arXiv subjects

Swagat S. Mishra

Publications and source records attributed to Swagat S. Mishra.

At least 19 recordsLinked to original sources

arXiv2026

Dark Matter as an Inflationary Relic in Warm Inflation

Warm inflation is usually expected to completely deplete the inflaton condensate by dissipating its energy into radiation. We show that this expectation fails in a simple and observationally viable regime. In a strongly dissipative warm inflationary scenario, the dissipative ratio, $Q=Υ/(3H)$, can fall rapidly after the end of inflation as the system approaches radiation domination, thereby suppressing further energy transfer to the thermal bath. This leads to a residual inflaton condensate, which subsequently evolves as an effectively non-dissipative scalar field. For potentials with a stable quadratic minimum, this remnant inflaton manifests as a cold dark matter component. We establish this mechanism for the minimal renormalizable potential, with a dissipative coefficient $Υ\propto T^3$. In this case, current cosmological data allow strong dissipation while leaving the inflaton mass weakly constrained by inflationary observables. The observed dark matter abundance then fixes its mass to be $m \approx 0.02\,{\rm MeV}$, while larger masses overclose the Universe. The transition to matter-like scaling occurs well before BBN, avoiding a long-lived inflaton dark radiation component. Relic inflaton dark matter therefore turns the post-inflationary dynamics of warm inflation into a new late time constraint on its parameter space.

astro-ph.CO↗

arXiv2026

Eigenvalue formulation of Stochastic Inflation and application to large perturbation generating inflationary features

Stochastic inflation is a powerful technique for calculating the probability distribution function (PDF) of large inflationary perturbations, which may collapse to form Primordial Black Holes. The PDF, $P({\cal N})$, of the stochastic number of e-folds, ${\cal N}$, satisfies an adjoint Fokker-Planck Equation. We develop a new self-contained eigenvalue technique which can be used to determine $P({\cal N})$. First we apply this method to the simple case of quantum diffusion along a flat potential without any classical drift. We recover the expression for the PDF that has previously been found using characteristic functions, with an exponential tail, and a power-law behaviour, $P({\cal N}) \propto {\cal N}^{-3/2}$, in the intermediate regime between the peak and the tail of the PDF. Finally we apply the method to constant drift inflation, in the narrow- and broad-well limits. In the narrow-well limit, there is an analytic solution and the PDF is similar to the drift-free case, with a mildly suppressed tail. In the broad-well limit, determining the full set of eigenvalues and eigenfunctions requires a piecewise construction of the spectrum, and the broad-well PDF is qualitatively different, with an enhanced peak and a strongly suppressed tail.

astro-ph.CO↗

arXiv2026

Effective Phantom Dark Energy: What Cosmological Reconstruction Does and Does Not Imply

In observational cosmology, the dark energy density and equation of state are effective quantities reconstructed at the background level under a set of assumptions. These include the FLRW framework, the standard Friedmann equation of General Relativity, and separately conserved non-relativistic matter at late times. Recent analyses involving DESI BAO measurements combined with CMB and supernova data have shown mild preference for dynamical dark energy featuring phantom or phantom-crossing behaviour. While the statistical significance of these trends remains limited, and unresolved systematics or modelling uncertainties may still be important, the resulting discussions have highlighted the need for a clearer interpretation of effective dark energy reconstruction. In particular, effective phantom behaviour does not necessarily imply the existence of a fundamental phantom field, microscopic ghost instabilities, violation of the null energy condition by the fundamental stress tensor, or a catastrophic cosmic future. The purpose of this work is to clarify these distinctions, independently of whether the current observational preference for dynamical dark energy survives future data. We discuss the definition of effective dark energy in cosmology, the interpretation of phantom and phantom-crossing behaviour, introduce a simple kinematic criterion for identifying effective phantom evolution directly from the expansion history, and review physical mechanisms through which effective phantom behaviour may arise without fundamental pathologies. While familiar within the dark energy reconstruction community, these distinctions are often left implicit in broader discussions of dynamical dark energy. We hope that this work will remain useful beyond the present observational situation as a clarification of what observationally reconstructed dark energy does and does not imply.

