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# ASTR 160: Frontiers and Controversies in Astrophysics

## Lecture 22

## - Supernovae

### Overview

Professor Bailyn offers a review of what is known so far about the expansion of the universe from observing galaxies, supernovae, and other celestial phenomena. The rate of the expansion of the universe is discussed along with the Big Rip theory and the balance of dark energy and dark matter in the universe over time. The point at which the universe shifts from accelerating to decelerating is examined. Worries related to the brightness of high redshift supernovae and the effects of gravitational lensing are explained. The lecture also describes current project designs for detecting supernovae at high or intermediate redshift, such as the Joint Dark Energy Mission (JDEM) and Large Synoptic Survey Telescope (LSST).

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html## Frontiers and Controversies in Astrophysics## ASTR 160 - Lecture 22 - Supernovae## Chapter 1. From Acceleration to Deceleration of Universe Expansion [00:00:00]
So, let’s go all the way back to what we know about the Universe from observing galaxies, supernovas, things like that. We know that the Universe is expanding and that, we know. Hubble already figured that out. We know this from the Hubble Law, and from other–just, in general, from the study of standard candles at relatively low redshift. So, distances out to, I don’t know, Now, the next question then becomes the rate of expansion. Is the rate of expansion changing? Is it accelerating? Is it decelerating? What’s going on there? And so, you want to compare acceleration versus deceleration, which is a fancy word, as you know, for slowing down. And deceleration: this is relatively easy to grasp. This is what happens because of matter. Matter exists. It’s got gravitational force, and gravitational force tends to hold things together. So, if you’ve got something that’s moving apart, and there’s some gravitational force, it’ll tend to hold it together, and thus, slow it down. Most of the matter, it turns out, is this weird dark matter, which we don’t understand, but that’s what’s responsible for the deceleration. If you’re going to make it go faster, you need something much weirder that pushes outward, that has repulsive force. This, we believe, actually exists. This, we label dark energy. And so, the question of, “is it accelerating or is it decelerating,” is basically a question of how much dark energy is there versus how much matter is there, because the more–if you have more of this than this, then it will accelerate and vice versa. And what we have discovered by supernovae observations with redshifts of greater than 0.3, and by now, out to about a redshift of 1, or so, demonstrate that–well, what exactly does this say? Let’s go slowly. In the past, the Universe was expanding more slowly than it is now, and therefore–those three dots are the mathematical symbol for therefore. Therefore, the Universe is accelerating, and therefore, there’s more dark energy than there is matter. And you’ll recall, we did the little pie chart of the Universe, and it turns out, it’s ¾ dark energy and ¼ matter. And those proportions are determined by the rate at which the Universe is accelerating. Not the rate at which it’s expanding, but the rate at which it’s accelerating, because that’s the change in the expansion. Fine, so far–or maybe it isn’t fine. I should pose that as a question. Fine? This is the basic premise of where we had gotten to about a week ago. Yes sir?
What it is, physically? Very hard to understand. The only physical explanation that’s been offered–well, many have been offered. But the only one that connects to anything else we know is the idea that it’s the vacuum energy predicted by quantum mechanics. And it turns out that if that’s true, it ought to be 10 And so, exactly how this works–what the mechanism is, what the nature of this stuff is? Completely unknown. And the only reason we think it’s there is that we see its effect. We see the Universe getting faster. And so, it’s just a name that’s attached to whatever is causing that effect. Okay. Yes?
Here’s now. Here’s 1. We’re at 1 and now. And so, if it were not accelerating, if it were just coasting along, that would be a straight line. And what happens is, we can look back into the past and what we see is this. So, at this point, if you think about it–believe me, for a sec, that this is true. Supposing you were sitting here. The rate of expansion is the slope of this line. That’s how much you’re expanding. And so, at this point, the slope is shallower than it is up here. And so, one interpretation of this line is that in the past, it used to be expanding, you know, like this, and now, it’s expanding like this.
Just to complete the thought here, the way this works out in the observational plane, you’ll recall, looks like this. Here’s the empty Universe in this set of units. And you observe a bunch of supernovae and they seem to be doing that. And that line, if you, then, transform redshift and distance–this is kind of a weird measure of distance–into scale factor and time. Let me make this line solid, so that the two graphs correspond. This line and that line correspond with each other. And so, as you observe many points along here, that’s essentially observing many points along here. Okay, yes?
