arXiv subjects

Varun Sahni

Publications and source records attributed to Varun Sahni.

At least 19 recordsLinked to original sources

arXiv2026

Revisiting Metastable Dark Energy in Light of DESI DR2 BAO and DESI DR1 Full-Shape Measurements

We revisit metastable dark energy (DE) models described by a radioactive-like decay law. We consider three scenarios: an effective, exponentially decaying DE component; decay of DE into non-baryonic dark matter (DM); and decay of DE into dark radiation (DR). We constrain the metastable DE models using DESI DR2 baryon acoustic oscillation (BAO) data, Type Ia supernovae (SNIa), cosmic microwave background (CMB) observations, and, for the first time, the current available DESI DR1 full-shape (FS) clustering measurements. The BAO+SNIa combinations show a mild preference for positive $\Gamma/H_0$, with a deviation from the $\Lambda$CDM limit at the $\gtrsim 2\sigma$ level. This corresponds to a decaying DE density and an effective quintessence-like behaviour at low redshift. Once CMB information from either Planck or P-ACT is included, however, the constraints become statistically consistent with $\Gamma/H_0=0$. The FS measurements probe the growth sector and help distinguish the interacting DM-DE behaviour of Model 2 from the effective decaying-DE response of Model 1 and the weaker DR-induced response of Model 3. For CMB+FS+DES-Dovekie, Model 1 shows a slight deviation from $\Gamma/H_0=0$ at the $\gtrsim 2\sigma$ level, while Models 2 and 3 remain consistent with the $\Lambda$CDM limit within $2\sigma$. Overall, metastable DE remains phenomenologically viable: current data allow late-time dynamics but do not provide decisive evidence for a nonzero decay rate. These results motivate extending our analysis to the upcoming DESI DR2 FS data to obtain tighter constraints on metastable dynamics.

astro-ph.CO↗

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, $Q=\Upsilon/(3H)$, 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 $\Upsilon\propto T^3$. 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

$w_0$-probe: A new diagnostic of dark energy based on $Om$

Recent DESI data suggest that dark energy may be evolving and motivate the use of model-independent diagnostics such as $Om(z)$ and probes of the equation of state (EoS) of dark energy, $w(z)$. Traditional reconstructions of $w(z)$ rely on differentiating the expansion history, $h(z)=H(z)/H_0$, which amplifies noise and systematic uncertainties. In this work, we introduce a new diagnostic, the $w_0$-probe, which is constructed from $Om(z)$, and which enables a direct determination of the current EoS from $h(z)$ without any additional differentiation. While retaining the null-test capability of $Om(z)$ for $\Lambda$CDM, the $w_0$-probe also provides a direct estimate of $w_0$ -- the current EoS of dark energy. We demonstrate that this reconstruction of $w_0$ is robust for any smooth underlying $w(z)$. We apply this method to Gaussian-process (GP) reconstructions of $h(z)$ using current SNe Ia+BAO+CMB data. Both $Om(z)$ and the $w_0$-probe exclude $\Lambda$CDM at the $95\%$ confidence level (C.L.), with the latter favouring $w_0\simeq-0.62 \pm 0.03$ at $95\%$ C.L. To mitigate potential over-constraining from GP priors, we additionally analyze $\chi^2$-limited reconstructions with likelihoods exceeding the $95\%$ CPL threshold. The $w_0$-probe obtained from these high-likelihood samples again predominantly excludes $\Lambda$CDM and yields $w_0\in(-0.8,-0.5)$ at $z\to 0$, demonstrating the robustness of our results. The $w_0$-probe therefore provides a simple, model-independent, and robust diagnostic of the current EoS of dark energy.

astro-ph.CO↗

arXiv2025

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

Braneworld Dark Energy in light of DESI DR2

Recent observational results from the DESI collaboration reveal tensions with the standard $\Lambda$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 $\chi^2$ 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.

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arXiv2025

Remembering Alexei Starobinsky -- the gentle giant of cosmology

I share fond memories of my former PhD advisor Alexei Starobinsky with whom I was closely associated for nearly 45 years. I reflect upon my early years in Moscow when I worked with him on my thesis, and touch upon the seminal work on inflation which he did during that period. Alexei visited India often and actively interacted with Indian scientists and students on issues relating to inflation, large scale structure and dark energy. This extensive collaboration, which lasted several decades, resulted in the publication of over a dozen important papers, several PhD's, and the development of the Statefinder and Om diagnostics, which I briefly discuss.

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 $V(\phi) = V_0 \, v(\phi)$ is a plateau potential then the inverse potential $V(\phi) = V_0 \, \left[v(\phi)\right]^{-1}$ describes hilltop QI. A simple example is provided by the KKLT-inspired potential $v(\phi) = \left\lbrack \frac{M^{2n} + \phi^{2n}}{N^{2n} + \phi^{2n}}\right\rbrack \,$. When $M/N \ll 1$ this potential describes plateau QI, while its inverse, $\left[v(\phi)\right]^{-1}$ describes hilltop QI. Other simple models of QI arise for the class of potentials $V(\phi) \sim \exp\left\lbrack\mp f(\phi)\right\rbrack$, 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.