astro-ph.CO↗

arXiv2026

Witten-O'Raifeartaigh potential revisited in the context of Warm Inflation

Warm Inflation is a scenario in which the inflaton field dissipates its energy during inflation to maintain a subdominant constant radiation bath. Two of its remarkable features are (i) inflation can be realized even by very steep potentials and (ii) such a scenario doesn't call for a separate post-inflation reheating phase. We exploit the first feature to show that Warm Inflation can successfully take place on the very steep left wing of the Witten-O'Raifeartaigh potential while remaining in excellent agreement with current cosmological data (joint analysis of Planck, ACT and DESI). The Witten-O'Raifeartaigh potential has a flatter right wing as well, which opens up the possibility of dark energy when the field rolls along this wing. However in order to successfully realize quintessential inflation one needs to (i) normalize the two wings of the Witten-O'Raifeartaigh potential differently in order to bridge between the two extreme energy scales of inflation and dark energy, (ii) allow the quintessence field to be dissipative, which is consistent with the presence of a dissipative term in warm inflation. The dissipative dynamics of the quintessence field is needed in order to sustain slow-roll in the right wing. With these modifications, we demonstrate that the Witten-O'Raifeartaigh potential can give rise to a unified model of warm inflation (on the left wing) and transient dark energy (on the right wing).

astro-ph.CO↗

arXiv2025

Morphology of Inflationary Gravitational Wave Spectra imprinted by a Sequence of Post-Inflationary Epochs $via$ ${\rm GWInSpect}$

The expansion history of the Universe prior to Big Bang Nucleosynthesis (BBN) remains largely unconstrained. The high-energy post-inflationary era may involve multiple distinct epochs, each characterized by a different equation of state (EoS). A key prediction of inflation is the generation of tensor perturbations that later manifest as a stochastic background of primordial gravitational waves (GWs). The large-scale amplitude and small-scale spectral tilt ($n_{\rm GW}$) of these GWs encode the inflationary energy scale and the subsequent expansion history, respectively. A soft post-inflationary EoS ($w<1/3$) yields red-tilted GW spectra ($n_{\rm GW}<0$), while a stiff EoS ($w>1/3$) results in a blue-tilt ($n_{\rm GW}>0$). In our previous work [arXiv:2407.07956], we developed an analytical framework for computing the GW spectral energy density, $Ω_{\rm GW}(f)$, for multiple post-inflationary transitions ($w_1 \to w_2 \to \cdots \to w_n \to 1/3$), focusing on the parameter space relevant for future GW observations. In this paper, we extend that framework to systematically investigate the $morphological~diversity$ of inflationary GW spectra generated by multi-epoch post-inflationary histories. Remaining model agnostic, we demonstrate that a wide variety of spectral shapes, ranging from convex and concave monotonic profiles to multi-peaked non-monotonic spectra, can naturally emerge depending on the sequence and duration of these epochs. We also introduce GWInSpect, a publicly available Python package that computes $Ω_{\rm GW}(f)$ for arbitrary sequences of EoS transitions, providing a practical tool to study the pre-BBN expansion history of the Universe.

astro-ph.CO↗

arXiv2025

Braneworld Dark Energy in light of DESI DR2

Recent observational results from the DESI collaboration reveal tensions with the standard $Λ$CDM model and favour a scenario in which dark energy (DE) decays over time. The DESI DR2 data also suggest that the DE equation of state (EoS) may have been phantom-like ($w < - 1$) in the past, evolving to $w > - 1$ at present, implying a recent crossing of the phantom divide at $w = - 1$. Scalar field models of DE naturally emerge in ultraviolet-complete theories such as string theory, which is typically formulated in higher dimensions. In this work, we investigate a broad class of $thawing~scalar~field~models$, including the simple quadratic, quartic, exponential, symmetry-breaking and axion potentials, propagating on a (4+1)-dimensional ghost-free phantom braneworld, and demonstrate that their effective EoS exhibits a phantom-divide crossing. Alongside the Hubble parameter and EoS of DE, we also analyse the evolution of the $Om$ diagnostic, and demonstrate that the time dependence of these quantities is in excellent agreement with the DESI DR2 observations. Furthermore, we perform a comprehensive parameter estimation using Markov Chain Monte Carlo sampling, and find that the $χ^2$ values for all our models are remarkably close to that of the widely used CPL parametrisation, indicating that our models fit the data very well.

