In 1998, astronomers made an astounding discovery that shook the foundations of modern physics: contrary to expectations, the expansion of our Universe is revving up. We live in a runaway Universe, where the most distant observable galaxies are racing away from us at ever increasing speeds.
But what is causing this cosmic acceleration? No one knows for certain, but whatever dark energy actually is, detailed measurements reveal that it comprises a whopping 74% of our Universe's total mass-energy budget!
As the Universe's dominant form of energy, dark energy plays a crucial role in determining how the cosmos evolves, and it will determine whether our Universe expands forever or collapses upon itself.
According to NASA’s website article on Dark Energy - Dark energy has the cosmologists scratching their heads. Observations taken by NASA's Hubble Space Telescope and future space telescopes will be needed in order to determine the properties of dark energy.
Probing dark energy, the energy in empty space causing the expanding universe to accelerate, calls for accurately measuring how that expansion rate is increasing with time. Dark energy is thought to drive space apart.
In physical cosmology and astronomy, dark energy is a hypothetical form of energy that permeates all of space and tends to increase the rate of expansion of the universe. Dark energy is the most popular way to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 74% of the total mass-energy of the universe.
NASA has developed the Beyond Einstein Program, a series of missions designed to probe fundamental questions about dark energy, black holes, and the very early Universe.
One of the missions is the Joint Dark Energy Mission (JDEM), which will study dark energy.
Two other Beyond Einstein missions, International X-ray Observatory (IXO, formerly Con-X) and the Laser Interferometer Space Antenna (LISA), will provide crucial independent measurements of dark energy.
The exact nature of this dark energy is a matter of speculation. It is known to be very homogeneous, not very dense and is not known to interact through any of the fundamental forces other than gravity. Since it is not very dense — roughly 10−29 grams per cubic centimeter — it is hard to imagine experiments to detect it in the laboratory. Dark energy can only have such a profound impact on the universe, making up 74% of all energy, because it uniformly fills otherwise empty space. The two leading models are quintessence and the cosmological constant. Both models include the common characteristic that dark energy must have negative pressure.
This accelerating expansion effect is sometimes labeled "gravitational repulsion", which is a colorful but possibly confusing expression. In fact a negative pressure does not influence the gravitational interaction between masses - which remains attractive - but rather alters the overall evolution of the universe at the cosmological scale, typically resulting in the accelerating expansion of the universe despite the attraction among the masses present in the universe.
The simplest explanation for dark energy is that it is simply the "cost of having space": that is, a volume of space has some intrinsic, fundamental energy. This is the cosmological constant.
Since energy and mass are related by E = mc2, Einstein's theory of general relativity predicts that it will have a gravitational effect. It is sometimes called a vacuum energy because it is the energy density of empty vacuum. In fact, most theories of particle physics predict vacuum fluctuations that would give the vacuum this sort of energy.
Some theorists think that dark energy and cosmic acceleration are a failure of general relativity on very large scales, larger than super-clusters. It is a tremendous extrapolation to think that our law of gravity, which works so well in the solar system, should work without correction on the scale of the universe. Most attempts at modifying general relativity, however, have turned out to be either equivalent to theories of quintessence, or inconsistent with observations. It is of interest to note that if the equation for gravity were to approach r instead of r2 at large, intergalactic distances, then the acceleration of the expansion of the universe becomes a mathematical artifact, negating the need for the existence of Dark Energy.
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 super-cluster 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 space-time. Relative speed and velocity can only be meaningfully defined in flat space-time or in sufficiently small (infinitesimal) regions of curved space-time). Rather, it prevents any communication between them as the objects pass out of contact.
The Earth, the Milky Way and the Virgo super cluster, however, would remain virtually undisturbed while the rest of the universe recedes. In this scenario, the local super cluster 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.
However, we need not get overly worried. The timescales being discussed towards the end of the universe range between 30 billion to 50 billion earth years (at least as we know them in present era).
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