Winter 2016 Bulletin

Exploding Stars and the Accelerating Universe

On October 11, 2015, as part of the Academy’s 2015 Induction weekend program, Alexei V. Filippenko (Professor of Astronomy and the Richard & Rhoda Goldman Distinguished Professor in the Physical Sciences at the University of California, Berkeley) discussed supernovae and the accelerating expansion of the universe. A condensed version of his remarks, not including the many supporting images and graphics he showed, appears below.

Alexei V. Filippenko

Alexei V. Filippenko

Alexei V. Filippenko is Professor of Astronomy and the Richard & Rhoda Goldman Distinguished Professor in the Physical Sciences at the University of California, Berkeley. He was elected a Fellow of the American Academy of Arts and Sciences in 2015.

I am honored to have been inducted into the American Academy of Arts and Sciences and especially to have been chosen as today’s featured speaker. I am also a bit frightened, however, because I have been given such an incredibly diverse audience. This is, after all, the American Academy of Arts and Sciences. So, I hope to have something for everyone in my talk this morning.

I will mostly be discussing supernovae: stars that explode. Only a small minority of stars do this at the end of their lives. They can become millions or even billions of times brighter than the Sun. If the Sun were to do this – don’t worry, it won’t – sunblock of 50 wouldn’t cut it! You would need sunblock, or supernova block, of a few billion to protect yourself.

So, why am I focusing on supernovae today? How did I get here? Well, in my youth I was very interested in nature. The first interest I can recall is magnets. I had this incredible obsession with magnets. You can hold them and feel that some sort of force is either attracting them together or repelling them. But you can’t see it.

I longed to understand this force. When I played with magnets in the sandbox at school, I noticed that they picked up little bits of black stuff. I later found out that they are iron filings. The school I was attending at the time thought I was weird. I wasn’t just memorizing a bunch of facts; I was doing all this other stuff. Also, I only ate peanut butter and jelly sandwiches back then. That disturbed them. They thought my behavior was kind of crazy, and they told my parents I should see a child psychologist! Now, please understand, I’m not against psychology; it’s a good thing for many reasons, but being a kid in the first grade who is curious about nature isn’t one of them!

So I started with this fascination with magnets. Then I played with electronics kits and microscopes and anything else my parents would give me – anything science oriented. After a while, I became really interested in chemistry. From age ten through seventeen, chemistry was my main passion. I built up an amazing home chemistry lab. Any dime I could earn I spent on equipment and chemicals. And I did a lot of quite sophisticated experiments, like making synthetic rubber and Bakelite and extracting mercury from cinnabar. It was lots of fun. But I was also sort of a basement bomber; I was interested in explosives.

At Dos Pueblos High School in Goleta, California, I was president of the science club, and occasionally I would have these explosion demos at lunchtime. Everyone loves flash powders. Anyone who is a chemist is interested, to some degree, in explosives. Seeing flash powders go off is just fun!

I remember one day I was holding one of my noontime demonstrations, and I didn’t know that the ventilation system in my high school was interconnected among all the buildings. Apparently, smoke started pouring into the administration building. They were about to call the fire department when someone said, “Wait, let’s look at the weekly bulletin. Isn’t Alex Filippenko doing something with the science club today?” So, they came to the room, opened the door, and said, “Oh, it’s you. Okay.” Then they left. Nowadays it wouldn’t work that way, would it?

When I was fourteen and a freshman in high school, my parents gave me a small telescope because I had done other types of science but had not yet explored astronomy. Chemistry was still my passion, but I figured it would be good to explore astronomy as well.

I clearly remember going out that first night, setting up the telescope. I pointed it to a bright star, and the star looked even brighter. That was kind of thrilling: my first view through a telescope. And then I looked at a second bright star, and it looked brighter. That is what a telescope does; it makes things look brighter.

The novelty was beginning to wear off, though, so I thought, “Okay, tomorrow or the next day, I will go to the library and look up where the good stuff is.” (The Internet didn’t exist yet!) Then I would go and use my telescope for real. But before packing it in for the night, I thought I would look at one final bright star. I chose one and it looked a bit fuzzy, so I released the telescope to let the vibrations damp out, and I realized I was looking at Saturn.

