arXiv subjects

Sai Chaitanya Tadepalli

Publications and source records attributed to Sai Chaitanya Tadepalli.

12 recordsLinked to original sources

arXiv2026

Axions on a Hyperbolic Ride: Geometric Suppression of CMB Isocurvature and a Blue-Tilted Spectrum

CMB limits on cold-dark-matter isocurvature are often interpreted as excluding the simultaneous realization of high-scale inflation and large QCD axion decay constants in pre-inflationary Peccei--Quinn (PQ) scenarios. This conclusion can be evaded by exploiting \emph{field-space geometry}. For a minimal complex PQ scalar with a $U(1)$-symmetric potential and a nonlinear sigma-model kinetic term $dσ^{2}=dR^{2}+f^{2}(R)\,dθ^{2}$, the observable axion fluctuation is $δθ\sim H_{\rm inf}/f(R)$, so an enhanced effective decay constant $f(R)$ suppresses isocurvature without explicit PQ breaking, extreme radial displacements, or additional couplings. We specialize to a hyperbolic metric $f(R)\propto \sinh(R/L)$ with curvature scale $L$. The same geometry also induces a time-dependent $\mathcal{O}(H_{\rm inf})$ effective mass for the canonical axial mode during radial slow-roll, and fixing the tilt and running of isocurvature. Thus, CMB-scale isocurvature is suppressed while a characteristic blue-tilted spectrum is generated. As a result, inflationary Hubble scales as large as $H_{\rm inf}\sim 10^{13}\,\mathrm{GeV}$ can be compatible with $f_a\sim 10^{14}$--$10^{16}\,\mathrm{GeV}$, reopening parameter space usually regarded as excluded. We present `observable' benchmarks and a semi-analytic template that relates the scale-dependence of isocurvature to the geometric lever arm $R/L$, providing a direct phenomenological probe on PQ field-space geometry.

hep-ph↗

arXiv2026

New Isocurvature Constraints from JWST UV Luminosity Function

We constrain uncorrelated primordial isocurvature perturbations using a combination of large- and small-scale cosmological probes, with the small-scale data provided by the ultraviolet luminosity function (UVLF) -- a measure of number density of galaxies as a function of UV brightness. We consider several isocurvature modes, including cold dark matter, baryon, neutrino density, neutrino velocity, and dark radiation perturbations. The isocurvature power spectrum is modeled using two independent parameterizations: a broken power law and a running power law, without fixing the spectral index a priori. Our analysis combines large-scale data from the Cosmic Microwave Background (CMB), baryon acoustic oscillations, and Type Ia supernovae with small-scale constraints from UVLF measurements obtained by \textit{HST} and \textit{JWST}. The UVLF probes matter fluctuations over a continuous range of intermediate scales, $k \sim 0.5$--$10~\mathrm{Mpc}^{-1}$ over a wide range of redshift $4\lesssim z \lesssim 13$, providing a direct handle on structure formation in a regime where constraints on the scale dependence of isocurvature perturbations remain comparatively limited. Our result represents the first UVLF-based constraint on model-agnostic isocurvature perturbations carried by various components. We construct $68\%$ and $95\%$ credible envelopes in $k$-space for the allowed isocurvature power and find good agreement between the envelopes for the $95\%$ envelope across a wide range of scales, indicating that our constraints are mostly insensitive to the assumed power-law form.

