Chung-Pei Ma: Welcome to the 2018 Distinguished Lecture in Astronomy. This is an annual public lecture sponsored by the Department of Astronomy at UC Berkeley. As the chair of this department, I have the honor to introduce our distinguish speaker this year, Professor Bob Kirshner. Bob received his undergraduate degree at Harvard and a PhD in astronomy at Caltech. He was a post-doctoral fellow at the Kitt Peak National Observatory and a faculty member at University of Michigan before joining the Harvard Astronomy faculty in 1986. Bob was the president of the American Astronomical Society from 2004 to 2006.
And in 2015, Bob moved to our coast to become the chief program officer for science of the Gordon and Betty Moore Foundation, overseeing the distributions of more than $100 million per year for the research and technology that enable fundamental scientific discoveries. Bob has been honored by many, many awards. To name just a few recent ones, the 2014 Watson Award, the medal from the National Academy of Sciences, awarded ever two years for outstanding contributions to the science of astronomy. The 2014 Breakthrough Prize in Fundamental Physics given to the High-Z Supernova Search Team. And the 2015 Wolf Prize in Physics, which Bob shared with Bjorken. It is well known to this audience that the 2011 Nobel Prize in Physics was awarded to Saul Perlmutter. I don’t know if he’s here.
Bob Kirshner: I’ll show a picture.
Chung-Pei Ma: Good, yeah. Of the Supernova Cosmology Project, and Adam Riess and Brian Schmidt of the High-Z Supernova Search Team for the discovery of the accelerating expansion of the universe through observations of distant supernovae. Bob and our own Alex Filippenko here were part of the High-Z Supernova team. And Adam Riess and Brian Schmidt were Bob’s Ph.D. students at Harvard University. And Adam Riess was a Miller Postdoctoral Fellow here at Cal when the accelerating universe result was announced in 1998, 20 years ago. So I thought to set a stage for Bob’s talk about this remarkable discovery in 1998. It would be useful to sort of bring everyone back to the early ’90s just briefly.
I happen to be a Ph.D. student in theoretical cosmology at MIT at that time. And being a student of Alan Guth who proposed the inflationary theory, I was taught an early stage that a cosmic doomsday would have occurred long ago, unless the universe as a whole is flat with zero curvature, and if the density parameter of the universe, this so-called omega parameter, has the value of exactly one. What made life interesting back then was down the hall from these MIT theorists were observers like Paul Schechter and John Tonry who actually knew something about real data. And they would take every opportunity they could find to remind us that real data simply didn’t add up to omega being one. They said, “No, no, no. “The universe is open with a low omega value of .2 or .3.” I didn’t witness blatant hostility between these groups of faculty members at MIT. But the tension in their cosmic views, contrasting cosmic views, was palpable. And as a student, I found it confusing, yet stimulating.
Of course within a few years, Bob, Alex, Saul, and other on these two teams showed that everyone was half right hand half wrong. The theorists were right, and that the total omega budget is probably one, but they were wrong to assume that all that is in the form of attractive dark matter, which would make the universe decelerate. The observers on the other hand were right about omega and matter being low, .3-ish, but they missed 70 percent of the energy budget of the universe which we know believe to be in the form of the repulsive dark energy. In fact, the concept of dark energy was so repulsive that when I wrote by 1995 paper based on my PhD work Ed Bertschinger in which we spelled out how to calculate the anisotropies in the cosmic microwave background. In that paper, we did not even bothered to include the dark energy terms in our equations, at first. And it was in the last revision, when we have halfheartedly added all these lambda terms into our equations, just for the sake of being complete, this was 1993 to five, I’m so glad we did that.
So one last point I’d like to just to highlight is how much of the action leading to this extraordinary discovery occurred here at Berkeley in Campbell Hall right across and right up at LBL, the Lawrence Berkeley Lab. So I will just read one email taken from Bob’s eloquent popular book, The Extravagant Universe. And this is an excerpt from a long email on January 12, 1998, at 6:36 p.m. from Adam Riess, then a Miller Fellow in Campbell Hall to the High-Z Team. He said, “The results are very surprising, shocking even. “I have avoided telling anyone about them “for a few reasons. “I wanted to do some cross checks, “and I wanted to get a ways into writing the results up “before Saul at all got wind of it.” You see? “I feel like the tortoise raising the hare. “Every day I see the LBL folks running around, “but I think if I keep quiet I can sneak up. “Shh. “The data require a non-zero cosmological constant. “Approach these results not with your heart or head, “but with your eyes. “We are observers after all.” So I’ll now let Bob tell us the actual story.
Bob Kirshner: Thank you. Thank you. That’s great. That was great. I just wish you had left me something to say. No. After that wonderful introduction, I can barely wait to hear myself talk. It’ll be great. So I’m going to talk about this story of finding that we live in an accelerating universe, and I thought I would also say a little bit about the Gordon and Betty Moore Foundation, since that’s kind of an interesting thing and it’s a California thing. So, just to say, I still have an appointment at Harvard. I am the Clowes Research Professor of Science. That means they don’t pay me. They don’t expect me to teach and they collect overhead on my research grants. From the point of view of a university administrator, wouldn’t you say that’s pretty close to perfect. Now that —
Audience member: Didn’t you hear, people like to move?
Bob Kirshner: Now that I’ve moved out of my office and don’t need a parking space, I’m really a model citizen of the university community. One thing I did when I came to the Moore Foundation is I asked if they would please give me 20 percent of my time to do my scientific work, and they said, “Oh sure, 20% of your time, no problem.” So then I thought, well, you know, suppose you have 20 percent of your time, how exactly would you do that? Would you say take a couple of months of every summer? Well, that sounds good, but it’s hard to do. Well, alright, maybe you would take a day off every week. That would be doable. Or, you could do as I currently do, which is take 12 seconds out of each minute to try to think about astronomical stuff while other kinds of things are buzzing around me.
Alright, well, the agenda for today is to tell you about what the world is made of, and a good place to start is with Galileo. You can’t go wrong with Galileo. We got Galileo. We got Einstein. Oh it’s gonna be good. So here’s Galileo out in front of the palace in Venice, and he’s showing the citizens how to use that telescope. Galileo of course aspired to discovery. And as he said, “All truths are easy to understand “once they are discovered. “The point is to discover them.” And so, I wanna talk about a kind of science of discovery or discovery science anyway that from Galileo and a little bit that we’re involved with.
And I think from Galileo’s early work with the telescope, there are many things that he did. He saw the moons of Jupiter, and he saw that the Milky Way is made of stars. But one thing that he did that was very convincing is that he made it seem as if the moon, seen through a telescope anyway, is a really place, that is it’s not made of something different from the stuff of the earth as the Greeks kind of model. They had air, earth, fire, and water. Wait a minute, shown here. Now a musical group. Air, earth, fire, and water, plus a fifth essence, the quintessence, which was the stuff of the heavens. But from the observations that Galileo had, you could see that there were mountains on the moon and craters on the moon, and it looked like a real place. So, in a way, what I go to talk about tonight is finding out what the current story is for the big picture of what the world is made of.
