729 research outputs found
Vector and Tensor Contributions to the Luminosity Distance
We compute the vector and tensor contributions to the luminosity distance
fluctuations in first order perturbation theory and we expand them in spherical
harmonics. This work presents the formalism with a first application to a
stochastic background of primordial gravitational waves.Comment: 14 pages, 3 figure
Tensor Microwave Anisotropies from a Stochastic Magnetic Field
We derive an expression for the angular power spectrum of cosmic microwave
background anisotropies due to gravity waves generated by a stochastic magnetic
field and compare the result with current observations; we take into account
the non-linear nature of the stress energy tensor of the magnetic field.
For almost scale invariant spectra, the amplitude of the magnetic field at
galactic scales is constrained to be of order 10^{-9} Gauss. If we assume that
the magnetic field is damped below the Alfven damping scale, we find that its
amplitude at
0.1 h^{-1}Mpc, B_\lambda, is constrained to be B_\lambda<7.9 x10^{-6} e^{3n}
Gauss, for n-3/2, where
n is the spectral index of the magnetic field and H_0=100h km s^{-1}Mpc^{-1} is
the Hubble constant today.Comment: 6 pages, 1 figure, accepted for publication in Phys. Rev.
Tensor Bounds on the Hidden Universe
During single clock inflation, hidden fields (i.e. fields coupled to the
inflaton only gravitationally) in their adiabatic vacua can ordinarily only
affect observables through virtual effects. After renormalizing background
quantities (fixed by observations at some pivot scale), all that remains are
logarithmic runnings in correlation functions that are both Planck and slow
roll suppressed. In this paper we show how a large number of hidden fields can
partially compensate this suppression and generate a potentially observable
running in the tensor two point function, consistently inferable courtesy of a
large resummation. We detour to address certain subtleties regarding loop
corrections during inflation, extending the analysis of [1]. Our main result is
that one can extract bounds on the hidden field content of the universe from
bounds on violations of the consistency relation between the tensor spectral
index and the tensor to scalar ratio, were primordial tensors ever detected.
Such bounds are more competitive than the naive bound inferred from requiring
inflation to occur below the strong coupling scale of gravity if deviations
from the consistency relation can be bounded to within the sub-percent level.
We discuss how one can meaningfully constrain the parameter space of various
phenomenological scenarios and constructions that address naturalness with a
large number of species (such as `N-naturalness') with CMB observations up to
cosmic variance limits, and possibly future 21cm and gravitational wave
observations.Comment: 14 pages, 4 figures, 3 appendices. Version accepted to JHEP;
references added, updated bounds on incorporate
Skewness as a probe of non-Gaussian initial conditions
We compute the skewness of the matter distribution arising from non-linear
evolution and from non-Gaussian initial perturbations. We apply our result to a
very generic class of models with non-Gaussian initial conditions and we
estimate analytically the ratio between the skewness due to non-linear
clustering and the part due to the intrinsic non-Gaussianity of the models. We
finally extend our estimates to higher moments.Comment: 5 pages, 2 ps-figs., accepted for publication in PRD, rapid com
Generation of helical magnetic fields from inflation
The generation of helical magnetic fields during single field inflation due
to an axial coupling of the electromagnetic field to the inflaton is discussed.
We find that such a coupling always leads to a blue spectrum of magnetic fields
during slow roll inflation. Though the helical magnetic fields further evolve
during the inverse cascade in the radiation era after inflation, we conclude
that the magnetic fields generated by such an axial coupling can not lead to
observed field strength on cosmologically relevant scales.Comment: 4 pages, 1 figure; Contribution to the proceedings of the
International Conference on Gravitation and Cosmology (ICGC), Goa, India,
December, 201
Are there static texture?
