Deep into the darkness peering,
long I stood there, wondering, fearing,
Doubting, dreaming dreams no mortal ever dared to dream before.
-Edgar Allen Poe, The Raven
Gazing out into the dark abyss of the night sky, stars, galaxies, and clusters shine like tiny islands of light against the blackness of deep space. Trillions upon trillions of protons and neutrons fuse together in stars across the Universe, producing all of this light, and decorating the sky above.
But, as we learned in part one of this series, the starlight that we see only accounts for 2% of all the matter that gravity tells us is there. What’s more than that, is that we can figure out how much normal matter (i.e., stuff made up of protons, neutrons, and electrons, etc.) is around. We explored this in part 2, and found that this brings us up to maybe 15-20% of the total, but no more.
So, where do we go from here? Do we invent a new type of matter, and give it some bland, generic name like dark matter, or do we conclude that gravity is lying to us, because we’ve been using the wrong theory?
In principle, either one of these two explanations would suffice (although I’m hard-pressed to think of a third). If you’re willing to add some new type of matter that doesn’t behave the same way that normal matter does, you could explain all of the observations we discussed in the first two parts. But can you do this by modifying gravity?
I am telling you now, the answer appears to be no, as long as you demand that your new theory of gravity be sensible. Here’s the three killer reasons why.
1.) General relativity works. In all the instances where Newton’s theories failed observationally — for the orbit of Mercury, for the bending of light by stars and galaxies, for pulsar timing variations and orbital decay, for the Lense-Thirring effect, and more — Einstein’s general theory of relativity saves us. If you’re going to modify gravity, you’d better make sure your new theory can reproduce all of the successes of general relativity.
This immediately means that MOND (MOdified Newtonian Dynamics) is out as a replacement theory, as it fails to solve each and every one of the problems above. But maybe there’s a way to do it. Maybe there’s a way to modify Einstein’s gravity (Bekenstein and Moffat have tried) to still keep all of the good stuff of general relativity and also to get rid of dark matter. But then this observation came along.
There are two clusters of galaxies here, interacting with each other. These galaxy clusters have just collided, clashing into one another. How do we know? The X-ray data shows us the evidence of a big SPLAT! that happened just a few million years ago:
This splat tells us where the majority of the normal matter is: right in the center of the image. You can even see the cone-like shockwave on the right side. But what if we look for the evidence of gravity? We can look at the lensing data to tell us where the mass is.
And this is the key find. When you lay these images on top of one another, you find out, very clearly, that the majority of the mass is not where the majority of the gravity is. Which brings us to point number 2.
2.) Gravity does not always line up with where the normal matter lives. This tells us that either, again, we need dark matter, or we need to accept that not only is General Relativity wrong, but that gravity works in a very bizarre way. We’d have to modify gravity an additional way to make it non-local, or to make things gravitate towards places where there isn’t any mass! This is very unsettling, but it’s the only way to reconcile these observations without using dark matter.
And it can be done! In fact, Moffat (described above) made a version of this that works. Although it’s extremely messy mathematically, he can get this “dumbbell” shape for gravity out of this configuration of mass without dark matter. Sure, it’s non-local, but maybe that’s not an impossibility.
There are, however, two interesting things that — for me — are convincing enough to kill this idea. Think about this, if you will: what did the bullet cluster look like before these two clusters collided? Wherever the normal matter (pink) was must have lined up precisely with where the gravity (blue) was. But after the collision, they don’t anymore. Take a look at the animation below (use the control bar and double-click on the image to start/stop the animation):
3.) You either need a separate theory of gravity for each new geometry of cluster collisions, including for each cluster before and after the collision, or you need to add dark matter to your modified gravity anyway. So all you need to kill modified gravity? A second example of colliding galaxy clusters. If you’d need a second, incompatible theory of modified gravity to explain it, that tells you that modified gravity isn’t the answer.
Well, say hello to the colliding cluster MACS J0025, in the optical:
in the X-ray:
to the gravitational lensing data:
and to all of them combined.
The configurations are incompatible. So either you need a different theory of non-local modified gravity for every colliding galaxy cluster in the Universe, or you need dark matter.
Not, mind you, that we know what dark matter is, just that we need it. When I write part IV, I’ll go over some of the leading candidates, what I favor, and why. In the meantime, have you convinced yourselves that we need dark matter? If so, what was it that convinced you, and if not, where have you wriggled out of my reasoning?