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Daksh Lohiya

Publications and source records attributed to Daksh Lohiya.

At least 19 recordsLinked to original sources
arXiv2023

Coulomb screening in linear coasting nucleosynthesis

We investigate the impact of coulomb screening on primordial nucleosynthesis in a universe having scale factor that evolves linearly with time. Coulomb screening affects primordial nucleosynthesis via enhancement of thermonuclear reaction rates. This enhancement is determined by the solving Poisson equation within the context of mean field theory (under appropriate conditions during the primordial nucleosynthesis). Using these results, we claim that the mean field estimates of coulomb screening hardly affect the predicted element abundances and nucleosynthesis parameters$, \{η_9,ξ_e\}$. The deviations from mean field estimates are also studied in detail by boosting genuine screening results with the screening parameter ($ω_s$). These deviations show negligible effect on the element abundances and on nucleosynthesis parameters. This work thus rules out the coulomb screening effects on primordial nucleosynthesis in slow evolving models and confirms that constraints in ref.[7] on nucleosynthesis parameters remain unaltered.

astro-ph.CO↗
arXiv2020

Accounting for anomalous velocity curve in galaxies as a possible frame dragging effect

We investigate the possibility of accounting for the observed anomalous velocities of stars in galaxies to be a result of a dynamic prescription for having a galaxy generate and prescribe its own inertial frame. It is demonstrated that within observational uncertainties, an overall rotation of an inertial frame dragged by a galaxy is a comfortable fit to the observed anomalous velocity curves. A case for looking for a natural law for prescribing an inertial frame by a mass distribution over a finite (galaxy size) range is made out.

astro-ph.GA↗
arXiv2015

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 ($ξ_e$) determines the $n/p$ (neutron to proton) ratio, which in turn determines the abundance of $^4$He in a manner quite distinct from the Standard Scenario. The observed abundances of $^4$He, $\mathrm{Y}_P$=0.254$\pm$0.003, and the minimum metallicity that is essential for fragmentation and cooling processes in star forming prestellar gas clouds (Z = Z$_{cr}$ = 10$^{-6}$Z$_\odot$), constrain the baryon to photon ratio, $η_B$=(3.927$\pm$0.292)10$^{-9}$, corresponding to a baryonic matter density, $Ω_B$=0.263$\pm$ 0.026 and $ξ_e$=-2.165$\pm$0.171. This closes the dynamic mass estimates of matter in the universe 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.

astro-ph.CO↗
arXiv2015

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.

gr-qc↗
arXiv2008

Power law cosmology - a viable alternative

A power law cosmology is defined by the cosmological scale factor evolving as $t^α$. In this work, we put bounds on $α$ by using the joint test of the SNe Ia data from Supernova Legacy Survey (SNLS) and H(z) data with curvature constant $ k = 0, \pm 1$. We observe that the combined analysis with SNLS and H(z) data favours the open power cosmology with $α= 1.31^{+0.06}_{-0.05}. It is also interesting to note that an Einstein - de Sitter model $(α= 2/3)$ is ruled out at $2σ$ level.

astro-ph↗
arXiv2008

Nucleosynthesis in slowly evolving Cosmologies

We explore aspects of Cosmological Nucleosynthesis in an FRW universe in which the scale factor evolves linearly with time: $a(t) \sim t$. A high Lepton number density during the period when significant nucleosynthesis takes place would lead to a dominant screening of the Coulomb potential of colliding nucleii. This would lead to a significant enhancement of nucleosynthesis rates. We demonstrate how adequate amount of $^4He$ and a collataral metallicity, close to the lowest metallicity observed in metal poor Pop II stars and clouds, can be produced with such an evolution.

astro-ph↗
arXiv2005

A case for nucleosynthesis in slowly evolving models

We present a case for Cosmological Nucleosynthesis in an FRW universe in which the scale factor expands linearly with time: $a(t) \sim t$. It is demonstrated that adequate amount of $^4He$ requires a baryon density that saturates mass bounds from galactic clusters. There is a collataral metallicity production that is quite close to the lowest metallicity observed in metal poor Pop II stars and clouds. On the other hand, sites for incipient low metallicity (Pop II) starformation can support environments conducive to Deuterium production up to levels observed in the universe. A profile of a revised ``Standard Cosmology'' is outlined.

astro-ph↗
arXiv2003

A Concordant "Freely Coasting Cosmology"

A strictly linear evolution of the cosmological scale factor is surprisingly an excellent fit to a host of cosmological observations. Any model that can support such a coasting presents itself as a falsifiable model as far as classical cosmological tests are concerned. Such evolution is known to be comfortably concordant with the Hubble diagram as deduced from data of recent supernovae 1a and high redshift objects, it passes constraints arising from the age and gravitational lensing statistics and clears basic constraints on nucleosynthesis. Such an evolution exhibits distinguishable and verifiable features for the recombination era. This article discusses the concordance of such an evolution in relation to minimal requirements for large scale structure formation and cosmic microwave background anisotropy along with the overall viability of such models. While these results should be of interest for a host of alternative gravity models that support a linear coasting, we conjecture that a linear evolution would emerge naturally either from General Relativity itself or from a General Relativistic theory of a non-minimally coupled scalar field theory.