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arXiv2022

Testing $\Lambda$CDM with eBOSS data using a model independent diagnostic

The $Om3$ diagnostic (Shafieloo et al. 2012) tests the consistency of the cosmological constant as a candidate for dark energy using Baryon Acoustic Oscillation (BAO) data. An important feature of $Om3$ is that it is independent of any parametric assumption for dark energy, neither does it depend upon the dynamics of the Universe during the pre-recombination nor post-recombination eras. In other words, $Om3$ can be estimated using BAO observables and used either to confirm or falsify the cosmological constant independently of the value of the Hubble constant $H_0$ (expansion rate at $z=0$), and the comoving sound horizon at the baryon drag epoch, $r_d$ (which is a function of the physics of the Universe prior to recombination). Consequently, $Om3$ can play a key role in identifying the nature of dark energy (DE) regardless of the existing tensions in the standard model of cosmology and the possible presence of systematics in some of the data sets. We revisit $Om3$ using the most recent BAO observables from the eBOSS survey in order to test the consistency of the cosmological constant with this data. Our results show the consistency of dark energy being the cosmological constant. Moreover, with eBOSS data, we have achieved a precision of $1.5\%$ for this three-point diagnostic. This demonstrates that $Om3$ can be a potent diagnostic of dark energy when used in conjunction with the high precision data expected from forthcoming large scale structure surveys such as the Dark Energy Spectroscopic Instrument (DESI) and Euclid.

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arXiv2022

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 $\phi > 0$.) 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↗

arXiv2022

Dark Matter and Dark Energy

I briefly review our current understanding of dark matter and dark energy. The first part of this paper focusses on issues pertaining to dark matter including observational evidence for its existence, current constraints and the `abundance of substructure' and `cuspy core' issues which arise in CDM. I also briefly describe MOND. The second part of this review focusses on dark energy. In this part I discuss the significance of the cosmological constant problem which leads to a predicted value of the cosmological constant which is almost $10^{123}$ times larger than the observed value $\la/8πG \simeq 10^{-47}$GeV$^4$. Setting $\la$ to this small value ensures that the acceleration of the universe is a fairly recent phenomenon giving rise to the `cosmic coincidence' conundrum according to which we live during a special epoch when the density in matter and $\la$ are almost equal. Anthropic arguments are briefly discussed but more emphasis is placed upon dynamical dark energy models in which the equation of state is time dependent. These include Quintessence, Braneworld models, Chaplygin gas and Phantom energy. Model independent methods to determine the cosmic equation of state and the Statefinder diagnostic are also discussed. The Statefinder has the attractive property $\atridot/a H^3 = 1 $ for LCDM, which is helpful for differentiating between LCDM and rival dark energy models. The review ends with a brief discussion of the fate of the universe in dark energy models.

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arXiv2022

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 ${\cal L} = X^\alpha - V(\phi)$. The observational upper bound on the tensor-to-scalar ratio, $r \leq 0.036$, implies that the whole family of monomial power law potentials $V(\phi) \sim \phi^p$ are now ruled out in the canonical framework at $95\%$ confidence, which includes the simplest classic inflationary potentials such as $\frac{1}{2}m^2 \phi^2$ and $\lambda \phi^4$. 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 $\alpha$-attractors in the canonical framework. Significantly, $V(\phi)$ can originate from Planck scale initial values $V(\phi) \simeq m_p^4$ in non-canonical models while in plateau-like canonical inflation the initial value of the potential is strongly suppressed $V_{\rm plat}(\phi) \leq 10^{-10} m_p^4$. 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↗

arXiv2021

Phantom braneworld and the Hubble tension

Braneworld models with induced gravity exhibit phantom-like behaviour of the effective equation of state of dark energy. They can, therefore, naturally accommodate higher values of $H_0$, preferred by recent local measurements, while satisfying the CMB constraints. We test the background evolution in such phantom braneworld scenarios with the current observational datasets. We find that the phantom braneworld prefers a higher value of $H_0$ even without the R19 prior, thereby providing a much better fit to the local measurements. Although this braneworld model cannot fully satisfy all combinations of cosmological observables, among existing dark energy candidates the phantom brane provides one of the most compelling explanations of cosmic evolution.

astro-ph.CO↗

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

Studying the morphology of HI isodensity surfaces during reionization using Shapefinders and percolation analysis

