arXiv subjects

Raul Jimenez

Publications and source records attributed to Raul Jimenez.

At least 37 records · Page 2Linked to original sources

arXiv2025

Inflation without an Inflaton II: observational predictions

We present a complete computation of the scalar power spectrum in the \emph{inflation without inflaton} (IWI) framework, where the inflationary expansion is driven solely by a de~Sitter (dS) background and scalar fluctuations arise as second-order effects sourced by tensor perturbations. By explicitly deriving and numerically integrating the full second-order kernel of the Einstein equations, we obtain a scale-invariant scalar spectrum without invoking a fundamental scalar field. In this framework, the amplitude of the scalar fluctuations is directly linked to the scale of inflation. More precisely, we show that matching the observed level of scalar fluctuations, $Δ_ϕ^2(k_\ast)\approx 10^{-9}$ at Cosmic Microwave Background (CMB) scales, fixes the inflationary energy scale $H_{\rm inf}$ as a function of the number of observed e-folds $N_{\rm obs}$. For $N_{\rm obs}\simeq 30 - 60$, we find $H_{\rm inf} \simeq 5\times 10^{13}\,\mathrm{GeV} - 2\times 10^{10}\,\mathrm{GeV}$, corresponding to a tensor-to-scalar ratio $r \simeq 0.01 - 5\times 10^{-9}$. In particular, requiring consistency with instantaneous reheating, we predict a number of e-folds of order~$\mathcal{O}(50)$ and an inflationary scale $H_{\rm inf} \simeq 10^{11}\,\mathrm{GeV}$. We also incorporate in our framework the quantum break-time of the dS state and show that it imposes an upper bound on the number of particle species. Specifically, using laboratory constraints on the number of species limits the duration of inflation to $N_{\rm obs}\lesssim 126$ e-folds. These results establish the IWI scenario as a predictive and falsifiable alternative to standard inflaton-driven models, linking the observed amplitude of primordial fluctuations directly to the quantum nature and finite lifetime of dS space.

astro-ph.CO↗

arXiv2025

Cross-correlations of the Cosmic Neutrino Background: HR-DEMNUni simulation analysis

We use the high-resolution HR-DEMNUni simulations to compute cross-correlations of the Cosmic Neutrino Background quantities, like neutrino density, deflection angle, and velocity, with other quantities, like cold dark matter density and effective weak lensing convergence, by accounting for the space-time delay between signals on Earth. We provide this to theoretically illustrate how much can be learned from these cross-correlation signals, once the cosmic neutrino background is detected with instruments in multiple locations. Against a naive expectation of null cross-correlation, we show that the signal is non zero, specially at the largest scales. We also discuss the scenario of co-located cross-correlations between dark matter, weak lensing and a future neutrino-induced photon emission signal. As cross-correlations will be comparable to auto-correlations of the cosmic neutrino background itself and are less affected by cosmic variance and shotnoise, these might be the ones to be measured first. Our predictions thus provide the imprint of what cosmological massive neutrinos, with total mass $\sum{m_ν} \sim 0.1$ eV, should look like from cosmological observations.

astro-ph.CO↗

arXiv2025

The Denario project: Deep knowledge AI agents for scientific discovery

We present Denario, an AI multi-agent system designed to serve as a scientific research assistant. Denario can perform many different tasks, such as generating ideas, checking the literature, developing research plans, writing and executing code, making plots, and drafting and reviewing a scientific paper. The system has a modular architecture, allowing it to handle specific tasks, such as generating an idea, or carrying out end-to-end scientific analysis using Cmbagent as a deep-research backend. In this work, we describe in detail Denario and its modules, and illustrate its capabilities by presenting multiple AI-generated papers generated by it in many different scientific disciplines such as astrophysics, biology, biophysics, biomedical informatics, chemistry, material science, mathematical physics, medicine, neuroscience and planetary science. Denario also excels at combining ideas from different disciplines, and we illustrate this by showing a paper that applies methods from quantum physics and machine learning to astrophysical data. We report the evaluations performed on these papers by domain experts, who provided both numerical scores and review-like feedback. We then highlight the strengths, weaknesses, and limitations of the current system. Finally, we discuss the ethical implications of AI-driven research and reflect on how such technology relates to the philosophy of science. We publicly release the code at https://github.com/AstroPilot-AI/Denario. A Denario demo can also be run directly on the web at https://huggingface.co/spaces/astropilot-ai/Denario, and the full app will be deployed on the cloud.

