“When you make the finding yourself — even if you’re the last person on Earth to see the light — you’ll never forget it.” -Carl Sagan
When we talk about dark matter and its alternatives, we are talking about no less a task than explaining the structure of every large object in the Universe. This means every one of the billions of galaxies, including the way they form, merge, and cluster together.
On the largest scales — where each pixel in the map above represents an entire galaxy — dark matter blows all of its competitors away. In terms of explaining the large-scale structure of the Universe, not a single one of dark matter’s alternatives comes close to mirroring its success.
But of course, that doesn’t stop the sensationalist headlines from rolling in. We are understandably uncomfortable with the notion that we are not the most important thing in the Universe. We were against the Earth not being the Universe’s center, we were against the Sun just being another run-of-the-mill star, we were against the spiral nebulae in the sky being other galaxies just like our own, and now we’re against all the matter we know in the Universe — protons, neutrons, and electrons — being relatively unimportant compared to the amount of dark matter in our Universe.
And while dark matter’s been the only successful game in town on large scales — for galaxy clusters, for supercluster and filaments, for the fluctuations in the microwave background, for big bang nucleosynthesis, for gravitational lensing, etc. — its alternatives have held the advantage in one spot: for individual galaxies.
In what way is this the case?
Above is spiral galaxy NGC 6744, often referred to as the Milky Way’s twin. Although somewhat larger than our own galaxy, to the best of our measurements the structure of our galaxy, including the central bar and sweeping spiral arms, are the best match to this one out of all the known galaxies surveyed.
When we look out at spiral galaxies, we discover a relationship between the speed at which the galaxy rotates and the distance from the galaxy’s core. This relationship is very simple, and is better described by an empirical, phenomenological (i.e., not physically motivated, but data-motivated) model known as MOdified Newtonian Dynamics. There are a number of ways to obtain MOND, including by postulating some type of gravitational dielectric medium in the vacuum of space.
When you shove an insulator in an electric field, it’s made out of matter, which in turn is comprised of positive (nuclear) and negative (electron) charges. The insulator responds to the electric field, polarizes as shown above, and changes the electric potential of space.
The analogy would hold for gravitation if there were negative gravitational masses, or some type of gravitational dielectric. This is an idea that’s been around for a while, and Dragan Hajdukovic’s new paper has been getting quite the buzz for claiming that perhaps virtual antimatter particles in fact do just this, and behave as though they have negative mass. This could, in fact, explain why individual galaxies look the way they do.
What are the problems? Well, when we’ve modeled our structure formation for individual galaxies, using dark matter and normal matter in the expected ratios, we arrive at a few problems.
First off, it takes a while for these galaxies to become very bright. When we look back in the Universe, we see extremely bright galaxies — some even brighter than the Milky Way is now — when the Universe is just one or two billion years old! It was thought that Milky Way-like galaxies would need more time to form than that, and that they’d run out of gas to form new stars too early if they formed so brightly back then. Furthermore, simulations always gave us central bulges that were too big and too bright to be explained by conventional dark matter, and perhaps not-quite-right structure for the grand spiral arms.
Wouldn’t it be some news if we could solve these problems, and explain the galaxies in our Universe — the last great difficulty for dark matter — without having to resort to any of the alternatives?
Well, as I said at the start, haters to the left! We’ve just successfully figured out where the new material to form the Milky Way’s young stars is coming from: high-velocity intergalactic gas clouds! About a Sun’s worth of gas falls into the Milky Way (on average) every year, and this resupplies the Milky Way’s gas reserves, which get eaten up as new stars form over billions of years.
But what about the other, larger mystery? What about reproducing the structure of the Milky Way itself?
I know that some of you will never be convinced, not until a ball of dark matter can physically hit you in the face and make you cry, but this is a huge advance for dark matter! There are still alternatives worth considering, but they’ve got a long way to go before they stack up.
And so on this day, even as my parent company disses dark matter and promotes the alternatives, I say, “Haters to the left,” and happily bask in yet another outstanding success for the best theory for structure in the Universe: dark matter.