In today’s blog we chat with Dr. Ethan Siegel, PhD from the University of Florida, astrophysicist, science communicator, and blogger at Forbes.com.
David: Oftentimes I’m asked to describe the evidence for the Big Bang model. How do we get the layperson to understand our confidence in this picture that goes further back in time?
Ethan: One of the things I love about being a cosmologist is that you have to learn all these different areas of physics in order to synthesize together what the universe was like. You need to know about gravity and black holes that happen in the late universe. You need to know about stars and stellar evolution but to a cosmologist that’s still stuff that happens in the late universe. If you go further back you see that galaxies are smaller, there are more of them, they are less evolved, and they have younger stars in them as compared to the ones today. And if you go back further you come to a place where the only galaxies that exist are super small and super young, and made of almost completely pristine materials. We know if you go before a certain point that we are not going to find any stars or galaxies because gravity takes time to attract mass into a region and form stars. But you’re saying ‘let’s go older’. Let’s go back, back, back. Yeah, there are no stars, no galaxies. What do we have? The Big Bang says that the universe was smaller. If you have radiation, the wavelength of that radiation determines its energy. Well, if the universe is expanding, that means it was smaller in the past and the wavelength of light in an expanding universe stretches, which means that light had shorter wavelength in the past. Shorter wavelength means hotter and more energetic. And this is more true earlier in the universe. If you just take normal matter, the stuff that I know, and I just turned up the temperature dial like we do when we go back in time in this smaller, hotter, denser, universe, what happens? You end up coming to a time when these light particles flying around are going to strike neutral atoms and kick the electrons off. That transition, when you cool and you can finally form neutral atoms, that’s where we see the cosmic microwave background. Now that we understand.
But now I say let’s go even further back. Let’s go back to when maybe you have energetic enough photons they could blast atomic nuclei apart. You would have protons and neutrons that want to be bound together, but every time you put them together, bam! A photon comes along and blasts them apart. So we can go further back to when protons and neutrons form because it gets cool enough, then what happens? What happens in the early universe? Well, it is hot enough and dense enough that they can undergo nuclear fusion. But the photons say, no! Because as soon a proton and a neutron come together to make deuterium, the photon comes along and blasts it apart. When you first form protons and neutrons, we’re talking fractions of a second after the Big Bang. But these photons have enough energy to split deuterium apart for about the first 3 minutes of the universe. So the weak interactions transform the amount of protons and neutrons that you have. Early on things were energetic enough that you had 50% neutrons, 50% protons. If you allowed all of that to fuse, you’d end up with a universe that is 100% helium, with two protons and two neutrons in every helium nucleus and 0% hydrogen. But that’s not the universe we got. What happened instead is that protons are a little less massive than neutrons so it is easier to have a neutron collide with, say, a neutrino, and become a proton and an electron, than for a proton and an electron to combine to have a neutron and a neutrino because neutron and neutrino is heavier. When energies are big compared to that mass difference, it doesn’t matter. So you get 50-50 very early on. But as things move on, the universe is half a second old, and then one second old, and then 3 seconds old, this matters. And all of a sudden you’ve got a universe that’s maybe 70% protons and only 30% neutrons. And then you want to form deuterium and fuse up in a chain reaction to helium but you can’t yet because these photons are still blasting your proton-neutron combinations apart. Free neutrons radioactively decay.
David: All of this atomic physics that you’re describing was learned in the 1930’s after quantum physics was formed?
Ethan: It started in the 30’s but continued in the 40’s, 50’s, and even into the 60’s. The measurements didn’t really get exquisite until the 70’s and 80’s. So this was something that we argued about theoretically: where did the elements come from? Was it just the light ones that were made in the Big Bang or heavier ones too? But this whole field of Big Bang nucleosynthesis started coming of age in the 50’s and 60’s. So when the observations started coming in we realized that if we go back, back, back, to the pristine gas, we’ll find that about 75% of that is hydrogen by mass, 92% by number because hydrogen is much lighter than helium, 25% helium by mass, and 8% by number. But then you also have predictions for rare things that were made. Some Helium 3 is made. Some of it is deuterium. Some of it is Lithium 7. So you get these tiny amounts, about 0.01% of Helium 3 and deuterium and a fraction of 10^-9 Lithium 7. And we go out and we observe this. We observe these ratios of the light elements. And they match the predictions of what we call Big Bang nucleosynthesis. This is how the nuclear heavy elements are synthesized in the early stages of the hot Big Bang. Steven Weinberg wrote a book about this in the 70’s called “The First Three Minutes”. That was one of the major books that got me into cosmology. The four cornerstones of the hot Big Bang are 1) the expanding universe that started it all, 2) the cosmic microwave background that vindicated it against the alternatives, 3) the growth and formation of structure and 4) you also have this abundance of the elements, this Big Bang nucleosynthesis. And all four of those combined is why the Big Bang is a tremendous success and no alternatives can match it.
