Dark Energy: Facts about the Biggest Mystery in the Universe
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VIVA – In the early 1990s, one thing was quite certain about the expansion of the universe. It may have enough energy density to stop its expansion and collapse, it may have an energy density so small that it will never stop expanding, but gravity will slow down expansion over time.
Indeed, the slowdown is not observed, but, theoretically, the universe should slow down. The universe is full of matter and the gravitational attraction pulls all matter together. Then came 1998 and the Hubble Space Telescope's (HST) observations of very distant supernovae showed that, long ago, the universe was actually expanding more slowly than now.
So, the expansion of the universe isn't slowing down because of gravity, as everyone thinks, it's accelerating. No one expected this, no one knew how to explain it. But something caused it.
Finally, theorists came up with three kinds of explanations. Perhaps it was the result of an old version of Einstein's theory of gravity, which contained the so-called "cosmological constant."
Perhaps there was some kind of strange energy liquid filling the space. Maybe something went wrong with Einstein's theory of gravity and the 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. This is referred to as dark energy. Dark Energy is a property of space. Albert Einstein was the first to realize that empty space is nothing. Space has extraordinary properties, many of which are just beginning to be understood.
The first property that Einstein discovered was that it was possible for more space to arise. Then one version of Einstein's theory of gravity, the version containing the cosmological constant, made a second prediction: "empty space" could have its own energy.
Since this energy belongs to space itself, it will not decrease as space expands. When more space appears, more of this space energy will appear.
As a result, this form of energy would cause the universe to expand faster. Unfortunately, no one understands why the cosmological constant has to be there, much less why it has the right value to cause the observed acceleration of the universe.
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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 try to calculate how much energy this empty space will give, the answer is wrong - a lot wrong. The number came out 10120 times too big. That's 1 with 120 zeros after it. It's hard to get a bad answer. So, the mystery continues.
Another explanation for dark energy is that it is a new type of fluid or dynamic energy field, something that fills all space but something whose effect on the expansion of the universe is the opposite of normal matter and energy.
Some theorists have named this "essence," after the fifth element of the Greek philosophers. But, if quintessence is the answer, we still don't know what it looks like, interacts with, or why it exists. So, the mystery continues.
The final possibility is that Einstein's theory of gravity is incorrect. Not only would that affect the expansion of the universe, but it would also affect the way normal matter in galaxies and clusters of galaxies behaved.
This fact will provide a way to decide whether the solution to the dark energy problem is a new theory of gravity or not. What it takes to decide between the possibilities of dark energy, a property of space, a new dynamic fluid, or a new theory of gravity is more data, better data.
Meanwhile, according to the astronomy website, Dark Energy is a hypothetical form of energy that exerts negative pressure and repulses, behaving like the opposite of gravity. It has been hypothesized to explain the observed nature of distant type of supernovae, which suggest the universe is going through a period of accelerated expansion.
Like Dark Matter, Dark Energy is not observed directly, but is inferred from observations of gravitational interactions between astronomical objects.
Here's an explanation of dark matter:
First, it is dark, meaning that it is not in the form of the stars and planets that we see. Observations show that there is too little visible matter in the universe to meet the 27% required by observations.
Second, not in the form of a normal dark cloud of matter, matter is made up of particles called baryons. Nasa knows this because they can detect baryon clouds by the absorption of radiation passing through them.
Third, dark matter is not antimatter, because we don't see the unique gamma rays that are produced when antimatter is annihilated with matter.
Finally, we can rule out black holes the size of large galaxies based on how many gravitational lenses we see.
High concentrations of matter bend light passing near them from more distant objects, but we didn't see enough lensing events to show that these objects contributed the required 25% dark matter.
However, at this point, there are still some viable dark matter possibilities. Baryonic matter can still form dark matter if it's all bound up in brown dwarfs or in small, dense chunks of heavy elements.
These possibilities are known as massive compact halo objects, or "MACHOS". But the most common view is that dark matter is not baryonic at all, but consists of other, more exotic particles such as axioms or WIMPS (Weakly Interacting Massive Particles).
