Date: October 12, 2011
Title: The Discovery of the Accelerating Universe
Podcaster: Rob Knop
Organization: Quest University Canada
Links: My home page : http://www.questu.ca/academics/faculty/rob_knop.php
Description: The 2011 Nobel Prize in Physics has been awarded for the discovery of the acceleration of the Universe’s expansion. This podcast, by one of the members of one of the teams whose leaders were granted to Nobel, gives an insider’s view on the discovery, and describes what sort of astronomical observations were made in order to conclude that in fact the Universe is accelerating.
Bio: Rob Knop obtained a PhD in Physics from Caltech in 1997. He then worked with the Supernova Cosmology Project and was part of the discovery that the expansion of the Universe is accelerating. After six years as an assistant professor at Vanderbilt University, he worked in the computer industry for two years. He now teaches physics the new college Quest Unviersity in British Columbia. He gives regular astronomy talks in Second Life in association with the Meta-Institute of Computational Astronomy.
Sponsor: This episode of “365 Days of Astronomy” is sponsored by — NO ONE. We still need sponsors for many days in 2011, so please consider sponsoring a day or two. Just click on the “Donate” button on the lower left side of this webpage, or contact us at signup@365daysofastronomy.org.
Transcript:
Hello, and thank you for listening to 365 Days of Astronomy! This is Dr. Rob Knop, professor of physical science at Quest University Canada.
On October 4th of this year, the 2011 Nobel Prize in Physics was announced. The recipients were three astrophysicists. Saul Perlmutter received half of the prize, and the other half was shared between Brian Schmidt and Adam Riess. The prize was awarded for the 1998 discovery that the expansion of our Universe is accelerating. This was quite an exciting announcement for me, because back in 1996 I started working with Saul as part of the Supernova Cosmology Project at Lawrence Berkeley Laboratory. I was a postdoctoral fellow, and was one of the core members of the team when we did the final work leading up to the discovery of the accelerating Universe.
Although there were hints coming out from both Saul Perlmutter’s team and Brian Schmidt’s team in late 1997, the world first really learned about the two team’s results at the American Astronomical Society meeting in Washington D.C., in January 1998. At this meeting, both teams had posters and talks that presented consistent results. We had measured the expansion history of the Universe, and found that our results could not be explained by a matter-dominated decelerating Universe. That there were two teams with consistent results was key. Very recently, a team has announced preliminary data that would seem to suggest neutrinos moving faster than the speed of light. However, most physicists, myself included, are skeptical of this result. And, indeed, when the accelerating Universe was announced, a lot of astronomers were skeptical. However, the fact that two different teams, working separately, had come up with consistent results meant that the community had to take it at least somewhat seriously. In the 13 years since then, the results have held up to further investigation. What’s more, independent ways of looking at the Universe have given consistent results, which is why today most astronomers and physicists, at least if they’re in a field where it matters, accept the evidence that, indeed, the Universe is accelerating.
January 1998 was a heady time. Perlmutter’s team made two scientific presentations with the cosmological results (in addition to a press meeting, which I and fellow attending post-doc Peter Nugent missed as a result of a miscommunication with Saul about what time it was). Saul had a poster, and I gave one of the signature AAS ten-minute talks. There was a lot of attention, both from other astronomers, and from the scientific press. Michael Turner, a well known cosmology theorist who would later coin the term “dark energy” to describe the mysterious substance that is causing the acceleration, was present near Saul’s poster for much of the day, speaking with scientists and reporters about the theoretical implications of the results that Saul and his team were reporting. The next day, a number of newspaper reports came out describing these results, and Peter Nugent’s parents, impressed, told Peter that he and I should go out to a fancy DC restaurant to have dinner on them in order to celebrate the results and all of the attention we were receiving.
What exactly were the measurements that led to the Nobel Prize-winning discovery of the Universe’s acceleration? What we need to measure is the expansion history of the Universe. We’ve known since early in the twentieth century that the Universe is expanding. What Perlmutter and Schmidt’s teams did at the close of the twentieth century was look far enough back in time, measuring the expansion rate of the Universe over time, to see how the expansion rate had been changing in the previous six or seven billion years.