cs.AI↗

arXiv2025

Efficient PINNs via Multi-Head Unimodular Regularization of the Solutions Space

Non-linear differential equations are a fundamental tool to describe different phenomena in nature. However, we still lack a well-established method to tackle stiff differential equations. Here we present a machine learning framework to facilitate the solution of nonlinear multiscale differential equations and, especially, inverse problems using Physics-Informed Neural Networks (PINNs). This framework is based on what is called \textit{multi-head} (MH) training, which involves training the network to learn a general space of all solutions for a given set of equations with certain variability, rather than learning a specific solution of the system. This setup is used with a second novel technique that we call Unimodular Regularization (UR) of the latent space of solutions. We show that the multi-head approach, combined with Unimodular Regularization, significantly improves the efficiency of PINNs by facilitating the transfer learning process thereby enabling the finding of solutions for nonlinear, coupled, and multiscale differential equations.

cs.LG↗

arXiv2025

The Age of the Universe with Globular Clusters III: Gaia distances and hierarchical modeling

This is the third article in a series aimed at computing accurate and precise ages of galactic globular clusters from their full color-magnitude diagram in order to estimate the age of the Universe and in turn constrain the cosmological model. We update previous constraints using additional data and an improved methodology which allows us to vary the helium abundance and the reddening law in addition to the usual parameters (age, metallicity, alpha enhancement, distance and absorption) in the analysis. Even with the additional degrees of freedom, using the full color-magnitude diagram, now described as a Gaussian mixture Bayesian hierarchical model, a tight constraint on the age(s) of the globular clusters and on the other parameters can be obtained, and the statistical errors are fully subdominant to the systematic errors. We find that the age of the oldest globular clusters is $t_{\rm GC} = 13.39 \pm 0.10 ({\rm stat.}) \pm 0.23 ({\rm sys.})$ Gyr, resulting in an age of the Universe $t_{\rm U}=13.57^{+ 0.16}_{-0.14} ({\rm stat})\pm 0.23 ({\rm sys.})$ and a robust 95\% confidence upper limit of $t_U^{\leq}=13.92_{-0.1}^{+0.13}({\rm stat}) \pm{0.23} ({\rm sys})$. This is fully compatible with our previous estimates and with the model-dependent, Cosmic Microwave Background-derived age for the Universe of $t_{\rm U} =13.8 \pm 0.02$ Gyr for a $Λ$CDM model.

astro-ph.CO↗

arXiv2025

Effects of the Cosmic Neutrino Background Capture on Astrophysical Objects

Low-energy neutrinos from the cosmic background are captured by objects in the sky that contain material susceptible of single beta decay. Neutrons, which compose most of a neutron star, capture low-energy neutrinos from the cosmic neutrino background and release a high-energy electron in the MeV range. Also, planets contain unstable isotopes that capture the cosmic neutrinos. We show that this process is feasible and results in a non-negligible flux of electrons in the MeV range in neutron stars. We present a novel observable, the redshift evolution of the temperature of neutron stars due to neutrino capture, that could provide a route for detection of the cosmic neutrino background from future gravitational waves observatories. For planets the flux is significantly smaller and a measurement is not possible with currently envisioned technology. While the signature from neutron stars is small and challenging, it could result in a novel way to detect the cosmic neutrino background.

astro-ph.CO↗

arXiv2024

Inflation without an Inflaton

We propose a novel scenario in which scalar perturbations, that seed the large scale structure of the Universe, are generated without relying on a scalar field (the inflaton). In this framework, inflation is driven by a de Sitter space-time (dS), where tensor metric fluctuations (i.e., gravitational waves) naturally arise from quantum vacuum oscillations, and scalar fluctuations are generated via second-order tensor effects. We compute the power spectrum of such scalar fluctuations and show it to be consistent with near scale-invariance. We derive the necessary conditions under which scalar perturbations become significant and much larger than the tensor modes, and we identify a natural mechanism to end inflation via a transition to a radiation-dominated phase. Our proposed mechanism could remove the need for a model-dependent scenario: the choice of a scalar field, as the inflaton, to drive inflation.

