22 research outputs found
Baryon Oscillations and Consistency Tests for Photometrically-Determined Redshifts of Very Faint Galaxies
Weak lensing surveys that can potentially place strong constraints on dark
energy parameters can only do so if the source redshift means and error
distributions are very well known. We investigate prospects for controlling
errors in these quantities by exploiting their influence on the power spectra
of the galaxies. Although, from the galaxy power spectra alone, sufficiently
precise and simultaneous determination of redshift biases and variances is not
possible, a strong consistency test is. Given the redshift error rms, galaxy
power spectra can be used to determine the mean redshift of a group of galaxies
to subpercent accuracy. Although galaxy power spectra cannot be used to
determine the redshift error rms, they can be used to determine this rms
divided by the Hubble parameter, a quantity that may be even more valuable for
interpretation of cosmic shear data than the rms itself. We also show that
galaxy power spectra, due to the baryonic acoustic oscillations, can
potentially lead to constraints on dark energy that are competitive with those
due to the cosmic shear power spectra from the same survey.Comment: 8 pages, 6 figures, submitted to Ap
Improved forecasts for the baryon acoustic oscillations and cosmological distance scale
We present the cosmological distance errors achievable using the baryon
acoustic oscillations as a standard ruler. We begin from a Fisher matrix
formalism that is upgraded from Seo & Eisenstein (2003). We isolate the
information from the baryonic peaks by excluding distance information from
other less robust sources. Meanwhile we accommodate the Lagrangian displacement
distribution into the Fisher matrix calculation to reflect the gradual loss of
information in scale and in time due to nonlinear growth, nonlinear bias, and
nonlinear redshift distortions. We then show that we can contract the
multi-dimensional Fisher matrix calculations into a 2-dimensional or even
1-dimensional formalism with physically motivated approximations. We present
the resulting fitting formula for the cosmological distance errors from galaxy
redshift surveys as a function of survey parameters and nonlinearity, which
saves us going through the 12-dimensional Fisher matrix calculations. Finally,
we show excellent agreement between the distance error estimates from the
revised Fisher matrix and the precision on the distance scale recovered from
N-body simulations.Comment: Submitted to ApJ, 21 pages, LaTe
Dark energy and curvature from a future baryonic acoustic oscillation survey using the Lyman-alpha forest
We explore the requirements for a Lyman-alpha forest (LyaF) survey designed
to measure the angular diameter distance and Hubble parameter at 2~<z~<4 using
the standard ruler provided by baryonic acoustic oscillations (BAO). The goal
would be to obtain a high enough density of sources to probe the
three-dimensional density field on the scale of the BAO feature. A
percent-level measurement in this redshift range can almost double the Dark
Energy Task Force Figure of Merit, relative to the case with only a similar
precision measurement at z~1, if the Universe is not assumed to be flat. This
improvement is greater than the one obtained by doubling the size of the z~1
survey, with Planck and a weak SDSS-like z=0.3 BAO measurement assumed in each
case. Galaxy BAO surveys at z~1 may be able to make an effective LyaF
measurement simultaneously at minimal added cost, because the required number
density of quasars is relatively small. We discuss the constraining power as a
function of area, magnitude limit (density of quasars), resolution, and
signal-to-noise of the spectra. For example, a survey covering 2000 sq. deg.
