arXiv subjects

Utkarsh Kumar

Publications and source records attributed to Utkarsh Kumar.

At least 19 recordsLinked to original sources

arXiv2026

Supernova Time Dilation in Hybrid Expansion-Tired-Light Cosmologies

Dark Energy Survey has shown that the emission light curve widths $\Delta t_{\rm em}$ of supernovae increase to $\Delta t_{\rm obs}$, obeying $\Delta t_{\rm obs}/\Delta t_{\rm em}=(1+z)^b$ with $b=1.003\pm0.011$, excluding a nondilating redshift. We test hybrid models with $1+z=(1+z_p)(1+z_t)$ and the phenomenological stretch $(1+z_p)R(z)$, where $z_p$ is the redshift associated with the expanding parent universe cosmology, $z_t$ is a tired-light (TL) contribution, and $R$ is the hybrid-to-parent lookback-time ratio. At $z=1$, $R$ and $1+z_t$ differ by $1.3\%$ for CCC+TL and $0.5\%$ for $\Lambda$CDM+TL, and their 95\% bands overlap the observed relation. This compatibility shows that the observations do not exclude the hybrid models, only fully nonexpanding ones.

physics.gen-ph↗

arXiv2026

Cosmological consequences of a dynamical dark matter in the light of DESI DR2 measurements

Recent DESI results exhibit preference for a Null Energy Condition violating dynamical dark energy, with early phantom behaviour. We explore an alternative interpretation in which this preference arises from unconventional dark matter dynamics rather than from dynamical dark energy. We propose a dynamical dark matter (DDM) model, with a non-zero equation of state (EoS) that smoothly interpolates between early time and late time asymptotes across a transition scale factor $a_t$, and study its consequences against cosmological datasets including CMB, DESI DR2 BAO, SNeIa (PantheonPlus, Union3, and DESY5) and growth rate data. We find the early time EoS to be consistent with zero, while the present day value is negative at a significance ranging from $0.42\sigma$ to $3.02\sigma$ depending on the dataset combination. The strongest preference occurs from the combination of CMB, DESI, and DESY5 giving the present day EoS to be $-0.060^{+0.013}_{-0.028}$ and $a_t = 0.41^{+0.088}_{-0.13}$ at 68\% CL. This preference for a non-zero, late time DM EoS persists when growth rate data are included and across all three SNeIa compilations considered, while the matter density $\Omega_m$ mildly shifts to higher values relative to $\Lambda$CDM. The model also predicts a lower $\sigma_8$ and $S_8$ than $\Lambda$CDM, in better agreement with weak-lensing data, while $H_0$ remains unchanged and in tension with local distance-ladder measurements. The DDM model is preferred over $\Lambda$CDM ($\Delta \chi^2_{\rm MAP} = -14.093$, $\Delta {\rm DIC} = -7.838$ for Planck+DESI+DESY5) but disfavored relative to the CPL parameterization of DE ($\Delta \chi^2_{\rm MAP} = 6.755$, $\Delta {\rm DIC} = 7.966$). This preference is consistent among other combination of datasets as well.

astro-ph.CO↗

arXiv2026

Primordial Black Hole from Tensor-induced Density Fluctuation: First-order Phase Transitions and Domain Walls

Gravitational waves generated by violent processes in the early Universe necessarily couple to scalar perturbations beyond linear order. We show that tensor perturbations produced by first-order phase transitions and by annihilating domain-wall networks source density fluctuations at second order, thereby opening a distinct channel for primordial-black-hole (PBH) formation. We compute the tensor-induced density spectrum for both sources and map the resulting PBH abundance onto the macroscopic parameters $(\alpha,\beta/H,T_\star)$ of a first-order phase transition and $(\alpha_{\rm ann},V_{\rm bias},\sigma)$ of a domain-wall network. Existing PBH limits therefore impose complementary constraints on early-Universe sources of stochastic gravitational waves. We find viable regions where one can have an observable gravitational-wave background and an appreciable abundance of asteroid-mass PBHs, including benchmark points that saturate the dark-matter abundance. Our results establish a direct, testable correlation among the source scale, the gravitational-wave spectrum, and the PBH mass function, distinguishing this tensor-induced channel from PBHs formed through delayed vacuum decay or direct defect collapse. We also discuss viable particle physics origin of such FOPT and DW, and therefore, constraints on such microphysics, either in the visible, or in dark sector models.

