11,853 research outputs found
On the Rabinowitz Floer homology of twisted cotangent bundles
Consider the cotangent bundle of a Riemannian manifold of dimension 2
or more, endowed with a twisted symplectic structure defined by a closed weakly
exact 2-form on whose lift to the universal cover of admits a
bounded primitive. We compute the Rabinowitz Floer homology of energy
hypersurfaces of mechanical (kinetic energy + potential)
Hamiltonians for the case when the energy value k is greater than the Mane
critical value c.
Under the stronger condition that k>c_{0}, where c_{0} denotes the strict
Mane critical value, Abbondandolo and Schwarz recently computed the Rabinowitz
Floer homology of such hypersurfaces, by means of a short exact sequence of
chain complexes involving the Rabinowitz Floer chain complex and the Morse
(co)chain complex associated to the free time action functional. We extend
their results to the weaker case k>c, thus covering cases where is not
exact.
As a consequence, we deduce that the hypersurface corresponding to the energy
level k is never displaceable for any k>c. Moreover, we prove that if dim M >
1, the homology of the free loop space of is infinite dimensional, and if
the metric is chosen generically, a generic Hamiltonian diffeomorphism has
infinitely many leaf-wise intersection points in .Comment: V4 - final version, accepted for publication in CVPD
Large Scale Structure Observations
Galaxy Surveys are enjoying a renaissance thanks to the advent of
multi-object spectrographs on ground-based telescopes. The last 15 years have
seen the fruits of this experimental advance, including the 2-degree Field
Galaxy Redshift Survey (2dFGRS; Colless et al. 2003) and the Sloan Digital Sky
Survey (SDSS; York et al. 2000). Most recently, the Baryon Oscillation
Spectroscopic Survey (BOSS; Dawson et al. 2013), part of the SDSS-III project
(Eisenstein et al. 2011), has provided the largest volume of the low-redshift
Universe ever surveyed with a galaxy density useful for high-precision
cosmology. This set of lecture notes looks at some of the physical processes
that underpin these measurements, the evolution of measurements themselves, and
looks ahead to the next 15 years and the advent of surveys such as the enhanced
Baryon Oscillation Spectroscopic Survey (eBOSS), the Dark Energy Spectroscopic
Instrument (DESI) and the ESA Euclid satellite mission.Comment: Lectures given at Post-Planck Cosmology, Ecole de Physique des
Houches, Les Houches, July 8-Aug 2, 2013, eds. B. Wandelt, C. Deffayet, P.
Peter, to be published by Oxford University Press, and New Horizons for
Observational Cosmology, International School of Physics Enrico Fermi,
Varenna, July 1-6, 2013, eds. A. Melchiorri, A. Cooray, E. Komatsu, to be
published by the Italian Society of Physic
Galaxy 2-Point Covariance Matrix Estimation for Next Generation Surveys
We perform a detailed analysis of the covariance matrix of the spherically
averaged galaxy power spectrum and present a new, practical method for
estimating this within an arbitrary survey without the need for running mock
galaxy simulations that cover the full survey volume. The method uses
theoretical arguments to modify the covariance matrix measured from a set of
small-volume cubic galaxy simulations, which are computationally cheap to
produce compared to larger simulations and match the measured small-scale
galaxy clustering more accurately than is possible using theoretical modelling.
We include prescriptions to analytically account for the window function of the
survey, which convolves the measured covariance matrix in a non-trivial way. We
also present a new method to include the effects of supersample covariance and
modes outside the small simulation volume which requires no additional
simulations and still allows us to scale the covariance matrix. As validation,
we compare the covariance matrix estimated using our new method to that from a
brute force calculation using 500 simulations originally created for analysis
of the Sloan Digital Sky Survey Main Galaxy Sample (SDSS-MGS). We find
excellent agreement on all scales of interest for large scale structure
analysis, including those dominated by the effects of the survey window, and on
scales where theoretical models of the clustering normally break-down, but the
new method produces a covariance matrix with significantly better
signal-to-noise. Although only formally correct in real-space, we also discuss
how our method can be extended to incorporate the effects of Redshift Space
Distortions.Comment: 18 pages, 9 figures. Accepted for publication in MNRAS. Added new
references to introduction and slightly updated text accordingl
Quantum revivals and carpets in some exactly solvable systems
We consider the revival properties of quantum systems with an eigenspectrum
E_{n} proportional to n^{2}, and compare them with the simplest member of this
class - the infinite square well. In addition to having perfect revivals at
integer multiples of the revival time t_{R}, these systems all enjoy perfect
fractional revivals at quarterly intervals of t_{R}. A closer examination of
the quantum evolution is performed for the Poeschel-Teller and Rosen-Morse
potentials, and comparison is made with the infinite square well using quantum
carpets.Comment: 5 pages, 5 figures (1 new), minor additions, to appear in J. Phys.
