44 research outputs found
Inflation with an antisymmetric tensor field
We investigate the possibility of inflation with models of antisymmetric
tensor field having minimal and nonminimal couplings to gravity. Although the
minimal model does not support inflation, the nonminimal models, through the
introduction of a nonminimal coupling to gravity, can give rise to stable
de-Sitter solutions with a bound on the coupling parameters. The values of
field and coupling parameters are sub-planckian. Slow roll analysis is
performed and slow-roll parameters are defined which can give the required
number of e-folds for sufficient inflation. Stability analysis has been
performed for perturbations to antisymmetric field while keeping the metric
unperturbed, and it is found that only the sub-horizon modes are free of ghost
instability for de-Sitter space.Comment: 10 pages; minor correction to the claim of Sec. IV; to appear in EPJ
An optimal method for scheduling observations of large sky error regions for finding optical counterparts to transients
The discovery and subsequent study of optical counterparts to transient
sources is crucial for their complete astrophysical understanding. Various
gamma ray burst (GRB) detectors, and more notably the ground--based
gravitational wave detectors, typically have large uncertainties in the sky
positions of detected sources. Searching these large sky regions spanning
hundreds of square degrees is a formidable challenge for most ground--based
optical telescopes, which can usually image less than tens of square degrees of
the sky in a single night. We present algorithms for optimal scheduling of such
follow--up observations in order to maximize the probability of imaging the
optical counterpart, based on the all--sky probability distribution of the
source position. We incorporate realistic observing constraints like the
diurnal cycle, telescope pointing limitations, available observing time, and
the rising/setting of the target at the observatory location. We use
simulations to demonstrate that our proposed algorithms outperform the default
greedy observing schedule used by many observatories. Our algorithms are
applicable for follow--up of other transient sources with large positional
uncertainties, like Fermi--detected GRBs, and can easily be adapted for
scheduling radio or space--based X--ray followup.Comment: Submitted to ApJ. 18 pages, 15 figure
Constant-Roll Inflation in modified gravity model using Palatini Formalism
In this work, we study a constant-roll inflationary model in the Palatini
formalism using modified gravity. Here our action consists a non-minimal
coupling of a scalar field with Ricci scalar in a general form of
. Using Palatini approach, we write its equivalent scalar-tensor
form in the Einstein frame and then apply the constant-roll condition in the
equation of motion for the inflaton field. Later the tensor-to-scalar ratio and
the spectral index are calculated using the slow-roll parameters and the
results obtained are matched with the Planck 2018 data. We found that the
results agree nicely with the observations within the parameter regime under
consideration.Comment: 13 Pages and 10 Figure
Towards mitigation of apparent tension between nuclear physics and astrophysical observations by improved modeling of neutron star matter
Observations of neutron stars (NSs) by the LIGO-Virgo and NICER
collaborations have provided reasonably precise measurements of their various
macroscopic properties. In this paper, we employ a Bayesian framework to
combine them and place improved joint constraints on the properties of NS
equation of state (EoS). We use a hybrid EoS formulation that employs a
parabolic expansion-based nuclear empirical parameterization around the nuclear
saturation density augmented by a generic 3-segment piecewise polytrope model
at higher densities. Within the credible level this parameterization
predicts km and for the radius and dimensionless tidal deformability,
respectively, of a NS. Finally, we show how the construction of
the full NS EoS based solely on the nuclear empirical parameters at saturation
density leads to certain tension with the astrophysical data, and how the
hybrid approach provides a resolution to it
Anisotropic Inflation in Dipolar Bose-Einstein Condensates
Early during the era of cosmic inflation, rotational invariance may have been
broken, only later emerging as a feature of low-energy physics. This motivates
ongoing searches for residual signatures of anisotropic space-time, for example
in the power spectrum of the cosmic microwave background. We propose that
dipolar Bose-Einstein condensates (BECs) furnish a laboratory quantum
simulation platform for the anisotropy evolution of fluctuation spectra during
inflation, exploiting the fact that the speed of dipolar condensate sound waves
depends on direction. We construct the anisotropic analogue space-time metric
governing sound, by linking the time-varying strength of dipolar and contact
interactions in the BEC to the scale factors in different coordinate
directions. Based on these, we calculate the dynamics of phonon power spectra
during an inflation that renders the initially anisotropic universe isotropic.
We find that the expansion speed provides an experimental handle to control and
study the degree of final residual anisotropy. Gravity analogues using dipolar
condensates can thus provide tuneable experiments for a field of cosmology that
was until now confined to a single experiment, our universe.Comment: Updated Format, More References Adde