiated under the zero angle has been obtained. That allows to find the gain expression of the stimulated radiation.

physics.class-ph↗

arXiv2011

Doppler effect in the oscillator radiation process in the medium

The purpose of this paper is to investigate the radiation process of the charged particle passing through an external periodic field in a dispersive medium. In the optical range of spectrum we will consider two cases: first, the source has not eigenfrequency, and second, the source has eigenfrequency. In the first case, when the Cherenkov radiation occurs, the non-zero eigenfrequency produces a paradox for Doppler effect. It is shown that the absence of the eigenfrequency solves the paradox known in the literature. The question whether the process is normal (i.e. hard photons are being radiated under the small angles) or anomalous depends on the law of the medium dispersion. When the source has an eigenfrequency the Doppler effects can be either normal or anomalous. In the X-ray range of the oscillator radiation spectrum we have two photons radiated under the same angle- soft and hard. In this case the radiation obeys to so-called complicated Doppler effect, i.e. in the soft photon region we have anomalous Doppler effect and in the hard photon region we have normal Doppler effect.

hep-ph↗