So, it’s all a balance of dark energy versus dark matter, or matter, in general, but the dark matter predominates. And, at the moment, it is true that the dark energy is–there’s more of it. It exerts more, more of an influence on the Universe; therefore, the Universe is not just expanding, but also accelerating. We know that because these points are above the line, not below the line. But this didn’t always have to be true. This balance changes with time. ## Chapter 2. The Balance between Dark Energy and Dark Matter [00:10:20]So, currently, DE blows away the DM, but this can change. This changes. In fact, this is almost certain to change with time. And the reason is that the energy density of the dark energy and the matter density of the dark matter behave differently as you change the size of the Universe. It’s all about the density here, so, matter density in the past. You have the same amount of matter, but the Universe was smaller. Density is equal to mass over volume. So, if you have the same So, in the past, there was–the matter density was much greater than it is right now, and therefore, the gravitational force trying to hold the Universe together was greater than it is right now. But the Cosmological Constant is constant, and what that means is that its density is constant. And what that means is that, in the past, the dark energy density was the same as it is now. That’s just the way this particular piece of Einstein’s equations works. It’s a constant. Now, if you’re about to ask the question, well, how does anything behave that way, I’m going to give you the same answer. You know, they have this wonderful thing in the British Parliament where they ask the Prime Minister questions–obnoxious questions. And when they start getting too many obnoxious questions in a row, what he’ll do is, he’ll look at the camera and say, “I refer the Honorable Gentleman to the answer I gave some moments ago.” So, if you’re going to ask what this is and why it behaves that way, I will refer you to the answer I gave to the Honorable Gentleman some moments ago; namely, I don’t have a clue. Something they don’t generally say in political situations. All right. So, it’s the same as now. But look at the implication of this. If the matter density gets bigger and bigger and bigger as you go into the past, and the dark energy density does not, then at some point in the past, it must have been true that the matter density overwhelms the dark energy. And then, at some point in the past, the dark energy and the dark matter exert comparable effects on the Universe. And before that, dark matter wins–that is to say, the Universe was decelerating. Now turn this around and go in chronological order. From the start of the Universe until some moment, the Universe decelerated. It was expanding, but it kept getting slower. Then, after some moment–and we know that this moment was in our past, because after this moment, it starts to accelerate. And we know that it’s accelerating now. And so, at some point in the past, there was a magic moment where everything balanced, and then, it started to accelerate. So, now, what does this look like on our graphs? What does this look like on the graphs, here? Well, let’s do the And so, if you make those two assumptions, this is the curve you get. And so, this is what you expect for Ω And so, you can predict, given these two quantities, and a measurement of
## Chapter 3. Complications from Supernovae Brightness and Gravitational Lensing Effects [00:18:59]Okay, let’s see. Right. Okay. So, yes. Let’s go there. Let’s go for the observational evidence. What would you expect it to be? Here we go. This one, again. We keep going back and forth between these two plots. That’s what it’s all about. So, here’s empty Universe, as always. And then, in this plot, acceleration means a positive slope. So, if you’re moving up in this plot, the Universe has been accelerating–is accelerating at that particular redshift, at that particular time in the past. So, you know, the data we’ve looked at so far kind of look like points along a line like this. That’s the demonstration that the Universe is now accelerating. So, here’s the prediction. And, you know, here’s a magic moment where, at that point, this slope is flat, and so, on this side, it’s decelerating, and on this side, it’s accelerating. And the question is, is it getting further away from–is it moving up compared to the empty Universe, or is it moving down compared to the empty Universe? Because the empty Universe neither accelerates nor decelerates. That’s the definition of an empty Universe. It doesn’t have any mass. It doesn’t have any energy. The expansion rate is constant forever. So, if you’re going this way with respect to that line, or going this way with respect to that line, you’re accelerating, and if you’re coming back, going in the other direction, you’re decelerating. So, here’s a closed Universe, right? This is a Big Crunch Universe, which is going to re-collapse, because it’s decelerating all the time. Now, interestingly, this turnaround point, where you turn around between accelerating and decelerating, is not so far back into the past. If you believe this set of parameters, the standard model, it’s at around 0.8. We can see stuff at 0.8. And you could hope to go and see things even further out. Turns out, it’s kind of–now, right at–if you only go out to about 0.8, which is about as far out as they did for the first time round, you’re not going to see the turnaround, because, you know, it’s just more or less flat. You’ve got a few points out there, and you’re going to have a tough time telling the difference between that and that at this point here. But if you go out just a little bit further, you might really be able to see this turnaround happening. Unfortunately, that turns out to be hard, observationally. It’s hard to see supernovae with redshifts greater than 