71,310 research outputs found
Biased-estimations of the Variance and Skewness
Nonlinear combinations of direct observables are often used to estimate
quantities of theoretical interest. Without sufficient caution, this could lead
to biased estimations. An example of great interest is the skewness of
the galaxy distribution, defined as the ratio of the third moment \xibar_3
and the variance squared \xibar_2^2. Suppose one is given unbiased estimators
for \xibar_3 and \xibar_2^2 respectively, taking a ratio of the two does
not necessarily result in an unbiased estimator of . Exactly such an
estimation-bias affects most existing measurements of . Furthermore,
common estimators for \xibar_3 and \xibar_2 suffer also from this kind of
estimation-bias themselves: for \xibar_2, it is equivalent to what is
commonly known as the integral constraint. We present a unifying treatment
allowing all these estimation-biases to be calculated analytically. They are in
general negative, and decrease in significance as the survey volume increases,
for a given smoothing scale. We present a re-analysis of some existing
measurements of the variance and skewness and show that most of the well-known
systematic discrepancies between surveys with similar selection criteria, but
different sizes, can be attributed to the volume-dependent estimation-biases.
This affects the inference of the galaxy-bias(es) from these surveys. Our
methodology can be adapted to measurements of analogous quantities in quasar
spectra and weak-lensing maps. We suggest methods to reduce the above
estimation-biases, and point out other examples in LSS studies which might
suffer from the same type of a nonlinear-estimation-bias.Comment: 28 pages of text, 9 ps figures, submitted to Ap
Cosmological constraints from the CMB and Ly-alpha forest revisited
The WMAP team has recently highlighted the usefulness of combining the
Ly-alpha forest constraints with those from the cosmic microwave background
(CMB). This combination is particularly powerful as a probe of the primordial
shape of the power spectrum. Converting between the Ly-alpha forest
observations and the linear mass power spectrum requires a careful treatment of
nuisance parameters and modeling with cosmological simulations. We point out
several issues which lead to an expansion of the errors, the two most important
being the range of cosmological parameters explored in simulations and the
treatment of the mean transmitted flux constraints. We employ a likelihood
calculator for the current Ly-alpha data set based on an extensive
6-dimensional grid of simulations. We show that the current uncertainties in
the mean transmission and the flux power spectrum define a degeneracy line in
the amplitude-slope plane. The CMB degeneracy due to the primordial power
spectrum shape follows a similar relation in this plane. This weakens the
statistical significance of the primordial power spectrum shape constraints
based on combined CMB+Ly-alpha forest analysis. Using the current data the
simplest n=1 scale invariant model with dn/dln k=0 and no tensors has a Delta
chi^2=4 compared to the best fitting model in which these 3 parameters are
free. Current data therefore do not require relaxing these parameters to
improve the fit.Comment: 7 pages, 2 figures, expanded, matches the accepted version in MNRA
Holographic Principle bounds on Primordial Black Hole abundances
The generalized Second Law of thermodynamics and the Holographic Principle
are combined to obtain the maximum mass of black holes formed inside a static
spherical box of size filled with radiation at initial temperature .
The final temperature after the formation of black holes is evaluated, and we
show that a critical threshold exists for the radiation to be fully consumed by
the process. We next argue that if some form of Holographic Principle holds,
upper bounds to the mass density of PBHs formed in the early universe may be
obtained. The limits are worked out for inflationary and non-inflationary
cosmological models. This method is independent of the known limits based on
the background fluxes (from cosmic rays, radiation and other forms of energy)
and applies to potentially important epochs of PBH formation, resulting in
quite strong constraints to .Comment: Latex file, 2 .ps figures. To appear in Classical and Quantum Gravit
Eulerian bias and the galaxy density field
We investigate the effects on cosmological clustering statistics of empirical
biasing, where the galaxy distribution is a local transformation of the
present-day Eulerian density field. The effects of the suppression of galaxy
numbers in voids, and their enhancement in regions of high density, are
considered, independently and in combination. We compare results from numerical
simulations with the predictions of simple analytic models. We find that the
bias is generally scale-dependent, so that the shape of the galaxy power
spectrum differs from that of the underlying mass distribution. The degree of
bias is always a monotonic function of scale, tending to an asymptotic value on
scales where the density fluctuations are linear. The scale dependence is often
rather weak, with many reasonable prescriptions giving a bias which is nearly
independent of scale. We have investigated whether such an Eulerian bias can
reconcile a range of theoretical power spectra with the twin requirements of
fitting the galaxy power spectrum and reproducing the observed mass-to-light
ratios in clusters. It is not possible to satisfy these constraints for any
member of the family of CDM-like power spectra in an Einstein - de Sitter
universe when normalised to match COBE on large scales and galaxy cluster
abundances on intermediate scales. We discuss what modifications of the mass
power spectrum might produce agreement with the observational data.Comment: 14 pages, LaTeX (using mn.sty, epsfig), 17 Postscript figures
included. Accepted for publication in MNRA
A first direct measurement of the intergalactic medium temperature around a quasar at z=6
The thermal state of the intergalactic medium (IGM) provides an indirect
probe of both the HI and HeII reionisation epochs. Current constraints on the
IGM temperature from the Lya forest are restricted to the redshift range
2<z<4.5, limiting the ability to probe the thermal memory of HI reionisation
toward higher redshift. In this work, we present the first direct measurement
of the IGM temperature around a z=6 quasar by analysing the Doppler widths of
Lya absorption lines in the proximity zone of SDSS J0818+1722. We use a high
resolution (R= 40000) Keck/HIRES spectrum in combination with detailed
numerical modelling to obtain the temperature at mean density,
T_0=23600\pm^5000_6900K (\pm^9200_9300K) at 68 (95) per cent confidence
assuming a prior probability 13500K<T_0<38500 K following HI and HeII
reionisation. This enables us to place an upper limit on the redshift of HI
reionisation, z_H, within 33 comoving Mpc of SDSS J0818+1722. If the quasar
reionises the HeII in its vicinity, then in the limit of instantaneous
reionisation we infer z_H<9.0 (11.0) at 68 (95) per cent confidence assuming
photoheating is the dominant heat source and that HI reionisation is driven by
ionising sources with soft spectra, typical of population II stars. If the HI
and HeII in the IGM around SDSS J0818+1722 are instead reionised simultaneously
by a population of massive metal-free stars, characterised by very hard
ionising spectra, we obtain a tighter upper limit of z_H<8.4 (9.4). Initiating
reionisation at higher redshifts produces temperatures which are too low with
respect to our constraint unless the HI ionising sources or the quasar itself
have spectra significantly harder than typically assumed.Comment: 15 pages, 9 figures, accepted to MNRA
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