“The best way to escape from a problem is to solve it.” –Alan Saporta
One of the greatest puzzles in the Universe today is just why the Universe is structured the way it is.
For the individual galaxies that we see, the puzzle is why they rotate at the speeds they do. If the only matter in these galaxies were normal matter (made out of protons, neutrons, electrons, etc.), the outskirts of these galaxies would rotate around their centers much more slowly than they actually do.
For clusters of galaxies, the above puzzle is not only there for each of the individual galaxies in the cluster, but there’s another one on top of it: these galaxies move through the cluster far more quickly than expected!
Based on what we know about matter and gravity, if all that were in these clusters were protons, neutrons, and electrons obeying the laws of gravity, galaxies moving at these high speeds would escape from the cluster!
But galaxies aren’t escaping from their clusters.
So what is going on? Well, something’s got to give, and the two ideas that are always considered are:
- Either there’s more mass out there than the stuff we can conventionally detect,
- or the laws of gravity need to be modified.
Any one observation, of course, could always be fit well by either of these options. The modifying gravity option does a little better at the individual galaxies, while having extra mass — known as dark matter — does better for the clusters. The key test, though, is whether we can fit all the observations with just one thing.
So what are some other observations we can make?
We can look to gravitational lensing, which is how general relativity was first confirmed in our Solar System! How is the light from background objects bent by the gravity of objects — like galaxies and clusters — that are in between us and the light source?
What else can we look for?
We can find galaxy clusters in the process of colliding! Normal matter, as you can verify for yourself by trying to pass one hand through your other hand, interacts with other normal matter. When a proton, neutron, or electron runs into another one, they don’t just pass right through one another, they exchange energy and momentum, and effectively experience friction. But not dark matter! There’s no exchange of energy or momentum; dark matter simply coasts through both normal matter and other dark matter particles.
So we can look at X-ray emissions to see where the protons, neutrons, and electrons are (in pink, below), and gravitational lensing to search for overall mass and matter (blue, below). The results?
So far, these (and other observations, like large-scale structure, clustering statistics and fluctuations in the cosmic microwave background) come down heavily in favor of dark matter, but it’s always important to find new ways to test this.
In science, even when you’re convinced that you know the right answer, you keep testing your understanding in new ways. You keep looking for phenomena that might do something different than what your best ideas and theories predict. As long as there’s a Universe out there to investigate, science doesn’t end.
So what else, if there’s dark matter in these galaxy clusters, should we see?
Well, the deeper into a gravitational potential well you are, the more energy it takes to climb out! According to general relativity, that means the deeper you are towards the center of a galaxy, or in the case of a galaxy cluster, towards the center of the cluster, the lower the energy (and hence, the redder) your light will be.
If there is dark matter, you’d get one prediction.
But if there isn’t dark matter, or if there’s something else instead, you’d get a different prediction. While gravitational redshift has been tested and confirmed on our planet, in our Solar System and within our galaxy, we’ve never tested it for a galaxy cluster before.
First off, they took data (from the Sloan Digital Sky Survey and the associated Gaussian Mixture Brightest Cluster Galaxy catalogue), and tested whether you could see the effect of galaxies being gravitationally bound in the clusters. (After, that is, they subtracted out the overall Hubble flow; see the paper’s supplemental section for more details.) If you couldn’t, the above curves (showing the velocity distributions) would be flat, but the quasi-Gaussian shape of these curves shows, in fact, that these galaxies are gravitationally bound, as expected, in these clusters.
So then you ask, now that you know they’re bound, what is their gravitational redshift? This is incredibly difficult to measure, as it’s the departure of the data (in red) from those Gaussian curves (in black), which is tiny, but amazingly, measurable! And is that gravitational redshift consistent with General Relativity + Dark Matter + Dark Energy, or with its alternatives?
With the data collected to this point, General Relativity with dark matter (and dark energy) is the best fit, in red, above. One of the alternatives, f(R) gravity (which replaces dark energy, but not dark matter; if you heard otherwise, see here), is a slightly worse fit but isn’t ruled out.
But TeVeS — the relativistic generalization of MoND, which seeks to do away with dark matter — is ruled out by this analysis!
The authors’ conclusion from this?
The measurement agrees with the predictions of general relativity and its modiﬁcation created to explain cosmic acceleration without the need for dark energy (f(R) theory), but is inconsistent with alternative models designed to avoid the presence of dark matter.
And there you have it. While this isn’t a good measure of dark energy, it concludes that there’s yet another independent test of dark matter, and the evidence not only supports it, it explicitly comes out against modified gravity as an alternative.
So how do you build the Universe, from the grand cosmic web down to individual galaxies?
You do it with dark matter.