arXiv subjects

J. A. R. Cembranos

Publications and source records attributed to J. A. R. Cembranos.

At least 19 recordsLinked to original sources

arXiv2026

Probing Cosmic Expansion and Early Universe with Einstein Telescope

Over the next two decades, gravitational-wave (GW) observations are expected to evolve from a discovery-driven endeavour into a precision tool for astrophysics, cosmology, and fundamental physics. Current second-generation ground-based detectors have established the existence of compact-binary mergers and enabled GW multi-messenger astronomy, but they remain limited in sensitivity, redshift reach, frequency coverage, and duty cycle. These limitations prevent them from addressing many fundamental open questions in cosmology. By the 2040s, wide-field electromagnetic surveys will have mapped the luminous Universe with unprecedented depth and accuracy. Nevertheless, key problems including the nature of dark matter, the physical origin of cosmic acceleration, the properties of gravity on cosmological scales, and the physical conditions of the earliest moments after the Big Bang will remain only partially constrained by electromagnetic observations alone. Progress on these fronts requires access to physical processes and epochs that do not emit light. Gravitational waves provide a unique and complementary observational channel: they propagate over cosmological distances largely unaffected by intervening matter, probe extreme astrophysical environments, and respond directly to the geometry of spacetime. In this context, next-generation GW observatories such as the Einstein Telescope (ET) will be transformative for European astronomy. Operating at sensitivities and frequencies beyond existing detectors, ET will observe binary black holes and neutron stars out to previously inaccessible redshifts, enable continuous high signal-to-noise monitoring of compact sources, and detect gravitational-wave backgrounds of astrophysical and cosmological origin. Together with space-based detectors, ET will play a central role in advancing our understanding of cosmic evolution and fundamental physics.

astro-ph.CO↗

arXiv2021

LHC constraints on hidden gravitons

We analyze LHC data in order to constrain the parameter space of new spin-2 particles universally coupled to the energy-momentum tensor. These new hypothetical particles are the so-called hidden gravitons, whose phenomenology at low energies is determined by two parameters: its mass and its dimensional coupling constant. Hidden gravitons arise in many different extensions of the Standard Model of particles and interactions and General Relativity. Their phenomenology has been studied mainly in relation to modifications of gravity and astrophysical signatures. In this work, we extend the constraints for heavy hidden gravitons, with masses larger than $1$ GeV, by taking into account events collected by ATLAS and CMS in the WW channel, Drell-Yan processes, and the diphoton channel from proton-proton collisions at $\sqrt{s}=8$ TeV.

hep-ph↗

arXiv2020

SKA-Phase 1 sensitivity for synchrotron radio emission from multi-TeV Dark Matter candidates

In the era of radio astronomy, the high sensitivity of the Square Kilometre Array (SKA) could play a decisive role in the detection of new radio sources. In this work, we study the SKA sensitivity to the synchrotron radio emission expected by the annihilation of TeV DM candidate in the Draco dwarf spheroidal galaxy. On the one hand, we consider model-independent DM candidates: we find out that with 1000 hours of data-taking, SKA1-MID will be able to exclude up to 10 TeV thermal DM candidates that annihilate in $W^+W^-$ and $b\bar b$ channels. We also study as these constraints improve by including a density enhancement due to a DM-spike associated with an intermediate-mass black hole in Draco. On the other hand, we consider extra-dimensional brane-world DM candidates, dubbed branons. In this specific scenario, SKA allows us to set constraints on the branon parameter space ($f$,$M$), where $f$ is related to the coupling of the branon to the Standard Model particles and $M$ is the mass of the branon itself. In particular, we consider two different branon DM candidates. We find out that SKA will be able to set more stringent constraints on the branon DM candidate required in order to fit the AMS-02 data, yet the sensitivity of the instrument should be improved in order to study the branon candidate for the Galactic Centre. Nonetheless, we show that SKA represents - among other detectors - the most promising instrument for multi-wavelength detection of synchrotron radio emission by annihilating multi-TeV DM.

