5 research outputs found
Gravitational waves in the presence of a cosmological constant
We derive the effects of a non-zero cosmological constant on
gravitational wave propagation in the linearized approximation of general
relativity. In this approximation we consider the situation where the metric
can be written as , , where is
the background perturbation and is a modification
interpretable as a gravitational wave. For this linearization
of Einstein equations is self-consistent only in certain coordinate systems.
The cosmological Friedmann-Robertson-Walker coordinates do not belong to this
class and the derived linearized solutions have to be reinterpreted in a
coordinate system that is homogeneous and isotropic to make contact with
observations. Plane waves in the linear theory acquire modifications of order
, both in the amplitude and the phase, when considered in FRW
coordinates. In the linearization process for , we have also
included terms of order . For the background
perturbation the difference is very small but when the
term is retained the equations of motion can be
interpreted as describing massive spin-2 particles. However, the extra degrees
of freedom can be approximately gauged away, coupling to matter sources with a
strength proportional to the cosmological constant itself. Finally we discuss
the viability of detecting the modifications caused by the cosmological
constant on the amplitude and phase of gravitational waves. In some cases the
distortion with respect to gravitational waves propagating in Minkowski
space-time is considerable. The effect of could have a detectable
impact on pulsar timing arrays.Comment: 20 pages, 1 figur
Do GOLD stages of COPD severity really correspond to differences in health status?
The purpose of this study was to assess whether different stages of
chronic obstructive pulmonary disease (COPD) severity defined according to the Global
Initiative for Chronic Obstructive Lung Disease (GOLD) criteria correlate with
meaningful differences in health status.
A total of 381 COPD patients, aged 73¡6 yrs, were classified in the five GOLD
stages. Disease-specific (St George Respiratory Questionnaire (SGRQ)) and generic
indexes of health status were measured in all patients. Multivariate analysis of
covariance or Kruskal Wallis tests were used to compare health status indexes across
the spectrum of GOLD stages of COPD severity.
GOLD stages of COPD severity significantly differed in SGRQ components and
Barthel9s index, but not in the indexes assessing cognitive and affective status and
quality of sleep. The largest variation in health status was observed at the transition
from stage IIa to stage IIb, while there were no other significant differences between
consecutive stages. Both female sex and comorbidity were associated with a greater
impact of COPD on the health status.
In conclusion, the upper limit of stage IIb (forced expiratory volume in one second of
49%) marks a threshold for dramatic worsening of health status. Progression of chronic
obstructive pulmonary disease severity from stage 0 to stage IIa does not correspond to
any meaningful difference in health status