533 research outputs found

### A Note on Quantum Field Theories with a Minimal Length Scale

The aim of this note is to address the low energy limit of quantum field
theories with a minimal length scale. The essential feature of these models is
that the minimal length acts as a regulator in the asymptotic high energy limit
which is incorporated through an infinite series of higher order derivatives.
If one investigates a perturbative expansion in inverse powers of the Planck
mass, one generically obtains extra poles in the propagator, and instabilities
connected with the higher order derivative Lagrangian, that are however
artifacts of truncating the series

### Inflation from Geometrical Tachyons

We propose an alternative formulation of tachyon inflation using the
geometrical tachyon arising from the time dependent motion of a BPS $D3$-brane
in the background geometry due to $k$ parallel $NS$5-branes arranged around a
ring of radius $R$. Due to the fact that the mass of this geometrical tachyon
field is $\sqrt{2/k}$ times smaller than the corresponding open-string tachyon
mass, we find that the slow roll conditions for inflation and the number of
e-foldings can be satisfied in a manner that is consistent with an effective
4-dimensional model and with a perturbative string coupling. We also show that
the metric perturbations produced at the end of inflation can be sufficiently
small and do not lead to the inconsistencies that plague the open string
tachyon models. Finally we argue for the existence of a minimum of the
geometrical tachyon potential which could give rise to a traditional reheating
mechanism.Comment: Latex, 20 pages, 4 figures; correction of algebraic errors in section
5 concerning the tachyon potential near its minimum. Conclusions unchange

### Reheating the Universe After Multi-Field Inflation

We study in detail (p)reheating after multi-field inflation models with a
particular focus on N-flation. We consider a variety of different couplings
between the inflatons and the matter sector, including both quartic and
trilinear interactions with a light scalar field. We show that the presence of
multiple oscillating inflatons makes parametric resonance inefficient in the
case of the quartic interactions. Moreover, perturbative processes do not
permit a complete decay of the inflaton for this coupling. In order to recover
the hot big bang, we must instead consider trilinear couplings. In this case we
show that strong nonperturbative preheating is possible via multi-field
tachyonic resonance. In addition, late-time perturbative effects do permit a
complete decay of the condensate. We also study the production of gauge fields
for several prototype couplings, finding similar results to the trilinear
scalar coupling. During the course of our analysis we develop the mathematical
theory of the quasi-periodic Mathieu equation, the multi-field generalization
of the Floquet theory familiar from preheating after single field inflation. We
also elaborate on the theory of perturbative decays of a classical inflaton
condensate, which is applicable in single-field models also.Comment: 46+1 pages, 19 figure

### Tachyon Defect Formation and Reheating in Brane-Antibrane Inflation

We study analytically the dynamical formation of lower dimensional branes at
the endpoint of brane-antibrane inflation through the condensation of
topological defects of the tachyon field which describes the instability of the
initial state. We then use this information to quantify the efficiency of the
reheating which is due to the coupling of time dependent tachyon background to
massless gauge fields which will be localized on the final state branes. We
improve upon previous estimates indicating that this can be an efficient
reheating mechanism for observers on the brane.Comment: 9 pages. Talk given at the 26th annual
Montreal-Rochester-Syracuse-Toronto Conference on High-Energy Physics: MRST
200

### Fisher Equation for a Decaying Brane

We consider the inhomogeneous decay of an unstable D-brane. The dynamical
equation that describes this process (in light-cone time) is a variant of the
non-linear reaction-diffusion equation that first made its appearance in the
pioneering work of (Luther and) Fisher and appears in a variety of natural
phenomena.Comment: 8 pages, 4 figure

