Galactic Interactions

I managed to get through my 15-20 minute "talk," and just as I threw it open for questions Second Life had a database problem and everbody in-world had to be logged out.... We got back in 40 minutes or so later, and I answered questions for a while for people who came back. However, if you were at the talk and wanted to ask questions but didn't come back, I'll be doing a follow-up Q&A session tomorrow (Wednesday August 1) at 10AM PDT at the same location.

Below, I've got a transcript of the talk I gave. Other than fixing some typos and merging things into paragraphs, I haven't edited what I said/typed.

What I want to do is spend about 15 minutes going over these slides I have hanging around the room. I've given 70 minute talks on this stuff before and still had more to talk about, so obviously this isn't everything :). What I'll try to do is explain how we actually *measure* the expansion history of the Universe, which is the evidence that led us to believe that the Universe is accelerating.

First, though, a few brief words about myself, and then a few brief words on SL training. I got my PhD from Caltech in 1997, although I had basically finished all the work in Fall 1996. I worked on infrared spectroscopy of active galactic nuclei. It was when I went on to my post-doc that I started working on the Universe as a whole. I worked with Saul Perlmutter and the Supernova Cosmology Project at Lawrence Berkeley Lab. It was an exciting time to be there, for it was in 1998 that we and our competitors both announced data that showed that the Universe was accelerating. This has been one of the truly exciting discoveries in Cosmology in recent decades. Not *quite* as exciting as the data from the 1960's that confirmed the Big Bang, but it's getting up there. This has also excited a lot of the particle physics community, because for the Universe to accelerate, it must be filled with something-- that we call Dark Energy-- that is not in the standard model of particle physics.

So what I'll do now is talk about how one actually goes about measuring the expansion of the Universe, and figuring out that the expansion is accelerating. First, though, I want to make sure that people know how to zoom in on the slides I have. I just highlighted one side -- the border on the side is a lighter color. If you're sitting, you can use the rotate left and rotate right buttons (A and D) to look around and find it. Move your mouse over the highlighted slide, hold ALT, click and hold the left mouse button, and you can zoom in on the slide. (By moving the mouse.) The writing is too small to read from where you're sitting, but if you do that you should be able to read it. Is anybody having trouble reading the slide with the highlighted border?

Measuring distances in astronomy is really hard. There aren't tape measures long enough to measure the distances between stars.... As such, there are a whole slew of methods for measuring distances, and sometimes they come with huge uncertainties. One of the most reliable is the "method of standard candles," which this slide outlines. Conceptually, it's very simple. Basically, you find something whose luminosity -- the intrinsic amount of light it puts out -- is known. Then, from how bright it is, you can figure out how far away it is.

On this slide, I've go two candles -- although a 100W lightbulb might be a better example, because two 100W lightbulbs from the same manufacturer will put out the same amount of light. The dimmer one is farther away-- and we can quantify that. The standard candle we used for the accelerating universe work was a Supernova.

Everybody press ESC to reset your view ;) Hanging in the middle of the room is a "Nova/Supernova Progenitor."

There's a big red star that's bulged out on one side. Orbiting around the star is a white dwarf. The big red star is perhaps 100 to 1000 times the radius of the Sun. The white dwarf is no bigger than the Earth. The gravity of the white dwarf pulls some of the outer layers off of the red star, which goes into an "accretion disk" swirling around the white dwarf.

As material builds up on the white dwarf, it reaches a critical mass where it suddenly explodes and completely disrupts itself in a massive thermonuclear explosion. Each time one of these puppies explode, they put out pretty close to the same amount of energy. They are also, for a few weeks, as bright as a whole galaxy. Thus, we can see them very far away. These are the standard candles we've used.

Move on to the next slide i've highlighted.

Astronomers have a time machine-- and, indeed, our time machine is way better than the one geologists and paleontologists have :). Because light moves at a well-known finite speed, if you see something far away, you are seeing it as it was in the past. However long the light took to reach you, that's how far in the past you're seeing it.