David: When we observe the universe, we infer properties of the universe as a whole by looking at parts of the universe. Do we have direct observational evidence from looking at one part of the universe that the elemental composition is the same as other parts, or do we infer that given our cosmological model?
Ethan: So you can’t look everywhere all at once. You’re just limited so what we do is survey in different directions. For the cosmic microwave background we measure the entire sky and we have to subtract out parts of the galactic plane but what we see in different areas on average is the same. We do our surveys of the universe by looking at different patches of the sky and the number of deep galaxy counts is the same. And on the largest of scales, the universe is both what we call homogeneous, which means the same in every location, and isotropic, which means the same in all directions to about the 99.99% level. If you start looking on smaller scales, you see bigger variations. If you take a big chunk of the sky, and ask what is the universe like over here versus what it is like over there, it’s the same to about 0.01%, which is really, really, good at being the same. If you want to measure deuterium, you need a special configuration where you need a quasar and clouds of gas that intervene. We only have a few good quasar absorbers. But wherever they are on the sky, the indicate the same universe. So, I won’t say that we’ve surveyed everything, but wherever we’ve looked, and we’ve looked in many different places, the universe appears to be the same.
David: What about these baryon acoustic oscillations. This is telling us something about the way structure formed and how it moved in some sense. How do we give people a sense of the importance of this? What about this beautiful connection between the observations and the theoretical power spectrum?
Ethan: This is actually my bread and butter but this is also something that I don’t think scientists that work on it have done a great job of communicating. Baryon acoustic oscillations is correct but it’s a mouthful and it’s not really intuitive. So let’s imagine you’re in the early universe and you’ve got stuff everywhere. You’ve got photons, protons, neutrons, and electrons, and you have dark matter. Wherever you have a little bit of extra mass compared to the average, that’s where gravitation is a little bit stronger. So you’re going to attract the matter around you more successfully than all the other regions. So you are going to draw more matter in but when radiation dominates your universe, what’s going to happen? You start to make something more massive and denser and radiation inside that is a little hotter than the radiation outside. When you make something massive and radiation falls in, it gets more energetic. So you have something with a little bit higher pressure inside compared to outside. So you gravitationally collapse and you grow but you also get hotter and more pressure pushes that back outward. So the normal matter gets pushed back out because radiation interacts with normal matter. But not the dark matter much because the dark matter responds to the pressure but it doesn’t interact with the photon. So the normal matter and the dark matter behave slightly differently. So on small scales you will get expansion followed by collapse and again expansion and so on multiple times. But there should be a scale where all the stuff is coming in and collapsing, and right when it would be pushed out, what happens instead is that the universe has cooled enough that we can make neutral atoms. The CMB can be released. So there comes a time when all this stuff is collapsing and right when it’s time for the photons to push it back out, you don’t get it anymore because neutral atoms have formed. So if you look at the fluctuations in temperature in the CMB, you see they’re constant as you go to smaller scales, and then they rise and peak, and that first peak corresponds to that matter that came in and was about to get pushed out but wasn’t. Now, you fast forward in time and ask what those big temperature and density fluctuations, that big peak that happened on a certain scale, what does that translate into when you form matter structures, when you form galaxies? You brought up the matter power spectrum. What I think about when I think about the matter power spectrum is the thing that underlies it, which is called the correlation function. And that just means that if I put my finger down on a galaxy, and I move radially outward, how likely am I to find another galaxy at that distance? And if I were to graph that, what would you get? Well you would say that close by I’m likely to find another while as I move away I get less and less likely, until I get to today’s universe with all the expansion that happened. At about five million light years away I see a bump. All of a sudden I’m more likely to see a galaxy 500 million light years away than I am 400 million or 600 million light years away. Why is that? That’s what the baryon acoustic oscillations are. Everything came in on this scale and this scale that it came in on is separated from other regions that are doing that same thing. Overall I get this little peak. As the universe expands, that distance also expands. So when I say that today I’m more likely to find one 500 million light years away, when I look back in time, the universe was smaller, so that baryon acoustic oscillation feature will be at a different scale the farther back in time I look. So that’s the physics behind baryon acoustic oscillations and how they can help us understand both the expanding universe and dark matter.
