“The only relevant test of the validity of a hypothesis is comparison of prediction with experience.” -Milton Friedman
Dark matter is one of the most important components of the Universe today. And yet in the public’s eye, almost no one accepts it the way, say, the Big Bang is accepted. But it should be, and I’ll show you why.
I’ve talked before about what it took to convince me that dark matter was the best theory out there (and gave a simpler version here), but — with alternative theories making big headlines — it’s important to truly know why dark matter is so successful and so thoroughly accepted by the scientific community, and why the alternatives really don’t stack up.
When you ask the question, “what’s the Universe made out of,” some part of that answer is obvious. There are atoms — protons, neutrons, and electrons — that make up, at a fundamental level, everything you’ve ever experienced here on Earth.
There is light! Photons, coming from the Sun and the stars, not only come in visible wavelengths, but also in ones invisible to our eyes, from ultra-high-energy gamma rays down to ultra-low-frequency radio waves.
And there are some less obvious — but still detectable — things, such as neutrinos, heavy unstable particles (like the muon, tau, and Z-boson), and the quarks and gluons that come together to form the protons and neutrons themselves.
And your first — and very reasonable — instinct would be to try and model our Universe as being made out of these constituents only, subject to our known laws of electromagnetism, nuclear and particle physics, and gravity. If our Universe is subject to these known laws and is made out of the stuff we know about, we should be able to be quite successful at modeling it so.
As a graduate student in physical cosmology, it was one of the first things I, personally tried to do. (Nearly a decade ago now; yikes!)
Now, here’s the truly interesting this: it doesn’t just fail, it fails spectacularly on a large number of levels.
It’s very frustrating, because these fundamental laws of physics are correct in every way that we’ve ever been able to test them. General relativity, quantum field theory, and classical electromagnetism have never failed us when they’ve been applied properly. And yet, something as simple as the shape of a spiral galaxy is unexplainable in this fashion.
So are galaxy clusters, in which the individual galaxies move far too quickly to be explained by this conventional, simplistic picture of the particles we know combined with the laws of physics that we know.
The large-scale clustering of galaxies in the Universe cannot be explained, either. We get a definitive set of predictions, and despite however we choose to tweak the densities of the various known particles, we cannot reproduce what is seen using the known particles and the known laws.
But it isn’t just the structure of the Universe.
The fluctuations in the temperature of the Universe don’t match up with any known way of tweaking these parameters. (In fact, the way to get closest is if you fill up the Universe with a huge percentage of neutrinos!) And in perhaps the most simple test…
How much hydrogen and helium are in the Universe? Given the amount of matter that we have, it won’t give us the right, observed ratio of hydrogen to helium if it’s all made out of “normal” stuff.
(There are plenty of other observational tests that fail to line up, including the unacceptably short lifetime of the Universe, but these are some of the major ones to look at.)
So we have a few options for things we can try, and they’re all dissatisfying at some level. Let’s look at the leading ones, plus one new one.
1.) There’s dark matter. Try throwing some cold (i.e., slow-moving) dark matter into your Universe, and it fixes all of these problems. The simulated clustering of galaxies (above) lines up with what we observe, the patterns of fluctuations in the microwave background suddenly match what theory predicts…
The predictions of gravitational lensing in clusters of galaxies — as well as the speeds of galaxies in clusters — match up with what we see, and the abundance of the light elements work out in an agreeable way with what the theory now predicts given the presence of dark matter…
Every galaxy gets outfitted with a halo of this dark matter, and this fixes the problem of it rotating (or winding up) too quickly on the inside. And when we apply the idea of dark matter to a very bizarre case, such as two colliding clusters of galaxies:
Dark matter manages to correctly predict what we’ll observe even when the gravitational dark matter gets physically separated from the normal, light-producing matter!
In short, by adding this one novelty to the Universe — dark matter — we can suddenly solve all of these unexplained problems in cosmology.
But some people will never be convinced of this until we figure out what this dark matter actually is, and learn how to detect it directly, not merely indirectly via the influence of its gravity. And so we can try to consider other options.
2.) Modify the Laws of Gravity. The second leading option is to assume that there are no extra particles or sources of mass or gravitation in the Universe, but to assume instead that — on large scales in the Universe — the laws of gravity are different from Einstein’s General Relativity.
The one huge success of this is that this idea gives you the velocities within individual galaxies, and the predictions work in a superior fashion to those made by dark matter!
But modifying gravity does not explain any of the other observations sufficiently, or in the case of the microwave background and the light element abudances, at all. In addition, you have to give something up: general relativity.
So in addition to all these things you still need to explain, you also need a new way of explaining all the things general relativity currently explains, such as gravitational lensing, shown above. Perhaps someday a modified theory of gravity will be able to sufficiently explain all of these things, and if so, it will be competitive with dark matter. But until it can do those things, it will (and should) be severely disfavored when compared with dark matter, the same way that a bowl of flour and sugar is disfavored when compared with a freshly baked cake.
And the new one…
3.) Maybe antimatter has negative mass. This one could, conceivably, explain the same things that dark matter does, although it would need to be worked out.
But you have to give up a ridiculous number of things that are known to be necessary, including:
E = mc2, or the equivalence between mass and energy. In other words, sometimes E = -mc2. The fact that positrons (anti-electrons) have been observed experimentally to be attracted towards (and not away from) the center of the Earth pretty much rules this one out.
But if this were true, then we would also have to accept that…
You need to get rid of energy conservation! Not in the general relativistic sense, but in terms of fundamental particles and their interactions, which is hugely problematic, and flies in the face of every single high-energy physics experiment ever done!
(It would also violate the CPT theorem, which we believe is necessary for the Universe to exist the way that it does.)
It’s important to challenge the accepted model, to test it, poke at its holes and frailties, and to consider all conceivable alternatives.
But it’s also important to realize why it’s the accepted model, and that the alternatives fail so spectacularly. Rob Knop has taken a go at this as well, where he adds:
In the face of evidence otherwise, many still insist that most of the Universe must be made up of baryonic stuff that interacts with other baryons and our familiar photons. Is this not just as much hubris as insisting that the Earth, where we live, must be the center about which all the other Solar System bodies orbit?
And that’s why — despite some people’s predilections for denying dark matter — it really, truly, most likely exists, and the current challenge for cosmological physics is to figure out how to detect it. In the meantime, appreciate what we know and how we know it; it’s the only Universe we’ve got!