astro-ph.CO↗

arXiv2025

Disentangling the Origins of the NANOGrav Signal: Early Universe Models and $\Delta N_{eff}$ Bounds

We investigate whether an Early-Universe stochastic gravitational-wave background (SGWB) can account for the common spectrum process reported by NANOGrav, while also being consistent with current and projected CMB measurements of extra radiation. We compute the contribution of effective number of relativistic species, $\Delta N_{eff}$, for a number of Early-Universe models proposed to explain the pulsar timing array (PTA) spectrum. We demonstrate that models predicting $\Delta N_{eff}$ above the CMB limit would be firmly excluded, implying that the NANOGrav signal in tension with these bounds must instead arise from astrophysical sources. We find that current NANOGrav 15-year dataset, sensitive up to 60 nHz, gives a negligible contribution to $\Delta N_{eff}$ and remains well below the present and future CMB detection threshold. However, when we project future PTA capabilities reaching upto 1 $\mu$Hz, even with our conservative estimate we find that Inflation, Scalar Induced Gravitational Waves (SIGW), and metastable cosmic strings can induce a $\Delta N_{eff}$ large enough for $>3.5\sigma$ detection by the Simons Observatory.

astro-ph.CO↗

arXiv2025

Primordial Gravitational Wave Background as a Probe of the Primordial Black Holes

We study the formation of primordial black holes (PBHs) from the collapse of density perturbations induced by primordial gravitational waves (PGWs). The PGWs' interpretation of the stochastic gravitational wave background (SGWB) detected by the Pulsar Timing Array (PTA) corresponds to PBHs formation in the mass range $[10^{-12}-10^{-3}] M_{\odot}$. Importantly, our analysis shows that PGWs' interpretation of recent PTA data remains viable, as it does not lead to PBH overproduction. We derive the amplitude of PGWs by leveraging existing constraints on the PBH abundance across a wide mass range. Notably, these constrained amplitudes predict SGWB signals that would be detectable by future gravitational wave observatories.

gr-qc↗

arXiv2025

Evidence for non-cold dark matter from DESI DR2 measurements

We investigate potential deviations from cold dark matter (CDM) using the latest Baryon Acoustic Oscillations (BAO) measurements from the Dark Energy Spectroscopic Instrument (DESI). Analyzing DESI data alone constrains the dark matter equation of state parameter $w_{\mathrm{dm}} = -0.042^{+0.047}_{-0.024}$, revealing a mild preference for non-cold dark matter. This preference strengthens significantly in combined analyses, but reveals a striking tension in the inferred $w_{\mathrm{dm}}$ values from observations of different nature. The DESI+DESY5 combination yields $w_{\mathrm{dm}} = -0.084 \pm 0.035$, excluding CDM ($w_{\mathrm{dm}}=0$) at 2.4$\sigma$ significance. In contrast, Planck+DESI gives $w_{\mathrm{dm}} = 0.00077\pm0.00038$, differing from concordance model at 2$\sigma$ significance. The non-vanishing $w_{\mathrm{dm}}$ preference is particularly driven by low-redshift BAO measurements ($z<1.1$), while higher redshift data remain consistent with $\Lambda$CDM. The evidence for non-cold dark matter is more pronounced in DESI compared to the previous BAO surveys. All dataset combinations show significant improvement over the $\Lambda$CDM paradigm, providing compelling evidence for non-cold dark matter scenario.