astro-ph.CO↗

arXiv2024

Time to Sparkler. Accurate ages of lensed globular clusters at $z=1.4$ with JWST photometry

Determining reliable ages for old stellar objects at different redshifts offers a powerful means to constrain cosmology without relying on a specific cosmological model: this is known as the cosmic clocks method. Globular clusters (GCs), long recognised as hosts of the Universe's oldest stars, have served as the archetypical cosmic clocks. However, their age estimates have traditionally been confined to redshift z=0, limiting their role to constraining the present-day age of the Universe. Here we explore how to measure reliable ages of GCs well beyond $z=0$, leveraging their potential to extend cosmic clock measurements to earlier epochs. Specifically, we use 6-band JWST/NIRCam high-precision photometry of candidate stellar clusters in the Sparkler galaxy, located at redshift $z$=1.378 and strongly lensed by the galaxy cluster SMACS J0723.3-7327. By employing stellar population models within a Bayesian inference framework, we constrain the GCs' ages, star formation histories, metallicities, and dust attenuation. The five compact sources previously identified as GCs, based on their red spectral energy distributions being consistent with the colours of old stellar systems, yield a formation age of $1.9\pm0.4$ Gyr on average. This result implies a total age of the Universe that aligns well with the $Λ$CDM model derived from Planck18 data. Recent space-based observations have uncovered a wealth of lensed GCs as well as globulars within the member galaxies of the clusters themselves. These findings suggest that the pool of objects available for cosmic clock studies is enormous. A systematic multi-band photometric survey of GCs in and behind galaxy clusters, using facilities like Euclid and JWST, would therefore be a powerful tool for estimating cluster ages across a large range of redshifts, allowing the Universe to be dated across an unprecedented range of epochs.

astro-ph.GA↗

arXiv2024

Globular clusters as cosmic clocks: new cosmological hints from their integrated light

We explore the reliability and robustness in measuring the age, metallicity and mass of a sample of old Milky Way globular clusters (GCs) from their integrated light, setting the stage for using GCs as cosmic clocks at high redshift. We analyse 77 GCs from the WAGGS project, first by measuring Lick indices and spectroscopic breaks with PyLick, then performing full-spectral-fitting (FSF) with BAGPIPES. The analysis of Lick indices offers an estimate of the GCs' age and [Z/H], generally aligning with literature values, but highlights a subset of old GCs for which we estimate younger ages. This discrepancy is attributed to the presence of blue horizontal branches (HB), which are not accounted for in the stellar population models. With FSF we measure the GCs' ages, [Z/H], and masses, also testing the cosmological prior's impact on ages. Compared to isochrone fitting estimates, ages are best recovered when the cosmological prior is removed, with a 20% increase in GCs' ages compatible with literature values (within $\pm$1.5 Gyr). The derived [Z/H] and mass agree with the reference values, regardless of HB morphology or fit setting, with average discrepancies across the entire sample of $Δ$[Z/H]=-0.02$\pm$0.24 dex and $Δlog(M/M_{\odot})=0.04\pm 0.28$ dex. Ages are best recovered for metal-rich GCs ([Z/H]$\geq$-0.4) showing a red HB (HBR>0), with 70% of the results compatible with literature values. Using a Gaussian Mixture Model, we identify a tail of 24 old GCs with age=13.4$\pm$1.1 Gyr. Being a natural lower limit to the age of the Universe, we use this value to constrain $H_0$, obtaining $H_0 = 70.5^{+7.7}_{-6.3}$ km/s/Mpc (stat+syst) when a flat $Λ$CDM with $Ω_m =0.30 \pm 0.02$ is assumed. Validating the study of GCs based on integrated light lays the foundation to extend this type of study to high-z, where lensed GCs have begun to appear, thanks to JWST. (abridged)

astro-ph.CO↗

arXiv2024

Neutrino Halo profiles: HR-DEMNUni simulation analysis

Using the high-resolution HR-DEMNUni simulations, we computed neutrino profiles within virialized dark matter haloes. These new high-resolution simulations allowed us to revisit fitting formulas proposed in the literature and provided updated fitting parameters that extend to less massive haloes and lower neutrino masses than previously in the literature, in accordance with new cosmological limits. The trend we observe for low neutrino masses is that, for dark matter halo masses below $\sim 4\times10^{14}$$h^{-1}M_\odot$, the presence of the core becomes weaker and the profile over the whole radius is closer to a simple power law. We also characterized the neutrino density profile dependence on the solid angle within clustered structures: a forward-backward asymmetry larger than 10% was found when comparing the density profiles from neutrinos along the direction of motion of cold dark matter particles within the same halo. In addition, we looked for neutrino wakes around halo centres produced by the peculiar motion of the halo itself. Our results suggest that the wakes effect is observable in haloes with masses greater than $3\times10^{14}$ $h^{-1}M_\odot$ where a mean displacement of $0.06$\hmpc was found.

