Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userGravitational Theory with a Dynamical Time
308
0
0.0
(
0
)
Added by
Guendelman Eduardo I
Publication date
2009
fields
Physics
and research's language is
English
Authors
E.I. Guendelman
Ask ChatGPT about the research
No Arabic abstract
A gravitational theory involving a vector field $chi^{mu}$, whose zero component has the properties of a dynamical time, is studied. The variation of the action with respect to $chi^{mu}$ gives the covariant conservation of an energy momentum tensor $ T^{mu u}_{(chi)}$. Studying the theory in a background which has killing vectors and killing tensors we find appropriate shift symmetries of the field $chi^{mu}$ which lead to conservation laws. The energy momentum that is the source of gravity $ T^{mu u}_{(G)}$ is different but related to $ T^{mu u}_{(chi)}$ and the covariant conservation of $ T^{mu u}_{(G)}$ determines in general the vector field $chi^{mu}$. When $ T^{mu u}_{(chi)}$ is chosen to be proportional to the metric, the theory coincides with the Two Measures Theory, which has been studied before in relation to the Cosmological Constant Problem. When the matter model consists of point particles, or strings, the form of $ T^{mu u}_{(G)}$, solutions for $chi^{mu}$ are found. For the case of a string gas cosmology, we find that the Milne Universe can be a solution, where the gas of strings does not curve the spacetime since although $ T^{mu u}_{(chi)} eq 0$, $ T^{mu u}_{(G)}= 0$, as a model for the early universe, this solution is also free of the horizon problem. There may be also an application to the time problem of quantum cosmology.
rate research
Read More
According to General Relativity gravity is the result of the interaction between matter and space-time geometry. In this interaction space-time geometry itself is dynamical: it can store and transport energy and momentum in the form of gravitational waves. We give an introductory account of this phenomenon and discuss how the observation of gravitational waves may open up a fundamentally new window on the universe.
Cosmological solutions with a scalar field behaving as radiation are obtained, in the context of gravitational theory with dynamical time. The solution requires the spacial curvature of the universe k, to be zero, unlike the standard radiation solutions, which do not impose any constraint on the spacial curvature of the universe. This is because only such $ k=0 $ radiation solutions poses a homothetic Killimg vector. This kind of theory can be used to generalize electromagnetism and other gauge theories, in curved space time, and there are no deviations from standard gauge filed equation (like Maxwell equations) in the case there exist a conformal Killing vector. But there could be departures from Maxwell and Yang Mills equations, for more general space times.
This decade will see the first direct detections of gravitational waves by observatories such as Advanced LIGO and Virgo. Among the prime sources are coalescences of binary neutron stars and black holes, which are ideal probes of dynamical spacetime. This will herald a new era in the empirical study of gravitation. For the first time, we will have access to the genuinely strong-field dynamics, where low-energy imprints of quantum gravity may well show up. In addition, we will be able to search for effects which might only make their presence known at large distance scales, such as the ones that gravitational waves must traverse in going from source to observer. Finally, coalescing binaries can be used as cosmic distance markers, to study the large-scale structure and evolution of the Universe. With the advanced detector era fast approaching, concrete data analysis algorithms are being developed to look for deviations from general relativity in signals from coalescing binaries, taking into account the noisy detector output as well as the expectation that most sources will be near the threshold of detectability. Similarly, several practical methods have been proposed to use them for cosmology. We explain the state of the art, including the obstacles that still need to be overcome in order to make optimal use of the signals that will be detected. Although the emphasis will be on second-generation observatories, we will also discuss some of the science that could be done with future third-generation ground-based facilities such as Einstein Telescope, as well as with space-based detectors.
We study a collapsing system attracted by a spherically symmetric gravitational source, with an increasing mass, that generates back-reaction effects that are the source of space-time waves. As an example, we consider an exponential collapse and the space-time waves emitted during this collapse due to the back-reaction effects, originated by geometrical deformation driven by the increment of the gravitational attracting mass during the collapse.
The direct detection of gravitational waves provides the opportunity to measure fundamental aspects of gravity which have never been directly probed before, including the polarization of gravitational waves. In the context of searches for continuous waves from known pulsars, we present novel methods to detect signals of any polarization content, measure the modes present and place upper-limits on the amplitude of non-tensorial components. This will allow us to obtain new model-independent, dynamical constraints on deviations from general relativity. We test this framework on multiple potential sources using simulated data from three advanced-era detectors at design sensitivity. We find that signals of any polarization will become detectable and distinguishable for characteristic strains $hgtrsim 3times10^{-27} sqrt{1~{rm yr}/T}$, for an observation time $T$. We also find that our ability to detect non-tensorial components depends only on the power present in those modes, irrespective of the strength of the tensorial strain.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
