7,660 research outputs found

### Restoring Time Dependence into Quantum Cosmology

Mini superspace cosmology treats the scale factor $a(t)$, the lapse function
$n(t)$, and an optional dilation field $\phi(t)$ as canonical variables. While
pre-fixing $n(t)$ means losing the Hamiltonian constraint, pre-fixing $a(t)$ is
serendipitously harmless at this level. This suggests an alternative to the
Hartle-Hawking approach, where the pre-fixed $a(t)$ and its derivatives are
treated as explicit functions of time, leaving $n(t)$ and a now mandatory
$\phi(t)$ to serve as canonical variables. The naive gauge pre-fix $a(t)=const$
is clearly forbidden, causing evolution to freeze altogether, so pre-fixing the
scale factor, say $a(t)=t$, necessarily introduces explicit time dependence
into the Lagrangian. Invoking Dirac's prescription for dealing with
constraints, we construct the corresponding mini superspace time dependent
total Hamiltonian, and calculate the Dirac brackets, characterized by
$\{n,\phi\}_D\neq 0$, which are promoted to commutation relations in the
quantum theory.Comment: Honorable Mentioned essay - Gravity Research Foundation 201

### A Toy Model for Open Inflation

The open inflation scenario based on the theory of bubble formation in the
models of a single scalar field suffered from a fatal defect. In all the
versions of this scenario known so far, the Coleman-De Luccia instantons
describing the creation of an open universe did not exist. We propose a simple
one-field model where the CDL instanton does exist and the open inflation
scenario can be realized.Comment: 7 pages, 4 figures, revtex, a discussion of density perturbations is
extende

### False Vacuum Chaotic Inflation: The New Paradigm?

Recent work is reported on inflation model building in the context of
supergravity and superstrings, with special emphasis on False Vacuum (`Hybrid')
Chaotic Inflation. Globally supersymmetric models do not survive in generic
supergravity theories, but fairly simple conditions can be formulated which do
ensure successful supergravity inflation. The conditions are met in some of the
versions of supergravity that emerge from superstrings.Comment: 4 pages, LATEX, LANCASTER-TH 94-1

### Unambiguous probabilities in an eternally inflating universe

``Constants of Nature'' and cosmological parameters may in fact be variables
related to some slowly-varying fields. In models of eternal inflation, such
fields will take different values in different parts of the universe. Here I
show how one can assign probabilities to values of the ``constants'' measured
by a typical observer. This method does not suffer from ambiguities previously
discussed in the literature.Comment: 7 pages, Final version (minor changes), to appear in Phys. Rev. Let

### Realizations of Hybrid Inflation in Supergravity with natural initial conditions

We present viable F-term realizations of the hybrid inflationary scenario in
the context of supergravity addressing at the same time the well-known problems
of the initial conditions and of the adequate suppression of the inflaton mass.
An essential role in our construction is played by "decoupled" superheavy
fields without superpotential acquiring large vevs due to D-terms associated
with "anomalous" U(1) gauge symmetries. The naturalness of the initial
conditions is achieved through a "chaotic" inflation starting at energies close
to the Planck scale and driven by the "anomalous" D-terms. We discuss two
distinct mechanisms leading to such an early "chaotic" D-term inflation which
depend on the choice of the K\"ahler potential involving the superheavy fields.
The one relies on a choice of the K\"ahler potential of the $SU(1,1)/U(1)$
K\"ahler manifold of the type encountered in no-scale supergravity whereas the
other employs a more "conventional" choice for the K\"ahler potential of the
$SU(1,1)/U(1)$ or $SU(2)/U(1)$ K\"ahler manifold but invokes rather specific
valuesof the Fayet-Iliopoulos $\xi$ term.Comment: 32 pages LATEX, no figure

