By some counts as much as 96% of everything in the universe comes from dark matter and dark energy. And, up to now, both are completely invisible. Few in the field question this theory, but could there be alternatives? Dean chats with Professor Stacy McGaugh from the Department of Astronomy at Case Western Reserve University to learn more.
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Episode Transcript: Looking Up is transcribed using a combination of AI speech recognition and human editors. It may contain errors. Please check the corresponding audio before quoting in print.
Dean Regas: Maybe you've heard this headline. Astronomers believe that most of the universe is not made up of galaxies, stars, planets, and atoms, but it is mostly composed of dark matter and dark energy. By some counts, as much as 96 percent of everything in the universe comes from dark matter and dark energy.
[Futurama byte]: It's dark matter.
[NASA Universe byte]: Dark energy is another very mysterious phenomenon...
Dean Regas: And up to now both are completely invisible. Yeah, so, we can only see 4 percent of everything, huh?
[Buffy the Vampire Slayer byte]: Hey, moron! I'm invisible.
Dean Regas: How does that make you feel? I know for me, I don't like it one bit.
From the studios of Cincinnati Public Radio, I'm your host, Dean Regas, and this is Looking Up a show that takes you deep into the cosmos or just to the telescope in your backyard to learn more about what makes this amazing universe of ours so, great.
My guest today is Professor Stacy McGaugh from the Department of Astronomy at Case Western Reserve University, and an astronomer who studies galaxies, cosmology, dark matter, and modified gravity with alternatives to dark matter and energy.
I don't think my guest today likes that 96 percent of the universe is invisible either. And if it's invisible, how do we know it's there? It all comes down to gravity, also, an invisible force, and how astronomers observe how large structures like galaxies and galaxy clusters behave as if they had more mass than what we can actually see. The mass and energy are invisible to all of our detection methods. So, what are they and are they really there?
On my snarkiest of days, I nickname this solution adopted by the majority of astronomers as the fudge theory. All right, come with me on this. So, if a galaxy is spinning differently than it should with the normal visible amount of matter we can see, then you just add more invisible matter, aka dark matter. How much do you add? Just enough to fudge the numbers. And what is dark matter? No one knows because you can't see it but just add it. Or you see a galaxy traveling away from us faster than it should. Add some invisible fudge, I mean dark energy but not too much. Fudge the numbers with invisible stuff. Just enough to make the data work with our known laws.
Yeah, I mean, come on, isn't that a little too convenient? The whole dark matter and dark energy thing rests on one, are our observations accurate? And two, are our laws accurate? But few in the field question any of those things. In fact, I've noticed a surprisingly dogmatic view that some astronomers have in supporting dark matter and dark energy, something bordering on the religious. But is there a better answer out there?
Dr. Stacy McGaugh: Hi, I'm Stacy McGaugh, professor of astronomy at Case Western Reserve University in Cleveland, Ohio.
Dean Regas: Well, Stacy, thanks so much for joining me today.
Dr. Stacy McGaugh: My pleasure.
Dean Regas: I personally cannot wait to talk about alternate theories, but let's start off with dark matter and dark energy first. What is their theory behind these two invisible things?
Dr. Stacy McGaugh: Right. So, the global statement is that things don't add up when we apply the known laws of gravity, as taught to us by Newton and Einstein, to the stuff we see in the universe. It doesn't add up. So, there's something extra going on. And the sort of most obvious first inference is that there's some extra mass there, what we call dark matter. A simple way to say it is a particle we don't know about. On the other hand, it's not just a new particle waiting to be discovered. It's new physics. It's really got to be something beyond anything we've experienced so far.
Dark energy is a bit of a different beast. It is more like an anti-gravity. So, whereas dark matter gives us extra attraction, dark energy has a net push. And that is what we attribute to not only the universe observed to be expanding, but it appears that that expansion rate is accelerating, speeding up more mass. And gravity as we know it won't do that. So, you have to do something that pushes instead of pulls, basically. And that's what the dark energy does.
Dean Regas: Well, so, then I got to ask, where does dark matter and dark energy fall short in explaining things?
Dr. Stacy McGaugh: So, where it struggles most is what people call the small-scale problems. By small, cosmologists mean a galaxy, individual galaxies like the Milky Way. Those are small compared to the universe. I think of them as being rather large, so, it's all a matter of perspective. But size isn't the important dividing line. There is a scale in the data that is not a length scale, but an acceleration scale. And that's weird from the perspective of cold dark matter, because cold dark matter is scale free. It does the same thing for really big structures as for mid-sized structures. It's for small structures. So, it really shouldn't matter if you're a dwarf galaxy or a giant cluster of galaxies, the basic structure should be the same. And there's no physical scale in that. Not a length scale, not an acceleration scale, not a frequency scale, not any kind of scale you can think of and yet are imprinted on the data for galaxies. And their kinematics is a very clear acceleration scale. And that is a new thing that we do not understand.