Okay, well, astronomy has a very good way to tell about the nature of glowing objects. We can tell from looking at the light from a glowing object how hot it is. That’s very useful. And we can also tell from taking the light and breaking it up into a spectrum what it’s made of. So, you all know that you can take the light from a light bulb or from the sun, and you can use a chunk of glass in the right way to make a rainbow. And it’s an astronomer’s job, it’s a scientist’s job to take something beautiful like a rainbow and turn it into a graph. And so, here I show you a kind of spectrum of the sun or some star. And what you could see is that it’s a rainbow. But in fact, if you image it through a spectrograph, there are these wavelengths or colors where the light is all missing. There are lines in the spectrum. We talk about it, or if you look up at that graph up there, you can see that there are dips at certain places where the atoms in the atmosphere of the star absorb the light. And the wavelengths or colors at which the atoms absorb the light are exactly the same in stars as they are on the earth. So you can identify the chemicals that are present in the sun.
Now it turns out it’s a little tricker than that. Showing that it was mostly made of hydrogen took more than just looking at the spectrum, but we do know that the chemical elements that make up the earth are present in the sun, they’re present in other stars, and we have some idea of how the explosion of stars make those elements. Okay. So people knew that. They’ve gotten passed this idea that it was all fifth essence, and they knew it was made of elements that are in the periodic table. One thing that people didn’t have a very good grip on in the early 20th century was the layout of the universe, that is where is the stuff.
And in 1915, which is a date that corresponds more or less to when this story begins, people thought that the Milky Way Galaxy, the band of light that goes across the northern sky in the summer, which Galileo had looked at and noticed was made up of individual stars is a system of stars, a band of stars that’s really something like this. This is a view form inside our galaxy. The center of the galaxy is that bulgy thing there. And the dark material is dust that is in the galaxy that’s blocking out the light from some of the stars. There’s no place on earth where you can go to see this. This is a picture put together from images from the southern hemisphere where you can see some things that you can’t see here, you can’t see from Berkeley, and the northern hemisphere, which has things that you can’t see when you’re in Chile. And it turns out, interestingly enough, the most important things for this story anyway is not the big band of light across the middle, but this little over here, which was a nebula, a fuzzy thing that is in the direction of the constellation cassio… Andromeda, one of those. Andromeda. And which turn out to be kind of the key to understanding how our universe is organized.
So how did that work? Well, this demure person, Henrietta Leavitt was working at the Harvard observatory. She was making 25 cents an hour, which is middling wages at that time. The director of the observatory got paid $2 an hour, so that’s eight times as much. I’m thinking about it. Graduate student, eight times as much. Not so different. Okay, anyway. And Henrietta Leavitt was looking at variable stars, the great technological breakthrough which I’ll talk about in a minute. You shouldn’t say break when you talk about glass though, should you?
Anyway, the great technological change at that time was to take images on photographic plates, and I’ll show you one in second. Glass plates with a silver halide emulsion, and by chemistry, the white would allow you to record the images as you know. People use to have experience with film, and cameras, and stuff, but that is going away very rapidly. Anyway, she noticed that there were certain kinds of stars that were all at the same distance because they were in a cloud of stars, large Magellanic cloud and a small Magellanic clouds. And she noticed that they’re all the same distance. So the ones that look brighter were really brighter. And she noticed that they varied in a way that was very systematic. The ones that were really bright varied slowly like a big bell as sort of slow vibration. And the ones that were less bright had quicker vibrations.
Now the point about that is that shows you there’s something you can measure, that is the period of the vibration, that doesn’t depend on the distance, and it will tell you whether the object you’re looking at is intrinsically bright or intrinsically dim. And that allows you to figure out how far away that object is. So here’s the director of the observatory on his $2 an hour salary, it’s quite a nice suite. Harlow Shapley. And he was interested in this question of the layout of the Milky Way and use variable stars like these and other types to try to map it out. Here is a kind of map. This is not a picture of our Milky Way Galaxy again, because you can’t get out of it. It’s too big. But what it shows is a typical galaxy. And the dimensions where what Shapley was able to work out.
So, astronomers talk about distances in a kind of funny way we talk about time, because all the information that we get is coming to us at the speed of light, or the light that we get is coming to us at the speed of light. And the speed of light, as everybody knows, who’s ever been in a science quiz, is a foot. that’s a unitive distance used in the United Stats and in Myanmar, a foot in a nanosecond, so in a billionth of a second. So, astronomers talk about the distance to something as in the length of time it would take light to get here.
So, for example, in this room, I’m looking at the front row. I don’t see you the way you are. I see you the way you were 14 nanoseconds ago. Curiously, the people in the back look younger. That’s because I see them the way they were 40 nanoseconds ago. Okay. Just a joke in the room, but not a joke in the universe. The light takes time to get here. We don’t see everything simultaneously. We sea nearby stuff more or less now, and distant stuff, the way it was in the past. So that means a telescope is a kind of time machine that lets us see into the past, or see the light that comes from the past, and see what the universe was doing back then.
When I describe this story of cosmic expansion, we’ll compare how the universe was expanding back then with how it’s expanding now by looking at things that are very distant or nearby. That was a pretty good job of mapping out the size of the galaxy. Shapley thought it was something like 100,000 light years. I’ve been talking about nanoseconds. In a year, light travels a certain distance. And it turns out when you go outside, the bright stars that you see are some light years away or maybe 100 light years away. That means the light left that star 100 years ago or more. It could be like when Democrats controlled the Congress and something, I don’t know. Anyway, the light left some length of time ago and the universe was quite different. And it’s arriving here tonight. The distance across this galaxy is about 100,000 of those light years. So that gives you the scale. Alright. And we’re not in the center. Berkeley is located out here, kind of across the bay. Not in the center of the Milky Way. Okay, so I was talking about, I was talking about photographic plates, and here’s an example of a photographic plate.
This is an image, black and white, taken of Albert Einstein. And you can see that the exposure time was actually quite long, because the person who was sitting next to Einstein in this chair got up and left. The person behind him is shaking his head. “No, no, no. “Relativity, a bunch of nonsense.” Anyway. The interesting thing here is that the sensitivity of photographic plates to light is kinda limited. It’s chemistry in a gel applied to a piece of glass. You swish it around in the right way and you reduce the silver salts to silver metal, and that makes the negative and all that stuff that you do. It turns out only a small faction of the light that falls on the photographic plate really does something. And so, in the old day, you had to keep the shutter open quite a long time. Now when you use a digital detector in your cellphone, you can take pictures in low light without a flash like this, and the exposures are pretty short because the light makes an electric signal that is much more efficiently converted to information that these photographic plates.
