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Name: Rick
Status: other
Grade: other
Location: FL
Country: USA
Date: Summer 2011


Question:
This week astronomers discovered the largest reservoir of water the farthest distance away in our universe. They say it is from a quasar some 12 billion light years away. That means we are seeing what it looked like 12 billion years ago when the universe was approximately 1.6 billion years old.

If the universe is expanding, not static, and it all started from a cosmic bang wherein everything was together at first, if light from the object takes 12 billion years to reach us, then how could we physically travel 12 billion light years apart in only 13.6 billion years?

Replies:
Rick,

It sounds like you might be confusing the difference between [objects moving in space] and [the expansion of space] - the arithmetic relationship you assumed between time and distance (which works for fixed space) is no longer correct with expanding space.

To help illustrate the difference, let me suggest a simple experiment. Take a balloon and blow it up just a little. Now draw two dots on the balloon with a marker. The dots are fixed on the rubber surface of the balloon. Now blow up the balloon all the way. The dots are now much farther apart on the balloon surface. Even though the dots are fixed on the balloon surface, they are now farther apart after blowing up the balloon. This is a good way to illustrate how objects in the universe can end up very far apart even without actually 'moving'. Watch the rate at which the dots separate too -- as you blow up the balloon, their relative distance grows larger faster and faster as the balloon expands.

In the example of the quasar, the light from the quasar is moving through space (from the quasar to us), but the space is expanding while the light is moving. Imagine drawing a line from one dot on the balloon to the other while you are blowing up the balloon... at first, the dots might be just an inch apart, but by the time the balloon is blown up, the line might be, say six inches long. Again, the dots haven't moved, but that's the importance of the expansion of the balloon. (and how the expansion of space behaves differently than fixed/non-expanding space.

You might conclude that, at some distance, the expansion of the universe will be faster than the speed of light -- and you would be correct! This creates a concept of the "observable universe" -- the farthest distance at which light could possible reach us. That distance is reported to be 40-90 billion light years across (with the quasar you mention being about 12 billion light years away). Of course, that also begs the question about how big is the universe beyond what we (here) can observe. But that is outside the scope of your question...

Hope this helps,
Burr Zimmerman


Rick,

Many of the laws of physics learned in class do not apply in the first seconds of the universe. At the level of individual particles (quantum physics), things can travel faster than the speed of light. Near extreme masses, such as black holes, a curved path can be the shortest path between two points. Even without these difficulties, it is possible for the light to take 12 billion years to reach us.

First, realize that almost everything traveled extremely fast for quite some time, essentially the speed of light. The quasar releases the light more than 12 billion years ago. We and the quasar are moving apart. At one time, we were separating at almost the speed of light. This was true for many billions of years. As we slow down, the light finally reaches us. If someone throws a baseball to you when you are running away from that person, the ball takes longer to reach you than if you were not moving. If you travel at the ball's speed, it will never reach you.

Dr. Ken Mellendorf
Physics Instructor
Illinois Central College


I am not familiar with the observations you mention. The age of the Universe (best current estimate is ~13.6 billion years). These estimates are based on the shift in the frequency (or wavelength) of the source of the radiation compared to its frequency (or wavelength) of the same source in the Earth-bound lab -- called the Doppler shift. This "connects" the age of the origin of the source. In this case ~12 billion years. These calculations, after various "corrections" have been applied, assume the speed of light in a vacuum is constant. Consequently, the "age" and the "distance" are coupled, that is, relative. So "Age" is "Distance" and "Distance" is "Age". We did not travel while the rest of the Universe stood still. The entire Universe expanded. So the age/distance are correlated with one another, that is, they are relative.

Vince Calder


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