Unbiased clustering estimation in the presence of missing observations
In order to be efficient, spectroscopic galaxy redshift surveys do not obtain
redshifts for all galaxies in the population targeted. The missing galaxies are
often clustered, commonly leading to a lower proportion of successful
observations in dense regions. One example is the close-pair issue for SDSS
spectroscopic galaxy surveys, which have a deficit of pairs of observed
galaxies with angular separation closer than the hardware limit on placing
neighbouring fibers. Spatially clustered missing observations will exist in the
next generations of surveys. Various schemes have previously been suggested to
mitigate these effects, but none works for all situations. We argue that the
solution is to link the missing galaxies to those observed with statistically
equivalent clustering properties, and that the best way to do this is to rerun
the targeting algorithm, varying the angular position of the observations.
Provided that every pair has a non-zero probability of being observed in one
realisation of the algorithm, then a pair-upweighting scheme linking targets to
successful observations, can correct these issues. We present such a scheme,
and demonstrate its validity using realisations of an idealised simple survey
strategy.Comment: 14 pages, 8 figures, published in MNRA
An accurate linear model for redshift space distortions in the void-galaxy correlation function
Redshift space distortions within voids provide a unique method to test for
environmental dependence of the growth rate of structures in low density
regions, where effects of modified gravity theories might be important. We
derive a linear theory model for the redshift space void-galaxy correlation
that is valid at all pair separations, including deep within the void, and use
this to obtain expressions for the monopole and quadrupole
contributions. Our derivation highlights terms that have previously been
neglected but are important within the void interior. As a result our model
differs from previous works and predicts new physical effects, including a
change in the sign of the quadrupole term within the void radius. We show how
the model can be generalised to include a velocity dispersion. We compare our
model predictions to measurements of the correlation function using mock void
and galaxy catalogues modelled after the BOSS CMASS galaxy sample using the Big
MultiDark -body simulation, and show that the linear model with dispersion
provides an excellent fit to the data at all scales, Mpc. While the RSD model matches simulations, the linear bias
approximation does not hold within voids, and care is needed in fitting for the
growth rate. We show that fits to the redshift space correlation recover the
growth rate to a precision of using the simulation volume
of .Comment: 16 pages, 12 figures. v3: updated to match version published in
MNRAS. Several minor changes to text for better explanations, with reference
to subsequent results (arXiv:1805.09349). No changes to theory, results or
conclusion
Using correlations between CMB lensing and large-scale structure to measure primordial non-Gaussianity
We apply a new method to measure primordial non-Gaussianity, using the
cross-correlation between galaxy surveys and the CMB lensing signal to measure
galaxy bias on very large scales, where local-type primordial non-Gaussianity
predicts a divergence. We use the CMB lensing map recently published by
the Planck collaboration, and measure its external correlations with a suite of
six galaxy catalogues spanning a broad redshift range. We then consistently
combine correlation functions to extend the recent analysis by Giannantonio et
al. (2013), where the density-density and the density-CMB temperature
correlations were used. Due to the intrinsic noise of the Planck lensing map,
which affects the largest scales most severely, we find that the constraints on
the galaxy bias are similar to the constraints from density-CMB temperature
correlations. Including lensing constraints only improves the previous
statistical measurement errors marginally, and we obtain (1) from the combined data set. However, the lensing
measurements serve as an excellent test of systematic errors: we now have three
methods to measure the large-scale, scale-dependent bias from a galaxy survey:
auto-correlation, and cross-correlation with both CMB temperature and lensing.
As the publicly available Planck lensing maps have had their largest-scale
modes at multipoles removed, which are the most sensitive to the
scale-dependent bias, we consider mock CMB lensing data covering all
multipoles. We find that, while the effect of indeed
increases significantly on the largest scales, so do the contributions of both
cosmic variance and the intrinsic lensing noise, so that the improvement is
small.Comment: 5 pages, 3 figures. Additional references added. Submitted to MNRA
Post-Newtonian constraints on f(R) cosmologies in metric formalism
We compute the complete post-Newtonian limit of the metric form of f(R)
gravities using a scalar-tensor representation. By comparing the predictions of
these theories with laboratory and solar system experiments, we find a set of
inequalities that any lagrangian f(R) must satisfy. The constraints imposed by
those inequalities allow us to find explicit bounds to the possible nonlinear
terms of the lagrangian. We conclude that the lagrangian f(R) must be almost
linear in R and that corrections that grow at low curvatures are incompatible
with observations. This result shows that modifications of gravity at very low
cosmic densities cannot be responsible for the observed cosmic speed-up.Comment: 10 pages, no figures, revtex
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