INTERVIEW OF THE WEEK
JEREMIAH OSTRIKER, co-discoverer of dark matter, Professor of Astrophysics,
Princeton University, Chairman, Department of Astrophysics and Director of Princeton
This weeks 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 Stars president, Fred Heeren, went to Jeremiah Ostriker, known as one of Princetons 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?
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.
WILSON: When you do this [apply the laws of gravity], you
find that there is more mass than you can account forfrom 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 matterit's not a question of whether it's thereit'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 lightfrom a quasar, I believein 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 thingswe 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 usfor those of you who are still waiting to see what all this has to do with lifes big questionsis that lifeyour lifeis 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 existenceor for any form of life. Day Stars book, Show Me God, lists many unnatural selections, somehow made at the beginning, that became nature for us.
DAY STARS DISCUSSION KICKER OF THE WEEK:
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