7,449 research outputs found
Cosmological Evolution on Self-Tuned Branes and the Cosmological Constant
We consider the cosmological evolution of a bulk scalar field and ordinary
matter living on the brane world in the light of the constraints imposed by the
matter dominated cosmological evolution and a small cosmological constant now.
We rule out models with a self-tuned minimum of the four dimensional potential
as they would lead to rapid oscillations of the Hubble parameter now. A more
natural framework is provided by supergravity in singular spaces where the
brane coupling and the bulk potential are related by supersymmetry leading to a
four dimensional run-away potential. For late times we obtain an accelerating
universe due to the breaking of supersymmetry on the brane with an acceleration
parameter of q_0=-4/7 and associated equation of state omega=-5/7.Comment: 24 pages, JHEP styl
CMB Anisotropies in the Presence of Extra Dimensions
We discuss the effect of the time evolution of extra dimensions on CMB
anisotropies and large-scale structure formation. We study the impact of scalar
fields in a low-energy effective description of a general class of brane world
models on the temperature anisotropy power spectrum. We show that when the
coupling between these scalar fields and matter evolves over cosmological
timescales, current observations of the CMB anisotropies can constrain
primordial values of the fields in a manner complementary to local, late-time
tests of gravity. We also present the effect of these fields on the
polarization anisotropy spectra and the growth of large-scale structure,
showing that future CMB observations will constrain theories of the Universe
involving extra dimensions even further.Comment: 17 pages, 15 figure
Varying Constants in Brane World Scenarios
Higher-dimensional theories imply that some constants, such as the
gravitational constant and the strength of the gauge-couplings, are not
fundamental constants. Instead they are related to the sizes of the
extra--dimensional space, which are moduli fields in the four--dimensional
effective theory. We study the cosmological evolution of the moduli fields
appearing in brane world scenarios and discuss the implications for varying
constants.Comment: 5 pages, LaTeX, 5 figures; based on a talk given by C. van de Bruck
at JENAM 2002, Porto; typos correcte
Cosmic D-Strings and Vortons in Supergravity
Recent developments in string inspired models of inflation suggest that
D-strings are formed at the end of inflation. Within the supergravity model of
D-strings there are 2(n-1) chiral fermion zero modes for a D-string of winding
n. Using the bounds on the relic vorton density, we show that D-strings with
winding number n>1 are more strongly constrained than cosmic strings arising in
cosmological phase transitions. The D-string tension of such vortons, if they
survive until the present, has to satisfy 8\pi G_N \mu \lesssim p 10^{-26}
where p is the intercommutation probability. Similarly, D-strings coupled with
spectator fermions carry currents and also need to respect the above bound.
D-strings with n=1 do not carry currents and evade the bound. We discuss the
coupling of D-strings to supersymmetry breaking. When a single U(1) gauge group
is present, we show that there is an incompatibility between spontaneous
supersymmetry breaking and cosmic D-strings. We propose an alternative
mechanism for supersymmetry breaking, which includes an additional U(1), and
might alleviate the problem. We conjecture what effect this would have on the
fermion zero modes.Comment: 11 page
Chameleon Dark Energy
Chameleons are scalar fields whose mass depends on the environment,
specifically on the ambient matter density. While nearly massless in the
cosmos, where the matter density is tiny, their mass is of order of an inverse
millimeter on Earth, where the density is high. In this note, we review how
chameleons can satisfy current experimental constraints on deviations from
General Relativity (GR). Moreover, we study the cosmological evolution with a
chameleon field and show the existence of an attractor solution, akin to the
tracker solution in quintessence models. We discuss how chameleons can
naturally drive the observed acceleration of the universeComment: 5 pages, 2 figures. To appear in the proceedings of the "Phi in the
Sky" conference, 8-10 July 2004, Porto, Portugua
Events and Piecewise Deterministic Dynamics in Event-Enhanced Quantum Theory
We enhance the standard formalism of quantum theory to enable events. The
concepts of experiment and of measurement are defined. Dynamics is given by
Liouville's equation that couples quantum system to a classical one. It implies
a unique Markov process involving quantum jumps, classical events and
describing sample histories of individual systems.Comment: to appear in Phys. Lett. A, 12 pages, Latex article.st
Strongly Coupled Quantum and Classical Systems and Zeno's Effect
A model interaction between a two-state quantum system and a classical
switching device is analysed and shown to lead to the quantum Zeno effect for
large values of the coupling constant k . A minimal piecewise deterministic
random process compatible with the Liouville equation is described, and it is
shown that 1/k can be interpreted as the jump frequency of the classical deviceComment: 9 page
Gauge Coupling Variation in Brane Models
We consider the space-time variation of gauge couplings in brane-world models
induced by the coupling to a bulk scalar field. A variation is generated by the
running of the gauge couplings with energy and a conformal anomaly while going
from the Jordan to the Einstein frame. We indicate that the one-loop
corrections cancel implying that one obtains a variation of the fine structure
constant by either directly coupling the gauge fields to the bulk scalar field
or having bulk scalar field dependent Yukawa couplings. Taking into account the
cosmological dynamics of the bulk scalar field, we constrain the strength of
the gauge coupling dependence on the bulk scalar field and relate it to
modifications of gravity at low energy.Comment: 4 pages, 1 figur
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