1, for a number of different reasons. First of all, they’re faint. The further away they are, the fainter they are. Second of all, they appear on top of galaxies. Obviously, they live in galaxies. The further away you look, the galaxies look smaller, and so, the supernova appear to be–the further away you look, the more light from the galaxy the supernova is superimposed on. And it becomes harder to separate the light from the supernova and the light from the galaxy that it lives in. And the third part is that the light is redshifted. Very redshifted. Redshifted by a factor of two. So, supernovae give off most of their light in optical light, which we can observe. But by the time they’re at a redshift of 1, all the wavelengths are doubled. And so, most of the radiation is in the infrared, which is much harder to see from the ground, because everything glows in the infrared. And, as I said before, it’s like looking in the daytime. The whole sky is glowing. Your telescope is glowing. It’s just really hard to make those kinds of observations. It is possible, however, to do–it’s much easier to do this from space. Now, faint is faint. It doesn’t matter whether you’re in orbit or not. But the other two things are greatly aided by looking from space. Point two. You get much better images from space. And you may recall, last time, I showed you a picture of a supernova, and from the ground, you know, it was a little extra light on top of some galaxy. And then, when you looked from space, the galaxy was clearly delineated. And then, the supernova was a tiny point in the outskirts of that galaxy. So, it makes it much, much easier to separate the galaxy from the supernova. And it is also true that the infrared background light is much fainter, because you can keep stuff cold out there. The infrared background light is much fainter and there’s no atmospheric absorption. The atmosphere actually absorbs a large fraction of the infrared light that hits us. That’s actually why the greenhouse effect works, because light comes in as optical light and it makes it all the way through. Then, it heats up the surface of the Earth. The Earth radiates heat, which doesn’t make it through the atmosphere and it gets trapped. So, there’s no atmospheric absorption out there, and so, you have a much easier time doing this from space. So, what they have been doing is, recently, people, and in particular, a guy named Adam Riess, have been using the space telescope, the Hubble Space Telescope, to find high redshift supernovae. There is a flaw in the way the space telescope is designed from the point of view of trying to do this experiment. And the flaw is that it doesn’t look at very much of the sky at once. It only looks at a small field of view. So, you don’t find many, because you’re just not looking at that big a part of the sky. And so, you only look at a tiny piece of the sky at once. However, they have spent–because this is of some clear importance, they have spent many, many, many hundreds of observing hours with the space telescope trying to track down a few supernovae at a redshift greater than 1. And they have now succeeded in doing so. So, here’s what the data look like. This is from Adam’s recent paper, 2004. I guess it’s no longer quite so recent. And this is the plot. Let me see if I can focus this. It’s actually the Xerox quality. We have a joke in astronomy that many important objects turn out to be LSXTS, which stands for Low Significant Xerox Transient Source. That means you’ve made a Xerox and a little blip appears on your graph and somebody says, what’s that point in the upper right-hand corner? And you say, oh, that’s the Xerox machine. All right. So, I apologize for that. But look what’s happening. This is redshift. This is the plot we’ve been looking at before, 0, .5, 1.0, 1.5, and 2. And this is the Delta ( Here are–these are all supernovae–all the supernovae they know. These kinds of fuzzy gray points are the ones they found with the space telescope. The ones in here are the ground-based ones known at the time. And what they’ve done is they’ve averaged them together in groups of redshift, grouping by redshift. That’s the bottom thing. So, each one of these points is an average of many supernovae. This point is an average of these two. And then, as you get down to lower redshifts where they have lots and lots of them, the precision gets much better. But you can definitely see what’s happening. Here, out to about, you know, .5, .8, it’s going up. And then, it does definitely seem like it’s going down. And so, there really is, now, some evidence for this turnaround. Now, it kind of all depends on these two points. I have a rule of thumb. My rule of thumb is you can put your thumb over any one point, because you never know what screw up might have happened in any one measurement. So, I can’t really do that on the top. But if I do this, and I make that point go away, you could still kind of draw a line that keeps going up. It would miss this point by a little, but not by a lot. It’s really this that does it. It’s these last two individual supernovae down there, which really make the case that the thing is turning over and coming back down. Now, you have to worry a little bit–and there’s actually a good reason to worry about the brightness of high redshift supernovae. And let me give you one particular reason to worry, namely, gravitational lensing. Remember gravitational lensing? You stick some mass in the middle, between you and the object you’re looking at. It focuses the light. Gravitational lensing makes things look brighter. The further away you look at something, the more likely it is that there’s something in between that will do the lensing. So, first of all, point one: more distant objects, more likely to be lensed. Point two: if you’re out looking for very distant, very faint objects, which are the ones you’re going to see first?
