45 research outputs found
Dark Matter, Modified Gravity and the Mass of the Neutrino
It has been suggested that Einstein's theory of General Relativity can be
modified to accomodate mismatches between the gravitational field and luminous
matter on a wide range of scales. Covariant theories of modified gravity
generically predict the existence of extra degrees of freedom which may be
interpreted as dark matter. We study a subclass of these theories where the
overall energy density in these extra degrees of freedom is subdominant
relative to the baryon density and show that they favour the presence of
massive neutrinos. For some specific cases (such as a flat Universes with a
cosmological constant) one finds a conservative lower bound on the neutrinos
mass of eV.Comment: 5 pages, 2 figures, 2 tables, submitted to Phys. Rev.
Response to ``Comment on `Primordial magnetic seed field amplification by gravitational waves' "
Here we respond to the comment by Tsagas (gr-qc/0503042) on our paper
gr-qc/0503006. We show that the results in that comment are flawed and cannot
be used for drawing conclusion about the nature of magnetic field amplification
by gravitational waves, and give further support that the results of
gr-qc/0503006 are correct.Comment: 4 pages, 2 figures, to appear in Physical Review
Cosmological implications of the KATRIN experiment
The upcoming Karlsruhe Tritium Neutrino (KATRIN) experiment will put
unprecedented constraints on the absolute mass of the electron neutrino,
\mnue. In this paper we investigate how this information on \mnue will
affect our constraints on cosmological parameters. We consider two scenarios;
one where \mnue=0 (i.e., no detection by KATRIN), and one where
\mnue=0.3eV. We find that the constraints on \mnue from KATRIN will affect
estimates of some important cosmological parameters significantly. For example,
the significance of and the inferred value of depend
on the results from the KATRIN experiment.Comment: 13 page
Robustness to systematics for future dark energy probes
We extend the Figure of Merit formalism usually adopted to quantify the
statistical performance of future dark energy probes to assess the robustness
of a future mission to plausible systematic bias. We introduce a new robustness
Figure of Merit which can be computed in the Fisher Matrix formalism given
arbitrary systematic biases in the observable quantities. We argue that
robustness to systematics is an important new quantity that should be taken
into account when optimizing future surveys. We illustrate our formalism with
toy examples, and apply it to future type Ia supernova (SNIa) and baryonic
acoustic oscillation (BAO) surveys. For the simplified systematic biases that
we consider, we find that SNIa are a somewhat more robust probe of dark energy
parameters than the BAO. We trace this back to a geometrical alignement of
systematic bias direction with statistical degeneracy directions in the dark
energy parameter space.Comment: Added clarifications following referee report, main results
unchanged. Matched version accepted by MNRA
The sensitivity of BAO Dark Energy Constraints to General Isocurvature Perturbations
Baryon Acoustic Oscillation (BAO) surveys will be a leading method for
addressing the dark energy challenge in the next decade. We explore in detail
the effect of allowing for small amplitude admixtures of general isocurvature
perturbations in addition to the dominant adiabatic mode. We find that
non-adiabatic initial conditions leave the sound speed unchanged but instead
excite different harmonics. These harmonics couple differently to Silk damping,
altering the form and evolution of acoustic waves in the baryon-photon fluid
prior to decoupling. This modifies not only the scale on which the sound waves
imprint onto the baryon distribution, which is used as the standard ruler in
BAO surveys, but also the shape, width and height of the BAO peak. We discuss
these effects in detail and show how more general initial conditions impact our
interpretation of cosmological data in dark energy studies. We find that the
inclusion of these additional isocurvature modes leads to an increase in the
Dark Energy Task Force Figure of merit by 140% and 60% for the BOSS and ADEPT
experiments respectively when considered in conjunction with Planck data. We
also show that the incorrect assumption of adiabaticity has the potential to
bias our estimates of the dark energy parameters by () for a
single correlated isocurvature mode, and up to () for three
correlated isocurvature modes in the case of the BOSS (ADEPT) experiment. We
find that the use of the large scale structure data in conjunction with CMB
data improves our ability to measure the contributions of different modes to
the initial conditions by as much as 100% for certain modes in the fully
correlated case.Comment: 20 pages, 17 figure
Effects of inhomogeneities on apparent cosmological observables: "fake" evolving dark energy
Using the exact Lemaitre-Bondi-Tolman solution with a non-vanishing
cosmological constant , we investigate how the presence of a local
spherically-symmetric inhomogeneity can affect apparent cosmological
observables, such as the deceleration parameter or the effective equation of
state of dark energy (DE), derived from the luminosity distance under the
assumption that the real space-time is exactly homogeneous and isotropic. The
presence of a local underdensity is found to produce apparent phantom behavior
of DE, while a locally overdense region leads to apparent quintessence
behavior. We consider relatively small large scale inhomogeneities which today
are not linear and could be seeded by primordial curvature perturbations
compatible with CMB bounds. Our study shows how observations in an
inhomogeneous CDM universe with initial conditions compatible with the
inflationary beginning, if interpreted under the wrong assumption of
homogeneity, can lead to the wrong conclusion about the presence of "fake"
evolving dark energy instead of .Comment: 22 pages, 19 figures,Final version to appear in European Physical
Journal
Dynamical Dark Energy or Simply Cosmic Curvature?
