2,666 research outputs found
Solution for a local straight cosmic string in the braneworld gravity
In this work we deal with the spacetime shaped by a straight cosmic string,
emerging from local gauge theories, in the braneworld gravity context. We
search for physical consequences of string features due to the modified
gravitational scenario encoded in the projected gravitational equations. It is
shown that cosmic strings in braneworld gravity may present significant
differences when compared to the general relativity predictions since its
linear density is modified and the deficit angle produced by the cosmic string
is attenuated. Furthermore, the existence of cosmic strings in that scenario
requires a strong restriction to the braneworld tension: , in Planck units.Comment: 7 pages, 3 figure
Static Domain Wall in the Braneworld gravity
In this paper we consider a static domain wall inside a 3-brane. Differently
of the standard achievement obtained in General Relativity, the analysis
performed here gives a consistency condition for the existence of static domain
walls in a braneworld gravitational scenario. It is also shown the behavior of
the domain wall gravitational field in the newtonian limit.Comment: 11 pages, no figures, accepted for publication in EPJ
Darth Fader: Using wavelets to obtain accurate redshifts of spectra at very low signal-to-noise
We present the DARTH FADER algorithm, a new wavelet-based method for
estimating redshifts of galaxy spectra in spectral surveys that is particularly
adept in the very low SNR regime. We use a standard cross-correlation method to
estimate the redshifts of galaxies, using a template set built using a PCA
analysis on a set of simulated, noise-free spectra. Darth Fader employs wavelet
filtering to both estimate the continuum & to extract prominent line features
in each galaxy spectrum. A simple selection criterion based on the number of
features present in the spectrum is then used to clean the catalogue: galaxies
with fewer than six total features are removed as we are unlikely to obtain a
reliable redshift estimate. Applying our wavelet-based cleaning algorithm to a
simulated testing set, we successfully build a clean catalogue including
extremely low signal-to-noise data (SNR=2.0), for which we are able to obtain a
5.1% catastrophic failure rate in the redshift estimates (compared with 34.5%
prior to cleaning). We also show that for a catalogue with uniformly mixed SNRs
between 1.0 & 20.0, with realistic pixel-dependent noise, it is possible to
obtain redshifts with a catastrophic failure rate of 3.3% after cleaning (as
compared to 22.7% before cleaning). Whilst we do not test this algorithm
exhaustively on real data, we present a proof of concept of the applicability
of this method to real data, showing that the wavelet filtering techniques
perform well when applied to some typical spectra from the SDSS archive. The
Darth Fader algorithm provides a robust method for extracting spectral features
from very noisy spectra. The resulting clean catalogue gives an extremely low
rate of catastrophic failures, even when the spectra have a very low SNR. For
very large sky surveys, this technique may offer a significant boost in the
number of faint galaxies with accurately determined redshifts.Comment: 22 pages, 15 figures. Accepted for publication in Astronomy &
Astrophysic
Degradation analysis in the estimation of photometric redshifts from non-representative training sets
We perform an analysis of photometric redshifts estimated by using a
non-representative training sets in magnitude space. We use the ANNz2 and GPz
algorithms to estimate the photometric redshift both in simulations as well as
in real data from the Sloan Digital Sky Survey (DR12). We show that for the
representative case, the results obtained by using both algorithms have the
same quality, either using magnitudes or colours as input. In order to reduce
the errors when estimating the redshifts with a non-representative training
set, we perform the training in colour space. We estimate the quality of our
results by using a mock catalogue which is split samples cuts in the -band
between . We obtain slightly better results with GPz on single
point z-phot estimates in the complete training set case, however the
photometric redshifts estimated with ANNz2 algorithm allows us to obtain mildly
better results in deeper -band cuts when estimating the full redshift
distribution of the sample in the incomplete training set case. By using a
cumulative distribution function and a Monte-Carlo process, we manage to define
a photometric estimator which fits well the spectroscopic distribution of
galaxies in the mock testing set, but with a larger scatter. To complete this
work, we perform an analysis of the impact on the detection of clusters via
density of galaxies in a field by using the photometric redshifts obtained with
a non-representative training set.Comment: 19 pages, 9 figures. Accepted for publication in MNRA
Optimising Spectroscopic and Photometric Galaxy Surveys: Efficient Target Selection and Survey Strategy
