Dark Energy, 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 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 Is Dark Energy?
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.
NASA/STSci/Ann Feild
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.
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 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.
What Is Dark Matter?
One of the most complicated and dramatic collisions between
galaxy clusters ever seen is captured in this new composite image of
Abell 2744. The blue shows a map of the total mass concentration (mostly
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. 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 25% 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).
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