3,664 research outputs found

### Intrinsic/Extrinsic Density-Ellipticity Correlations and Galaxy-Galaxy Lensing

We compute both extrinsic (lensing) and intrinsic contributions to the
(galaxy-)density-ellipticity correlation function, the latter done using
current analytic theories of tidal alignment. The gravitational lensing
contribution has two components: one is analogous to galaxy-galaxy lensing and
the other arises from magnification bias -- that gravitational lensing induces
a modulation of the galaxy density as well as ellipticity. On the other hand,
the intrinsic alignment contribution vanishes, even after taking into account
source clustering corrections, which suggests the density-ellipticity
correlation might be an interesting diagnostic in differentiating between
intrinsic and extrinsic alignments. {\it However}, an important assumption,
commonly adopted by current analytic alignment theories, is the Gaussianity of
the tidal field. Inevitable non-Gaussian fluctuations from gravitational
instability induces a non-zero intrinsic density-ellipticity correlation, which
we estimate. We also argue that non-Gaussian contributions to the intrinsic
{\it ellipticity-ellipticity} correlation are often non-negligible. This leads
to a linear rather than, as is commonly assumed, quadratic scaling with the
power spectrum on sufficiently large scales. Finally, we estimate the
contribution of intrinsic alignment to low redshift galaxy-galaxy lensing
measurements (e.g. SDSS), due to the partial overlap between foreground and
background galaxies: the intrinsic contamination is about 10 - 30 % at 10'.
Uncertainties in this estimate are discussed.Comment: 14 pages, 5 figures, submitted to Ap

### Local Approximations to the Gravitational Collapse of Cold Matter

We investigate three different local approximations for nonlinear
gravitational instability in the framework of cosmological Lagrangian fluid
dynamics of cold dust. They include the Zel'dovich approximation (ZA), the
``non-magnetic'' approximation of Bertschinger \& Jain (1994, NMA), and a new
``local tidal'' approximation (LTA). The LTA is exact for any perturbations
whose gravitational and velocity equipotentials have the same constant shape
with time, including spherical, cylindrical, and plane-parallel perturbations.
We tested all three local approximations with the collapse of a homogeneous
triaxial ellipsoid, for which an exact solution exists for an ellipsoid
embedded in empty space and an excellent approximation is known in the
cosmological context. We find that the LTA is significantly more accurate in
general than the ZA and the NMA. Like the ZA, but unlike the NMA, the LTA
generically leads to pancake collapse. For a randomly chosen mass element in an
Einstein-de Sitter universe, assuming a Gaussian random field of initial
density fluctuations, the LTA predicts that at least 78\% of initially
underdense regions collapse owing to nonlinear effects of shear and tides.Comment: 29 pages of latex, uses aaspp4.sty (AASTeX v4.0), submitted to Ap

### Complementarity + Back-reaction is enough

We investigate a recent development of the black hole information problem, in
which a practical paradox has been formulated to show that complementarity is
insufficient. A crucial ingredient in this practical paradox is to distill
information from the early Hawking radiation within the past lightcone of the
black hole. By causality this action can back-react on the black hole. Taking
this back-reaction into account, the paradox could be resolved without invoking
any new physics beyond complementarity. This resolution requires a certain
constraint on the S-matrix to be satisfied. Further insights into the S-matrix
could potentially be obtained by effective-field-theory computations of the
back-reaction on the nice slice.Comment: v2, 21 pages, 4 figure

### Lagrangian space consistency relation for large scale structure

Consistency relations, which relate the squeezed limit of an (N+1)-point
correlation function to an N-point function, are non-perturbative symmetry
statements that hold even if the associated high momentum modes are deep in the
nonlinear regime and astrophysically complex. Recently, Kehagias & Riotto and
Peloso & Pietroni discovered a consistency relation applicable to large scale
structure. We show that this can be recast into a simple physical statement in
Lagrangian space: that the squeezed correlation function (suitably normalized)
vanishes. This holds regardless of whether the correlation observables are at
the same time or not, and regardless of whether multiple-streaming is present.
The simplicity of this statement suggests that an analytic understanding of
large scale structure in the nonlinear regime may be particularly promising in
Lagrangian space.Comment: 19 pages, no figure

### A non-perturbative test of consistency relations and their violation

In this paper, we verify the large scale structure consistency relations
using N-body simulations, including modes in the highly non-linear regime.
These relations (pointed out by Kehagias & Riotto and Peloso & Pietroni) follow
from the symmetry of the dynamics under a shift of the Newtonian potential by a
constant and a linear gradient, and predict the absence of certain poles in the
ratio between the (equal time) squeezed bispectrum and power spectrum. The
consistency relations, as symmetry statements, are exact, but have not been
previously checked beyond the perturbative regime. Our test using N-body
simulations not only offers a non-perturbative check, but also serves as a
warm-up exercise for applications to observational data. A number of subtleties
arise when taking the squeezed limit of the bispectrum--we show how to
circumvent or address them. An interesting by-product of our investigation is
an explicit demonstration that the linear-gradient symmetry is unaffected by
the periodic boundary condition of the simulations. Lastly, we verify using
simulations that the consistency relations are violated when the initial
conditions are non-gaussian (of the local fNL type). The methodology developed
here paves the way for constraining primordial non-gaussianity using large
scale structure data, including (numerous) highly non-linear modes that are
otherwise hard to interpret and utilize.Comment: 10 pages, 5 figures, 1 tabl

### Measurement of the dipole in the cross-correlation function of galaxies

It is usually assumed that in the linear regime the two-point correlation
function of galaxies contains only a monopole, quadrupole and hexadecapole.
Looking at cross-correlations between different populations of galaxies, this
turns out not to be the case. In particular, the cross-correlations between a
bright and a faint population of galaxies contain also a dipole. In this paper
we present the first attempt to measure this dipole. We discuss the four types
of effects that contribute to the dipole: relativistic distortions, evolution
effect, wide-angle effect and large-angle effect. We show that the first three
contributions are intrinsic anti-symmetric contributions that do not depend on
the choice of angle used to measure the dipole. On the other hand the
large-angle effect appears only if the angle chosen to extract the dipole
breaks the symmetry of the problem. We show that the relativistic distortions,
the evolution effect and the wide-angle effect are too small to be detected in
the LOWz and CMASS sample of the BOSS survey. On the other hand with a specific
combination of angles we are able to measure the large-angle effect with high
significance. We emphasise that this large-angle dipole does not contain new
physical information, since it is just a geometrical combination of the
monopole and the quadrupole. However this measurement, which is in excellent
agreement with theoretical predictions, validates our method for extracting the
dipole from the two-point correlation function and it opens the way to the
detection of relativistic effects in future surveys like e.g. DESI.Comment: 15 pages, 17 figures. v2: 20 pages, 17 figures. Section IIIc partly
rewritten, new section IIId, new figures 16 and 17. Main results unchanged.
Matches published version in JCA

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