hep-ph↗

arXiv2019

Magnetic fields from cosmological bulk flows

We explore the possibility that matter bulk flows could generate the required vorticity in the electron-proton-photon plasma to source cosmic magnetic fields through the Harrison mechanism. We analyze the coupled set of perturbed Maxwell and Boltzmann equations for a plasma in which the matter and radiation components exhibit relative bulk motions at the background level. We find that, to first order in cosmological perturbations, bulk flows with velocities compatible with current Planck limits ($β<8.5\times 10^{-4}$ at $95\%$ CL) could generate magnetic fields with an amplitude $10^{-21}$ G on 10 kpc comoving scales at the time of completed galaxy formation which could be sufficient to seed a galactic dynamo mechanism.

astro-ph.CO↗

arXiv2019

Fundamental Physics with the Square Kilometre Array

The Square Kilometre Array (SKA) is a planned large radio interferometer designed to operate over a wide range of frequencies, and with an order of magnitude greater sensitivity and survey speed than any current radio telescope. The SKA will address many important topics in astronomy, ranging from planet formation to distant galaxies. However, in this work, we consider the perspective of the SKA as a facility for studying physics. We review four areas in which the SKA is expected to make major contributions to our understanding of fundamental physics: cosmic dawn and reionisation; gravity and gravitational radiation; cosmology and dark energy; and dark matter and astroparticle physics. These discussions demonstrate that the SKA will be a spectacular physics machine, which will provide many new breakthroughs and novel insights on matter, energy and spacetime.

astro-ph.CO↗

arXiv2019

Dark photon searches with atomic transitions

Dark matter could be made up of dark photons, massive but very light particles whose interactions with matter resemble those of usual photons but suppressed by a small mixing parameter. We analyze the main approaches to dark photon interactions and how they can be applied to direct detection experiments which test different ranges of masses and mixings. A new experiment based on counting dark photons from induced atomic transitions in a target material is proposed. This approach appears to be particularly appropriate for dark photon detection in the meV mass range, extending the constraints in the mixing parameter by up to eight orders of magnitude with respect to previous experiments.

hep-ph↗

arXiv2019

Fermion dynamics in torsion theories

In this work we have studied the non-geodesical behaviour of particles with spin 1/2 in Poincaré gauge theories of gravity, via the WKB method and the Mathisson-Papapetrou equation. We have analysed the relation between the two approaches and we have argued the different advantages associated with the WKB approximation. Within this approach, we have calculated the trajectories in a particular Poincaré gauge theory, discussing the viability of measuring such a motion.

gr-qc↗

arXiv2019

Non-comoving Cosmology

One of the fundamental assumptions of the standard $Λ$CDM cosmology is that, on large scales, all the matter-energy components of the Universe share a common rest frame. This seems natural for the visible sector, that has been in thermal contact and tightly coupled in the primeval Universe. The dark sector, on the other hand, does not have any non-gravitational interaction known to date and therefore, there is no a priori reason to impose that it is comoving with ordinary matter. In this work we explore the consequences of relaxing this assumption and study the cosmology of non-comoving fluids. We show that it is possible to construct a homogeneous and isotropic cosmology with a collection of fluids moving with non-relativistic velocities. Our model extends $Λ$CDM with the addition of a single free parameter $β_0$, the initial velocity of the visible sector with respect to the frame that observes a homogeneous and isotropic universe. This modification gives rise to a rich phenomenology, while being consistent with current observations for $β_0<1.6\times 10^{-3} \text{(95% CL)}$. This work establishes the general framework to describe a non-comoving cosmology and extracts its first observational consequences for large-scale structure. Among the observable effects, we find sizeable modifications in the density-velocity and density-lensing potential cross-correlation spectra. These corrections give rise to deviations from statistical isotropy with a dipolar structure. The relative motion between the different fluids also couples the vector and scalar modes, the latter acting as sources for metric vector modes and vorticity for all the species.

astro-ph.CO↗

arXiv2019

Non-geodesic incompleteness in Poincaré gauge gravity

In this work we review the study of singularities in Poincaré gauge theories of gravity. Since one of the most recent studies uses the appearance of black hole regions of arbitrary dimension as an indicator of singular behaviour, we also give some explicit examples of these structures and study how particles behave around them.