Well, this knocked my socks off. That night, I discovered Saturn! It didn’t matter that millions of people had seen it before. No one had told me to look at that bright star. I did it on my own, and I discovered Saturn.

That thrill of discovery has never left me. If it can be so amazing to discover something on your own – even when lots of other people know about it – how thrilling must it be to truly discover or understand something that no human on Earth has ever seen or understood. That must be really special. And that is what drives all of us who are scientists – the thrill of discovery.

So astronomy became a growing hobby when I was in high school. Chemistry was still my main passion, though, and I entered my freshman year at UC Santa Barbara in the College of Creative Studies as a chemistry major. Because I already knew quite a bit of chemistry, I was put in charge of helping design experiments for a junior physical chemistry laboratory. Most of the equipment that was purchased was of inferior quality. (That’s why they had an undergraduate trying to fix it.)

I soon got bored with it, though. After hours, when my supervisor had gone home . . . well, I had this incredible storehouse of material. I could play with explosives. I also did legitimate experiments. But I did things on my own. And one time I had a bad accident.

It was not the first. I had had an accident a number of years earlier, in my basement lab. But now I was a freshman in college; I should have known better. That taught me that I don’t have the self-discipline to stay away from this stuff. If I became a chemist, I would have all these dangerous chemicals at my disposal, and one of these days I would blow myself up, kill myself, or lose my eyesight. And I didn’t want to do that.

So I thought, as a matter of self-preservation, if nothing else, I have to move out of this field. I took a class on astronomy from Stan Peale. He taught me that the structure and evolution of the universe as a whole, and its contents including the stars, depend on an understanding of microphysics – atoms and subatomic particles. I realized that by switching to astrophysics I could have it all. I could have the very small – the physical chemistry, the quantum chemistry in which I was most interested – and the very large. So at the end of my freshman year at Santa Barbara, I switched to physics with the intention of becoming an astrophysicist, and I have never looked back.

As a graduate student at Caltech, I did a lot of my work at the Palomar Observatory. I was studying galaxies at the time with Wallace (“Wal”) Sargent. Not normal galaxies, but a type of galaxy known as an active galaxy – in particular, Seyfert galaxies, which have a very bright central region.

What we think is happening in these galaxies is that they have a giant black hole, a region of space where matter is compressed so much that nothing – not even light – can escape. The black hole is sucking in material, and as that material is being devoured it glows. No radiation is escaping from within the black hole. Rather, it is escaping from the vicinity of the black hole.

I was trying to find evidence of these gigantic black holes in nearby normal galaxies by looking for faint activity in the galaxies. This is done by sending the light of the nuclei of the galaxies – the center of the galaxies – through a prism or related object and producing a spectrum, then measuring the brightness of the light as a function of color, or wavelength, and plotting the two.

A normal galaxy that is just forming hot, massive stars has bright emission lines because a lot of gas excited by ultraviolet radiation from these stars emits light. But a Seyfert galaxy has broadened lines, in some cases, because the gas is moving very, very rapidly in the vicinity of a massive black hole, whose gravity is pulling on it.

That is how you can distinguish an active galaxy from a more-or-less normal galaxy, which is what I did as part of my doctoral thesis (and also for some of my postdoctoral work at UC Berkeley). We used the “Big Eye,” the Palomar Observatory’s 200-inch, 5-meter Hale telescope. At the time, it was the biggest and best in the world.

Well, in February 1985 – I had recently become a postdoc at Berkeley, but I was still finishing projects related to my thesis – I had one hour left at the end of a long, five-night observing run. It was still early in this particular survey of galaxies, so we had hundreds of galaxies to observe. In this last hour, however, I had time to observe two galaxies. I said to Wal, “Well, let’s do this one: NGC 4618.” Some other astronomer had classified it as a peculiar galaxy, and there is no particular order in which you need to observe the galaxies, so why not choose this one?

In NGC 4618, we saw an extra star that looked like it didn’t belong. It didn’t seem to be in any of the charts we had. So we decided to take a spectrum of that star, as well as of the galaxy itself. That spectrum knocked my socks off, just like Saturn had done many years earlier.