astro-ph.CO↗

arXiv2025

Reviving sub-keV warm dark matter: a UVLF-based analysis

Thermal warm dark matter (WDM) particles with $m_{\rm WDM} \leq 1~\mathrm{keV}$ are ruled out at more than $4σ$ by multiple observational probes, owing to the strong suppression of small-scale structure induced by early-time free-streaming. Recently, it was highlighted that a small admixture of $\sim1\%$ ($f_{\rm CDM} \sim\!0.01$) cold dark matter (CDM) endowed with a blue-tilted isocurvature spectrum could offset the WDM-induced suppression and relax the WDM mass bound by a factor of $\mathcal{O}(10)$. If viable, this ''warm + cold-isocurvature'' scenario would allow sub-keV WDM particles to constitute nearly the full dark matter abundance while potentially alleviating some small-scale tensions. In this work, we test this mechanism by constraining the WDM mass $m_{\rm WDM}$ while marginalizing over CDM isocurvature parameters. We combine ultraviolet luminosity function measurements from the \textit{Hubble Space Telescope} and \textit{James Webb Space Telescope} over redshift $4 \leq z \leq 11$ with CMB, BAO, and SNe data. For a pure WDM model, our joint analysis yields a lower bound $m_{\rm WDM} > 1.8~\mathrm{keV}$ (95% credible intervals). When CDM isocurvature is introduced at $f_{\rm CDM} = 0.01$, the limit relaxes to $m_{\rm WDM} > 0.27~\mathrm{keV}$ (95% credible intervals), reflecting a shallow degeneracy in which blue-tilted isocurvature fluctuations partially compensate for WDM suppression. These results provide new constraints on thermal WDM in the presence of CDM isocurvature fluctuations and quantify the extent to which such fluctuations can mask the small-scale signatures of light relics.

astro-ph.CO↗

arXiv2025

Warm Dark Matter meets Cold Dark Matter Isocurvature

Isocurvature fluctuations can be generated in various scenarios in the early Universe. In particular, some specific models predict those with a blue-tilted spectrum, which is consistent with the constraints from cosmic microwave background such as Planck, although isocurvature fluctuations with an almost scale-invariant spectrum are severely constrained. We argue that cold dark matter (CDM) isocurvature fluctuations with blue-tilted spectrum are not only consistent with current cosmological data, but also can loosen the bound on the masses of warm dark matter (WDM), which suppresses small-scale power. In pure thermal WDM models with the adiabatic initial condition, a combination of the data from Lyman-$α$, gravitational lensing, and Milky Way satellites gives a lower bound on the WDM mass as $6~{\rm keV}$ at $95\%$ C.L. while mixed WDM+CDM models loosen these bounds to $m_{\rm WDM}\sim1$ keV for a warm-fraction $f_{\rm WDM}\lesssim0.14$ and $m_{\rm WDM}\sim600$ eV for $f_{\rm WDM}\lesssim0.08$. On the other hand, as we demonstrate, WDM scenarios with a blue-tilted CDM isocurvature power spectrum, even with only $1\%$ CDM contribution ($f_{\rm WDM}\sim0.99$), can allow WDM masses as low as $600$ eV. We further assess the implications of this ``warm + cold-isocurvature'' extension for the small-scale structure by performing $N$-body simulations, particularly focusing on nonlinear matter power spectrum and halo mass function.

astro-ph.CO↗

arXiv2025

Test-time Scaling Techniques in Theoretical Physics -- A Comparison of Methods on the TPBench Dataset

Large language models (LLMs) have shown strong capabilities in complex reasoning, and test-time scaling techniques can enhance their performance with comparably low cost. Many of these methods have been developed and evaluated on mathematical reasoning benchmarks such as AIME. This paper investigates whether the lessons learned from these benchmarks generalize to the domain of advanced theoretical physics. We evaluate a range of common test-time scaling methods on the TPBench physics dataset and compare their effectiveness with results on AIME. To better leverage the structure of physics problems, we develop a novel, symbolic weak-verifier framework to improve parallel scaling results. Our empirical results demonstrate that this method significantly outperforms existing test-time scaling approaches on TPBench. We also evaluate our method on AIME, confirming its effectiveness in solving advanced mathematical problems. Our findings highlight the power of step-wise symbolic verification for tackling complex scientific problems.

cs.LG↗

arXiv2025

Theoretical Physics Benchmark (TPBench) -- a Dataset and Study of AI Reasoning Capabilities in Theoretical Physics