David: What happens to the match between theory and observation if we assume there’s no dark matter?
Ethan: Well, the biggest problem from my perspective is that first off I get the wrong number of peaks. You’re going to get normal matter peaks and you’re going to get dark matter peaks and they’re all going to show up in the CMB. If you look at what the Planck satellite observed, down to angular scales of about a tenth of a degree, about 0.07 degrees, you get 7 different peaks. If there were no dark matter, you’d get 4. So the number of peaks is wrong if there were no dark matter. The height of the peaks is wrong. The position of the peaks is wrong. You cannot get the same features in the CMB without dark matter. And you cannot get the same matter power spectrum because that’s where that comes from. These density fluctuations which is what you’re seeing in temperature fluctuations in the CMB, grow into the structure we have today in the universe. If we didn’t have dark matter, we wouldn’t get the same cosmic web. So both the CMB and what we call the large scale structure of the universe, tell us there has to be dark matter. If we didn’t have dark matter, we wouldn’t have those features in the CMB or the cosmic web that we see.
David: Is it possible to recover the number of peaks by changing the gravitational paradigm?
Ethan: You can say, ok, I want to change the law of gravity. Any change you make to the law of gravity, won’t get you those same sets of peaks, unless the change you make to the law of gravity specifically is indistinguishable from the way that gravity would change if you threw dark matter in. So you can change the law of gravity in a way that mimics dark matter, It has to specifically be that exact change. So your change has to act like a cold, collisionless, particle, that to me is like adding dark matter and you called it something else. You added something that was equivalent to dark matter.
David: Can’t we turn that around in galaxies and say that dark matter needs to distribute itself in a very specific or counterintuitive way, to mimic this MOND phenomenology?
Ethan: Ok. What MOND was motivated by is to say look if all I did was instead of looking at cosmic structures, instead of looking at the CMB, at the abundances of the light elements, at the peaks and fluctuations, at the power spectrum of the universe, at the large scale structure, at two clusters of galaxies that smash together where the normal matter sticks together and produces X-rays while the dark matter just passes right through, which is where you see the mass bending, the effects of general relativity, but only look at individual galaxies, that’s where MOND comes in. Only look at galaxies themselves. This is hard for dark matter because in an individual galaxy things get messy for dark matter. It’s very easy in physics to say, hey, what are things like when they’re first starting to collapse? When you get what we call small effects, right? It’s very easy to calculate that. So, on large cosmic scales, the predictions of cosmology are very good. On smaller scales, things get messier because things collapse and they form stars. And there’s feedback because you produce radiation and things get heated up out from the center and how the normal matter moves is going to change how the dark matter moves because everything experiences gravitation. And things merge and grow over time. Long story short, this is a messy environment.
We know that if you ignore the normal matter and just simulate the dark matter, that doesn’t line up with what we get. But there is this phenomenological thing that if you just took Newton’s law and you add this extra term, say when accelerations get below a certain amount, then when we look at the way galaxies rotate, and we look at the way structures form, on the scale of an individual galaxy, we reproduce what we expect. This is interesting. That is where MOND succeeds tremendously. MOND does better than dark matter for individual galaxies. But people who are studying individual galaxies, for the most part, aren’t enamored with MOND. What they say, instead, is that if you look at the full suite of evidence, there has to be dark matter. So I can try to learn something about the nature of dark matter by looking at these galaxies. For example, is dark matter being dynamically heated by interactions affecting the normal matter? That’s something that Justin Read specializes in. And then you have people who say, well, what if this is an indication that dark matter isn’t purely cold and collisionless, but it interacts with itself? What if it collides with itself? And then you have people who say there could be some sort of quantum exclusion rule that keeps dark matter from getting too dense in the centers of galaxies. So, you have two approaches. You can either say that I’m going to use MOND for these individual galaxies because it works well for them. And then I don’t have very much to say about the rest of the universe. Or you can say, well, I know that dark matter exists so I am going to use what I learned about these individual galaxies to try and say something smarter about the nature of dark matter.
David: Given the messiness, as you’ve described, in galaxies, is it not surprising that we have this MOND phenomenology that manages to capture so much of what’s happening in galaxies?