astro-ph.CO↗

arXiv2024

An Entangled Universe

We propose a possible quantum signature of the early Universe that could lead to observational imprints of the quantum nature of the inflationary period. Graviton production in the presence of an inflaton scalar field results in entangled states in polarization. This is because of a non-trivial effect due to the derivatives on two scalar fluctuations and it provides a fingerprint that depends on the polarization of the graviton that Alice and/or Bob measured in their patch. At horizon crossing, interactions between the gravitons and inflatons perform the required Bell experiments leading to a definitive measure. We hint how this signature could be measure in the high-order correlation function of galaxies, in particular on the halo bias and the intrinsic alignment.

astro-ph.CO↗

arXiv2024

Gravitational Duals from Equations of State

Holography relates gravitational theories in five dimensions to four-dimensional quantum field theories in flat space. Under this map, the equation of state of the field theory is encoded in the black hole solutions of the gravitational theory. Solving the five-dimensional Einstein's equations to determine the equation of state is an algorithmic, direct problem. Determining the gravitational theory that gives rise to a prescribed equation of state is a much more challenging, inverse problem. We present a novel approach to solve this problem based on physics-informed neural networks. The resulting algorithm is not only data-driven but also informed by the physics of the Einstein's equations. We successfully apply it to theories with crossovers, first- and second-order phase transitions.

hep-th↗

arXiv2023

Why is zero spatial curvature special?

Evidence for almost spatial flatness of the Universe has been provided from several observational probes, including the Cosmic Microwave Background (CMB) and Baryon Acoustic Oscillations (BAO) from galaxy clustering data. However, other than inflation, and in this case only in the limit of infinite time, there is no strong a priori motivation for a spatially flat Universe. Using the renormalization group (RG) technique in curved spacetime, we present in this work a theoretical motivation for spatial flatness. Starting from a general spacetime, the first step of the RG, coarse-graining, gives a Friedmann-Lemaître-Robertson-Walker (FLRW) metric with a set of parameters. Then, we study the rescaling properties of the curvature parameter, and find that zero spatial curvature of the FLRW metric is singled out as the unique scale-free, non-singular background for cosmological perturbations.

astro-ph.CO↗

arXiv2023

Cosmic Chronometers with Photometry: a new path to $H(z)$

We present a proof-of-principle determination of the Hubble parameter $H(z)$ from photometric data, obtaining a determination at an effective redshift of $z=0.75$ ($0.65<z<0.85$) of $H(0.75) =105.0\pm 7.9(stat)\pm 7.3(sys)$ km s$^{-1}$ Mpc$^{-1}$, with 7.5\% statistical and 7\% systematic (10\% with statistical and systematics combined in quadrature) accuracy. This is obtained in a cosmology model-independent fashion, but assuming a linear age-redshift relation in the relevant redshift range, as such, it can be used to constrain arbitrary cosmologies as long as $H(z)$ can be considered slowly varying over redshift. In particular, we have applied a neural network, trained on a well-studied spectroscopic sample of 140 objects, to the {\tt COSMOS2015} survey to construct a set of 19 thousand near-passively evolving galaxies and build an age-redshift relation. The Hubble parameter is given by the derivative of the red envelope of the age-redshift relation. This is the first time the Hubble parameter is determined from photometry at $\lesssim 10$\% accuracy. Accurate $H(z)$ determinations could help shed light on the Hubble tension; this study shows that photometry, with a reduction of only a factor of two in the uncertainty, could provide a new perspective on the tension.

