9,961 research outputs found
Imprints of deviations from the gravitational inverse-square law on the power spectrum of mass fluctuations
Deviations from the gravitational inverse-square law would imprint
scale-dependent features on the power spectrum of mass density fluctuations. We
model such deviations as a Yukawa-like contribution to the gravitational
potential and discuss the growth function in a mixed dark matter model with
adiabatic initial conditions. Evolution of perturbations is considered in
general non-flat cosmological models with a cosmological constant, and an
analytical approximation for the growth function is provided. The coupling
between baryons and cold dark matter across recombination is negligibly
affected by modified gravity physics if the proper cutoff length of the
long-range Yukawa-like force is > 10 h^{-1} Mpc. Enhancement of gravity affects
the subsequent evolution, boosting large-scale power in a way that resembles
the effect of a lower matter density. This phenomenon is almost perfectly
degenerate in power-spectrum shape with the effect of a background of massive
neutrinos. Back-reaction on density growth from a modified cosmic expansion
rate should however also affect the normalization of the power spectrum, with a
shape distortion similar to the case of a non-modified background.Comment: 8 pages, 7 figures; submitted to MNRA
Baryonic Signatures in Large-Scale Structure
We investigate the consequences of a non-negligible baryon fraction for
models of structure formation in Cold Dark Matter dominated cosmologies,
emphasizing in particular the existence of oscillations in the present-day
matter power spectrum. These oscillations are the remnants of acoustic
oscillations in the photon-baryon fluid before last scattering. For acceptable
values of the cosmological and baryon densities, the oscillations modulate the
power by up to 10%, with a `period' in spatial wavenumber which is close to
Delta k approximately 0.05/ Mpc. We study the effects of nonlinear evolution on
these features, and show that they are erased for k > 0.2 h/ Mpc. At larger
scales, the features evolve as expected from second-order perturbation theory:
the visibility of the oscillations is affected only weakly by nonlinear
evolution. No realistic CDM parameter combination is able to account for the
claimed feature near k = 0.1 h/ Mpc in the APM power spectrum, or the excess
power at 100 Mpc/h wavelengths quoted by several recent surveys. Thus baryonic
oscillations are not predicted to dominate existing measurements of clustering.
We examine several effects which may mask the features which are predicted, and
conclude that future galaxy surveys may be able to detect the oscillatory
features in the power spectrum provided baryons comprise more than 15% of the
total density, but that it will be a technically challenging achievement.Comment: 16 pages, 13 Figures, to be published in MNRA
Solar sailing - mission opportunities and innovative technology demonstration
Solar sailing is a unique and elegant form of propulsion that transcends reliance on reaction mass. Rather than carrying propellant, solar sails acquire momentum from photons, the quantum packets of energy from which sunlight is composed. In addition, since solar sails are not limited by reaction mass, they can provide continual acceleration, limited only by the lifetime of the sail film in the space environment. Therefore, solar sails can expand the envelope of possible missions, enabling new high-energy mission concepts that are essentially impossible with conventional reaction propulsion, and enhancing current mission concepts by lowering launch mass and reducing trip times
Measuring the galaxy power spectrum with multiresolution decomposition -- II. diagonal and off-diagonal power spectra of the LCRS galaxies
The power spectrum estimator based on the discrete wavelet transform (DWT)
for 3-dimensional samples has been studied. The DWT estimator for
multi-dimensional samples provides two types of spectra with respect to
diagonal and off-diagonal modes, which are very flexible to deal with
configuration-related problems in the power spectrum detection. With simulation
samples and mock catalogues of the Las Campanas redshift survey (LCRS), we show
(1) the slice-like geometry of the LCRS doesn't affect the off-diagonal power
spectrum with ``slice-like'' mode; (2) the Poisson sampling with the LCRS
selection function doesn't cause more than 1- error in the DWT power
spectrum; and (3) the powers of peculiar velocity fluctuations, which cause the
redshift distortion, are approximately scale-independent. These results insure
that the uncertainties of the power spectrum measurement are under control. The
scatter of the DWT power spectra of the six strips of the LCRS survey is found
to be rather small. It is less than 1- of the cosmic variance of mock
samples in the wavenumber range h Mpc. To fit the detected
LCRS diagonal DWT power spectrum with CDM models, we find that the best-fitting
redshift distortion parameter is about the same as that obtained from
the Fourier power spectrum. The velocity dispersions for SCDM and
CDM models are also consistent with other detections with
the LCRS. A systematic difference between the best-fitting parameters of
diagonal and off-diagonal power spectra has been significantly measured. This
indicates that the off-diagonal power spectra are capable of providing
information about the power spectrum of galaxy velocity field.Comment: AAS LaTeX file, 41 pages, 10 figures included, accepted for
publication in Ap
Old Galaxies at High Redshift and the Cosmological Constant
In a recent striking discovery, Dunlop {\bf \it et al} observed a galaxy at
redshift z=1.55 with an estimated age of 3.5 Gyr. This is incompatible with age
estimates for a flat matter dominated universe unless the Hubble constant is
less than . While both an open universe, and a universe
with a cosmological constant alleviate this problem, I argue here that this
result favors a non-zero cosmological constant, especially when considered in
light of other cosmological constraints. In the first place, for the favored
range of matter densities, this constraint is more stringent than the globular
cluster age constraint, which already favors a non-zero cosmological constant.
