596 research outputs found
Survey design for Spectral Energy Distribution fitting: a Fisher Matrix approach
The spectral energy distribution (SED) of a galaxy contains information on
the galaxy's physical properties, and multi-wavelength observations are needed
in order to measure these properties via SED fitting. In planning these
surveys, optimization of the resources is essential. The Fisher Matrix
formalism can be used to quickly determine the best possible experimental setup
to achieve the desired constraints on the SED fitting parameters. However,
because it relies on the assumption of a Gaussian likelihood function, it is in
general less accurate than other slower techniques that reconstruct the
probability distribution function (PDF) from the direct comparison between
models and data. We compare the uncertainties on SED fitting parameters
predicted by the Fisher Matrix to the ones obtained using the more thorough PDF
fitting techniques. We use both simulated spectra and real data, and consider a
large variety of target galaxies differing in redshift, mass, age, star
formation history, dust content, and wavelength coverage. We find that the
uncertainties reported by the two methods agree within a factor of two in the
vast majority (~ 90%) of cases. If the age determination is uncertain, the
top-hat prior in age used in PDF fitting to prevent each galaxy from being
older than the Universe needs to be incorporated in the Fisher Matrix, at least
approximately, before the two methods can be properly compared. We conclude
that the Fisher Matrix is a useful tool for astronomical survey design.Comment: Accepted by ApJ; online Fisher Matrix tool available at
http://galfish.physics.rutgers.ed
Halo Clustering with Non-Local Non-Gaussianity
We show how the peak-background split can be generalized to predict the
effect of non-local primordial non-Gaussianity on the clustering of halos. Our
approach is applicable to arbitrary primordial bispectra. We show that the
scale-dependence of halo clustering predicted in the peak-background split
(PBS) agrees with that of the local-biasing model on large scales. On smaller
scales, k >~ 0.01 h/Mpc, the predictions diverge, a consequence of the
assumption of separation of scales in the peak-background split. Even on large
scales, PBS and local biasing do not generally agree on the amplitude of the
effect outside of the high-peak limit. The scale dependence of the biasing -
the effect that provides strong constraints to the local-model bispectrum - is
far weaker for the equilateral and self-ordering-scalar-field models of
non-Gaussianity. The bias scale dependence for the orthogonal and folded models
is weaker than in the local model (~ 1/k), but likely still strong enough to be
constraining. We show that departures from scale-invariance of the primordial
power spectrum may lead to order-unity corrections, relative to predictions
made assuming scale-invariance - to the non-Gaussian bias in some of these
non-local models for non-Gaussianity. An Appendix shows that a non-local model
can produce the local-model bispectrum, a mathematical curiosity we uncovered
in the course of this investigation.Comment: 12 pages, 4 figures; submitted to Phys. Rev. D; v2: references added;
v3: some more comments on kernel-bispectrum relation in appendi
Quantum fields in gravity
We give a brief description of some compelling connections between general
relativity and thermodynamics through i) the semi-classical tunnelling
method(s) and ii) the field-theoretical modelling of Unruh-DeWitt detectors. In
both approaches it is possible to interpret some quantities in a
thermodynamical frame.Comment: 4 pages, no figures, contribution to the proceedings of the
conference "Relativity and Gravitation - 100 years after Einstein in Prague
Gauge-Invariant Temperature Anisotropies and Primordial Non-Gaussianity
We provide the gauge-invariant expression for large-scale cosmic microwave
background temperature fluctuations at second-order in perturbation theory. It
enables to unambiguously define the nonlinearity parameter f_NL which is used
by experimental collaborations to pin down the level of Non-Gaussianity in the
temperature fluctuations. Furthermore, it contains a primordial term encoding
all the information about the Non-Gaussianity generated at primordial epochs
and about the mechanism which gave rise to cosmological perturbations, thus
neatly disentangling the primordial contribution to Non-Gaussianity from the
one caused by the post-inflationary evolution.Comment: 4 pages, LaTeX file. Revised to match the version to appear in Phys.
Rev. Let
Time variable cosmological constant of holographic origin with interaction in Brans-Dicke theory
Time variable cosmological constant (TVCC) of holographic origin with
interaction in Brans-Dicke theory is discussed in this paper. We investigate
some characters for this model, and show the evolutions of deceleration
parameter and equation of state (EOS) for dark energy. It is shown that in this
scenario an accelerating universe can be obtained and the evolution of EOS for
dark energy can cross over the boundary of phantom divide. In addition, a
geometrical diagnostic method, jerk parameter is applied to this model to
distinguish it with cosmological constant.Comment: 10 pages, 9 figure
Observational signatures of Jordan-Brans-Dicke theories of gravity
We analyze the Jordan-Brans-Dicke model (JBD) of gravity, where deviations
from General Relativity (GR) are described by a scalar field non-minimally
coupled to gravity. The theory is characterized by a constant coupling
parameter, ; GR is recovered in the limit . In such theories, gravity modifications manifest at early times,
so that one cannot rely on the usual approach of looking for inconsistencies in
the expansion history and perturbations growth in order to discriminate between
JBD and GR. However, we show that a similar technique can be successfully
applied to early and late times observables instead. Cosmological parameters
inferred extrapolating early-time observations to the present will match those
recovered from direct late-time observations only if the correct gravity theory
is used. We use the primary CMB, as will be seen by the Planck satellite, as
the early-time observable; and forthcoming and planned Supernov{\ae}, Baryonic
Acoustic Oscillations and Weak Lensing experiments as late-time observables. We
find that detection of values of as large as 500 and 1000 is
within reach of the upcoming (2010) and next-generation (2020) experiments,
respectively.Comment: minor revision, references added, matching version published in JCA
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