Eugenio Megías

A bstract: Cold dark matter may be a fluid (or plasma) residing in a strongly-interacting hidden sector, rather than a population of weakly-coupled particles. Such a scenario admits a holographic description in terms of a cosmological braneworld embedded in the linear dilaton five-dimensional (5D) spacetime. In this framework, dark matter originates from the linear dilaton bulk black hole, whose phase we show to be thermodynamically favored at all temperatures. We present a natural freeze-in mechanism for the production of holographic dark matter, in which the bulk black hole is fed by energy leaking from the brane after inflation. Our model is characterized by two free parameters, one of which, the position of the black hole horizon, is fixed by the observed dark matter abundance. The remaining parameter, the 5D Planck scale M ₅ , is consistent with all current experimental bounds provided that M ₅ ≳ 3 × 10 ⁵ TeV.

We consider the possibility of dark matter in a warped extra-dimensional theory in the presence of a linear dilaton background, with a gapped continuum spectrum, in a brane-world cosmological scenario. Firstly, triggered by self-energy radiative corrections, we study the existence of an isolated resonance of massive gravitons, and its realization as a long-lived feebly interacting dark matter candidate, produced by the freeze-in mechanism. This massive graviton is proved to satisfy all theoretical and experimental constraints, in the sub-MeV mass range. We further consider the close relationship between the existence of this component of dark matter and the presence of an inflaton localized on the brane, with a mass around 10 11 GeV and a sub-TeV reheating temperature, in a brane inflationary scenario that allows one to reproduce the most recent cosmological observables. Secondly, the gapped continuum of gravitons, a particular five-dimensional realization of the physics of unparticles, is identified as a holographic fluid which can play the role of holographic dark matter. The production of the holographic fluid goes by an ultraviolet freeze-in mechanism, with an abundance mainly depending on the reheating temperature. Depending on the values of the mass gap and the reheating temperature, one or both components of dark matter can be present.

A bstract: We show that spacetime curvature alone can classically stabilize black strings. Working within a consistent five-dimensional dilaton-gravity system with a flat brane, we find that sufficiently large black strings are classically stable when they extend from the brane to a timelike boundary, which may be either regular or conformal. Black strings are also classically stable in the critical case of the linear dilaton spacetime. In some of the curved backgrounds considered, black strings are stable despite having infinite horizon area.

A warped extra dimension in a five-dimensional (5D) anti de Sitter (AdS) background was introduced in 1999 by Lisa Randall and Raman Sundrum to solve the gauge hierarchy problem in particle physics. As a bonus, a holographic interpretation in terms of four dimensional (4D) conformal field theories (CFT) was found. Interestingly enough, another 5D background, the linear dilaton (LD), was found to have a holographic interpretation in terms of Little String Theory. In this review we will show how a set of 5D backgrounds, parametrized in terms of a real parameter ν, generalizes both theories and gives rise, in particular, to AdS for ν=0 and to LD for ν=1. Furthermore, working in the 5D theory, we will consider applications of the LD background to: i) Particle Physics, so that the 5D Planck scale can be lowered to sub-Planckian values, ii) Brane World Cosmology (BWC), based on the appearance of an extra vacuum characterized by a 5D black hole. In all cases we find a gapped continuum for bulk propagating fields, which makes connection with unparticles. In the case of BWC we also point out on the existence of a pressureless holographic fluid which could play the role of dark matter (DM), with feeble (gravitational) interactions to the Standard Model (SM), decoupled from the thermal SM bath, and generated by a freeze-in mechanism after inflation. We also point out the additional possibility of identifying DM with a long-lived feebly interacting massive graviton, as an isolated resonance generated by radiative corrections to the continuum graviton propagator self-energy.

A bstract: We study the transport properties of relativistic fluids induced by quantum anomalies in presence of explicit symmetry breaking. To this end we consider a holographic Einstein-Maxwell model in 5 dimensions with pure gauge and a mixed gauge-gravitational Chern-Simons terms, coupled with a scalar field. To study the chiral vortical effects and the energy transport sector, apart from the chiral magnetic effects, we have considered the full backreaction of the gauge field on the metric. We have studied the anomalous effects by using Kubo formulae involving correlators of the charged currents and the energy current. Our findings reveal that, in the presence of explicit symmetry breaking, anomaly-induced transport phenomena can extend beyond anomalous currents and affect non-anomalous sectors as well. In particular, we find that all the conductivities display a distinct sensitivity to the mass parameter controlling the symmetry breaking, thus reflecting the interplay between anomaly coefficients and explicit symmetry breaking terms. These findings highlight the role played by pure gauge and mixed gauge-gravitational anomalies in holographic transport, and their importance for strongly coupled systems with broken symmetries.

A bstract: We study the formation of primordial black holes (PBHs) and stochastic gravitational waves background (SGWB) produced by the supercooled radion phase transition (PT) in warped extra-dimension models solving the gauge hierarchy problem. We first determine how the SGWB and the produced PBH mass and abundance depend on the warped model’s infrared energy scale ρ , and the number of holographic colors N . With this finding, we recast on the plane { ρ , N } the current SGWB and PBH constraints, as well as the expected parameter reaches of GW detectors, as LISA and ET, and the gravitational lensing ones, such as NGRST. On the same plane, we also map the collider bounds on massive graviton production, and cosmological bounds on the radion phenomenology. We find that, for N ∼ 10 − 50, the considered PT predicts a PBH population mass in the range M _PBH ∼ (10 ⁻¹ − 10 ⁻²⁵ ) M _⊙ for ρ ∼ (10 ⁻⁴ − 10 ⁸ ) TeV. In the range ρ ≃ (0.05 − 0.5) GeV, it can explain the recent SGWB hint at nHz frequencies and generate PBH binaries with mass M _PBH ∼ (0.1 – 1) M _⊙ detectable at LISA and ET. The experimentally allowed mass region where PBHs can account for the whole dark matter abundance, and are produced with a tuning ≲ 10 ⁻⁴ , corresponds to 10 TeV ≲ ρ ≲ 10 ⁴ TeV. These PBHs can compensate the lack of natural candidates for dark matter in warped extra dimensional models. Such a region represents a great science case where forthcoming and future colliders like HE-LHC and FCC-hh, gravitational-wave observatories and other PBHs probes play a key complementary role.