astro-ph.CO↗

arXiv2025

Inflationary Gravitational Waves as a probe of the unknown post-inflationary primordial Universe

One of the key predictions of the standard inflationary paradigm is the quantum mechanical generation of the transverse and traceless tensor fluctuations due to the rapid accelerated expansion of space, which later constitute a stochastic background of primordial gravitational waves (GWs). The amplitude of the (nearly) scale-invariant inflationary tensor power spectrum at large scales provides us with crucial information about the energy scale of inflation in the case of the minimal inflaton coupling to gravity. Furthermore, the spectral energy density, $Ω_{_{\rm GW}}(f)$, of the GWs at sufficiently small scales (or, large frequencies $f$) serves as an important observational probe of post-inflationary primordial dynamics. In fact, the small-scale spectral tilt, $n_{_{\rm GW}} = \frac{{\rm d}\log{Ω_{_{\rm GW}}}}{{\rm d}\log{f}}$, of the spectral energy density of GWs is sensitive to the (unknown) post-inflationary equation of state (EoS), $w$, of the universe; with a softer EoS ($w < 1/3$) leading to a red tilt: $n_{_{\rm GW}} < 0$, while a stiffer EoS ($w > 1/3$) resulting in a blue tilt: $n_{_{\rm GW}} > 0$. The post-inflationary dynamics, however, is generically expected to be quite complex, potentially involving a number of distinct phases. Hence, in this work, we discuss the possibility of multiple sharp transitions, namely $w_1 \to w_2 \to w_3 \to ... \to w_n$, in the EoS of the post-inflationary universe and compute the corresponding spectral energy density of the inflationary GWs. We explicitly determine the region of the parameter space $\lbrace{ w_1, \, w_2, \, w_3, ..., w_n\rbrace}$ which leads to a potentially detectable signal in the upcoming GW detectors, without violating the current constraints.

gr-qc↗

arXiv2024

Numerical simulations of inflationary dynamics: slow roll and beyond

Numerical simulations of the inflationary dynamics are presented here for a single canonical scalar field minimally coupled to gravity. We spell out the basic equations governing the inflationary dynamics in terms of cosmic time $t$ and define a set of dimensionless variables convenient for numerical analysis. We then provide a link to our simple numerical Python code on GitHub that can be used to simulate the background dynamics as well as the evolution of linear perturbations during inflation. The code computes both scalar and tensor power spectra for a given inflaton potential $V(ϕ)$. We discuss a concrete algorithm to use the code for various purposes, especially for computing the enhanced scalar power spectrum in the context of Primordial Black Holes and scalar-induced Gravitational Waves. We also compare the efficiency of different variables used in the literature to compute the scalar fluctuations. We intend to extend the framework to simulate the dynamics of a number of different quantities, including the computation of scalar-induced second-order tensor power spectrum in the near future.

gr-qc↗

arXiv2024

Formation and decay of oscillons after inflation in the presence of an external coupling, Part-I: Lattice simulations

We investigate the formation and decay of oscillons during the post-inflationary reheating epoch from inflaton oscillations around asymptotically flat potentials $V(φ)$ in the presence of an external coupling of the form $\frac{1}{2}\, g^2 \, φ^2 \, χ^2$. It is well-known that in the absence of such an external coupling, the attractive self-interaction term in the potential leads to the formation of copious amounts of long-lived oscillons both for symmetric and asymmetric plateau potentials. We perform a detailed numerical analysis to study the formation of oscillons in the $α$-attractor E- and T-model potentials using the publicly available lattice simulation code ${\cal C}$osmo${\cal L}$attice. We observe the formation of nonlinear oscillon-like structures with the average equation of state $\langle w_φ\rangle \simeq 0$ for a range of values of the inflaton self-coupling $λ$ and the external coupling $g^2$. Our results demonstrate that oscillons form even in the presence of an external coupling and we determine the upper bound on $g^2$ which facilitates oscillon formation. We also find that eventually, these oscillons decay into the scalar inflaton radiation as well as into the quanta of the offspring field $χ$. Thus, we establish the possibility that reheating could have proceeded through the channel of oscillon decay, along with the usual decay of the oscillating inflaton condensate into $χ$ particles. For a given value of the self-coupling $λ$, we notice that the lifetime of a population of oscillons decreases with an increase in the strength of the external coupling, following an (approximately) inverse power-law dependence on $g^2$.