Okay, a long-winded thing about photographic plates. The amazing things is that we have now developed the method not just to see what people look like back then, but to see what they were thinking. And here’s what Einstein was thinking about the universe. He was thinking, “Well, it must be static.” He had been working with gravity, and he knew that gravity pulled things in. But he thought, “That just doesn’t sound “like the right answer.” So he put in by hand, when he was thinking about this world of the whole universe, he put in by hand an extra term, this cosmological term that we’ve been talking about, that had the effect of a kinda antigravity, or, as you described it, is a repulsive term, yes. Here’s why he did it. He said… Can you read this in the back? Oh, too bad, because I was gonna say I was translating from the German. But okay.
He says, “That term,” the Greek letter lambda, that’s his fraternity, that is the cosmological term. He says, “That term is necessary only for the purpose “of making possible a quasi-static distribution of matter, “as required by the fact “of the small velocities of the stars.” So it turns out if you measure the velocities, I’ll mention that in a minute of the stars in the Milky Way, they’re not very large, and some are coming towards us and some are going away from us. And so he thought we lived in a static universe. He wanted to make sure that his gravity theory, which was the general theory of relativity, agreed with the facts as he knew them in 1915 or so. So, here he is. Here is Einstein and de Sitter, and they are working at the blackboard. We have this photograph. And again, the most important thing of course is not those big hulking people with their pipes, but the lambda that’s in there, unless that’s just a Lagrange multiplier, I think. Anyway. Alright, so how do you decide whether that’s right or not?
Well, the answer has gotta be observation. It can’t just be theoretical argument no matter how articulate or kinda convincing to people or how loudly it is pronounced. So here is the tool that really helped change things, which is the 100-inch telescope at Mount Wilson. Andrew Carnegie set up an observatory. George Ellery Hale built the world’s largest telescope. Well, he had built it in Wisconsin, then he realized California, much better. Built a 60-inch telescope. That’s the diameter of the mirror. 100-inch telescope, a tremendous feat of engineering at the time. And then later the 200-inch telescope, the Palomar Telescope.
So again, what’s the most important thing in this picture. By now, you’re on to me, and you know it’s not the big thing in the middle. It’s something else. It’s this chair over here. Up here is a platform where the astronomer would sit, and here’s the astronomer sitting in it. That’s Edwin Hubble sitting on that chair, on that dangerous platform. And what he’s got there is a gizmo which holds a big photographic plate so that time exposure can accumulate the signal for, I don’t know, 20 minutes or something like that. And then he’s holding some knobs that allow you to compensate for the errors in the motion of the telescope. The earth is turning, you know that, and so that means star seems to rise and set. And a big telescope like the one at Mount Wilson, or any telescope, is designed to pivot at the rate that the earth turns, so that you can keep the star on the photographic plate and make a picture like this one. So this is a picture of the Andromeda galaxy. It was taken on, it’s not just memory, 6th of October 1923.
So about this of year, almost 100 years ago. And Hubble was looking for novae, new stars, that would pop off in this cloud of gas, this cloud of gas and stars. And he found one of these new stars. And then he realized that wasn’t new, he’d seen it before. It was a variable star, so he crossed out N and he wrote for he rest of us to see in big letters V-A-R exclamation point. I don’t think that just was a notation to himself. I think he knew this was really important. And he wants you to know about it 100 years later.
Anyway, there it is, VAR. And what that is, he found out, is one of these stars that Henrietta Leavitt had been working. What he was able to do by going back night after night, week after week, he had a lot of telescope time, he could go back and he could measure the periods of these stars. Then he knew whether they were intrinsically bright or intrinsically dim. It turns out even though they were apparently dim because they’re far away, they were luminous stars that appears dim. That meant they were far away, and the distance to the Andromeda galaxy is on the order of millions of light ears. So, for the stars you could see with your naked eye, it’s a few, or a dozen, or a hundred, or a thousand. But for the galaxies, it’s a thousand times farther away, a million times dimer. And that’s why people needed telescopes to be able to get on this trail.
Okay, one more thing before we get to the… Well, we’ll still be in the 20th century for a little while. Here’s a kind of exciting sort of astronomy. Here is a guy wearing a hat who has got this kind of bent thing, which is a spectrograph. I talked about taking the light from the sun and spreading it out into a rainbow. That’s a spectrograph that has prisms down there. And that bendy part, that make one of those images, only on a photographic plate, in black and white, but that’s okay, to see whether the spectrum lines that I talked about that belonged to the chemical element same as the one in the periodic table, where they show up because… I didn’t tell you, but it’s true that if they object is moving away from you, it stretches out the spectrum to the red. And if it’s moving towards you, it crunches it up toward the blue end of the spectrum.
It’s a small effect for ordinary things on the earth because the speed of light is so big as I was talking about a minute ago. If the speed of light was a million time slower, highways would look like this. Cars coming toward you would look kinda blue. Cars going way from you would look kinda red. Well, in fact they do, but it’s not for that reason. It’s not for that reason. We have red taillight and you know it. Okay. So, it turns out that Slipher work incredibly hard, the fellow I just showed you. Vesto Slipher worked at the Lowell Observatory which is in Arizona and he used that spectrograph, and he took specter of the same things that Hubble was measuring distances to. They’re interested in these objects, were they? He saw they’re mostly beta stars and gas, and he could measure their velocities.
And the interesting thing was he found that almost all of them were moving away from us. That’s interesting. So, Hubble, by 1925, he had the list that Slipher had measured, and he had distances which were first from the variable stars and then from the properties of the galaxies themselves. And that was 1925. And I’ve always wondered, how come he was so slow to notice that if you took the distances, and you took the velocities, and you did the thing that scientists always do with a list of two numbers, two list of numbers, and that is make a graph. How come he didn’t do that?
And it turns out, I’d forgotten this, I’d read this book when I was a freshman in college, which is Hubbles book about how I did this, The Realm of the Nebulae. And in it he explains why he did not plot the distance against the redshift, the thing for which he is so famous now. Here is his reason. He said, he said it was natural inertia. Are there graduate students here? If you’re kinda not making very good progress on your thesis work, natural inertia. Now the sort of psychological physics. “A natural inertia in the face of revolutionary ideas,” Einstein’s ideas they turn out, “couched in the unfamiliar language “of general relativity discouraged immediate investigation.” The passive voice was used. This is the school of letters and science. When I read this book I realized I had to write a foreword it. I’m the fourth person to do that. The first three forewords were pretty good I thought.