Minkowski functionals and Shapefinders shed light on the connectedness of large-scale structure by determining its topology and morphology. We use a sophisticated code, SURFGEN2, to measure the Minkowski functionals and Shapefinders of individual clusters by modelling cluster surfaces using the 'Marching Cube 33' triangulation algorithm. In this paper, we study the morphology of simulated neutral hydrogen (HI) density fields using Shapefinders at various stages of reionization from the excursion set approach. Accompanying the Shapefinders, we also employ the 'largest cluster statistic' (LCS) to understand the percolation process. Percolation curves demonstrate that the non-Gaussianity in the HI field increases as reionization progresses. The large clusters in both the HI overdense and underdense excursion sets possess similar values of "thickness" ($T$), as well as "breadth" ($B$), but their third Shapefinder - "length" ($L$) - becomes almost proportional to their volume. The large clusters in both HI overdense and underdense segments are overwhelmingly filamentary. The 'cross-section' of a filamentary cluster can be estimated using the product of the first two Shapefinders, $T \times B$. Hence the cross sections of the large clusters at the onset of percolation do not vary much with volume and their sizes only differ in terms of their lengths. This feature appears more vividly in HI overdense regions than in underdense regions and is more pronounced at lower redshifts which correspond to an advanced stage of reionization.

astro-ph.CO↗

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↗

arXiv2019

Revisiting Metastable Dark Energy and Tensions in the Estimation of Cosmological Parameters

We investigate constraints on some key cosmological parameters by confronting metastable dark energy models with different combinations of the most recent cosmological observations. Along with the standard $Λ$CDM model, two phenomenological metastable dark energy models are considered: (\romannumeral1) DE decays exponentially, (\romannumeral2) DE decays into dark matter. We find that: (1) when considering the most recent supernovae and BAO data, and assuming a fiducial $Λ$CDM model, the inconsistency in the estimated value of the $Ω_{\rm{m,0}}h^2$ parameter obtained by either including or excluding Planck CMB data becomes very much substantial and points to a clear tension~\citep{sahni2014model,zhao2017dynamical}; (2) although the two metastable dark energy models that we study provide greater flexibility in fitting the data, and they indeed fit the SNe Ia+BAO data substantially better than $Λ$CDM, they are not able to alleviate this tension significantly when CMB data are included; (3) while local measurements of the Hubble constant are significantly higher relative to the estimated value of $H_0$ in our models (obtained by fitting to SNe Ia and BAO data), the situation seems to be rather complicated with hints of inconsistency among different observational data sets (CMB, SNe Ia+BAO and local $H_0$ measurements). Our results indicate that we might not be able to remove the current tensions among different cosmological observations by considering simple modifications of the standard model or by introducing minimal dark energy models. A complicated form of expansion history, different systematics in different data and/or a non-conventional model of the early Universe might be responsible for these tensions.

astro-ph.CO↗

arXiv2018

Initial conditions for Inflation in an FRW Universe

We examine the class of initial conditions which give rise to inflation. Our analysis is carried out for several popular models including: Higgs inflation, Starobinsky inflation, chaotic inflation, axion monodromy inflation and non-canonical inflation. In each case we determine the set of initial conditions which give rise to sufficient inflation, with at least $60$ e-foldings. A phase-space analysis has been performed for each of these models and the effect of the initial inflationary energy scale on inflation has been studied numerically. This paper discusses two scenarios of Higgs inflation: (i) the Higgs is coupled to the scalar curvature, (ii) the Higgs Lagrangian contains a non-canonical kinetic term. In both cases we find Higgs inflation to be very robust since it can arise for a large class of initial conditions. One of the central results of our analysis is that, for plateau-like potentials associated with the Higgs and Starobinsky models, inflation can be realised even for initial scalar field values which lie close to the minimum of the potential. This dispels a misconception relating to plateau potentials prevailing in the literature. We also find that inflation in all models is more robust for larger values of the initial energy scale.

gr-qc↗

arXiv2018

The Shape and Size distribution of HII Regions near the percolation transition

Using Shapefinders, which are ratios of Minkowski functionals, we study the morphology of neutral hydrogen (HI) density fields, simulated using semi-numerical technique (inside-out), at various stages of reionization. Accompanying the Shapefinders, we also employ the 'largest cluster statistic' (LCS), originally proposed in Klypin and Shandarin (1993), to study the percolation in both neutral and ionized hydrogen. We find that the largest ionized region is percolating below the neutral fraction $x_{HI} \lesssim 0.728$ (or equivalently $z \lesssim 9$). The study of Shapefinders reveals that the largest ionized region starts to become highly filamentary with non-trivial topology near the percolation transition. During the percolation transition, the first two Shapefinders - 'thickness' ($T$) and 'breadth' ($B$) - of the largest ionized region do not vary much, while the third Shapefinder - 'length' ($L$) - abruptly increases. Consequently, the largest ionized region tends to be highly filamentary and topologically quite complex. The product of the first two Shapefinders, $T\times B$, provides a measure of the 'cross-section' of a filament-like ionized region. We find that, near percolation, the value of $T\times B$ for the largest ionized region remains stable at $\sim 7$ Mpc$^2$ (in comoving scale) while its length increases with time. Interestingly all large ionized regions have similar cross-sections. However their length shows a power-law dependence on their volume, $L\propto V^{0.72}$, at the onset of percolation.

astro-ph.CO↗