arXiv subjects

Antonio L. Maroto

Publications and source records attributed to Antonio L. Maroto.

At least 55 records · Page 3Linked to original sources

arXiv2013

Cosmic Rays from Heavy Dark Matter from the Galactic Center

The gamma-ray fluxes observed by the High Energy Stereoscopic System (HESS) from the J1745-290 Galactic Center source is well fitted by the secondary photons coming from Dark Matter (DM) annihilation in particle-antiparticle standard model pairs over a diffuse power-law background. The spectral features of the signal are consistent with different channels: light quarks, electro-weak gauge bosons and top-antitop production. The amount of photons and morphology of the signal localized within a region of few parsecs, require compressed DM profiles as those resulting from baryonic contraction, which offer large enhancements in the signal over DM alone simulations. The fits return a heavy WIMP, with a mass above 10 TeV, but well below the unitarity limit for thermal relic annihilation. The fitted background spectral index is compatible with the Fermi-Large Area Telescope (LAT) data from the same region. This possibility can be potentially tested with the observations of other high energy cosmic rays.

astro-ph.HE↗

arXiv2013

Vector dark energy and high-z massive clusters

The detection of extremely massive clusters at $z>1$ such as SPT-CL J0546-5345, SPT-CL J2106-5844, and XMMU J2235.3-2557 has been considered by some authors as a challenge to the standard \LCDM$\;$cosmology. In fact,assuming Gaussian initial conditions, the theoretical expectation of detecting such objects is as low as $\leq 1%$. In this \textit{Letter} we discuss the probability of the existence of such objects in the light of the Vector Dark Energy (VDE) paradigm, showing by means of a series of $N$-body simulations that chances of detection are substantially enhanced in this non-standard framework.

astro-ph.CO↗

arXiv2013

N-body simulations with a cosmic vector for dark energy

We present the results of a series of cosmological $N$-body simulations of a Vector Dark Energy (VDE) model, performed using a suitably modified version of the publicly available \texttt{GADGET}-2 code. The setups of our simulations were calibrated pursuing a twofold aim: 1) to analyze the large scale distribution of massive objects and 2) to determine the properties of halo structure in this different ramework.We observe that structure formation is enhanced in VDE, since the mass function at high redshift is boosted up to a factor of ten with respect to \LCDM, possibly alleviating tensions with the observations of massive clusters at high redshifts and early reionization epoch. Significant differences can also be found for the value of the growth factor, that in VDE shows a completely different behaviour, and in the distribution of voids, which in this cosmology are on average smaller and less abundant. We further studied the structure of dark matter haloes more massive than $5\times10^{13}$\hMsun, finding that no substantial difference emerges when comparing spin parameter, shape, triaxiality and profiles of structures evolved under different cosmological pictures. Nevertheless, minor differences can be found in the concentration-mass relation and the two point correlation function; both showing different amplitudes and steeper slopes.Using an additional series of simulations of a \LCDM\ scenario with the same $Ω_M$ and $σ_8$ used in the VDE cosmology, we have been able to establish whether the modifications induced in the new cosmological picture were due to the particular nature of the dynamical dark energy or a straightforward consequence of the cosmological parameters.

astro-ph.CO↗

arXiv2011

The Dark Magnetism of the Universe

Despite the success of Maxwell's electromagnetism in the description of the electromagnetic interactions on small scales, we know very little about the behaviour of electromagnetic fields on cosmological distances. Thus, it has been suggested recently that the problems of dark energy and the origin of cosmic magnetic fields could be pointing to a modification of Maxwell's theory on large scales. Here, we review such a proposal in which the scalar state which is usually eliminated be means of the Lorenz condition is allowed to propagate. On super-Hubble scales, the new mode is essentially given by the temporal component of the electromagnetic potential and contributes as an effective cosmological constant to the energy-momentum tensor. The new state can be generated from quantum fluctuations during inflation and it is shown that the predicted value for the cosmological constant agrees with observations provided inflation took place at the electroweak scale. We also consider more general theories including non-minimal couplings to the space-time curvature in the presence of the temporal electromagnetic background. We show that both in the minimal and non-minimal cases, the modified Maxwell's equations include new effective current terms which can generate magnetic fields from sub-galactic scales up to the present Hubble horizon. The corresponding amplitudes could be enough to seed a galactic dynamo or even to account for observations just by collapse and differential rotation in the protogalactic cloud.