Ethan: I would say the history of astronomy is rife with what we call empirical correlation where we look at two properties of an object and find they’re related. And this happens observationally long before we puzzle out what that relationship is. Or what the physics is that underlies that relationship. It’s a lot easier to go measure a bunch of things and find two things are related to each other. For MOND they did something that is a little more clever than that. They looked at certain properties of a galaxy like how a galaxy rotates versus how far away I am. And, I’ll notice that if I make this change to Newton’s law of gravity, then I actually predict this relationship. And for me that’s very clever. But that’s also really the full extent of it. That’s as far as MOND goes. It tells you that if you make this addition, it explains this stuff that happens on small galactic scales. And that’s what MOND does. Unfortunately, that’s really all MOND does. If you try to extend this to larger scales, it fails. If you try to apply it to things that happen in the solar system, things in the solar system don’t have that. If you try to do experiments on Earth, we don’t really see that. When you slow objects down, like in an ion trap or something, they don’t exhibit this extra acceleration to get below a certain value.
David: You mentioned the problems in clusters on larger scales with MOND. Some people would say that we haven’t accounted for all the baryons and that could be distributed in clusters.
Ethan: No. That’s not true. That’s a disingenuous argument. There are galaxy clusters that emit X-rays. The cluster is full of hot gas and hot gas emits X-rays. At the same time, you have individual galaxies moving around that cluster. So, if all the matter is emitting X-rays, and I know how much normal matter there is, and if galaxies are moving around, then I know how much total mass there has to be. And there’s a mismatch there by about a factor of 5 or 6. So there is too much mass to be accounted for by the matter even in cases where all the matter is emitting X-rays. Even in cases where all the matter is emitting light.
Remember also that we talked about the abundance of the light elements from Big Bang nucleosynthesis. When we look at the ratio of those different light elements, it turns out from a theoretical perspective that there’s really only one free parameter that determines how much of this stuff you get. How much helium and deuterium and lithium you get. That parameter is the normal matter to photon ratio. How many protons and neutrons do I have for every photon in the universe. That tells us how much total normal matter there has to be. And again, it’s not enough by that same factor of 5 or 6. Although we can’t look at everything and say this much is plasma, and this much is gas, and this much is stars, and this much is dust, and this much is black holes. We can’t break everything down to that level of granularity but we know how much total there is and it is not enough. It is not even close to enough. It makes up 15 to 17 % of the mass that is out there and the rest has to be something else and that’s what we call dark matter.
David: The impression is that education is changing rapidly and in maybe decades or so, the university campus will not be the hub of one’s education. It already isn’t to some extent. You already can’t really be a university professor without producing online content. What is your sense of where education is heading? Are universities going to be obsolete or are they going to change drastically?
Ethan: I take a look at what universities are doing right now and they are recognizing that there is a change happening, that there is a problem with their current model. But what I see as their problem is not what they see as the problem. What I see is that universities are way too focused on administrative stuff, on growing administrations, on money towards more administration, and less, and less, and less, on the interactions that actually matter. When you say that you can see things moving away from in-person and towards universality where things are online, yeah, I can see that happening. But when I look at where I got the valuable parts of my education, that was from small interactions. From small group interactions; from one-on-one interactions, from interactions with professors, interactions with other students. Working on problem sets together. Asking questions to help identify and overcome my misconceptions. If all you have is one way communication, you will lose that. You will absolutely lose that. And that for me was the most valuable part of my education. So what I think universities should be doing is focusing on improving in-person experience. And that starts with valuing everyone who is a part of the student experience which is grad students, researchers, professors, instructors, people that the students interact with for their education.
At the University of Kansas, for instance, they just cut 40% of graduate assistantships, which is disproportionately going to affect international students who can’t get jobs elsewhere. It’s going to affect people who are reliant on teaching assistantships for their graduate studies. And it’s going to affect students who are in these classes where graduate students teach the course or help with problem sets. I think this is absolutely wrongheaded. We don’t need 50 administrators for every thousand students. We do need to improve the student-to-faculty ratio. We do need to have more personal interactions, more one-on-one time, more places where students can take advantage of the resources that their professors are, where people just have time to be curious and explore what they like. I want to push back against this idea that everyone go to the same online professors. Everyone hear the same interpretation of the subject. Everyone get the same toolkit. So you get a screwdriver, a hammer, and a drill press. All these tools. That’s not good for anyone. I don’t want a million graduates in a field going into the world with the same exact toolkit. Having a diversity of toolkits, and a diversity of perspectives, and a diversity of instructors, and of experiences, is what leads people to develop different skills and interests and allows them to contribute in different ways. If everyone has the same cookie cutter experience, the same education, I see this as a loss of diversity. Just like we don’t like monocultures in agriculture, you probably don’t want monocultures in education either. I think I’m a great educator and I think that many people would benefit from having my education. But I don’t want the whole world to have only my education. I think that would be bad for everyone. So, as you see this change happening where things become democratized, on the internet, available to everyone, don’t push for everyone to have the same experience.
