Cosmologists estimate that the acceleration
began roughly 5 billion years ago. Before that, it is thought that the
expansion was decelerating, due to the attractive influence of dark matter and baryons.
The density of dark matter in an expanding universe decreases more
quickly than dark energy, and eventually the dark energy dominates.
Specifically, when the volume of the universe doubles, the density of dark matter is halved but the density of dark energy is nearly unchanged (it is exactly constant in the case of a cosmological constant).
If the acceleration continues indefinitely, the ultimate result will be that galaxies outside the local supercluster will move beyond the cosmic horizon: they will no longer be visible, because their line-of-sight velocity becomes greater than the speed of light. This is not a violation of special relativity,
and the effect cannot be used to send a signal between them. (Actually
there is no way to even define "relative speed" in a curved spacetime.
Relative speed and velocity can only be meaningfully defined in flat
spacetime or in sufficiently small (infinitesimal) regions of curved
spacetime).
Rather, it prevents any communication between them and the
objects pass out of contact. The Earth, the Milky Way and the Virgo supercluster,
however, would remain virtually undisturbed while the rest of the
universe recedes. In this scenario, the local supercluster would
ultimately suffer heat death, just as was thought for the flat, matter-dominated universe, before measurements of cosmic acceleration.
There are some very speculative ideas about the future of the universe. One suggests that phantom energy causes divergent
expansion, which would imply that the effective force of dark energy
continues growing until it dominates all other forces in the universe.
Under this scenario, dark energy would ultimately tear apart all
gravitationally bound structures, including galaxies and solar systems,
and eventually overcome the electrical and nuclear forces to tear apart atoms themselves, ending the universe in a "Big Rip".
On the other hand, dark energy might dissipate with time, or even
become attractive. Such uncertainties leave open the possibility that
gravity might yet rule the day and lead to a universe that contracts in
on itself in a "Big Crunch". Some scenarios, such as the cyclic model
suggest this could be the case.
While these ideas are not supported by
observations, they are not ruled out. Measurements of acceleration are
crucial to determining the ultimate fate of the universe in big bang
theory.
Kunjungi Juga:
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