462 research outputs found
Phantom Energy and the Cosmic Horizon: Rh is still not a horizon!
There has been a recent spate of papers on the Cosmic Horizon, an apparently
fundamental, although unrecognised, property of the universe. The
misunderstanding of this horizon, it is claimed, demonstrates that our
determination of the cosmological makeup of the universe is incorrect, although
several papers have pointed out key flaws in these arguments. Here, we identify
additional flaws in the most recent claims of the properties of the Cosmic
Horizon in the presence of phantom energy, simply demonstrating that it does
not act as a horizon, and that its limiting of our view of the universe is a
trivial statement.Comment: 4 pages, 1 figure. Accepted for publication in MNRAS Letter
Gravitational microlensing of planets: the influence of planetary phase and caustic orientation
Recent studies have demonstrated that detailed monitoring of gravitational
microlensing events can reveal the presence of planets orbiting the microlensed
source stars. With the potential of probing planets in the Galactic Bulge and
Magellanic Clouds, such detections greatly increase the volume over which
planets can be found. This paper expands on these original studies by
considering the effect of planetary phase on the form of the resultant
microlensing light curve. It is found that crescent-like sources can undergo
substantially more magnification than a uniformly illuminated disk, the model
typically employed in studying such planets. In fact, such a circularly
symmetric model is found to suffer a minimal degree of magnification when
compared to the crescent models. The degree of magnification is also a strong
function of the planet's orientation with respect to the microlensing caustic.
The form of the magnification variability is also strongly dependent on the
planetary phase and from which direction it is swept by the caustic, providing
further clues to the geometry of the planetary system. As the amount of light
reflected from a planet also depends on its phase, the detection of extreme
crescent-like planets requires the advent of 30-m class telescopes, while light
curves of planets at more moderate phases can be determined with today's 10-m
telescopes.Comment: 7 pages, 7 figures, to appear in the MNRA
Producing the Deuteron in Stars: Anthropic Limits on Fundamental Constants
Stellar nucleosynthesis proceeds via the deuteron (D), but only a small
change in the fundamental constants of nature is required to unbind it. Here,
we investigate the effect of altering the binding energy of the deuteron on
proton burning in stars. We find that the most definitive boundary in parameter
space that divides probably life-permitting universes from probably
life-prohibiting ones is between a bound and unbound deuteron. Due to neutrino
losses, a ball of gas will undergo rapid cooling or stabilization by electron
degeneracy pressure before it can form a stable, nuclear reaction-sustaining
star. We also consider a less-bound deuteron, which changes the energetics of
the and reactions. The transition to endothermic and
reactions, and the resulting beta-decay instability of the deuteron, do not
seem to present catastrophic problems for life.Comment: 19 pages, 5 figures. Accepted to JCAP. Revised to match the published
version; corrected to better take into account free neutron
Microlensing of Broad Absorption Line Quasars
The physical nature of the material responsible for the high--velocity,
broad-absorption line features seen in a small fraction of quasar spectra has
been the subject of debate since their discovery. This has been especially
compounded by the lack of observational probes of the absorbing region. In this
paper we examine the role of ``microlenses'' in external galaxies on observed
variability in the profiles of broad absorption lines in multiply-imaged
quasars. Utilizing realistic models for both the broad absorption line region
and the action of an ensemble of microlensing masses, we demonstrate that stars
at cosmological distances can provide an important probe of the physical state
and structure of material at the heart of these complex systems. Applying these
results to the macrolensed BAL quasar system, H1413+117, the observed spectral
variations are readily reproduced, but without the fine-tuning requirements of
earlier studies which employ more simplistic models.Comment: 7 pages, 4 figures (included), Latex (mn.sty), Submitted to MNRA
Was SN1997ff at z~1.7 magnified by gravitational lensing?
The quest for the cosmological parameters has come to fruition with the
identification of a number of supernovae at a redshift of . Analyses of
the brightness of these standard candles reveal that the Universe is dominated
by a large cosmological constant. The recent identification of the
SN1997ff in the northern Hubble Deep Field has provided further evidence for
this cosmology. Here we examine the case for gravitational lensing of SN1997ff
due to the presence of galaxies lying along our line of sight. We find that,
while the alignment of SN1997ff with foreground masses was not favorable for it
to be multiply imaged and strongly magnified, two galaxies did lie close enough
to result in significant magnification: for the case where these
elliptical galaxies have velocity dispersion . Given the small
difference between supernova brightnesses in different cosmologies, detailed
modeling of the gravitational lensing properties of the intervening matter is
therefore required before the true cosmological significance of SN1997ff can be
deduced.Comment: 3 pages, 2 figures. Accepted for publication in MNRAS. Missing
reference adde
Matter Matters: Unphysical Properties of the Rh = ct Universe
It is generally agreed that there is matter in the universe and, in this
paper, we show that the existence of matter is extremely problematic for the
proposed Rh = ct universe. Considering a dark energy component with an equation
of state of w=-1/3, it is shown that the presence of matter destroys the strict
expansion properties that define the evolution of Rh = ct cosmologies,
distorting the observational properties that are touted as its success. We
further examine whether an evolving dark energy component can save this form of
cosmological expansion in the presence of matter by resulting in an expansion
consistent with a mean value of = -1/3, finding that the presence of mass
requires unphysical forms of the dark energy component in the early universe.
We conclude that matter in the universe significantly limits the fundamental
properties of the Rh = ct cosmology, and that novel, and unphysical, evolution
of the matter component would be required to save it. Given this, Rh = ct
cosmology is not simpler or more accurate description of the universe than
prevailing cosmological models, and its presentation to date possesses
significant flaws.Comment: 7 pages, 5 figures, to appear in MNRA
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