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touqra
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If our universe is expanding and light travels at a finite speed, then, will there be a time in future, where we won't see any light from stars and our whole sky will be black, since all the lights have not reach us?
Let’s consider we are located in a spatially infinite universe which contains a homogeneous distribution of light sources in space and let’s assume that no new light sources are created nor destroyed as time passes. Consider a spherical volume with a given radius. Due to expansion of space, it is true that this volume will contain a decreasing number of light sources as time increases and that it will be void sometime. However, the light we are receiving is not determined by the number of light sources contained in this volume as [itex]t \rightarrow \infty[/itex], but by the shape of our past lightcone as [itex]t \rightarrow \infty[/itex].touqra said:If our universe is expanding and light travels at a finite speed, then, will there be a time in future, where we won't see any light from stars and our whole sky will be black, since all the lights have not reach us?
jhe1984 said:So if I (being able to live forever without aging) were standing a meter away from a lamp (which also could go unchanging forever) - PRESUMING LOCAL GRAVITATION DID NOT INTERFERE WITH UNIVERSE EXPANSION - would the light from the lamp appear dimmer to me, say, a million or five hundred million years later?
If it does, is this because the lamp is further away from me or because the strength of the light "dims" (which I guess would mean redshifts) over the course of millions of years?
jhe1984 said:So if I (being able to live forever without aging) were standing a meter away from a lamp (which also could go unchanging forever) - PRESUMING LOCAL GRAVITATION DID NOT INTERFERE WITH UNIVERSE EXPANSION - would the light from the lamp appear dimmer to me, say, a million or five hundred million years later?
More over, the universe can only expand if it also redshifts, because to expand without redshifting (or contract without blueshifting, I guess) would be a violation of the total conservation of a finite universe's energy?
SpaceTiger said:That question can't really be answered because it depends primarily on the local distribution of matter instead of the Hubble flow. If you were instead to ask about a "lamp" one hundred million light years away, then yes, it would appear dimmer. If we only consider cosmology, its apparent brightness will depend on its redshift.
Actually, energy is not always conserved in GR. The fact that light redshifts to begin with could be considered a violation of the energy conservation law, depending on your definition of "energy". See here for more details:
Is Energy Conserved in General Relativity?
In general relativity, on the other hand, it has no meaning to speak of a definite
localization of energy. One may define a quantity which is divergence free analogous to the
energy-momentum density tensor of special relativity , but it is gauge dependent: i.e., it
is not covariant under general coordinate transformations. Consequently the fact that it is
divergence free does not yield a meaningful law of local energy conservation. Thus one has,
as Hilbert saw it, in such theories ‘improper energy theorems’.
A key feature for physics of Noether’s I.V. paper is the clarity her theorems brought
to our understanding of the principle of energy conservation. As Feza Gursey wrote [18]:
“Before Noether’s Theorem the principle of conservation of energy was shrouded in mystery,
leading to the obscure physical systems of Mach and Ostwald. Noether’s simple and
profound mathematical formulation did much to demystify physics.” Noether showed in her
theorem I that the principle of energy conservation follows from symmetry under time translations.
This applies to theories having a finite continuous symmetry group; theories that are
Galilean or Poincar`e invariant, for example. In general relativity, on the other hand, energy
conservation takes a different form as will be shown below. Noether’s theorem II applies in
the case of general relativity and one sees that she has proved Hilbert’s assertion that in
this case one has ‘improper energy theorems’, and that this is a characteristic feature of the
theory. It is owing to the fact that the theory is a gauge theory; i.e., that it has an infinite
continuous group of symmetries of which time translations are a subgroup. Indeed generally
she defines as “improper” divergence relationships, which vanish when the field equations are
satisfied, which correspond to a finite continuous subgroup of an infinite continuous group.
Generally they do not have the required invariance or covariance properties under the larger
group. For example, in general relativity a divergence free energy-momentum (pseudo) tensor
can be constructed but it is gauge dependent (see below). Because it is not covariant
under general coordinate transformations, it is more properly called a pseudotensor. Such
pseudotensors are covariant with respect to the linear transformations of the Poincar`e group
and may be used in asymptotic spacetime regions far from gravitating sources to derive a
principle of energy conservation.
Therein lies the problem of a Quantum Gravity, may energy be represented by an operator or not? Does this not require a preferred foliation of space-time in contradiction to the principle of relativity?pervect said:In general relativity, on the other hand, it has no meaning to speak of a definite localization of energy. One may define a quantity which is divergence free analogous to the energy-momentum density tensor of special relativity , but it is gauge dependent: i.e., it is not covariant under general coordinate transformations.
heliocentricprose said:What do you mean by negative pressure? Could you give an analogy perhaps?
Universe expansion refers to the theory that the universe is continuously expanding, with all galaxies and celestial bodies moving away from each other at an accelerating rate.
Scientists have observed the redshift of light from distant galaxies, which indicates that they are moving away from us. This is known as the Doppler effect and provides evidence for the universe's expansion.
Based on current observations and theories, it is believed that the universe will continue to expand forever. This is due to the presence of dark energy, a mysterious force that is causing the expansion to accelerate.
The expanding universe means that galaxies and celestial bodies will continue to move away from each other, making them appear dimmer and further apart in the night sky. This will result in darker skies and fewer visible stars over time.
Unfortunately, the expansion of the universe is a natural process that cannot be stopped or reversed. However, efforts can be made to reduce light pollution and preserve dark skies for stargazing and scientific research.