12,084 research outputs found
Cosmic acceleration: Inhomogeneity versus vacuum energy
In this essay, I present an alternative explanation for the cosmic
acceleration which appears as a consequence of recent high redshift Supernova
data. In the usual interpretation, this cosmic acceleration is explained by the
presence of a positive cosmological constant or vacuum energy, in the
background of Friedmann models. Instead, I will consider a Local Rotational
Symmetric (LRS) inhomogeneous spacetime, with a barotropic equation of state
for the cosmic matter. Within this framework the kinematical acceleration of
the cosmic fluid or, equivalently, the inhomogeneity of matter, is just the
responsible of the SNe Ia measured cosmic acceleration. Although in our model
the Cosmological Principle is relaxed, it maintains local isotropy about our
worldline in agreement with the CBR experiments.Comment: LATEX, 7 pags, no figs, Honorable Mention in the 1999 Essay
Competition of the Gravity Research Foundatio
Stars and brown dwarfs in the sigma Orionis cluster. III. OSIRIS/GTC low-resolution spectroscopy of variable sources
Context. Although many studies have been performed so far, there are still
dozens of low-mass stars and brown dwarfs in the young sigma Orionis open
cluster without detailed spectroscopic characterisation. Aims. We look for
unknown strong accretors and disc hosts that were undetected in previous
surveys. Methods. We collected low-resolution spectroscopy (R ~ 700) of ten
low-mass stars and brown dwarfs in sigma Orionis with OSIRIS at the Gran
Telescopio Canarias under very poor weather conditions. These objects display
variability in the optical, infrared, Halpha, and/or X-rays on time scales of
hours to years. We complemented our spectra with optical and near-/mid-infrared
photometry. Results. For seven targets, we detected lithium in absorption,
identified Halpha, the calcium doublet, and forbidden lines in emission, and/or
determined spectral types for the first time. We characterise in detail a
faint, T Tauri-like brown dwarf with an 18 h-period variability in the optical
and a large Halpha equivalent width of -125+/-15 AA, as well as two M1-type,
X-ray-flaring, low-mass stars, one with a warm disc and forbidden emission
lines, the other with a previously unknown cold disc with a large inner hole.
Conclusions. New unrevealed strong accretors and disc hosts, even below the
substellar limit, await discovery among the list of known sigma Orionis stars
and brown dwarfs that are variable in the optical and have no detailed
spectroscopic characterisation yet.Comment: A&A, in press (accepted for publication in section 14. Catalogs and
data of Astronomy and Astrophysics
Curvature(s) of a light wavefront in a weak gravitational field
The geometry of a light wavefront evolving from a flat wavefront under the
action of weak gravity field in the 3-space associated to a post-Newtonian
relativistic spacetime, is studied numerically by means of the ray tracing
method.Comment: 3 pages, 1 fig, Talk given by JFPS at the 12th Marcel Grossmann
conference (Paris, July, 2009), submitted to the Proceeding
Geometry of an accelerated rotating disk
We analyze the geometry of a rotating disk with a tangential acceleration in
the framework of the Special Theory of Relativity, using the kinematic linear
differential system that verifies the relative position vector of time-like
curves in a Fermi reference. A numerical integration of these equations for a
generic initial value problem is made up and the results are compared with
those obtained in other works.Comment: 10 pages, LaTeX, 2 eps figs; typos corrected, added reference, minor
changes; submitte
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