461 research outputs found
Designing Cyclic Universe Models
Recent advances in understanding the propagation of perturbations through the
transition from big crunch to big bang (esp. Tolley et al. hep-th/0306109) make
it possible for the first time to consider the full set of phenomenological
constraints on the scalar field potential in cyclic models of the universe. We
show that cyclic models require a comparable degree of tuning to that needed
for inflationary models. The constraints are reduced to a set of simple design
rules including "fast-roll" parameters analogous to the "slow-roll" parameters
in inflation.Comment: 4 pages, 2 figures. Minor typos and figure correcte
Mixmaster Horava-Witten Cosmology
We discuss various superstring effective actions and, in particular, their
common sector which leads to the so-called pre-big-bang cosmology (cosmology in
a weak coupling limit of heterotic superstring). Then, we review the main ideas
of the Horava-Witten theory which is a strong coupling limit of heterotic
superstring theory. Using the conformal relationship between these two theories
we present Kasner asymptotic solutions of Bianchi type IX geometries within
these theories and make predictions about possible emergence of chaos. Finally,
we present a possible method of generating Horava-Witten cosmological solutions
out of the well-known general relativistic pre-big-bang solutions.Comment: 7 pages, 2 figures, based on the talks given at Marcel Grossmann
Meeting IX, Rome 2000 and at "Supersymmetry and Quantum Field Theory" (D.V.
Volkov Memorial), Kharkov 2000, espcrc2.sty include
On the Friedmann Equation in Brane-World Scenarios
The Friedmann law on the brane generically depends quadratically on the brane
energy density and involves a ``dark radiation'' term due to the bulk Weyl
tensor. Despite its unfamiliar form, we show how it can be derived from a
standard four-dimensional Brans-Dicke theory at low energy. In particular, the
dark radiation term is found to depend linearly on the brane energy densities.
For any equation of state on the branes, the radion evolves such as to generate
radiation-dominated cosmology. The radiation-dominated era is conventional and
consistent with nucleosynthesis.Comment: 4 pages. v2,v3: discussion on BBN extended, minor correction
M-Theory Through the Looking Glass: Tachyon Condensation in the E_8 Heterotic String
We study the spacetime decay to nothing in string theory and M-theory. First
we recall a nonsupersymmetric version of heterotic M-theory, in which bubbles
of nothing -- connecting the two E_8 boundaries by a throat -- are expected to
be nucleated. We argue that the fate of this system should be addressed at weak
string coupling, where the nonperturbative instanton instability is expected to
turn into a perturbative tachyonic one. We identify the unique string theory
that could describe this process: The heterotic model with one E_8 gauge group
and a singlet tachyon. We then use worldsheet methods to study the tachyon
condensation in the NSR formulation of this model, and show that it induces a
worldsheet super-Higgs effect. The main theme of our analysis is the
possibility of making meaningful alternative gauge choices for worldsheet
supersymmetry, in place of the conventional superconformal gauge. We show in a
version of unitary gauge how the worldsheet gravitino assimilates the goldstino
and becomes dynamical. This picture clarifies recent results of Hellerman and
Swanson. We also present analogs of R_\xi gauges, and note the importance of
logarithmic CFT in the context of tachyon condensation.Comment: 36 pages, 1 figur
General Covariance in Quantum Gravity at a Lifshitz Point
In the minimal formulation of gravity with Lifshitz-type anisotropic scaling,
the gauge symmetries of the system are foliation-preserving diffeomorphisms of
spacetime. Consequently, compared to general relativity, the spectrum contains
an extra scalar graviton polarization. Here we investigate the possibility of
extending the gauge group by a local U(1) symmetry to "nonrelativistic general
covariance." This extended gauge symmetry eliminates the scalar graviton, and
forces the coupling constant in the kinetic term of the minimal
formulation to take its relativistic value, . The resulting theory
exhibits anisotropic scaling at short distances, and reproduces many features
of general relativity at long distances.Comment: 41 pages; v2: small clarifications, references adde
Uniqueness of static spherically symmetric vacuum solutions in the IR limit of Ho\v{r}ava-Lifshitz gravity
We investigate static spherically symmetric vacuum solutions in the IR limit
of projectable nonrelativistic quantum gravity, including the renormalisable
quantum gravity recently proposed by Ho\v{r}ava. It is found that the
projectability condition plays an important role. Without the cosmological
constant, the spacetime is uniquely given by the Schwarzschild solution. With
the cosmological constant, the spacetime is uniquely given by the Kottler
(Schwarzschild-(anti) de Sitter) solution for the entirely vacuum spacetime.
However, in addition to the Kottler solution, the static spherical and
hyperbolic universes are uniquely admissible for the locally empty region, for
the positive and negative cosmological constants, respectively, if its
nonvanishing contribution to the global Hamiltonian constraint can be
compensated by that from the nonempty or nonstatic region. This implies that
static spherically symmetric entirely vacuum solutions would not admit the
freedom to reproduce the observed flat rotation curves of galaxies. On the
other hand, the result for locally empty regions implies that the IR limit of
nonrelativistic quantum gravity theories does not simply recover general
relativity but includes it.Comment: 10 pages, accepted for publication in International Journal of Modern
Physics
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
Fermion Mass Hierarchy in Lifshitz Type Gauge Theory
We study the origin of fermion mass hierarchy and flavor mixing in a Lifshitz
type extension of the standard model including an extra scalar field. We show
that the hierarchical structure can originate from renormalizable interactions.
In contrast to the Froggatt-Nielsen mechanism, the higher the dimension of
associated operators, the heavier the fermion masses. Tiny masses for
left-handed neutrinos are obtained without introducing right-handed neutrinos.Comment: 13 pages; clarifications of some point
Anisotropic Conformal Infinity
We generalize Penrose's notion of conformal infinity of spacetime, to
situations with anisotropic scaling. This is relevant not only for
Lifshitz-type anisotropic gravity models, but also in standard general
relativity and string theory, for spacetimes exhibiting a natural asymptotic
anisotropy. Examples include the Lifshitz and Schrodinger spaces (proposed as
AdS/CFT duals of nonrelativistic field theories), warped AdS_3, and the
near-horizon extreme Kerr geometry. The anisotropic conformal boundary appears
crucial for resolving puzzles of holographic renormalization in such
spacetimes.Comment: 11 page
The Big Bang as a Phase Transition
We study a five-dimensional cosmological model, which suggests that the
universe bagan as a discontinuity in a (Higgs-type) scalar field, or
alternatively as a conventional four-dimensional phase transition.Comment: 10 pages, 2 figures; typo corrected in equation (18); 1 reference
added; version to appear in International Journal of Modern Physics
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