“It used to be that talking to people about cosmology was really like talking to them about theology.”

Well, as you may know, the standard model for the universe now has both dark matter and dark energy in it. This was perhaps the first paper that stated that the correct model of the universe is one dominated by dark energy. It was quite revolutionary.

Well, I have been an amateur in most fields in astrophysics. I wasn’t trained as a cosmologist and I don’t have strong views on what the universe ought to be. And there were essentially two camps on that question for quite a long time. On one side were the astronomical observers and the theorists steeped in astronomical knowledge, and they knew how much mass there was in the universe. They knew that the mass in stars corresponded to only a few percent of the "closure density," the critical density in the Einstein universe. Even including the dark matter, it only went up to 20 or 25 percent. It still wasn’t anywhere near 100 percent. So they had their preconceptions. Then there were the physicists who didn’t know a lot of astrophysics, but they knew there were good aesthetic arguments for having a universe that was just closed, with omega equal to one. So they were arguing that there must be much more mass of some kind out there. I was working with Paul Steinhardt and we just started looking back and revising this old idea of Einstein’s of the cosmological constant. The modernization of that is dark energy. And so we explored this possibility that you could get a closed universe by adding a lot of dark energy to the ordinary matter and dark matter. And we found that everything worked better if we allowed for it. It was really as simple as that.

Well, we looked at a whole bunch of different arguments. For example, you can get the age of the universe from the Hubble Constant, but it depends on which model universe you’re using. With different model universes, you get different ages using the same value of the Hubble Constant. One of the things that had been noted is that if you took the physicists’ model, where omega equals one, you get an age of the universe that is less than the age of the stars. As it turns out, this is all straightened out if the missing amount of matter or energy is dark energy. That causes the universe to accelerate and it changes the relationship between the Hubble Constant and the age of the Universe. You now get an answer that is considerably larger and in agreement with the other observations.

It used to be that talking to people about cosmology was really like talking to them about theology. They had definite ideas about what it ought to be, and these ideas were unshakable. It wasn’t an evidence-driven field. It was certainly like that when I started working in the 1960s. By the mid-1990s, there were more and more observational constraints, so it was becoming a data-driven subject. You couldn’t just imagine anything. You could actually rule out theories and argue for others based on the evidence. We said in the abstract of our Nature paper that observations were providing tighter constraints, and these included the recent determinants of the Hubble Constant and the anisotropy in the cosmic microwave background. The abstract ends by saying "a universe having a critical energy density and large cosmological constant appears to be favored." That’s the dramatic sentence, and then the paper goes on to make the arguments to show that the observations are consistent with that.

In some sense it had. Einstein had proposed it way back in the beginning as a cosmological constant, but that was in order to make a static model of the universe. He later said that this was the biggest blunder he ever made, because at the time he wrote that, astronomers already knew it wasn’t a static universe from Hubble’s observations. He didn’t know about those observations, though, and they showed that we lived in an expanding universe. His cosmological constant provided the outward force that could balance gravity. After that, the idea of a cosmological constant fell into disfavor. From time to time people attempted to revive it, to see if it would fit the observations. I won’t claim that we were totally original, but there weren’t a lot of people saying a cosmological constant was a requirement. To some extent it took two outsiders to make that claim. Neither Paul Steinhardt nor I had written many papers in cosmology.

Well, among my more-cited papers is one on the interstellar medium in our galaxy. I then went from that to doing the intergalactic medium. I was interested in the large-scale structure of the universe, but there’s a sort of trade school of people doing cosmology and I wasn’t in it. We were two outsiders who looked at this and said, "Hey, this will work." The idea was really to see without prejudice which model seemed to fit all data best.

