6,640 research outputs found
The graceful exit in pre-big bang string cosmology
We re-examine the graceful exit problem in the pre-big bang scenario of
string cosmology, by considering the most general time-dependent classical
correction to the Lagrangian with up to four derivatives. By including possible
forms for quantum loop corrections we examine the allowed region of parameter
space for the coupling constants which enable our solutions to link smoothly
the two asymptotic low-energy branches of the pre-big bang scenario, and
observe that these solutions can satisfy recently proposed entropic bounds on
viable singularity free cosmologies.Comment: 14 pages, 6 figures, JHEP class. Added new section on the classical
correction and reference
Cosmological perturbations and the transition from contraction to expansion
We investigate both analytically and numerically the evolution of scalar
perturbations generated in models which exhibit a smooth transition from a
contracting to an expanding Friedmann universe. We find that the resulting
spectral index in the late radiation dominated universe depends on which of the
or \ variables passes regularly through the transition. The
results can be parameterized through the exponent defining the rate of
contraction of the universe. For we find that there are no stable
cases where both variables are regular during the transition. In particular,
for , we find that the resulting spectral index is close to scale
invariant if is regular, whereas it has a steep blue behavior if
is regular. We also show that as long as , perturbations arising
from the Bardeen potential remain small during contraction in the sense that
there exists a gauge in which all the metric and matter perturbation variables
are small.Comment: 30 pages, 16 figures. Version to appear in Phys. Rev. D. Slight
modifications, but no change in the conclusio
The SERI solar energy storage program
In support of the DOE thermal and chemical energy storage program, the solar energy storage program (SERI) provides research on advanced technologies, systems analyses, and assessments of thermal energy storage for solar applications in support of the Thermal and Chemical Energy Storage Program of the DOE Division of Energy Storage Systems. Currently, research is in progress on direct contact latent heat storage and thermochemical energy storage and transport. Systems analyses are being performed of thermal energy storage for solar thermal applications, and surveys and assessments are being prepared of thermal energy storage in solar applications. A ranking methodology for comparing thermal storage systems (performance and cost) is presented. Research in latent heat storage and thermochemical storage and transport is reported
Computer simulation of radiation damage in gallium arsenide
A version of the binary-collision simulation code MARLOWE was used to study the spatial characteristics of radiation damage in proton and electron irradiated gallium arsenide. Comparisons made with the experimental results proved to be encouraging
The Isometries of Low-Energy Heterotic M-Theory
We study the effective D=4, N=1 supergravity description of five-dimensional
heterotic M-theory in the presence of an M5 brane, and derive the Killing
vectors and isometry group for the Kahler moduli-space metric. The group is
found to be a non-semisimple maximal parabolic subgroup of Sp(4,R), containing
a non-trivial SL(2,R) factor. The underlying moduli-space is then naturally
realised as the group space Sp(4,R)/U(2), but equipped with a nonhomogeneous
metric that is invariant only under that maximal parabolic group. This
nonhomogeneous metric space can also be derived via field truncations and
identifications performed on Sp(8,R)/U(4) with its standard homogeneous metric.
In a companion paper we use these symmetries to derive new cosmological
solutions from known ones.Comment: 11 pages, 1 table; two foonotes added, minor corrections to conten
Human mitochondrial DNA replication machinery and disease.
