arXiv subjects

Rajeev Kumar Jain

Publications and source records attributed to Rajeev Kumar Jain.

At least 37 records · Page 2Linked to original sources

arXiv2022

On the primordial correlation of gravitons with gauge fields

We calculate the primordial correlation of gravitons with an abelian gauge field non-minimally coupled through a dynamical dilaton field or a volume moduli during inflation in the early universe. In particular, we compute the cross-correlation of a tensor mode with two gauge field modes and the corresponding correlation functions for the associated magnetic and electric fields using the in-in formalism. Moreover, using semi-classical methods, we show that the three-point cross-correlation functions satisfy new consistency relations (soft theorems) in the squeezed limit. Our findings exhibit a complete agreement of the full in-in results with the new consistency relations. An interesting consequence of our scenario is the possibility of a novel correlation of the primordial tensor mode with the primordial curvature perturbation induced by higher order quantum gravity corrections. The anisotropic background created by long wavelength gauge field modes makes this correlation function non-vanishing. Finally, we discuss how these three-point correlation functions are imprinted on cosmological observables today and the applications to scenarios of inflationary magnetogenesis.

hep-th↗

arXiv2021

Small scale induced gravitational waves from primordial black holes, a stringent lower mass bound, and the imprints of an early matter to radiation transition

In all inflationary scenarios of primordial black holes (PBH) formation, amplified scalar perturbations inevitably accompany an induced stochastic gravitational waves background (ISGWB) at smaller scales. In this paper, we study the ISGWB originating from the inflationary model, introduced in our previous paper [1] wherein PBHs can be produced with a nearly monochromatic mass fraction in the asteroid mass window accounting for the total dark matter in the universe. We numerically calculate the ISGWB in our scenario for frequencies ranging from nanoHz to KHz that covers the observational scales corresponding to future space based GW observatories such as IPTA, LISA, DECIGO and ET. Interestingly, we find that ultralight PBHs ($M_{\rm PBH} \sim 10^{-20} M_\odot$) which shall completely evaporate by today with exceedingly small contribution to dark matter, would still generate an ISGWB that may be detected by a future design of the ground based Advanced LIGO detector. Using a model independent approach, we obtain a stringent lower mass limit for ultralight PBHs which would be valid for a large class of ultra slow roll inflationary models. Further, we extend our formalism to study the imprints of a reheating epoch on both the ISGWB and the derived lower mass bound. We find that any non-instantaneous reheating leads to an even stronger lower bound on PBHs mass and an epoch of a prolonged matter dominated reheating shifts the ISGWB spectrum to smaller frequencies. In particular, we show that an epoch of an early matter dominated phase leads to a secondary amplification of ISGWB at much smaller scales corresponding to the smallest comoving scale leaving the horizon during inflation or the end of inflation scale. Finally, we discuss the prospects of the ISGWB detection by the proposed and upcoming GW observatories.

astro-ph.CO↗

arXiv2021

Impact of Helical Electromagnetic Fields on the Axion Window

Primordial electromagnetic fields can strongly affect the cosmic evolution of axions, and vice versa. We show that if helical electromagnetic fields are coherently produced in the early universe, their remnants source a field velocity to the coupled axions and enhance the relic abundance of axion dark matter. We discuss the implications for the QCD axion and axion-like particles that are coupled to the SM or hidden gauge groups. For a QCD axion coupled to hidden photons, we find that the conventional window for the axion decay constant $10^{8}\, \mathrm{GeV} \lesssim f \lesssim 10^{12}\, \mathrm{GeV}$ can be completely closed due to overproduction of axion dark matter by helical electromagnetic fields as little as $α\, ΔN_{\mathrm{eff}} \gtrsim 10^{-12}$, where $α$ is the gauge coupling and $ΔN_{\mathrm{eff}}$ is the effective extra relativistic degrees of freedom of the hidden photons.

astro-ph.CO↗

arXiv2020

Primordial black holes dark matter from inflection point models of inflation and the effects of reheating