Dean Regas: Well, I gotta say, I personally don't like invisible forces and unviewable things. So, I tend to be a little skeptical of this stuff. And am I a little too harsh in saying it's not much different than saying God did it? You know, we see a galaxy spinning faster, add more stuff that we can't see. Am I way off on that?
Dr. Stacy McGaugh: It gets dangerously close to that. So, it's certainly not that bad. But one has to make a distinction between the inference that we see extra motion. So, we do exactly what you said. Just throw in some extra mass that we can't see. And that's sort of how the whole paradigm started. Since that time, people have developed more careful theories or predictions for what the dark matter halos in which galaxies reside should be like. And they're sort of in the ballpark, but they don't really work in detail. It is a problem for the standard picture, and there are lots of solutions people have come up with, none of which I find particularly satisfactory. But that's where most of the community is at.
Dean Regas: Well, that's what I'm hoping to find out, you know, what alternate theories are out there, especially ones that you think are more promising to follow up on.
Dr. Stacy McGaugh: So, the thing I struggled a long time with myself, because I came out of the dark matter paradigm I sort of grew up with it as a grad student, and that's what I believed in as much as any of my colleagues do. And so, it seemed outrageous to suggest that something else could be going on, but I had a very shocking experience in my own data. There were contradictions to the dark matter interpretation that I could not explain away. I tried, and I tried really hard. Try as I might, it just didn't make sense.
And then I happened to hear a talk by Mordy Milgrom on his theory of modified gravity called MOND. And I almost didn't go to that talk because I was like, oh, modified gravity. That's nonsense. Who wants to hear about that? But I did go, and not knowing who I was or what I worked on which was the surface brightness galaxies he sort of did an aside and said, OK, maybe it's not dark matter, maybe it's the force law that changes. That is, we're using the wrong equation. So, if we change the equation in this particular way, then it makes these predictions. And he basically on a few lines on the chalkboard derived exactly what I was seeing that was causing me just fits trying to understand in terms of dark matter.
So, I sort of scraped my jaw off the floor and went back to my office and thought, well, I should look up this theory. And reading his original papers, which appeared in 1983, I came across this statement that low surface brightness galaxies the very things I was working on provide strong tests of this hypothesis that he was making. And the reason for that is that he was suggesting a force law that changed at an acceleration scale. But this idea of having a change in acceleration scale was new and weird to me. And so, I tucked it away and didn't think about it again. When I came back to it later, it was OK. Here's what he predicted. Let's check the data. And it was just like, check, check, check. You know, I could just go down the list and everything he had predicted more than a decade before came true. So, what am I supposed to say? That he's wrong?
Dr. Stacy McGaugh: That was a very shocking experience. And so, I spent a lot of time sleepless, staring at the ceiling, thinking, how could this stupid theory have any predictions come true when there's so much evidence for dark matter? And I was really hung up on that for a while. And I see a lot of my colleagues are still hung up on that. And it's a linguistic problem. What we see is a discrepancy. If we assume that gravity works the way Einstein taught us, then you have to have dark matter. But maybe there's more to it. Maybe the equations changed, and a lot of the data at the time did not really distinguish between those options. And so, by calling it the dark matter problem, we had prejudiced our thinking about it to be on that one side of it. But really, it's a discrepancy.
And to follow Jacob Bekenstein, who was a gravitational theorist who worked on this, he called it the acceleration discrepancy. And that is an experimental realm. We have no experiments that probe that realm. The only experiments that test this are the data for galaxies and other extragalactic systems that show the need for dark matter or something. And so, that "or something" could be a theory like MOND.
Dean Regas: Well, I mentioned to you off air that I had some trouble finding somebody to talk about alternate theories. So, it sounds like maybe you're a little bit of a rogue voice compared to your colleagues. How do you stay motivated in this field as the voice of a different idea?
Dr. Stacy McGaugh: It's a rich sociological fabric. I recognized a long time ago people weren't going to go along easily with this change of ideas. I didn't, and I only was forced to think about it because it came up in my data. I felt it would be intellectually dishonest to go around saying, oh no, dark matter is better, when this alternate theory nailed all its predictions, and dark matter is manifestly not as successful for those observations. So, you know, I realized I could be popular or I could be honest, and I chose to be honest.
And so, what it comes down to is how do you weigh the various lines of evidence? And there are many. Part of the problem I face is that since I know this is out of the mainstream, I have been at pains to evaluate all the evidence, and other people have not. Nobody else has done that exercise not in the U.S. because it's a lot of work. And why would you do that? Because it's some stupid theory that's unlikely to be true, which was, you know, exactly the attitude I came to it with.
