14 research outputs found
Cosmic Acceleration, Dark Energy and Fundamental Physics
A web of interlocking observations has established that the expansion of the
Universe is speeding up and not slowing, revealing the presence of some form of
repulsive gravity. Within the context of general relativity the cause of cosmic
acceleration is a highly elastic (p\sim -rho), very smooth form of energy
called ``dark energy'' accounting for about 75% of the Universe. The
``simplest'' explanation for dark energy is the zero-point energy density
associated with the quantum vacuum; however, all estimates for its value are
many orders-of-magnitude too large. Other ideas for dark energy include a very
light scalar field or a tangled network of topological defects. An alternate
explanation invokes gravitational physics beyond general relativity.
Observations and experiments underway and more precise cosmological
measurements and laboratory experiments planned for the next decade will test
whether or not dark energy is the quantum energy of the vacuum or something
more exotic, and whether or not general relativity can self consistently
explain cosmic acceleration. Dark energy is the most conspicuous example of
physics beyond the standard model and perhaps the most profound mystery in all
of science.Comment: 10 pages, 8 figures, invited review for Journal of the Physical
Society of Japan, in pres
Decay of the free-theory vacuum of scalar field theory in de Sitter spacetime in the interaction picture
A free-theory vacuum state of an interacting field theory, e.g. quantum
gravity, is unstable at tree level in general due to spontaneous emission of
Fock-space particles in any spacetime with no global timelike Killing vectors,
such as de Sitter spacetime, in the interaction picture. As an example, the
rate of spontaneous emission of Fock-space particles is calculated in phi^4
theory in de Sitter spacetime. It is possible that this apparent spontaneous
emission does not correspond to any physical processes because the states are
not evolved by the true Hamiltonian in the interaction picture. Nevertheless,
the constant spontaneous emission of Fock-space particles in the interaction
picture clearly demonstrates that the in- and out-vacuum states are orthogonal
to each other as emphasized by Polyakov and that the in-out perturbation
theory, which presupposes some overlap between these two vacuum states, is
inadequate. Other possible implications of apparent vacuum instability of this
kind in the interaction picture are also discussed.Comment: title changed, 7 page
New Constraints from High Redshift Supernovae and Lensing Statistics upon Scalar Field Cosmologies
We explore the implications of gravitationally lensed QSOs and high-redshift
SNe Ia observations for spatially flat cosmological models in which a
classically evolving scalar field currently dominates the energy density of the
Universe. We consider two representative scalar field potentials that give rise
to effective decaying (``quintessence'') models:
pseudo-Nambu-Goldstone bosons () and an inverse
power-law potential (). We show that a
large region of parameter space is consistent with current data if . On the other hand, a higher lower bound for the matter density
parameter suggested by large-scale galaxy flows, ,
considerably reduces the allowed parameter space, forcing the scalar field
behavior to approach that of a cosmological constant.Comment: 6 pages, 2 figures, submitted to PR
Probing Dark Energy with Supernovae: Exploiting Complementarity with the Cosmic Microwave Background
A primary goal for cosmology and particle physics over the coming decade will
be to unravel the nature of the dark energy that drives the accelerated
expansion of the Universe. In particular, determination of the
equation-of-state of dark energy, w equivalent p/rho, and its time variation,
dw/dz, will be critical for developing theoretical understanding of the new
physics behind this phenomenon. Type Ia supernovae (SNe) and cosmic microwave
background (CMB) anisotropy are each sensitive to the dark energy
equation-of-state. SNe alone can determine w(z) with some precision, while CMB
anisotropy alone cannot because of a strong degeneracy between the matter
density Omega_M and w. However, we show that the Planck CMB mission can
significantly improve the power of a deep SNe survey to probe w and especially
dw/dz. Because CMB constraints are nearly orthogonal to SNe constraints in the
Omega_M-w plane, for constraining w(z) Planck is more useful than precise
determination of Omega_M. We discuss how the CMB/SNe complementarity impacts
strategies for the redshift distribution of a supernova survey to determine
w(z) and conclude that a well-designed sample should include a substantial
number of supernovae out to redshifts z ~ 2.Comment: More discussion of CMB systematics and many new references added.
Matches the PRD versio
Probing the dark energy: methods and strategies
The presence of dark energy in the Universe is inferred directly from the
accelerated expansion of the Universe, and indirectly, from measurements of
cosmic microwave background (CMB) anisotropy. Dark energy contributes about 2/3
of the critical density, is very smoothly distributed, and has large negative
pressure. Its nature is very much unknown. Most of its discernible consequences
follow from its effect on evolution of the expansion rate of the Universe,
which in turn affects the growth of density perturbations and the age of the
Universe, and can be probed by the classical kinematic cosmological tests.
Absent a compelling theoretical model (or even a class of models), we describe
the dark energy by an effective equation-of-state w=p_X/\rho_X which is allowed
to vary with time. We describe and compare different approaches for determining
w(t), including magnitude-redshift (Hubble) diagram, number counts of galaxies
and clusters, and CMB anisotropy, focusing particular attention on the use of a
sample of several thousand type Ia supernova with redshifts z\lesssim 1.7, as
might be gathered by the proposed SNAP satellite. Among other things, we derive
optimal strategies for constraining cosmological parameters using type Ia
supernovae. While in the near term CMB anisotropy will provide the first
measurements of w, supernovae and number counts appear to have the most
potential to probe dark energy.Comment: 20 pages and 23 figures, revtex, submitted to Phys. Rev.
Radiative neutrino mass generation and dark energy
We study the models with radiative neutrino mass generation and explore the
relation between the neutrino masses and dark energy. In these models, the
pseudo-Nambu-Goldston bosons (pNGBs) arise at two-loop level via the Majorana
neutrino masses. In particular, we demonstrate that the potential energy of the
pNGB can be the dark energy potential and the observed value of the equation of
state (EoS) parameter of the universe, , , can be realized.Comment: 10 pages, 1 figure, a minor correction in Eq. (17
The Cosmological Constant
This is a review of the physics and cosmology of the cosmological constant.
Focusing on recent developments, I present a pedagogical overview of cosmology
in the presence of a cosmological constant, observational constraints on its
magnitude, and the physics of a small (and potentially nonzero) vacuum energy.Comment: 50 pages. Submitted to Living Reviews in Relativity
(http://www.livingreviews.org/), December 199
Agegraphic dark energy as a quintessence
Recently, a dark energy model characterized by the age of the universe,
dubbed ``agegraphic dark energy'', was proposed by Cai. In this paper, a
connection between the quintessence scalar-field and the agegraphic dark energy
is established, and accordingly, the potential of the agegraphic quintessence
field is constructed.Comment: 9 pages, 3 figures; accepted by Eur. Phys. J.
Holographic dilatonic model of dark energy
We present a dilatonic description of the holographic dark energy by
connecting the holographic dark energy density with the dilaton scalar field
energy density in a flat Friedmann-Robertson-Walker universe. We show that this
model can describe the observed accelerated expansion of our universe with the
choice and reconstruct the kinetic term as well as the dynamics of the
dilaton scalar field.Comment: 7 pages, 3 figures, changed content, added references, accepted for
publication at Eur.Phys.J.
The growth factor of matter perturbations in an f(R) gravity
The growth of matter perturbations in the model proposed by
Starobinsky is studied in this paper. Three different parametric forms of the
growth index are considered respectively and constraints on the model are
obtained at both the and confidence levels, by using the
current observational data for the growth factor. It is found, for all the
three parametric forms of the growth index examined, that the Starobinsky model
is consistent with the observations only at the confidence level.Comment: 15 pages, 5 figure