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The Question

(Submitted June 30, 1997)

I'm a college graduate with a degree in computer science. However, my favorite pastime has always been reading about astronomy, quantum mechanics, etc. that's my background. My question is:

When astronomers speak of the estimated size of the "known Universe", are they setting this distance (from us) based upon the furthest visible object, or upon calculation? This is in reference to the fact that quasars (as far as I know) are the furthest observable objects. Yet they travel at speeds approaching that of light away from us. Obviously, if there was anything further than the distance at which the expansion of the Universe = c, it would be impossible for us to detect it, now or ever. To sum up the question: how can one estimate the size of the Universe if any part of it past this critical distance is forever cut off from our measurement? One could argue that since we cannot ever reach these locations, for us they do not exist, but I think that's a horrible cop-out.

The Answer

What astronomers mean when they speak of the "known Universe" depends on the astronomer. Most often it refers to the region of the Universe from which light could travel to us since shortly after the Big Bang.

The farthest observable discrete objects are the quasars (visible at such great distances because they are so bright). However, the cosmic microwave background radiation, at 3 degrees Kelvin, comes from even further away. It has a redshift of about 1000, and comes from the time when the Universe was much smaller, and filled with hot ionized gas (plasma) at 3000 Kelvin, as hot as the surface of some stars. Dense plasma blocks light, and so we cannot see anything beyond that distance.

If the theory known as "inflation" is true, the size of the "known Universe" is much smaller than that of the Universe as a whole. If you look at the "known Universe", every part of it looks about the same, as far as we can tell. As an analogy, if you look at a typical cornfield in Kansas, it all looks the same as far as the eye can see. For there to be as much variety as you would expect in a world, the world has to be much larger than the size of a Kansas cornfield. Likewise, inflation says that the Universe is much larger than the known Universe.

How much larger is hard to determine, and theories are untrustworthy since we can never confirm them by observations. (Actually, 'never' is a bit of an overstatement. If you waited long enough, the Universe would slow its expansion and you may be able to see a bit further. But that would take billions of years.)

For more information on inflation, look at the references on

David Palmer
for Ask an Astrophysicist

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