We study the generation of primordial black holes (PBH) in a single field inflection point model of inflation wherein the effective potential is expanded up to the sextic order and the inversion symmetry is imposed such that only even powers are retained in the potential. Such a potential allows the existence of an inflection point which leads to a dynamical phase of ultra slow roll evolution, thereby causing an enhancement of the primordial perturbation spectrum at smaller scales. Working with a quasi-inflection point in the potential, we find that PBHs can be produced in our scenario in the asteroid-mass window with a nearly monochromatic mass fraction which can account for the total dark matter in the universe. For different choices of quasi-inflection points and other parameters of our model, we can also generate PBHs in higher mass windows but the primordial spectrum of curvature perturbations becomes strongly tilted at the CMB scales. Moreover, we study the effects of a reheating epoch after the end of inflation on the PBHs mass fraction and find that an epoch of a matter dominated reheating can shift the mass fraction to a larger mass window as well as increase their fractional contribution to the total dark matter even for the case of a monochromatic mass fraction.

astro-ph.CO↗

arXiv2018

Effective chemical potential in spontaneous baryogenesis

Models of spontaneous baryogenesis have an interaction term $\partial_μθj^μ_B$ in the Lagrangian, where $j^μ_B$ is the baryonic current and $θ$ can be a pseudo-Nambu-Goldstone boson. Since the time component of this term, $\dotθ j^0_B$, equals $\dotθ n_B$ for a spatially homogeneous current, it is usually argued that this term implies a splitting in the energy of baryons and antibaryons thereby providing an effective chemical potential for baryon number. In thermal equilibrium, one {then obtains} $n_B \sim \dotθ T^2$. We however argue that a term of this form in the Lagrangian does not contribute to the single particle energies of baryons and antibaryons. We show this for both fermionic and scalar baryons. But, similar to some recent work, we find that despite the above result the baryon number density obtained from a Boltzmann equation analysis can be proportional to $\dotθ T^2$. Our arguments are very different from that in the standard literature on spontaneous baryogenesis.

hep-ph↗

arXiv2018

A Unified Universe

We present a unified evolution of the universe from very early times until the present epoch by including both the leading local correction $R^2$ and the leading non-local term $R\frac{1}{\square^2}R$ to the classical gravitational action. We find that the inflationary phase driven by $R^2$ term gracefully exits in a transitory regime characterized by coherent oscillations of the Hubble parameter. The universe then naturally enters into a radiation dominated epoch followed by a matter dominated era. At sufficiently late times after radiation-matter equality, the non-local term starts to dominate inducing an accelerated expansion of the universe at the present epoch. We further exhibit the fact that both the leading local and non-local terms can be obtained within the covariant effective field theory of gravity. This scenario thus provides a unified picture of inflation and dark energy in a single framework by means of a purely gravitational action without the usual need of a scalar field.

gr-qc↗

arXiv2017

On the covariant formalism of the effective field theory of gravity and its cosmological implications

Following our previous work wherein the leading order effective action was computed in the covariant effective field theory of gravity, here we specialize the effective action to the FRW spacetime and obtain the effective Friedmann equations. In particular, we focus our attention on studying the cosmological implications of the non-local terms when each of them is combined with the Einstein-Hilbert action. We obtain both analytical and iterative solutions to the effective background equations in all the cases and also briefly comment on the consistency between the iterative and numerical solutions whenever possible. We find that among all the non-local terms, the imprints induced by $R\frac{1}{\square^2}R$ are very significant. Interpreting these corrections as an effective dark energy component characterized by an equation of state parameter, we find that the $R\frac{1}{\square^2}R$ correction can indeed lead to an accelerated expansion of the universe at the present epoch even in the absence of a cosmological constant. We briefly discuss some phenomenological consequences of our results.

astro-ph.CO↗

arXiv2016

On the covariant formalism of the effective field theory of gravity and leading order corrections

We construct the covariant effective field theory of gravity as an expansion in inverse powers of the Planck mass, identifying the leading and next-to-leading quantum corrections. We determine the form of the effective action for the cases of pure gravity with cosmological constant as well as gravity coupled to matter. By means of heat kernel methods we renormalize and compute the leading quantum corrections to quadratic order in a curvature expansion. The final effective action in our covariant formalism is generally non-local and can be readily used to understand the phenomenology on different spacetimes. In particular, we point out that on curved backgrounds the observable leading quantum gravitational effects are less suppressed than on Minkowski spacetime.

gr-qc↗

arXiv2016

Duality and scale invariant magnetic fields from bouncing universes

Recently, we had numerically shown that, for a non-minimal coupling that is a simple power of the scale factor, scale invariant magnetic fields arise in a class of bouncing universes. In this work, we {\it analytically} evaluate the spectrum of magnetic and electric fields generated in a sub-class of such models. We illustrate that, for cosmological scales which have wavenumbers much smaller than the wavenumber associated with the bounce, the shape of the spectrum is preserved across the bounce. Using the analytic solutions obtained, we also illustrate that the problem of backreaction is severe at the bounce. Finally, we show that the power spectrum of the magnetic field remains invariant under a two parameter family of transformations of the non-minimal coupling function.

