Distance, Time, and Expanding Universe
Date: Summer 2011
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?
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
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,
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
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.
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Update: June 2012