2,521 research outputs found
Interpretations of the Accelerating Universe
It is generally argued that the present cosmological observations support the
accelerating models of the universe, as driven by the cosmological constant or
`dark energy'. We argue here that an alternative model of the universe is
possible which explains the current observations of the universe. We
demonstrate this with a reinterpretation of the magnitude-redshift relation for
Type Ia supernovae, since this was the test that gave a spurt to the current
trend in favour of the cosmological constant.Comment: 12 pages including 2 figures, minor revision, references added, a
paragraph on the interpretation of the CMB anisotropy in the QSSC added in
conclusion, general results unchanged. To appear in the October 2002 issue of
the "Publications of the Astronmical Society of the Pacific
The cosmological BCS mechanism and the Big Bang Singularity
We provide a novel mechanism that resolves the Big Bang Singularity present
in FRW space-times without the need for ghost fields. Building on the fact that
a four-fermion interaction arises in General Relativity when fermions are
covariantly coupled, we show that at early times the decrease in scale factor
enhances the correlation between pairs of fermions. This enhancement leads to a
BCS-like condensation of the fermions and opens a gap dynamically driving the
Hubble parameter to zero and results in a non-singular bounce, at least in
some special cases.Comment: replaced to match the journal versio
Properties of a future susy universe
In the string landscape picture, the effective potential is characterized by
an enormous number of local minima of which only a minuscule fraction are
suitable for the evolution of life. In this "multiverse", random transitions
are continually made between the various minima with the most likely
transitions being to minima of lower vacuum energy. The inflationary era in the
very early universe ended with such a transition to our current phase which is
described by a broken supersymmetry and a small, positive vacuum energy.
However, it is likely that an exactly supersymmetric (susy) phase of zero
vacuum energy as in the original superstring theory also exists and that, at
some time in the future, there will be a transition to this susy world. In this
article we make some preliminary estimates of the consequences of such a
transition.Comment: 17 pages, 3 figures; intermediate extensions/revisions available at
http://www.bama.ua.edu/~lclavell/Susyria.pd
How Protostellar Outflows Help Massive Stars Form
We consider the effects of an outflow on radiation escaping from the
infalling envelope around a massive protostar. Using numerical radiative
transfer calculations, we show that outflows with properties comparable to
those observed around massive stars lead to significant anisotropy in the
stellar radiation field, which greatly reduces the radiation pressure
experienced by gas in the infalling envelope. This means that radiation
pressure is a much less significant barrier to massive star formation than has
previously been thought.Comment: 4 pages, 2 figures, emulateapj, accepted for publication in ApJ
Letter
Astrophysical implications of hypothetical stable TeV-scale black holes
We analyze macroscopic effects of TeV-scale black holes, such as could
possibly be produced at the LHC, in what is regarded as an extremely
hypothetical scenario in which they are stable and, if trapped inside Earth,
begin to accrete matter. We examine a wide variety of TeV-scale gravity
scenarios, basing the resulting accretion models on first-principles, basic,
and well-tested physical laws. These scenarios fall into two classes, depending
on whether accretion could have any macroscopic effect on the Earth at times
shorter than the Sun's natural lifetime. We argue that cases with such effect
at shorter times than the solar lifetime are ruled out, since in these
scenarios black holes produced by cosmic rays impinging on much denser white
dwarfs and neutron stars would then catalyze their decay on timescales
incompatible with their known lifetimes. We also comment on relevant lifetimes
for astronomical objects that capture primordial black holes. In short, this
study finds no basis for concerns that TeV-scale black holes from the LHC could
pose a risk to Earth on time scales shorter than the Earth's natural lifetime.