astro-ph.CO↗

arXiv2011

On the electromagnetic nature of dark energy and the origin of cosmic magnetic fields

In this work we consider quantum electromagnetic fields in an expanding universe. We start by reviewing the difficulties found when trying to impose the Lorenz condition in a time-dependent geometry. Motivated by this fact, we explore the possibility of extending the electromagnetic theory by allowing the scalar state which is usually eliminated by means of the Lorenz condition to propagate, preserving at the same time the dynamics of ordinary transverse photons. We show that the new state cannot be generated by charged currents, but it breaks conformal invariance and can be excited gravitationally. In fact, primordial quantum fluctuations produced during inflation can give rise to super-Hubble temporal electromagnetic modes whose energy density behaves as a cosmological constant. The value of the effective cosmological constant is shown to agree with observations provided inflation took place at the electroweak scale. The theory is compatible with all the local gravity tests and is free from classical or quantum instabilities. Thus we see that, not only the true nature of dark energy can be established without resorting to new physics, but also the value of the cosmological constant finds a natural explanation in the context of standard inflationary cosmology. On sub-Hubble scales, the new state generates an effective charge density which, due to the high electric conductivity of the cosmic plasma after inflation, gives rise to both vorticity and magnetic fields. Present upper limits on vorticity coming from CMB anisotropies are translated into lower limits on the present value of cosmic magnetic fields. We find that magnetic fields $B_λ> 10^{-12}$ G can be typically generated with coherence lengths ranging from sub-galactic scales up to the present Hubble radius.

astro-ph.CO↗

arXiv2011

Cosmic magnetic fields and dark energy in extended electromagnetism

We discuss an extended version of electromagnetism in which the usual gauge fixing term is promoted into a physical contribution that introduces a new scalar state in the theory. This new state can be generated from vacuum quantum fluctuations during an inflationary era and, on super-Hubble scales, gives rise to an effective cosmological constant. The value of such a cosmological constant coincides with the one inferred from observations as long as inflation took place at the electroweak scale. On the other hand, the new state also generates an effective electric charge density on sub-Hubble scales that produces both vorticity and magnetic fields with coherent lengths as large as the present Hubble horizon.

astro-ph.CO↗

arXiv2011

Cosmological magnetic fields from inflation in extended electromagnetism

In this work we consider an extended electromagnetic theory in which the scalar state which is usually eliminated by means of the Lorenz condition is allowed to propagate. This state has been shown to generate a small cosmological constant in the context of standard inflationary cosmology. Here we show that the usual Lorenz gauge-breaking term now plays the role of an effective electromagnetic current. Such a current is generated during inflation from quantum fluctuations and gives rise to a stochastic effective charge density distribution. Due to the high electric conductivity of the cosmic plasma after inflation, the electric charge density generates currents which give rise to both vorticity and magnetic fields on sub-Hubble scales. Present upper limits on vorticity coming from temperature anisotropies of the CMB are translated into lower limits on the present value of cosmic magnetic fields. We find that, for a nearly scale invariant vorticity spectrum, magnetic fields $B_λ> 10^{-12}$ G are typically generated with coherence lengths ranging from sub-galactic scales up to the present Hubble radius. Those fields could act as seeds for a galactic dynamo or even account for observations just by collapse and differential rotation of the protogalactic cloud.

astro-ph.CO↗

arXiv2010

Dark energy, non-minimal couplings and the origin of cosmic magnetic fields

In this work we consider the most general electromagnetic theory in curved space-time leading to linear second order differential equations, including non-minimal couplings to the space-time curvature. We assume the presence of a temporal electromagnetic background whose energy density plays the role of dark energy, as has been recently suggested. Imposing the consistency of the theory in the weak-field limit, we show that it reduces to standard electromagnetism in the presence of an effective electromagnetic current which is generated by the momentum density of the matter/energy distribution, even for neutral sources. This implies that in the presence of dark energy, the motion of large-scale structures generates magnetic fields. Estimates of the present amplitude of the generated seed fields for typical spiral galaxies could reach $10^{-9}$ G without any amplification. In the case of compact rotating objects, the theory predicts their magnetic moments to be related to their angular momenta in the way suggested by the so called Schuster-Blackett conjecture.