astro-ph.CO↗

arXiv2023

Cosmic Background Neutrinos Deflected by Gravity: DEMNUni Simulation Analysis

The local supercluster acts as a gravity deflection source for cosmic background neutrinos. This deflection by gravity changes the neutrino helicity and therefore has important consequences for ground based tritium capture experiments aimed at determining if the neutrino is Dirac or Majorana. Here we explore the deflection effect of the local supercluster using the higher resolution DEMNUni simulation suite and reaffirm our previous results. We show that the lightest neutrinos are ultra-relativistic enough to suffer little deflection by gravity and at the same time not relativistic enough to achieve the same capture rate for Dirac and Majorana cases. This means that the capture rate in Ptolemy-like experiments will be sensitive to the neutrino nature and that gravity deflection enlarges the difference between Majorana and Dirac rates. Moreover, using the relation between mass and momentum of the neutrinos frozen Fermi-Dirac distribution, we are able to calculate the deflection angle for different neutrino masses from the same set of neutrinos obtained from the simulation. Doing so, we provide a formula to compute the deflection angle for any neutrino mass, such that when cosmology detects an absolute neutrino mass, precise predictions can be made for tritium ground-based detectors on Earth aimed to determine neutrinos nature.

astro-ph.CO↗

arXiv2022

Unveiling the Universe with Emerging Cosmological Probes

The detection of the accelerated expansion of the Universe has been one of the major breakthroughs in modern cosmology. Several cosmological probes (CMB, SNe Ia, BAO) have been studied in depth to better understand the nature of the mechanism driving this acceleration, and they are being currently pushed to their limits, obtaining remarkable constraints that allowed us to shape the standard cosmological model. In parallel to that, however, the percent precision achieved has recently revealed apparent tensions between measurements obtained from different methods. These are either indicating some unaccounted systematic effects, or are pointing toward new physics. Following the development of CMB, SNe, and BAO cosmology, it is critical to extend our selection of cosmological probes. Novel probes can be exploited to validate results, control or mitigate systematic effects, and, most importantly, to increase the accuracy and robustness of our results. This review is meant to provide a state-of-art benchmark of the latest advances in emerging beyond-standard cosmological probes. We present how several different methods can become a key resource for observational cosmology. In particular, we review cosmic chronometers, quasars, gamma-ray bursts, standard sirens, lensing time-delay with galaxies and clusters, cosmic voids, neutral hydrogen intensity mapping, surface brightness fluctuations, stellar ages of the oldest objects, secular redshift drift, and clustering of standard candles. The review describes the method, systematics, and results of each probe in a homogeneous way, giving the reader a clear picture of the available innovative methods that have been introduced in recent years and how to apply them. The review also discusses the potential synergies and complementarities between the various probes, exploring how they will contribute to the future of modern cosmology.

astro-ph.CO↗

arXiv2022

Black Holes as "Time Capsules": A Cosmological Graviton Background and the Hubble Tension

Minuscule primordial black holes before the end and after inflation can serve as "time capsules" bringing back energy from the past to a later epoch when they evaporate. As these black holes behave like matter, while the rest of the Universe content behaves like radiation, the mass fraction of these black holes, that is tiny at formation, becomes significant later. If sufficiently small, these black holes will evaporate while the Universe is still radiation dominated. We revisit this process and point out that gravitons produced during the evaporation behave as ``dark radiation". If the initial black holes are uniformly distributed so will be the gravitons and in this case they will be free of Silk damping and avoid current limits on "dark radiation" scenarios. Seeds for such black holes can arise during the last phases of inflation. We show here that with suitable parameters, this background graviton field can resolve the Hubble tension. We present current observational constraints on this scenario and suggest upcoming observational tests to prove or refute it. Finally, we also elaborate on the graviton background produced by particle annihilation during the Planck era or shortly after inflation.

astro-ph.CO↗

arXiv2022

Neutrino Masses and Mass Hierarchy: Evidence for the Normal Hierarchy

The latest cosmological constraints on the sum of neutrino masses, in combination with the latest laboratory measurements on oscillations, provide ``decisive" Bayesian evidence for the normal neutrino mass hierarchy. We show that this result holds across very different prior alternatives by exploring two extremes on the range of prior choices. In fact, while the specific numerical value for the Evidence depends on the choice of prior, the Bayesian odds remain greater than 140:1 across very different prior choices. For Majorana neutrinos this has important implications for the upper limit of the neutrino-less double beta decay half life and thus for the technology and resources needed for future double beta decay experiments.

hep-ph↗