13,412 research outputs found

### Symmetric Vacua in Heterotic M-Theory

Symmetric vacua of heterotic M-theory, characterized by vanishing cohomology
classes of individual sources in the three-form Bianchi identity, are analyzed
on smooth Calabi-Yau three-folds. We show that such vacua do not exist for
elliptically fibered Calabi-Yau spaces. However, explicit examples are found
for Calabi-Yau three-folds arising as intersections in both unweighted and
weighted projective space. We show that such symmetric vacua can be combined
with attractive phenomenological features such as three generations of quarks
and leptons. Properties of the low energy effective actions associated with
symmetric vacua are discussed. In particular, the gauge kinetic functions
receive no perturbative threshold corrections, there are no corrections to the
matter field Kahler metric and the associated five-dimensional effective theory
admits flat space as its vacuum.Comment: 22 pages, Late

### Moving Five-Branes in Low-Energy Heterotic M-Theory

We construct cosmological solutions of four-dimensional effective heterotic
M-theory with a moving five-brane and evolving dilaton and T modulus. It is
shown that the five-brane generates a transition between two asymptotic
rolling-radii solutions. Moreover, the five-brane motion always drives the
solutions towards strong coupling asymptotically. We present an explicit
example of a negative-time branch solution which ends in a brane collision
accompanied by a small-instanton transition. The five-dimensional origin of
some of our solutions is also discussed.Comment: 16 pages, Latex, 3 eps figure

### Cosmological Solutions of Horava-Witten Theory

We discuss simple cosmological solutions of Horava-Witten theory describing
the strongly coupled heterotic string. At energies below the grand-unified
scale, the effective theory is five- not four-dimensional, where the additional
coordinate parameterizes a S^1/Z_2 orbifold. Furthermore, it admits no
homogeneous solutions. Rather, the vacuum state, appropriate for a reduction to
four-dimensional supersymmetric models, is a BPS domain wall. Relevant
cosmological solutions are those associated with this BPS state. In particular,
such solutions must be inhomogeneous, depending on the orbifold coordinate as
well as on time. We present two examples of this new type of cosmological
solution, obtained by separation of variables rather that by exchange of time
and radius coordinate applied to a brane solution, as in previous work. The
first example represents the analog of a rolling radii solution with the radii
specifying the geometry of the domain wall. This is generalized in the second
example to include a nontrivial ``Ramond-Ramond'' scalar.Comment: 21 pages, Latex 2e with amsmath, minor addition

### U-duality Covariant M-theory Cosmology

A manifestly U-duality covariant approach to M-theory cosmology is developed
and applied to cosmologies in dimensions D=4,5. Cosmological properties such as
expansion powers and Hubble parameters turn out to be U-duality invariant in
certain asymptotic regions. U-duality transformations acting on cosmological
solutions, on the other hand, shift the transition time between two asymptotic
regions and determine the details of the transition. Moreover, in D=5, we show
that U-duality can map expanding negative and positive branch solutions into
each other.Comment: 18 pages, LATEX, 1 Postscript figure include

### Decaying Cosmological Constant of the Inflating Branes in the Randall-Sundrum -Oda Model

We examine the issue of the cosmological constant in the $many$ $inflating$
branes scenario, extending on two recent models by I.Oda and Randall-Sundrum.
The exact solution in a closed form is found in the slow roll approximation of
the radion. Defining an effective expansion rate, which depends on the location
of each brane in the fifth dimension and demanding stability for this case we
show that each positive tension brane has a localized, decaying cosmological
constant (the opposite process applies to the negative energy branes [4]) . The
reason is that the square of the effective expansion rate enters as a source
term in the Einstein equations for the branes.Thus the brane has two scale
factors depending on time and the fifth dimnesion respectively .The brane will
roll along the fifth dimension in order to readjust its effective expansion
rate in such a way that it compensates for its internal energy changes due to
inflation and possible phase transitions.Comment: 9 pages, comments and ref.added, solution replaced with the exact
one, submitted to PR

### Heterotic M-Theory Cosmology in Four and Five Dimensions

We study rolling radii solutions in the context of the four- and
five-dimensional effective actions of heterotic M-theory. For the standard
four-dimensional solutions with varying dilaton and T-modulus, we find
approximate five-dimensional counterparts. These are new, generically
non-separating solutions corresponding to a pair of five-dimensional domain
walls evolving in time. Loop corrections in the four-dimensional theory are
described by certain excitations of fields in the fifth dimension. We point out
that the two exact separable solutions previously discovered are precisely the
special cases for which the loop corrections are time-independent. Generically,
loop corrections vary with time. Moreover, for a subset of solutions they
increase in time, evolving into complicated, non-separating solutions. In this
paper we compute these solutions to leading, non-trivial order. Using the
equations for the induced brane metric, we present a general argument showing
that the accelerating backgrounds of this type cannot evolve smoothly into
decelerating backgrounds.Comment: 15 pages, Latex, 1 eps figur

### Homotopy Structure of 5d Vacua

It is shown that flat zero-energy solutions (vacua) of the 5d Kaluza-Klein
theory admit a non-trivial homotopy structure generated by certain Kaluza-Klein
excitations. These vacua consist of an infinite set of homotopically different
spacetimes denoted by $\mathcal{M}^{(n)}_5$, among which $\mathcal{M}^{(0)}_5$
and $\mathcal{M}^{(1)}_5$ are especially identified as $M_{4} \times S^{1}$ and
$M_5$, the ground states of the 5d Kaluza-Klein theory and the 5d general
relativity, respectively (where $M_k$ represents the $k$-dimensional Minkowski
space).Comment: 8 page

### Kink-boundary collisions in a two dimensional scalar field theory

In a two-dimensional toy model, motivated from five-dimensional heterotic
M-theory, we study the collision of scalar field kinks with boundaries. By
numerical simulation of the full two-dimensional theory, we find that the kink
is always inelastically reflected with a model-independent fraction of its
kinetic energy converted into radiation. We show that the reflection can be
analytically understood as a fluctuation around the scalar field vacuum. This
picture suggests the possibility of spontaneous emission of kinks from the
boundary due to small perturbations in the bulk. We verify this picture
numerically by showing that the radiation emitted from the collision of an
initial single kink eventually leads to a bulk populated by many kinks.
Consequently, processes changing the boundary charges are practically
unavoidable in this system. We speculate that the system has a universal final
state consisting of a stack of kinks, their number being determined by the
initial energy

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