astro-ph.CO↗

arXiv2010

The electromagnetic dark sector

We consider electromagnetic field quantization in an expanding universe. We find that the covariant (Gupta-Bleuler) method exhibits certain difficulties when trying to impose the quantum Lorenz condition on cosmological scales. We thus explore the possibility of consistently quantizing without imposing such a condition. In this case there are three physical states, which are the two transverse polarizations of the massless photon and a new massless scalar mode coming from the temporal and longitudinal components of the electromagnetic field. An explicit example in de Sitter space-time shows that it is still possible to eliminate the negative norm state and to ensure the positivity of the energy in this theory. The new state is decoupled from the conserved electromagnetic currents, but is non-conformally coupled to gravity and therefore can be excited from vacuum fluctuations by the expanding background. The cosmological evolution ensures that the new state modifies Maxwell's equations in a totally negligible way on sub-Hubble scales. However, on cosmological scales it can give rise to a non-negligible energy density which could explain in a natural way the present phase of accelerated expansion of the universe.

astro-ph.CO↗

arXiv2010

Dark Energy in vector-tensor theories of gravity

We consider a general class of vector-tensor theories of gravity and show that solutions with accelerated expansion and a future type III singularity are a common feature in these models. We also show that there are only six vector-tensor theories with the same small scales behavior as General Relativity and, in addition, only two of them can be made completely free from instabilities. Finally, two particular models as candidates for dark energy are proposed: on one hand, a cosmic vector that allows to alleviate the usual naturalness and coincidence problems and, on the other hand, the electromagnetic field is shown to give rise to an effective cosmological constant on large scales whose value can be explained in terms of inflation at the electroweak scale.

astro-ph.CO↗

arXiv2010

Black holes in modified gravity theories

In the context of $f(R)$ gravity theories, the issue of finding static and spherically symmetric black hole solutions is addressed. Two approaches to study the existence of such solutions are considered: first, constant curvature solutions, and second, the general case (without imposing constant curvature) is also studied. Performing a perturbative expansion around the Einstein-Hilbert action, it is found that only solutions of the Schwarzschild-(Anti-) de Sitter type are present (up to second order in perturbations) and the explicit expressions for these solutions are provided in terms of the $f(R)$ function. Finally we consider the thermodynamics of black holes in Anti-de Sitter space-time and study their local and global stability.

gr-qc↗

arXiv2009

Electromagnetic nature of dark energy

Out of the four components of the electromagnetic field, Maxwell's theory only contains two physical degrees of freedom. However, in an expanding universe, consistently eliminating one of the "unphysical" states in the covariant (Gupta-Bleuler) formalism turns out to be difficult to realize. In this work we explore the possibility of quantization without subsidiary conditions. This implies that the theory would contain a third physical state. The presence of such a new (temporal) electromagnetic mode on cosmological scales is shown to generate an effective cosmological constant which can account for the accelerated expansion of the universe. This new polarization state is completely decoupled from charged matter, but can be excited gravitationally. In fact, primordial electromagnetic quantum fluctuations produced during electroweak scale inflation could naturally explain the presence of this mode and also the measured value of the cosmological constant. The theory is compatible with all the local gravity tests, it is free from classical or quantum instabilities and reduces to standard QED in the flat space-time limit. Thus we see that, not only the true nature of dark energy can be established without resorting to new physics, but also the value of the cosmological constant finds a natural explanation in the context of standard inflationary cosmology. Possible signals, discriminating this model from LCDM, are also discussed.

astro-ph.CO↗

arXiv2009

Perturbations in electromagnetic dark energy

It has been recently proposed that the presence of a temporal electromagnetic field on cosmological scales could explain the phase of accelerated expansion that the universe is currently undergoing. The field contributes as a cosmological constant and therefore, the homogeneous cosmology produced by such a model is exactly the same as that of $Λ$CDM. However, unlike a cosmological constant term, electromagnetic fields can acquire perturbations which in principle could affect CMB anisotropies and structure formation. In this work, we study the evolution of inhomogeneous scalar perturbations in this model. We show that provided the initial electromagnetic fluctuations generated during inflation are small, the model is perfectly compatible with both CMB and large scale structure observations at the same level of accuracy as $Λ$CDM.

