Hey, it’s Professor Dave, I wanna tell you about dark matter.
When you really think about everything we have discussed in this astronomy series, you
have to admit that our current understanding of the universe is nothing short of astounding.
All of those objects in the night sky that we’ve wondered about since we could talk,
they are no longer gods and demons.
They are real, physical objects, with properties and characteristics that we can measure, categorize,
and predict with incredible accuracy.
We know about planets, asteroids, comets, stars, black holes, and galaxies.
We know what they’re made of.
We know how they behave.
We’ve sent objects through space, put men on the moon, and built sophisticated telescopes
that can see nearly to the edge of the observable universe, giving us a solid picture of how
and when the entire universe began.
But even with all this, we don’t know everything.
Just as with any other field of science, there are areas of active study, with brilliant
minds all over the world trying to answer perplexing questions about the universe.
One of the most puzzling of these areas regards our understanding of matter and energy.
Over the past few decades, it has become increasingly clear that there must be some kind of matter
out there in space that we can’t see.
This isn’t as mysterious as it sounds, as long as we remember what it means to see something.
An object emits electromagnetic radiation when electrons in its atoms relax from some
excited state down to some lower energy state, generating photons in the process.
When those photons hit our eyes, we see the object.
So if something is not capable of producing photons in this manner, or related nuclear
processes, it can’t produce visible light, or any other kind of light that we can detect,
like UV rays, radio waves, microwaves, or any of the rest.
But the particles that make up atoms aren’t the only kinds of particles there are.
There are lots of different kinds of particles, so it’s not outrageous to think that there
could be macroscopic quantities of matter that don’t interact with light.
Because of this behavior, we assigned it the name dark matter.
And although we don’t know what it is, we know what it’s not.
It can’t be any kind of baryonic matter, which is all the protons and neutrons that
make up all the ordinary stuff we can see.
How much dark matter is there?
A lot, apparently.
Even though we can’t see it, it still exerts gravitational influence, so we can measure
the mass of some amount of dark matter by analyzing the motion of nearby objects.
The best example of this has to do with the orbital velocities of stars within a galaxy.
Stars should travel at a speed dictated by how much mass is in between them and the center
of the galaxy.
So stars very close will go a little slower, as there isn’t as much mass in there.
Moving farther out, there is more mass tugging a star inward, and they should orbit faster.
Going towards the edge, stars should slow back down a bit, as despite the additional
mass, they are getting very far away, and gravity weakens rapidly with distance.
So we should expect velocity to rise rapidly and then decrease as we move from the center
of the galaxy to the edge.
But in fact, this is not what we observe.
Velocities rise initially as expected, but at a certain distance they level out, maintaining
roughly the same velocity all the way to the edge, violating Kepler’s second law.
This seemed puzzling, and it was proposed that there must be large amounts of unseen
mass in the outer regions of a galaxy, including ours.
After doing the math, it was realized that there actually must be up to ten times more
dark matter than regular matter, with every galaxy being embedded in a dark matter halo.
By making this assumption, the behavior of all the galaxies began to make perfect sense.
Now you may be wondering how we can jump to such a nonsensical conclusion, rather than
just restructuring our notion of orbital velocities.
Beyond the certainty of the math, there are other ways of demonstrating that dark matter
exists.
When we learned about general relativity, we saw that matter warps spacetime, and if
light travels through warped spacetime, its trajectory will be affected.
This is observed through a phenomenon called gravitational lensing, where the light from
a distant object curves around a nearer object, sometimes in more than one direction, making
several copies of the object in the sky, or fanning the light source out into smaller
regions of light.
When this nearer object is something visible, like a cluster of galaxies, we can perform
important calculations from such an observation, and calculations show that the magnitude of
the lensing requires much more mass than appears to be in the cluster.
Beyond this, gravitational lensing is sometimes witnessed around seemingly empty space.
In addition, when we look at the structure of the observable universe, galaxies seem
to be gathered into filaments, with huge voids in between containing almost no galaxies,
as though some network of massive objects is responsible for directing them there, and
calculations suggest that the amount of luminous matter we see could not have been enough to
cause matter to initially accumulate into stars and galaxies in the time that they did.
The notion that cold dark matter, unaffected by radiation, may have experienced perturbations
in its uniform density first, accumulating to form large-scale structures with a gravitational
potential that luminous matter was then attracted to later, perfectly accounts for a huge amount
of observation.
So between galactic orbital velocities, gravitational lensing, and a wide variety of other data
regarding the evolution of galaxies and other such phenomena, dark matter seems to be here
to stay.
So what are the best candidates for dark matter?
We know it’s non-baryonic, so what does that leave us with?
There could exist subatomic particles that go beyond the standard model of particle physics,
which could account for these observations.
One such type are called WIMPs, or weakly-interacting massive particles.
It may even be some kind of neutrino, supersymmetric particles, or any number of other exotic particles
that have been postulated but unconfirmed by experiment.
So this is a prime example of a situation where astronomers and particle physicists
have to work together to solve a problem.
Unfortunately dark matter is not the only burning question in astronomy.
There appears to be a similarly mysterious agent at work, which we call dark energy.
This concept arose as a result of measuring the rate of expansion of the universe.
We had thought that because of the gravitational attraction between all matter, the rate of
expansion should be slowing over time, as all the galaxies tug on each other, potentially
even halting the expansion and eventually contracting back into a single point.
This event would be called the Big Crunch, essentially the precise opposite of the Big Bang.
However, observations show that this is not happening.
Instead, the rate of expansion is actually accelerating.
What could possibly be causing this acceleration?
Gravity is only attractive, but is there some repulsive counterpart that becomes apparent
at very large scales?
Is it some other thing totally unrelated to gravity?
We don’t know, so we have dubbed this phenomenon dark energy.
Do we have any ideas as to what it could be?
We do.
One of them brings back Einstein’s cosmological constant, his self-professed biggest mistake.
It turns out that it might not have been such a mistake after all.
Einstein initially invoked this concept to support the steady state model for the universe,
describing a constant energy density that fills up every point in space evenly.
In his model, it was necessary to counteract any expansion or contraction to yield a static universe.
But this concept could just as readily apply to this problem, with such an energy that
actually accelerates expansion.
We represent this constant with a capital lambda, and we can think of it as a kind of
vacuum energy, the energy contained within space itself.
It’s extremely weak, and was completely masked in the earlier stages of the universe
when matter was more dense.
But as everything continued to drift apart, the universe becoming more sparse, and the
effects of gravity weakening, lambda continued to occupy every cubic centimeter of empty space.
Around five billion years ago the universe passed below a density beyond which lambda
began to be significant, pushing everything apart even faster, and today we see a universe
expanding at an accelerating rate.
But all of this is even more hypothetical than what we have already said about dark matter.
We really don’t know what dark energy is or how it does what it does.
There are competing models, all of which are based on some set of observations, but remain
largely speculative.
So to put things into perspective, based on present calculations, we believe that dark
energy makes up about 68 percent of all the energy in the observable universe.
Dark matter makes up about 27 percent, by mass-energy equivalence, while only the remaining
five percent is represented by all the stuff we can see.
Even still, while we must admit that there are plenty of things about the universe that
we don’t understand, there are also plenty of things that we do.
Enough even to understand how the universe began, so it’s not that surprising that
we can propose hypotheses regarding how the universe will end.
Let’s move forward and see what some of these possibilities could be.