gr-qc↗

arXiv2016

Leading Gravitational Corrections and a Unified Universe

Leading order gravitational corrections to the Einstein-Hilbert action can lead to a consistent picture of the universe by unifying the epochs of inflation and dark energy in a single framework. While the leading local correction induces an inflationary phase in the early universe, the leading non-local term leads to an accelerated expansion of the universe at the present epoch. We argue that both the leading UV and IR terms can be obtained within the framework of a covariant effective field theory of gravity. The perturbative gravitational corrections therefore provide a fundamental basis for understanding a possible connection between the two epochs.

gr-qc↗

arXiv2016

White Dwarf Critical Tests for Modified Gravity

Scalar-tensor theories of gravity can lead to modifications of the gravitational force inside astrophysical objects. We exhibit that compact stars such as white dwarfs provide a unique set-up to test beyond Horndeski theories of ${\rm G}^3$ type. We obtain stringent and independent constraints on the parameter $Υ$ characterizing the deviations from Newtonian gravity using the mass-radius relation, the Chandrasekhar mass limit and the maximal rotational frequency of white dwarfs. We find that white dwarfs impose stronger constraints on $Υ$ than red and brown dwarfs.

astro-ph.CO↗

arXiv2015

Generation of scale invariant magnetic fields in bouncing universes

We consider the generation of primordial magnetic fields in a class of bouncing models when the electromagnetic action is coupled non-minimally to a scalar field that, say, drives the background evolution. For scale factors that have the power law form at very early times and non-minimal couplings which are simple powers of the scale factor, one can easily show that scale invariant spectra for the magnetic field can arise before the bounce for certain values of the indices involved. It will be interesting to examine if these power spectra retain their shape after the bounce. However, analytical solutions for the Fourier modes of the electromagnetic vector potential across the bounce are difficult to obtain. In this work, with the help of a new time variable that we introduce, which we refer to as the ${\rm e}$-${\cal N}$-fold, we investigate these scenarios numerically. Imposing the initial conditions on the modes in the contracting phase, we numerically evolve the modes across the bounce and evaluate the spectra of the electric and magnetic fields at a suitable time after the bounce. As one could have intuitively expected, though the complete spectra depend on the details of the bounce, we find that, under the original conditions, scale invariant spectra of the magnetic fields do arise for wavenumbers much smaller than the scale associated with the bounce. We also show that magnetic fields which correspond to observed strengths today can be generated for specific values of the parameters. But, we find that, at the bounce, the backreaction due to the electromagnetic modes that have been generated can be significantly large calling into question the viability of the model. We briefly discuss the implications of our results.

astro-ph.CO↗

arXiv2015

Radiative Corrections from Heavy Fast-Roll Fields during Inflation

We investigate radiative corrections to the inflaton potential from heavy fields undergoing a fast-roll phase transition. We find that a logarithmic one-loop correction to the inflaton potential involving this field can induce a temporary running of the spectral index. The induced running can be a short burst of strong running, which may be related to the observed anomalies on large scales in the cosmic microwave spectrum, or extend over many e-folds, sustaining an effectively constant running to be searched for in the future. We implement this in a general class of models, where effects are mediated through a heavy messenger field sitting in its minimum. Interestingly, within the present framework it is a generic outcome that a large running implies a small field model with a vanishing tensor-to-scalar ratio, circumventing the normal expectation that small field models typically lead to an unobservable small running of the spectral index. An observable level of tensor modes can also be accommodated, but, surprisingly, this requires running to be induced by a curvaton. If upcoming observations are consistent with a small tensor-to-scalar ratio as predicted by small field models of inflation, then the present study serves as an explicit example contrary to the general expectation that the running will be unobservable.

hep-th↗

arXiv2014

Inflationary Magnetogenesis without the Strong Coupling Problem II: Constraints from CMB anisotropies and B-modes