Moreover, the age-redshift relation for redshifts of order unity implies that
the ratio between the age associated with redshift 1.55 and the present age is
also generally larger for a cosmological constant dominated universe than for
an open universe. In addition, structure formation is generally suppressed in
low density cosmologies, arguing against early galaxy formation. The additional
constraints imposed by the new observation on the parameter space of vs
(where ) are derived for both
cosmologies. For a cosmological constant dominated universe this constraint is
consistent with the range allowed by other cosmological constraints, which also
favor a non-zero value.Comment: latex, 10 pages, including two embedded postscript figure
Is space really expanding? A counterexample
In all Friedman models, the cosmological redshift is widely interpreted as a
consequence of the general-relativistic phenomenon of EXPANSION OF SPACE. Other
commonly believed consequences of this phenomenon are superluminal recession
velocities of distant galaxies and the distance to the particle horizon greater
than c*t (where t is the age of the Universe), in apparent conflict with
special relativity. Here, we study a particular Friedman model: empty universe.
This model exhibits both cosmological redshift, superluminal velocities and
infinite distance to the horizon. However, we show that the cosmological
redshift is there simply a relativistic Doppler shift. Moreover, apparently
superluminal velocities and `acausal' distance to the horizon are in fact a
direct consequence of special-relativistic phenomenon of time dilation, as well
as of the adopted definition of distance in cosmology. There is no conflict
with special relativity, whatsoever. In particular, INERTIAL recession
velocities are subluminal. Since in the real Universe, sufficiently distant
galaxies recede with relativistic velocities, these special-relativistic
effects must be at least partly responsible for the cosmological redshift and
the aforementioned `superluminalities', commonly attributed to the expansion of
space. Let us finish with a question resembling a Buddhism-Zen `koan': in an
empty universe, what is expanding?Comment: 12 pages, no figures; added Appendix with a calculation of the
cosmological redshift in `private space
Spontaneous and induced hyperplasia and neoplasia in the mouse lung.
,SINCE Livingood (1896) first described a spontaneous pulmonary tumour in a mouse, much has been written on the histogenesis of this common tumour, vet it is still difficult to decide whether an early lesion should be classified as hyperplastic or neoplastic. Since the introduction of line-bred strains of mice by Strong (1936) and their general use by research workers, genetic differences in susceptibility have been extensively investigated by Heston (1940) and manv others. It is accepted that the susceptibility to spontaneous development of pulmonary tumours is high in A Strain and low in C57 Black, but that there is no essential difference between the type or range of tumours seen in different strains (Stewart, 1953). As to the sites of origin and distribution of these tumours, most authors describe them as subpleural and as originating in the alveolar epithelium. As far as we know, there has been no evidence as to localising factors which cause one alveolus rather than another to develop neoplastic growth
An engineering approach to modelling of dynamic insulation using ESP-r
The use of Dynamic Insulation (DI) can enable recovery of conduction heat loss through a building envelope. This is an active process that allows air to move through the fabric against the temperature gradient. Additionally it promises better indoor air quality, primarily due to filtration properties of the construction material [11]. This paper is concerned with quantifying the energy savings and enhancement of human comfort if this technology is integrated into a building. To ascertain the impact of the technology on whole-building performance, it is necessary to undertake detailed dynamic modelling. A suitable building and plant simulation computer tool (ESP-r) was employed to do this. A technique for modelling the dynamic insulation was developed and validated against known analytical solutions. A full-size test house was then simulated, in the UK climate, with and without DI. Comparative results show that better thermal comfort and energy savings are possible with the use of DI. The results obtained have been translated into suggestions for best practice
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