Brief Answers to Cosmic Questions Structure of the Universe<br>Does the Universe have an edge, beyond which there is nothing?<br>Are the galaxies arranged on the surface of a sphere?<br>Why can't we see the whole universe?<br>Does the term "universe" refer to space, or to the matter in it, or to both?<br>Evolution of the Universe<br>Did the Universe expand from a point? If so, doesn't the universe have to have an edge?<br>Then where did the idea that the universe was once a point come from?<br>If the universe started out so dense, why didn't it collapse into a black hole?<br>Why does looking out in space mean looking back in time?<br>I've heard the expansion of the universe may be speeding up. Is there an "anti-gravity" force?<br>More about the Big Bang<br>When they say "the universe is expanding," what exactly is expanding?<br>But if you can't see space, or feel it or touch it- how can it be expanding?<br>Why did anyone ever think that space should be expanding? Isn't it a far-fetched idea?<br>Then how do we know that space really is expanding?<br>Are the galaxies in the universe moving through space?<br>But I heard that our Milky Way galaxy may one day collide with a neighboring galaxy. If galaxies are all moving apart from each other, how can they collide?<br>Where did the Big Bang scenario come from?<br>How do we know when the Big Bang took place?<br>Do we know where, in space, the Big Bang took place?<br>How do we know there really was a Big Bang?<br>But I've heard on the news there are problems with the Big Bang theory. Is it still just a "theory"?<br>Was the Big Bang the origin of the universe?<br>Are there theories that go beyond the Big Bang?

Structure of the Universe<br>Does the Universe have an edge, beyond which there is nothing?<br>Galaxies extend as far as we can detect... with no sign of diminishing.There is no evidence that the universe has an edge. 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. 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. 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.<br>^ back to top<br>Are the galaxies arranged on the surface of a sphere?<br>No. Galaxies are not actually arranged on the surface of a sphere. Many students and teachers mistakenly believe that the galaxies in the universe are arranged on the surface of a sphere. One origin of this misconception is the common demonstration of blowing up a balloon to model the expansion of the universe. Another is the (mistaken) belief that during the Big Bang, matter expanded into space from a point (see below). A third is the finding that many clusters of galaxies appear to be arranged around the outside of "bubble-like" voids in the universe. 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.<br>^ back to top<br>Why can't we see the whole universe?<br>We can see just about as far as nature allows us to see. Two things prevent us from seeing further. First, the universe has been evolving with time. Stars and galaxies did not always exist. Therefore light from MOST of the galaxies in the universe has not yet had time to reach us. Second, the universe has been expanding with time. Again, light from MOST of the universe has not yet had time to reach us.<br>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. But how far, no one knows.<br>^ back to top<br>Does the term "universe" refer to space, or to the matter in it, or to both?<br>Just a hundred years ago, scientists thought of the universe in terms of matter. Space was just the "emptiness" in which matter lived.<br>Today, the situation is reversed. 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. (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. For example, matter and anti-matter are routinely created in the laboratory from space itself (and an energy source); the kinds of particles that can exist reflect the structure of space. In fact, there is now evidence that space itself MAY possess some slight amount of energy of its own, of a form previously unknown. If so, space may actually have weight!<br>Discovering the properties of space remains one of the deepest and...