and achieving S/N=1.8 per Ang. at g=23 (~40 quasars per sq. deg.) with an
R~>250 spectrograph is sufficient to measure both the radial and transverse
oscillation scales to 1.4% from the LyaF (or better, if fainter magnitudes and
possibly Lyman-break galaxies can be used). At fixed integration time and in
the sky-noise-dominated limit, a wider, noisier survey is generally more
efficient; the only fundamental upper limit on noise being the need to identify
a quasar and find a redshift. Because the LyaF is much closer to linear and
generally better understood than galaxies, systematic errors are even less
likely to be a problem.Comment: 18 pages including 6 figures, submitted to PR
Dark matter clustering: a simple renormalization group approach
I compute a renormalization group (RG) improvement to the standard
beyond-linear-order Eulerian perturbation theory (PT) calculation of the power
spectrum of large-scale density fluctuations in the Universe. At z=0, for a
power spectrum matching current observations, lowest order RGPT appears to be
as accurate as one can test using existing numerical simulation-calibrated
fitting formulas out to at least k~=0.3 h/Mpc; although inaccuracy is
guaranteed at some level by approximations in the calculation (which can be
improved in the future). In contrast, standard PT breaks down virtually as soon
as beyond-linear corrections become non-negligible, on scales even larger than
k=0.1 h/Mpc. This extension in range of validity could substantially enhance
the usefulness of PT for interpreting baryonic acoustic oscillation surveys
aimed at probing dark energy, for example. I show that the predicted power
spectrum converges at high k to a power law with index given by the fixed-point
solution of the RG equation. I discuss many possible future directions for this
line of work. The basic calculation of this paper should be easily
understandable without any prior knowledge of RG methods, while a rich
background of mathematical physics literature exists for the interested reader.Comment: much expanded explanation of basic calculatio
On the Robustness of the Acoustic Scale in the Low-Redshift Clustering of Matter
We discuss the effects of non-linear structure formation on the signature of
acoustic oscillations in the late-time galaxy distribution. We argue that the
dominant non-linear effect is the differential motion of pairs of tracers
separated by 150 Mpc. These motions are driven by bulk flows and cluster
formation and are much smaller than the acoustic scale itself. We present a
model for the non-linear evolution based on the distribution of pairwise
Lagrangian displacements that provides a quantitative model for the degradation
of the acoustic signature, even for biased tracers in redshift space. The
Lagrangian displacement distribution can be calibrated with a significantly
smaller set of simulations than would be needed to construct a precise power
spectrum. By connecting the acoustic signature in the Fourier basis with that
in the configuration basis, we show that the acoustic signature is more robust
than the usual Fourier-space intuition would suggest because the beat frequency
between the peaks and troughs of the acoustic oscillations is a very small
wavenumber that is well inside the linear regime. We argue that any possible
shift of the acoustic scale is related to infall on 150 Mpc scale, which is
O(0.5%) fractionally at first-order even at z=0. For the matter, there is a
first-order cancellation such that the mean shift is O(10^{-4}). However,
galaxy bias can circumvent this cancellation and produce a sub-percent
systematic bias.Comment: 27 pages, LaTeX. Submitted to the Astrophysical Journa
Modeling scale-dependent bias on the baryonic acoustic scale with the statistics of peaks of Gaussian random fields
Models of galaxy and halo clustering commonly assume that the tracers can be
treated as a continuous field locally biased with respect to the underlying
mass distribution. In the peak model pioneered by BBKS, one considers instead
density maxima of the initial, Gaussian mass density field as an approximation
to the formation site of virialized objects. In this paper, the peak model is
extended in two ways to improve its predictive accuracy. Firstly, we derive the
two-point correlation function of initial density peaks up to second order and
demonstrate that a peak-background split approach can be applied to obtain the
k-independent and k-dependent peak bias factors at all orders. Secondly, we
explore the gravitational evolution of the peak correlation function within the
Zel'dovich approximation. We show that the local (Lagrangian) bias approach
emerges as a special case of the peak model, in which all bias parameters are
scale-independent and there is no statistical velocity bias. We apply our
formulae to study how the Lagrangian peak biasing, the diffusion due to large
scale flows and the mode-coupling due to nonlocal interactions affect the scale
dependence of bias from small separations up to the baryon acoustic oscillation
(BAO) scale. For 2-sigma density peaks collapsing at z=0.3, our model predicts