In plotting the brightness versus wavelength, we saw broad emission lines, which I identified with neutral oxygen, singly ionized calcium, and neutral sodium, among other things. This was a spectrum like none other; no one had ever published such a spectrum before. We quickly realized we were probably looking at an exploding star but of a type never before recognized.

I could have chosen some other galaxy, one without that supernova. But I happened to choose this galaxy. I was handed an opportunity, and I ran with it. It is good to be lucky, but you also have to be prepared to be lucky and to take advantage of the opportunities that come your way.

What we observed that night turned out to be a new type of explosion. We now understand that it was a massive star that exploded in more or less the same way as many other massive stars are known to explode. They develop an iron core at the end of their lives. The core collapses, then rebounds and blows the outer layers outward. You end up getting what is called a neutron star. Then the ejected materials go flying out. But in this case, the star had lost its hydrogen shell, and maybe even its helium shell, leaving the denuded core of a massive star, prior to exploding. Stars can do this in various ways. Winds can blow out the outer atmospheres of very massive stars on their own, or the star can be in a gravitationally bound pair with another star, which steals the hydrogen and helium layers away from it.

What we found was a known type of explosion in unknown clothing. The clothing had basically been stripped away, leaving a naked exploding star.

This discovery got me all jazzed up about supernovae. I thought, “Wow, these things explode!” And this brought me back to my old chemistry days. I’m still a chemist at heart, in some ways!

Besides being thrilling to watch and study, these explosions are critical to our existence. Stars build up heavy elements in their cores. Yet if all stars remained dead, inert things at the ends of their lives, without releasing those elements, then those heavy elements would never become available for the production of other stars, planets, and life.

But we need some way of getting these elements out. And exploding stars are the answer. That is how those elements get out. (The explosions themselves also produce additional heavy elements.) The carbon in your cells, the oxygen you breathe, the calcium in your bones, the iron in your red blood cells – all of those heavy elements were cooked up in the cores of stars through nuclear reactions long ago and then ejected into the cosmos by these incredible explosions.

We can tell that this is happening, that the ejected debris from supernovae is chemically enriched, because we can take spectra of these gases and see that they have a large quantity of heavy elements in them, elements that did not exist in any significant quantity prior to the star’s birth and explosion. And we can see these supernova remnants expanding for thousands of years. Gradually they merge with other clouds of gas, some enriched, some not, and you get gravitationally bound clouds like the Orion Nebula, which then start gravitationally collapsing. In the central regions of such a collapsing cloud, the cloud fragments into little pockets that themselves form stars.

The Orion Nebula is chemically enriched. About 2 percent of its mass comprises elements heavier than hydrogen and helium. All those other elements were produced by generation after generation of stars, because the Big Bang basically produced only hydrogen and helium.

Dusty disks form around some of the new stars. The disks are undoubtedly coalescing to form new planetary systems. Some of those planets will be rocky, Earth-like planets, because now the gases have been sufficiently enriched by previous generations of stars to contain enough of the heavy elements needed to form rocky, Earth-like planets.

On at least one such planet – Earth – life somehow formed. I will leave the mystery of life’s origin to the biologists to solve! By the way, everything in biology is more complex than in astrophysics, because even the simplest cell is far more complex than any inanimate object. My fellow astrophysicists and I have an easy time compared to biologists.

So, somehow life formed and evolved sentient creatures that have the intelligence to understand complex things; that have the curiosity, the inquisitiveness, to ask questions; that have the ability and the dexterity – the opposable thumbs – to build machines with which to answer those questions. “Atoms with consciousness,” Richard Feynman called us.

What I have been talking about is the origin of the elements – the elements in our DNA. This is why supernovae are important to you. Without them, you would not be here.

Now we want to understand this explosive process in more detail, and learn which elements are produced in each explosion. But a galaxy might produce a star like this only every thirty or forty years. If I were a really cruel advisor, I would have each of my students staring through the eyepiece of a telescope at one, and only one, galaxy, until that student found a supernova. Only then would we let that student graduate and move on to greener pastures. Meanwhile, I would have decades’ worth of slave labor from said student. Fortunately for my students, some crimes are so egregious that even a tenured professor cannot get away with them!

