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STRUCTURE OF THE UNIVERSE
Does the Universe have an edge, beyond which there is nothing?
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.
Galaxies extend as far as we can detect... with no sign of diminishing.
Are the galaxies arranged on the surface of a sphere?
No. 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.
Be careful of introducing misconceptions if you model the expanding universe using a balloon. Galaxies are not actually arranged on the surface of a sphere.
Why can't we see the whole universe?
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.
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.
Does the term "universe" refer to space, or to the matter in it, or to both?
Just a hundred years ago, scientists thought of the universe in terms of matter. Space was just the "emptiness" in which matter lived.
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!
Discovering the properties of space remains one of the deepest and most important problems in modern science.
EVOLUTION OF THE UNIVERSE
Did the Universe expand from a point? If so, doesn't the universe have to have an edge?
No. This misconception causes more confusion than any other in cosmology. Unfortunately, many students, teachers, and scientists(!) mistakenly picture the "Big Bang" as an explosion that took place at some location in space, hurtling matter outward (see illustration, left).
The Big Bang was not an explosion IN space (left). It was a process that involved ALL of space (right).
In reality, ALL of space was filled with energy right from the beginning. There was no center to the expansion, and no magical point from which matter hurtled outward. The confusion arises in part because of the amazing conclusion that the OBSERVABLE portion of the universe was once packed into an incredibly tiny volume. But that primordial pellet of matter and energy was NOT surrounded by empty space... it was surrounded by more matter and energy (which today is beyond the region we can observe.) In fact, if the whole universe is infinitely large now, then it was always infinite, including during the Big Bang as well.
To put it another way, the current evidence indicates only that the early universe -- the WHOLE universe -- was extremely DENSE -- but not necessarily extremely small. Thus the Big Bang took place everywhere in space, not at a particular point in space.
Then where did the idea that the universe was once a point come from?
For much of the twentieth century, astronomers and physicists believed that space might NOT be infinitely large -- that is, space might actually curve around on itself to form a "closed universe." This unusual three-dimensional shape was discovered in the mid-1800's by the great mathematician Bernhard Riemann. The shape was later favored by Einstein as a possible shape for the universe. Such a closed universe would have a finite volume, yet no boundaries or edges. Although closed universes cannot be visualized from the outside, they CAN be visualized from the inside. For example, the image at right gives an idea of what a tiny closed universe might look like. (In a real closed universe, you cannot see the back of your head, the way you can here.)
If you shrink such a space down, then everything in it gets closer together, and the volume of the closed universe gets closer and closer to zero. But there is still nowhere OUTSIDE the space for an observer.
The view inside a simple kind of closed universe. If the universe were not expanding, you could see the back of your head. (Courtesy: U. of Minnesota)
Current evidence shows that our part of the universe appears not to be curved. This tells us that either the universe is infinitely large, or else is so large that we cannot detect its curvature from the tiny portion we can observe -- just as we could not tell that the Earth was curved if our measurements were confined to a sandbox!
If the universe started out so dense, why didn't it collapse into a black hole?
A large enough clump of matter will collapse to form a black hole, but ONLY if it is surrounded by (relatively) empty space. During the Big Bang, there WAS NO empty space: ALL of space was filled more or less uniformly with matter and energy; there was no "center of attraction" around which matter could coalesce. Under these circumstances, a cosmic-scale black hole will not form (and lucky for us!).
Why does looking out in space mean looking back in time?
Because it takes time for light from distant objects to reach us. We see the sun as it looked about 8 minutes ago... other stars as they looked years ago... and distant galaxies as they looked millions or even billions of years ago.
I've heard the expansion of the universe may be speeding up. Is there an "anti-gravity" force?
Current studies of distant exploding stars have led astronomers to conclude that the universe is not only expanding -- the expansion may be accelerating with time. This is not due to an "anti-gravity force" but rather to gravity itself. In fact, the effect was predicted as a possibility on the basis of Einstein's theory of gravity.
(It may seem strange that gravity can be "repulsive" as well as attractive. The secret is that the expansion applies to the fabric of space itself -- not to the matter within it; space behaves very differently from matter. For example, no chunk of matter can travel through space at the speed of light. Yet SPACE itself can expand faster than the speed of light. Similarly, while matter is attracted to other matter by gravity, space behaves differently: Space can either expand or contract as a consequence of gravity.)
MORE ABOUT THE BIG BANG
When they say "the universe is expanding," what exactly is expanding?