Recent observational claims of magnetic fields stronger than $10^{-16}$ G in the extragalactic medium motivate a new look for their origin in the inflationary magnetogenesis models. In this work we shall review the constraints on the simplest gauge invariant model $f^2(ϕ)F_{μν}F^{μν}$ of inflationary magnetogenesis, and show that in the optimal region of parameter space the anisotropic constraints coming from the induced bispectrum, due to the generated electromagnetic fields, yield the strongest constraints. In this model, only a very fine tuned scenario at an energy scale of inflation as low as $10^{-2}$ GeV can explain the observations of void magnetic fields. These findings are consistent with the recently derived model independent constraints. However, if the detection of primordial tensor modes by BICEP2 is confirmed, the possibility of low scale inflation is excluded. Assuming the validity of the BICEP2 claim of a tensor-to-scalar ratio $r=0.2^{+0.07}_{-0.05}$, we provide the updated constraints on inflationary magnetogenesis. On the Mpc scale, the maximal allowed magnetic field strength from inflation is less than $10^{-30}$ G.

astro-ph.CO↗

arXiv2013

Inflationary Magnetogenesis without the Strong Coupling Problem

The simplest gauge invariant models of inflationary magnetogenesis are known to suffer from the problems of either large backreaction or strong coupling, which make it difficult to self-consistently achieve cosmic magnetic fields from inflation with a field strength larger than $10^{-32} G$ today on the $\Mpc$ scale. Such a strength is insufficient to act as seed for the galactic dynamo effect, which requires a magnetic field larger than $10^{-20} G$. In this paper we analyze simple extensions of the minimal model, which avoid both the strong coupling and back reaction problems, in order to generate sufficiently large magnetic fields on the Mpc scale today. First we study the possibility that the coupling function which breaks the conformal invariance of electromagnetism is non-monotonic with sharp features. Subsequently, we consider the effect of lowering the energy scale of inflation jointly with a scenario of prolonged reheating where the universe is dominated by a stiff fluid for a short period after inflation. In the latter case, a systematic study shows upper bounds for the magnetic field strength today on the Mpc scale of $10^{-13} G$ for low scale inflation and $10^{-25} G$ for high scale inflation, thus improving on the previous result by 7-19 orders of magnitude. These results are consistent with the strong coupling and back reaction constraints.

astro-ph.CO↗

arXiv2013

On the non-Gaussian correlation of the primordial curvature perturbation with vector fields

We compute the three-point cross-correlation function of the primordial curvature perturbation generated during inflation with two powers of a vector field in a model where conformal invariance is broken by a direct coupling of the vector field with the inflaton. If the vector field is identified with the electromagnetic field, this correlation would be a non-Gaussian signature of primordial magnetic fields generated during inflation. We find that the signal is maximized for the flattened configuration where the wave number of the curvature perturbation is twice that of the vector field and in this limit, the magnetic non-linear parameter becomes as large as |b_{NL}| ~ 10^3. In the squeezed limit where the wave number of the curvature perturbation vanishes, our results agree with the magnetic consistency relation derived in arXiv:1207.4187.

astro-ph.CO↗

arXiv2013

Cosmological Ohm's law and dynamics of non-minimal electromagnetism

The origin of large-scale magnetic fields in cosmic structures and the intergalactic medium is still poorly understood. We explore the effects of non-minimal couplings of electromagnetism on the cosmological evolution of currents and magnetic fields. In this context, we revisit the mildly non-linear plasma dynamics around recombination that are known to generate weak magnetic fields. We use the covariant approach to obtain a fully general and non-linear evolution equation for the plasma currents and derive a generalised Ohm law valid on large scales as well as in the presence of non-minimal couplings to cosmological (pseudo-)scalar fields. Due to the sizeable conductivity of the plasma and the stringent observational bounds on such couplings, we conclude that modifications of the standard (adiabatic) evolution of magnetic fields are severely limited in these scenarios. Even at scales well beyond a Mpc, any departure from flux freezing behaviour is inhibited.

astro-ph.CO↗

arXiv2013

Consistency relation for cosmic magnetic fields

If cosmic magnetic fields are indeed produced during inflation, they are likely to be correlated with the scalar metric perturbations that are responsible for the Cosmic Microwave Background anisotropies and Large Scale Structure. Within an archetypical model of inflationary magnetogenesis, we show that there exists a new simple consistency relation for the non-Gaussian cross correlation function of the scalar metric perturbation with two powers of the magnetic field in the squeezed limit where the momentum of the metric perturbation vanishes. We emphasize that such a consistency relation turns out to be extremely useful to test some recent calculations in the literature. Apart from primordial non-Gaussianity induced by the curvature perturbations, such a cross correlation might provide a new observational probe of inflation and can in principle reveal the primordial nature of cosmic magnetic fields.

astro-ph.CO↗