A more humane option would be to have the students look at thousands of galaxies. Statistically, if there is one supernova per galaxy per century, that’s the same thing as one supernova per one hundred galaxies per year. Each of those one hundred galaxies will produce a supernova sometime in the next century; we just don’t know when. But if we look at one thousand galaxies, we are likely to find ten supernovae. I could have my students view thousands of galaxies at night, but even that would still be considered cruel and unusual punishment.

Thanks to modern technology we have a better technique. We can attach digital cameras, like the CCD camera in your iPhone, to the eyepiece end of the telescope, take photographs of thousands of galaxies, and then simply look for arrows. Wherever you see an arrow, you see an exploding star. By rigorous mathematical induction, I conclude that this process must work every time!

Well, obviously it cannot be that simple. What we did at Lick Observatory was develop a robotic telescope that looks at nearly ten thousand galaxies each week or two, then repeats the process and automatically compares the new pictures with the old ones.

Once we have something that looks like it might be real, the undergraduate students eliminate the bad candidates by eye. Our software is getting better and better, though. Someday it will be sufficiently good to replace all of us. Until then our students get to look at those candidates and help discover supernovae. They get their hands dirty with research early. Most of them do not go on to become astrophysicists. Instead, they pursue areas that are more immediately useful to society, such as computer science, applied physics, and engineering. But they get research experience on my team.

For a decade, we found more relatively nearby supernovae than all other teams in the world combined. Recently, however, the numbers have been dribbling down because other teams have been finding lots of supernovae; we have altered our strategy to find not many, but few, and early when they are very young. We do this by looking at the same galaxies the same night or every other night, rather than every week or two.

I am very proud of my team. We found the first supernova of the new millennium (regardless of your definition of the new millennium): both “Supernova 2000A” and “Supernova 2001A.” Our very first discovery was “Supernova 1997bs.” The name refers to the order of discovery (they go A, B, C, through Z, then aa, ab, ac, through az, and so on). But, interestingly, although we thought it was a legitimate supernova, our recent studies suggest it may have been an imposter. Sometimes stars burp in such a way that they do not completely destroy themselves. They are not really supernovae, even though they look like one.

Using spectra, astronomers have grouped supernovae into two main types and several subtypes. The one I am most interested in is the so-called Type Ia. Supernovae of this subtype come from stars called white dwarfs. Our own Sun will become a white dwarf in about seven billion years. A white dwarf consists of a type of matter known as degenerate matter – not because it is morally reprehensible; this is just the term quantum physicists give to a very compactified type of matter.

But our Sun won’t explode as a Type Ia supernova. For a white dwarf to explode, it needs to gather mass from a companion star in a binary system. There are various ways it can do this. One mechanism is for the more-or-less normal star, near the end of its life, to give mass to the white dwarf, which eventually explodes. Subrahmanyan Chandrasekhar is the one who basically figured out that white dwarf stars have a maximum possible mass.

Because they all explode at the same mass, the explosions were thought to be virtually identical, and observational studies of Type Ia supernovae showed that they are indeed all quite similar, to a first approximation. This is like one light bulb looking roughly like every other light bulb.

This observation increased the prospects for using Type Ia supernovae as yardsticks. If you can measure the observed brightness of a distant object and compare that with how powerful it really is, then you can figure out the object’s distance. However, it turns out that observationally, they are not all exactly the same, and my group in the early 1990s made some rather major contributions to this field by getting high-quality spectra of Type Ia supernovae with the 3-meter Shane telescope at Lick Observatory.

We found that different Type Ia supernovae actually do look spectroscopically different. They weren’t all the same, just as not all light bulbs are the same: some are bicycle light bulbs, some are the headlights of big trucks, and so on. Spectroscopically different Type Ia supernovae also have differences in peak luminosity or power. But if they aren’t all the same, that means maybe we can’t use them to determine accurate distances, right?