gr-qc↗

arXiv2018

Constraints on anharmonic corrections of Fuzzy Dark Matter

The cold dark matter (CDM) scenario has proved successful in cosmology. However, we lack a fundamental understanding of its microscopic nature. Moreover, the apparent disagreement between CDM predictions and subgalactic-structure observations has prompted the debate about its behaviour at small scales. These problems could be alleviated if the dark matter is composed of ultralight fields $m \sim 10^{-22}\ \text{eV}$, usually known as fuzzy dark matter (FDM). Some specific models, with axion-like potentials, have been thoroughly studied and are collectively referred to as ultralight axions (ULAs) or axion-like particles (ALPs). In this work we consider anharmonic corrections to the mass term coming from a repulsive quartic self-interaction. Whenever this anharmonic term dominates, the field behaves as radiation instead of cold matter, modifying the time of matter-radiation equality. Additionally, even for high masses, i.e. masses that reproduce the cold matter behaviour, the presence of anharmonic terms introduce a cut-off in the matter power spectrum through its contribution to the sound speed. We analyze the model and derive constraints using a modified version of CLASS and comparing with CMB and large-scale structure data.

astro-ph.CO↗

arXiv2017

Constraints on hidden gravitons from fifth-force experiments and stellar energy loss

We study different phenomenological signatures associated with new spin-2 particles. These new degrees of freedom, that we call hidden gravitons, arise in different high-energy theories such as extra-dimensional models or extensions of General Relativity. At low energies, hidden gravitons can be generally described by the Fierz-Pauli Lagrangian. Their phenomenology is parameterized by two dimensionful constants: their mass and their coupling strength. In this work, we analyze two different sets of constraints. On the one hand, we study potential deviations from the inverse-square law on solar-system and laboratory scales. To extend the constraints to scales where the laboratory probes are not competitive, we also study consequences on astrophysical objects. We analyze in detail the processes that may take place in stellar interiors and lead to emission of hidden gravitons, acting like an additional source of energy loss.

hep-ph↗

arXiv2017

Singularities and n-dimensional black holes in torsion theories

In this work we have studied the singular behaviour of gravitational theories with non symmetric connections. For this purpose we introduce a new criteria for the appearance of singularities based on the existence of black/white hole regions of arbitrary codimension defined inside a spacetime of arbitrary dimension. We discuss this prescription by increasing the complexity of the particular torsion theory under study. In this sense, we start with Teleparallel Gravity, then we analyse Einstein-Cartan theory, and finally dynamical torsion models.

gr-qc↗

arXiv2016

Perturbations of ultralight vector field dark matter

We study the dynamics of cosmological perturbations in models of dark matter based on ultralight coherent vector fields. Very much as for scalar field dark matter, we find two different regimes in the evolution: for modes with $k^2\ll {\cal H}ma$, we have a particle-like behaviour indistinguishable from cold dark matter, whereas for modes with $k^2\gg {\cal H}ma$, we get a wave-like behaviour in which the sound speed is non-vanishing and of order $c_s^2\simeq k^2/m^2a^2$. This implies that, also in these models, structure formation could be suppressed on small scales. However, unlike the scalar case, the fact that the background evolution contains a non-vanishing homogeneous vector field implies that, in general, the evolution of the three kinds of perturbations (scalar, vector and tensor) can no longer be decoupled at the linear level. More specifically, in the particle regime, the three types of perturbations are actually decoupled, whereas in the wave regime, the three vector field perturbations generate one scalar-tensor and two vector-tensor perturbations in the metric. Also in the wave regime, we find that a non-vanishing anisotropic stress is present in the perturbed energy-momentum tensor giving rise to a gravitational slip of order $(Φ-Ψ)/Φ\sim c_s^2$. Moreover in this regime the amplitude of the tensor to scalar ratio of the scalar-tensor modes is also $h/Φ\sim c_s^2$. This implies that small-scale density perturbations are necessarily associated to the presence of gravity waves in this model. We compare their spectrum with the sensitivity of present and future gravity waves detectors.