So, let me now go back to this plot. Here’s the plot. So, if they’re lensed, then they’re brighter–then they seem to be brighter than they actually are. They’re actually fainter than they look. That would have the effect that this one actually ought to be up here, and this one actually ought to be up here, if they were lensed by some amount. So, if these things are lenses, then the true, correct position of these points would be higher up in the graph than it actually is. So, this is the effect of lensing. Now, when it was only one of these points, people were very concerned about this. Now that there’s two, people are a lot less concerned, because, you know, it would be pretty bad luck to have both of the supernovae that you know about at high redshift turn out to be lensed. If they had twenty, this whole problem would go away, because there would be no possible way that you could be so unlucky–well, you could calculate the probability that you would be so unlucky as to have the first twenty high redshift supernovae you know happen to line up right behind some massive object, and it would be some incredibly small probability. So, each time you observe another one of these things out there and it’s low, you get around this lensing problem. I should say this is only one of a number of problems. These things are very hard to observe. They’re very faint objects. But, nevertheless, I think, at the moment, the evidence for this turnaround is highly suggestive, but not yet wholly conclusive. All right. But now, suppose you actually want to get higher accuracy than just seeing the thing turnaround. Because, suppose dark energy isn’t constant. Suppose the Big Rip really is going to happen. Then, DE density increases with time. This leads to the Big Rip, as we discussed last time. And therefore, the dark energy density was less in the past than in now. And therefore, the deceleration, the moment of balance between deceleration and acceleration was more recent. Because, as you go back in the past under this new scenario where the density of dark energy is increasing with time, it’s therefore decreasing as you go into the past. So, as you go into the past, two things happen ‒ the density of matter gets bigger, and the density of dark energy gets less. So, there’s an additional effect that will make that crossover happen earlier. And so, to summarize on these, by now, almost boring and ubiquitous plots, here. All right. So, here’s the, kind of, standard cosmological model, and if it’s going to be a–oh, and then, this predicts in the future that it’s going to go something like, if it’s a Big Rip, then what happens is, it’s doing this, and it’s going to do that. Because the dark matter takes over from the dark energy more recently in the past. Because you’ve also lost oomph in your dark energy, but the dark energy’s getting bigger and bigger with time. This, then, translates onto this plot. And here is what we expect, and here’s the kind of prediction into the future. Future of observations. Further into the past in time. And what you would expect is that this would sort of keel over earlier. So, this is a Big Rip cosmology. Big Rip cosmology has the same–Big Rip would have the same expansion rate now, the same acceleration now, but more acceleration in the future than you’d expect, and more deceleration in the past. And so, it’s important, not just to see this turnaround, but to actually plot, in detail, where that line goes. Because, if it turns out that your supernovae are kind of lined up like this, or they’re kind of lined up like this–you already know that there are a bunch of them like this–or that they’re kind of lined up like this, makes a huge difference in how you understand the dark energy, and tells you something new about the dark energy. In fact, it tells you the first thing you know about the dark energy other than that it exists–namely, that it would be getting bigger or less big, or maybe staying constant as a function of time. Yes?
## Chapter 4. The Joint Dark Energy Mission (JDEM) and Large Synoptic Survey Telescope (LSST) [00:37:33]Okay. So, how are we going to figure this out? The space telescope, kind of, finds these things one at a time. It’s going to be awfully tough to make this distinction. But let me introduce you to two projects currently underway, the goal of which is to figure this out. They do the exact two things that we’ve just been talking about. One is designed to go deep-to-high redshift, and the other is designed to do many, many, many, at intermediate redshift. So, there is something called the–so, there’s a space mission, space telescope. This is called JDEM, stands for Joint Dark Energy Mission. And the joint means joint between NASA and the Department of Energy. Whoever named this stuff dark energy gets a little prize, because now we get money from the Department of Energy to study it. Let’s see. And the idea–of which an example is something called Snap 1 Proposal. That’s the supernova acceleration probe.
