Interview of the Week


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JEREMIAH OSTRIKER, co-discoverer of dark matter, Professor of Astrophysics, Princeton University, Chairman, Department of Astrophysics and Director of Princeton University Observatory.

ROBERT WILSON, co-winner of the 1978 Nobel prize for physics for his co-discovery of the cosmic background radiation. Astronomer, Smithsonian Astrophysical Observatory.

This week’s theme centers around the question of the critical density of the universe, one of the finely tuned parameters that physicists have turned up that, if tweaked slightly differently, would make our existence impossible. To find out how scientists know this is true, Day Star’s president, Fred Heeren, went to Jeremiah Ostriker, known as one of Princeton’s observational sticklers, and to the Nobel prize-winning physicist, Robert Wilson. Here are the relevant portions of their conversations:

How Do We Know Our Universe Is Very Near to the “Critical Density”?

Note from Fred Heeren: When I once asked Arno Penzias the above question, he told me: “If I put on my physics hat, I think it's so unlikely as to make me shudder as a scientist. On the other hand, as an astronomer, boy, all I have to do is look at the parameters. If this had been anything but galaxies in the universe, everyone would have taken the data at face value long ago and said the universe doesn't have enough matter to pull it together. There'd be no reason.”

Evidently the existence of galaxies takes some explaining, and science has had to scramble for theories (like exotic matter and inflation, even though they have no observational support) in order to explain how our universe might possibly have reached very close to the precise density required for our existence. Calculations easily tell us that the likeliest scenario should either be a universe that collapses too soon or disperses too fast to permit life. Any other result has a likelihood of one in billions, by anyone's estimate.  Yet actual measurements tell us that the universe has very close to the required mass for critical density (at least a tenth or so of the needed amount). This means that the ratio between the universe's actual density and the critical density had to be either 1 or within billionths of 1 at the very beginning. But how do astronomers come up with these actual measurements? How, the skeptic might ask, do astronomers even begin to “observe” the amount of mass in the universe?

HEEREN: How does one measure the amount of mass in the universe?

OSTRIKER: The way I like to think about it is this: suppose you want to know what you weigh. You can say, "Well, my pants are getting tight, so I guess I put on weight. Or I look in the mirror or I ask my friends what they think. Any number of different ways of doing it. But there's really only one real way to find out. And that's standing on the scale. There's only one way to find out what things weigh and that's through gravity. It's the only way. Everything else is indirect. Okay?

And so you use gravitational measures. And an example that we're familiar with is we get the mass of the sun by the orbits of the planets around it. It's basically their velocity squared times the distance divided by Newton's constant of gravity. This gives you the mass of the sun. Well, we do the same thing for galaxies and groups of galaxies and clusters of galaxies, etc., etc., and we find the mass that way. And that, I've always believed, is the only way.

Sometimes people use the light, and they take the observed light times the mass to light ratio for the object, and they get the mass that way. But that's an observation times an assumption, if you wish, because who knows what the mass to light ratio is?. . . . So the answer is that you should always use gravity, and it always involves velocities and distances and Newton's laws.

When I asked Robert Wilson about how we know that the density of the universe is very close to the critical density, he gave me a good summary of the modern scientific consensus. After explaining how the mass of galaxies is measured in similar terms that Ostriker discussed (that is, by using gravity), he went on to discuss the need for dark matter.

PhotoWILSON: When you do this [apply the laws of gravity], you find that there is more mass than you can account for—from the mass to light ratio. And that's the first evidence for dark matter. If you were to simply take the amount of light that is observed in galaxies and sort of add it up over the universe, you come out with about one-hundredth of the critical mass. . . .

HEEREN: So this dark matter—it's not a question of whether it's there—it's a matter of what it is? No one disputes the fact that it's there?

WILSON: Right. I know one other measurement that has been made other than just the dynamics: that is that Tony Tyson looked at the gravitational deflection of light—from a quasar, I believe—in a very distant cluster, and came out again with the requirement for dark matter in that cluster. So in addition to the gravitational attraction of the galaxies in the cluster, there also was the effect on light passing through one. So if you put in all of the dark matter, you're missing sort of a factor of ten in closing the universe.


WILSON: Now when you go back to general relativity and look at the expansion, what you find is if there's a tiny deviation on either side, it will exponentially depart from critical density. So if it started out with one part in 10 to the 60th too little mass, there wouldn't be stars and galaxies and planets and things—we just wouldn't be here to observe it. And if there were the same amount more, it never would have expanded to this point; it would have re-collapsed. So the requirement after inflation is very exacting, as to the density of the universe, to come out near the critical density.... But the observations put us within a factor of ten now, and therefore within some very tiny fraction, back at the earliest time we can think about.

The relevance of this to all of us—for those of you who are still waiting to see what all this has to do with life’s big questions—is that life—your life—is apparently no accident. Science now tells us that our universe was critically adjusted at the beginning. And this is only one of many finely tuned parameters required for our existence—or for any form of life. Day Star’s book, Show Me God, lists many “unnatural selections,” somehow made at the beginning, that became nature for us.

Inflation is the name of one theory that physicists use to explain how the universe was launched at the critical density, though it doesn’t explain why such a perfect balance among physical constants should have permitted inflation in the first place. All cosmologists agree that many other critical parameters remain a mystery. How do you explain these mysteries? Do you take your existence as a one-in-a-million fluke? Do you take it as a brute fact, not worthy of exploration? Or do you see evidence of purpose behind the universe, and if so, what kind of purpose?

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