88 research outputs found
Isolating relativistic effects in large-scale structure
We present a fully relativistic calculation of the observed galaxy number
counts in the linear regime. We show that besides the density fluctuations and
redshift-space distortions, various relativistic effects contribute to
observations at large scales. These effects all have the same physical origin:
they result from the fact that our coordinate system, namely the galaxy
redshift and the incoming photons' direction, is distorted by inhomogeneities
in our universe. We then discuss the impact of the relativistic effects on the
angular power spectrum and on the two-point correlation function in
configuration space. We show that the latter is very well adapted to isolate
the relativistic effects since it naturally makes use of the symmetries of the
different contributions. In particular, we discuss how the Doppler effect and
the gravitational redshift distortions can be isolated by looking for a dipole
in the cross-correlation function between a bright and a faint population of
galaxies.Comment: 26 pages, 7 figures. Invited contribution to a Classical and Quantum
Gravity focus issue on "Relativistic Effects in Cosmology", edited by Kazuya
Koyam
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
Optimising the measurement of relativistic distortions in large-scale structure
It has been shown recently that relativistic distortions generate a dipolar
modulation in the two-point correlation function of galaxies. To measure this
relativistic dipole it is necessary to cross-correlate different populations of
galaxies with for example different luminosities or colours. In this paper, we
construct an optimal estimator to measure the dipole with multiple populations.
We show that this estimator increases the signal-to-noise of the dipole by up
to 35 percent. Using 6 populations of galaxies, in a survey with halos and
number densities similar to those of the millennium simulation, we forecast a
cumulative signal-to-noise of 4.4. For the main galaxy sample of SDSS at low
redshift z<0.2 our optimal estimator predicts a cumulative signal-to-noise of
2.4. Finally we forecast a cumulative signal-to-noise of 7.4 in the upcoming
DESI survey. These forecasts indicate that with the appropriate choice of
estimator the relativistic dipole should be detectable in current and future
surveys.Comment: 20 pages, 8 figures. v2: 19 pages, 6 figures. Explicit derivation of
the cancellation of the cosmic variance added. Some references added, minor
changes to the text, main results unchanged. Matches version published in
JCA
No-go theorem for k-essence dark energy
We demonstrate that if k-essence can solve the coincidence problem and play
the role of dark energy in the universe, the fluctuations of the field have to
propagate superluminally at some stage. We argue that this implies that
successful k-essence models violate causality. It is not possible to define a
time ordered succession of events in a Lorentz invariant way. Therefore,
k-essence cannot arise as low energy effective field theory of a causal,
consistent high energy theory.Comment: 4 pages, 2 figures. Replaced with revised version accepted for
publication in Phys. Rev. Let
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