Anyway. So I really had to read it. And I noticed that every time he used the passive voice, he was talking about himself. You’ll see, it comes up again. Alright, so, some people will say, “What is he talking about?” Here is the graph which we call, uh… I think we can still call it that for a little while until the IAU counts the votes to see how much we have to talk about Lemaitre. Anyway, we’ll come back to that. Here’s velocity going up this way, distance coming out that way. This is what we would call a Hubble diagram. This is what Hubble called figure one. And what you can see distance out here. This is in parsecs. Well, okay, there was like three layers and parsecs. Yeah, three layers and parsecs. Six million, three million light I should say. And velocity, those are 500 kilometers a second or a thousand kilometers a second, so those are pretty good speeds. New Tesla 3 isn’t gonna get up to that speed. I’ll show you why I would think that this is the evidence that we live in an expanding universe.
But I wanna just mention one thing for the people who are kind of really tuned into this, which is that it’s widely rumored and probably true that Hubble never really bought in to the idea that they were measuring actual expansion velocities or the stretching out of the universe. And he thought there might still be some other reason for that. And I think the reason he was so ambivalent on that is given away in his, if not foreword, it’s a preface, the thing he wrote at the front of the book. This is kind of interesting. “In the field of cosmology, “the writer has had the privilege. “The writer has had “the privilege of consulting Richard Tolman and Fritz Zwicky “Of the California Institute of Technology. “Daily contact with these men “has engendered a common atmosphere “in which ideas develop that cannot always be assigned “to particular sources. “The individual, “in a sense, speaks for the group.” No citations for you. Okay. So, we’ll come back to that because I’ll show you that Zwicky was very uncertain in the mid-30s about whether these redshifts that I described a minute ago were really measuring velocities or something else. I’ll show you the evidence for that.
Anyway, here’s the conventional picture. Nevermind all those highfalutin scientist. If you’re taking a freshman course from Alex Filippenko, you must interpret the Hubble diagram this way, that the universe is expanding away from you. The nearby galaxies moving away slowly, we’re just the ones more rapidly, and that’s the evidence for the expanding universe. Well, okay, at Harvard of course this is normal, certainly for the faculty. They all think they’re, well nevermind. It turns out you can make a picture where the whole universe is stretching out in all directions, and for every observer it would look like that. If I had the power, if you gave me the power, Dean Hellman, to make this room twice as big, what would you see? You’d see the people are one seat away from you would move to be two, and the people who are 10 seats away from you would end up 20 seats away if I just stretched everything out. You’d see everybody moving away from you not because they don’t like you, but because the whole space is stretching out. The nearby things would stretch out a little, the distant ones a lot more.
So that’s the picture we have. Okay. And the thing that determines how this expansion is gonna go is gravity and… Einstein wrote down a kind of theory for this, which was very helpful, in which people wanted to use to understand the expansion of the universe. So, here’s the kind of class, what we call yearbook picture. Yeah, here’s the yearbook book picture. Einstein is visiting 813 Santa Barbara Street, the headquarters of the Carnegie Observatories, Mount Wilson Observatories. Campbell of Campbell Hall is next to him. Isn’t that Campbell? I think that’s Campbell. I think that’s Campbell. I don’t know where Hall is, he’s not in here? Hubble is over there on the left, and you can see that George Ellery Hale who is up there in the portrait is patting him on the head, “Good work. “I’m glad we built that telescope “so you could measure the distances “and connect that to the velocities “and see that we’re living in an expanding universe.” And Einstein who had gone to a lot of trouble to make this equation with the cosmological constant in it, for completeness no doubt, decided that, well, maybe after his own observations shown here, after his own observations, he became convinced that Hubble was onto something. Hubble is in every picture, like a ghost coming out of, anyway.
And here’s what I… Well, the legend is if you read a book and you gotta be, you must read Alex’s book. I’m sure it talks about, I don’t know actually, it might be, he ought to, say something about Einstein calling this his greatest blunder, that’s what everybody says. I was trying to, the cosmology constant, his greatest blunder, I tried to track down who said it first. This is incomplete and possible not the best historical work ever, but George Gamow, who wrote the books that I read when I was in high school about math and science, and cosmology too, wrote an autobiography. So who else could write his autobiography?
And here’s what he said: “Einstein’s original gravity equation was correct, “and changing it was a mistake.” That is putting in the cosmological constant. “Much later, when I was discussing “cosmological problems with Einstein.” I really wanted to put that in my book, hasn’t happened yet. “Much later, when I was discussing “cosmological problems with Einstein, “he remarked that the introduction of the cosmological term “was the biggest blunder he ever made in his life. “But this blunder, rejected by Einstein, “is still sometimes used by cosmologists even today,” this is 1970 I think, “even today, and the cosmological constant “denoted by the Greek letter lambda “rears its ugly head again, and again, and again.” Gamow, you can’t beat it.
Okay, so this is the picture now. We have a universe made of galaxies. The distances between the galaxies are millions of light years. The equations that govern the history of expansion are Einstein’s equations, maybe with, maybe without the cosmological constant, With Einstein giving it the curse, you’d probably think, “Well, I don’t think I’ll do that, “except for completeness of course.” And there were people who thought this might be a real physical story of the universe of expansion from a high density place in the past to this universe that we’re seeing today. And the person who really talked about that is Georges Lemaitre. The controversy is Lemaitre made a Hubble diagram before Hubble did. So whose diagram is it? That’s the short form of it. The answer is Lemaitre was too polite after it. He says, “The evolution of the world,” he means the whole universe, “the evolution of the world can be compared “to a display of fireworks that has just ended, “some few red wisps, ashes, and smokes. “standing on a well-chilled cinder, “we see the slow fading of the suns, “and we try to recall the vanished brilliance “of the origins of the worlds.” He thought that this expansion meant there was a time when the universe was dense, and small, and hot. Well, we’ll see. Yes, hot. That led to the universe that we see today. So he was really onto it. Okay.
But just let me say even after Hubble’s expansion law was found and after Einstein said we can get along without it, there were some people who thought maybe lambda had something to do with the blowing up of the world. So here is Professor Willem de Sitter. This is from a Dutch newspaper. English is a dialect of Frisian, college of letters. And so, it turns out you can read this. What blows up the ball? The ball up. What makes the universe expand or swell up? That has to be lambda. No other answer can be given. Well, okay, so at least some people were still thinking about it. And of course I’ll show you that lambda is back rearing it’s ugly head, only now with data. Okay, better get moving. So here’s Einstein. It’s not Einstein, it’s a statue of Einstein at the National Academy, and he’s holding a tablet. Some people say he brought it down from a mountain, but he’s holding a tablet with some equations on it.
Let’s take a closer look. Let’s take a closer look. There’s an equation everybody knows, E equals m c squared, the way energy is related to the conversion of mass. If you wanna build a nuclear weapon, that is a good equation to know. Or power the sun, come on. The Nobel Prize committee was afraid of all that stuff, and they thought this middle equation here that has to do with the photoelectric effect. Very good. Okay. And up at the top is the equation of general relativity, kinda complicated. It’s got all thew mu new things on it, but there’s no lambda in it. No lambda on the tablet, and I’ll come back to that at the very end. Alright. So I keep saying you don’t decide these things about what’s the right picture of the world just by talking, by convincing people, or intimidating people as people we’re trying to do in the hallways at MIT. I’ve already got that in here, so ha ha ha. But by evidence.
