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Pull and Push

When you throw a stone into the air, how high it goes depends on how much energy you put into it. It also depends on the mass of the Earth. On a less massive body, like the Moon, you could throw a stone much higher. Throw hard enough, and the stone will go on forever.

It turns out to be similar for the Universe. The "push" in the Universe is the energy it started with, measured by astronomers as the rate at which galaxies are separating. The "pull" is the gravitational force of all the galaxies (and other stuff), trying to pull the matter in the Universe back together and stop the expansion. Cosmologists measure the ratio of pull to push with a symbol they call Omega, appropriately, the last letter of the Greek alphabet. It's appropriate because this single number actually describes the fate of the Universe. 

If Omega is bigger than one, there is more pull than push in the Universe. Eventually, the expansion will slow down and then reverse, leading to the Big Bang in reverse. Sometimes this is called the Big Crunch. This type of Universe is called "closed". It's finite in much the same way as the surface of the Earth is finite. You can't go on forever without eventually getting back to where you started.

If Omega is less than one, the Universe will go on expanding forever. This is an "open", infinite Universe. Future observers would find themselves in an almost empty Universe, in the company of only the few nearby galaxies that happened to be gravitationally bound to their own. If you set off for a journey in this Universe, you could go on forever.

So you might say that the value of Omega is of some significance, and indeed a lot of scientists have tried to measure it. It is a hard project, for two reasons. You have to be able to measure distances in the Universe, and you have to be able to count up how much stuff there is in it.

The Hunt for Omega: How Much and How Far?

You need distances to make sense of the expansion. Think of the experiment with the balloon and the stickers. Observers living on the stickers would notice that the other stickers were moving away - for instance, by seeing that they were getting smaller. However, without knowing their distances, they could not measure the rate of expansion of their Universe, the rate (in liters per second, say) at which the balloon was being inflated. Measuring distances - to planets, stars, and galaxies - is an audacious and ingenious project for astronomy.  The Hubble Space Telescope has made a big contribution in this area. Since it orbits the earth above the atmosphere, it can see much further and with greater clarity than earthbound telescope whose vision is obstructed by the air and everything floating in it.

For galaxies and other faraway things, distance can be measured from the redshift - the reddening of the light from them. You can think of this as being the result of them appearing to be rushing away from us because of the expansion of the Universe: the further, the faster. Most galaxies show distinctive features in their spectra (the "rainbow" you get if you put their light through a prism). These features get redder for galaxies that are further-away.

This illustration shows how the light from distant objects gets reddened. The spectrum at the top is from furthest away. The one at the bottom is from the the closest star.

Every substance produces a characteristic pattern of colors. Often this includes dark or bright lines which are unique to particular atoms or molecules. If the object being observed is at a fixed distance from the viewer the pattern appears in one position on the spectrum. But if the object is moving away from the viewer, the perceived wavelengths all get longer.

The pattern is essentially the same in each case, but shifts further to the right -- the red end of the spectrum -- the more distant the observed object. This is because the more distant objects in the universe are moving away from us faster than the closer ones.

Finding out how much stuff there is in the Universe is much harder. For a long time astronomers have known that stars can't account for the gravitational forces acting in our neighborhood in the Galaxy. The essence of this measurement is very simple; stars near us are moving so fast that they ought to have escaped from the Galaxy. But they haven't, and there must be extra mass, over and above the stars we can count, that is holding things together. This is often called the "missing mass" problem. Some astronomers say dryly that it ought to be called the "missing light" problem - we are sure the mass is there, but we can't see it. So how do we go about measuring this invisible but crucial piece of the puzzle?

Weighing techniques always deal with some sort of distinct system, separate from its surroundings. It turns out that they can't detect smoothly spread-out stuff that might fill all of space.  Given the attractive nature of gravity, though, we don't expect any smoothly-spread distribution of stuff to stay smooth for very long. The tiniest lumpiness - one atom out of place - will make extra gravitational pull, which will make the lump bigger . . . and so on.

On larger and larger scales - galaxies, clusters of galaxies, superclusters - astronomers can use the same type of measurements of velocity to weigh things. Always, there is far more mass than we can see in stars. What this extra stuff might be is a separate question, but the best bet at present is that Omega is about 0.3. That means the Universe will go on expanding forever, and is infinite - and always was! 

Is that it?

Well, no. For one thing, the determination of Omega is at the cutting edge of science, and measurements are continually being improved - and mistakes found. There are also some big conceptual problems with Omega being about a third. This is a very arbitrary number. Why isn't it something else? The only special value for Omega is 1.0, as it happens, because the pull-to-push ratio always then stays the same.

There are other problems with Omega being less than one. If this is the case, the Universe must have been infinite even at the Big Bang and that easily observable parts of the Universe - on opposite sides of the sky, say - could never have been in any sort of contact. It's then very hard to explain why these opposite sides look so similar, rather than having developed in totally different ways.