When you go outside and look at the sun -- not a good idea, by the way, if you don't want to go blind -- you aren't seeing it as it is right now, you're seeing it as it was 8 minutes ago. The discovery of the accelerating Universe was based on supernovae that exploded as much as 8 or 9 billion years ago. Because we can measure the distance using the method of standard candles, we can also measure exactly how far back in time we're looking.

That's the first piece of the puzzle. The second thing we want to measure is just how much the Universe has expanded since them time of the explosion.

Move on to the next (now highlighted) slide.

Again, hold ALT, move the mouse over the slide, and hold the left mouse button to zoom in. Many of you have probably heard of redshift -- probably because of the Doppler shift. Something that is moving away from us will show a redshift -- light (or sound) emitted by it will be shifted to longer wavelengths. Often, the redshift from the expansion of the Universe is described this way, but in fact that's not really what it is. In fact, the dynamics of the Universe as described by Einstein's General Relativity tell us that as the Universe expands, wavelengths of light expand at *exactly the same rate*. This is called the "cosmological redshift".

In the diagram at the bottom of the slide, at "Time of emission" there are two galaxies; we are the one on the right, and the one we want to observe is on the left. At emission time, some light is emitted. Time passes, the light travels, and the Universe expands. By the time the light reaches us, the Universe has expanded -- so the galaxies are farther apart -- and the light's wavelength has also expanded; longer wavelength light is redder. The amount of the redshift we observe is a *direct* measure of how much the Universe has expanded.

Go to the next slide, on the other side of the mollusk... :) [Edit added to transcript: The mollusk was Joshua Linden.]

The two things I've told you give us everything we need to measure the expansion history of the Universe. Measure the distance of a standard candle to figure out how far back in time we're looking; that's the X coordinate of a point on the graph. Measure the redshift to figure out how much the Universe has expanded since that time; you can use that to figure out the relative size of the Universe at the time of emission as compared to now. That relative size is the Y coordinate of each point. Plot all your points for supernovae at different distances -- that is, different lookback times -- and you have your expansion history.

At that point, we can compare it to what theory predicts. On the next slides are the predictions from theory that we *thought* we were going to be comparing to when the project was started in the 1990's.

Basically, mass creates gravity; all the galaxies are pulling towards each other, which will tend to put the brakes on the expansion. If there is a *lot* of mass, there's a lot of gravity, so the expansion will be slowing down a lot. In that case, there may be enough mass to stop the expansion and cause the Universe to recollapse. On the other hand, if there isn't very much mass (the "low-mass" Universe), the expansion should be slowing down less, and the expansion will continue forever. However, because gravity is attractive, no matter what you should expect the expansion to be *slowing down*-- decelerating.

Go to the last slide.

Again, hold ALT, move the mouse over the slide, push and hold the LMB, and move the mouse. When we really made the measurement, we can up with the result that was unexpected by most of us.... For the last 6 or 7 billion years, the expansion of the Universe has been *speeding up*.

A lot of people didn't believe this at first. One prominent theorist, Rocky Kolb, apparently said in one talk that "the Supernova people had better figure out what is wrong with their data, or somebody is going to get hurt." However, there were two independent teams that came up with the same result, so people took it seriously. Now, almost a decade later, independent methods have confirmed that it seems that the accelerating Universe measurement is correct.

Because gravity is attractive, normal stuff can't make this happen. For the Universe's expansion to be accelerating, there must be *something else* in the Universe. It turns out that Einstein's General Relativity *does* allow for a very exotic material which will have (effectively) a negative gravitational effect. Today we call that Dark Energy. From the fact that the Universe's expansion is accelerating, we know that Dark Energy makes up 2/3 to 3/4 of the total energy density of the Universe.

Wacky stuff.

The big slide over the DJ's stage summarizes some of this again, and shows a bunch of the actual supernova data that I worked on back in 1996-1999 (and may also include some additional data form a 2003 paper I wrote -- I'm not sure).

At any rate, I'll stop there -- that's the whirlwind tour of just how one goes about measuring the expansion history of the Universe.

For the rest of the time we have, I'd be happy to answer questions about any of this, or about the discovery.

...and right then was when the grid crashed. I couldn't have timed it better if I tried.

If you didn't come back at about 11AM PDT to ask questions, drop by the same spot tomorrow at 10AM for a Q&A.