4,067 research outputs found
Can the Universe escape eternal acceleration?
Recent astronomical observations of distant supernovae light-curves suggest
that the expansion of the universe has recently begun to accelerate.
Acceleration is created by an anti-gravitational repulsive stress, like that
produced by a positive cosmological constant, or universal vacuum energy. It
creates a rather bleak eschatological picture. An ever-expanding universe's
future appears to be increasingly dominated by its constant vacuum energy. A
universe doomed to accelerate forever will produce a state of growing
uniformity and cosmic loneliness. Structures participating in the cosmological
expansion will ultimately leave each others' horizons and
information-processing must eventually die out. Here, we examine whether this
picture is the only interpretation of the observations. We find that in many
well-motivated scenarios the observed spell of vacuum domination is only a
transient phenomenon. Soon after acceleration starts, the vacuum energy's
anti-gravitational properties are reversed, and a matter-dominated decelerating
cosmic expansion resumes. Thus, contrary to general expectations, once an
accelerating universe does not mean always an accelerating universe.Comment: 6 pages, 2 figure
Opportunities for future supernova studies of cosmic acceleration
We investigate the potential of a future supernova dataset, as might be
obtained by the proposed SNAP satellite, to discriminate among different ``dark
energy'' theories that describe an accelerating Universe. We find that many
such models can be distinguished with a fit to the effective
pressure-to-density ratio, , of this energy. More models can be
distinguished when the effective slope, , of a changing is also fit,
but only if our knowledge of the current mass density, , is improved.
We investigate the use of ``fitting functions'' to interpret luminosity
distance data from supernova searches, and argue in favor of a particular
preferred method, which we use in our analysis.Comment: Four pages including figures. Final published version. No significant
changes from v
Dynamics of Massive Scalar Fields in dS Space and the dS/CFT Correspondence
Global geometric properties of dS space are presented explicitly in various
coordinates. A Robertson-Walker like metric is deduced, which is convenient to
be used in study of dynamics in dS space. Singularities of wavefunctions of
massive scalar fields at boundary are demonstrated. A bulk-boundary propagator
is constructed by making use of the solutions of equations of motion. The
dS/CFT correspondence and the Strominger's mass bound is shown.Comment: latex, 14 pages and 3 figure
Cosmology with two compactification scales
We consider a (4+d)-dimensional spacetime broken up into a (4-n)-dimensional
Minkowski spacetime (where n goes from 1 to 3) and a compact (n+d)-dimensional
manifold. At the present time the n compactification radii are of the order of
the Universe size, while the other d compactification radii are of the order of
the Planck length.Comment: 16 pages, Latex2e, 7 figure
Zeldovich flow on cosmic vacuum background: new exact nonlinear analytical solution
A new exact nonlinear Newtonian solution for a plane matter flow superimposed
on the isotropic Hubble expansion is reported. The dynamical effect of cosmic
vacuum is taken into account. The solution describes the evolution of nonlinear
perturbations via gravitational instability of matter and the termination of
the perturbation growth by anti-gravity of vacuum at the epoch of transition
from matter domination to vacuum domination. On this basis, an `approximate' 3D
solution is suggested as an analog of the Zeldovich ansatz.Comment: 9 pages, 1 figure
Community Development as a Public Sector Agenda
This paper documents how one Department of Recreation in a large city underwent a planned change effort which transformed some 200 site managers into active community organizers. Based on the philosophy of a very dynamic new commissioner (a former community organizer), city employees enhanced local civic participation and forged a partnership between the public and private sectors as well as with local residents. The paper presents the guiding philosophy, implementation steps, an assessment of the impact of the change, and a discussion of the potential of such a process which can be adopted nationwide
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