### Predictability crisis in inflationary cosmology and its resolution

Models of inflationary cosmology can lead to variation of observable
parameters ("constants of Nature") on extremely large scales. The question of
making probabilistic predictions for today's observables in such models has
been investigated in the literature. Because of the infinite thermalized volume
resulting from eternal inflation, it has proven difficult to obtain a
meaningful and unambiguous probability distribution for observables, in
particular due to the gauge dependence. In the present paper, we further
develop the gauge-invariant procedure proposed in a previous work for models
with a continuous variation of "constants". The recipe uses an unbiased
selection of a connected piece of the thermalized volume as sample for the
probability distribution. To implement the procedure numerically, we develop
two methods applicable to a reasonably wide class of models: one based on the
Fokker-Planck equation of stochastic inflation, and the other based on direct
simulation of inflationary spacetime. We present and compare results obtained
using these methods.Comment: 23 pages, 13 figure

### Hunting Down the Best Model of Inflation with Bayesian Evidence

We present the first calculation of the Bayesian evidence for different
prototypical single field inflationary scenarios, including representative
classes of small field and large field models. This approach allows us to
compare inflationary models in a well-defined statistical way and to determine
the current "best model of inflation". The calculation is performed numerically
by interfacing the inflationary code FieldInf with MultiNest. We find that
small field models are currently preferred, while large field models having a
self-interacting potential of power p>4 are strongly disfavoured. The class of
small field models as a whole has posterior odds of approximately 3:1 when
compared with the large field class. The methodology and results presented in
this article are an additional step toward the construction of a full numerical
pipeline to constrain the physics of the early Universe with astrophysical
observations. More accurate data (such as the Planck data) and the techniques
introduced here should allow us to identify conclusively the best inflationary
model.Comment: 12 pages, 2 figures, uses RevTeX. Misprint corrected, references
added. Matches published versio

### Towards a gauge invariant volume-weighted probability measure for eternal inflation

An improved volume-weighted probability measure for eternal inflation is
proposed. For the models studied in this paper it leads to simple and
intuitively expected gauge-invariant results.Comment: 16 pages, 3 figs, few misprints corrected, comments adde

### Pre-Big-Bang Requires the Universe to be Exponentially Large From the Very Beginning

We show that in a generic case of the pre-big-bang scenario, inflation will
solve cosmological problems only if the universe at the onset of inflation is
extremely large and homogeneous from the very beginning. The size of a
homogeneous part of the universe at the beginning of the stage of pre-big-bang
(PBB) inflation must be greater than $10^{19}$ $l_s$, where $l_s$ is the
stringy length. The total mass of an inflationary domain must be greater than
$10^{72} M_{s}$, where $M_{s} \sim l_s^{-1}$. If the universe is initially
radiation dominated, then its total entropy at that time must be greater than
$10^{68}$. If the universe is closed, then at the moment of its formation it
must be uniform over $10^{24}$ causally disconnected domains. The natural
duration of the PBB stage in this scenario is $M_p^{-1}$. We argue that the
initial state of the open PBB universe could not be homogeneous because of
quantum fluctuations. Independently of the issue of homogeneity, one must
introduce two large dimensionless parameters, $g_0^{-2} > 10^{53}$, and $B >
10^{91}$, in order to solve the flatness problem in the PBB cosmology. A regime
of eternal inflation does not occur in the PBB scenario. This should be
compared with the simplest versions of the chaotic inflation scenario, where
the regime of eternal inflation may begin in a universe of size $O(M_{p}^{-1})$
with vanishing initial radiation entropy, mass $O(M_p)$, and geometric entropy
O(1). We conclude that the current version of the PBB scenario cannot replace
usual inflation even if one solves the graceful exit problem in this scenario.Comment: 14 pages, a discussion of the flatness problem in the PBB cosmology
is adde

### Dynamical renormalization group methods in theory of eternal inflation

Dynamics of eternal inflation on the landscape admits description in terms of
the Martin-Siggia-Rose (MSR) effective field theory that is in one-to-one
correspondence with vacuum dynamics equations. On those sectors of the
landscape, where transport properties of the probability measure for eternal
inflation are important, renormalization group fixed points of the MSR
effective action determine late time behavior of the probability measure. I
argue that these RG fixed points may be relevant for the solution of the gauge
invariance problem for eternal inflation.Comment: 11 pages; invited mini-review for Grav.Cos

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