“We are just an advanced breed of monkeys on a minor planet of a very average star. But we can understand the Universe. That makes us something very special.” -Stephen Hawking
You’re probably familiar with the standard picture of our Universe. You’ve heard it all before: that the Universe we know of — stars, planets, atoms, etc. — is less than 5% of the Universe’s total energy. That most of the matter is dark matter, and that most of the energy in the Universe isn’t matter at all, but dark energy.
But recently, we’ve started to discover a couple of interesting things about the atoms in the Universe. First off, take a look up at the night sky, and you’ll be greeted by a familiar sight.
Stars! Our galaxy — like all galaxies — is full of them, as you can see by looking at M46 and M47, above. We’ve known for a long time that there aren’t enough stars in the galaxy to explain what we see gravity doing, so we know that most of the mass in the Universe isn’t stars.
At least, it isn’t conventional stars. But you might start to wonder, what if there are lots of small, low-mass, very dim stars out there? What if, in fact, there are failed stars out there?
Not just dim, red, M-stars, which still fuse hydrogen into helium, as long as they’re about 1/12th as massive as the Sun. But even smaller, lower mass ones. Maybe they can fuse deuterium, which makes them brown dwarfs, or maybe they’re just big, Jupiter-like blobs on their own. Regardless, this could be considered some type of “dark” matter, because we don’t see it with standard telescopes.
However, there is a very clever way to detect such objects.
When one of these “rogue planets” passes in between us and a background star, we’ll see that star briefly brighten and then dim again, thanks to a process called gravitational microlensing. While searches such as MACHO and EROS showed that these objects can’t be most of the missing matter, they can still be a significant amount.
And recently, a team has found many more of these planets — freely floating through space and not attached to any star — than we thought! Again, it isn’t enough to be all (or even most) of the dark/missing matter, but it’s something!
We can also look at other things that have mass: things that aren’t stars, planets, or other collapsed objects. Things like interstellar gas and dust, like Bok Globule B68.
After all, if there’s plenty of gas and dust, maybe that could be some of the dark matter, too! In fact, it’s just been discovered that there’s plenty of this, too, in the Universe. It’s actually really cool. When we look far out in the Universe, we can map out where the galaxies we can see are.
You’ll notice that the shape of this looks like some type of web, or network. (Someone has even pointed out to me that it looks a lot like a series of neurons in the brain!) If we try to simulate structure in the Universe, we get something that matches observations (and neurons) very well.
If you look at the “nodes” above, that’s where you’re going to find the greatest concentrations of galaxies clustered together. But if you look between the nodes, along the imaginary lines connecting them, you’ll find a few, small galaxies, sure. But you’ll also find X-rays, which come from the collapsing gas clouds!
So, if there are more rogue planets than we thought, and more dim stars than we thought, and more intergalactic gas and dust than we thought, is it possible that we don’t need dark matter? Or, a little more conservatively, is it possible that we need less dark matter?
There’s only one way to decide: let’s ask the Universe! We can look at cosmic structure formation, above, as well as…
The fluctuations in the Cosmic Microwave Background, and…
The primordial abundances of the light elements: Hydrogen, Helium-3, Helium-4, Deuterium, and Lithium.
These are observations we can make that tell us how much “atomic” matter there is — stuff made out of protons, neutrons, and electrons — versus how much is truly some new type of matter that doesn’t emit light.
And all of these observations — these independent observations — point to the same thing: a Universe that’s about 4.5% atoms.
Not only can we not get rid of dark matter, we can’t even make a dent in it!
But then, what do these extra planets, dim stars, or gas mean for our Universe?
The truth of the matter is, they simply tell us how that 4.5% of atoms is divided up.
It’s important and fun to know how the normal matter in the Universe is divided up, and how much of our Universe is made of stars, planets, gas, dust, or anything else you can think of, but no matter how it’s divided, you can’t replace dark matter with it.
Too many things would be different. Large-scale structure would be all wrong; you’d see too much Silk damping. Nucleosynthesis would be all wrong; you’d have too much helium and too little deuterium. And the fluctuations in the microwave background would be all wrong; the third peak wouldn’t be there.
It’s why we do the measurements we do, and this is what we learn from them: physical cosmology requires a Universe with 20-25% dark matter, and with just 4-5% of normal (atomic) matter. And what an interesting thing to learn: no matter how the 4.5% of the Universe that’s made out of atoms is split up, we still need just as much dark matter to make the Universe the way it is.