Cosmological implications of interacting Group Field Theory models: cyclic Universe and accelerated expansion

We study the cosmological implications of interactions between spacetime quanta in the Group Field Theory (GFT) approach to Quantum Gravity from a phenomenological perspective. Our work represents a first step towards understanding Early Universe Cosmology by studying the dynamics of the emergent continuum spacetime, as obtained from a fundamentally discrete microscopic theory. In particular, we show how GFT interactions lead to a recollapse of the Universe while preserving the bounce replacing the initial singularity, which has already been shown to occur in the free case. It is remarkable that cyclic cosmologies are thus obtained in this framework without any a priori assumption on the geometry of spatial sections of the emergent spacetime. Furthermore, we show how interactions make it possible to have an early epoch of accelerated expansion, which can be made to last for an arbitrarily large number of e-folds, without the need to introduce an ad hoc potential for the scalar field.Comment: 11 pages, 6 figure

Local density of states in metal - topological superconductor hybrid systems

We study by means of the recursive Green's function technique the local density-of-states of (finite and semi-infinite) multi-band spin-orbit coupled semiconducting nanowires in proximity to an s-wave superconductor and attached to normal-metal electrodes. When the nanowire is coupled to a normal electrode, the zero-energy peak, corresponding to the Majorana state in the topological phase, broadens with increasing transmission between the wire and the leads, eventually disappearing for ideal interfaces. Interestingly, for a finite transmission a peak is present also in the normal electrode, even though it has a smaller amplitude and broadens more rapidly with the strength of the coupling. Unpaired Majorana states can survive close to a topological phase transition even when the number of open channels (defined in the absence of superconductivity) is even. We finally study the Andreev-bound-state spectrum in superconductor-normal metal-superconductor junctions and find that in multi-band nanowires the distinction between topologically trivial and non-trivial systems based on the number of zero-energy crossings is preserved.Comment: 11 pages, 12 figures, published versio

The evolution of the disc variability along the hard state of the black hole transient GX 339-4

We report on the analysis of hard-state power spectral density function (PSD) of GX 339-4 down to the soft X-ray band, where the disc significantly contributes to the total emission. At any luminosity probed, the disc in the hard state is intrinsically more variable than in the soft state. However, the fast decrease of disc variability as a function of luminosity, combined with the increase of disc intensity, causes a net drop of fractional variability at high luminosities and low energies, which reminds the well-known behaviour of disc-dominated energy bands in the soft state. The peak-frequency of the high-frequency Lorentzian (likely corresponding to the high-frequency break seen in active galactic nuclei, AGN) scales with luminosity, but we do not find evidence for a linear scaling. In addition, we observe that this characteristic frequency is energy-dependent. We find that the normalization of the PSD at the peak of the high-frequency Lorentzian decreases with luminosity at all energies, though in the soft band this trend is steeper. Together with the frequency shift, this yields quasi-constant high frequency (5-20 Hz) fractional rms at high energies, with less than 10 percent scatter. This reinforces previous claims suggesting that the high frequency PSD solely scales with BH mass. On the other hand, this constancy breaks down in the soft band (where the scatter increases to ~30 percent). This is a consequence of the additional contribution from the disc component, and resembles the behaviour of optical variability in AGN.Comment: 12 pages, 8 figures, accepted for publication in MNRA

Tracing the reverberation lag in the hard state of black hole X-ray binaries

We report results obtained from a systematic analysis of X-ray lags in a sample of black hole X-ray binaries, with the aim of assessing the presence of reverberation lags and studying their evolution during outburst. We used XMM-Newton and simultaneous RXTE observations to obtain broad-band energy coverage of both the disc and the hard X-ray Comptonization components. In most cases the detection of reverberation lags is hampered by low levels of variability signal-to-noise ratio (e.g. typically when the source is in a soft state) and/or short exposure times. The most detailed study was possible for GX 339-4 in the hard state, which allowed us to characterize the evolution of X-ray lags as a function of luminosity in a single source. Over all the sampled frequencies (~0.05-9 Hz) we observe the hard lags intrinsic to the power law component, already well-known from previous RXTE studies. The XMM-Newton soft X-ray response allows us to detail the disc variability. At low-frequencies (long time scales) the disc component always leads the power law component. On the other hand, a soft reverberation lag (ascribable to thermal reprocessing) is always detected at high-frequencies (short time scales). The intrinsic amplitude of the reverberation lag decreases as the source luminosity and the disc-fraction increase. This suggests that the distance between the X-ray source and the region of the optically-thick disc where reprocessing occurs, gradually decreases as GX 339-4 rises in luminosity through the hard state, possibly as a consequence of reduced disc truncation.Comment: 15 pages, 9 figures, 2 tables, accepted for publication in Ap

Percorsi culturali e antropologici nel 'Palazzo Enciclopedico'

Articolo di critica d'arte sulla Biennale di Venezi

Dynamics of anisotropies close to a cosmological bounce in quantum gravity

We study the dynamics of perturbations representing deviations from perfect isotropy in the context of the emergent cosmology obtained from the group field theory formalism for quantum gravity. Working in the mean field approximation of the group field theory formulation of the Lorentzian EPRL model, we derive the equations of motion for such perturbations to first order. We then study these equations around a specific simple isotropic background, characterised by the fundamental representation of \mbox{SU(2)}, and in the regime of the effective cosmological dynamics corresponding to the bouncing region replacing the classical singularity, well approximated by the free GFT dynamics. In this particular example, we identify a region in the parameter space of the model such that perturbations can be large at the bounce but become negligible away from it, i.e. when the background enters the non-linear regime. We also study the departures from perfect isotropy by introducing specific quantities, such as the surface-area-to-volume ratio and the effective volume per quantum, which make them quantitative.Comment: 45 pages, 4 figure

Dressed Polyakov loop and phase diagram of hot quark matter under magnetic field

We evaluate the dressed Polyakov loop for hot quark matter in strong magnetic field. To compute the finite temperature effective potential, we use the Polyakov extended Nambu-Jona Lasinio model with eight-quark interactions taken into account. The bare quark mass is adjusted in order to reproduce the physical value of the vacuum pion mass. Our results show that the dressed Polyakov loop is very sensitive to the strenght of the magnetic field, and it is capable to capture both the deconfinement crossover and the chiral crossover. Besides, we compute self-consistently the phase diagram of the model. We find a tiny split of the two aforementioned crossovers as the strength of the magnetic field is increased. Concretely, for the largest value of magnetic field investigated here, $eB=19 m_\pi^2$, the split is of the order of $10\%$. A qualitative comparison with other effective models and recent Lattice results is also performed.Comment: 10 pages, 3 figures, RevTeX4-1 styl