Indeed, conservative arguments based on detailed calculations and the
best-available scientific knowledge, including solid astronomical data,
conclude, from multiple perspectives, that there is no risk of any significance
whatsoever from such black holes.Comment: Version2: Minor corrections/fixed typos; updated reference
Modeling Repulsive Gravity with Creation
There is a growing interest in the cosmologists for theories with negative
energy scalar fields and creation, in order to model a repulsive gravity. The
classical steady state cosmology proposed by Bondi, Gold and Hoyle in 1948, was
the first such theory which used a negative kinetic energy creation field to
invoke creation of matter. We emphasize that creation plays very crucial role
in cosmology and provides a natural explanation to the various explosive
phenomena occurring in local (z<0.1) and extra galactic universe. We exemplify
this point of view by considering the resurrected version of this theory - the
quasi-steady state theory, which tries to relate creation events directly to
the large scale dynamics of the universe and supplies more natural explanations
of the observed phenomena. Although the theory predicts a decelerating universe
at the present era, it explains successfully the recent SNe Ia observations
(which require an accelerating universe in the standard cosmology), as we show
in this paper by performing a Bayesian analysis of the data.Comment: The paper uses an old SNeIa dataset. With the new improved data, for
example the updated gold sample (Riess et al, astro-ph/0611572), the fit
improves considerably (\chi^2/DoF=197/180 and a probability of
goodness-of-fit=18%
Cosmic Needles versus Cosmic Microwave Background Radiation
It has been suggested by a number of authors that the 2.7K cosmic microwave
background (CMB) radiation might have arisen from the radiation from Population
III objects thermalized by conducting cosmic graphite/iron needle-shaped dust.
Due to lack of an accurate solution to the absorption properties of exceedingly
elongated grains, in existing literature which studies the CMB thermalizing
process they are generally modelled as (1) needle-like spheroids in terms of
the Rayleigh approximation; (2) infinite cylinders; and (3) the antenna theory.
We show here that the Rayleigh approximation is not valid since the Rayleigh
criterion is not satisfied for highly conducting needles. We also show that the
available intergalactic iron dust, if modelled as infinite cylinders, is not
sufficient to supply the required opacity at long wavelengths to obtain the
observed isotropy and Planckian nature of the CMB. If appealing to the antenna
theory, conducting iron needles with exceedingly large elongations (10^4)
appear able to provide sufficient opacity to thermalize the CMB within the iron
density limit. But the applicability of the antenna theory to exceedingly thin
needles of nanometer/micrometer in thickness needs to be justified.Comment: 13 pages, 4 figures; submitted to ApJ
Vacuum friction in rotating particles
We study the frictional torque acting on particles rotating in empty space.
At zero temperature, vacuum friction transforms mechanical energy into light
emission and produces particle heating. However, particle cooling relative to
the environment occurs at finite temperatures and low rotation velocities.
Radiation emission is boosted and its spectrum significantly departed from a
hot-body emission profile as the velocity increases. Stopping times ranging
from hours to billions of years are predicted for materials, particle sizes,
and temperatures accessible to experiment. Implications for the behavior of
cosmic dust are discussed.Comment: 4 figures, 10 pages, includes paper and supplementary information in
the appendi
Galactic Center Extinction: Evidence for Metallic Needles in the General Interstellar Medium
The extinction curve derived from ISO mid-infrared (IR) observations of the
Galactic center (GC) exhibits a surprisingly flat behavior in the ~ 3 to 8
micron region, contrary to the deep minimum expected from standard interstellar
dust models consisting of bare silicate and graphite dust particles. We show
that this extinction is likely caused by the presence of metallic needles in
the interstellar medium (ISM) towards the Galactic center. If the needles
contribute only to the 3 - 8 micron extinction, they must have a long
wavelength cutoff at ~ 8 microns, and therefore a typical length over radius
ratio of about 600, smaller than the 3000 aspect ratio determined for the
needles in Cas A. Homogeneously distributed throughout the ISM, they comprise
only a minor mass fraction of the ISM, with a needle-to-H mass ratio of
\~5x10^{-6}, which is equivalent to 0.14% of the silicate dust mass. Their
total ISM abundance can then be readily explained by the combined production in
SNe and O-rich stellar outflows. It is currently unclear how universal the GC
extinction law is. Local 2 - 5 micron extinction measurements seem to be
consistent with the standard extinction law, suggesting a non uniform
distribution of needles in the ISM. The GC observations show that metallic
needles, in spite of their low abundance or non uniform distribution, can be
the dominant source of opacity in the 3 - 8 micron wavelength region. However,
expelled into the intergalactic medium, their abundance is too low to cause any
dimming of cosmological sources, and their length is too short to make them a
significant source of submillimeter emission.Comment: 9 pages, 1 figure, to appear in the ApJ Letter
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