So here is an iPhone picture taken of Tycho Brahe. And you’ll notice they’re looking up. There’s a new star up there in Cassiopeia, that was on my mind. And they’re pointing to it. The most important thing in the picture is not the castle, it’s the little dot up there, which is a new star. It turns out this is a supernova explosion, a star exploding, destroying itself, turning the oxygen in its core into iron, and emitting a bunch of light. And if you go to the same place that where Tycho saw this in 1572, if you go there tonight and use a radio telescope, and an x-ray telescope, and engineer optical telescope, because we can, and look at it, you can see there’s this wonderful thing where the emission from the silicon and iron, which is the fluffy stuff in there, which is the inner parts of that star is being blasted out into the space between the stars. And it’s a very nearly spherical kind of a thing that we’re seeing. This is what we call Tycho’s supernova remnant. And if you look at there today, you can see that it’s expanding. And if you work back when did it explode, it’s the right one. These things are seen in our own Milky Way galaxy with your eyes, but you can see them in distant galaxies with telescopes. Alex is a great expert on finding supernovae. So here is a galaxy not too far away. That dot, we know because we measured that the light from it shows the stars expanding at 10,000 kilometers a second, so it’s ripping the star apart, and it glows brighter and dimmer over a period of a month or so, and then trails on for a long time because it makes radioactive elements. Okay.
So, what’s the story here? How do you use these things? And it turns out Hubble was onto that too. I had the experience of reading Great Expectations in high school. Maybe some of you also endured that. I remember reading Great Expectations. Reading Great Expectations and thinking Pip, the protagonist, was kinda sort of my age, which he was because he gets older. But then I had to read it again in college. I thought, I read this, geez, Dickens. Okay. Anyway, I read it, and I thought, Pip, he’s kind of my age now. When you re-read something, you kinda see it differently because of what’s happened to you. And in this case, it turns out this book, which is allegedly about galaxies, is in fact entirely about supernovae. Read this. “Supernovae can be detected, can be detected. “Super novae can be detected at immense distance, “and, in principle, they are a criterion of distance “about as reliable as that of “the total luminosities of the nebulae.” That’s the tool he was using to figure out how far things were. And he said, “Gee, that’d be great.”
Actually, however, actually, however, Hubble was one of the very first Road Scholars, and he went to Oxford. So even though he grew up in Missouri, he spoke with an English Oxford accent. And said things like “Actually, however.” “Actually, however, the maxima,” the brightness of the supernovae, “are so seldom observed “and the supernovae themselves are so rare “they contribute very little to the present problem.” The problem wasn’t that the supernovae weren’t good. It’s that you couldn’t find enough of them. And so, I wanna show you that technology has come to our rescue. Here is Bev Oak who was my thesis adviser at Caltech. And you can see he is a happy guy because he is holding in his hand one of the very largest silicon detectors used in astronomy, a CCD, like the things in digital cameras. This is a .24 megapixel device. And it cost a ton of money, and he’s very happy to have it. This is in the ’70s.
And now I’m going to the Gordon and Betty Moore Foundation. The Gordon and Betty Moore Foundation, somebody said I give away $100 million a year. It’s not my money. It’s money that Gordon Moore of Moore’s law and one of the founders of Intel set up a philanthropic foundation 15 years ago with money that had to do with the technology of building chips. And here’s Moore’s law, the one you’re familiar with, where on the horizontal axis is the year of introduction, and on the vertical axis is the number of transistors in the processor. And it’s pretty amazing. The vertical scale is a logarithmic scale, so each tick is a doubling. And the horizontal scale is a linear scale, so each tick is tick of a clock, years. And it turns out that the doubling time is about two years, starting back in the ’70s when that chip was made. And now it’s up here.
So over here, a few thousand devices on a chip, and that’s what you get, now worth billions of transistors on a chip. It turns out that through the miracle of capitalism, you can just change the axis. Roughly speaking, this is Gordon Moore’s net worth. It went into the billions. And he decided that’s enough. There’s nothing I need. And he set up this foundation. So we’re in Palo Alto. We give away all of the earnings on $7 billion. Well, not really. We give away 5% on seven, around $350 million a year. The things that we do are environmental conservation, we do basic scientific research. That’s the chunk I’m in charge of. We do patient care. It turns out that Gordon and Betty, Betty was in the hospital, and she got the wrong medication, and nearly died. You would think normally, a rich person would then sue for malpractice. No, that’s not what they did. They set up part of the foundation to improve the outcome of patient care and they endowed the Betty Irene Moore School of Nursing at UC Davis. Very generous people who’ve taken the high road in life. Anyway. Alright.
So what’s the point of this diagram? Well, okay, there’s a Moore Foundation. That’s part of it. And the other is that detectors have gone from being very expensive and very small to very expensive and very big. So here’s silicon detector now as big as a photographic plate, only it works 100 times more efficiently. It sends out the signals as electronic signals that you can process in a computer. The computers are bigger, and better, and faster. So you can actually do this, add pictures, subtract pictures, multiply them. So, for example, you can take a picture of a piece of the sky and go back, take a picture of it a week later, and you can subtract the epoch two, that’s tonight. You can subtract epoch one. That was a couple of weeks ago from tonight. And what’s left over is a dot which is a new object, in fact, in this case a supernovae. So, the technology that has replaced photographic imaging has also allowed you to go straight to the computers and do this digital processing to find things. Everything else that stayed the same went away, because we did a real good job on this little piece of it. And only the dot that changed is visible. Okay, so I’ll come back to that because the technology underpins being able to do this. Let me go a little faster. Okay.
There’s one more thing I wanted to say about Zwicky, which is of course there’s another big component with the universe that we heard about a little bit in the introduction, which is that there’s gravitating stuff that we don’t see very well. It’s not the stars. It turns out it’s not even stuff from the period table. But Fritz Zwicky looked at clusters of galaxy. So here you see those big fuzzy things, and then there are a bunch of ones around. It turns out these are all in the same part of the sky. The density of galaxy is much, much higher than it is elsewhere. And if you go and measure the velocities of some of these, you can estimate what the mass is that would bind this bunch of whizzing things together. So here’s Fritz Zwicky, as he was seen later. Here’s book, Morphological Astronomy. I had to buy a used copy and somebody had written in it. And they underlined the most interesting things.
