Most of us have heard of "dark matter" and "dark energy", but do you really know what these terms mean? Modern Cosmology can be extremely difficult to
grasp with all these different mechanisms which seem to be little understood. The truth is scientists still don't know much about dark energy or dark
matter, hence their names.
It's taken me quite some time to get a real handle on these concepts, now my aim is to explain these ideas in the simplest way possible, as I seem to
have a knack for that. These ideas really aren't that difficult to grasp, it's just that they have many different names and there are many theories
concerning what dark energy and dark matter are.
We will start with dark matter, because amazingly enough, this is probably the easiest to explain and understand out of all the following concepts.
Dark matter is similar to normal matter as far as we know, except that we can't detect it. The reason we know it must be there is because it has a
gravitational effect just like normal matter. Since huge objects bend space, photons (light) travelling near those large objects will follow a curved
path around the object.
This essentially causes large objects (such as galaxies or clusters of galaxies) to act as huge lenses. Objects behind the galaxies can be magnified
and distorted by the gravitational lensing effect. So we can use this to measure the mass of far off galaxies simply by looking at the degree to which
they bend the space around them. Our calculations show us that there is much more mass in those galaxies than we are able to directly observe with out
In fact we can't even detect this so called dark matter. After all it must exist in our own galaxy, but to date we haven't been able to detect it with
our instruments. It's basically invisible mass which seems to be completely undetectable other than the gravitational effects it has on space. It is
estimated that dark matter makes up about 85% of all matter. The total energy-density of the universe is estimated to consist of 70% dark energy, 25%
dark matter, 5% normal matter.
We will move onto dark energy next, but it's important to note that dark energy and dark matter have no real connection other than their names. Dark
matter might be called dark matter because we can't detect it, but it's also probably partly because we don't fully understand the real nature of it.
We also don't full understand dark energy so they are similar in that respect, but as we'll see they are two very different concepts (they may have
some deeper connection however).
Dark energy is the energy responsible for the expansion of the universe. There are many theories about what it could be, it may be some type of energy
fluid or field or it may be an inherent energy of empty space (vacuum energy caused by virtual particles). A more familiar name for dark energy is
Einstein's "cosmological constant". The cosmological constant is a mathematical constant which describes how much energy empty space must have to
account for inflation.
The simplest explanation for dark energy is that it is simply the "cost of having space": that is, a volume of space has some intrinsic,
fundamental energy. This is the cosmological constant, sometimes called Lambda (hence Lambda-CDM model) after the Greek letter Λ, the symbol used to
mathematically represent this quantity. Since energy and mass are related by E = mc2, Einstein's theory of general relativity predicts that this
energy 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. This is related to the Casimir Effect, in
which there is a small suction into regions where virtual particles are geometrically inhibited from forming (e.g. between plates with tiny
Inflation, if you are unaware, is a term describing the expansion of space. Yes, the space
between galaxies is expanding, which moves us
further away from all galaxies around us. It's not actually that all other galaxies are zooming away from us due to motion (some may be), but the main
reason they appear to move away is because the space between our galaxies gets stretched out. Gravity counter acts the expansion of space so that's
why the space inside our galaxies doesn't expand in the same way.
So when space expands (due to the vacuum energy), it causes inflation. However the new space which is created in that expansion process also contains
this vacuum energy. The energy is an inherent property of space so when the space expands the energy doesn't become diluted. The new space brings with
it new vacuum energy. Thus the rate of expansion doesn't slow down, creating the illusion of an accelerated expansion; the further away from us the
object is the faster it seems to be moving away from us.
Remember that it's the space expanding, so when the galaxy is further away from us there is more space between us undergoing expansion. Once there is
enough space between us it will be moving away from us faster than the speed of light (this is consistent with the general theory of relativity), and
the light from that galaxy will no longer be able to reach us. This represents the end of the "observable universe", it is like the event horizon of
our universe (from our frame of reference).
So at some distant point in the future we wont be able to see anything outside of our own galaxy because everything will be beyond the event horizon.
There are many reasons we know the space in the universe is expanding, most prominently the work done by Hubble in measuring the brightness of special
distant supernovae and the work following on from his. In order to explain this expansion we need some sort of mysterious energy, and that's where
dark energy/vacuum energy comes into play.
You also have to remember that if the Universe is "flat" and infinite (flat in a 3D sense) it means nothing is really "getting bigger" unless you
consider one infinity larger than another infinity (debatable). But if the universe is not infinite (meaning the universe curves back in on its self)
then we can say it is expanding at an exponential rate, since the larger it gets, the more space it will contain, and the new space is also expanding.
But from our perspective the end result is the same.
However we do have ways of measuring the shape or geometry of the universe. We can look at the afterglow of the big bang (cosmic microwave background
radiation) and with some clever tricks which I don't have time to get into we can measure the shape of the universe. Recent measurements by the WMAP
spacecraft indicate that the universe is probably "flat", or so close to flat we can't measure the curvature. And we know that for the universe to be
this flat it must have a certain critical density.
We have ways of measuring the total amount of matter in the universe, but when we measure the total amount of matter in the universe (including dark
matter) we find that there's only about 30% of the critical density required. This indicates the existence of another type of "dark" energy which can
account for the remaining 70% and also help us explain inflation. However we find that 70% dark energy would cause a far greater acceleration of the
universe than what we measure.
To reconcile this we require yet another mysterious force/energy which acts to provide a positive pressure to cancel out the negative pressure of the
dark energy (the negative pressure causes the expansion). But this mysterious new energy must not entirely cancel out the dark energy, it must leave
the amount we were looking for to account for the correct rate of expansion which we measure. You may begin to see why this is hard to learn.
The cosmological constant is estimated by cosmologists to be on the order of 10^-29g/cm3, or about 10^-120 in reduced Planck units. Particle
physics predicts a natural value of 1 in reduced Planck units, leading to a large discrepancy.
A major outstanding problem is that most quantum field theories predict a huge cosmological constant from the energy of the quantum vacuum, more than
100 orders of magnitude too large. This would need to be cancelled almost, but not exactly, by an equally large term of the opposite sign.
edit on 22/2/2013 by ChaoticOrder because: (no reason given)