As bizarre as it may seem, space itself is expanding specifically, the vast regions of space between galaxies.
But if you can't see space, or feel it or touch it how can it be expanding?
According to Einstein, space is not simply emptiness; it's a real, stretchable, flexible thing. In fact, understanding the properties and behavior of space is a major goal of modern physics.
Why did anyone ever think that space should be expanding? Isn't it a far-fetched idea?
The notion that space is expanding is a prediction of Einstein's theory of gravity, which describes a simple but universal relationship between space, time, and matter. But it was a prediction that Einstein didn't believe; in fact, he tried to modify his theory to get rid of it.
Then how do we know that space really is expanding?
In the late 1920's, the astronomer Edwin Hubble first observed that distant galaxies are moving away from us, just as would be expected if the space between galaxies were growing in volume and just as predicted by Einstein's theory of gravity. Since then, astronomers have measured this recession for millions of galaxies. But there's other evidence as well.
Are the galaxies in the universe moving through space?
No, the galaxies sit more or less passively in the space around them. As the space between galaxies expands, it carries the galaxies further apart like raisins in an expanding dough.
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?
The universe is a chaotic place and the gravity from one galaxy, or from a group of galaxies, may disturb the motion of its near neighbors, causing them to collide. However, on average, when you compare two large enough chunks of space, the galaxies in one are moving away from the galaxies in the other.
Where did the Big Bang scenario come from?
If space (and everything with it) is expanding now, then the universe must have been much denser in the past. That is, all the matter and energy (such as light) that we observe in the universe would have been compressed into a much smaller space in the past. Einstein's theory of gravity enables us to run the "movie" of the universe backwards i.e., to calculate the density that the universe must have had in the past. The result: any chunk of the universe we can observe no matter how large must have expanded from an infinitesimally small volume of space.
How do we know when the Big Bang took place?
By determining how fast the universe is expanding now, and then "running the movie of the universe" backwards in time, using Einstein's theory of gravity. The result is that space started expanding about 15 billion years ago, give or take a few billion years. This number is uncertain, in part because of uncertainties in our current measurements of how fast the universe is expanding, how much matter and energy there is, and even what kind of energy there is in the universe.
Do we know where, in space, the Big Bang took place?
It's a common misconception that the Big Bang was an "explosion" that took place somewhere in space. But the Big Bang was an expansion of space itself. Every part of space participated in it. For example, the part of space occupied by the Earth, the Sun, and our Milky Way galaxy was once, during the Big Bang, incredibly hot and dense. The same holds true of every other part of the universe we can see.
Artists may find it more dramatic to draw a "fireball" expanding into space, but as far as we know, there would have been no such "ball."
How do we know there really was a Big Bang?
As mentioned above, we observe that galaxies are rushing apart in just the way predicted by the Big Bang scenario. But there are other important clues.
Astronomers have detected, throughout the universe, two chemical elements that could only have been created during the Big Bang: hydrogen and helium. Furthermore, these elements are observed in just the proportions (roughly 75% hydrogen, 25% helium) predicted to have been produced during the Big Bang. This prediction is based on our well-established understanding of nuclear reactions independent of Einstein's theory of gravity.
Second, we can actually detect the light left over from the era of the Big Bang. The blinding light that was present in our region of space has long since traveled off to the far reaches of the universe. But light from distant parts of the universe is just now arriving here at Earth, billions of years after the Big Bang. This light is observed to have all the characteristics expected from the Big Bang scenario and from our understanding of heat and light.
But I've heard on the news there are problems with the Big Bang theory. Is it still just a "theory"?
The Big Bang is actually not a "theory" at all, but rather a scenario about the early moments of our universe, for which the evidence is overwhelming. But the Big Bang scenario cannot be the whole story, and its details are a subject of intense research.
Was the Big Bang the origin of the universe?
It is a common misconception that the Big Bang was the origin of the universe. In reality, the Big Bang scenario is completely silent about how the universe came into existence in the first place. In fact, the closer we look to time "zero," the less certain we are about what actually happened, because our current description of physical laws do not yet apply to such extremes of nature.
The Big Bang scenario simply assumes that space, time, and energy already existed. But it tells us nothing about where they came from or why the universe was born hot and dense to begin with.
Are there theories that go beyond the Big Bang?
Yes, there are theories that build on the Big Bang scenario by adding insights from physics about the structure of space itself. Watch this space for more details.
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