How do you look back in time? It turns out that astronomers have a very convenient time machine. The speed of light may be a frustrating limit for futurists and science fiction writers as it means that even supposing engineering way beyond our current capabilities, it would take a voyage of several years to visit the nearest star. However, because the speed of light is finite, the farther away something is, the longer it takes for light to reach you. Therefore, the farther away something is, the further back in time you are looking. When we look at the Andromeda Galaxy, the light takes two million years to reach us. Therefore, we are seeing it as it was two million years ago, not as it is right now.
As best we can tell, our Universe is pretty much the same everywhere. That’s not to say that it’s the same where you are sitting and at the center of the Sun. However, on largest scales– scales of hundreds of millions of light-years– one place in the Universe is pretty much the same as any other. Everywhere you find collections of galaxies in clusters and groups, and you find pretty much the same sorts of galaxies.
Put the uniformity of the Universe together with the fact that the speed of light is finite. When you look at very distant galaxies, you’re seeing them as they were a long time ago. However, because galaxies everywhere are pretty much the same, you’re seeing galaxies which are very much like the ones near us were like a long time ago. Indeed, as you look farther and farther away, you’re looking further and further back into the History of the Universe. What’s more, because we know the speed of light, if you can measure just how far away you’re looking, you can figure out how far back in time you’re looking. We know, for example, that M101, the site of a recent supernova, is about 20 million light-years away. Thus, we know that the supernova exploded 20 million years ago. Which isn’t exactly what I meant by “recent”, but we discovered it recently.
I mention supernova for a reason. Supernovae are extremely bright. They’re exploding stars, and for a period of a couple of weeks, they can shine as brightly as an entirely galaxy. That means that we can see them nearly as far away as we can see galaxies. What’s more, a certain class of supernova, known as a thermonuclear supernova, always reaches the same absolute maximum intrinsic luminosity every time it explodes; such an object is what astronomers call a “standard candle”. The dimmer it looks to us, the farther away it is. Therefore, by just measuring how bright it is, we can figure out how far away it is. And, therefore, we can figure out how far back in time we’re looking.
So, great! We have a way to look back in time, and to know how far back in time we’re looking, by observing distant objects, and by observing objects whose brightness allows us to calculate the distance and thus the lookback time. The other piece of the puzzle is figuring out how much the Universe expanded in between when a distant supernova exploded and now. If we can put those two things together, we can put together an expansion history of the Universe.
In order to measure the expansion, we measure something known as “cosmological redshift.” You may have heard of redshift before. Astronomers talk about it all the time, as we measure it for all sorts of reasons. You may also have heard of the Doppler shift, which is one way you can get redshift. If an object is flying away from you, you will observe any light emitted from it shifted to longer wavelengths– that is, shifter to redder light. Cosmological redshift, it turns out, isn’t quite the same thing, although for galaxies that aren’t too far away it does behave as if it were a Doppler shift. There’s a better way to think about Cosmological redshift. What happens is that the wavelengths of light, as the light travels through the Universe, expand at exactly the same rate as the Universe itself is expanding. Indeed, anything that isn’t held together by other means (such as you, held together by your sinews, or the Earth, held together by its self-gravity) will expand at the same rate as the Universe.
We can easily measure the redshift of a supernova, or of a galaxy in which a supernova exploded. All we have to do is find something whose wavelength we know– and both supernovae and galaxies have characteristic features at well-known wavelengths as the result of different sorts of atoms in the expanding gas of the supernova, and in the gas and stars of the galaxies. We then measure the wavelength of these features as they reach us, and compare the wavelength to the known wavelength that the light was emitted at. However much longer the wavelength is, we know that during the time the light was traveling from the distant galaxy to us, the Universe expanded by exactly the same amount.
Let’s review where we are. Supernova are exploding stars that briefly are very bright. If we can observe one that’s very far away, we may be able to look at the Universe as it was a long time ago, several billion years ago. From how bright it looks, we can figure out how far away it is, and therefore how far back in time we’re looking. From its redshift, we can figure out how much the Universe expanded during that time. If we find a bunch of supernovae at a bunch of different distances, we will be able to build up an expansion history of the Universe. This is exactly what the Nobel Prize winners and their teams did in the late 1990’s. Perlmutter’s team reported on results of 42 high-redshift supernovae, and in a paper by Riess the other team reported on results of 16 high-redshift supernovae. Both results were consistent: to very high statistical significance, the expansion history we measured with the data were inconsistent with a Universe that was coasting or slowing down. Instead, the expansion of the Universe had to be speeding up.