Mark Phillips at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, and later Mario Hamuy (also at CTIO), figured out a way to calibrate the power of these supernovae, as did Adam Riess in his PhD thesis work with Bob Kirshner at Harvard. They found that the more luminous Type Ia supernovae take longer to brighten and longer to fade than the less luminous ones. If you figure out this relationship by observing a bunch of Type Ia supernovae in nearby galaxies whose distances have already been determined, and then you measure the light curve of a distant Type Ia supernova, you then know from where in the distribution of luminosities this particular supernova is. Is it average? Is it overluminous? Underluminous?

By greatly decreasing the dispersion in the relationship and also figuring out more accurately what the supernova’s luminosity is, we can precisely calibrate nearby Type Ia supernovae. And that is what made them incredibly useful as cosmological distance indicators.

In 1929, Edwin Hubble showed that nearby galaxies, though moving away from us, are moving more slowly than distant galaxies. All galaxies are moving away from us, but the more distant ones are moving faster than the nearby ones. You can see this from a spectrum. A nearby galaxy might have a spectrum that indicates a very low speed or even close to being at rest. A distant galaxy has the same, or more or less the same, pattern of lines, but the spectrum is shifted toward longer wavelengths.

We now understand that the redshift is caused by the expansion of space itself. Basically, objects such as observatories and light bulbs and human beings are not expanding because we are held together by electromagnetic forces. The Earth and our Milky Way Galaxy are held together by gravity. But things that aren’t tied down by stronger forces expand. In particular, a light wave not tied down by anything in the universe expands as the universe expands, and this is the fundamental cause of what is called the cosmological redshift: it is not a motion through preexisting space but rather an expansion of space itself.

Nevertheless, from our perspective in the Milky Way, all the other galaxies are moving away from us. The more distant ones, at a given time, are moving away faster than the nearby ones. And we are at the center. Why would we be at the center? Do the other galaxies not like us? Is it something we said? Does the Milky Way smell? Are all these other galaxies lactose intolerant?

Actually, we don’t think we are in any central position. We think we live in a uniformly expanding universe. Imagine an expanding loaf of raisin bread. Yeast is spread uniformly through the dough. After sitting around for an hour, the loaf doubles in size. (Imagine it is an infinite loaf or that it wraps around itself.) From the perspective of any single raisin, the other raisins move away. That one raisin thinks it is at the center, but so do all the other raisins. Of course, none of them is at the unique center. There is no unique center, at least not in dimensions that we can physically probe. There may be a unique center in a mathematically describable dimension, but we can’t see it or physically access it.

So, the universe is expanding. If we extrapolate that expansion back in time, we get the Big Bang: the moment of origin, when the universe was hot and compressed. With big telescopes such as the Hubble Space Telescope, we have been able to measure the current rate of expansion.

Based on principles going all the way back to Isaac Newton, we expect the expansion of the universe to be changing with time. Newton supposedly saw an apple fall from a tree while he was at his parent’s countryside home escaping the plague. He wondered whether whatever caused the apple to fall was related to the orbit of the Moon around Earth. With his law of universal gravitation he tied together terrestrial phenomena with celestial phenomena.

All galaxies have visible matter in them, the gravitational attraction of which should cause them to slow down in their recession away from one another. However, they have even more of what is called dark matter. Dark matter was first proposed by Fritz Zwicky, a hero of mine at Caltech. He was way ahead of his time on a number of issues, and one of his ideas was dark matter. He basically said that clusters of galaxies appear to be gravitationally bound, but the galaxies within them are moving so quickly that they would fly apart from one another unless some extra gravity were holding them in. He presented this idea in the 1930s and was uniformly ignored. So was Vera Rubin in the 1970s, when she reintroduced the idea using spiral-galaxy rotation curves.

Perhaps one reason why Zwicky’s ideas were often rejected was that he was not a very friendly guy. He was frequently arrogant and abrasive. He didn’t think highly of the intellectual capacity of his Caltech colleagues, and they in turn did not look kindly upon this guy who thought they were all a bit dim. (He is on record as having referred to his colleagues as “spherical bastards.” Because, you know, they’re bastards any way you look at them.)

What is dark matter? We don’t really know. We think probably it is weakly interactive massive particles – little particles left over from the Big Bang that interact only through gravity and the weak nuclear force. No one has compellingly detected one yet. Like neutrinos (another weakly interacting particle), they are very difficult to detect. In the case of neutrinos, if I could send a beam of them through a block of lead ten trillion kilometers thick (one light-year thick), about half of them would make it through without having bounced off of anything.

