5,558 research outputs found

### Origin of FRW cosmology in slow-roll inflation from noncompact Kaluza-Klein theory

Using a recently introduced formalism we discuss slow-roll inflaton from
Kaluza-Klein theory without the cylinder condition. In particular, some
examples corresponding to polynomic and hyperbolic $\phi$-potentials are
studied. We find that the evolution of the fifth coordinate should be
determinant for both, the evolution of the early inflationary universe and the
quantum fluctuations.Comment: (final version) to be published in EPJ

### Energy-Momentum Restrictions on the Creation of Gott Time Machines

The discovery by Gott of a remarkably simple spacetime with closed timelike
curves (CTC's) provides a tool for investigating how the creation of time
machines is prevented in classical general relativity. The Gott spacetime
contains two infinitely long, parallel cosmic strings, which can equivalently
be viewed as point masses in (2+1)-dimensional gravity. We examine the
possibility of building such a time machine in an open universe. Specifically,
we consider initial data specified on an edgeless, noncompact, spacelike
hypersurface, for which the total momentum is timelike (i.e., not the momentum
of a Gott spacetime). In contrast to the case of a closed universe (in which
Gott pairs, although not CTC's, can be produced from the decay of stationary
particles), we find that there is never enough energy for a Gott-like time
machine to evolve from the specified data; it is impossible to accelerate two
particles to sufficiently high velocity. Thus, the no-CTC theorems of Tipler
and Hawking are enforced in an open (2+1)-dimensional universe by a mechanism
different from that which operates in a closed universe. In proving our result,
we develop a simple method to understand the inequalities that restrict the
result of combining momenta in (2+1)-dimensional gravity.Comment: Plain TeX, 41 pages incl. 9 figures. MIT-CTP #225

### A Relativistic Description of Gentry's New Redshift Interpretation

We obtain a new expression of the Friedmann-Robertson-Walker metric, which is
an analogue of a static chart of the de Sitter space-time. The reduced metric
contains two functions, $M(T,R)$ and $\Psi(T,R)$, which are interpreted as,
respectively, the mass function and the gravitational potential. We find that,
near the coordinate origin, the reduced metric can be approximated in a static
form and that the approximated metric function, $\Psi(R)$, satisfies the
Poisson equation. Moreover, when the model parameters of the
Friedmann-Robertson-Walker metric are suitably chosen, the approximated metric
coincides with exact solutions of the Einstein equation with the perfect fluid
matter. We then solve the radial geodesics on the approximated space-time to
obtain the distance-redshift relation of geodesic sources observed by the
comoving observer at the origin. We find that the redshift is expressed in
terms of a peculiar velocity of the source and the metric function, $\Psi(R)$,
evaluated at the source position, and one may think that this is a new
interpretation of {\it Gentry's new redshift interpretation}.Comment: 11 pages. Submitted to Modern Physics Letters

### Towards Resolution of Hierarchy Problems in a Cosmological Context

A cosmological scenario is proposed, which simultaneously solves the mass
hierarchy and the small dark energy problem. In the present scenario an
effective gravity mass scale (inverse of the Newton's constant) increases
during the inflationary period. The small cosmological constant or the dark
energy density in the present universe is dynamically realized by introducing
two, approximately O(2) symmetric dilatons, taking the fundamental mass scale
at TeV.Comment: 12 pages, no figur

### Geometry and Destiny

The recognition that the cosmological constant may be non-zero forces us to
re-evaluate standard notions about the connection between geometry and the fate
of our Universe. An open Universe can recollapse, and a closed Universe can
expand forever. As a corollary, we point out that there is no set of
cosmological observations we can perform that will unambiguously allow us to
determine what the ultimate destiny of the Universe will be.Comment: 7 pages, Gravity Research Foundation Essa

### Post-Newtonian expansion for Gauss-Bonnet Gravity

The Parametrized Post-Newtonian expansion of gravitational theories with a
scalar field coupled to the Gauss-Bonnet invariant is performed and
confrontation of such theories with Solar system experiments is discussed.Comment: 4 pages; typos corrected, published versio

### Persistent Topology of Syntax

We study the persistent homology of the data set of syntactic parameters of
the world languages. We show that, while homology generators behave erratically
over the whole data set, non-trivial persistent homology appears when one
restricts to specific language families. Different families exhibit different
persistent homology. We focus on the cases of the Indo-European and the
Niger-Congo families, for which we compare persistent homology over different
cluster filtering values. We investigate the possible significance, in
historical linguistic terms, of the presence of persistent generators of the
first homology. In particular, we show that the persistent first homology
generator we find in the Indo-European family is not due (as one might guess)
to the Anglo-Norman bridge in the Indo-European phylogenetic network, but is
related to the position of Ancient Greek and the Hellenic branch within the
network.Comment: 15 pages, 25 jpg figure

### Metric fluctuations and its evolution during inflation

W discuss the evolution of the fluctuations in a symmetric
$\phi_c$-exponential potential which provides a power-law expansion during
inflation using both, the gauge invariant field $\Phi$ and the Sasaki-Mukhanov
field.Comment: version accepted in EPJ

### Large Scale Inhomogeneities from the QCD Phase Transition

We examine the first-order cosmological QCD phase transition for a large
class of parameter values, previously considered unlikely. We find that the
hadron bubbles can nucleate at very large distance scales, they can grow as
detonations as well as deflagrations, and that the phase transition may be
completed without reheating to the critical temperature. For a subset of the
parameter values studied, the inhomogeneities generated at the QCD phase
transition might have a noticeable effect on nucleosynthesis.Comment: 15 LaTeX pages + 6 PostScript figures appended at the end of the
file, HU-TFT-94-1

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