14,588 research outputs found
Dilaton cosmology and phenomenology
This paper is dedicated to Gabriele Veneziano on his 65th birthday. Most of
the results reported here are known results, due to Gabriele, or obtained in
collaboration with him, or inspired by our joint work on string cosmology. A
few new results are also presented concerning the duality invariance of a
non-local dilaton coupling to the matter sources, and its possible cosmological
applications in the context of the dark-energy scenario.Comment: 60 pages, 10 figures, to appear in "String theory and fundamental
interactions: celebrating Gabriele Veneziano on his 65th birthday" (Lecture
Notes in Physics, Springer-Verlag, Berlin/Heidelberg, 2007). A few references
added and some typos correcte
Late-time effects of Planck-scale cosmology: dilatonic interpretation of the dark energy field
We present a model of dark energy based on the string effective action, and
on the assumption that the dilaton is strongly coupled to dark matter. We
discuss the main differences between this class of models and more conventional
models of quintessence,uncoupled to dark matter. This paper is based on talks
presented at the "VII Congresso Nazionale di Cosmologia" (Osservatorio
Astronomico di Roma, Monte Porzio Catone, November 2002), and at the Meeting
"Dark Energy Day" (University of Milano-Bicocca, November 2002). To appear in
the Proc. of the International Conference on ``Thinking, Observing and Mining
the Universe" (Sorrento, September 2003), eds. G. Longo and G. Miele (World
Scientific, Singapore).Comment: 8 pages, latex, 4 figures included using eps
Cosmology of Axions and Moduli: A Dynamical Systems Approach
This paper is concerned with string cosmology and the dynamics of multiple
scalar fields in potentials that can become negative, and their features as
(Early) Dark Energy models. Our point of departure is the "String Axiverse", a
scenario that motivates the existence of cosmologically light axion fields as a
generic consequence of string theory. We couple such an axion to its
corresponding modulus. We give a detailed presentation of the rich cosmology of
such a model, ranging from the setting of initial conditions on the fields
during inflation, to the asymptotic future. We present some simplifying
assumptions based on the fixing of the axion decay constant , and on the
effective field theory when the modulus trajectory is adiabatic, and find the
conditions under which these assumptions break down. As a by-product of our
analysis, we find that relaxing the assumption of fixed leads to the
appearance of a new meta-stable de-Sitter region for the modulus without the
need for uplifting by an additional constant. A dynamical systems analysis
reveals the existence of many fixed point attractors, repellers and saddle
points, which we analyse in detail. We also provide geometric interpretations
of the phase space. The fixed points can be used to bound the couplings in the
model. A systematic scan of certain regions of parameter space reveals that the
future evolution of the universe in this model can be rich, containing multiple
epochs of accelerated expansion.Comment: 27 pages, 12 figures, comments welcome, v2 minor correction
A Phase Space Diagram for Gravity
In modified theories of gravity including a critical acceleration scale,
, a critical length scale, , will naturally
arise, with the transition from the Newtonian to the dark matter mimicking
regime occurring for systems larger than . This adds a second critical
scale to gravity, in addition to the one introduced by the criterion of
the Shwarzschild radius, . The distinct dependencies of the
two above length scales give rise to non-trivial phenomenology in the (mass,
length) plane for astrophysical structures, which we explore here.
Surprisingly, extrapolation to atomic scales suggests gravity should be at the
dark matter mimicking regime there.Comment: 9 pages, 1 figure, Final version accepted for publication in Entrop
Phenomenology of Hybrid Scenarios of Neutrino Dark Energy
We study the phenomenology of hybrid scenarios of neutrino dark energy, where
in addition to a so-called Mass Varying Neutrino (MaVaN) sector a cosmological
constant (from a false vacuum) is driving the accelerated expansion of the
universe today. For general power law potentials we calculate the effective
equation of state parameter w_{eff}(z) in terms of the neutrino mass scale. Due
to the interaction of the dark energy field (acceleron) with the neutrino
sector, w_{eff}(z) is predicted to become smaller than -1 for z>0, which could
be tested in future cosmological observations. For the considered scenarios,
the neutrino mass scale additionally determines which fraction of the dark
energy is dynamical, and which originates from the cosmological constant like
vacuum energy of the false vacuum. On the other hand, the field value of the
acceleron field today as well as the masses of the right-handed neutrinos,
which appear in the seesaw-type mechanism for small neutrino masses, are not
fixed. This, in principle, allows to realise hybrid scenarios of neutrino dark
energy with a high-scale seesaw where the right-handed neutrino masses are
close to the GUT scale. We also comment on how MaVaN Hybrid Scenarios with
high-scale seesaw might help to resolve stability problems of dark energy
models with non-relativistic neutrinos.Comment: 22 pages, 5 figures; references and comments added; version to be
published in JCA
- …