All this visible matter and dark matter should be slowing down the expansion of the universe – just as the mutual gravitational attraction between Earth and the apple slows down the apple. If there is enough matter in the universe – if the density is high enough – the universe should expand and then collapse. What began with a Big Bang should end with a Big Crunch (or you could call it a “gnaB giB” – the opposite of a Big Bang!).

That is one possible fate. The other possible fate is that the density of the universe isn’t sufficiently high to cause a recollapse. As everything moved apart, the expansion would continue to slow down, because you can never cut off the effects of gravity – but it would never reverse its motion. The universe would be eternally expanding.

We would like to know what kind of a universe we live in regardless of any practical application. How can we do that? Imagine an apple thrown straight up, with a speed either below or above Earth’s escape speed. As it moves away from Earth it will gradually slow down because of the effects of gravity. If I were to measure the speed of the apple at many times, I could figure out how much it has been slowing down, and thus I could use the laws of physics to predict the future – whether it will someday come back down or continue going away forever.

In a similar way, if we measure the expansion history of the universe, we can predict what it will do in the future. We know the current rate of expansion. We next have to place ourselves back in time, to measure what the rate of expansion used to be, and then we can compare the two.

How do we effectively go back in time? We look at distant objects. We see the Sun as it was a little over eight minutes ago, because that is how long the light took to travel to us. The stars we can see with the naked eye appear to us as they were some tens or hundreds of years ago. Galaxies that are a billion, four billion, nine billion light-years away, we see as they were one, four, nine billion years ago.

We can also measure how much the universe has stretched during the time the light was in transit. That is the redshift. You get the redshift as a function of distance, or equivalently as a function of look-back time. That gives you the expansion history of the universe.

Back to the apple: if I throw an apple in the air, it comes back down thanks to gravity. But if there were no gravity, the apple would not slow down at all; it would move away from me linearly, neither accelerating nor decelerating. If there is some gravity, then the apple slows down with time. Similarly, how much the universe slows, and whether it turns around, depends on the average density of the universe divided by some critical value, and that ratio is called Ωm (“Omega matter”). For dense universes (Ωm > 1), you have the Big Crunch. For empty universes (Ωm = 0), you have no deceleration at all. For medium-density universes (say, Ωm = 0.3), you have an intermediate amount of deceleration.

That is the theorist’s explanation. Since we want to translate this into actual measurements of the past, we look at the redshift. It turns out that one plus the redshift is simply the size of the universe – or the distance between galaxies – now divided by the distance they were apart at the time the light was emitted. So, for example, redshift one means you are looking at light that was emitted in the universe at a time when the universe was half its present size.

Different expansion histories correspond to different look-back times and thus to different distances. For a given galaxy’s redshift, depending on what the universe used to be doing, that galaxy will be at different distances from us. The smallest distance, smallest look-back time, for the densest universe; bigger distances, bigger look-back times, for less-dense universes.

If you measure galaxies at many different redshifts and you figure out their distances, you can plot what the universe has done. All of the redshift versus distance curves have the same slope now, because all galaxies are moving apart from one another at whatever speed they have right now. But at bigger redshifts, the curves diverge from one another.

Fortunately, we can measure redshifts easily from spectra. We can measure distances from Type Ia supernovae. In a nearby galaxy, you just find a star whose properties you know. Let’s say we found a star in such a galaxy. We will call it Jonathan. Jonathan is like Betelgeuse, the left shoulder of Orion – a big, powerful, mighty star. We know Betelgeuse’s distance and its apparent brightness, and that allows us to determine its true power. We know Jonathan is the same kind of star, so compare the two. That gives Jonathan’s distance, and hence the distance of the galaxy. Choose another star, Donald, and get the same distance; that gives you some confidence in your technique, especially if even more stars yield the same result.

What you are doing is similar to estimating the distance of a car by looking at its headlights. If you know how bright the headlights are when the car is only 2 meters away, then you can make distance estimates based on how bright the headlights appear to be. Most of you can make this calculation intuitively, almost instinctively. You are actually using the inverse-square law of light, and if you are not very good at doing it quickly, you should not be driving at night.

