7,449 research outputs found
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
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
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
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
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
The Evolution of Radio Galaxies at Intermediate Redshift
We describe a new estimate of the radio galaxy 1.4 GHz luminosity function
and its evolution at intermediate redshifts (z~0.4). Photometric redshifts and
color selection have been used to select Bj<23.5 early-type galaxies from the
Panoramic Deep Fields, a multicolor survey of two 25 sq deg fields.
Approximately 230 radio galaxies have then been selected by matching early-type
galaxies with NVSS radio sources brighter than 5 mJy. Estimates of the 1.4 GHz
luminosity function of radio galaxies measure significant evolution over the
observed redshift range. For an Omega_M=1 cosmology the evolution of the radio
power is consistent with luminosity evolution where P(z)=P(0)(1+z)^{k_L} and
3<k_L<5. The observed evolution is similar to that observed for UVX and X-ray
selected AGN and is consistent with the same physical process being responsible
for the optical and radio luminosity evolution of AGN.Comment: 26 pages, 9 Figures, Accepted for Publication in A
Radio Galaxy Clustering at z~0.3
Radio galaxies are uniquely useful as probes of large-scale structure as
their uniform identification with giant elliptical galaxies out to high
redshift means that the evolution of their bias factor can be predicted. As the
initial stage in a project to study large-scale structure with radio galaxies
we have performed a small redshift survey, selecting 29 radio galaxies in the
range 0.19<z<0.45 from a contiguous 40 square degree area of sky. We detect
significant clustering within this sample. The amplitude of the two-point
correlation function we measure is consistent with no evolution from the local
(z<0.1) value. This is as expected in a model in which radio galaxy hosts form
at high redshift and thereafter obey a continuity equation, although the
signal:noise of the detection is too low to rule out other models. Larger
surveys out to z~1 should reveal the structures of superclusters at
intermediate redshifts and strongly constrain models for the evolution of
large-scale structure.Comment: 7 pages, 3 figures, accepted by ApJ Letter
Measuring the galaxy power spectrum with future redshift surveys
Precision measurements of the galaxy power spectrum P(k) require a data
analysis pipeline that is both fast enough to be computationally feasible and
accurate enough to take full advantage of high-quality data. We present a
rigorous discussion of different methods of power spectrum estimation, with
emphasis on the traditional Fourier method, the linear (Karhunen-Loeve; KL),
and quadratic data compression schemes, showing in what approximations they
give the same result. To improve speed, we show how many of the advantages of
KL data compression and power spectrum estimation may be achieved with a
computationally faster quadratic method. To improve accuracy, we derive
analytic expressions for handling the integral constraint, since it is crucial
that finite volume effects are accurately corrected for on scales comparable to
the depth of the survey. We also show that for the KL and quadratic techniques,
multiple constraints can be included via simple matrix operations, thereby
rendering the results less sensitive to galactic extinction and mis-estimates
of the radial selection function. We present a data analysis pipeline that we
argue does justice to the increases in both quality and quantity of data that
upcoming redshift surveys will provide. It uses three analysis techniques in
conjunction: a traditional Fourier approach on small scales, a pixelized
quadratic matrix method on large scales and a pixelized KL eigenmode analysis
to probe anisotropic effects such as redshift-space distortions.Comment: Major revisions for clarity. Matches accepted ApJ version. 23 pages,
with 2 figs included. Color figure and links at
http://www.sns.ias.edu/~max/galpower.html (faster from the US), from
http://www.mpa-garching.mpg.de/~max/galpower.html (faster from Europe) or
from [email protected]
Correlations in the Spatial Power Spectrum Inferred from Angular Clustering: Methods and Application to APM
We reconsider the inference of spatial power spectra from angular clustering
data and show how to include correlations in both the angular correlation
function and the spatial power spectrum. Inclusion of the full covariance
matrices loosens the constraints on large-scale structure inferred from the APM
survey by over a factor of two. We present a new inversion technique based on
singular value decomposition that allows one to propagate the covariance matrix
on the angular correlation function through to that of the spatial power
spectrum and to reconstruct smooth power spectra without underestimating the
errors. Within a parameter space of the CDM shape Gamma and the amplitude
sigma_8, we find that the angular correlations in the APM survey constrain
Gamma to be 0.19-0.37 at 68% confidence when fit to scales larger than k=0.2h
Mpc^-1. A downturn in power at k<0.04h Mpc^-1 is significant at only 1-sigma.
These results are optimistic as we include only Gaussian statistical errors and
neglect any boundary effects.Comment: 37 pages, LaTex, 9 figures. Submitted to Ap
Simulated Extragalactic Observations with a Cryogenic Imaging Spectrophotometer
In this paper we explore the application of cryogenic imaging
spectrophotometers. Prototypes of this new class of detector, such as
superconducting tunnel junctions (STJs) and transition edge sensors (TESs),
currently deliver low resolution imaging spectrophotometry with high quantum
efficiency (70-100%) and no read noise over a wide bandpass in the visible to
near-infrared. In order to demonstrate their utility and the differences in
observing strategy needed to maximize their scientific return, we present
simulated observations of a deep extragalactic field. Using a simple analytic
technique, we can estimate both the galaxy redshift and spectral type more
accurately than is possible with current broadband techniques. From our
simulated observations and a subsequent discussion of the expected migration
path for this new technology, we illustrate the power and promise of these
devices.Comment: 30 pages, 10 figures, accepted for publication in the Astronomical
Journa
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