“Nothing in life is to be feared, it is only to be understood. Now is the time to understand more, so that we may fear less.” -Marie Curie
After all, when you look at what we call small-scale structure, from dwarf spheroidal galaxies up to the scales of some very small galaxy clusters, MOND works even better than dark matter does!
So what is dark matter, and why am I so dismissive of MOND? Let’s go through it.
This is the Coma Cluster of galaxies, the closest huge galaxy cluster to us in the Universe. In 1933, Fritz Zwicky looked at the speeds of the moving galaxies inside the cluster and realized they were moving too quickly; unless there were far more mass than the galaxies themselves were showing, the cluster itself would be unstable and fly apart!
The invisible mass that must be there, holding it together, he reasoned, was the birth of what we now call dark matter.
And so we can test it. Put some extra matter into Einstein’s general theory of relativity (GR), and what should you get? Well, that extra mass should bend light according to GR by a certain amount, and when we observe galaxy clusters, we see huge arcs due to this effect!
How much extra, dark mass is there? About 5 times as much as there is normal matter. Which, perhaps coincidentally, the same number we get for pretty much all large clusters.
We physical cosmologists really like our large scale structure and our general relativity, so we also go back and imagine a Universe filled with 5 times as much dark matter as there is normal matter, and we do simulations re-creating the formation of large scale structure in the Universe.
And we get some predictions. How do they line up with what we actually observe, from big sky surveys such as 2dF and SDSS?
Again, the same number there: about five times as much dark matter as normal matter gives the expected clustering. And this works for really, really detailed analysis, ranging from looking at peculiar velocities of galaxy pairs to the magnitude of Silk Damping in the transfer function.
And so we look at the last major piece of cosmic evidence: the microwave background.
Again, that same “five times the amount of dark matter as normal matter” as before. It gives the correct scales for the formation of structure in the Universe, from galaxies up to superclusters. It gives the right amount of gravitational lensing in the right places. It gives the cosmic web down to the smallest measurable details. And, in glorious gory detail, it even gives you the imperfections in the microwave background!
That’s our predictions, on the line, with our data, shown as points, courtesy of the WMAP team. What’s amazing is that this not only works on large scales in every observable fashion in a self-consistent manner, but it even has something intelligible to say about individual galaxies; namely, that they all should have a halo surrounding them, on average, of about five times as much dark matter as normal matter.
And it is this last point only that MOND argues on. Because in order to accept MOND as superior to dark matter, you need to ignore all of physical cosmology.
And the best argument they’ve got is a 50 year old quote from Lev Landau:
Cosmologists are often in error but seldom in doubt.
Well, that quote is back from before the development of physical cosmology due to Dicke, Peebles, Zel’dovich, Ed Groth, David Wilkinson, and many, many others. Hell, that quote is from before they even discovered the microwave background and confirmed the Big Bang! So maybe we should point more frequently to Mike Turner’s quote from a decade ago:
I am certain that it is time to retire Landau’s quote.
So you can doubt dark matter, and there’s no problem in being skeptical, but do you have a better explanation? The answer is no, and until you can explain the large scales, all you’re doing is cosmological guesswork, and insulting the entire field of physical cosmology.
Which isn’t to say that dark matter is the only option. But it’s remarkable that just by adding some cold matter to the Universe, you get a compelling model that you can use to make predictions from sub-galactic scales all the way up to the largest superclusters and filaments in the Universe.
Dark matter is the only option that has an answer for all of these scales, even if we still don’t understand galactic dynamics — or small scale structure — completely. And that’s the burden that any alternative must address: how do you deal with all of these structures on all of these different scales?
And until any alternative can do that, I’ll stick with my dark matter. Here’s hoping that — by its nature — it turns out to be something we can easily find and directly detect, because it doesn’t have to be!