But for galaxies that are billions of light-years away, you might think there are no individually visible stars, so how can you use this technique? Well, it’s true: no normal stars are visible because they’re faint and blurred together. But supernovae can be seen, and distinguished from their neighbors, even billions of light-years away.

So, we try to find faint supernovae in distant galaxies. Those allow us to determine the distance of the galaxy using the inverse-square law. Moreover, the spectrum tells us how much redshift there is: how much the universe has expanded while the supernova light was on its way.

My main job on the High-Redshift Supernova Search Team led by Brian Schmidt and on the Supernova Cosmology Project led by Saul Perlmutter was to get spectra of supernova candidates. Two teams meant there was competition; both wanted to be first, both wanted to be the best. That accelerated progress in the field and improved the quality of the work. Plus, it lent credibility when we came up with a crazy result.

Using CTIO in Chile, both teams would take wide-angle pictures of the sky containing thousands of galaxies. Then they would repeat the procedure three weeks later and subtract the earlier picture from the later picture. You get a bunch of noise, but that’s okay; any measurement process necessarily has some noise associated with it.

Sometimes we got something that looked like it might be real. To make sure it’s a Type Ia supernova, and to measure the redshift, I used the world’s most powerful optical telescopes, the two 10-meter telescopes at Keck Observatory in Hawaii. With those gigantic mirrors, we collected light from these faint supernova candidates, spread the light out into a spectrum, and examined the data. For example, the spectrum of SN 1999ff shows that it is a Type Ia supernova at a redshift of 0.455, or about five billion light-years away.

The Type Ia supernovae we found were really, really faint. Looking at images of some examples, you might say, “Well, sure. They are in these faint, pathetic-looking galaxies that are obviously very distant.” That’s true, but these supernovae are fainter than they had any right to be. Given their redshifts, they could not have been that faint in a decelerating universe, or even in a universe expanding at a constant speed. They didn’t fit any of the theoretically expected curves. They were too far away to be consistent with a dense universe or a medium-density universe or even an empty universe.

So, instead of deceleration caused by attractive matter, it looks like the opposite is going on: acceleration caused by negative matter. What? Wrong sign! You can see why we were afraid of announcing this result!

My postdoc at Berkeley at the time, Adam Riess, was charged with analyzing the data. His analysis showed, “Ωm = −0.36.” The negative sign indicates negative deceleration, which means acceleration. This was the eureka moment, but when Adam showed me the results I didn’t really believe him at first. But then other people on the team did the measurements and analysis, and Adam redid them, and we couldn’t find anything wrong.

What this finding implies is perhaps not negative matter – which seems pretty crazy – but an idea that Albert Einstein came up with in 1917. Einstein said, “You know, the universe appears to be static.” He and others thought the universe was neither expanding nor collapsing; there was no evidence for either possibility back then. So he conjured up something that he called the cosmological constant. He came up with the idea of a repulsive effect that negates gravity and, in his view, had exactly the same magnitude as gravity. The net force is 0, meaning no acceleration.

But he never liked this solution; it relied on something of unknown physical origin for which there was no laboratory evidence. And it made his equations less pretty. All in all, it looked arbitrary, ad hoc. So twelve years later, when Hubble revealed the expansion of the universe, Einstein renounced his own idea as having been the biggest blunder of his career.

Well, what have we done, the better part of a century later? We have reincarnated Einstein’s idea. Not to make a static universe, but rather one that over the biggest distances accelerates with time. One that is consistent with these data. Not with some form of matter that has negative gravitation but with a new type of energy – let’s say Einstein’s cosmological constant – that causes a repulsion.

The news headlines about our work said, “Astronomers see a cosmic antigravity force at work.” We used the term “antigravity” hesitantly, however, because people then want to know if they can attach this stuff – whatever it is – to their cars and levitate over the traffic jams in Boston or LA. And the answer is no. This stuff is either a property of space itself, something that can’t be harnessed; or it is a new type of energy, but one that will essentially never be harnessed because there is so little of it.

