3,240 research outputs found
A Century of Cosmology
In the century since Einstein's anno mirabilis of 1905, our concept of the
Universe has expanded from Kapteyn's flattened disk of stars only 10 kpc across
to an observed horizon about 30 Gpc across that is only a tiny fraction of an
immensely large inflated bubble. The expansion of our knowledge about the
Universe, both in the types of data and the sheer quantity of data, has been
just as dramatic. This talk will summarize this century of progress and our
current understanding of the cosmos.Comment: Talk presented at the "Relativistic Astrophysics and Cosmology -
Einstein's Legacy" meeting in Munich, Nov 2005. Proceedings will be published
in the Springer-Verlag "ESO Astrophysics Symposia" series. 10 pages Latex
with 2 figure
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%
QCD Signatures of Narrow Graviton Resonances in Hadron Colliders
We show that the characteristic p_\perp spectrum yields valuable information
for the test of models for the production of narrow graviton resonances in the
TeV range at LHC. Furthermore, it is demonstrated that in those scenarios the
parton showering formalism agrees with the prediction of NLO matrix element
calculations.Comment: 6 pages, 9 figures, LaTe
Active Galactic Nuclei in Void Regions
We present a comprehensive study of accretion activity in the most underdense
environments in the universe, the voids, based on the SDSS DR2 data. Based on
investigations of multiple void regions, we show that AGN's occurrence rate and
properties differ from those in walls. AGN are more common in voids than in
walls, but only among moderately luminous and massive galaxies (M_r < -20, log
M_*/M_sun < 10.5), and this enhancement is more pronounced for the weakly
accreting systems (i.e., L_[O III] < 10^39 erg/s). Void AGN hosted by
moderately massive and luminous galaxies are accreting at equal or lower rates
than their wall counterparts, show less obscuration than in walls, and
similarly aged stellar populations. The very few void AGN in massive bright
hosts accrete more strongly, are more obscured, and are associated with younger
stellar emission than wall AGN. Thus, accretion strength is probably connected
to the availability of fuel supply, and accretion and star-formation co-evolve
and rely on the same source of fuel. Nearest neighbor statistics indicate that
the weak accretion activity (LINER-like) is not influenced by the local
environment. However, H IIs, Seyferts, and Transition objects prefer more
grouped small scale structures, indicating that the rate at which galaxies
interact with each other affects their activity. These trends support a
potential H II -> Seyfert/Transition Object -> LINER evolutionary sequence that
we show is apparent in many properties of actively line-emitting galaxies, in
both voids and walls. The subtle differences between void and wall AGN might be
explained by a longer, less disturbed duty cycle of these systems in voids.Comment: 19 pages, 7 figures (1 color); to appear in ApJ, submitted on May 11,
200
On the stability of self-gravitating accreting flows
Analytic methods show stability of the stationary accretion of test fluids
but they are inconclusive in the case of self-gravitating stationary flows. We
investigate numerically stability of those stationary flows onto compact
objects that are transonic and rich in gas. In all studied examples solutions
appear stable. Numerical investigation suggests also that the analogy between
sonic and event horizons holds for small perturbations of compact support but
fails in the case of finite perturbations.Comment: 10 pages, accepted for publication in PR
Come to Daddy? Claiming Chris Cunningham for British Art Cinema
Twenty years after he came to prominence via a series of provocative, ground-breaking music videos, Chris Cunningham remains a troubling, elusive figure within British visual culture. His output – which includes short films, advertisements, art gallery commissions, installations, music production and a touring multi-screen live performance – is relatively slim, and his seemingly slow work rate (and tendency to leave projects uncompleted or unreleased) has been a frustration for fans and commentators, particularly those who hoped he would channel his interests and talents into a full-length ‘feature’ film project. There has been a diverse critical response to his musical sensitivity, his associations with UK electronica culture – and the Warp label in particular – his working relationship with Aphex Twin, his importance within the history of the pop video and his deployment of transgressive, suggestive imagery involving mutated, traumatised or robotic bodies. However, this article makes a claim for placing Cunningham within discourses of British art cinema. It proposes that the many contradictions that define and animate Cunningham's work – narrative versus abstraction, political engagement versus surrealism, sincerity versus provocation, commerce versus experimentation, art versus craft, a ‘British’ sensibility versus a transnational one – are also those that typify a particular terrain of British film culture that falls awkwardly between populism and experimentalism
Let's talk about varying G
It is possible that fundamental constants may not be constant at all. There
is a generally accepted view that one can only talk about variations of
dimensionless quantities, such as the fine structure constant . However, constraints on the strength of
gravity tend to focus on G itself, which is problematic. We stress that G needs
to be multiplied by the square of a mass, and hence, for example, one should be
constraining , where is
the proton mass. Failure to focus on such dimensionless quantities makes it
difficult to interpret the physical dependence of constraints on the variation
of G in many published studies. A thought experiment involving talking to
observers in another universe about the values of physical constants may be
useful for distinguishing what is genuinely measurable from what is merely part
of our particular system of units.Comment: 6 pages, Gravity Research Foundation essa
Planary Symmetric Static Worlds with Massless Scalar Sources
Motivated by the recent wave of investigations on plane domain wall
spacetimes with nontrivial topologies, the present paper deals with (probably)
the most simple source field configuration which can generate a spatially
planary symmetric static spacetime, namely a minimally coupled massless scalar
field that depends only upon a spacelike coordinate, . It is shown that the
corresponding exact solutions are
algebraically special, type , and represent globally
pathologic spacetimes with a - group of motion acting on orbits. In spite of the model simplicity, these
- generated worlds possess naked timelike singularities (reached within
a finite universal time by normal non-spacelike geodesics), are completely free
of Cauchy surfaces and contain into the - leveled sections points which can
not be jointed by - trajectories images of oblique non-spacelike
geodesics. Finally, we comment on the possibility of deriving from two other physically interesting ^^ ^^ -
generated'' spacetimes, by appropiate jonction conditions in the -
plane.Comment: 14 pages, LaTeX format, figures not include
Brane World Dynamics and Adiabatic Matter creation
We have treated the adiabatic matter creation process in various three-brane
models by applying thermodynamics of open systems. The matter creation rate is
found to affect the evolution of scale factor and energy density of the
universe. We find modification at early stages of cosmic dynamics. In GB and RS
brane worlds, by chosing appropriate parameters we obtain standard scenario,
while the warped DGP model has different Friedmann equations. During later
stages, since the matter creation is negligible the evolution reduces to FRW
expansion, in RS and GB models.Comment: Replaced with TEX file,No figures, Corrected References and typos.
Accepted in IJMP
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