We show that the assumption of a flat universe induces critically large
errors in reconstructing the dark energy equation of state at z>~0.9 even if
the true cosmic curvature is very small, O(1%) or less. The spuriously
reconstructed w(z) shows a range of unusual behaviour, including crossing of
the phantom divide and mimicking of standard tracking quintessence models. For
1% curvature and LCDM, the error in w grows rapidly above z~0.9 reaching
(50%,100%) by redshifts of (2.5,2.9) respectively, due to the long cosmological
lever arm. Interestingly, the w(z) reconstructed from distance data and Hubble
rate measurements have opposite trends due to the asymmetric influence of the
curved geodesics. These results show that including curvature as a free
parameter is imperative in any future analyses attempting to pin down the
dynamics of dark energy, especially at moderate or high redshifts.Comment: 5 pages, 2 figures. To appear in JCA
Non-Gaussian statistics of critical sets in 2 and 3D: Peaks, voids, saddles, genus and skeleton
The formalism to compute the geometrical and topological one-point statistics
of mildly non-Gaussian 2D and 3D cosmological fields is developed. Leveraging
the isotropy of the target statistics, the Gram-Charlier expansion is
reformulated with rotation invariant variables. This formulation allows us to
track the geometrical statistics of the cosmic field to all orders. It then
allows us to connect the one point statistics of the critical sets to the
growth factor through perturbation theory, which predicts the redshift
evolution of higher order cumulants. In particular, the cosmic non-linear
evolution of the skeleton's length, together with the statistics of extrema and
Euler characteristic are investigated in turn. In 2D, the corresponding
differential densities are analytic as a function of the excursion set
threshold and the shape parameter. In 3D, the Euler characteristics and the
field isosurface area are also analytic to all orders in the expansion.
Numerical integrations are performed and simple fits are provided whenever
closed form expressions are not available. These statistics are compared to
estimates from N-body simulations and are shown to match well the cosmic
evolution up to root mean square of the density field of ~0.2. In 3D,
gravitational perturbation theory is implemented to predict the cosmic
evolution of all the relevant Gram-Charlier coefficients for universes with
scale invariant matter distribution. The one point statistics of critical sets
could be used to constrain primordial non-Gaussianities and the dark energy
equation of state on upcoming cosmic surveys; this is illustrated on idealized
experiments.Comment: 41 pages, 13 figures, submitted to Phys Rev
Cosmological constraints on neutrino plus axion hot dark matter
We use observations of the cosmological large-scale structure to derive
limits on two-component hot dark matter consisting of mass-degenerate neutrinos
and hadronic axions, both components having velocity dispersions corresponding
to their respective decoupling temperatures. We restrict the data samples to
the safely linear regime, in particular excluding the Lyman-alpha forest. Using
standard Bayesian inference techniques we derive credible regions in the
two-parameter space of m_a and sum(m_nu). Marginalising over sum(m_nu) provides
m_a < 1.2 eV (95% C.L.). In the absence of axions the same data and methods
give sum(m_nu) < 0.65 eV (95% C.L.). We also derive limits on m_a for a range
of axion-pion couplings up to one order of magnitude larger or smaller than the
hadronic value.Comment: 13 pages, 2 figures, uses iopart.cl