But is it really the cosmological constant? Is it Einstein’s idea? Well, it might be. That would be a property of space itself, the vacuum: particles and antiparticles coming into existence and then disappearing a short time later. That, in and of itself, is not strange. In fact, it is the basis for a field of physics called quantum electrodynamics, for which Richard Feynman is famous.

However, physicists had always assumed that the net energy density of the vacuum is zero. If it is not zero, then it would actually have the desired effect of accelerating the universe. But a lot of physicists still think the net effect of this is zero and that the accelerating expansion is caused by something else. The general term for that something else is “dark energy.”

One form of dark energy, in a sense, is the cosmological constant, but it is a qualitatively different form than other dark-energy candidates. It is a vacuum energy, whereas most of the other candidates would be a new type of Higgs field. Not the Higgs field that gives mass to particles, but another type.

There is some sort of a dark energy density of the universe that is repulsive. Its value is greater than zero. Early in the universe, it was actually of negligible importance, so the universal expansion was slowing down. But about four or five billion years ago, its total cumulative effect became comparable to, or exceeded, the effect of visible and dark matter, and so the universe started accelerating, and this is what it is doing now.

To test whether the universe actually went through this period of early deceleration, we used the Hubble Space Telescope to find and study very distant supernovae. We showed that this period of deceleration really did occur, for roughly the first 9 billion years of the universe’s existence.

You might justifiably ask, “If this acceleration result is based just on Type Ia supernovae, could it be wrong? What if we are misinterpreting the data in some way? What if supernovae evolve in some way we haven’t taken into account?” In science we all recognize that we have to verify using independent techniques. And the more important the result, the more important it is to independently verify. Our results have now been confirmed in many different ways, including by studies of the afterglow of the Big Bang using the Wilkinson Microwave Anisotropy Probe.

Those studies found “freckles” in the early universe that correspond to temperature variations, which themselves correspond to density variations. The observed angular sizes of the typical variations, together with their known physical sizes and their distance, show that something must be making the universe spatially flat. According to Einstein’s general theory of relativity, there has to be an extra density of the universe to make it Euclidean in its global properties. This is consistent with the presence of dark energy as revealed by the Type Ia supernovae.

Moreover, if you take these tiny density variations and propagate them through time using computer simulations, you can look at the growth of what is called large-scale structure: the galaxies, the clusters of galaxies, and the giant voids between them. If you don’t include dark energy, the results of computer simulations end up not looking quite like the observed universe. But if you include dark energy when simulating the growth of large-scale structure, then you get a computer prediction that looks a lot like the observed universe.

The census of the universe is now the following: dark energy is 70 percent and dark matter is 25 percent, which means we don’t understand 95 percent of the universe. So, for anyone who says physics and astrophysics are dead, you can ask them, “Well, what about the origin and nature of most of the universe?” The physical origin of repulsive dark energy is, in the opinion of many, probably the most important observationally motivated, unsolved problem in all of physics. Dark energy even provides clues to the much-desired unification of quantum physics and general relativity.

We are now trying to measure more precisely the expansion history of the universe in order to rule out some of the candidates for dark energy. Different types of dark energy will lead to slightly different past histories. So we are trying to set observational constraints on what the dark energy might be.

How will the universe end? If the dark energy continues to be repulsive, then the universe will expand faster and faster with time – a runaway universe. This means that if you want to look at a galaxy with your very own eyes through a telescope, you had better do it pretty soon, at most within the next few tens of billions of years. After that time, the galaxies will have been whisked away beyond any distance from which they can be seen.

On the other hand, we don’t really know that this will happen; it’s possible that the dark energy will someday reverse sign, becoming gravitationally attractive. In that case the universe could still, ultimately, recollapse, ending in fire rather than ice. Though these two cosmological possibilities had not yet been articulated, Robert Frost’s famous 1920 poem, “Fire and Ice,” seems entirely appropriate in retrospect:

Some say the world will end in fire,
Some say in ice.
From what I’ve tasted of desire
I hold with those who favor fire.
But if it had to perish twice,
I think I know enough of hate
To say that for destruction ice
Is also great
And would suffice.

© 2016 by Alexei V. Filippenko