268 research outputs found
Characteristically American: Memorial Architecture, National Identity, and the Egyptian Revival
Civil War Americans Commemorated in Egyptian Architectural Style
Joy M. Giguere’s book is a welcome addition to the history of American architecture, particularly to the revival styles that were so popular in antebellum American. The last authoritative scholarly work on the Egyptian Reviv...
Lost Plantations of the South
Studying the Old Plantations Marc Matrana’s book brings together, in one handy volume, short histories of many well-known and lesser-known antebellum plantations that, as the title suggests, have been lost due to natural disasters including fire and flood, but most often to “demolition ...
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Cabin, Quarter, Plantation: Architecture and Landscapes of North American Slavery
Dark matter and dark energy proposals: maintaining cosmology as a true science?
I consider the relation of explanations for the observed data to testability
in the following contexts: observational and experimental detection of dark
matter; observational and experimental detection of dark energy or a
cosmological constant ; observational or experimental testing of the
multiverse proposal to explain a small non-zero value of ; and
observational testing of the possibility of large scale spatial inhomogeneity
with zero .Comment: 14 pages. Paper for CRAL-IPNL conference "Dark Energy and Dark
Matter", Lyon 200
An exact quantification of backreaction in relativistic cosmology
An important open question in cosmology is the degree to which the
Friedmann-Lemaitre-Robertson-Walker (FLRW) solutions of Einstein's equations
are able to model the large-scale behaviour of the locally inhomogeneous
observable universe. We investigate this problem by considering a range of
exact n-body solutions of Einstein's constraint equations. These solutions
contain discrete masses, and so allow arbitrarily large density contrasts to be
modelled. We restrict our study to regularly arranged distributions of masses
in topological 3-spheres. This has the benefit of allowing straightforward
comparisons to be made with FLRW solutions, as both spacetimes admit a discrete
group of symmetries. It also provides a time-symmetric hypersurface at the
moment of maximum expansion that allows the constraint equations to be solved
exactly. We find that when all the mass in the universe is condensed into a
small number of objects (<10) then the amount of backreaction in dust models
can be large, with O(1) deviations from the predictions of the corresponding
FLRW solutions. When the number of masses is large (>100), however, then our
measures of backreaction become small (<1%). This result does not rely on any
averaging procedures, which are notoriously hard to define uniquely in general
relativity, and so provides (to the best of our knowledge) the first exact and
unambiguous demonstration of backreaction in general relativistic cosmological
modelling. Discrete models such as these can therefore be used as laboratories
to test ideas about backreaction that could be applied in more complicated and
realistic settings.Comment: 13 pages, 9 figures. Corrections made to Tables IV and
Cosmology Without Averaging
We construct cosmological models consisting of large numbers of identical,
regularly spaced masses. These models do not rely on any averaging procedures,
or on the existence of a global Friedmann-Robertson-Walker (FRW) background.
They are solutions of Einstein's equations up to higher order corrections in a
perturbative expansion, and have large-scale dynamics that are well modelled by
the Friedmann equation. We find that the existence of arbitrarily large density
contrasts does not change either the magnitude or scale of the background
expansion, at least when masses are regularly arranged, and up to the
prescribed level of accuracy. We also find that while the local space-time
geometry inside each cell can be described as linearly perturbed FRW, one could
argue that a more natural description is that of perturbed Minkowski space (in
which case the scalar perturbations are simply Newtonian potentials). We expect
these models to be of use for understanding and testing ideas about averaging
in cosmology, as well as clarifying the relationship between global
cosmological dynamics and the static space-times associated with isolated
masses.Comment: 24 pages, 3 figures. Corrected and expande
Local and non-local measures of acceleration in cosmology
Current cosmological observations, when interpreted within the framework of a
homogeneous and isotropic Friedmann-Lemaitre-Robertson-Walker (FLRW) model,
strongly suggest that the Universe is entering a period of accelerating
expansion. This is often taken to mean that the expansion of space itself is
accelerating. In a general spacetime, however, this is not necessarily true. We
attempt to clarify this point by considering a handful of local and non-local
measures of acceleration in a variety of inhomogeneous cosmological models.
Each of the chosen measures corresponds to a theoretical or observational
procedure that has previously been used to study acceleration in cosmology, and
all measures reduce to the same quantity in the limit of exact spatial
homogeneity and isotropy. In statistically homogeneous and isotropic
spacetimes, we find that the acceleration inferred from observations of the
distance-redshift relation is closely related to the acceleration of the
spatially averaged universe, but does not necessarily bear any resemblance to
the average of the local acceleration of spacetime itself. For inhomogeneous
spacetimes that do not display statistical homogeneity and isotropy, however,
we find little correlation between acceleration inferred from observations and
the acceleration of the averaged spacetime. This shows that observations made
in an inhomogeneous universe can imply acceleration without the existence of
dark energy.Comment: 19 pages, 10 figures. Several references added or amended, some minor
clarifications made in the tex
Shear dynamics in Bianchi I cosmologies with R^n-gravity
We give the equations governing the shear evolution in Bianchi spacetimes for
general f(R)-theories of gravity. We consider the case of R^n-gravity and
perform a detailed analysis of the dynamics in Bianchi I cosmologies which
exhibit local rotational symmetry. We find exact solutions and study their
behaviour and stability in terms of the values of the parameter n. In
particular, we found a set of cosmic histories in which the universe is
initially isotropic, then develops shear anisotropies which approaches a
constant value.Comment: 25 pages LaTeX, 6 figures. Revised to match the final version
accepted for publication in CQ
The magnetic part of the Weyl tensor, and the expansion of discrete universes
42 pages, 27 figures42 pages, 27 figures42 pages, 27 figures42 pages, 27 figures42 pages, 27 figures42 pages, 27 figuresTC is supported by the STFC and DG by an AARMS postdoctoral fellowship
Optimal Measurements of Magnetic Flux in Superconducting Circuits and Macroscopic Quantum Mechanics
A model of repeated quantum measurements of magnetic flux in superconducting
circuits manifesting tunneling is discussed. The perturbation due to the
previous measurements of magnetic flux is always present unless quantum
nondemolition measurements are performed. By replacing the classical notion of
noninvasivity with this condition, temporal Bell-like inequalities allows one
to test the observability at the macroscopic level of the conflict between
realism and quantum theory.Comment: 11 pages (no figures), to be published in Physics Letters
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