Dark Energy and Dark Matter in Universe
Overview of Dark Energy and Dark Matter
In the early 1990’s, one thing was fairly certain about the expansion of the Universe. It might have enough energy density to stop its expansion and re collapse, 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.
The Universe’s Dark Matter and Dark Energy
Two of the biggest mysteries of both string theory and cosmology are the presence of unseen dark matter and of repulsive gravity in the form of dark energy.
Dark Matter – The Source of Extra Gravity
Astronomers have discovered that the gravitational effects observed in our universe don’t match the amount of matter seen. To account for these differences, it appears that the universe contains a mysterious form of matter that we can’t observe, called dark matter. Throughout the universe, there’s approximately six times as much dark matter as normal visible matter — and string theory may explain where it comes from!
In the 1930s, Swiss astronomer Fritz Zwicky first observed that some galaxies were spinning so fast that the stars in them should fly away from each other. Unfortunately, Zwicky had personality clashes with many in the astronomy community, so his views weren’t taken very seriously.
In 1962, astronomer Vera Rubin made the same discoveries and had nearly the same outcome. Though Rubin didn’t have the same issues of temperament that Zwicky did, many disregarded her work because she was a woman.
Rubin maintained her focus on the problem and, by 1978, had studied 11 spiral galaxies, all of which ( including our own Milky Way ) were spinning so fast that the laws of physics said they should fly apart. Together with work from others, this was enough to convince the astronomy community that something strange was happening.
Whatever is holding these galaxies together, observations now indicate that there has to be far more of it than there is the visible matter that makes up the baryonic matter that we’re used to — the matter that comprises you, your computer, this planet, and the stars.
Physicists have made several suggestions about what could make up this dark matter, but so far no one knows for sure.
Dark Energy : Pushing the Universe Apart
Einstein’s cosmological constant allowed for a uniform repulsive energy throughout the universe. Since Hubble discovered the expansion of the universe, most scientists have believed that the cosmological constant was zero ( or possibly slightly negative ). Recent findings have indicated that the expansion rate of the universe is actually increasing, meaning that the cosmological constant has a positive value. This repulsive gravity — or dark energy — is actually pushing the universe apart. This is one major feature of the universe that string theory may be able to explain.
In 1998, two teams of astronomers announced the same results : Studies of distant supernovas ( exploding stars ) showed that stars looked dimmer than expected. The only way to account for this was if the stars were somehow farther away than expected, but the physicists had already accounted for the expansion of the universe. The explanation eventually found was startling: The rate of expansion of the universe was accelerating.
What is Dark Energy ?
Eventually theorists came up with three sorts of explanations. May be 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.
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 70% of the Universe is dark energy. Dark matter makes up about 25%. 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.
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 10120 times too 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.
The greatest discoveries are the unexpected ones, which was the case in the late 1990s when two teams of astronomers competing to measure the rate at which the expansion of the universe is slowing down ( as virtually everyone thought it must be ) discovered that it is speeding up instead. A previously unknown, all – pervasive dark energy must be at work, representing 70% of the energy density of the universe.
What 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,
- 70% dark energy
- 25% dark matter
- 5% normal 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 25% required by the observations.
- First proposed in the 1930s, the idea that there is missing mass influencing the behavior of galaxies began to look more and more likely from the 1970s on. We know that it is matter because we can detect its gravitational influence on visible matter, but we cannot see it. An inventory of the distribution of dark matter throughout space shows that it constitutes 25% of the energy density of the universe.
- 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.
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