hep-ph↗

arXiv2024

Canonical and Non-canonical Inflation in the light of the recent BICEP/Keck results

We discuss implications of the latest BICEP/Keck data release for inflationary models, with particular emphasis on scalar fields with non-canonical Lagrangians of the type ${\cal L} = X^α- V(ϕ)$. The observational upper bound on the tensor-to-scalar ratio, $r \leq 0.036$, implies that the whole family of monomial power law potentials $V(ϕ) \sim ϕ^p$ are now ruled out in the canonical framework at $95\%$ confidence, which includes the simplest classic inflationary potentials such as $\frac{1}{2}m^2 ϕ^2$ and $λϕ^4$. Instead, current observations strongly favour asymptotically flat plateau potentials. However, working in the non-canonical framework, we demonstrate that monomial potentials, as well as the Higgs potential with its Standard Model self-coupling, can easily be accommodated by current CMB data. We find striking similarities between the $\lbrace n_{_S}, r\rbrace$ flow lines of monomial potentials in the non-canonical framework and the T-model $α$-attractors in the canonical framework. Significantly, $V(ϕ)$ can originate from Planck scale initial values $V(ϕ) \simeq m_p^4$ in non-canonical models while in plateau-like canonical inflation the initial value of the potential is strongly suppressed $V_{\rm plat}(ϕ) \leq 10^{-10} m_p^4$. This has bearing on the issue of initial conditions for inflation and allows for the equipartition of the kinetic and potential terms in non-canonical models.

astro-ph.CO↗

arXiv2024

Cosmic Inflation: Background dynamics, Quantum fluctuations and Reheating

These lecture notes provide a pedagogical introduction to some aspects of the inflationary cosmology, including the background scalar field dynamics, generation of primordial seed perturbations via quantum fluctuations during inflation, and the process of reheating after inflation in the single-field inflationary paradigm.

gr-qc↗

arXiv2024

New models of Quintessential Inflation featuring plateau and hilltop potentials

We introduce a new class of hilltop and plateau potentials which can successfully unify inflation and dark energy resulting in Quintessential Inflation (QI). Interestingly these new potentials are related through an inverse transformation. Namely, if $V(ϕ) = V_0 \, v(ϕ)$ is a plateau potential then the inverse potential $V(ϕ) = V_0 \, \left[v(ϕ)\right]^{-1}$ describes hilltop QI. A simple example is provided by the KKLT-inspired potential $v(ϕ) = \left\lbrack \frac{M^{2n} + ϕ^{2n}}{N^{2n} + ϕ^{2n}}\right\rbrack \,$. When $M/N \ll 1$ this potential describes plateau QI, while its inverse, $\left[v(ϕ)\right]^{-1}$ describes hilltop QI. Other simple models of QI arise for the class of potentials $V(ϕ) \sim \exp\left\lbrack\mp f(ϕ)\right\rbrack$, where the $-$ ($+$) sign is associated with a plateau (hilltop). A key feature of this new class of QI models is the near absence of small parameters which are usually associated with the presence of dark energy. A forecast for the gravitational wave background generated in these models is provided.

astro-ph.CO↗

arXiv2023

Primordial black holes and stochastic inflation beyond slow roll: I -- noise matrix elements

