2,310 research outputs found
Naked Singularity in a Modified Gravity Theory
The cosmological constant induced by quantum fluctuation of the graviton on a
given background is considered as a tool for building a spectrum of different
geometries. In particular, we apply the method to the Schwarzschild background
with positive and negative mass parameter. In this way, we put on the same
level of comparison the related naked singularity (-M) and the positive mass
wormhole. We discuss how to extract information in the context of a f(R)
theory. We use the Wheeler-De Witt equation as a basic equation to perform such
an analysis regarded as a Sturm-Liouville problem . The application of the same
procedure used for the ordinary theory, namely f(R)=R, reveals that to this
approximation level, it is not possible to classify the Schwarzschild and its
naked partner into a geometry spectrum.Comment: 8 Pages. Contribution given to DICE 2008. To appear in the
proceeding
Simple Model of Propagating Flame Pulsations
A simple model which exhibits dynamical flame properties in 1D is presented.
It is investigated analytically and numerically. The results are applicable to
problems of flame propagation in supernovae Ia.Comment: 10 pages, 8 figures, revised version accepted by MNRA
Zeldovich flow on cosmic vacuum background: new exact nonlinear analytical solution
A new exact nonlinear Newtonian solution for a plane matter flow superimposed
on the isotropic Hubble expansion is reported. The dynamical effect of cosmic
vacuum is taken into account. The solution describes the evolution of nonlinear
perturbations via gravitational instability of matter and the termination of
the perturbation growth by anti-gravity of vacuum at the epoch of transition
from matter domination to vacuum domination. On this basis, an `approximate' 3D
solution is suggested as an analog of the Zeldovich ansatz.Comment: 9 pages, 1 figure
Dark Viscous Fluid coupled with Dark Matter and future singularity
We study effects of viscous fluid coupled with dark matter in our universe.
We consider bulk viscosity in the cosmic fluid and we suppose the existence of
a coupling between fluid and dark matter, in order to reproduce a stable de
Sitter universe protected against future-time singularities. More general
inhomogeneous fluids are studied related to future singularities.Comment: 11 page
Cepheid calibration of type Ia Supernovae and the Hubble constant
We investigate how a different calibration of the Cepheid Period-Luminosity
(PL) relation taking into account the metallicity corrections, affects the
absolute magnitude calibration of Supernovae (SNe) Ia and, in turn, the
determination of the Hubble constant H_0. We exploit SN Ia light curves from
literature and previously unpublished data, to build the M_B -- Delta m_{15}(B)
relation and we calibrate the zero point by means of 9 type Ia SNe with Cepheid
measured distances. This relation was then used to build the Hubble diagram and
in turn to derive H_0. In the attempt to correct for the host galaxy
extinction, we found that the data seems to suggest a value for the total to
selective absorption ratio, R_B=3.5, which is smaller than the standard value
for our own Galaxy R_B=4.315. Depending on different metallicity corrections
for the Cepheids P-L relation, values of R_B and SN sample selection criteria,
we found that the values of the Hubble constant H_0 is in the range 68--74 km
s^{-1} Mpc^{-1}, with associated uncertainties of the order of 10%. Unpublished
photometry is also presented for 18 SNe of our sample (1991S, 1991T, 1992A,
1992K, 1993H, 1993L, 1994D, 1994M, 1994ae, 1995D, 1995ac, 1995bd, 1996bo,
1997bp, 1997br, 1999aa, 1999dk, 2000cx), which are the results of a long
standing effort for supernova monitoring at ESO - La Silla and Asiago
Observatories.Comment: 19 pages, 9 figures, accepted for publication in MNRA
Can MONDian vector theories explain the cosmic speed up ?
Generalized Einstein - Aether vector field models have been shown to provide,
in the weak field regime, modifications to gravity which can be reconciled with
the successfull MOND proposal. Very little is known, however, on the function
F(K) defining the vector field Lagrangian so that an analysis of the viability
of such theories at the cosmological scales has never been performed. As a
first step along this route, we rely on the relation between F(K) and the MOND
interpolating function to assign the vector field Lagrangian thus
obtaining what we refer to as "MONDian vector models". Since they are able by
construction to recover the MOND successes on galaxy scales, we investigate
whether they can also drive the observed accelerated expansion by fitting the
models to the Type Ia Supernovae data. Should be this the case, we have a
unified framework where both dark energy and dark matter can be seen as
different manifestations of a single vector field. It turns out that both
MONDian vector models are able to well fit the low redshift data on Type Ia
Supernovae, while some tension could be present in the high z regime.Comment: 15 pages, 5 tables, 4 figures, accepted for publication on Physical
Review
Cosmological Models with Variable Gravitational and Cosmological constants in Gravity
We consider the evolution of a flat Friedmann-Roberstson-Walker Universe in a
higher derivative theories, including terms to the
Einstein-Hilbert action in the presence of a variable gravitational and
cosmological constants. We study here the evolution of the gravitational and
cosmological constants in the presence of radiation and matter domination era
of the universe. We present here new cosmological solutions which are
physically interesting for model building.Comment: 14 pages, no figure. to be published in Int. J. Mod. Phys.
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