Post-inflationary phases stiffer than radiation and Palatini formulation

If the inflaton and the quintessence fields are identified, the background geometry evolves through a stiff epoch undershooting the expansion rate of a radiation-dominated plasma. For some classes of inflationary potentials this scenario is at odds with the current observational evidence since the corresponding tensor-to-scalar ratio is too large. Quintessential inflation is analyzed when the gravitational action is supplemented by a contribution quadratic in the Einstein-Hilbert term. In the Palatini formulation the addition such a term does not affect the scalar modes during the inflationary phase and throughout the course of the subsequent stiff epoch but it suppresses the tensor power spectrum and the tensor-to-scalar ratio. While in the Palatini formulation the power-law potentials leading to a quintessential inflationary dynamics are again viable, the high-frequency spike of the relic graviton spectrum is squeezed and the whole signal is suppressed at least when the higher-order contributions appearing in the action are explicitly decoupled from the inflaton.Comment: 25 page

Effective horizons, junction conditions and large-scale magnetism

The quantum mechanical generation of hypermagnetic and hyperlectric fields in four-dimensional conformally flat background geometries rests on the simultaneous continuity of the effective horizon and of the extrinsic curvature across the inflationary boundary. The junction conditions for the gauge fields are derived in general terms and corroborated by explicit examples with particular attention to the limit of a sudden (but nonetheless continuous) transition of the effective horizon. After reducing the dynamics to a pair of integral equations related by duality transformations, we compute the power spectra and deduce a novel class of logarithmic corrections which turn out to be, however, numerically insignificant and overwhelmed by the conductivity effects once the gauge modes reenter the effective horizon. In this perspective the magnetogenesis requirements and the role of the postinflationary conductivity are clarified and reappraised. As long as the total duration of the inflationary phase is nearly minimal, quasi-flat hypermagnetic power spectra are comparatively more common than in the case of vacuum initial data.Comment: 12 pages, 1 figur

Seventy Years of Getting Transistorized

Vacuum tubes appeared at the break of the twentieth century giving birth to electronics. By the 1930s, they had become established as a mature technology, spreading into areas such as radio communications, long distance radiotelegraphy, radio broadcasting, telephone communication and switching, sound recording and playing, television, radar, and air navigation. During World War II, vacuum tubes were used in the first electronic computers, which were built in the United Kingdom and the United States. Although vacuum tubes had been a successful technology, they were also bulky, fragile and expensive, had a short life, and consumed a lot of power to heat the thermo-emitters. These drawbacks promoted the search for completely new devices. Alternative solutions had long been considered, but without significant developments

Hypermagnetic knots and gravitational radiation at intermediate frequencies

The maximally gyrotropic configurations of the hypermagnetic field at the electroweak epoch can induce a stochastic background of relic gravitational waves with comoving frequencies ranging from the $\mu$Hz to the kHz. Using two complementary approaches we construct a physical template family for the emission of the gravitational radiation produced by the hypermagnetic knots. The current constraints and the presumed sensitivities of the advanced wide-band interferometers (both terrestrial and space-borne) are combined to infer that the lack of observations at intermediate frequencies may invalidate the premise of baryogenesis models based (directly or indirectly) on the presence of gyrotropic configurations of the hypermagnetic field at the electroweak epoch. Over the intermediate frequency range the spectral energy density of the gravitational waves emitted by the hypermagnetic knots at the electroweak scale can exceed the inflationary signal even by nine orders of magnitude without affecting the standard bounds applicable on the stochastic backgrounds of gravitational radiation. The signal of hypermagnetic knots can be disambiguated, at least in principle, since the the produced gravitational waves are polarized.Comment: 19 pages, 2 figures; to appear in Classical and Quantum Gravity as a regular articl

The First Galaxies and the Likely Discovery of their Fossils in the Local Group

In cold dark matter cosmologies, small mass halos outnumber larger mass halos at any redshift. However, the lower bound for the mass of a galaxy is unknown, as are the typical luminosity of the smallest galaxies and their numbers in the universe. The answers depend on the extent to which star formation in the first population of small mass halos may be suppressed by radiative feedback loops operating over cosmological distance scales. If early populations of dwarf galaxies did form in significant number, their relics should be found today in the Local Group. These relics have been termed "fossils of the first galaxies". This paper is a review that summarizes our ongoing efforts to simulate and identify these fossils around the Milky Way and Andromeda. It is widely believed that reionization of the intergalactic medium would have stopped star formation in the fossils of the first galaxies. Thus, they should be among the oldest objects in the Universe. However, here we dispute this idea and discuss a physical mechanism whereby relatively recent episodes of gas accretion and star formation would be produced in some fossils of the first galaxies. We argue that fossils may be characterized either by a single old population of stars or by a bimodal star formation history. We also propose that the same mechanism could turn small mass dark halos formed before reionization into gas-rich but starless "dark galaxies". We believe that current observational data support the thesis that a fraction of the new ultra-faint dwarfs recently discovered in the Local Group are in fact fossils of the first galaxies.Comment: Invited review/tutorial paper, 18 pages, 11 figures. Accepted to Advances in Astronomy, special issue on "Dwarf-Galaxy Cosmology

PageRank: Standing on the shoulders of giants

PageRank is a Web page ranking technique that has been a fundamental ingredient in the development and success of the Google search engine. The method is still one of the many signals that Google uses to determine which pages are most important. The main idea behind PageRank is to determine the importance of a Web page in terms of the importance assigned to the pages hyperlinking to it. In fact, this thesis is not new, and has been previously successfully exploited in different contexts. We review the PageRank method and link it to some renowned previous techniques that we have found in the fields of Web information retrieval, bibliometrics, sociometry, and econometrics

Study of θ\theta dependence in Yang-Mills theories on the lattice

We discuss the use of field theoretical techniques in the lattice determination of the free energy dependence on the $\theta$ angle in SU(N) Yang-Mills theories.Comment: 5 pages. Talk at the International Workshop on QCD: QCD@Work 2003 - Conversano (Italy) 14-18 June 2003 (eConf C030614