Inquiring Minds

Questions About Physics

Density of the Universe

Dear Quincy,

Thanks for your great questions on the future of the Universe. They made me think a bit. It turns out that I am an experimental physicist at Fermilab who is presently involved with an experiment to detect the dark matter, which you also asked about.

You asked:

It is known that if the universe's average density is less than a critical value, it will expand forever. If the density is higher than it, the expansion will stop at sometime and the universe will start to recollapse. Have any conclusion of this been made now?

The answer is that we don't yet know the whether the density of the Universe is above, below, or exactly equal to the critical value. Surprisingly, theory tells us there are good reasons to believe the density is exactly equal to the critical value. If this is true, then the Universe is balanced precariously between collapse and infinite expansion. What does this mean? It won't collapse, and it will almost stop expanding one day. To say this mathematically, as time --> infinity, the universe stops expanding. Anyway, we don't know that this is actually the case. There is much work to be done by experimentalists or observers before we will know the correct answer.

Your next question:

Suppose the density is slightly higher than the critical value, that means the expansion of the universe will finally stop. But before that time, it's still expanding and so the density of it should fall. Then is it a contradiction that if the density drops below the value before the expansion stop?

If the density is higher than the critical value, it will remain above the critical value for all time (and the Universe will eventually collapse). This is because the critical density varies with time; i.e., at any given time you can calculate the critical density and determine whether the universe will collapse or not. It is very similar to calculating whether a ball thrown into the air has enough velocity to escape earth's gravitational pull. Initially it is going very fast, but it slows down. However, you can take the velocity at any point on its trajectory and decide whether it is greater than or less than the critical escape velocity for that point in its trajectory. Similarly, you will get the same result no matter what time you decide to calculate the density. Therefore, you will never come up with you contradiction.

Now my favorite subject, dark matter:

I've heard of something called dark matter which exists between galaxies. But I wonder if this is just another case of "ether". Is it proposed just because some scientists want to satisfy some theories (e.g. the expansion of universe will stop at some time)?

First of all dark matter is not believed to exist just between the galaxies. It must exist within the galaxies as well. One of the prime reasons for believing it must be there is based on a theory-- a very old and well established theory-- Newton's law of gravity. It turns out if you apply the simple calculations to the galaxy that are used to predict when the moon will come up or to send a space probe to Jupiter they will fail unless there is a lot of matter out there that is not visible. The rotational curves of all the galaxies are constant as a function of distance from the center of the galaxy, which can only be explained by lots of unseen mass that extends well beyond the visible portion of the galaxy. If we look at the motions of the galaxies about one another, we find that there must be even more unseen mass than the galactic rotation curves indicate. So the evidence is quite strong, and I think a better comparison would be to the time when physicists invented the neutrino to preserve energy and momentum conservation rather than the creation of ethe r.

There are plenty of cosmological and particle physics theories which give us some clues as to the nature of this matter, but we won't really know the truth until the experiments are done. Already, astronomers have been using gravitational lensing to detect dark objects between our galaxy and the Large Magellenic Cloud (a satellite galaxy). The have found seven or eight objects-- not enough to explain away the problem, but they help. Meanwhile, people like myself are trying to detect weakly interacting particles, which are very similar to the neutrinos, but have large mass. They aren't seen because they interact with normal matter only through two very weak forces, gravity and the weak interaction force. There are good theoretical reasons to believe that such particles might exist and explain the behavior of the galaxies, but we won't rack it up as scientific fact until we have observed and confirmed their existence.

Well, thanks again for the great questions. If anything is not clear, or you want to follow up, please send me email at I'll respond when I have time. (I spend a lot of time out at Stanford these days where the initial version of our experiment is beginning to take data. And, much of the rest of the time is spent in an iron mine full of bats in northern Minnesota where we hope to install an upgraded version of the experiment sometime next year.)


Roger L. Dixon

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last modified 5/9/1997