Expert: Jayendra Upadhye - 4/10/2006

Question
Hi!
I was doing some research on the expansion of the universe. And I came across an article that had a paragraph that read:

"After the rise of modern astronomy, another paradox began to puzzle astronomers. In the early 1800s, German astronomer Heinrich Olbers argued that the universe must be finite. If the Universe were infinite and contained stars throughout, Olbers said, then if you looked in any particular direction, your line-of-sight would eventually fall on the surface of a star. Although the apparent size of a star in the sky becomes smaller as the distance to the star increases, the brightness of this smaller surface remains a constant. Therefore, if the Universe were infinite, the whole surface of the night sky should be as bright as a star. Obviously, there are dark areas in the sky, so the universe must be finite."


I didn't quite understand why the "surface of the night sky should be as bright as a star" if the Universe were infinite. If you could please explain why that would happen it would be much appreciated.

Thank You for your time,
Eric Snyder


Answer
Hi Eric,
In fact there was this nice html site with a java application on it.
What Olbers suggested was that if the universe (finit or infinite) has a "proverbial star" at every point, then technically looking anywhere in any direction, one will be looking "directly" at a stellar surface!

Theoretically, the temperature of absolute space would in such a universe rise to the mean temperature of stellar surfaces!

All of spave would be awash in a radiance at a temperaure approx 6000 deg celcius. [black body].

But the absolute temperature of space is at about 3 deg kelvin. [temp of the cosmic background radiation].

And the universe is observed dark.

Scientists say that is precisely because it is an expanding universe, with more voids and less matter!

Secondly, due to the expansion, there has not been time ENOUGH for ALL space to heat up to the thermal equilibrium temperature of stellar surfaces! [In a closed universe, that would be expected over time].

the "surface of the sky" is a word play to refer to the "spherical sky" one observes when one looks "up"!

Take that with a pinch of salt.

Olbers wanted to say the universe is dark and cold because it is finite and hence doesnot have stars in all possible locations!

It turned out otherwise! It is cold and sark chiefly because it is ALMOST INFINITE and GROWING!

Hope that suffices.
Jayen