And what this is going to be is, it’s a space telescope that differs from the current space telescope in two major ways. One is, it has a wide field of view, dozens of times bigger than the current space telescope. So, it can do twenty–let me see if I can remember. It’s like twenty–it’s over 100 times bigger field of view than the current space telescope. So, the discovery rate of supernovae will be much, much greater. And it’s optimized for the infrared. And so, its goal is to find many supernovae with redshift greater than 1, perhaps out to a redshift of 2. That’s its purpose in life. And proposals are currently being evaluated for this. Launch time is supposed to be‒well, the optimists say 2013, but that means congressional funding this year, which is actually unlikely. More likely 2016 or 2017, and then, we’ll know. Yes?
And there’s one other project, at least, which I’ll describe next week, that these guys are interested in doing, but there’s a real tradeoff between making it a general purpose telescope like the Hubble, or doing this one thing especially well. And that’s, you know, the kind of thing that’s currently under discussion. So, SNAP is one example of where people might go in the future. Another is a ground-based project called the Large Synoptic Survey Telescope. This is otherwise known as LSST. LSST. The plan is this. And this has a lot of other science besides this. It’s going to do a full survey of the sky every three days. So, it’s going to–every three nights, it marches across the entire sky, takes a really deep image of the sky. And so, the consequence of this for cosmology is that you find every low and intermediate redshift supernova. That’s tens to hundreds of thousands–tens of–maybe 10,000 a year, approximately. So, you find lots of these things. So, first of all, you build up incredible statistics out to a redshift of about ½. So, huge statistics to So, this is important, because you know if you’re fooling yourself, because the distant supernovae are different, somehow, from the local supernovae. They’re fainter for some reason. But in the local sample, only one in 100 is of the faint kind. If you’ve got 10,000, that means you’ve got, you know, a sample of 100 of the weird subclass that’s causing you trouble. So, this will be hugely helpful, not only in beating down the statistics so that you can tell the difference between lines that are really quite close to each other, theoretical predictions that are close to each other, but it also will test the systematic problems that you might have different kinds of supernovae and generally enhance your overall confidence that you know what you’re talking about. Also, these kinds of sky surveys are of benefit for many, many other fields of astronomy. The issue with this is the following statistic. Thirty terabytes of data per night, right? That’s 30,000 gig per night. Thirty million megabytes every night. And so, this is not something that us normal astronomers can handle, and so, the recent news from the LSST project is, Google has joined the project. And so, we’re bringing in the–yeah, right. We’re bringing in the big boys for this. And I think this is kind of Google Universe, right? Because it’s a–you know, you take all–you do this for a few years, and you’ve done 100 sky surveys. You can then add them all up and get incredibly deep data. And so, I guess Google figured, you know, if somebody’s going to be piling up a catalog of the entire known Universe, they’d better be a part of it. And it’s fortunate for us, because they’re probably the only people in the world who can create a database of data coming in at this rate. And, I should say, keep in mind that you have to actually, not just acquire this data, you have to actually look at it each night. Because you’ve got to actually discover these supernovae in real time, because they’re going to be gone three weeks from now, or three months now. And so, you not only have to pile up 30 terabytes of data every night, you’ve got to actually look at it and find all the interesting objects. So -
And they have a whole design planned for this. It’s all ready to go, but it’s going to cost a fair amount of money to build. And what’s even more important: it’s going to cost an enormous amount of money to run, just on a night-to-night basis. So, actually, the construction costs of the telescope are not the dominant cost. The dominant costs are building the software and running the program. And it turns out, that’s harder to get some rich guy to give you a $100,000,000 for. And it’s ground-based, so it doesn’t fall into the NASA category. And so, it isn’t clear where the money is going to come from. They actually have a fair amount of private money already piled up. And, as I say, they have detailed, elaborate plans for this thing. You can look at their website. In fact, you will have to look at their website, because that’s going to be part of the problem set. And so, these are two of the projects, which are currently being designed and hopefully soon will be built, that are going to try and push this a little further and try and find out more about the dark energy than simply the fact that it exists. Okay, let me put back up this piece of paper with details. Remember, no sections on Monday. No sections on Monday. [end of transcript] Back to Top |
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