What
are the explanations
for dark energy?
1) 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. In another version of Einstein's gravity
theory, the version that contains a cosmological
constant, he 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.
And so
one can say that dark
energy is simply the "cost of having space": that is, a volume
of space has some intrinsic, fundamental energy. This is the
cosmological constant. Since energy and mass are related by
E = mc2, Einstein's theory of general
relativity predicts that it will have a gravitational effect.
It is sometimes called a vacuum energy because it is the energy
density of empty vacuum. In fact, most theories of particle
physics predict vacuum fluctuations that would give the vacuum
this sort of energy.
3) 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.
4) Einsteins
theory of gravitation is not correct ----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).
|
In astronomy
and cosmology, dark matter is matter that neither emits
nor scatters light or other electromagnetic radiation, and
so cannot be directly detected via optical or radio astronomy.
Its existence is inferred from gravitational effects on visible
matter and gravitational lensing of background radiation,
and was originally hypothesized to account for discrepancies
between calculations of the mass of galaxies, clusters of
galaxies and the entire universe made through dynamical and
general relativistic means, and calculations based on the
mass of the visible "luminous" matter these objects contain:
stars and the gas and dust of the interstellar and intergalactic
medium. Many experiments to detect dark matter through non-gravitational
means are underway.
 |
|
Estimated
distribution of dark matter and dark energy in the universe
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.
|
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. W/e 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.
A small
proportion of dark matter may be baryonic dark matter: astronomical
bodies, such as massive compact halo objects, that are composed
of ordinary matter but which emit little or no electromagnetic
radiation. The vast majority of dark matter in the universe
is believed to be nonbaryonic, and thus not formed out of
atoms. It is also believed that it does not interact with
ordinary matter via electromagnetic forces; in particular,
dark matter particles do not carry any electric charge. The
nonbaryonic dark matter includes neutrinos, and possibly hypothetical
entities such as axions, or supersymmetric particles. Unlike
baryonic dark matter, nonbaryonic dark matter does not contribute
to the formation of the elements in the early universe ("Big
Bang nucleosynthesis") and so its presence is revealed only
via its gravitational attraction. In addition, if the particles
of which it is composed are supersymmetric, they can undergo
annihilation interactions with themselves resulting in observable
by-products such as photons and neutrinos ("indirect detection").
Nonbaryonic
dark matter is
classified in
terms of the mass
of the particle(s)
that is assumed
to make it up,
and/or the typical
velocity dispersion
of those particles
(since more massive
particles move
more slowly).
There are three
prominent hypotheses
on nonbaryonic
dark matter, called
Hot Dark Matter
(HDM), Warm Dark
Matter (WDM),
and Cold Dark
Matter (CDM);
some combination
of these is also
possible. The
most widely discussed
models for nonbaryonic
dark matter are
based on the Cold
Dark Matter hypothesis,
and the corresponding
particle is most
commonly assumed
to be a neutralino.
Hot dark matter
might consist
of (massive) neutrinos.
Cold dark matter
would lead to
a "bottom-up"
formation of structure
in the universe
while hot dark
matter would result
in a "top-down"
formation scenario.
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).
The
tug of war between dark matter and dark energy
"...If
the expansion of the universe is the result of a battle between
dark energy speeding things up and dark matter slowing things
down, then the history of cosmic expansion will have a record
of which entity was winning at various points. Because light
takes time to get to us, we can see into the past by observing
distant objects. In the recent past (say, the last five billion
years) we see acceleration. But if we could look far enough
into the past, then the balance should tip — the dark matter
should be denser when the universe was a smaller place, while
the dark energy, if it resembles the cosmological constant,
should hold steady. This would make the universe slow down.
Mr. Riess led a group that carried out these observations
with the Hubble Space Telescope. In 2004 and in 2007, his
team showed that the change from deceleration to acceleration
really happened: the predictions for a dark energy/dark matter
universe match the observations..." from NYtimes
article from Robert Kirshner
NOTE:
The Nobel Prize in Physics 2011 was divided, one half awarded
to Saul Perlmutter, the other half jointly to Brian P. Schmidt
and Adam G. Riess "for the discovery of the accelerating expansion
of the Universe through observations of distant supernovae".
