Today, we will talk about dark energy.
What’s the difference between dark energy
and dark matter?
What does dark energy have to do with the
cosmological constant and is the cosmological
constant really the worst prediction ever?
At the end of this video, you will know.
First things first, what is dark energy?
Dark energy is what causes the expansion of
the universe to accelerate.
It’s not only that astrophysicists think
the universe expands, but that the expansion
is actually getting faster.
And, here’s the important thing, matter
alone cannot do that.
If there was only matter in the universe,
the expansion would slow down.
To make the expansion of the universe accelerate,
it takes negative pressure, and neither normal
matter nor dark matter has negative pressure
– but dark energy has it.
We do not actually know that dark matter is
really made of anything, so interpreting this
pressure in the normal way as by particles
bumping into each other may be misleading.
This negative pressure is really just something
that we write down mathematically and that
fits to the observations.
It is similarly misleading to call dark energy
“dark”, because “dark” suggests that
it swallows light like, say, black holes do.
But neither dark matter nor dark energy is
actually dark in this sense.
Instead, light just passes through them, so
they are really transparent and not dark.
What’s the difference between dark energy
and dark matter?
Dark energy is what makes the universe expands,
dark matter is what makes galaxies rotate
faster.
Dark matter does not have the funny negative
pressure that is characteristic of dark energy.
Really the two things are different and have
different effects.
There are of course some physicists speculating
that dark energy and dark matter might have
a common origin, but we don’t know whether
that really is the case.
What does dark energy have to do with the
cosmological constant?
The cosmological constant is the simplest
type of dark energy.
As the name says, it’s really just a constant,
it doesn’t change in time.
Most importantly this means that it doesn’t
change when the universe expands.
This sounds innocent, but it is a really weird
property.
Think about this for a moment.
If you have any kind of matter or radiation
in some volume of space and that volume expands,
then the density of the energy and pressure
will decrease just because the stuff dilutes.
But dark energy doesn’t dilute!
It just remains constant.
Doesn’t this violate energy conservation?
I get this question a lot.
The answer is yes, and no.
Yes, it does violate energy conservation in
the way that we normally use the term.
That’s because if the volume of space increases
but the density of dark energy remains constant,
then it seems that there is more energy in
that volume.
But energy just is not a conserved quantity
in general relativity, if the volume of space
can change with time.
So, no, it does not violate energy conservation
because in general relativity we have to use
a different conservation law, that is the
local conservation of all kinds of energy
densities.
And this conservation law is fulfilled even
by dark energy.
So the mathematics is all fine, don’t worry.
The cosmological constant was famously already
introduced by Einstein and then discarded
again.
But astrophysicists think today that is necessary
to explain observations, and it has a small,
positive value.
But I often hear physicists claiming that
if you try to calculate the value of the cosmological
constant, then the result is 120 orders of
magnitude larger than what we observe.
This, so the story has it, is the supposedly
worst prediction ever.
Trouble is, that’s not true!
It just isn’t a prediction.
If it was a prediction, I ask you, what theory
was ruled out by it being so terribly wrong?
None, of course.
The reason is that this constant which you
can calculate – the one that is 120 orders
of magnitude too large – is not observable.
It doesn’t correspond to anything we can
measure.
The actually measureable cosmological constant
is a free parameter of Einstein’s theory
of general relativity that cannot be calculated
by the theories we currently have.
Dark energy now is a generalization of the
cosmological constant.
This generalization allows that the energy
density and pressure of dark energy can change
with time and maybe also with space.
In this case, dark energy is really some kind
of field that fills the whole universe.
What observations speak for dark energy?
Dark energy in form of a cosmological constant
is one of the parameters in the concordance
model of cosmology.
This model is also sometimes called LambdaCDM.
The Lambda in this name is the cosmological
constant and CDM stands for cold dark matter.
The cosmological constant in this model is
not extracted from one observation in particular,
but from a combination of observations.
Notably that is the distribution of matter
in the universe, the properties of the cosmic
microwave background, and supernovae redshifts.
Dark energy is necessary to make the concordance
model fit to the data.
At least that’s what most physicists say.
But some of them are claiming that really
the data has been wrongly analyzed and the
expansion of the universe doesn’t speed
up after all.
Isn’t science fun?
If I come around to do it, I’ll tell you
something about this new paper next week,
so stay tuned.
