15 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
Children road safety training with augmented reality (AR) [Demo]
© 2017 International Ambient Media Association (iAMEA). All rights reserved. Children killed or seriously injured through road accidents can be avoided through an appropriate safety training. Through play and engagement children learn and understand hazards at i.e. railway stations, bus stops, crossings, school zones, train stations, footpaths, or while cycling. We developed a rapid prototype of an Augmented Reality (AR) safety training proof-of-concept demonstrator for a scaled real-world model of dangerous road hazards. Two scenarios have been picked to give children the possibility to apply, and acquire knowledge of road safety: 1. handling emergency situations and informing authorities; 2. correct behavior at a bus stop on arrival/departure of a bus. In this paper we discuss our design approach, outline the technical implementation of the system, and give a brief overview of our lessons learned