### Tachyon cosmology with non-vanishing minimum potential: a unified model

We investigate the tachyon condensation process in the effective theory with
non-vanishing minimum potential and its implications to cosmology. It is shown
that the tachyon condensation on an unstable three-brane described by this
modified tachyon field theory leads to lower-dimensional branes (defects)
forming within a stable three-brane. Thus, in the cosmological background, we
can get well-behaved tachyon matter after tachyon inflation, (partially)
avoiding difficulties encountered in the original tachyon cosmological models.
This feature also implies that the tachyon inflated and reheated universe is
followed by a Chaplygin gas dark matter and dark energy universe. Hence, such
an unstable three-brane behaves quite like our universe, reproducing the key
features of the whole evolutionary history of the universe and providing a
unified description of inflaton, dark matter and dark energy in a very simple
single-scalar field model.Comment: 18 p

### Phenomenology of a Pseudo-Scalar Inflaton: Naturally Large Nongaussianity

Many controlled realizations of chaotic inflation employ pseudo-scalar
axions. Pseudo-scalars \phi are naturally coupled to gauge fields through c
\phi F \tilde{F}. In the presence of this coupling, gauge field quanta are
copiously produced by the rolling inflaton. The produced gauge quanta, in turn,
source inflaton fluctuations via inverse decay. These new cosmological
perturbations add incoherently with the "vacuum" perturbations, and are highly
nongaussian. This provides a natural mechanism to generate large nongaussianity
in single or multi field slow-roll inflation. The resulting phenomenological
signatures are highly distinctive: large nongaussianity of (nearly) equilateral
shape, in addition to detectably large values of both the scalar spectral tilt
and tensor-to-scalar ratio (both being typical of large field inflation). The
WMAP bound on nongaussianity implies that the coupling, c, of the pseudo-scalar
inflaton to any gauge field must be smaller than about 10^{2} M_p^{-1}.Comment: 45 pages, 7 figure

### Dynamics with Infinitely Many Derivatives: The Initial Value Problem

Differential equations of infinite order are an increasingly important class
of equations in theoretical physics. Such equations are ubiquitous in string
field theory and have recently attracted considerable interest also from
cosmologists. Though these equations have been studied in the classical
mathematical literature, it appears that the physics community is largely
unaware of the relevant formalism. Of particular importance is the fate of the
initial value problem. Under what circumstances do infinite order differential
equations possess a well-defined initial value problem and how many initial
data are required? In this paper we study the initial value problem for
infinite order differential equations in the mathematical framework of the
formal operator calculus, with analytic initial data. This formalism allows us
to handle simultaneously a wide array of different nonlocal equations within a
single framework and also admits a transparent physical interpretation. We show
that differential equations of infinite order do not generically admit
infinitely many initial data. Rather, each pole of the propagator contributes
two initial data to the final solution. Though it is possible to find
differential equations of infinite order which admit well-defined initial value
problem with only two initial data, neither the dynamical equations of p-adic
string theory nor string field theory seem to belong to this class. However,
both theories can be rendered ghost-free by suitable definition of the action
of the formal pseudo-differential operator. This prescription restricts the
theory to frequencies within some contour in the complex plane and hence may be
thought of as a sort of ultra-violet cut-off.Comment: 40 pages, no figures. Added comments concerning fractional operators
and the implications of restricting the contour of integration. Typos
correcte

### Warming up brane-antibrane inflation

We show that, in constructions with additional intersecting D-branes,
brane-antibrane inflation may naturally occur in a warm regime, such that
strong dissipative effects damp the inflaton's motion, greatly alleviating the
associated eta-problem. We illustrate this for D3-antiD3 inflation in flat
space with additional flavor D7-branes, where for both a Coulomb-like or a
quadratic hybrid potential a sufficient number of e-folds may be obtained for
perturbative couplings and O(10-10^4) branes. This is in clear contrast with
the corresponding cold scenarios, thus setting the stage for more realistic
constructions within fully stabilized compactifications. Such models
generically predict a negligible amount of tensor perturbations and
non-gaussianity f_NL \sim O(10).Comment: 8 pages, 2 figures; version to be published in Physical Review

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