David: In physics, faculty on average have not learned that learning happens via direct interaction between students and faculty. These problem-solving sessions are crucial. I try to avoid lecturing excessively in the abstract without applying the ideas, because that’s where you lose students.
Ethan: It’s a lot like anything. If you want to get good at it, you have to practice. No one gets good at playing the piano watching other people play the piano. You get good at the piano by playing it yourself. Same thing with physics. You get good at physics by solving physics problems. By practicing physics. I remember being in graduate school and finally figuring out how I was going to do well in my classes. I’m going to read the material we’re going to cover in class before I go to class. I’m going to take notes on everything that goes on in class as we work through the stuff. After class I’m going to go back and look at the book and I’m going to look at my notes and I’m going to work through it until I can do this myself. That level of self-direction requires a level of maturity that I didn’t have as an undergrad, that most undergrads I’ve taught didn’t have as undergrads. So you have to put up as much scaffolding as possible to make them engage in the activities that are going to get them to learn. I think you have to be an individual and treat your students as individuals. Find out where they are and meet them there. Some students just need a pep talk. Some students need to spend time with you during office hours before they get it. Other students need to be motivated. Each student has a different set of what will motivate them. This is the sort of thing that I think in-person education offers that’s unique. Online you can get farther than others but no one will get as far as they would in an in-person environment.
David: I guess we need to get away from university professors, at least in the hard sciences, just lecturing in the abstract. Because that part can be done online.
Ethan: I hesitate to tell university professors how to do their jobs. And I say this as someone who has had my share of university professors whose experiences I did not appreciate. If you had asked me when I was younger I would have said that they are bad at their jobs. And they don’t care that they’re doing a bad job. But having seen professors excel at giving lectures, having been to lectures where they are just this firehose of awesome information, where I don’t want that firehose to turn off, I am a bigger fan of not telling competent professionals how to do their job. You are a competent professional. You are listening to the research out there on education. You are doing your best to educate your students. You are making yourself available. You are teaching the material, giving them opportunities to work on the material themselves. And that’s it. I am not a fan of micro managing. You can tell the lessons you’ve learned. You can demonstrate the way you like to see things done. But I am not a fan of being a prescriptivist. We’re going to trust that you’re going to do your job to the best of your ability. We’re going to give you feedback. We expect you to adjust and learn and grow. We can give you general advice but this idea that we should be telling everyone exactly how to do their jobs, I think is more of this push towards everyone gets a hammer, screwdriver, and drill press. And nobody gets a meat tenderizer. That’s all well and good until somebody needs that oddly shaped mallet. So, I don’t know why we have this obsession with telling people the right way to do their job and the wrong way to do their job. I was a professional and I resented it when people told me how to do my job cause I wanted to do my own thing. I had an idea of the way I could make a course that was superior to the way that course had ever been taught before, and I appreciated when I was a professor that I had the freedom to do it. That I had the freedom to innovate, that I had a long leash. And part of that was I had a supportive department chair and part of that was I was young and confident and brash and I was going to steamroll anyone who got in my way and I didn’t care what the consequences would be to me. I want to advocate for that level of academic freedom for professors. I have incorporated a lot of lessons from education research. But not all of them because I have my own preferences. I have preferences for how I like students to have an experience in the classroom and I don’t think that some command from on high on how you do your job gives people a better experience in the end. And that’s maybe a little controversial in this day and age but I feel strongly about that.
David: Where do you see your outreach going in the future?
Ethan: I’m opened to possibilities but what I would like to do is bring this sense of wonder, this story of the universe, and of physics and astronomy and cosmology, in general, to as broad an audience as possible. I would like to see this transform from me making videos online, podcasts, articles, and books, to expand into video series, television, film. I would like to reach as wide an audience as possible. I think there is a whole universe out there to discover and the story of that universe should be opened and available to everyone who wants to be a part of it. So go big and stay home until covid is over. That’s the plan.
David: Thank you so much. I’m going to transcribe this and send it to you and you can do what you want with it.
Ethan: That sounds great. And you can do whatever you want with it too.
David: Ha ha ha.
Ethan: Ha ha ha.
David: Thank you Dr. Siegel!