astro-ph↗
arXiv2002

Cosmological Tests for a Linear Coasting Cosmology

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

astro-ph↗
arXiv2001

Radiation in a Fractal Cosmology

It is shown that Homogeneous radiation can not be included in the Fractal Cosmological model obtained earlier by assuming an isotropic fractal cosmography, General Relativity and the Copernican Principle.

astro-ph↗
arXiv2001

From Fractal Cosmography to Fractal Cosmology

Assuming a fractal distribution of matter in the universe, consequences that follow from the General Theory of Relativity and the Copernican Principle for fractal cosmology are examined. The change in perspective necessary to deal with a fractal universe is highlighted. An ansatz that provides a concrete application of the Conditional Cosmological Principle is provided. This fractal cosmology is obtained by arguments closely following those used in standard cosmology. The resulting model may play a significant role in the debate on whether the universe is a fractal or crosses over to homogeneity at some scale. This model may also be regarded as an idealized fractal model around which more realistic models may be built.

astro-ph↗
arXiv2001

Dynamically tuning away the cosmological constant in effective scalar tensor theories

It is known that the cosmological constant can be dynamically tuned to an arbitrary small value in classes of scalar tensor theories. The trouble with such schemes is that effective gravity itself vanishes. We explore the possibility of avoiding this ``no-go'' with a spatially varying effective gravity. We demonstrate this in principle with the non-minimally coupled scalar field having an additional coupling to a fermionic field. The expectation value of the scalar field gets anchored to a non-trivial value inside compact domains. But for the non-minimal coupling to the scalar curvature, these configurations are analogous to the non-topological solutions suggested by Lee and Wick. With non-minimal coupling, this leads to a peculiar spatial variation of effective gravity. As before, one can dynamically have the long distance (global) gravitational constant $G$ and $Λ$, the cosmological constant, tending to zero. However, inside compact domains, $G$ can be held to a universal (non-vanishing) value. Long distance gravitational effects turn out to be indistinguishable from those expected of general theory of relativity (GTR). There are two ways in which the ensuing theory may lead to a viable effective gravity theory: (a) the compact domains could be of microscopic (sub-nuclear) size, or (b) the domains could be large enough to accommodate structures as large as a typical galaxy. Aspects of effective gravity and cosmology that follow are described. A toy Freidman - Robertson - Walker (FRW) model free from several standard model pathologies and characteristic features emerges.

astro-ph↗
arXiv2000

Linear Coasting in Cosmology and SNe Ia

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 article we demonstrate how such models present very good fits to the current supernovae 1a data. We discuss the overall viability of such models.

astro-ph↗
arXiv1999

Nucleosynhthesis in a Simmering Universe

Primordial nucleosynthesis is a success story of the standard big bang (SBB) cosmology. We explore nucleosynthesis in possible models very different from SBB in which the cosmological scale factor increases linearly with time right through the period during which nucleosynthesis occurs till the present.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 ensures 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 and consistency with classical cosmological tests.

nucl-th↗
arXiv1998

Behavior of Quasilocal Mass Under Conformal Transformations

We show that in a generic scalar-tensor theory of gravity, the ``referenced'' quasilocal mass of a spatially bounded region in a classical solution is invariant under conformal transformations of the spacetime metric. We first extend the Brown-York quasilocal formalism to such theories to obtain the ``unreferenced'' quasilocal mass and prove it to be conformally invariant. The appropriate reference term in this case is defined by generalizing the Hawking-Horowitz prescription, which was originally proposed for general relativity. For such a choice of reference term, the referenced quasilocal mass for a general spacetime solution is obtained. This expression is shown to be a conformal invariant provided the conformal factor is a monotonic function of the scalar field. We apply this expression to the case of static spherically symmetric solutions with arbitrary asymptotics to obtain the referenced quasilocal mass of such solutions. Finally, we demonstrate the conformal invariance of our quasilocal mass formula by applying it to specific cases of four-dimensional charged black hole spacetimes, of both the asymptotically flat and non-flat kinds, in conformally related theories.

gr-qc↗
arXiv1998

A program for a problem free Cosmology within a framework of a rich class of scalar tensor theories

A search for a problem free cosmology within the framework of an effective non - minimally coupled scalar tensor theory is suggested. With appropriate choice of couplings in variants of a Lee - Wick model [as also in a model supporting Q - ball solutions], non topological solutions [NTS's], varying in size upto 10 kpc to 1 Mpc can exist. We explore the properties of a ``toy'' Milne model containing a distribution of NTS domains. The interior of these domains would be regions where effective gravitational effects would be indistinguishable from those expected in standard Einstein theory. For a large class of non - minimal coupling terms and the scalar effective potential, the effective cosmological constant identically vanishes. The model passes classical cosmological tests and we describe reasons to expect it to fare well as regards nucleosynthesis and structure formation.

gr-qc↗
arXiv1998

Nucleosynthesis in a simmering univeerse

Primordial nucleosynthesis is considered a success story of the standard big bang (SBB) cosmology. The cosmological and elementary particle physics parameters are believed to be severely constrained by the requirement of correct abundances of light elements. We explore nucleosynthesis in a class of models very different from SBB. In these models the cosmological scale factor increases linearly with time right through the period during which nucleosynthesis occurs till the present. 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) temperature in SBB. Inverse beta decay of the proton can ensure adequate production of several light elements while producing primordial metalicity much higher than that produced in SBB. Other attractive features of these models are the absence of the horizon, flatness and the age problems and consistency with classical cosmological tests.

gr-qc↗