The human mitochondrial genome is replicated by DNA polymerase γ in concert with key components of the mitochondrial DNA (mtDNA) replication machinery. Defects in mtDNA replication or nucleotide metabolism cause deletions, point mutations, or depletion of mtDNA. The resulting loss of cellular respiration ultimately induces mitochondrial genetic diseases, including mtDNA depletion syndromes (MDS) such as Alpers or early infantile hepatocerebral syndromes, and mtDNA deletion disorders such as progressive external ophthalmoplegia, ataxia-neuropathy, or mitochondrial neurogastrointestinal encephalomyopathy. Here we review the current literature regarding human mtDNA replication and heritable disorders caused by genetic changes of the POLG, POLG2, Twinkle, RNASEH1, DNA2, and MGME1 genes
Scaling Laws for Non-Intercommuting Cosmic String Networks
We study the evolution of non-interacting and entangled cosmic string
networks in the context of the velocity-dependent one-scale model. Such
networks may be formed in several contexts, including brane inflation. We show
that the frozen network solution , although generic, is only a
transient one, and that the asymptotic solution is still as in the
case of ordinary (intercommuting) strings, although in the present context the
universe will usually be string-dominated. Thus the behaviour of two strings
when they cross does not seem to affect their scaling laws, but only their
densities relative to the background.Comment: Phys. Rev. D (in press); v2: final published version (references
added, typos corrected
Reconfiguring experimental archaeology using 3D reconstruction
Experimental archaeology has long yielded valuable insights into the tools and techniques that featured in past peoples’ relationship with the material world around them. We can determine, for example, how many trees would need to be felled to construct a large round-house of the southern British Iron Age (over one hundred), infer the exact angle needed to strike a flint core in order to knap an arrowhead in the manner of a Neolithic hunter-gatherer, or recreate the precise environmental conditions needed to store grain in underground silos over the winter months, with only the technologies and materials available to Romano-Briton villagers (see Coles 1973; Reynolds 1993). However, experimental archaeology has, hitherto, confined itself to rather rigid, empirical and quantitative questions such as those posed in these examples. This is quite understandable, and in line with good scientific practice, which stipulates that any ‘experiment’ must be based on replicable data, and be reproducible. Despite their potential in this area however, it is notable that digital reconstruction technologies have yet to play a significant role in experimental archaeology. Whilst many excellent examples of digital 3D reconstruction of heritage sites exist (for example the Digital Roman Forum project: http://dlib.etc.ucla.edu/projects/Forum) most, if not all, of these are characterized by a drive to establish a photorealistic re-creation of physical features. This paper will discuss possibilities that lie beyond straightforward positivist re-creation of heritage sites, in the experimental reconstruction of intangible heritage. Between 2010 and 2012, the authors led the Motion in Place Platform project (MiPP: http://www.motioninplace.org/), a capital grant under the AHRC's DEDEFI scheme developing motion capture and analysis tools for exploring how people move through spaces. In the course of MiPP, a series of experiments were conducted using motion capture hardware and software at the Silchester Roman town archaeological excavation in Hampshire, and at the Butser Ancient Farm facility, where Romano-British and Iron Age dwellings have been constructed according to the best experimental practice. As well as reconstructing such Roman and early British dwellings in 3D, the authors were able to use motion capture to reconstruct the kind of activities that – according to the material evidence – are likely to have been carried out by the occupants who used them. Bespoke motion capture suits developed for the project were employed, and the traces captured and rendered with a combination of Autodesk and Unity3D software. This sheds new light on how the reconstructed spaces - and, by inference, their ancient counterparts - were most likely to have been used. In particular the exercises allowed the evaluation and visualisation of changes in behaviour which occur as a result of familiarity with an environment and the acquisition of expertise over time; and to assess how interaction between different actors affects how everyday tasks are carried out
The stability of cosmological scaling solutions
We study the stability of cosmological scaling solutions within the class of
spatially homogeneous cosmological models with a perfect fluid subject to the
equation of state p_gamma=(gamma-1) rho_gamma (where gamma is a constant
satisfying 0 < gamma < 2) and a scalar field with an exponential potential. The
scaling solutions, which are spatially flat isotropic models in which the
scalar field energy density tracks that of the perfect fluid, are of physical
interest. For example, in these models a significant fraction of the current
energy density of the Universe may be contained in the scalar field whose
dynamical effects mimic cold dark matter. It is known that the scaling
solutions are late-time attractors (i.e., stable) in the subclass of flat
isotropic models. We find that the scaling solutions are stable (to shear and
curvature perturbations) in generic anisotropic Bianchi models when gamma <
2/3. However, when gamma > 2/3, and particularly for realistic matter with
gamma >= 1, the scaling solutions are unstable; essentially they are unstable
to curvature perturbations, although they are stable to shear perturbations. We
briefly discuss the physical consequences of these results.Comment: AMSTeX, 7 pages, re-submitted to Phys Rev Let
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