Primordial Black Holes (PBHs) may form in the early Universe, from the gravitational collapse of large density perturbations, generated by large quantum fluctuations during inflation. Since PBHs form from rare over-densities, their abundance is sensitive to the tail of the primordial probability distribution function (PDF) of the perturbations. It is therefore important to calculate the full PDF of the perturbations, which can be done non-perturbatively using the 'stochastic inflation' framework. In single field inflation models generating large enough perturbations to produce an interesting abundance of PBHs requires violation of slow roll. It is therefore necessary to extend the stochastic inflation formalism beyond slow roll. A crucial ingredient for this are the stochastic noise matrix elements of the inflaton potential. We carry out analytical and numerical calculations of these matrix elements for a potential with a feature which violates slow roll and produces large, potentially PBH generating, perturbations. We find that the transition to an ultra slow-roll phase results in the momentum induced noise terms becoming larger than the field noise whilst each of them falls exponentially for a few e-folds. The noise terms then start rising with their original order restored, before approaching constant values which depend on the nature of the slow roll parameters in the post transition epoch. This will significantly impact the quantum diffusion of the coarse-grained inflaton field, and hence the PDF of the perturbations and the PBH mass fraction.

astro-ph.CO↗

arXiv2023

Oscillon formation from preheating in asymmetric inflationary potentials

We investigate the possibility of oscillon formation during the preheating phase of asymmetric inflationary potentials. We analytically establish the existence of oscillon-like solutions for the Klein-Gordon equation for a polynomial potential of the form $V(ϕ)=\frac{1}{2}ϕ^2+Aϕ^3+Bϕ^4$ using the small amplitude analysis, which naturally arises as a Taylor expansion of the $α$-attractor E-model for $ϕ\ll M_\text{pl}$ and $α\sim\mathcal{O}(1)$. We perform a detailed numerical analysis to study the formation of nonlinear structures in the $α$-attractor E-model using the publicly available lattice simulation code $\mathcal{C}\text{osmo}\mathcal{L}\text{attice}$ for parameters in the range $10^{-5}\lesssimα\lesssim 5\times 10^{-4}$. We find the backreaction of the field fluctuations onto the evolution of the homogeneous inflaton condensate to be significant for $α\lesssim 2\times 10^{-4}$ for which we observe the formation of highly nonlinear structures with average equation of state $w\simeq 0$. These nonlinear structures maybe interpreted as oscillons, providing evidence that they can form during the inflaton oscillations around an asymmetric potential and are found to be present for the entirety of the runtime of our simulations, comprising $\gtrsim 40\%$ of the total energy density.

astro-ph.CO↗

arXiv2023

Tabletop potentials for inflation from $f(R)$ gravity

We show that a large class of modified gravity theories (MOG) with the Jordan-frame Lagrangian $f(R)$ translate into scalar-field (scalaron) models with hilltop potentials in the Einstein frame. (A rare exception to this rule is provided by the Starobinsky model for which the corresponding scalaron potential is plateau-like for $ϕ> 0$.) We find that MOG models featuring two distinct mass scales lead to scalaron potentials that have a flattened hilltop, or tabletop. Inflationary evolution in tabletop models agrees very well with CMB observations. Tabletop potentials therefore provide a new and compelling class of MOG-based inflationary models. By contrast, MOG models with a single mass scale generally correspond to steep hilltop potentials and fail to reproduce the CMB power spectrum. Inflationary evolution in hilltop/tabletop models can proceed in two alternative directions: towards the stable point at small $R$ describing the observable universe, or towards the asymptotic region at large $R$. The MOG models which we examine have several new properties including the fact that gravity can become asymptotically vanishing, with $G_{\rm eff} \to 0$, at infinite or large finite values of the scalar curvature $R$. A universe evolving towards the asymptotically vanishing gravity region at large $R$ will either run into a 'Big-Rip' singularity, or inflate eternally.

gr-qc↗

arXiv2021

Generality of Starobinsky and Higgs inflation in the Jordan frame

We revisit the problem of generality of Starobinsky and Higgs inflation. The known results obtained in the Einstein frame are generalized for the case of an arbitrary initial energy of the scalar field. These results are compared with the results obtained directly in the Jordan frame, which, to our knowledge, has not been thoroughly explored in the literature previously. We demonstrate that the qualitative picture of initial conditions zone in the $(ϕ, \dot ϕ)$ plane, which leads to sufficient amount of inflation, is quite similar for both the frames in the case of Higgs inflation. For Starobinsky inflation, the conformal transformation between the frames relates the geometrical variables in the Jordan frame with the properties of an effective scalar field in the Einstein frame. We show that the transformation $(H, R) \to (ϕ, \dot ϕ)$ is not regular everywhere, leading to some peculiarities in the zone of good initial conditions in the $(H, R)$ plane.

