As promised in my last post, I’ve now written the answer to the second of the three questions I posed about how the Large Hadron Collider [LHC] can search for dark matter. You can read the answers to the first two questions here. The first question was about how scientists can possibly look for something that passes through a detector without leaving any trace! The second question is how scientists can tell the difference between ordinary production of neutrinos — which also leave no trace — and production of something else. [The answer to the third question — how one could determine this “something else” really is what makes up dark matter — will be added to the article later this week.]
In the meantime, after Monday’s post, I got a number of interesting questions about dark matter, why most experts are confident it exists, etc. There are many reasons to be confident; it’s not just one argument, but a set of interlocking arguments. One of the most powerful comes from simulations of the universe’s history. These simulations
- start with what we think we know about the early universe from the cosmic microwave background [CMB], including the amount of ordinary and dark matter inferred from the CMB (assuming Einstein’s gravity theory is right), and also including the degree of non-uniformity of the local temperature and density;
- and use equations for known physics, including Einstein’s gravity, the behavior of gas and dust when compressed and heated, the effects of various forms of electromagnetic radiation on matter, etc.
The output of the these simulations is a prediction for the universe today — and indeed, it roughly has the properties of the one we inhabit.
Here’s a video from the Illustris collaboration, which has done the most detailed simulation of the universe so far. Note the age of the universe listed at the bottom as the video proceeds. On the left side of the video you see dark matter. It quickly clumps under the force of gravity, forming a wispy, filamentary structure with dense knots, which then becomes rather stable; moderately dense regions are blue, highly dense regions are pink. On the right side is shown gas. You see that after the dark matter structure begins to form, that structure attracts gas, also through gravity, which then forms galaxies (blue knots) around the dense knots of dark matter. The galaxies then form black holes with energetic disks and jets, and stars, many of which explode. These much more complicated astrophysical effects blow clouds of heated gas (red) into intergalactic space.
Meanwhile, the distribution of galaxies in the real universe, as measured by astronomers, is illustrated in this video from the Sloan Digital Sky Survey. You can see by eye that the galaxies in our universe show a filamentary structure, with big nearly-empty spaces, and loose strings of galaxies ending in big clusters. That’s consistent with what is seen in the Illustris simulation.
Now if you’d like to drop the dark matter idea, the question you have to ask is this: could the simulations still give a universe similar to ours if you took dark matter out and instead modified Einstein’s gravity somehow? [Usually this type of change goes under the name of MOND.]
In the simulation, gravity causes the dark matter, which is “cold” (cosmo-speak for “made from objects traveling much slower than light speed”), to form filamentary structures that then serve as the seeds for gas to clump and form galaxies. So if you want to take the dark matter out, and instead change gravity to explain other features that are normally explained by dark matter, you have a challenge. You are in danger of not creating the filamentary structure seen in our universe. Somehow your change in the equations for gravity has to cause the gas to form galaxies along filaments, and do so in the time allotted. Otherwise it won’t lead to the type of universe that we actually live in.
Challenging, yes. Challenging is not the same as impossible. But everyone one should understand that the arguments in favor of dark matter are by no means limited to the questions of how stars move in galaxies and how galaxies move in galaxy clusters. Any implementation of MOND has to explain a lot of other things that, in most experts’ eyes, are efficiently taken care of by cold dark matter.