37 research outputs found
Approximate Deadline-Scheduling with Precedence Constraints
We consider the classic problem of scheduling a set of n jobs
non-preemptively on a single machine. Each job j has non-negative processing
time, weight, and deadline, and a feasible schedule needs to be consistent with
chain-like precedence constraints. The goal is to compute a feasible schedule
that minimizes the sum of penalties of late jobs. Lenstra and Rinnoy Kan
[Annals of Disc. Math., 1977] in their seminal work introduced this problem and
showed that it is strongly NP-hard, even when all processing times and weights
are 1. We study the approximability of the problem and our main result is an
O(log k)-approximation algorithm for instances with k distinct job deadlines
A new view of k-essence
K-essence models, relying on scalar fields with non-canonical kinetic terms,
have been proposed as an alternative to quintessence in explaining the observed
acceleration of the Universe. We consider the use of field redefinitions to
cast k-essence in a more familiar form. While k-essence models cannot in
general be rewritten in the form of quintessence models, we show that in
certain dynamical regimes an equivalence can be made, which in particular can
shed light on the tracking behaviour of k-essence. In several cases, k-essence
cannot be observationally distinguished from quintessence using the homogeneous
evolution, though there may be small effects on the perturbation spectrum. We
make a detailed analysis of two k-essence models from the literature and
comment on the nature of the fine tuning arising in the models.Comment: 7 pages RevTeX4 file with four figures incorporate
Scenario of Accelerating Universe from the Phenomenological \Lambda- Models
Dark matter, the major component of the matter content of the Universe,
played a significant role at early stages during structure formation. But at
present the Universe is dark energy dominated as well as accelerating. Here,
the presence of dark energy has been established by including a time-dependent
term in the Einstein's field equations. This model is compatible with
the idea of an accelerating Universe so far as the value of the deceleration
parameter is concerned. Possibility of a change in sign of the deceleration
parameter is also discussed. The impact of considering the speed of light as
variable in the field equations has also been investigated by using a well
known time-dependent model.Comment: Latex, 9 pages, Major change
Condensate cosmology -- dark energy from dark matter
Imagine a scenario in which the dark energy forms via the condensation of
dark matter at some low redshift. The Compton wavelength therefore changes from
small to very large at the transition, unlike quintessence or metamorphosis. We
study CMB, large scale structure, supernova and radio galaxy constraints on
condensation by performing a 4 parameter likelihood analysis over the Hubble
constant and the three parameters associated with Q, the condensate field:
Omega_Q, w_f and z_t (energy density and equation of state today, and redshift
of transition). Condensation roughly interpolates between Lambda CDM (for large
z_t) and sCDM (low z_t) and provides a slightly better fit to the data than
Lambda CDM. We confirm that there is no degeneracy in the CMB between H and z_t
and discuss the implications of late-time transitions for the Lyman-alpha
forest. Finally we discuss the nonlinear phase of both condensation and
metamorphosis, which is much more interesting than in standard quintessence
models.Comment: 13 pages, 13 colour figures. Final version with discussion of TE
cross-correlation spectra for condensation and metamorphosis in light of the
WMAP result
Phantom with Born-Infield type Lagrangian
Recent analysis of the observation data indicates that the equation of state
of the dark energy might be smaller than -1, which leads to the introduction of
phantom models featured by its negative kinetic energy to account for the
regime of equation of state . In this paper, we generalize the idea to
the Born-Infield type Lagrangian with negative kinetic energy term and give the
condition for the potential, under which the late time attractor solution
exists and also analyze a viable cosmological model in such a scheme.Comment: 13 pages, 6 figures, Reference updated, the final version will be
published in Phys. Rev.
Generalized Ricci dark energy in Horava-Lifshitz gravity
In this letter, we have considered generalized Ricci dark energy in the
Horava-Lifshitz gravity. We have reconstructed the Hubble's parameter in terms
of fractional densities. We have viewed the equation of state parameter in this
situation. Also, we have examined the behavior of deceleration parameter and
investigated the nature of the statefinder diagnostics. The equation of state
parameter has exhibited quintessence-like behavior and from the plot of the
deceleration parameter we have observed an ever accelerating universe
Measuring the metric: a parametrized post-Friedmanian approach to the cosmic dark energy problem
We argue for a ``parametrized post-Friedmanian'' approach to linear
cosmology, where the history of expansion and perturbation growth is measured
without assuming that the Einstein Field Equations hold. As an illustration, a
model-independent analysis of 92 type Ia supernovae demonstrates that the curve
giving the expansion history has the wrong shape to be explained without some
form of dark energy or modified gravity. We discuss how upcoming lensing,
galaxy clustering, cosmic microwave background and Lyman alpha forest
observations can be combined to pursue this program, which generalizes the
quest for a dark energy equation of state, and forecast the accuracy that the
proposed SNAP satellite can attain.Comment: Replaced to match accepted PRD version. References and another
example added, section III omitted since superceded by astro-ph/0207047. 11
PRD pages, 7 figs. Color figs and links at
http://www.hep.upenn.edu/~max/gravity.html or from [email protected]
Reducing the environmental impact of surgery on a global scale: systematic review and co-prioritization with healthcare workers in 132 countries
Background
Healthcare cannot achieve net-zero carbon without addressing operating theatres. The aim of this study was to prioritize feasible interventions to reduce the environmental impact of operating theatres.
Methods
This study adopted a four-phase Delphi consensus co-prioritization methodology. In phase 1, a systematic review of published interventions and global consultation of perioperative healthcare professionals were used to longlist interventions. In phase 2, iterative thematic analysis consolidated comparable interventions into a shortlist. In phase 3, the shortlist was co-prioritized based on patient and clinician views on acceptability, feasibility, and safety. In phase 4, ranked lists of interventions were presented by their relevance to high-income countries and low–middle-income countries.
Results
In phase 1, 43 interventions were identified, which had low uptake in practice according to 3042 professionals globally. In phase 2, a shortlist of 15 intervention domains was generated. In phase 3, interventions were deemed acceptable for more than 90 per cent of patients except for reducing general anaesthesia (84 per cent) and re-sterilization of ‘single-use’ consumables (86 per cent). In phase 4, the top three shortlisted interventions for high-income countries were: introducing recycling; reducing use of anaesthetic gases; and appropriate clinical waste processing. In phase 4, the top three shortlisted interventions for low–middle-income countries were: introducing reusable surgical devices; reducing use of consumables; and reducing the use of general anaesthesia.
Conclusion
This is a step toward environmentally sustainable operating environments with actionable interventions applicable to both high– and low–middle–income countries