The name Dark Energy is given to the force that is believed to be making the universe larger. Distant galaxies appear to be moving away from us at high speed: the idea is that the universe is getting bigger and has been since the Big Bang.
In the early 1990s, one thing was fairly certain about the expansion of the universe. It might have enough energy density to stop its expansion and recollapse, it might have so little energy density that it would never stop expanding, but gravity was certain to slow the expansion as time went on. Granted, the slowing had not been observed, but, theoretically, the universe had to slow. The universe is full of matter and the attractive force of gravity pulls all matter together. Then came 1998 and the Hubble Space Telescope (HST) observations of very distant supernovae that showed that, a long time ago, the universe was actually expanding more slowly than it is today. So the expansion of the universe has not been slowing due to gravity, as everyone thought, it has been accelerating. No one expected this, no one knew how to explain it. But something was causing it.
Eventually theorists came up with three sorts of explanations. Maybe it was a result of a long-discarded version of Einstein's theory of gravity, one that contained what was called a "cosmological constant." Maybe there was some strange kind of energy-fluid that filled space. Maybe there is something wrong with Einstein's theory of gravity and a new theory could include some kind of field that creates this cosmic acceleration. Theorists still don't know what the correct explanation is, but they have given the solution a name. It is called dark energy.
What Actually is Dark Energy?
More is unknown than is known. We know how much dark energy there is because we know how it affects the universe's expansion. Other than that, it is a complete mystery. But it is an important mystery. It turns out that roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest - everything on Earth, everything ever observed with all of our instruments, all normal matter - adds up to less than 5% of the universe. Come to think of it, maybe it shouldn't be called "normal" matter at all, since it is such a small fraction of the universe.
The above diagram reveals changes in the rate of expansion since the universe's birth 15 billion years ago. The more shallow the curve, the faster the rate of expansion. The curve changes noticeably about 7.5 billion years ago, when objects in the universe began flying apart as a faster rate. Astronomers theorize that the faster expansion rate is due to a mysterious, dark force that is pulling galaxies apart.
(Credit: NASA/STSci/Ann Feild)
One explanation for dark energy is that it is a property of space. Albert Einstein was the first person to realize that empty space is not nothing. Space has amazing properties, many of which are just beginning to be understood. The first property that Einstein discovered is that it is possible for more space to come into existence. Then one version of Einstein's gravity theory, the version that contains a cosmological constant, makes a second prediction: "empty space" can possess its own energy. Because this energy is a property of space itself, it would not be diluted as space expands. As more space comes into existence, more of this energy-of-space would appear. As a result, this form of energy would cause the universe to expand faster and faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe.
This image shows the distribution of dark matter, galaxies, and hot gas in the core of the merging galaxy cluster Abell 520. The result could present a challenge to basic theories of dark matter.
Another explanation for how space acquires energy comes from the quantum theory of matter. In this theory, "empty space" is actually full of temporary "virtual" particles that continually form and then disappear. But when physicists tried to calculate how much energy this would give empty space, the answer came out wrong - wrong by a lot. The number came out 10 to the power 120 times big. That's a 1 with 120 zeros after it. It's hard to get an answer that bad. So the mystery continues.
Another explanation for dark energy is that it is a new kind of dynamical energy fluid or field, something that fills all of space but something whose effect on the expansion of the universe is the opposite of that of matter and normal energy. Some theorists have named this "quintessence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it is like, what it interacts with, or why it exists. So the mystery continues.
A last possibility is that Einstein's theory of gravity is not correct. That would not only affect the expansion of the universe, but it would also affect the way that normal matter in galaxies and clusters of galaxies behaved. This fact would provide a way to decide if the solution to the dark energy problem is a new gravity theory or not: we could observe how galaxies come together in clusters. But if it does turn out that a new theory of gravity is needed, what kind of theory would it be? How could it correctly describe the motion of the bodies in the Solar System, as Einstein's theory is known to do, and still give us the different prediction for the universe that we need? There are candidate theories, but none are compelling. So the mystery continues.
