Don Lincoln: prediction

Transcript context

…Oh gosh, what is dark matter? A, I don’t know. B, it’s terribly fascinating. All right? So first thing, and the most important thing ’cause I’m an experimentalist, by God, the first thing is why do we believe there’s dark matter? And the reason is that astronomical measurements do not agree with predictions by Newtonian or relativity theory. Galaxies spin too fast, clusters of galaxies move too quickly, and the distortion of very distant galaxies due to the gravitational field of nearer galaxies disagrees with the prediction from what we see from the observed matter. So there are three very distinct reasons why we are predicting that, that something is wrong in our understanding of either the laws of physics or the matter budget of the universe. The easiest one to talk about is the spinning galaxies. Now, this is– what I’m saying is not unique to spinning galaxies, just easiest to talk about. So galaxies are observed to spin more quickly than they should if we add up the gravity we see. By all rights, galaxies spinning that fast should blow themselves apart, and they don’t. So what can be the answer? Well, you have the force required for a star to orbit to move in a circle, and you have the force due to gravity, and they’re connected by an equal sign, and the prediction is wrong. So either the force due to gravity is wrong, the force needed to move in a circle is wrong, or the equal sign is wrong. I mean, this is really simple. One of those things is wrong. So one possibility is simply that Newton’s law of gravity, mass times the mass over R squared times a constant, that’s just wrong. Another possibility is Newton’s F equals ma that we are taught in introductory physics is wrong. Both of those are eminently possible. Over here, maybe we don’t understand gravity, or maybe there’s more mass than we can see. So these, you know, I mean, it’s nice that you can look at this really simply and come up with a list, you know, a cookbook of things we can test. And so we’ve done that. We’ve gone and said, “What are the possibilities?” Well, the most obvious possibility is that there is more mass than we can see. There’s black holes, there’s hydrogen gas that we can’t see, whatever. There’s something out there. So that was the first thing. So you go and you look, and there’s no hydrogen gas because we can see that with radio waves. That’s not it. In the ’90s, we went looking for black holes, rogue planets, things like that. Those exist, but not enough of them. and there’s no hydrogen gas because we can see that with radio waves. That’s not it. In the ’90s, we went looking for black holes, rogue planets, things like that. Those exist, but not enough of them. That’s not it. And so now we’re left with there’s some sort of matter that we can’t see, or we don’t understand gravity, or we don’t understand inertia. Now, I personally, if you asked me this, oh, I don’t know, 25 years ago, I would’ve said the most likely answer is that we don’t understand inertia or gravity. You know, I– if 20 years ago, 25 years ago, that’s what I would’ve said, no problem. However, there have been a couple of observations that have caused me to change my thinking, and I think that dark matter is more likely. One of them is called the Bullet Cluster. So the Bullet Cluster, there are two large clusters of galaxies. In these large clusters of galaxies, well, any galaxy consists of a couple of components. There are the galaxies themselves, there is the hydrogen gas that surrounds the galaxies, and maybe there is dark matter. And if dark matter is real or dark matter is not real, you will get different answers if those two galaxies pass through one another. The galaxies themselves should pass through one another basically not interacting, but the big thing is the gas clouds. So if there’s big clouds of gas, as the galaxies pass through one another, the clouds should interact, and the gas clouds should stop in the middle and be really, really hot. So then you would see, if there were no dark matter, you would see a cluster of galaxies, cluster of galaxies, a big gas cloud in the middle, and because the big gas cloud in the middle is much more massive than the galaxies themselves, you would expect to see distortions that we call dark matter distortions in the middle. If, however, dark matter is real, the galaxies pass through one another, the cloud stops, dark matter doesn’t interact with the cloud, so it passes through. In that case, you would expect to see the distortions where the galaxies are. And that’s what we see. So that is a strong evidence in my mind. The Bullet Cluster is strong evidence that dark matter is a real thing. And there is another example which is much more recent (the Bullet Cluster was a while ago) called the Dragonfly galaxies. There’s Dragonfly 2 and Dragonfly 4. These are galaxies that rotate exactly according to Newton’s laws. And so the fact that they rotate exactly according to Newton’s laws says that whatever’s causing galaxies to rotate too fast is not a property of matter. But if you had a galaxy where there was no dark matter, for whatever reason it got stripped off or something, this is one of those lovely ironies that the existence of a galaxy with no dark matter is very strong evidence that dark matter is real because you can take the dark matter out. So the DF2 and DF4 also suggests to me that dark matter is real. he existence of a galaxy with no dark matter is very strong evidence that dark matter is real because you can take the dark matter out. So the DF2 and DF4 also suggests to me that dark matter is real. So now, while it remains possible that we need to modify the laws of inertia or we need to modify the laws of gravity, those are possible still. In my opinion, and now this is Don’s opinion, but it’s probably the opinion of most of the scientific community, dark matter is likely a real thing. Now, that’s great. I’ve taken you all the way to dark matter. So now you’re gonna ask me, you’re gonna say, “Don, what is dark matter?” I’m gonna go, “I don’t know.” But I know what it isn’t. Okay? I know that it is not black holes. I know that it is not rogue planets. I know that we’ve done the measurements. We’ve looked across nearly every mass range for compact objects and ruled them out. So if dark matter is real, it can’t be made of those. So then you’re left with the idea that dark matter is a particle, and that’s what we’ve thought about. The name for the dark matter particle that we’ve called for a long time is a WIMP, for a weakly interacting massive particle, and we have spent the last, God, 30 years looking for them in the various ways. There are three ways that we might see dark matter, the direct way, which says that dark matter exists literally everywhere in this room, in our laboratory, and the dark matter is passing through the Earth like a wind, and we put up detectors trying to see those. We have done that, and we’ve seen nothing.…

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