The next generation of spectroscopic surveys will have a wealth of
photometric data available for use in target selection. Selecting the best
targets is likely to be one of the most important hurdles in making these
spectroscopic campaigns as successful as possible. Our ability to measure dark
energy depends strongly on the types of targets that we are able to select with
a given photometric data set. We show in this paper that we will be able to
successfully select the targets needed for the next generation of spectroscopic
surveys. We also investigate the details of this selection, including
optimisation of instrument design and survey strategy in order to measure dark
energy. We use color-color selection as well as neural networks to select the
best possible emission line galaxies and luminous red galaxies for a
cosmological survey. Using the Fisher matrix formalism we forecast the
efficiency of each target selection scenario. We show how the dark energy
figures of merit change in each target selection regime as a function of target
type, survey time, survey density and other survey parameters. We outline the
optimal target selection scenarios and survey strategy choices which will be
available to the next generation of spectroscopic surveys.Comment: 16 pages, 22 figures, accepted to MNRAS in dec 201
Cosmological systematics beyond nuisance parameters: form-filling functions
In the absence of any compelling physical model, cosmological systematics are often misrepresented as statistical effects and the approach of marginalizing over extra nuisance systematic parameters is used to gauge the effect of the systematic. In this article, we argue that such an approach is risky at best since the key choice of function can have a large effect on the resultant cosmological errors. As an alternative we present a functional form-filling technique in which an unknown, residual, systematic is treated as such. Since the underlying function is unknown, we evaluate the effect of every functional form allowed by the information available (either a hard boundary or some data). Using a simple toy model, we introduce the formalism of functional form filling. We show that parameter errors can be dramatically affected by the choice of function in the case of marginalizing over a systematic, but that in contrast the functional form-filling approach is independent of the choice of basis set. We then apply the technique to cosmic shear shape measurement systematics and show that a shear calibration bias of |m(z)| ≲ 10−3 (1 +z)0.7 is required for a future all-sky photometric survey to yield unbiased cosmological parameter constraints to per cent accuracy. A module associated with the work in this paper is available through the open source icosmo code available at http://www.icosmo.or
Positive tension 3-branes in an bulk
In this work, we review and extend the so-called consistency conditions for
the existence of a braneworld scenario in arbitrary dimensions in the
Brans-Dicke (BD) gravitational theory. After that, we consider the particular
case of a five-dimensional scenario which seems to have phenomenological
interesting implications. We show that, in the BD framework, it is possible to
achieve necessary conditions pointing to the possibility of accommodating
branes with positive tensions in an AdS bulk by the presence of the additional
BD scalar field, avoiding in this way the necessity of including unstable
objects in the compactification scheme. Furthermore, in the context of time
variable brane tension, it is shown that the brane tension may change its sign,
following the bulk cosmological constant sign.Comment: 15 pages, new version to appear in JHE
Large scale distribution of total mass versus luminous matter from Baryon Acoustic Oscillations: First search in the SDSS-III BOSS Data Release 10
Baryon Acoustic Oscillations (BAOs) in the early Universe are predicted to
leave an as yet undetected signature on the relative clustering of total mass
versus luminous matter. A detection of this effect would provide an important
confirmation of the standard cosmological paradigm and constrain alternatives
to dark matter as well as non-standard fluctuations such as Compensated
Isocurvature Perturbations (CIPs). We conduct the first observational search
for this effect, by comparing the number-weighted and luminosity-weighted
correlation functions, using the SDSS-III BOSS Data Release 10 CMASS sample.
When including CIPs in our model, we formally obtain evidence at of
the relative clustering signature and a limit that matches the existing upper
limits on the amplitude of CIPs. However, various tests suggest that these
results are not yet robust, perhaps due to systematic biases in the data. The
method developed in this Letter, used with more accurate future data such as
that from DESI, is likely to confirm or disprove our preliminary evidence.Comment: 6 pages, 2 figures, accepted for publication in PR
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