We consider harmonic maps from Minkowski space into the three sphere. We are
especially interested in solutions which are asymptotically constant, i.e.
converge to the same value in all directions of spatial infinity. Physical
3-space can then be compactified and can be identified topologically (but not
metrically!) with a three sphere. Therefore, at fixed time, the winding of the
map is defined. We investigate whether static solutions with non-trivial
winding number exist. The answer which we can proof here is only partial: We
show that within a certain family of maps no static solutions with non-zero
winding number exist. We discuss the existing static solutions in our family of
maps. An extension to other maps or a proof that our family of maps is
sufficiently general remains an open problem.Comment: 12 page Latex file, 1 postscript figure, submitted to PR
CMB temperature anisotropy at large scales induced by a causal primordial magnetic field
We present an analytical derivation of the Sachs Wolfe effect sourced by a
primordial magnetic field. In order to consistently specify the initial
conditions, we assume that the magnetic field is generated by a causal process,
namely a first order phase transition in the early universe. As for the
topological defects case, we apply the general relativistic junction conditions
to match the perturbation variables before and after the phase transition which
generates the magnetic field, in such a way that the total energy momentum
tensor is conserved across the transition and Einstein's equations are
satisfied. We further solve the evolution equations for the metric and fluid
perturbations at large scales analytically including neutrinos, and derive the
magnetic Sachs Wolfe effect. We find that the relevant contribution to the
magnetic Sachs Wolfe effect comes from the metric perturbations at
next-to-leading order in the large scale limit. The leading order term is in
fact strongly suppressed due to the presence of free-streaming neutrinos. We
derive the neutrino compensation effect dynamically and confirm that the
magnetic Sachs Wolfe spectrum from a causal magnetic field behaves as
l(l+1)C_l^B \propto l^2 as found in the latest numerical analyses.Comment: 31 pages, 2 figures, minor changes, matches published versio
Large Scale Structure Formation with Global Topological Defects. A new Formalism and its implementation by numerical simulations
We investigate cosmological structure formation seeded by topological defects
which may form during a phase transition in the early universe. First we derive
a partially new, local and gauge invariant system of perturbation equations to
treat microwave background and dark matter fluctuations induced by topological
defects or any other type of seeds. We then show that this system is well
suited for numerical analysis of structure formation by applying it to seeds
induced by fluctuations of a global scalar field. Our numerical results are
complementary to previous investigations since we use substantially different
methods. The resulting microwave background fluctuations are compatible with
older simulations. We also obtain a scale invariant spectrum of fluctuations
with about the same amplitude. However, our dark matter results yield a smaller
bias parameter compatible with on a scale of in contrast to
previous work which yielded to large bias factors. Our conclusions are thus
more positive. According to the aspects analyzed in this work, global
topological defect induced fluctuations yield viable scenarios of structure
formation and do better than standard CDM on large scales.Comment: uuencoded, compressed tar-file containing the text in LaTeX and 12
Postscript Figures, 41 page
Cosmic Microwave Background Anisotropies from Scaling Seeds: Global Defect Models
We investigate the global texture model of structure formation in cosmogonies
with non-zero cosmological constant for different values of the Hubble
parameter. We find that the absence of significant acoustic peaks and little
power on large scales are robust predictions of these models. However, from a
careful comparison with data we conclude that at present we cannot safely
reject the model on the grounds of present CMB data. Exclusion by means of
galaxy correlation data requires assumptions on biasing and statistics. New,
very stringent constraints come from peculiar velocities.
Investigating the large-N limit, we argue that our main conclusions apply to
all global O(N) models of structure formation.Comment: LaTeX file with RevTex, 27 pages, 23 eps figs., submitted to Phys.
Rev. D. A version with higher quality images can be found at
http://mykonos.unige.ch/~kunz/download/lam.tar.gz for the LaTeX archive and
at http://mykonos.unige.ch/~kunz/download/lam.ps.gz for the compiled
PostScript fil
Analytical modeling of large-angle CMBR anisotropies from textures
We propose an analytic method for predicting the large angle CMBR temperature
fluctuations induced by model textures. The model makes use of only a small
number of phenomenological parameters which ought to be measured from simple
simulations. We derive semi-analytically the -spectrum for together with its associated non-Gaussian cosmic variance error bars. A
slightly tilted spectrum with an extra suppression at low is found, and we
investigate the dependence of the tilt on the parameters of the model. We also
produce a prediction for the two point correlation function. We find a high
level of cosmic confusion between texture scenarios and standard inflationary
theories in any of these quantities. However, we discover that a distinctive
non-Gaussian signal ought to be expected at low , reflecting the prominent
effect of the last texture in these multipoles
- …