25 research outputs found
Is Brane Inflation Eternal?
In this paper, we show that eternal inflation of the random walk type is
generically absent in the brane inflationary scenario. Depending on how the
brane inflationary universe originated, eternal inflation of the false vacuum
type is still quite possible. Since the inflaton is the position of the
D3-brane relative to the anti-D3-brane inside the compactified bulk with finite
size, its value is bounded. In DBI inflation, the warped space also restricts
the amplitude of the scalar fluctuation. These upper bounds impose strong
constraints on the possibility of eternal inflation. We find that eternal
inflation due to the random walk of the inflaton field is absent in both the
KKLMMT slow roll scenario and the DBI scenario. A more careful analysis for the
slow-roll case is also presented using the Langevin equation, which gives very
similar results. We discuss possible ways to obtain eternal inflation of the
random walk type in brane inflation. In the multi-throat brane inflationary
scenario, the branes may be generated by quantum tunneling and roll out the
throat. Eternal inflation of the false vacuum type inevitably happens in this
scenario due to the tunneling process. Since these scenarios have different
cosmological predictions, more data from the cosmic microwave background
radiation will hopefully select the specific scenario our universe has gone
through.Comment: 32 pages; v2: references and comments adde
Cosmological Landscape From Nothing: Some Like It Hot
We suggest a novel picture of the quantum Universe -- its creation is
described by the {\em density matrix} defined by the Euclidean path integral.
This yields an ensemble of universes -- a cosmological landscape -- in a mixed
state which is shown to be dynamically more preferable than the pure quantum
state of the Hartle-Hawking type. The latter is dynamically suppressed by the
infinitely large positive action of its instanton, generated by the conformal
anomaly of quantum fields within the cosmological bootstrap (the
self-consistent back reaction of hot matter). This bootstrap suggests a
solution to the problem of boundedness of the on-shell cosmological action and
eliminates the infrared catastrophe of small cosmological constant in Euclidean
quantum gravity. The cosmological landscape turns out to be limited to a
bounded range of the cosmological constant . The domain is ruled out by the
back reaction effect which we analyze by solving effective Euclidean equations
of motion. The upper cutoff is enforced by the quantum effects of vacuum energy
and the conformal anomaly mediated by a special ghost-avoidance renormalization
of the effective action. They establish a new quantum scale
which is determined by the coefficient of the topological Gauss-Bonnet term in
the conformal anomaly. This scale is realized as the upper bound -- the
limiting point of an infinite sequence of garland-type instantons which
constitute the full cosmological landscape. The dependence of the cosmological
constant range on particle phenomenology suggests a possible dynamical
selection mechanism for the landscape of string vacua.Comment: Final version, to appear in JCA
Accelerating universe emergent from the landscape
We propose that the existence of the string landscape suggests the universe
can be in a quantum glass state, where an extremely large viscosity is
generated, and long distance dynamics slows down. At the same time, the short
distance dynamics is not altered due to the separation of time scales. This
scenario can help to understand some controversies in cosmology, for example
the natural existence of slow roll inflation and dark energy in the landscape,
the apparent smallness of the cosmological constant. We see also that moduli
stabilization is no longer necessary. We further identify the glass transition
point, where the viscosity diverges, as the location of the cosmic horizon. We
try to reconstruct the geometry of the accelerating universe from the structure
of the landscape, and find that the metric should have an infinite jump when
crossing the horizon. We predict that the static coordinate metric for dS space
breaks down outside the horizon.Comment: 20 pages, no figures, harvma
A radiation-like era before inflation
We show that the semiclassical approximation to the Wheeler-DeWitt equation
for the minisuperspace of a minimally coupled scalar field in the spatially
flat de Sitter Universe prompts the existence of an initial power-law evolution
driven by non-adiabatic terms from the gravitational wavefunction which act
like radiation. This simple model hence describes the onset of inflation from a
previous radiation-like expansion during which the cosmological constant is
already present but subleading.Comment: LaTeX, 8 pages, no figures; final version to be published in JCA
Exact spectrum of scalar field perturbations in a radiation deformed closed de Sitter universe
We observe that the equation of motion for a free scalar field in a closed
universe with radiation and a positive cosmological constant is given by
Lam\'e's equation. Computing the exact power spectrum of scalar field
perturbations, the presence of both curvature and radiation produces a red tilt
weakly dependent on the amount of radiation.Comment: 16 pages, 2 figure
Inflation as a probe of new physics
In this paper we consider inflation as a probe of new physics near the string
or Planck scale. We discuss how new physics can be captured by the choice of
vacuum, and how this leads to modifications of the primordial spectrum as well
as the way in which the universe expands during inflation. Provided there is a
large number of fields contributing to the vacuum energy -- as typically is
expected in string theory -- we will argue that both types of effects can be
present simultaneously and be of observational relevance. Our conclusion is
that the ambiguity in choice of vacuum is an interesting new parameter in
serious model building.Comment: 14 page