The thing that is needed to decide between dark energy possibilities - a property of space, a new dynamic fluid, or a new theory of gravity - is more data, better data.
What Now is Dark Matter?
By fitting a theoretical model of the composition of the universe to the combined set of cosmological observations, scientists have come up with the composition that we described above, ~68% dark energy, ~27% dark matter, ~5% normal matter. What is dark matter?
We are much more certain what dark matter is not than we are what it is. First, it is dark, meaning that it is not in the form of stars and planets that we see. Observations show that there is far too little visible matter in the universe to make up the 27% required by the observations. Second, it is not in the form of dark clouds of normal matter, matter made up of particles called baryons. We know this because we would be able to detect baryonic clouds by their absorption of radiation passing through them. Third, dark matter is not antimatter, because we do not see the unique gamma rays that are produced when antimatter annihilates with matter. Finally, we can rule out large galaxy-sized black holes on the basis of how many gravitational lenses we see. High concentrations of matter bend light passing near them from objects further away, but we do not see enough lensing events to suggest that such objects to make up the required 25% dark matter contribution.
Some Facts About Dark Matter
☻ Dark matter comprises around 84.5% of the total matter in the universe.
☻ It isn’t made up of baryons, unlike normal matter, which is a combination of protons and neutrons. Dark matter doesn’t interact with electromagnetic forces. It doesn’t absorb or emit light, nor does it reflect it like normal matter. The only way it can be detected is through its gravitational pull on visible matter.
☻ Jan Oort, a Dutch astronomer, was the first to detect the presence of dark matter, in 1932. While studying stellar motions in nearby galaxies, Oort found that the matter seen in the galaxies is quite less as compared to the size of the galaxies.
☻ The ‘missing mass’ problem suggested that, the stars, hot gases, and visible matter in the galaxy clusters accounted for hardly 20% of the total matter in the galaxy. The speed at which these galaxies moved was enough to flow them apart. According to Fritz Zwicky, a Swiss astronomer, these galaxies needed a lot more mass to hold themselves together, considering their speed. This meant that, there was some matter missing to the human eye, that was holding these galaxies together
☻ The strongest evidence for the presence of dark matter is provided by gravitational lensing studies of the bullet cluster. Collision of two galaxies was observed in this cluster, but it was astonishing that the galaxies merged together without any disaster, or the stars smashing into each other. This means that, there was some invisible gravitational field governing the stars of these galaxies.
☻ A dark matter halo surrounds the whole galaxy and extends beyond its edges. The Milky Way galaxy is enveloped by a much bigger halo than the actual galaxy, and is spherical in shape.
☻ Gravitational lensing is used to detect dark matter in the universe. Basically, the gravity of dark matter is responsible for bending light from distant light sources. To detect dark matter, astronomers check for bending of light, in cases of absence of visible matter.
☻ Recent studies and observations seem to point toward a universe with a mixture of both hot and cold dark matter. The temperature determines the speed of the particles; hot particles move nearly at the speed of light. A good example is neutrinos. Neutrinos are weakly-interacting subatomic particles which do not carry an electric charge, hence, leaving them unaffected by electromagnetic forces. On the other hand, cold particles move much slower than hot particles. The hypothesis of the Peccei-Quinn theory in 1977 talks about ‘Axions’. According to this theory, this hypothetical particle should have very low mass, and could be a possible component of cold dark matter.
Dark matter is just one of the several unknowns of space. A lot of research is being done on dark matter by the U.S. government. Projects like the Super Cryogenic Dark Matter Search (SuperCDMS), the LUX-ZEPLIN (LZ) Experiment, and the Axion Dark Matter Experiment Gen2 (ADMX-Gen2) have been initiated by the U.S. government for studying dark matter and its Weakly Interacting Massive Particles (WIMPS).
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