Daksh Lohiya

Cosmological tests for a linear coasting cosmology

Abstract A strictly linear evolution of the scale factor is a characteristic feature in several classes of alternative gravity theories. In this Letter we investigate the overall viability of an open linear coasting cosmological model. We report that this model is consistent with gravitational lensing statistics (within 1 σ ) and accommodates old high-redshift galaxies. We finally conclude that such a linear coasting, α ( t )= t , is not ruled out on basis of these observational tests.

Linear coasting in cosmology and SNe Ia

Abstract A strictly linear evolution of the cosmological expansion scale factor is a characteristic feature in several classes of alternative gravity theories as also in the standard (Big-bang) model with specially chosen equations of state of matter. Such an evolution has no free parameters as far as the classical cosmological tests are concerned and should therefore be easily falsifiable. In this Letter we demonstrate how such models present very good fits to the current supernovae Ia data. We discuss the overall viability of such models.

Particle emission in the de Sitter universe for massless fields with spin

The thermal flux emitted by a de Sitter due to interaction with massless fields with spin is calculated by examining the field equations of these fields. The field equations are obtained by perturbing the metric with the various fields and obtaining linearised equations. The expression for the power radiated indicates the absence of spin-half particle emission.

Classical self-force on an electron near a charged, rotating black hole

The author demonstrates a very concise method to determine the force on a charged particle on the theta =0 axis of a black hole. The relevance of this result to the emission of charged particles by mini black holes is discussed.

Stability of Einstein-Yang-Mills monopoles and dyons

Abstract The stabilities of Yang-Mills magnetic and Coulomb charges have been studied separately before, without laying much emphasis on the singular nature of the effective potential in the resulting radial equation. We investigate an analysis that is valid simultaneously for both Yang-Mills magnetic monopoles and dyons in flat as well as curved spacetime. We find the instability modes with their qualitative behaviour determined completely by the large distance behaviour of the radial equation. One of the results for the dyon is that we could get unstable modes even if there were no such modes for the constituent Yang-Mills “electric” and “magnetic” charges separately.

Constraints on Lepton Asymmetry from Nucleosynthesis in a Linearly Coasting Cosmology

We study the effect of neutrino degeneracy on primordial nucleosynthesis in a universe in which the cosmological scale factor evolves linearly with time. The degeneracy parameter of electron type neutrinos (ξe) determines the n/p (neutron to proton) ratio, which in turn determines the abundance of 4He in a manner quite distinct from the Standard Scenario. The observed abundances of 4He, YP = 0.254 ± 0.003, and the minimum metallicity that is essential for fragmentation and cooling processes in star forming prestellar gas clouds (Z = Zcr = 10−6Z⊙), constrain the baryon to photon ratio, ηB = (3.927±0.292)10−9, corresponding to a baryonic matter density, ΩB = 0.263 ± 0.026 and ξe = -2.165 ± 0.171. This closes the dynamic mass estimates of matter in the universe, obtained from large scale velocity dispersion in galaxy clusters, by baryons alone. Useful byproducts are the threshold X(CNO) abundances required to trigger the CNO cycle in the observed low metallicity stars in the universe.

NUCLEOSYNTHESIS IN A UNIVERSE WITH A LINEARLY EVOLVING SCALE FACTOR

Primordial nucleosynthesis is regarded as a success story of the standard big bang (SBB) cosmology. In this article nucleosynthesis is explored in models in which the cosmological scale factor R(t) increases linearly with time. This relationship of R(t) and t continues from the period when nucleosynthesis begins until the present time. It turns out that weak interactions remain in thermal equilibrium upto temperatures which are two orders of magnitude lower than the corresponding (weak interaction decoupling) temperatures in SBB. Inverse beta decay of the proton can ensure adequate production of helium while producing primordial metallicity much higher than that produced in SBB. Attractive features of such models are the absence of the horizon, flatness and age problems as well consistency with classical cosmological tests.

Anomalous production of chiral charge from black holes

Abstract The Reissner Nordstrom solution to Einstein equations coupled to a Yang-Mills field is unstable against the emission of light-charged fermions. We study the flux chirality in this decay process and illustrate the consistency of the result with the prediction of the axial anomaly.

Dynamics of a scalar field, with a double exponential potential, interacting with dark matter

We study the interaction between dark matter and dark energy, with dark energy described by a scalar field having a double exponential effective potential. We discover conditions under which such a scalar field driven solution is a late time attractor. We observe a realistic cosmological evolution which consists of sequential stages of dominance of radiation, matter and dark energy, respectively.

Reanalyzing the upper limit on the tensor-to-scalar perturbation ratio rT in a quartic potential inflationary model

Abstract We study the polynomial chaotic inflation model with a single scalar field in a double well quartic potential which has recently been shown to be consistent with Planck data. In particular, we study the effects of lifting the degeneracy between the two vacua on the inflationary observables, i.e., spectral index n s and tensor-to-scalar perturbation ratio r T . We find that removing the degeneracy allows the model to satisfy the upper limit constraints on r T from Planck data, provided the field starts near the local maximum. We also calculate the scalar power spectrum and non-Gaussianity parameter f NL for the primordial scalar perturbations in this model.