astro-ph.CO↗

arXiv2016

Cosmological perturbations in coherent oscillating scalar field models

The fact that fast oscillating homogeneous scalar fields behave as perfect fluids in average and their intrinsic isotropy have made these models very fruitful in cosmology. In this work we will analyse the perturbations dynamics in these theories assuming general power law potentials $V(ϕ)=λ\vertϕ\vert^{n}/n$. At leading order in the wavenumber expansion, a simple expression for the effective sound speed of perturbations is obtained $c_{\text{eff}}^2 = ω=(n-2)/(n+2)$ with $ω$ the effective equation of state. We also obtain the first order correction in $k^2/ω_{\text{eff}}^2$, when the wavenumber $k$ of the perturbations is much smaller than the background oscillation frequency, $ω_{\text{eff}}$. For the standard massive case we have also analysed general anharmonic contributions to the effective sound speed. These results are reached through a perturbed version of the generalized virial theorem and also studying the exact system both in the super-Hubble limit, deriving the natural ansatz for $δϕ$; and for sub-Hubble modes, exploiting Floquet's theorem.

astro-ph.CO↗

arXiv2015

Antiproton signatures from astrophysical and dark matter sources at the galactic center

The center of our Galaxy is a complex region characterized by extreme phenomena. The presence of the supermassive Sagittarius A* black hole, a high Dark Matter density and an even higher baryonic density are able to produce very energetic processes. Indeed, high energetic gamma rays have been observed by different telescopes, although its origin is not clear. In this work, we constrain the possible antiproton flux component associated to this signal. The expected secondary astrophysical antiproton background already saturates the observed data. It implies that any other important astrophysical source leads to an inconsistent excess, since the theoretical uncertainties corresponding to the mentioned background are small. The constraints depend on the diffusion model and the spectral features of the source. In particular, we consider antiproton spectra described by a power-law, a monochromatic signal and a Standard Model particle-antiparticle channel production.

astro-ph.HE↗

arXiv2015

Reissner-Nordstrom Black Holes in the Inverse Electrodynamics Model

We study electric and magnetic monopoles in static, spherically symmetric and constant curvature geometries in the context of the inverse electrodynamics model. We prove that this U(1) invariant Lagrangian density is able to support the standard metric of a Reissner-Nordstrom Black Hole, but with more complex thermodynamical properties than in the standard case. By employing the Euclidean Action approach we perform a complete analysis of its phase space depending on the sign and singularities of the heat capacity and the Helmholtz free energy.

gr-qc↗

arXiv2014

Vacuum energy as dark matter

We consider the vacuum energy of massive quantum fields in an expanding universe. We define a conserved renormalized energy-momentum tensor by means of a comoving cutoff regularization. Using exact solutions for de Sitter space-time, we show that in a certain range of mass and renormalization scales there is a contribution to the vacuum energy density that scales as nonrelativistic matter and that such a contribution becomes dominant at late times. By means of the WKB approximation, we find that these results can be extended to arbitrary Robertson-Walker geometries. We study the range of parameters in which the vacuum energy density would be compatible with current limits on dark matter abundance. Finally, by calculating the vacuum energy in a perturbed Robertson-Walker background, we obtain the speed of sound of density perturbations and show that the vacuum energy density contrast can grow on sub-Hubble scales as in standard cold dark matter scenarios.

gr-qc↗

arXiv2014

The large-scale structure of vacuum

The vacuum state in quantum field theory is known to exhibit an important number of fundamental physical features. In this work we explore the possibility that this state could also present a non-trivial space-time structure on large scales. In particular, we will show that by imposing the renormalized vacuum energy-momentum tensor to be conserved and compatible with cosmological observations, the vacuum energy of sufficiently heavy fields behaves at late times as non-relativistic matter rather than as a cosmological constant. In this limit, the vacuum state supports perturbations whose speed of sound is negligible and accordingly allows the growth of structures in the vacuum energy itself. This large-scale structure of vacuum could seed the formation of galaxies and clusters very much in the same way as cold dark matter does.

gr-qc↗