507 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
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
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.
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
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