astro-ph.CO↗

arXiv2024

Probe of spatial geometry from scalar induced gravitational waves

We investigate a novel probe of spatial geometry of the Universe through the observation of gravitational waves (GWs) induced by first order curvature perturbations. The existence of spatial curvature leaves imprints on the gravitational wave spectrum and formation of primordial black holes. Given the peaked scalar spectrum, the induced spectrum deviates from the flat space power spectrum and the deviation is dependent on the spatial curvature K and reheating temperature $T_{rh}$. For prolonged reheating and negative spatial curvature the spectrum is amplified enough and exhibits an additional peak solely due to K indicating a possible detection by future gravitational wave experiments including LISA and DECIGO. We also observe that the presence of negative spatial curvature improves the constraints on PBH formation, increasing the mass of black holes which are viable dark matter candidates.

astro-ph.CO↗

arXiv2024

Impact of Low ell's on Large Scale Structure Anomalies

We scrutinize the reported lensing anomaly of the CMB by considering several phenomenological modifications of the lensing consistency parameter, $A_{\rm L}$. Considering Planck spectra alone, we find statisically significant evidence for scale dependence (`running') of $A_{\rm L}$. We then demonstrate that the anomaly is entirely driven by Planck's low multipoles, $\ell \leq 30$. When these data points are excluded a joint analysis with several other datasets clearly favors $\Lambda$CDM over the extended $\Lambda \rm CDM+A_L$ model. Not only that the lensing anomaly and low $\ell$ anomaly of the CMB go away in this case, but also the $S_8$ tension is ameliorated, and only the Hubble tension persists.

astro-ph.CO↗

arXiv2024

Large Language Models have Intrinsic Self-Correction Ability

Large language models (LLMs) have attracted significant attention for their exceptional abilities in various natural language processing tasks, but they suffer from hallucinations that will cause performance degradation. One promising solution to improve the LLMs' performance is to ask LLMs to revise their answer after generation, a technique known as self-correction. Among the two types of self-correction, intrinsic self-correction is considered a promising direction because it does not utilize external knowledge. However, recent works doubt the validity of LLM's ability to conduct intrinsic self-correction. In this paper, we present a novel perspective on the intrinsic self-correction capabilities of LLMs through theoretical analyses and empirical experiments. In addition, we identify two critical factors for successful self-correction: zero temperature and fair prompts. Leveraging these factors, we demonstrate that intrinsic self-correction ability is exhibited across multiple existing LLMs. Our findings offer insights into the fundamental theories underlying the self-correction behavior of LLMs and remark on the importance of unbiased prompts and zero temperature settings in harnessing their full potential.

cs.CL↗

arXiv2023

Theoretical Priors and the Dark Energy Equation of State

We revisit the theoretical priors used for inferring Dark Energy (DE) parameters. Any DE model must have some form of a tracker mechanism such that it behaved as matter or radiation in the past. Otherwise, the model is fine-tuned. We construct a model-independent parametrization that takes this prior into account and allows for a relatively sudden transition between radiation/matter to DE behavior. We match the parametrization with current data, and deduce that the adiabatic and effective sound speeds of DE play an important role in inferring the cosmological parameters. We find that there is a preferred transition redshift of $1+z\simeq 29-30$, and some reduction in the Hubble and Large Scale Structure tensions.

astro-ph.CO↗

arXiv2023

Rotating black hole solutions for $f(R)$ gravity and Newman Janis Algorithm

We show that the $f(R)$-gravity theories with constant Ricci scalar in the Jordan/Einstein frame can be described by Einstein or Einstein-Maxwell gravity with a cosmological term and a modified gravitational constant. We also propose a modified Newmann-Janis algorithm to obtain the rotating axisymmetric solutions for the Einstein/Einstein-Maxwell gravity with a cosmological constant. Using the duality between the two gravity theories we show that the stationary or static solutions for the Einstein/Einstein-Maxwell gravity with a cosmological constant will also be the solutions for the dual $f(R)$-gravity with constant Ricci scalar.

