452 research outputs found
Scalar field dark matter and the Higgs field
We discuss the possibility that dark matter corresponds to an oscillating scalar field coupled to the Higgs boson. We argue that the initial field amplitude should generically be of the order of the Hubble parameter during inflation, as a result of its quasi-de Sitter fluctuations. This implies that such a field may account for the present dark matter abundance for masses in the range 10^-6 - 10^-4 eV, if the tensor-to-scalar ratio is within the range of planned CMB experiments. We show that such mass values can naturally be obtained through either Planck-suppressed non-renormalizable interactions with the Higgs boson or, alternatively, through renormalizable interactions within the Randall–Sundrum scenario, where the dark matter scalar resides in the bulk of the warped extra-dimension and the Higgs is confined to the infrared brane
Warming up brane-antibrane inflation
We show that, in constructions with additional intersecting D-branes,
brane-antibrane inflation may naturally occur in a warm regime, such that
strong dissipative effects damp the inflaton's motion, greatly alleviating the
associated eta-problem. We illustrate this for D3-antiD3 inflation in flat
space with additional flavor D7-branes, where for both a Coulomb-like or a
quadratic hybrid potential a sufficient number of e-folds may be obtained for
perturbative couplings and O(10-10^4) branes. This is in clear contrast with
the corresponding cold scenarios, thus setting the stage for more realistic
constructions within fully stabilized compactifications. Such models
generically predict a negligible amount of tensor perturbations and
non-gaussianity f_NL \sim O(10).Comment: 8 pages, 2 figures; version to be published in Physical Review
Superradiance in the sky
We discuss the conditions under which plane electromagnetic and gravitational waves can be amplified by a rotating black hole due to superradiant scattering. We show, in particular, that amplification can occur for low-frequency waves with an incidence angle parametrically close to 0 (or π) with respect to the black hole spin axis and with a parametrically small left (or right) polarization. This is the case of the radiation emitted by a spinning electric/magnetic dipole or gravitational quadrupole orbiting a black hole companion at large radius and co-rotating with the latter. This can yield observable effects of superradiance, for example, in neutron star-stellar mass black hole binaries, as well as in triple systems composed by a compact binary orbiting a central supermassive black hole. Due to superradiance, the total source luminosity in these systems exhibits a characteristic orbital modulation that may lead to significant observational signatures, thus paving the way for testing, in the near future, one of the most peculiar predictions of general relativity
Warm Inflation, Neutrinos and Dark matter: a minimal extension of the Standard Model
We show that warm inflation can be realized within a minimal extension of the
Standard Model with three right-handed neutrinos, three complex scalars and a
gauged lepton/B-L U(1) symmetry. This simple model can address all the
shortcomings of the Standard Model that are not related to fine-tuning, within
general relativity, with distinctive experimental signatures that can be probed
in the near future. The inflaton field emerges from the collective breaking of
the U(1) symmetry, and interacts with two of the right-handed neutrinos,
sustaining a high-temperature radiation bath during inflation. The discrete
interchange symmetry of the model protects the scalar potential against large
thermal corrections and leads to a stable inflaton remnant at late times which
can account for dark matter. Consistency of the model and agreement with Cosmic
Microwave Background observations naturally yield light neutrino masses below
0.1 eV, while thermal leptogenesis occurs naturally after a smooth exit from
inflation into the radiation era.Comment: 43 pages (30 main + 13 appendices), 8 figures. Comments are welcom
Superradiant axion clouds around asteroid-mass primordial black holes
We analyze the dynamics and observational signatures of axion clouds formed
via the superradiant instability around primordial black holes, focusing on the
mass range kg where the latter may account for all the dark
matter. We take into account the leading effects of axion self-interactions,
showing that, even though these limit the number of axions produced within each
cloud, a large number of superradiant axions become free of the black hole's
gravitational potential and accumulate in the intergalactic medium or even in
the host galaxy, depending on their escape velocity. This means that primordial
black hole dark matter may lead to a sizeable astrophysical population of
non-relativistic axions, with masses ranging from 0.1 eV to 1 MeV, depending on
the primordial black hole mass and spin. We then show that if such axions
couple to photons their contribution to the galactic and extragalactic
background flux, mainly in the X-ray and gamma-ray band of the spectrum, is
already beyond current observational limits for a large range of parameters
that are, therefore, excluded. We finish by showing the prospects of the Athena
X-ray telescope to further probe this co-existence of primordial black holes
and axions.Comment: 21 pages, 5 figure
Solvent-free process for the development of photocatalytic membranes
PTDC/EAM-AMB/30989/2017.This work described a new sustainable method for the fabrication of ceramic membranes with high photocatalytic activity, through a simple sol-gel route. The photocatalytic surfaces, prepared at low temperature and under solvent-free conditions, exhibited a narrow pore size distribution and homogeneity without cracks. These surfaces have shown a highly efficient and reproducible behavior for the degradation of methylene blue. Given their characterization results, the microfiltration photocatalytic membranes produced in this study using solvent-free conditions are expected to effectively retain microorganisms, such as bacteria and fungi that could then be inactivated by photocatalysis.publishe
Novel submerged photocatalytic membrane reactor for treatment of olive mill wastewaters
POCI-01-0145-FEDER-007265A new hybrid photocatalytic membrane reactor that can easily be scaled-up was designed, assembled and used to test photocatalytic membranes developed using the sol–gel technique. Extremely high removals of total suspended solids, chemical oxygen demand, total organic carbon, phenolic and volatile compounds were obtained when the hybrid photocatalytic membrane reactor was used to treat olive mill wastewaters. The submerged photocatalytic membrane reactor proposed and the modified membranes represent a step forward towards the development of new advanced treatment technology able to cope with several water and wastewater contaminants.publishersversionpublishe
Massive vector fields on the Schwarzschild spacetime: quasinormal modes and bound states
We study the propagation of a massive vector or Proca field on the
Schwarzschild spacetime. The field equations are reduced to a one-dimensional
wave equation for the odd-parity part of the field and two coupled equations
for the even-parity part of the field. We use numerical techniques based on
solving (scalar or matrix-valued) three-term recurrence relations to compute
the spectra of both quasi-normal modes and quasi-bound states, which have no
massless analogue, complemented in the latter case by a forward-integration
method. We study the radial equations analytically in both the near-horizon and
far-field regions and use a matching procedure to compute the associated
spectra in the small-mass limit. Finally, we comment on extending our results
to the Kerr geometry and its phenomenological relevance for hidden photons
arising e.g. in string theory compactifications.Comment: 15 pages, 8 figures; minor corrections, to be published in Phys. Rev.
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