2,759 research outputs found
Dark energy cosmologies for codimension-two branes
A six-dimensional universe with two branes in the "football-shaped" geometry
leads to an almost realistic cosmology. We describe a family of exact solutions
with time dependent characteristic size of internal space. After a short
inflationary period the late cosmology is either of quintessence type or turns
to a radiation dominated Friedmann universe where the cosmological constant
appears as a free integration constant of the solution. The radiation dominated
universe with relativistic fermions is analyzed in detail, including its
dimensional reduction.Comment: 18 page
Evolution equations for the effective four-quark interactions in QCD
A nonperturbative renormalization group equation describes how the momentum
dependent four-quark vertex depends on an infrared cutoff. We find a quasilocal
one-particle irreducible piece generated by (anomaly-free) multi-gluon
exchange. It becomes important at a cutoff scale where scalar and pseudoscalar
meson-bound states are expected to play a role. This interaction remains
subleading as compared to the effective one-gluon exchange contribution. The
local instanton induced four-quark interaction becomes dominant at a scale
around 800 MeV. In absence of a gluon mass the strong dependence of the
one-gluon exchange on the transferred momentum indicates that the pointlike
interactions of the Nambu-Jona-Lasinio model cannot give a very accurate
description of QCD. A pointlike effective four-quark interaction becomes more
realistic in case of spontaneous color symmetry breaking.Comment: 24 pages, LaTeX file, + 6 PS figure
Conformal fixed point, Cosmological Constant and Quintessence
We connect a possible solution for the ``cosmological constant problem'' to
the existence of a (postulated) conformal fixed point in a fundamental theory.
The resulting cosmology leads to quintessence, where the present acceleration
of the expansion of the universe is linked to a crossover in the flow of
coupling constants.Comment: More detailed discussion of quantum fluctuations,update with
WMAP-data,4 pages,LaTe
Universality of geometry
In models of emergent gravity the metric arises as the expectation value of
some collective field. Usually, many different collective fields with
appropriate tensor properties are candidates for a metric. Which collective
field describes the "physical geometry"? We resolve this "metric ambiguity" by
an investigation of the most general form of the quantum effective action for
several metrics. In the long-distance limit the physical metric is universal
and accounts for a massless graviton. Other degrees of freedom contained in the
various metric candidates describe very massive scalars and symmetric second
rank tensors. They only play a role at microscopic distances, typically around
the Planck length. The universality of geometry at long distances extends to
the vierbein and the connection. On the other hand, for distances and time
intervals of Planck size geometry looses its universal meaning. Time is born
with the big bang.Comment: 6 page
Spinor Gravity
A unified description of all interactions could be based on a
higher-dimensional theory involving only spinor fields. The metric arises as a
composite object and the gravitational field equations contain
torsion-corrections as compared to Einstein gravity. Lorentz symmetry in spinor
space is only global, implying new goldstone-boson-like gravitational particles
beyond the graviton. However, the Schwarzschild and Friedman solutions are
unaffected at one loop order. Our generalized gravity seems compatible with all
present observations.Comment: 6 pages, LaTeX, v3: extended discussion and new reference
On the origin of the difference between time and space
We suggest that the difference between time and space is due to spontaneous
symmetry breaking. In a theory with spinors the signature of the metric is
related to the signature of the Lorentz-group. We discuss a higher symmetry
that contains pseudo-orthogonal groups with arbitrary signature as subgroups.
The fundamental asymmetry between time and space arises then as a property of
the ground state rather than being put into the formulation of the theory a
priori. We show how the complex structure of quantum field theory as well as
gravitational field equations arise from spinor gravity - a fundamental spinor
theory without a metric.Comment: 4 page
Quantum particles from classical statistics
Quantum particles and classical particles are described in a common setting
of classical statistical physics. The property of a particle being "classical"
or "quantum" ceases to be a basic conceptual difference. The dynamics differs,
however, between quantum and classical particles. We describe position, motion
and correlations of a quantum particle in terms of observables in a classical
statistical ensemble. On the other side, we also construct explicitly the
quantum formalism with wave function and Hamiltonian for classical particles.
For a suitable time evolution of the classical probabilities and a suitable
choice of observables all features of a quantum particle in a potential can be
derived from classical statistics, including interference and tunneling.
Besides conceptual advances, the treatment of classical and quantum particles
in a common formalism could lead to interesting cross-fertilization between
classical statistics and quantum physics.Comment: New material on quantum formalism and wave function for classical
particles. Part of previous material is moved to a separate paper. 25 page
Holographic Branes
We discuss the properties of codimension-two branes and compare them to
codimension-one branes. In particular, we show that for deficit angle branes
the brane energy momentum tensor is uniquely related to integration constants
in the bulk solution. We investigate chiral fermions whose wave functions are
concentrated on the brane, while all their properties in the effective
four-dimensional world can be inferred from the tail of the wave function in
the bulk, thereby realizing a holographic principle. We propose holographic
branes for which the knowledge of the bulk geometry is sufficient for the
computation of all relevant properties of the observable particles,
independently of the often unknown detailed physics of the branes.Comment: 13 pages, references adde
Dilatation symmetry in higher dimensions and the vanishing of the cosmological constant
A wide class of dilatation symmetric effective actions in higher dimensions
leads to a vanishing four-dimensional cosmological constant. This requires no
tuning of parameters and results from the absence of an allowed potential for
the scalar dilaton field. The field equations admit many solutions with flat
four-dimensional space and non-vanishing gauge couplings. In a more general
setting, these are candidates for asymptotic states of cosmological runaway
solutions, where dilatation symmetry is realized dynamically if a fixed point
is approached as time goes to infinity. Dilatation anomalies during the runaway
can lift the degeneracy of solutions and lead to an observable dynamical dark
energy.Comment: 4 page
Zwitters: particles between quantum and classical
We describe both quantum particles and classical particles in terms of a
classical statistical ensemble, characterized by a probability distribution in
phase space. By use of a wave function in phase space both can be treated in
the same quantum formalism. The different dynamics of quantum and classical
particles resides then only from different evolution equations for the
probability distribution. Quantum particles are characterized by a specific
choice of observables and time evolution of the probability density. All
relations for a quantum particle in a potential, including interference and
tunneling, can be described in terms of the classical probability distribution.
We formulate the concept of zwitters - particles for which the time evolution
interpolates between quantum and classical particles. Experiments can test a
small parameter which quantifies possible deviations from quantum mechanics.Comment: extended discussion of possible realizations of zwitters, including
macroscopic droplets or BEC condensate
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