(In fact, when astronomers talk about the "expansion of the universe," they are referring to the stretching of space between clusters of galaxies - NOT to the motion of galaxies through space.) Later, scientists found other properties of space. During the twentieth century, scientists learned that space is not "nothingness." First, Einstein showed that space has structure: It is flexible and can be stretched. Space was just the "emptiness" in which matter lived.
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Just a hundred years ago, scientists thought of the universe in terms of matter. If you could suddenly freeze time everywhere in the universe, and magically survey all of creation, you would find galaxies extending out far beyond what we can see today. Again, light from MOST of the universe has not yet had time to reach us. Second, the universe has been expanding with time. Therefore light from MOST of the galaxies in the universe has not yet had time to reach us.
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First, the universe has been evolving with time. Two things prevent us from seeing further. We can see just about as far as nature allows us to see. But on the largest scales that astronomers have observed, each chunk of space appears to have just as much matter as any other equivalent chunk. A third is the finding that many clusters of galaxies appear to be arranged around the outside of "bubble-like" voids in the universe. Another is the (mistaken) belief that during the Big Bang, matter expanded into space from a point (see below). One origin of this misconception is the common demonstration of blowing up a balloon to model the expansion of the universe. Many students and teachers mistakenly believe that the galaxies in the universe are arranged on the surface of a sphere. Galaxies are not actually arranged on the surface of a sphere. When astronomers sometimes refer (carelessly!) to galaxies "near the edge of the universe," they are referring only to the edge of the OBSERVABLE universe - i.e., the part we can see. We know that the galaxies must extend much further than we can see, but we do not know whether the universe is infinite or not. The part of the universe we can observe from Earth is filled more or less uniformly with galaxies extending in every direction as far as we can see - more than 10 billion light-years, or about 6 billion trillion miles. with no sign of diminishing.There is no evidence that the universe has an edge. Was the Big Bang the origin of the universe?Īre there theories that go beyond the Big Bang? How do we know there really was a Big Bang?īut I've heard on the news there are problems with the Big Bang theory. How do we know when the Big Bang took place?ĭo we know where, in space, the Big Bang took place? Where did the Big Bang scenario come from?
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If galaxies are all moving apart from each other, how can they collide? Then how do we know that space really is expanding?Īre the galaxies in the universe moving through space?īut I heard that our Milky Way galaxy may one day collide with a neighboring galaxy. Why did anyone ever think that space should be expanding? Isn't it a far-fetched idea? When they say "the universe is expanding," what exactly is expanding?īut if you can't see space, or feel it or touch it- how can it be expanding? I've heard the expansion of the universe may be speeding up. Why does looking out in space mean looking back in time?
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If the universe started out so dense, why didn't it collapse into a black hole? Then where did the idea that the universe was once a point come from? Does the Universe have an edge, beyond which there is nothing?Īre the galaxies arranged on the surface of a sphere?ĭoes the term "universe" refer to space, or to the matter in it, or to both?ĭid the Universe expand from a point? If so, doesn't the universe have to have an edge?