gr-qc↗

arXiv2021

Curing inflationary degeneracies using reheating predictions and relic gravitational waves

It is well known that the inflationary scenario often displays different sets of degeneracies in its predictions for CMB observables. These degeneracies usually arise either because multiple inflationary models predict similar values for the scalar spectral index $n_{_S}$ and the tensor-to-scalar ratio $r$, or because within the same model, the values of $\lbrace n_{_S}, r \rbrace$ are insensitive to some of the model parameters, making it difficult for CMB observations alone to constitute a unique probe of inflationary cosmology. We demonstrate that by taking into account constraints on the post-inflationary reheating parameters such as the duration of reheating $N_{_{\rm re}}$, its temperature $T_{_{\rm re}}$ and especially its equation of state (EOS), $w_{_{\rm re}}$, it is possible to break this degeneracy in certain classes of inflationary models where identical values of $\lbrace n_{_S}, r \rbrace$ can correspond to different reheating $w_{_{\rm re}}$. In particular, we show how reheating constraints can break inflationary degeneracies in the T-model and the E-model $α$-attractors. Non-canonical inflation is also studied. The relic gravitational wave (GW) spectrum provides us with another tool to break inflationary degeneracies. This is because the GW spectrum is sensitive to the post-inflationary EOS of the universe. Indeed a stiff EOS during reheating $(w_{_{\rm re}} > 1/3)$ gives rise to a small scale blue tilt in the spectral index $n_{_{\rm GW}} = \frac{d\log{Ω_{_{\rm GW}}}}{d\log{k}} > 0$, while a soft EOS $(w_{_{\rm re}} < 1/3)$ results in a red tilt. Relic GWs therefore provide us with valuable information about the post-inflationary epoch, and their spectrum can be used to cure inflationary degeneracies in $\lbrace n_{_S}, r\rbrace$.

gr-qc↗

arXiv2020

Primordial Black Holes from a tiny bump/dip in the Inflaton potential

Scalar perturbations during inflation can be substantially amplified by tiny features in the inflaton potential. A bump-like feature behaves like a local speed-breaker and lowers the speed of the scalar field, thereby locally enhancing the scalar power spectrum. A bump-like feature emerges naturally if the base inflaton potential $V_b(ϕ)$ contains a local correction term such as $V_b(ϕ)\left[1+\varepsilon(ϕ)\right]$ at $ϕ=ϕ_0$. The presence of such a localised correction term at $ϕ_0$ leads to a large peak in the curvature power spectrum and to an enhanced probability of black hole formation. Remarkably this does not significantly affect the scalar spectral index $n_{_S}$ and tensor to scalar ratio $r$ on CMB scales. Consequently such models can produce higher mass primordial black holes ($M_{\rm PBH}\geq 1 M_{\odot}$) in contrast to models with `near inflection-point potentials' in which generating higher mass black holes severely affects $n_{_S}$ and $r$. With a suitable choice of the base potential - such as the string theory based (KKLT) inflation or the $α$-attractor models - the amplification of primordial scalar power spectrum can be as large as $10^7$ which leads to a significant contribution of primordial black holes (PBHs) to the dark matter density today, $f_{\rm PBH} = Ω_{0,\rm PBH}/Ω_{0,\rm DM} \sim O(1)$. Interestingly, our results remain valid if the bump is replaced by a dip. In this case the base inflaton potential $V_b(ϕ)$ contains a negative local correction term such as $V_b(ϕ)\left[1-\varepsilon(ϕ)\right]$ at $ϕ=ϕ_0$ which leads to an enhanced probability of PBH formation. We conclude that primordial black holes in the mass range $10^{-17} M_{\odot} \leq M_{\rm PBH} \leq 100\, M_{\odot}$ can easily form in single field inflation in the presence of small bump-like and dip-like features in the inflaton potential.

gr-qc↗