So, it turns out where Fritz was talking about that cluster of galaxies and how much mass it had and how it was so much more mass than was account for by the stars, that there must be dark matter, stuff that was present gravitating but not radiating. And so he calculates equation 82. He works out how much mass you’d get from this whirling round stuff. And then this unspeakable person who bought, from whom I guess I bought the book, indirectly, says. It talks about this, it says we arrive at the conclusion that the conversion factor from luminosity to mass is at the order of 500. So there’s 500 solar masses for every solar luminosity. That’s sort of the number. In contradistinction to much smaller conversion factors for galactic stars. For the sun, for example, that number is one. If the star is more massive than the sun, it’s less than one. So, that he scratches his head. He said there’s several alternatives to explore. He says, well, maybe it’s true indicators “of their total mass. Okay.
If you find an astronomer… Who’s an astronomer? Who consider him to? Yes. Alright, look around. What you do is you punch them in the arm and you says, “Who discovered dark matter in clusters in galaxies?” They’ll tell you Fritz Zwicky. Okay, because that’s explanation A. The second B beta. The second possibility is again that the universe is non-expanding. This does not mean, however, that the cluster are necessarily stationary. They observed, that’s confusing. The next one is gamma. The universal nebular redshift might be caused wholly or in part by some physical effect other than a doppler affect. As I’ve said at the beginning, Alex is gonna mark it wrong if you say that it’s caused by tired light or something like that. Fritz Zwicky said, “Well, you know, A, B. “I mean alpha, beta, gamma. “Delta, the fourth assumption.”
Okay. It goes on. Epsilon. He’s got five explanations. Looking backwards from today, we can only see one. Only thing, oh, it must be that he knew that the mass to light ratio was high. Okay, that’s probably the one he most believed. But the point I wanted to make was that he was still willing to entertain. This is 1957. He was still willing to entertain this gamma possibility that the redshift isn’t due to the motions. It’s some other story. Wow. Just to put your mind at ease. We’re pretty sure now that it really is, the motions really do tell us about the gravity. We know that because we see not just the motions of the galaxies, but we also see the emission from hot gas that’s in there, x-ray emission. And maybe the most simple thing to understand is you can see that gravity has the effect, Einstein’s gravity has the effect of curving the space and producing these beautiful arcs that you see, these images. There’s a Hubble Space Telescope picture of a cluster of galaxies. The big yellow things are galaxies, and these arcs are the bent light from objects behind the cluster of galaxies. So, there’s really mass there. And just to say it’s also present on the scale, this dark matter is also present on the scale of individual galaxies.
Here’s Vera Rubin measuring some of these photographic spectra that were taken in galaxies. And it showed one side of a galaxy coming toward you, the other side going away. It’s sort of depicted like this. This is a way a radio astronomer would do it. And you see that the galaxy is spinning, how fast it’s going. You can figure out what the mass is inside there, and it’s really big, mch more mass than the stars. Okay. Alright, that’s a long preface for how are we gonna find out if the universe is accelerating. I better go quicker. Alright. Here is a bad haiku. Here is a very fine etching. And the point is the way to understand the dark matter is what you see is the snow. But the thing that’s underneath it that gives it substance is the mountain.
So, in the same way, we see the stars, but the things that’s underneath that gives it substance is the dark matter. Deep snow traces rock, always winter never spring, mountains do not melt. Whoa! School of letters. This is prose, by the way. It’s a prose. Alright. Visible matter traces the dark universe. So that when you see a galaxy, what you see are the stars moving in the gravitational field of the dark stuff. Okay, why do we believe this, because we see they connect. Wouldn’t it be great though if we had a real experimental test? And it turns out the Berkeley is one of the places where people are working very hard to try to make detectors that will actually detect the dark matter that’s moving through this room. So the idea is we’re in the galaxy. I showed you we’re out there in the outskirts.
We’re going around, we’re going 200 kilometers a second. If the dark matter is there, it’s like driving through a rainstorm, there’ll be splattering going on. Dark matter does not interact as strongly as a drop of water with your windshield, but there ought to be a way to detect it. People here have been very clever about building detectors here, a gadget which is a cryogenic dark matter something rather. That’s I think what the S stands for. That is they’re hoping to use to actually see the dark matter that’s the earth is going through as we go around the galaxy. That would be great. You do wonder how long you’re gonna have to wait for this though. The speed of light, it was 150 years from the time that astronomers knew that the speed of light was finite, as I was talking about, until you could measure it in a lab, because it’s so fast on ordinary scales. Dark matter, well, you might say it was 1933, this stuff that Zwicky was doing. And everybody who has a dark matter experiment expects that when they open up the box, they will see the signal. It could happen next week. Maybe. People are working very hard on that. Okay. Alright.
So now let me come back. So now we’ve got the components. We’ve got an expanding universe with dark matter. Let’s get moving. We’ve got supernovae that allow us to measure distances and technology that allows us to find them at will. And so, Saul Perlmutter and his gang and Alex, and Brian Schmidt and our gang were working on this more or less separately, but we had contact with one another as shown here. Saul punches above his weight, I guess you could say. And so we measured distances in redshifts for distant supernovae, and we compared them to what you’d predict. Remember the story is if you look nearby, you’re seeing now, and if you look faraway, you’re seeing the past. And the light travels four billions of light years from the supernovae. It turns out the stars that we’re looking at are a million times brighter than the ones that Henrietta Leavitt was working on. That means we can see them a thousand times further away. Instead of millions of light years, we see them at billions of light years. It’s really as simple as that, except we have to figure out how to use the supernovae. That’s a whole story in itself. Okay.
So here’s the result. As it was back in 1998, the blue dots are ours, the red dots are Saul’s, and the thing that you’ll notice is there’s some kind of a diagram up there, but… Evidence that the universe is change its expansion in a way that’s kinda interesting is out here at large redshift, big distance, the early universe. It turns out the distances that you find are bigger than you’d expect in a universe that was coasting. If it was coasting, you get this horizontal line. The points are up there, even though it’s a mess and they’re not very good points. We do much better now. It was evidence that we’re living in a universe that was accelerating. So, this cosmological constant, which had the property of making the universe accelerate seemed like there was some evidence for that.
So just to recap, you could make a diagram, dark energy, how much of it going one way, and dark matter, how much of it, going the other way. And just to recap, if you ask a theorist in the right part of the hallway at MIT, they would say, “That’s my answer, that red star.” And if you went over to building 37, and you talk to, or nine, I forgot. Anyway, MIT is very confusing, all numbers. I took a course from professor 84. If you went and talk to the, no, that was the building. If you went to talk to the observers, they’d say it was here. Zero dark energy that has no cosmological constant and a low dark matter for the observers, zero dark energy because Einstein said you must do this, except for completeness, and all dark matter. And those are both wrong.
