417 research outputs found
Through the Looking Glass: Why the "Cosmic Horizon" is not a horizon
The present standard model of cosmology, CDM, contains some
intriguing coincidences. Not only are the dominant contributions to the energy
density approximately of the same order at the present epoch, but we note that
contrary to the emergence of cosmic acceleration as a recent phenomenon, the
time averaged value of the deceleration parameter over the age of the universe
is nearly zero. Curious features like these in CDM give rise to a
number of alternate cosmologies being proposed to remove them, including models
with an equation of state w = -1/3. In this paper, we examine the validity of
some of these alternate models and we also address some persistent
misconceptions about the Hubble sphere and the event horizon that lead to
erroneous conclusions about cosmology.Comment: Accepted for publication by MNRAS, 6 pages, 3 figure
No Way Back: Maximizing survival time below the Schwarzschild event horizon
It has long been known that once you cross the event horizon of a black hole,
your destiny lies at the central singularity, irrespective of what you do.
Furthermore, your demise will occur in a finite amount of proper time. In this
paper, the use of rockets in extending the amount of time before the collision
with the central singularity is examined. In general, the use of such rockets
can increase your remaining time, but only up to a maximum value; this is at
odds with the ``more you struggle, the less time you have'' statement that is
sometimes discussed in relation to black holes. The derived equations are
simple to solve numerically and the framework can be employed as a teaching
tool for general relativity.Comment: 7-pages, 5 figures, accepted for publication in the Publications of
the Astronomical Society of Australia (Journal name corrected.
Another coincidence problem for CDM?
Over the last nine years of cosmic microwave background observations, the
Wilkinson Microwave Anisotropy Probe () results were consistent with a
CDM cosmological model in which the age of the Universe is one Hubble
time, and the time-averaged value of the deceleration parameter is consistent
with zero. This curious observation has been put forward as a new coincidence
problem for the CDM concordance cosmology, which is in fact a
`greater' coincidence than the near equality of the density parameters of
matter and the cosmological constant. At the moment of writing these conference
proceedings, the Planck Collaboration has released its first cosmological data,
which revealed a small shift in the CDM cosmological parameters when
compared to . We show that under the assumption of a spatially flat
CDM cosmology, Planck's results remove this coincidence problem for
CDM at greater than 99\% confidence level.Comment: 3 pages, 1 figure, proceedings of the 13th Marcel Grossmann meetin
Chemical aspects related to using recycled geopolymers as aggregates
This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits unrestricted use, distribution, and reproduction in any medium, provided the original work is properly cited.Despite extensive research into sustainability of geopolymers, end-of-life aspects have been largely overlooked. A recycling scenario is examined in this study. This requires an investigation of alkali leaching potential from a geopolymeric matrix. To study the feasibility of geopolymer cement (GPC) recycling, the migration of alkalis was evaluated for the first time on a microstructural level through energy dispersive X-ray (EDX) scanning electron microscopy (SEM) elemental mapping and leaching tests. Macroscale impacts were assessed through an investigation of Portland cement (PC) mortar properties affected by alkali concentration. Leaching tests indicated that alkalis immediately become available in aqueous environments, but the majority remain chemically or physically bound in the matrix. This type of leaching accelerates the initial setting of PC paste. Elemental mapping and EDX/SEM analysis showed a complex paste-aggregate interfacial transition zone. Exchange of calcium and sodium, revealed by the maps, resulted in the migration of sodium into the PC paste and the formation of additional calcium-silicon-based phases in the geopolymeric matrix. Strength values of mortars with 25% and 50% recycled aggregates (RA) showed negligible differences compared with the reference sample. Screening tests indicated a low potential for GPC RA inducing alkali-silica reaction. Transport of GPC RA alkalis and the underlying mechanisms were observed. This transport phenomenon was found to have minor effects on the properties of the PC mortar, indicating that recycling of geopolymers is a viable reuse practice.Peer reviewedFinal Published versio
Cosmological Radar Ranging in an Expanding Universe
While modern cosmology, founded in the language of general relativity, is
almost a century old, the meaning of the expansion of space is still being
debated. In this paper, the question of radar ranging in an expanding universe
is examined, focusing upon light travel times during the ranging; it has
recently been claimed that this proves that space physically expands. We
generalize the problem into considering the return journey of an accelerating
rocketeer, showing that while this agrees with expectations of special
relativity for an empty universe, distinct differences occur when the universe
contains matter. We conclude that this does not require the expansion of space
to be a physical phenomenon, rather that we cannot neglect the influence of
matter, seen through the laws of general relativity, when considering motions
on cosmic scales.Comment: 6 Pages. To appear in MNRA
Fractal Bubble Cosmology: A concordant cosmological model?
The Fractal Bubble model has been proposed as a viable cosmology that does
not require dark energy to account for cosmic acceleration, but rather
attributes its observational signature to the formation of structure. In this
paper it is demonstrated that, in contrast to previous findings, this model is
not a good fit to cosmological supernovae data; there is significant tension in
the best fit parameters obtained from different samples, whereas LCDM is able
to fit all datasets consistently. Furthermore, the concordance between galaxy
clustering scales and data from the cosmic microwave background is not achieved
with the most recent supernova compilations. The validity of the FB formalism
as a sound cosmological model is further challenged as it is shown that
previous studies of this model achieve concordance by requiring a value for the
present day Hubble constant that is derived from supernovae data containing an
arbitrary distance normalisation.Comment: 6 pages, 3 figures, revised version published in MNRAS letter
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