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.
This 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.
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.
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.
But…what is
dark matter?
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.
However, at
this point, there are still a few dark matter possibilities that are viable.
Baryonic matter could still make up the dark matter if it were all tied 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 that it is made up of other, more
exotic particles like axions or WIMPS (Weakly Interacting
Massive Particles).
Info via NASA
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