We introduce a benchmark to evaluate the capability of AI to solve problems in theoretical physics, focusing on high-energy theory and cosmology. The first iteration of our benchmark consists of 57 problems of varying difficulty, from undergraduate to research level. These problems are novel in the sense that they do not come from public problem collections. We evaluate our data set on various open and closed language models, including o3-mini, o1, DeepSeek-R1, GPT-4o and versions of Llama and Qwen. While we find impressive progress in model performance with the most recent models, our research-level difficulty problems are mostly unsolved. We address challenges of auto-verifiability and grading, and discuss common failure modes. While currently state-of-the art models are still of limited use for researchers, our results show that AI assisted theoretical physics research may become possible in the near future. We discuss the main obstacles towards this goal and possible strategies to overcome them. The public problems and solutions, results for various models, and updates to the data set and score distribution, are available on the website of the dataset tpbench.org.

cs.LG↗

arXiv2024

Non-Gaussianity from explicit $U(1)$-breaking interactions

We investigate primordial non-Gaussianity (NG) arising from the explicit $U(1)$ symmetry-breaking interactions during inflation involving a nearly massless axial component of a complex scalar field $P$. We analyze the induced NG parameter $f_{\mathrm{NL}}$ under scenarios where the axial field functions as either a curvaton or cold dark matter (CDM). In the curvaton framework, there is a conventional contribution to the local NG of $f_{\rm NL} \simeq -O(1)$. Additional positive local NG can result from either the self-interactions of axial field fluctuations, their interactions with a light radial partner, or kinetic mixing with the inflaton via $U(1)$ symmetry-breaking terms. We identify parameter regions where the interactions lead to cancellations, suppressing the overall local NG to $|f^{\rm loc}_{\mathrm{NL}}| \lesssim O(0.1)$, while leaving the trispectrum largely unaffected. In the CDM scenario, these interactions enhance the NG in the isocurvature fluctuations. Moreover, interactions between the axial field and another light scalar, such as a curvaton, can generate $O(1)$ curvature NG signals and significant mixed curvature-isocurvature NGs that are within the reach of future experiments with $σ(f^{\rm loc}_{\rm NL})\sim1$. We also explore the role of a heavy radial field in generating oscillating correlation signals, noting that such signals can dominate the shape of the mixed adiabatic-isocurvature bispectrum. In certain cases, an oscillatory isocurvature bispectrum signal may be observable in the future, aiding in distinguishing between certain types of the $U(1)$-breaking self-interactions of the axial field.

astro-ph.CO↗

arXiv2024

Large Blue Spectral Index From a Conformal Limit of a Rotating Complex Scalar

One well known method of generating a large blue spectral index for axionic isocurvature perturbations is through a flat direction not having a quartic potential term for the radial partner of the axion field. In this work, we show how one can obtain a large blue spectral index even with a quartic potential term associated with the Peccei-Quinn symmetry breaking radial partner. We use the fact that a large radial direction with a quartic term can naturally induce a conformal limit which generates an isocurvature spectral index of 3. We point out that this conformal representation is intrinsically different from both the ordinary equilibrium axion scenario or massless fields in Minkowski spacetime. Another way to view this limit is as a scenario where the angular momentum of the initial conditions slows down the radial field or as a superfluid limit. Quantization of the non-static system in which derivative of the radial field and the derivative of the angular field do not commute is treated with great care to compute the vacuum state. The parametric region consistent with axion dark matter and isocurvature cosmology is discussed.