Dean Regas: I think that what you brought up this is a sociological issue as much as anything from my perspective, too because I see people that are ardent proponents of various theories. You know, I'm a student of history. I know how astronomy history has changed over time. And certainty of a theory there is this kind of hard line to it, too. So, I think you're on to something, that there's a human element to this theory that's getting maybe mixed up with the actual information.
Dr. Stacy McGaugh: Absolutely. So, dark matter is essential to cosmology. And if you come at it from a cosmological perspective, it makes more sense to have dark matter because you just don't have a theory without it. And so, you don't even have a way to think about things that otherwise you understand. So, the way I hear a lot of my colleagues put it is that the current cosmological paradigm, Lambda-CDM, works so, well, that it must be true. So, where is it? We've been doing experiments for decades now, and those experiments have succeeded brilliantly in failing. They should have seen it by now, and they have not.
I also think it's important to if you're going to stay rational and scientific then you always need to set goals and reasons why you would change your mind. So, when MOND had all of these successful predictions come true, I keep testing it and it has had many other predictions come true since then. But it doesn't have everything come true. And so, OK, what would make me change my mind back? And I would love to change my mind back, because then I wouldn't have to argue with all my colleagues about this anymore. It would be very convenient for me to do that. But it would not be honest at this time.
I have set those things and, you know, there are a couple of things that would help persuade me that in fact dark matter is right, and one is to detect this stuff. We have spent hundreds of millions and probably billions of dollars at this point thousands of man-years of effort in order to try to detect this stuff. And we have bupkis to show for it. So, you know, show me a piece if you're so, sure it's there.
Dean Regas: Well, I gotta say, you know, you got a face of a rebel. You know, I think that's what you yeah. I'm gonna be buzzing about this for a long time. I've been thinking. I've been thinking. I, you know, like, I don't have the expertise, the data, the observations in my head to argue well enough, but philosophically, I just understand what the evidence is that dark matter helps with. But boy, I just don't like it. There's something I don't like about it. And it is interesting. Thanks for doing this. Really appreciate it.
Dr. Stacy McGaugh: Sure. It's fun.
Dean Regas: Oh yeah. Absolutely. I love thinking about it too. So, let's go down this rabbit hole of reason for a moment. Let's say there is dark matter and energy and it's everywhere and it's not uniform. There's denser parts of them in some places. Wouldn't that mean there's also dark matter and energy on Earth and in the solar system?
Not much like almost none, but some!
[Ben Hur byte]: I tell you, there are strange forces at work here.
Dean Regas: So, if it is invisible and if it can only be observed at large scales, like with galaxies, how do we know how much dark matter and energy are around us here? Our gravitational laws are based on known masses, but if we don't know how much mass is from the invisible stuff and how much is from the regular stuff, can they be universally applied? I mean, maybe Earth is embedded in an unusual amount of dark matter. Maybe the opposite. We don't know because we can't see it.
[Forbidden Planet byte]: Some dark, terrible, incomprehensible force.
Dean Regas: And I know all the dark matter people like Dean, that amount in our solar system is negligible. Not true, man. It's something, but an unknown something.
So, I propose an addition to the law of gravity to include dark matter and energy. Newton's laws say the force of gravity is equal to the mass of the two objects divided by the square of the distance between the two objects. I think we need to add an extra symbol in there to represent the amount of dark matter and dark energy in the system. Let's combine them and call them, oh, I don't know dark resources. You know, give it the symbol like "DR.” Hmm. Yeah, I like that. That sounds good.
To make our laws work, the doctor for Earth and our solar system is equal to one. One. What? Never mind. We can't see it, measure it, or know what it is, but it's one. Cool. The DR scale can adjust to match all observations of distant objects, too. I think I can safely say my suggestion will be met by both eye-rolling and derision from both the dark matter camp and the Dr. McGaughs alike. Glad I can unify them against me. I mean something. Hey, I'm on your side. I'm just a guy in an expanding universe who just wants to find the fudge out there.
Dr. Stacy McGaugh: Show me a piece!
Dean Regas: Looking up with Dean Regas is a production of Cincinnati Public Radio. Kevin Reynolds and I created the podcast in 2017. Ella Rowen and Carlos Lopez Cornu produce and edit our show and have helped me with my new startup, Fudge Shop. Dark matter is by far our top-selling flavor.
Jenell Walton is our Vice President of Content, and Ronny Salerno is our digital platforms manager. Our theme song is, “Possible Light” by Ziv Moran. Our social media coordinator is Hannah McFarland, and our cover art is by Nicole Tiffany. I'm Dean Regas, keep looking up!
Producer note: Special thanks to Kevin Luce for providing the kitchen timer sound effect used in this episode.