gr-qc↗

arXiv2023

Probing The Early Universe Cosmology With NANOGrav: Possibilities and Limitations

A stochastic gravitational wave background is a prediction of a number of astrophysical and cosmological phenomena including early Universe Cosmology. Recently, the NANOGrav Collaboration reported conclusive evidence for a stochastic gravitational-wave background. We analyze the NANOGrav signal assuming it is of primordial origin including the reheating phase. We use the latest measurements from NANOGrav to constrain the Universe's reheating equation of state $w_{re}$ the reheating temperature, $T_{re}$, the tensor to scalar ratio $r$, and the tensor tilt $n_t$. Assuming the constant equation of state $w_{re}$ responsible for reheating phase, we find preference for instant reheating, $w_{re} = 0.36^{+0.15}_{-0.28}$, and a very blue tilt $n_t = 1.94^{+0.43}_{-0.88}$. We find a degeneracy between the tensor to scalar ratio $r$ and $T_{re}$ and suggest ways to break this degeneracy. In all cases where the reheating temperature is constrained, it is constrained to be very low with $T_{re}\leq 10^5 GeV$. We further find that a scale-invariant spectrum as suggested by inflation implies a stiff equation of state $w_{re}=19/3$. If extrapolated, the blue-tilted primordial spectrum that agrees with the NANOGrav signal at corresponding frequencies is incompatible with the LIGO bound. This incompatibility is another challenge for connecting NANOGrav with the primordial spectrum. We discuss a number of ways to circumvent this issue. We split the spectrum into a sum of astrophysical and primordial spectra and constrain the astrophysical and primordial components using NANOGrav data and the LIGO bound. In another attempt, we use the same data and constrain the running of the spectrum. Any of these or a combination of such methods can be used to reconcile the NANOGrav data and the LIGO bound with the primordial power spectrum.

astro-ph.CO↗

arXiv2023

Emergent Unparticles Dark Energy can restore cosmological concordance

Addressing the discrepancy between the late and early time measurements of the Hubble parameter, $H_0$, and the so-called $S_8$ parameter has been a challenge in precision cosmology. Several models are present to address these tensions, but very few of them can do so simultaneously. In the past, we have suggested Banks-Zaks/Unparticles as an emergent Dark Energy model and claimed that it can ameliorate the Hubble tension. In this work, we test this claim and perform a likelihood analysis of the model and its parameters are given current data and compare it to $\Lambda$CDM. The model offers a possible resolution of Hubble tension and softens the Large Scale Structure (LSS) tension without employing a scalar field or modifying the gravitational sector. Our analysis shows a higher value of $H_0 \sim 70 - 73$ km/sec/Mpc and a slightly lower value of $S_8$ for various combinations of data sets. Consideration of Planck CMB data combined with the Pantheon sample and SH0ES priors lowers the $H_0$ and $S_8$ tension to $0.96 \sigma$ and $0.94 \sigma$ respectively with best-fit $\Delta \chi^2 \approx -10$ restoring cosmological concordance. Significant improvement in the likelihood persists for other combinations of data sets as well. Evidence for the model is given by inferring one of its parameters to be $x_0\simeq-4.36$.

astro-ph.CO↗

arXiv2022

Small Field models with ACTPol and BICEP3 data -- Likelihood analysis

We perform a Bayesian analysis for small field models of inflation, using the most recent datasets produced by Planck`18, ACTPol, and BICEP3. We employ Artificial Neural Networks (ANN) to perform analyses with model coefficients, instead of their proxy slow-roll parameters. The ANN connects the models with their projected scalar index $n_s$ and index running $\alpha$, in lieu of the less accurate Lyth-Riotto expressions. We recover the most likely coefficients for a sixth degree polynomial inflationary potential, which yields a tensor-to-scalar ratio $r\lesssim 0.03$. We do so for the case of joint Planck and ACTPol datasets, and for each dataset alone. The BICEP3 data is included in all three analyses. We show that these models are likely, with coefficients that are tuned to about $\Delta\gtrsim 1/60$. Curiously, we also find a significant tension between ACTPol and Planck datasets, which we try to account for.

