Dark Energy is a force that is repulsive in nature, which physicists believe to exist
throughout the universe, and the evidence for has moved from the circumstantial to the likely. Just
as with dark matter, where although it can't be directly observed, its effects can be seen. In this case the
Wilkinson Microwave Anisotropy Probe has made observations on the largest scale possible, the entire
universe, that indicate dark energy is actually needed to explain what we see.
It all started when the Department of Energy's Lawrence Berkeley National Laboratory,
and the High-Z Supernova Search team in Siding Spring Observatory in Australia had been running a project in
1998, measuring the distance to certain kinds of supernova (type Ia), in an attempt to in turn calculate the
rate of expansion (or contraction) of the universe. They expected to find the the rate of expansion was in
fact slowing, what they actually found was completely unexpected, the rate of expansion had been increasing!
So far the most succesfull model of the universe is the Big Bang theory, describing a universe that expanded
from a hot, dense, (arbitarily) small point. The rate of expansion after any cosmological inflation is
determined by the mass/energy of the universe, and the curvature 1 of the universe. If there is more
than a critical density, the 'explosion' of the big bang will blow it up to a certain size, before
gravity stops it and it begins to contract again, at an ever increasing rate. If there is less, nothing will
halt the expansion and the universe will carry on growing, at a rate that decreases over time; settling
to a constant positive value. Until recently it was thought that the energy density, 'Omega' of the universe is
exactly one, corresponding to a flat universe where the rate of expansion slows down continually until
the expansion is so slow it appears to be static. To make it come out as one, admittedly 'dark matter' has to
be added to the mix, which was at first a strange concept, but not totally unexpected. Which is why the results
of observations were so perplexing, no model of the universe predicted an expansion that increased with
To give a bit more detail, their initial results showed that for supernovae of a given redshift, its
brightness is less than calculated assuming the expansion were slowing. Not to difficult to deal with, maybe
the universes negative curvature could explain that, or maybe dust clouds or some other un-accounted for effect
relying on 'traditional' physics could cut down the novas brightness. However, detailed analysis, and further
observations strengthened the hypothesis that the universe is expanding, and not only that, but at seemingly
an ever increasing rate!. Some new repulsive effect, radically different from anything physicists
expected, had to be at work; this new effect whose nature remained unknown began to be termed 'dark energy'.
The case for dark energy was strengthened by observations made by the Wilkinson Microwave Anisotropy Probe,
whose detailed measurements of the changes in the temperature of the cosmic fireball suggested the universe has
a flat geometry; meaning there is exactly enough 'stuff' (whatever it may be) for the expansion to slow down to
zero. Hang on you might think, this result contradicts the supernova one! Well yes, it does but not all the
facts are in place, nor all the questions answered. The last question has to be 'What does the universe
actually consist of?'
Astronomers have of course been answering this question for thousands of years; and have built up an accurate
measure of the amount of material tied up in stars. Recent detailed observations of the rotation speed of
galaxies and gravatational lensing by super-clusters have now put an upper bound on the amount of non-luminous
'dark matter'. But none the less a conservative estimate says that 65% of the mass needed to make
the universe flat is still missing.
All the pieces of the puzzle are now in place :-
1. Supernova brightness show the universe to be much larger than thought, and expanding faster and faster.
2. The spectrum of the cosmic background radiation prove the universe to be flat.
3. Measurement and calculation of the mass present to give a flat universe can only account for (at the
absolute most) 35% of the necessary value.
What can bring these facts together is a force similar to gravity; in that it works over large distances and is
quite weak, but unlike gravity in that it is repulsive. It now all ties up, the mass
assiocated with the 'dark energy' (E=MC2) is enough to make the universe flat at the big bang,
but unlike gravity, it's blowing the universe apart. The latest observations tend to put the amount of dark energy as constant over space and time, rather that as a varying 'scalar' field known as quintessence. But it might just be that Einstein was correct with his first description of the universe,
which included a 'cosmological constant' working against gravity, necessary to keep the universe how he
thought it was, static. What dark energy is, (like dark matter) nobody knows; but of course theories abound,
further detailed observation of the background radiation should be able to prove some of them wrong.
The end is nigh!
So, taking the properties and amount (whatever it is) of dark energy into account, this is how our universe
will end, far down the river of time....The matter in the universe will cool and thin, and as it spreads
out new stars will be less likely to form. The galaxies will move farther apart, and the skies will dim, in
fact as the expansion accelerates more of the universe will become causally disconnected. That is to
say you'd have to go faster than the speed of light to ever reach parts of the universe, which is impossible.
Matter itself will decay into elementary particles, which due to dark energy, will fly apart at an ever
Black Holes may provide a haven for a time, the massive concentration of matter in them can be
harnessed for energy production. Eventually Hawking Radiation will cause these to evaporate into elementary
particles, which will spread out leaving the universe utterly dark and cold. Heat won't be transferred, as the
particles at some point will be so far apart; light won't be able to move between them. This is all the fault
of dark energy....
As time goes on, the evidence for dark energy mounts; the first results from NASA's Wilkinson Microwave
Anisotropy Probe (WMAP) suggested that the universe is 'flat', 13.7 billion years old and that a massive
73% of the universe is 'dark energy', 23% dark matter, leaving only a froth of 4% ordinary matter.
In 2004 published studies of galaxy clusters by the Chandra X-ray space telescope, point towards the fact the
expansion rate of universe stopped slowing down 6 billion years ago and has been speeding up ever since. Taking
all the various results, the distribution of of 'stuff' breaks down as follows :-
75% Dark Energy
21% Dark Matter
4% Visible Matter
It really is looking like the universe is doomed to end in a 'Big Rip'
Thanks to tdent
for technical advise and corrections! You spot any more, msg me, any questions
you can try me!
1. If the universe has positive curvature,corresponding to an Omega value greater than 1,
then it could be a 4 dimensional sphere, where the angles of a
triangle will add up to more than 180o. Zero curvature (Omega=1) means a flat
universe, with 180o triangles, and a negative universe (omegaless than 180o.