238 research outputs found
The cosmic gravitational wave background in a cyclic universe
Inflation predicts a primordial gravitational wave spectrum that is slightly
``red,'' i.e., nearly scale-invariant with slowly increasing power at longer
wavelengths. In this paper, we compute both the amplitude and spectral form of
the primordial tensor spectrum predicted by cyclic/ekpyrotic models. The
spectrum is blue and exponentially suppressed compared to inflation on long
wavelengths. The strongest observational constraint emerges from the
requirement that the energy density in gravitational waves should not exceed
around 10 per cent of the energy density at the time of nucleosynthesis.Comment: 4 pages, 3 figuer
Schwinger-Keldysh Propagators from AdS/CFT Correspondence
We demonstrate how to compute real-time Green's functions for a class of
finite temperature field theories from their AdS gravity duals. In particular,
we reproduce the two-by-two Schwinger-Keldysh matrix propagator from a gravity
calculation. Our methods should work also for computing higher point Lorentzian
signature correlators. We elucidate the boundary condition subtleties which
hampered previous efforts to build a Lorentzian-signature AdS/CFT
correspondence. For two-point correlators, our construction is automatically
equivalent to the previously formulated prescription for the retarded
propagator.Comment: 16 pages, 1 figure, references added; to appear in JHE
Shared Care, Elder and Family Member Skills Used to Manage Burden
Aim. The aim of this paper is to further develop the construct of Shared Care by comparing and contrasting it to related research, and to show how the construct can be used to guide research and practice.
Background. While researchers have identified negative outcomes for family caregivers caused by providing care, less is known about positive aspects of family care for both members of a family dyad. Understanding family care relationships is important to nurses because family participation in the care of chronically ill elders is necessary to achieve optimal outcomes from nursing interventions. A previous naturalistic inquiry identified a new construct, Shared Care, which was used to describe a family care interaction that contributed to positive care outcomes.
Methods. A literature review was carried out using the databases Medline, CINAHL, and Psych-info and the keywords home care, care receiver, disability, family, communication, decision-making and reciprocity. The results of the review were integrated to suggest how Shared Care could be used to study care difficulties and guide interventions.
Results. The literature confirmed the importance of dyad relationships in family care. Shared Care extended previous conceptualizations of family care by capturing three critical components: communication, decision-making, and reciprocity. Shared Care provides a structure to expand the conceptualization of family care to include both members of a care dyad and account for positive and negative aspects of care.
Conclusions. The extended view provided by the construct of Shared Care offers practitioners and scholars tools to use in the context of our ageing population to improve the effectiveness of family care relationships
No Dynamics in the Extremal Kerr Throat
Motivated by the Kerr/CFT conjecture, we explore solutions of vacuum general
relativity whose asymptotic behavior agrees with that of the extremal Kerr
throat, sometimes called the Near-Horizon Extreme Kerr (NHEK) geometry. We
argue that all such solutions are diffeomorphic to the NHEK geometry itself.
The logic proceeds in two steps. We first argue that certain charges must
vanish at all times for any solution with NHEK asymptotics. We then analyze
these charges in detail for linearized solutions. Though one can choose the
relevant charges to vanish at any initial time, these charges are not
conserved. As a result, requiring the charges to vanish at all times is a much
stronger condition. We argue that all solutions satisfying this condition are
diffeomorphic to the NHEK metric.Comment: 42 pages, 3 figures. v3: minor clarifications and correction
Cosmological Perturbations in a Big Crunch/Big Bang Space-time
A prescription is developed for matching general relativistic perturbations
across singularities of the type encountered in the ekpyrotic and cyclic
scenarios i.e. a collision between orbifold planes. We show that there exists a
gauge in which the evolution of perturbations is locally identical to that in a
model space-time (compactified Milne mod Z_2) where the matching of modes
across the singularity can be treated using a prescription previously
introduced by two of us. Using this approach, we show that long wavelength,
scale-invariant, growing-mode perturbations in the incoming state pass through
the collision and become scale-invariant growing-mode perturbations in the
expanding hot big bang phase.Comment: 47 pages, 4 figure
Out of equilibrium: understanding cosmological evolution to lower-entropy states
Despite the importance of the Second Law of Thermodynamics, it is not
absolute. Statistical mechanics implies that, given sufficient time, systems
near equilibrium will spontaneously fluctuate into lower-entropy states,
locally reversing the thermodynamic arrow of time. We study the time
development of such fluctuations, especially the very large fluctuations
relevant to cosmology. Under fairly general assumptions, the most likely
history of a fluctuation out of equilibrium is simply the CPT conjugate of the
most likely way a system relaxes back to equilibrium. We use this idea to
elucidate the spacetime structure of various fluctuations in (stable and
metastable) de Sitter space and thermal anti-de Sitter space.Comment: 27 pages, 11 figure
Global Fluctuation Spectra in Big Crunch/Big Bang String Vacua
We study Big Crunch/Big Bang cosmologies that correspond to exact world-sheet
superconformal field theories of type II strings. The string theory spacetime
contains a Big Crunch and a Big Bang cosmology, as well as additional
``whisker'' asymptotic and intermediate regions. Within the context of free
string theory, we compute, unambiguously, the scalar fluctuation spectrum in
all regions of spacetime. Generically, the Big Crunch fluctuation spectrum is
altered while passing through the bounce singularity. The change in the
spectrum is characterized by a function , which is momentum and
time-dependent. We compute explicitly and demonstrate that it arises
from the whisker regions. The whiskers are also shown to lead to
``entanglement'' entropy in the Big Bang region. Finally, in the Milne orbifold
limit of our superconformal vacua, we show that and, hence, the
fluctuation spectrum is unaltered by the Big Crunch/Big Bang singularity. We
comment on, but do not attempt to resolve, subtleties related to gravitational
backreaction and light winding modes when interactions are taken into account.Comment: 68 pages, 1 figure; typos correcte
Clean Time-Dependent String Backgrounds from Bubble Baths
We consider the set of controlled time-dependent backgrounds of general
relativity and string theory describing ``bubbles of nothing'', obtained via
double analytic continuation of black hole solutions. We analyze their quantum
stability, uncover some novel features of their dynamics, identify their causal
structure and observables, and compute their particle production spectrum. We
present a general relation between squeezed states, such as those arising in
cosmological particle creation, and nonlocal theories on the string worldsheet.
The bubble backgrounds have various aspects in common with de Sitter space,
Rindler space, and moving mirror systems, but constitute controlled solutions
of general relativity and string theory with no external forces. They provide a
useful theoretical laboratory for studying issues of observables in systems
with cosmological horizons, particle creation, and time-dependent string
perturbation theory.Comment: 38 pages, harvmac big, 6 figure
- …