astro-ph.CO↗

arXiv2021

More on Emergent Dark Energy from Unparticles

In a recent paper \cite{Artymowski:2020zwy} we suggested the possibility that the present acceleration of the Universe is due to thermodynamical behavior of unparticles. The model is free of scalar fields, modified gravity, a Cosmological Constant (CC), the coincidence problem, initial conditions problem and possesses interesting distinct predictions regarding the equation of state of Dark Energy, the growth rate and the number of relativistic degrees of freedom at BBN and CMB decoupling. In this work, we relate to a recent paper \cite{Abchouyeh:2021wey}, which discusses a similar setup of unparticles with and without a CC as an external source of late-time acceleration. The authors have shown how such a model is inconsistent with the data. We show that these claims are viable only in a particular part of the parameter space and that model \cite{Artymowski:2020zwy} stands tall. We further suggest a consistency condition in terms of observables. We then fit publicly available supernovae data to derive the expected Hubble parameter and constrain the parameters of the model.

astro-ph.CO↗

arXiv2020

Emergent dark energy from unparticles

A limiting temperature of a species can cause the Universe to asymptote to it yielding a deSitter (dS) phase due to macroscopic emergent behavior. The limiting temperature is generic for theories slightly shifted from their conformal point. We demonstrate such behavior in the example of unparticles/Banks-Zaks theory. The unparticles behave like radiation at high energies reducing the Hubble tension, and a cosmological constant (CC) at low energies yielding a model that follows closely {\Lambda}CDM model but due to collective phenomenon. It is technically natural and avoids the no-dS conjecture. The model is free of the coincidence and initial conditions problems, of scalar fields and of modified gravity.

hep-ph↗

arXiv2019

Banks-Zaks Cosmology, Inflation, and the Big Bang Singularity

We consider the thermodynamical behavior of Banks-Zaks theory close to the conformal point in a cosmological setting. Due to the anomalous dimension, the resulting pressure and energy density deviate from that of radiation and result in various interesting cosmological scenarios. Specifically, for a given range of parameters one avoids the cosmological singularity. We provide a full "phase diagram" of possible Universe evolution for the given parameters. For a certain range of parameters, the thermal averaged Banks-Zaks theory alone results in an exponentially contracting universe followed by a non-singular bounce and an exponentially expanding universe, i.e. \textit{Inflation without a Big Bang singularity}, or shortly termed "dS Bounce". The temperature of such a universe is bounded from above and below. The result is a theory violating the classical Null Energy Condition (NEC). Considering the Banks-Zaks theory with an additional perfect fluid, yields an even richer phase diagram that includes the standard Big Bang model, stable single "normal" bounce, dS Bounce and stable cyclic solutions. The bouncing and cyclic solutions are with no singularities, and the violation of the NEC happens only near the bounce. We also provide simple analytical conditions for the existence of these non-singular solutions. Hence, within effective field theory, we have a new alternative non-singular cosmology based on the anomalous dimension of Bank-Zaks theory that may include inflation and without resorting to scalar fields.

hep-th↗

arXiv2019

Hamiltonian Formalism for Nonlocal Gravity Models

Nonlocal gravity models are constructed to explain the current acceleration of the universe. These models are inspired by the infrared correction appearing in Einstein Hilbert action. Here we develop the Hamiltonian formalism of a nonlocal model by considering only terms to quadratic order in Ricci tensor and Ricci scalar. We also show how to count the degree of freedom using Hamiltonian formalism in this model.

gr-qc↗