Why is this result so surprising? If the Universe is expanding, after all, why not speed up? If you think about gravity, you normally think about it as being attractive. We’re held down on the surface of the Earth by the Earth’s gravity. The Earth is held in orbit around the Sun by the gravity between the Earth and the Sun. The gravity of all the matter in our Galaxy holds it together. And so forth.
Now, imagine an expanding Universe. Everything is getting farther away from everything else. What you would expect gravity to do is try to pull everything back towards each other. You would expect gravity to try to “put the brakes” on the expansion, and lead to an expansion that is slowing down. Indeed, depending on how much mass there was in the Universe, the gravity might be enough to cause the Universe’s expansion to eventually stop and turn around, leading to a recollapse. However, this is not what we observed. We observed that the expansion of the Universe is speeding up. That means that there is something out there that is having a gravitational effect on the Universe that is backwards from what we would normally expect.
“Dark energy” is the name we give to whatever it is that is causing the Universe to accelerate. At some level, dark energy is just the name for the thing that we don’t know what it is. Dark energy is NOT the same as dark matter; dark matter has normal gravity, and indeed it’s the gravity of dark matter that is mostly responsible for holding together galaxies and clusters of galaxies. And, while we don’t know just what dark matter is, it’s much less mysterious than dark energy. We have observations that point directly to dark matter, whereas with dark energy we really don’t know what’s going on. Indeed, it is still possible that dark energy isn’t “stuff” at all, but the name we’re giving to our current misunderstanding about the nature of gravity on the largest scales.
What was it like to be part of this huge discovery? There was no eureka moment, there was no grand “aha”. A British science TV show came to film our team at work in 1999, to report on the discovery of the accelerating Universe. The woman who was leading it asked me, what did it feel like the moment you knew that the Universe was accelerating? The truth is, I couldn’t answer that question, for there was no such moment.
In 1997, I had multiple roles in the team, but there were two primary ones. I was one of the main people who maintained and updated the software that we used for our data reduction pipeline during supernova searches. That was always a frantic time, for we were getting in data on these supernovae that would fade within weeks. We had to figure out from our supernova search where any new supernovae were, so that we could get other telescopes, including the Hubble Space Telescope, the 10m Keck telescope, and a range of others, targeted for follow-up observations. My second role was in taking all of the images we had from this wide range of telescopes, and processing them to generate the lightcurves for the supernova. A lightcurve is a plot of brightness versus time. The maximum brightness of the supernova was what we used to measure the distance to the supernova, so of course this was of key importance. I was processing the data over and over again. As more data came in, I was able to refine each lightcurve. However, I was also refining the techniques we were using to extract the brightnesses from the images. A lot of what I was doing were cross-checks, making sure that we really were doing everything right.
At group meetings, starting a few months before the end of 1997, Gerson Goldhaber, a senior scientist who had previously been involved with the discovery of the first charmed meson, would report on his back-of-the-envelope calculations about what our data were telling us. My lightcurves would be sent on to Don Groom, who would fit a model to the lightcurve in order to get the best possible measurement of that maximum brightness, and Don would pass that maximum brightness on to Gerson. Gerson started telling us that our data were seeming to indicate that the Universe had to be accelerating. My response? “Yeah, that will go away when I finish all of my cross-checks and really make sure that we’ve got the lightcurves right.” But, we already had them right. The cross-checks didn’t change anything significant. The result was that I had a few months to go from not thinking that the accelerating Universe result was anything worth getting excited about yet to accepting it as the truth indicated by our data. By the time I was one of our team presenting about it at the American Astronomical Society meeting in 1998, I was already used to the result, and as such I was a little bit surprised by just how much excitement there was from people hearing about it for the first time.
Thank you for listening! This podcast has mostly been about the observations that we made that led to the discovery of the accelerating Universe. I personally feel privileged to have been a core member of Saul’s team back at the turn of the century when this discovery was made, and it’s exciting that this work has been honored with a Nobel Prize. In a future podcast, I’ll talk more about the Big Bang, and just what it really means to have a Universe that is expanding and a Universe that is accelerating. I’ll also talk about what little we do know about dark energy.
End of podcast:
365 Days of Astronomy
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