The contours there show you what the hint was from the early data from 1998, from both groups. And you can see the contours from the two groups are very similar. And they don’t lie where those blobs do. They light up, which means you have to have some dark energy in order to account for the supernova data. This was a big surprise to Einstein, because he’d been dead for so long. And he started walking around with sheets of paper under his arm that have lambda on it. I don’t know if you could see this revisionist stuff. He got lambda right there. Okay. So here’s the elementary idea. Oh, right, an elementary school. “What school did you last attend, Sherlock?” “Elementary, my dear Watson.” Okay. It’s the girls and the boys I think doing a tug of war. But the idea is it’s like the dark matter and the dark energy. The dark energy, the dark matter is working to slow things down. The dark… What did I say? The dark matter is working to slow things down. The dark energy is working to speed things up. And there’s a kinda tug of war that is reflected in the history of expansion for the universe. That’s pretty bad metaphor. Okay.
Today, the data is a lot better. Here’s a paper by Dan Scolnic and others. Data is way better, and you can see there’s some kid of a line up there. And then here, this shows how things are scattered about the line. It’s a curved line. I’ll show you in a second. That is one where the universe is accelerating over time, and the data has gotten way better. The constraints of the properties in the universe are much better. Here’s that old diagram I showed you. This is a fairly recent one, not even the one that I just showed you, because I couldn’t figure out how to make the colors plausible. Anyway, we’re doing a lot better. Okay. So here’s Galileo, let’s circle back. He said, “All truths are easy to understand “once they are discovered. “The point is to discover them.”
So now it’s your turn to show that it’s easy to see. Here I show you a bunch of points which are the measurements of supernovae, distance and redshift, so like distance and velocity. And the line that’s ticking through are sort of different values of what different fracturing the universe in the form of this crazy stuff, this dark energy that has this negative pressure that makes the universe expand faster over time. And you can see there. Not there, not there, not there, not there. Getting better, getting better. There. .75 or so. 3/4 of the universe in dark energy. Okay. The point is to discover it, not to see it later. Because if you discover it, the king of Sweden invites you to shake hands. Okay, so here’s our picture. It’s an early time in the universe. It was hot and dense. I’m sure they had the inflation before the big bang. Anyway, the timely expansion and slowing down due to gravity for a while. Gravity was winning. But now as the matter thins out, the density goes down, the dark energy is bound to win. So that’s our kind of picture up for it. This is not the world’s most precise diagram, but this is conventional thing, is to have a pie diagram. Now, are you the dean of all sciences?
Frances Hellman: No. No. just physical science.
Bob Kirshner: Alright then. So, I think you should look at this diagram, Dean Hellman. And you’ll see that atoms out of, so let’s say chemistry, roughly speaking, is about 4 percent. The dark matter, which turns out it can’t be made of anything from the periodic table, it’s too long story to tell you tonight, is about a quarter of the universe. So that would be, let’s say, experimental physics. And the astronomy in cosmology would be 73 percent of the university’s budget. Now I told you obliquely that I work at a philanthropic foundation. I didn’t tell you obliquely. I told you directly I work in a philanthropic foundation, and that makes me think of money. And so, I’ve redrawn this diagram. I don’t know if you’ve ever spent any time looking at the back of a $1 bill. I really do not recommend it. It is the weirdest thing. There’s this eye, and there’s some Latin phrases. Oh my goodness. It says, the Roman numerals says 1776. Anyway, dark energy, dark matter, but the intelligent brilliant part of the universe, the eye, it’s about 4 percent of that. Four cents on the dollar okay. So you might say, “Well, this is a crazy picture. “I don’t believe this.”
Well, can you read that? I’m hoping not. Okay. It says, that T-shirt which my wife is wearing says, “Although the universe “is under no obligation to make sense, “students in pursuit of the PhD are.” And I saw this for sale in a gift shop. And I said, I am Robert P. Kirshner. And they said, “Oh very nice.” They said, “We’ll give you a good discount.” Okay, well, if you go in the inter-webs, people say, “Oh, the scientists “don’t know what they’re talking about. “Dark matter, dark energy, they’re just names for things.” No, we really do know what we’re talking about. Here’s the metaphorical picture for the dark matter that we see where the light is distributed, but that’s not the substance. The real substance is like the mountain. And the dark energy, we also don’t see it exactly, but we see the effect of it. And you’re used to making arguments like that. If you look at trees outside and they’re moving. And somebody says, “what causes that?” You’ll say, “The wind.” You don’t see the wind, you see the effects of the wind. And we don’t see the dark energy. We don’t have a dark energy detector really, that’ll be a good thing, but we have the idea that if we measure the expansion history where you can detect the effect of dark energy.
Okay. So, I showed you Einstein’s statute. So I went back there and I was trying to put the lambda back in that equation when the park police came. Alright, well, how long do we have to wait? I don’t know. Till there’ll be real experimental evidence on these things. For the dark matter, it could be tomorrow, or it could be we’re looking in the wrong place. That’s something that we’re looking into at the Moore Foundation. I know people are working on at Berkeley, other ways to think about dark matter that might not be the ones will be found with the detector I showed you. For the dark energy, a laboratory experiment for dark energy seems unimaginably hard, but that’s only now. Maybe things will get better. On the astronomical side, the Moore Foundation has been helping the University of California and Caltech to participate in this incredible technological adventure of building the 30 meter telescope. And Moore Foundation has put $200 million into this project, and we sure hope we’ll get the land use permit.
But this is something which will help us work over the whole span of the universe from nearby to look at planets around, nearby stars, only a few light years away to the earliest starts and galaxies forming right at the dawn of cosmic time. So it is a fantastic instrument that will help us fill in this picture. Okay, well that’s enough. Let me just close by saying what kind of science is this. When I was president of the American Astronomical Society, every once in a while I would stand shoulder to shoulder with the presidents of other societies, and we would talk to congressmen basically, and explain to them the low road. We’d say, well, science is for technology and technology is gonna lead to economic growth, and we’d say it’s dangerous world and it’s important we have science for defense. And a lot of congressmen are kinda old guys, and so medicine, they’re big on that.
You know, there’s this idea that science should do all these things in the world. And honestly, it does do these things in the world, and those are pretty good reasons, but there’s more to it than that. Everybody, you came here. You didn’t come here because I was gonna tell you how to make a large detector or something. You came here because of the ideas. And we’re all interested. We wanna know where we came from. We wanna know where we’re going. We wanna know where we are. So, I think at least some science, a little bit, and the science that we do at the Moore Foundation is part of that story. Oh no. You didn’t wanna be bored. Sorry, I forgot my punchline. We don’t wanna be all those things. Some of the science we ought to do for the joy of finding out how the world works. Thank you very much. Thank you. Boy, you didn’t get cheated, did you? Get out of here.
Audience member: What’s stopping the original inflation? Why did it stop?
Bob Kirshner: Well, very interesting, because of course that also is a negative pressure or an energy density that doesn’t decline very rapidly with time. But whether this acceleration that we see now is closely related to the early phase or not, I don’t think anyone really knows. And so problems have been studied more or less separately. But you’re exactly right that they have the same character that as the volume expands, the energy density does not go down. It doesn’t go down very fast. And the question of how you come to an end of inflation is a really technical problem that people have worked on. And whether there’s some residue that’s left over, that is this stuff that shows up billions of years later as the dominant energy in the universe seems wildly improbable. But I would say we really don’t know. It’s not that people don’t have a story about the end of inflation, but the question of whether it’s connected to the late time acceleration I think is completely open.