a ~ 5% residual scale-dependent bias around the acoustic scale that arises
mostly from first-order Lagrangian peak biasing (as opposed to second-order
gravity mode-coupling). We also search for a scale dependence of bias in the
large scale auto-correlation of massive halos extracted from a very large
N-body simulation provided by the MICE collaboration. For halos with mass
M>10^{14}Msun/h, our measurements demonstrate a scale-dependent bias across the
BAO feature which is very well reproduced by a prediction based on the peak
model.Comment: (v1): 23 pages text, 8 figures + appendix (v2): typos fixed,
references added, accepted for publication in PR
Resumming Cosmological Perturbations via the Lagrangian Picture: One-loop Results in Real Space and in Redshift Space
We develop a new approach to study the nonlinear evolution in the large-scale
structure of the Universe both in real space and in redshift space, extending
the standard perturbation theory of gravitational instability. Infinite series
of terms in standard Eulerian perturbation theory are resummed as a result of
our starting from a Lagrangian description of perturbations. Delicate nonlinear
effects on scales of the baryon acoustic oscillations are more accurately
described by our method than the standard one. Our approach differs from other
resummation techniques recently proposed, such as the renormalized perturbation
theory, etc., in that we use simple techniques and thus resulting equations are
undemanding to evaluate, and in that our approach is capable of quantifying the
nonlinear effects in redshift space. The power spectrum and correlation
function of our approach are in good agreement with numerical simulations in
literature on scales of baryon acoustic oscillations. Especially, nonlinear
effects on the baryon acoustic peak of the correlation function are accurately
described both in real space and in redshift space. Our approach provides a
unique opportunity to analytically investigate the nonlinear effects on baryon
acoustic scales in observable redshift space, which is requisite in
constraining the nature of dark energy, the curvature of the Universe, etc., by
redshift surveys.Comment: 18 pages, 12 figures, replaced to match the published versio
Baryon acoustic signature in the clustering of density maxima
We reexamine the two-point correlation of density maxima in Gaussian initial
conditions. Spatial derivatives of the linear density correlation, which were
ignored in the calculation of Bardeen, Bond, Kaiser & Szalay (1986), are
included in our analysis. These functions exhibit large oscillations around the
sound horizon scale for generic CDM power spectra. We derive the exact
leading-order expression for the correlation of density peaks and demonstrate
the contribution of those spatial derivatives. In particular, we show that
these functions can modify significantly the baryon acoustic signature of
density maxima relative to that of the linear density field. The effect depends
upon the exact value of the peak height, the filter shape and size, and the
small-scale behaviour of the transfer function. In the LambdaCDM cosmology, for
maxima identified in the density field smoothed at mass scale M\approx
10^{12}-10^{14}M_sun/h and with linear threshold height \nu=1.673/\sigma(M),
the contrast of the BAO can be a few tens of percent larger than in the linear
matter correlation. Overall, the BAO is amplified (damped) for threshold
heights larger (less) than unity. Density maxima thus behave quite differently
than linearly biased tracers of the density field, whose acoustic signature is
a simple scaled version of the linear baryon acoustic oscillation. We also
calculate the mean streaming of peak pairs in the quasi-linear regime. We show
that the leading-order 2-point correlation and pairwise velocity of density
peaks are consistent with a nonlinear, local biasing relation involving
gradients of the density field. Biasing will be an important issue in
ascertaining how much of the enhancement of the BAO in the primeval correlation
of density maxima propagates into the late-time clustering of galaxies.Comment: 19 pages, 8 figures. minor corrections. In press in PR
Synthesis and third order nonlinear optics of a new soluble conjugated porphyrin polymer
The synthesis of a new soluble conjugated porphyrin polymer 4 is reported. The MALDI TOF mass spectrum shows the presence of oligomers with up to 13 repeat units and GPC gives a Mn of 53 kDa. The electronic absorption spectra of this polymer exhibit an intense Q band at 800 nm in solution and 853 nm in the solid state, demonstrating a high degree of conjugation. Electroabsorption spectroscopy shows that thin films of 4 have lower resonant third order NLO susceptibility than our previous conjugated porphyrin polymer 2, whereas closed z-scan measurements indicate that the off-resonance real susceptibility, at 1064 nm, is exceptionally large for both polymers (χ(3)R =-2 × 10-16 m2 V-2). Open z-scan measurements were also made at 1064 nm, demonstrating that the two polymers exhibit similar nonlinear absorption at this wavelength (β= 1 cm GW-1 at 0.2 mM concentration)