astro-ph.CO↗

arXiv2023

Cosmological constraints from harmonic space analysis of DES Y3 3x2 clustering

The large-scale distribution of matter, as mapped by photometric surveys like the Dark Energy Survey (DES), serves as a powerful probe into cosmology. It is especially sensitive to both the amplitude of matter clustering ($σ_8$) and the total matter density ($Ω_m$). The fiducial analysis of the two-point clustering statistics of these surveys is invariably done in configuration space where complex masking scheme is easier to handle. However, such an analysis inherently mixes different scales together, requiring special care in modeling. In this study, we present an analysis of DES Y3 3x2 clustering data in harmonic space where small and large scales are better separated and can be neatly modeled using perturbative techniques. Using conservative scale cuts together with Limber approximation and a Gaussian covariance assumption in a first study, we model the clustering data under a linear bias model for galaxies, incorporating comprehensive treatment for astrophysical effects. We subsequently extend this fiducial analysis to explore a third-order biasing prescription. For our fiducial analysis, we get $S_8=0.789\pm0.020$, consistent with the configuration space analysis presented by the DES collaboration, although under our different modeling choices, we find a preference for a lower $Ω_m$ and a higher $σ_8$. The analysis sets the stage for a future search for signatures of primordial non-Gaussianity and blue-tilted isocurvature perturbations from photometric surveys.

astro-ph.CO↗

arXiv2023

Power Spectrum in the Chaotic Regime of Axionic Blue Isocurvature Perturbations

Large blue tilted spectral index axionic isocurvature perturbations can be produced when the axion sector is far out of equilibrium during inflation through an initial Peccei-Quinn (PQ) symmetry breaking field displacement along a nearly flat direction in the effective potential. As a companion to a previous work, we present analytic formulae for the blue isocurvature spectrum for the case of the kinetic energy density of the PQ symmetry breaking field being larger than the quartic power of the final spontaneous PQ symmetry breaking scale. It corresponds to a regime in which the nonlinearities of the classical potential become important many times during the formation of the axion isocurvature quantum perturbations leading to interesting resonant behavior. One consequence of this nonlinearity-driven resonance is the chaotic nature of the map that links the underlying Lagrangian parameters to the isocurvature amplitudes. We point out an accidental duality symmetry between the perturbation equations and the background field equations that can be used to understand this. Finally, we present two types of analytic results. The first relies on a computation utilizing an effective potential wherein fast time scale fluctuations have been integrated out. The second is grounded in a functional ansatz, requiring only a limited set of fitting parameters. Both analytic results should be useful for carrying out forecasts and fits to the data.

astro-ph.CO↗

arXiv2023

Search for Isocurvature with Large-scale Structure: A Forecast for Euclid and MegaMapper using EFTofLSS

Isocurvature perturbations with a blue power spectrum are one of the natural targets for the future large scale structure observations which are probing shorter length scales with greater accuracy. We present a Fisher forecast for the Euclid and MegaMapper (MM) experiments in their ability to detect blue isocurvature perturbations. We construct the theoretical predictions in the EFTofLSS and bias expansion formalisms at quartic order in overdensities which allows us to compute the power spectrum at one loop order and bispectrum at tree level and further include theoretical error at the next to leading order for the covariance determination. We find that Euclid is expected to provide at least a factor of few improvement on the isocurvature spectral amplitude compared to the existing Planck constraints for large spectral indices while MM is expected to provide about 1 to 1.5 order of magnitude im provement for a broad range of spectral indices. We find features that are specific to the blue isocurvature scenario including the leading parametric degeneracy being with the Laplacian bias and a UV sensitive bare sound speed parameter.

astro-ph.CO↗

arXiv2021

An Analytic Treatment of Underdamped Axionic Blue Isocurvature Perturbations

Previous computations of strongly blue tilted axionic isocurvature spectra were computed in the parametric region in which the lightest time-dependent mass is smaller than the Hubble expansion rate during inflation, leading to an overdamped time evolution. Here we present the strongly blue tilted axionic isocurvature spectrum in an underdamped time evolution parametric regime. Somewhat surprisingly, there exist parametric regions with a strong resonant spectral behavior that leads to a rich isocurvature spectral shape. We focus on computing this resonant spectrum analytically in a large parametric region amenable to such computations. Because the spectrum is sensitive to nonperturbative classical field dynamics, a wide variety of analytic techniques are used including a time-space effective potential obtained by integrating out high frequency fluctuations.

astro-ph.CO↗