Chung-Pei Ma: There are many theoretical models, but they all require fine tuning.
Bob Kirshner: It’s like when you go to the doctor and the doctor says, “I’m gonna give four pills for this.” That means they don’t know which one works. But, you know, it’s good to have a variety of ideas and to let the observations or the experiments in the long run determine which of those is really the right one. Science works there.
Audience member: If you have a group of Ia supernovas, what is the variation of brightness of that group, and how does it affect this measurement?
Bob Kirshner: Yeah. It turns out that if you just take a sample of supernovae, and you don’t do anything, there’s a factor of two range and brightness. But we are much more clever than that. It turns out that if you look at the light curve, how it gets bright and how it gets dim. Just like Henrietta Leavitt, the ones that decline slowly are intrinsically brighter than the ones that decline fast. And so, empirically, we’ve been able to work that out we’ve been able to do things with the colors of the supernovae and account for dust. We have a whole elaborate way, which is what made this possible. When you do that, the scatter in the distance is about 10 percent. So that’s a big improvement, and it means that with a sample, as you saw, a couple of hands full of supernovae, you can measure an effect, which is the one due to the acceleration, which by itself is a 10 percent or 20 percent effect. But if you get a bunch of things, each which the distance in the 10%, by the time you got a couple of hands full, you really got a pretty good idea whether the universe is coasting along, slowing down, or speeding up. The answer is the supernovae are pretty good, provided you filter them properly and use these rather elaborate empirical techniques to improve the measurement, but that’s what we were working on in the ’90s, to get to the point to be able to do this.
Audience member: I’m from the letter side of the house, so I don’t know if my question will make sense. the big bang and following. Is that saying that the history of the cosmos is a one directional event that it’s going to go out further out, further out?
Bob Kirshner: Yes, interesting, isn’t it? So the idea is, like that tug of war, it turns out as the density of matter decreases as the universe gets bigger and lower in density, the cosmological constant, even if it’s constant will be bigger compared to the slowing down. So, you will get acceleration until the expansion speed. There’s no rule that says the expansion of the universe that the space can’t be faster than the speed of light. So that means galaxies will kinda get out of your view. And as time goes by, oh I can see he’s really getting this, and as times goes by you’ll see less and less of the universe, until finally it’s only us and Andromeda, just the way it was in 1915 when people — no, seriously. And it could be worse than that. That’s for a cosmological constant that obeys the simplest rule for the relation between the volume and the pressure. If the pressure changes more slowly than that, the expansion will be not just exponential, which this is, but worse than that. And so it means with people talking about the big rip that you would start to take apart things that are bound by other forces. We don’t know if that’s correct. We don’t know if that’s the universe we live in or not, because we don’t have precise enough measurements of how the acceleration is going. So the next thing to do is not to just find out if there’s some cosmological constant, but to see what its properties are, pressure and density. And also, whether it has changed over time, because that’s a possibility too.
Audience member: But the view from here is it’s going.
Bob Kirshner: Yeah, the view from here looks like a one-way picture and people have gotten very nervous about that. Because in a few hundred billion years, the view is gonna be really different. There won’t be any astronomy, well, hardly any.
Audience member: What are the current efforts to define a more complex model of gravity that might explain things like dark energy?
Bob Kirshner: Yeah. So, as many of you know, there’s a really problem in theoretical physics which is that gravity stands apart from the other forces of nature that we understand the electric force, and the weak force, weak nuclear force, and the strong nuclear force. It’s all connected to one another. You can write down a theory that has all of those. But there’s no quantum theory, and those are all quantum theories, there’s no quantum theory of gravity that people can agree on or that you can test. So, if we’re gonna get this right, it’s gotta be, that’s gotta be where the problem is. Now there is a big theoretical enterprise of string theory, in which gravity does get treated like the other forces. It’s very hard to use string theory to make actual predictions about how things will work. Although this summer, there was recently or there has been quite a big discussion about a string theory model that seems to imply that you would have a changing cosmological constant. Well, if that’s true, it’s very interesting. At the moment, that’s learned people on one side saying, “Here’s what I think.” and somebody else saying, “Oh no, that’s completely silly.” In the end, you’d like to have that adjudicated not by personalities, but by evidence out in the real world. So we’ll see, we’ll see. I think you’ve put your finger on the big problem.
The big problem is that physics is missing some important thing, which is how to understand gravity. The clues from astronomy seemed to be pointing in a really interesting direction. We have this cosmological constant. It’s the goal of a lot of theorists to try to get all of that together. At the moment, there are many ideas. It’s like having a garden that needs some weeding, but we don’t have very good tools, but we’re working on it. We’re gonna improve the supernova tools. I’m gonna give a talk in the astronomy department tomorrow about that. People are going to use other approaches, how galaxies cluster over time and to use that to figure out whether gravity is behaving like Einstein’s gravity does, or whether there’s some other aspect that somehow we’re just blind to. I think a little humility is probably good, but you got get up in the morning to do this stuff. It’s really hard. Self-doubt is not the most important thing. Zwicky taught us that. Yeah.
Audience member: About the experiments of inflation. Do we have something about that?
Bob Kirshner: Yeah, I know, inflation theory, there’s plenty. But one extremely interesting question is, is there a signature in the universe today that inflation really took place? Because it explains a lot of things that it was invented to explain, and it turns out there is. Yes, the answer is yes, and the answer is in the microwave background. The glow from the hot big bang itself should have in it a trace of whether inflation really happened or not. And it’s in this pattern in the polarization of the cosmic microwave background, the so-called B-modes. And there are experiments being built, including here at Berkeley, including funding from the Gordon and Betty Moore Foundation. There’s many efforts around the world to do it. It’s an exceedingly difficult experimental measurement. You may remember four of five years ago, there was an announcement that people had seen it.
Well, they did see polarization. They did see a pattern of polarization, but it’s probably due to scattering by dust. It’s not quite what they thought it was. They hadn’t subtracted the foreground well enough. But everybody had been scalded by that, and they’re not gonna do that again. They’re gonna make some other mistake, but they’re not gonna do that again. And people are working very hard to make that measurement. So I would say it’s possible that, in a couple years when these experiments have their data and it’s analyzed, that we’ll be cheerful that the evidence is really pointing to an era of inflation. This incredibly 10 to a minus 35 seconds compared to know at 12 billion years, 14 billion years. It’s an amazing thing if we can do that. If the signal is not found, this will not discourage people. Because they’ll say, well… Right, because they’ll say, “well, It’s a different kind of inflation. “It’s a different kind of inflation.”