Dark Matter Candidates: Running for 23% of your Universe!

“[I]f there were no light in the universe and therefore no creatures with eyes, we should never know it was dark. Dark would be without meaning.” –C.S. Lewis

(For readers Mu and Bjoern.)

I’ve told you the entire history of the Universe, from before the Big Bang into the far distant future, and yet many of you noticed there was a conspicuous missing piece to the story: dark matter.

Living in halos around our galaxies and clusters, dark matter, at present, makes up 23% of the energy density of the Universe. This makes it second in overall importance to dark energy, which has become important only recently, but makes it about five to six times as abundant as normal matter.

Yes, normal matter — protons, neutrons, electrons, etc. — all the stuff our everyday world is made out of, is only 4-5% of the total energy in the Universe.

But in the past, before dark energy was important, dark matter was the dominant thing, as far as energy is concerned, in the Universe. Hugely important, gravitationally, for the formation of structure in the Universe, including galaxies, clusters and superclusters, dark matter is mind-numbingly upsetting to physicists and astronomers everywhere for two simple reasons.

  1. We can see its gravitational effects. It is the best explanation for what we see in all aspects of physical cosmology, from fluctuations in the microwave background to large-scale structure, from supernovae data to gravitational lensing, dark matter is far and away the most superior explanation.
  2. And we have no idea how, other than gravitationally, to interact with it.
Image credit: NASA / ESA / Richard Massey.

But, if we knew what it was, we could figure out how to interact with it. Because dark matter wasn’t always important. Back when the Universe was just a few thousand years old and earlier radiation was the dominant thing. Not just photons (light), mind you, but any high-energy particle that moves close to the speed of light.

From the moment of the Big Bang and onwards, the Universe was full of it. Perhaps one of the more bizarre things, though, is if you think about all the particles we know exist in our Universe. They come with different spins and charges, they come in matter and anti-matter varieties, they come in lots of different types and flavors. You know most of them already: they’re in the standard model.

The thing is, they all acted like radiation in the Early Universe, even the heaviest one: the top quark. It was too hot, and they had too much energy. And they weren’t the only things around, either.

Image credit: University of Glasgow.

There were likely more, as-of-yet undiscovered particles, because the temperatures back then were far in excess of what we’ve ever created or observed on Earth. Supersymmetry is the leading example of this: a new set of particles that are heavier, but should be able to be created at high enough energies.

The Early Universe, shortly after the Big Bang, surely has enough energy to create these in abundance, if they exist. And as the Universe cools, they annihilate away and/or decay, leaving us with nothing.

Except, of course, if the lightest one is stable.

Image credit: Sandbox Studio.

Dubbed the “neutralino,” these are almost completely gone from annihilating away from the Universe when it’s just a second or so old. The Universe, at this time, is still dominated by radiation. But as we expand and cool, the radiation becomes less and less important, and the little bit of matter suddenly, by time the Universe is many thousands of years old, comes to dominate.

And at this moment, when you look up, you discover that these neutralinos, most of which were destroyed a long time ago, are now the dominant form of energy in the Universe, and will be for billions of years, until Dark Energy takes over.

And that’s one dark matter candidate, although many others (such as the Lightest Kaluza-Klein Particle) have very similar stories.

But dark matter has another interesting option.

Much like the Antarctic Fur Seal, dark matter could be born cold, due to something like a phase transition. An example of this is the axion, which — when it’s first produced, again when the Universe is less than a second old — is a tiny, tiny fraction of the total energy density. It’s completely negligible, as only the radiation is important. But as the Universe cools, the radiation’s energy density drops faster than the matter density, and by time the Universe is around 10,000 years old, these axions have surpassed radiation, and come to dominate the Universe.

In either of these cases, interacting with them should be possible: we just need to re-create the conditions of the Universe where their interactions were abundant, or, barring that, to be very clever about how it might interact today.

Image credit: CDMS collaboration.

So we search for dark matter. And we make assumptions as to how it interacts, and we poke around in the dark, trying to get it to interact with us. We haven’t successfully done it yet, but that’s ok: this is science in progress. I have every confidence that we’ll figure it out, just as hundreds of years ago we figured out first how to interact with electricity, and then later, how to master it.

And if you thought discovering how to interact with and master electricity and magnetism led to some advances, just imagine what learning how to control our interactions with dark matter would allow us to do.

Limitless energy. Not in some silly-but-untrue way, but in the 100% efficient, perfect sort of way. How’s that?

For most candidates for dark matter, including both of the leading ones explained above, dark matter is its own antiparticle, which means that if you collide it with another dark matter particle, it turns into pure energy, the same way as if you collide matter with antimatter.

But dark matter is innocuous, and doesn’t run the risk of annihilating with us the way antimatter does.

Safe, clean, abundant, 100% efficient energy. Yes, it’s on the very, very distant horizon, the same way Benjamin Franklin probably couldn’t have imagined the laptop computer I’m writing this post on. But it’s one of the goals we aspire towards, and if we can not only interact with, but master our interactions with dark matter, this is what awaits us.

And so we look. We look for neutralinos, above (credit: CDMS collaboration), and axions, below (credit: ADMX collaboration).

And, long-term, if we can figure out how to interact with it, control/harness it, and collide it with more dark matter under controlled conditions, we’ll have a free, virtually limitless source of energy, with no waste.

So if learning about the Universe wasn’t enough reason for you to want to look, now you’ve got a practical one, too.