"Neil Armstrong has died at age 82. Perhaps to honor his memory we can resolve as a society to continue to do inspiring things", wrote Sean Carroll, at Cosmic Variance.[AS] Yes. And so this accelerating expansion means that there has to be something, which has basically been termed dark energy, that's pushing the universe apart. Is there any idea what that is yet, do you think?
[SP] No. Not only do we not know what dark energy might be, that would be making the universe expand faster and faster, we don't even know whether really the answer will turn out to be a new energy in the universe. It's possible that we've just discovered an extra wrinkle in Einstein's Theory of Relativity, and that that would be the real final result. But at this point, the job is really back in our court again as observers, and we have to come up with more data that will help narrow in on what the answer is.
[AS] Because the theoretical people have been proposing solutions at a great rate, but more data is needed to know which if any of these proposals is correct.
[SP] Exactly. I think the papers have been coming out on the order of one per day for the last 10 years, from the theorists. And of course, all these theories can't be right! But the other thing is that the theorists themselves I think would tell you that they don't believe any one of their theories. They're just trying to expand the range of possibilities. And if it turns out that, you know ... What they're really looking for is for us – the observers and the experimentalists – to come up with some more data to help home in, help narrow in, on which range of possibilities could be right.
[AS] It seems to be one of the lovely things about physics; that sometimes it's the time of the experimentalists, and then it goes back to the theorists, and then back to the experimentalists again.
[SP] No, no, it makes for a great tennis game of science!
[AS] [Laughs] And so what do the experimentalists have to do next? What is the next big challenge?
[SP] So our job now is to develop ways to measure this very subtle effect. It seems like a big deal, something that can power the expansion of the universe, but it only makes a difference over the timescale of a billion years, and billions of light years. So now we have maybe three different techniques that we could use to get at what's going on. And each of them is a very challenging experiment that will require building new instruments, and perhaps even a new space telescope.
[AS] So in looking for these subtle effects, do we have to look at the fringes of the universe, or can we build experiments here on earth that will allow us to see it?
[SP] So far, nobody's figured out any way that we could detect it locally. So far, as far as we know, it takes very large distances and billions of light years to be able to see enough of the effect. But, who knows, it may turn out that there'll be new ideas that we haven't invented yet.
[AS] In a conversation with Saul Perlmutter just about an hour ago, I was asking about the relationship between the theoretical and the experimental physicists. The ball seems to be sort of batted back and forth between the two groups.[BS] Yes, that's often the way things go. The theorists come up with a great idea, we try to test it; most of the time we expect we'll be able to prove it wrong, but occasionally we show it's right, or it seems to be right, it doesn't seem to be wrong. And so it's a great interplay, when you can have theory and observations continually playing off each other. That means you make progress really quickly. If it's all theory, or it's all observation, then the progress is much slower.
[AS] And as an experimentalist, what's the new excitement? Is it technology that's improving to allow you to look deeper into space?
[BS] Yes, so the technology; these big telescopes, better detectors. Those types of things really enable us to do better and better experiments. So those are a huge driver. But I guess also just good ideas. Astronomy is not like a physics lab. You can't design an experiment. You need to go through and look up into the heavens, and sort of figure out what the cosmos has given you, and make an experiment out of that. So it's a very different process. And so there's always the chance of having a really good idea of how to put together things in space to do the experiment that you want to do.
[AS] That's a beautiful description of astronomy.
[BS] Yes, it is a very different thing than experimental physics that way.
[AS] And is the environment for it supportive? Is there lots of money around to help you with the new ideas?
[BS] So astronomy is really going through a heyday right now. That is, it's very well supported compared to what it was 30 or 40 years ago. But I think one thing to remind ourselves is that great ideas don't need billions of dollars, they need moderate support, and you need to have a lot of people with moderate support to get the good ideas. There are also needs for great big experiments and those are expensive, but they tend not to be so imaginative. They're sort of the brute force way of solving problems. And so, I'm a very strong believer that you want to try both things, but you want to make sure you keep a bunch of smaller groups with innovative ideas, at the same time as maybe a couple of brute force big experiments.
[AS] ... How far away are we talking?
[AR] We are talking about 5-billion light years.
[AS] [Laughs] Okay. Mind-boggling distances. To explain the acceleration, you had to propose some sort of force pushing galaxies apart, and that’s what we refer to as "dark energy".
[AR] Right. We actually did not have to propose that, seeing that there was, I would say, off-the-shelf and ready, a model from Albert Einstein, something he referred to as the cosmological constant, which would neatly do the trick. And so all we did was to say that that seemed like at that point the simplest hypothesis.
[AS] But Einstein had felt that he’d been wrong in proposing it, I gather.
[AR] That’s right, so maybe he should be getting the Nobel Prize again! [Both laugh]
[AS] And do we have any idea at all what dark energy is yet?
[AR] No. I wish we did! [Both Laugh] They didn’t give us the Nobel Prize for that!
[AS] [Laughs] What’s our best hope of finding out, do you think?
[AR] More experiments, and hopefully somebody else with some great ideas. But more experiments.
[AS] Okay.
[AR] Probably some space satellite, you know, the use of our current facilities, taking a lot more data.
[AS] Okay. Last question.
[AR] Yes.
[AS] When you’re tracking the infinite recesses of space in this way, does it not get a little bit scary thinking at such distances?
[AR] Is it scary? No, I would say I find it very calming. It just feels so big.
I started with the thirst of determining the fate of our universe: Will it expand forever, or will it halt and contract, resulting in the Big Crunch?To find the answer, we had to determine the mass of the universe. It can be calculated by measuring how much the expansion of the universe is slowing.
First, we had to find cosmic candles—distant objects of known brightness—and use them as yardsticks. On this page, I checked the reliability of the supernovae, or exploding stars, that we had collected to serve as our candles. I found that the results they yielded for the present expansion rate of the universe (known as the Hubble constant) did not appear to be affected by the age or dustiness of their host galaxies.
Next, I used the data to calculate ΩM, the relative mass of the universe.
It was significantly negative!
The result, if correct, meant that the assumption of my analysis was wrong. The expansion of the universe was not slowing. It was speeding up! How could that be?
I spent the next few days checking my calculation. I found one could explain the acceleration by introducing a vacuum energy, also called the cosmological constant, that pushes the universe apart. In March 1998, we submitted these results, which were published in September 1998.
Today, we know that 74 percent of the universe consists of this dark energy. Understanding its nature remains one of the most pressing tasks for physicists and astronomers alike.
Most astronomers had expected that the Universe's rapid growth following the Big Bang would gradually slow down as gravity pulled distant galaxies towards each other. Yet the discovery was accepted almost immediately by the astronomical community — in part because the idea of a cosmic pressure pushing the Universe outwards had already been mooted by Albert Einstein. - Nature news.So now, from those lines, we begin to think about gravity pulling back and hence slowing down the velocity of Expansion.
