28 research outputs found
Fine tuning and the ratio of tensor to scalar density fluctuations from cosmological inflation
The form of the inflationary potential is severely restricted if one requires
that it be natural in the technical sense, i.e. terms of unrelated origin are
not required to be correlated. We determine the constraints on observables that
are implied in such natural inflationary models, in particular on , the
ratio of tensor to scalar perturbations. We find that the naturalness
constraint does not require to be lare enough to be detectable by the
forthcoming searches for B-mode polarisation in CMB maps. We show also that the
value of is a sensitive discriminator between inflationary models.Comment: 8 pages LaTeX; clarifications and a reference added; to appear in
JCA
Completing Natural Inflation
If the inflaton is a pseudo-scalar axion, the axion shift symmetry can
protect the flatness of its potential from too large radiative corrections.
This possibility, known as natural inflation, requires an axion scale which is
greater than the (reduced) Planck scale. It is unclear whether such a high
value is compatible with an effective field theoretical description, and if the
global axionic symmetry survives quantum gravity effects. We propose a
mechanism which provides an effective large axion scale, although the original
one is sub-Planckian. The mechanism is based on the presence of two axions,
with a potential provided by two anomalous gauge groups. The effective large
axion scale is due to an almost exact symmetry between the couplings of the
axions to the anomalous groups. We also comment on a possible implementation in
heterotic string theory.Comment: 9 pages, 1 figur
Naturally Large Cosmological Neutrino Asymmetries in the MSSM
A large neutrino asymmetry is an interesting possibility for cosmology, which
can have significant observable consequences for nucleosynthesis and the cosmic
microwave background. However, although it is a possibility, there is no
obvious reason to expect the neutrino asymmetry to be observably large. Here we
note that if the baryon asymmetry originates via the Affleck-Dine mechanism
along a d=4 flat direction of the MSSM scalar potential and if the lepton
asymmetry originates via Affleck-Dine leptogenesis along a d=6 direction,
corresponding to the lowest dimension directions conserving R-parity, then the
ratio n_{L}/n_{B} is naturally in the range 10^{8}-10^{9}. As a result, a
potentially observable neutrino asymmetry is correlated with a baryon asymmetry
of the order of 10^{-10}.Comment: 10 pages LaTeX. Final version to be published in Physical Review
Letter
New constraints on neutrino physics from Boomerang data
We have performed a likelihood analysis of the recent data on the Cosmic
Microwave Background Radiation (CMBR) anisotropy taken by the Boomerang
experiment. We find that this data places a strong upper bound on the radiation
density present at recombination. Expressed in terms of the equivalent number
of neutrino species the bound is N_nu < 13, and the standard model
prediction, N_nu = 3.04, is completely consistent the the data. This bound is
complementary to the one found from Big Bang nucleosynthesis considerations in
that it applies to any type of radiation, i.e. it is not flavour sensitive. It
also applies to the universe at a much later epoch, and as such places severe
limits on scenarios with decaying neutrinos. The bound also yields a firm upper
limit on the lepton asymmetry in the universe.Comment: 4 pages, 2 postscript figures, matches version to appear in PR
Cosmic microwave background measurements can discriminate among inflation models
Quantum fluctuations during inflation may be responsible for temperature
anisotropies in the cosmic microwave background (CMB). Observations of CMB
anisotropies can be used to falsify many currently popular models. In this
paper we discuss the prospectus for observations of CMB anisotropies at the
accuracy of planned satellite missions to reject currently popular inflation
models and to provide some direction for model building.Comment: 25-page LaTeX file. Six postscript figure
Solar Wakes of Dark Matter Flows
We analyze the effect of the Sun's gravitational field on a flow of cold dark
matter (CDM) through the solar system in the limit where the velocity
dispersion of the flow vanishes. The exact density and velocity distributions
are derived in the case where the Sun is a point mass. The results are extended
to the more realistic case where the Sun has a finite size spherically
symmetric mass distribution. We find that regions of infinite density, called
caustics, appear. One such region is a line caustic on the axis of symmetry,
downstream from the Sun, where the flow trajectories cross. Another is a
cone-shaped caustic surface near the trajectories of maximum scattering angle.
The trajectories forming the conical caustic pass through the Sun's interior
and probe the solar mass distribution, raising the possibility that the solar
mass distribution may some day be measured by a dark matter detector on Earth.
We generalize our results to the case of flows with continuous velocity
distributions, such as that predicted by the isothermal model of the Milky Way
halo.Comment: 30 pages, 8 figure
Non-chaotic dynamics in general-relativistic and scalar-tensor cosmology
In the context of scalar-tensor models of dark energy and inflation, the
dynamics of vacuum scalar-tensor cosmology are analysed without specifying the
coupling function or the scalar field potential. A conformal transformation to
the Einstein frame is used and the dynamics of general relativity with a
minimally coupled scalar field are derived for a generic potential. It is shown
that the dynamics are non-chaotic, thus settling an existing debate.Comment: 20 pages, LaTeX, to appear in Class. Quantum Gra
Extended quintessence, inflation, and stable de Sitter spaces
A new gauge-invariant criterion for stability against inhomogeneous
perturbations of de Sitter space is applied to scenarios of dark energy and
inflation in scalar-tensor gravity. The results extend previous studies.Comment: 16 pages, LaTeX, to appear in Class. Quantum Gra
Cosmology From Random Multifield Potentials
We consider the statistical properties of vacua and inflationary trajectories
associated with a random multifield potential. Our underlying motivation is the
string landscape, but our calculations apply to general potentials. Using
random matrix theory, we analyze the Hessian matrices associated with the
extrema of this potential. These potentials generically have a vast number of
extrema. If the cross-couplings (off-diagonal terms) are of the same order as
the self-couplings (diagonal terms) we show that essentially all extrema are
saddles, and the number of minima is effectively zero. Avoiding this requires
the same separation of scales needed to ensure that Newton's constant is stable
against radiative corrections in a string landscape. Using the central limit
theorem we find that even if the number of extrema is enormous, the typical
distance between extrema is still substantial -- with challenging implications
for inflationary models that depend on the existence of a complicated path
inside the landscape.Comment: revtex, 3 figures, 10 pages v2 refs adde