I have noticed that each time I talk about dark energy or dark matter, I get a lot of
comments from people saying, oh that stuff doesn’t exist, you can’t just invent something
invisible each time there’s an inconvenient measurement. Physicists have totally lost
it. This is such a common misunderstanding that I thought I will dedicate a video to
sorting this out. Dark energy and dark matter are entirely normal, and perfectly scientific
hypotheses. They may turn out to be wrong, but that doesn’t mean it’s wrong to consider
them in the first place.
Before I say anything else, here is a brief reminder what dark energy and dark matter
are. Dark energy is what speeds up the expansion of the universe; it does not behave like normal
energy. Dark matter has a gravitational pull like normal matter, but you can’t see it.
Dark energy and dark matter are two different things. They may be related, but currently
we have no good reason to think that they are related.
Why have physicists come up with this dark stuff? Well, we have two theories to describe
the behavior of matter. The one is the standard model of particle physics, which describes
the elementary particles and the forces between them, except gravity. The other is Einstein’s
theory of general relativity, which describes the gravitational force that is generated
by all types of matter and energy. The problem is, if you use Einstein’s theory for the
matter that is in the standard model only, this does not describe what we see. The predictions
you get from combining those two theories do not fit to the observations.
It’s not only one prediction that does not fit to observations, it’s many different
ones. For dark matter it’s that galaxies rotate too fast, galaxies in clusters move
too fast, gravitational lenses bend light too strongly, and neither the cosmic microwave
background nor galactic filaments would look like we observe them without dark matter.
I explained this in detail in an earlier video. For dark energy the shooting gun signature
is that the expansion of the universe is getting faster, which you can find out by observing
how fast supernova in other galaxies speed away from us. The evidence for dark energy
is not quite as solid as for dark matter. I explained this too in an earlier video.
So, what’s the scientist to do when they are faced with such a discrepancy between
theory and observation? They look for new regularities in the observation and try to
find a simple way to explain them. And that’s what dark energy and dark matter are. They
are extremely simple terms to add to Einstein’s theory, that explain observed regularities,
and make the theory agrees with the data again.
This is easy to see when it comes to dark energy because the presently accepted version
of dark energy is just a constant, the so-called cosmological constant. This cosmological constant
is just a constant of nature and it’s a free parameter in General Relativity. Indeed,
it was introduced already by Einstein himself. And what explanation for an observation could
possibly be simpler than a constant of nature?
For dark matter it’s not quite as simple as that. I frequently hear the criticism that
dark matter explains nothing because it can be distributed in arbitrary amounts wherever
needed, and therefore can fit any observation. But that’s just wrong. It’s wrong for
two reasons. First, the word “matter” in “dark matter” doesn’t just vaguely
mean “stuff”. It’s a technical term that means “stuff with a very specific behavior”.
Dark matter behaves like normal matter, except that, for all we currently know, it doesn’t
have internal pressure. You cannot explain any arbitrary observation by attributing it
to matter. It just happens to be the case that the observations we do have can be explained
this way. That’s a non-trivial fact.
Let me emphasize that dark matter in cosmology is a kind of fluid. It does not have any substructure.
Particle physicists, needless to say, like the idea that dark matter is made of a particle.
This may or may not be case. We currently do not have any observation that tells us
dark matter must have a microscopic substructure.
The other reason why it’s wrong to say that dark matter can fit anything is that you cannot
distribute it as you wish. Dark matter starts with a random distribution in the early universe.
As the universe expands, and matter in it cools, dark matter starts to clump and it
forms structures. Normal matter then collects in the gravitational potentials generated
by the dark matter. So, you do not get to distribute matter as you wish. It has to fit
with the dynamical evolution of the universe.
This is why dark matter and dark energy are good scientific explanations. They are simple
and yet explain a lot of data.
Now, to be clear, this is the standard story. If you look into the details it is, as usual,
more complicated. That’s because the galactic structures that form with dark matter actually
do not fit the data all that well, and they do not explain some regularities that astronomers
have observed. So, there are good reasons for being skeptical that dark matter is ultimately
the right story, but it isn’t as simple as just saying “it’s unscientific”.
Thanks for watching. See you next week.