physics.gen-ph↗

arXiv2009

Cosmological evolution in vector-tensor theories of gravity

We present a detailed study of the cosmological evolution in general vector-tensor theories of gravity without potential terms. We consider the evolution of the vector field throughout the expansion history of the universe and carry out a classification of models according to the behavior of the vector field in each cosmological epoch. We also analyze the case in which the universe is dominated by the vector field, performing a complete analysis of the system phase map and identifying those attracting solutions which give rise to accelerated expansion. Moreover, we consider the evolution in a universe filled with a pressureless fluid in addition to the vector field and study the existence of attractors in which we can have a transition from matter-domination to vector-domination with accelerated expansion so that the vector field may play the role of dark energy. We find that the existence of solutions with late-time accelerated expansion is a generic prediction of vector-tensor theories and that such solutions typically lead to the presence of future singularities. Finally, limits from local gravity tests are used to get constraints on the value of the vector field at small (Solar System) scales.

astro-ph.CO↗

arXiv2009

Cosmic vector for dark energy: constraints from SN, CMB and BAO

It has been recently shown that the presence of a vector field over cosmological scales could explain the observed accelerated expansion of the universe without introducing neither new scales nor unnatural initial conditions in the early universe, thus avoiding the coincidence problem. Here, we present a detailed analysis of the constraints imposed by SNIa, CMB and BAO data on the vector dark energy model with general spatial curvature. We find that contrary to standard cosmology, CMB data excludes a flat universe for this model and, in fact, predicts a closed geometry for the spatial sections. We see that CMB and SNIa Gold data are perfectly compatible at the 1-sigma level, however SNIa Union dataset exhibits a 3-sigma tension with CMB. The same level of tension is also found between SNIa and BAO measurements.

astro-ph.CO↗

arXiv2009

Dark energy: the absolute electric potential of the universe

Is there an absolute cosmic electric potential?. The recent discovery of the accelerated expansion of the universe could be indicating that this is certainly the case. In this essay we show that the consistency of the covariant and gauge invariant theory of electromagnetism is truly questionable when considered on cosmological scales. Out of the four components of the electromagnetic field, Maxwell's theory only contains two physical degrees of freedom. However, in the presence of gravity, one of the "unphysical" states cannot be consistently eliminated, thus becoming real. This third polarization state is completely decoupled from charged matter, but can be excited gravitationally thus breaking gauge invariance. On large scales the new state can be seen as a homogeneous cosmic electric potential, whose energy density behaves as a cosmological constant.

physics.gen-ph↗

arXiv2009

Large-scale cosmic flows and moving dark energy

Large-scale matter bulk flows with respect to the cosmic microwave background have very recently been detected on scales 100 Mpc/h and 300 Mpc/h by using two different techniques showing an excellent agreement in the motion direction. However, the unexpectedly large measured amplitudes are difficult to understand within the context of standard LCDM cosmology. In this work we show that the existence of such a flow could be signaling the presence of moving dark energy at the time when photons decoupled from matter. We also comment on the relation between the direction of the CMB dipole and the preferred axis observed in the quadrupole in this scenario.

astro-ph↗

arXiv2009

Viability of vector-tensor theories of gravity

We present a detailed study of the viability of general vector-tensor theories of gravity in the presence of an arbitrary temporal background vector field. We find that there are six different classes of theories which are indistinguishable from General Relativity by means of local gravity experiments. We study the propagation speeds of scalar, vector and tensor perturbations and obtain the conditions for classical stability of those models. We compute the energy density of the different modes and find the conditions for the absence of ghosts in the quantum theory. We conclude that the only theories which can pass all the viability conditions for arbitrary values of the background vector field are not only those of the pure Maxwell type, but also Maxwell theories supplemented with a (Lorentz type) gauge fixing term.

astro-ph↗