Well, let me backtrack before I answer that. The first paper I did, my first step in this, was in the mid-1970s with Jim Peebles on dark matter. That idea was not accepted for a decade, even though the evidence was really overwhelming. People just couldn’t see it. If you look at the light distribution in galaxies and the light distribution in the solar system, it all comes from the center. In our solar system, it’s the sun. If you were to measure the mass within a set sphere, as you go further and further out, it doesn’t change very much. You get Kepler’s law: the velocity of rotation falls as one over the square root of the distance. If you do the same thing for the galaxy, the light falls off dramatically but the velocity is constant. This had been known for a long time. That means the mass is proportional to the radius. Light isn’t proportional to radius. So it was known for a long time that there must be something fishy. People would try to fit rotation curves, and they didn’t fit the data at all. This was done time and again. So you read the papers and you wonder what’s going on here. You’re trying to fit something, which doesn’t fit at all, and then you’re assuming it has to be right anyway. At that time we said there must be some other component, which is invisible. We don’t know what it is. We’ll call it dark matter, and it must increase in fraction as you go further and further out. This was so unappealing to people, it took them over a decade to accept it. Yet the evidence for this was overwhelming. It wasn’t until the mid-1980s when many other arguments came along that gave the same answer that it was accepted. Gravitational lensing gave the same answer, and so did the cosmic background curve, etc. So there was about 15 years of denial that followed our paper. Vera Rubin’s work on galaxy rotation curves, which followed ours, was very important and influential.

In this case, it’s been fairly different. There is a very direct way of seeing whether the universe is expanding faster or slower: if the Hubble Constant is declining with time, remaining constant, or increasing. You can use supernovae, good standard candles, to test this. When we wrote the paper, the supernovae evidence was indicating no dark energy. I absolutely didn’t believe it. I didn’t think there were enough statistics at the time. All the other arguments went the other way, so I thought this supernovae evidence must be wrong. Within about three or four years after the paper our came out, the supernovae evidence came in and gave strong evidence for exactly the arguments we were making. So when the supernovae came in and confirmed it, then later the cosmic background radiation confirmed it even better, it all fell into place. So it has been, in effect, completely accepted within a decade. And now the standard model of the universe is one with dark matter and dark energy.

I think skepticism is always reasonable if somebody proposes something new. My attitude toward it was to look at every single measurement, and see if it fits better or fits worse. If you look at the age of the universe, does it agree better with the age of the stars or worse when we do this? Does the Hubble Constant? All the things you can measure. In every case, they get better rather than worse when dark matter and dark energy are added. So there is a prima facie strong case for this. That doesn’t say it’s absolutely right. And so it wasn’t taken very seriously until the observations came in on the supernovae—from two groups, one at Berkeley and one at Harvard. Those two groups agreed, and then it became accepted. Now, oddly enough, it’s the new dogma. If you were to say that anything else is possible, you would be laughed out of the room.

My own guess is that there’s a 95 percent chance it’s right. The degree of acceptance is beyond that, and the degree of skepticism of alternatives is overdone. There are still loopholes in the argument.

That’s a subjective guess. You also have to remember that science, at least in my area, proceeds by successive approximations. Newton wasn’t proven wrong by Einstein. It’s just that he was right to a very high accuracy, and then there are certain circumstances where you can find departures from the theory. I’m sure the model with dark matter and dark energy is like that. We will find other components. It’s not 100 percent the truth. For example, dark energy might not be fixed lambda, but a variable field.

That’s the cosmological constant. It was introduced by Einstein with the Greek letter lambda. He did it as a constant of nature. Gravity was a capital "G." Lambda stood for this mysterious force that causes the expansion of the universe.

I think this is where we go: if you accept that there is some kind of dark energy, then different theories will give different time dependence for this extra force in the universe. If you now make measurements very accurately in the epochs between now and about red shift one, when the universe was about half its current size, you can distinguish between these models and tell something about the nature of this dark energy.

Both NASA and NSF have ground-based and space-based experiments looking for exactly this. There are "dark energy" telescopes being designed. Obviously, you’re not really looking at dark energy, because you can’t see it, but you can look at the traces, and the behavior of these traces will give you insight.

Jeremiah P. Ostriker
Princeton University
Princeton, NJ, USA