h2g2 The Hitchhiker's Guide to the Galaxy: Earth Edition

A bit that I was reading last week said that the discovery of the background radiation proved the Big Bang; that it was an echo of sorts. Wouldn't a steady state universe also result in such a radiation?
Someone asked the question 'Why is the night sky black? If space is filled with an infinite number of stars, wouldn't it be aglow with their combined light?'
Looking at the sky with the eyes that we have it seems to me that black is the colour you'd expect. Space isn't empty after all. There's all sorts of dust and such so it stands to reason that the only stars we'd see would be the ones closest to us. The rest would have their radiation absorbed eventually by the interstellar stuff and if energy is conserved then that stuff must re-emit it but at a lower temperature. You would expect to see just what we see which is an even glow-invisible to our eyes owing to the long wavelength-coming from every direction.

I don't see any reason why a Steady State Universe would have any background radiation, unless it had always been there.

The black night sky is known as Olber's Paradox. If the universe is infinite and stationary, then the night sky would be as bright as the sun over all its area. Dust absorbs light and heat from stars, but will heat up and will radiate as much energy as it absorbs. So there fact that there is dust in the way will not reduce the amount of light reaching you (in an infinite stationary universe).

The fact that the night sky is black means that the universe is finite or expanding or both.

Yes, that's my point. It will radiate as much energy as it absorbs but it will do that at wavelengths corresponding to the temperature of the interstellar dust etc., wavelengths much longer than what we can see with our eyes but which will be seen with radio telescopes. Meanwhile, the dust obscures the most distant stars so Olber's Paradox is resolved.

In any case, I'm not arguing for a steady state universe, my question was why is the background radiation used as proof for the other theory.

Just as I hit 'post' I realized what is wrong with my argument

Even assuming that the dust reemits the energy there would have to be an ultimate sink for that energy or the sky would just keep getting hotter. The only way out of that would be to find a way to turn that energy back into matter. Of course, if you could do that you could provide the matter needed to make up for what the black holes eat up.

Then you'd have a steady state universe. Except to explain that pesky red shift.

The term "Steady State Universe" is generally used to mean the Universe modelled by astrophysicist Fred Hoyle - expanding but eternal. Hoyle envisaged that as the galaxies got further from each other, new matter was created by some unexplained process in the gaps in between, and that this could go on for ever.

So "Steady State" doesn't necessarily mean "unchanging", just that the changes are local and the universe as a whole stays the same forever.

Thanks for that Gnomon, I learn a new thing every day.

There was a group of people, who tried to calculate how hot the universe should be, if it had expanded from a point. The universe should originally have been very hot, but as it expanded it would have undergone an adiabatic cooling of sorts. So they predicted that there should be a general temperature today of about 5 Kelvin above absolute zero. Later another group of people found a strange disturbance in their radio telescope, a constant background noise seemingly coming from everywhere. This radiation was consistent with black body radiation from an object about 3,5 Kelvin above absolute zero. It also showed an incredible uniformity, no matter where they looked. It was then realized that this was consistent with the predictions made by the first team, and helped support the Big Bang theory.

Incidentally what you´re talking about is usually known as the Cosmic Microwave Background Radiation. the term Background Radiation in itself usually refers to the kind of radioactivity you get from the surrounding bedrock in any given area, such as the uranium in granite.