9,072 research outputs found
Adiabatic regularization of the graviton stress-energy tensor in de Sitter space-time
We study the renormalized energy-momentum tensor of gravitons in a de Sitter
space-time. After canonically quantizing only the physical degrees of freedom,
we adopt the standard adiabatic subtraction used for massless minimally coupled
scalar fields as a regularization procedure and find that the energy density of
gravitons in the E(3) invariant vacuum is proportional to H^4, where H is the
Hubble parameter, but with a positive sign. According to this result the scalar
expansion rate, which is gauge invariant in de Sitter space-time, is increased
by the fluctuations. This implies that gravitons may then add to conformally
coupled matter in driving the Starobinsky model of inflation.Comment: 5 pages, revtex, final version accepted for publication in PR
Ultracold collisions of metastable helium atoms
We report scattering lengths for the singlet Sigma g +, triplet Sigma u + and
quintet Sigma g + adiabatic molecular potentials relevant to collisions of two
metastable (n=2 triplet S) helium atoms as a function of the uncertainty in
these potentials. These scattering lengths are used to calculate experimentally
observable scattering lengths, elastic cross sections and inelastic rates for
any combination of states of the colliding atoms, at temperatures where the
Wigner threshold approximation is valid.Comment: 20 pages, 8 figures, RevTeX, epsf. Small additions of tex
Stochastic growth of quantum fluctuations during slow-roll inflation
We compute the growth of the mean square of quantum fluctuations of test
fields with small effective mass during a slowly changing, nearly de Sitter
stage which took place in different inflationary models. We consider a
minimally coupled scalar with a small mass, a modulus with an effective mass (with as the Hubble parameter) and a massless non-minimally
coupled scalar in the test field approximation and compare the growth of their
relative mean square with the one of gauge-invariant inflaton fluctuations. We
find that in most of the single field inflationary models the mean square gauge
invariant inflaton fluctuation grows {\em faster} than any test field with a
non-negative effective mass. Hybrid inflationary models can be an exception:
the mean square of a test field can dominate over the gauge invariant inflaton
fluctuation one on suitably choosing parameters. We also compute the stochastic
growth of quantum fluctuation of a second field, relaxing the assumption of its
zero homogeneous value, in a generic inflationary model; as a main result, we
obtain that the equation of motion of a gauge invariant variable associated,
order by order, with a generic quantum scalar fluctuation during inflation can
be obtained only if we use the number of e-folds as the time variable in the
corresponding Langevin and Fokker-Planck equations for the stochastic approach.
We employ this approach to derive some bounds in the case of a model with two
massive fields.Comment: 9 pages, 4 figures. Added references, minor changes, matches the
version to be published in Phys. Rev.
Creation of a molecular condensate by dynamically melting a Mott-insulator
We propose creation of a molecular Bose-Einstein condensate (BEC) by loading
an atomic BEC into an optical lattice and driving it into a Mott insulator (MI)
with exactly two atoms per site. Molecules in a MI state are then created under
well defined conditions by photoassociation with essentially unit efficiency.
Finally, the MI is melted and a superfluid state of the molecules is created.
We study the dynamics of this process and photoassociation of tightly trapped
atoms.Comment: minor revisions, 5 pages, 3 figures, REVTEX4, accepted by PRL for
publicatio
High sensitivity low frequency radio observations of cD galaxies
We present the GMRT 235 MHz images of three radio galaxies and 610 MHz images
of two sources belonging to a complete sample of cD galaxies in rich and poor
galaxy clusters. The analysis of the spectral properties confirms the presence
of aged radio emission in two of the presented sources.Comment: 3 pages, 2 figures, To appear in the Proceedings of "Heating vs.
Cooling in Galaxies and Clusters of Galaxies", August 2006, Garching
(Germany
Radio halos in merging clusters of galaxies
We present the preliminary results of 235 MHz, 327 MHz and 610 MHz
observations of the galaxy cluster A3562 in the core of the Shapley
Concentration. The purpose of these observations, carried out with the Giant
Metrewave Radio Telescope (GMRT, Pune, India) was to study the radio halo
located at the centre of A3562 and determine the shape of its radio spectrum at
low frequencies, in order to understand the origin of this source. In the
framework of the re--acceleration model, the preliminary analysis of the halo
spectrum suggests that we are observing a young source (few yrs) at the
beginning of the re--acceleration phase.Comment: 3 pages, 2 figures. Proceedings of IAU Colloquium 195 - Outskirts of
Galaxy Clusters: intense life in the suburb
IGR J17488-2338: a newly discovered giant radio galaxy
We present the discovery of a large scale radio structure associated with IGR
J17488--2338, a source recently discovered by \emph{INTEGRAL} and optically
identified as a broad line AGN at redshift 0.24. At low frequencies, the source
properties are those of an intermediate-power FR II radio galaxy with a linear
size of 1.4\,Mpc. This new active galaxy is therefore a member of a class of
objects called Giant Radio Galaxies (GRGs), a rare type of radio galaxies with
physical sizes larger than 0.7\,Mpc; they represent the largest and most
energetic single entities in the Universe and are useful laboratories for many
astrophysical studies. Their large scale structures could be due either to
special external conditions or to uncommon internal properties of the source
central engine The AGN at the centre of IGR J17488--2338 has a black hole of
1.310 solar masses, a bolometric luminosity of
710erg\,s and an Eddington ratio of 0.3, suggesting that
it is powerful enough to produce the large structure observed in radio. The
source is remarkable also for other properties, among which its X-ray
absorption, at odds with its type 1 classification, and the presence of a
strong iron line which is a feature not often observed in radio galaxies.Comment: 4 pages, 3 figures, accepted for publication on Astronomy and
Astrophysic
Testing the radio halo-cluster merger scenario. The case of RXCJ2003.5-2323
We present a combined radio, X-ray and optical study of the galaxy cluster
RXCJ2003.5-2323. The cluster hosts one of the largest, most powerful and
distant giant radio halos known to date, suggesting that it may be undergoing a
strong merger process. The aim of our multiwavelength study is to investigate
the radio-halo cluster merger scenario. We studied the radio properties of the
giant radio halo in RXCJ2003.5-2323 by means of new radio data obtained at 1.4
GHz with the Very Large Array, and at 240 MHz with the Giant Metrewave Radio
Telescope, in combination with previously published GMRT data at 610 MHz. The
dynamical state of the cluster was investigated by means of X-ray Chandra
observations and optical ESO--NTT observations. Our study confirms that
RXCJ2003.5-2323 is an unrelaxed cluster. The unusual filamentary and clumpy
morphology of the radio halo could be due to a combination of the filamentary
structure of the magnetic field and turbulence in the inital stage of a cluster
merger.Comment: 10 page, 10 figures, accepted for publication on A&
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