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Message: <blockquote data-quote="fuindordm" data-source="post: 2388018" data-attributes="member: 5435">Olber's ParadoxThe name of that astronomer was Olber.  He postulated that if the universe were infinitely large and infinitely old, then every line of sight from a given point into space would eventually hit the surface of a star, and therefore the night sky would be as bright as the sun.You could counter that by arguing that a line of sight could end on other things such as planets, space dust or whatnot, but if the universe were infinitely old then such things would have had time to come to thermal equilibrium with all the stars and grow just as brightly.Olber's paradox was a serious concern up until the expansion of the universe became widely accepted in the late 20's, but it was still something that steady state universe proponents had to contend with in their theories up until the 90's.Once you give the universe (or at least that phase of the universe where matter has formed hydrogen and other light elements) a finite age, then Olber's paradox is doubly resolved by the appearance of a 'horizon' (the distance that light could have traveled to us from the time the universe became transparent) and by the redshifting of light that has traveled great distances down to the bottom of the electromagnetic spectrum.So at least in regards to our own universe, we know that it is (in its currently recognizable form, at least) finite in time.  We don't know this because of Olber's paradox, but because of three independent observational paths: the relative proportions of light elements in the universe conforms to numerical simulations of nuclear reactions in an expanding and cooling universe; the redshift of spectra from all recognizable objects in the universe is proportional to their distance, confirming a uniform expansion of the universe; and the cosmic microwave background is observed in all directions to have a completely uniform temperature and spectrum down to one part in 10 000, confirming that at one time the universe was a hot plasma in thermal equilibrium with itself.  All this adds up to the Big Bang theory, which starts the universal 'clock' at about 13 billion years ago, at a time when the universe consisted of a hot, dense, plasma of elementary particles.Note that we don't know whether this ball of plasma was finite or infinite.We also don't know what happened before that time.  Inflation is a good idea, but doesn't add much to the basic picture other than resolving some observational issues that appear to be very puzzling coincidences. (the uniformity of the microwave background over distance much larger than the horizon, the density of the universe being very close to the critical density needed to stop the expansion, etc.)Heck, we don't even know what most of the matter in the universe is.  That question made it into the top ten of the 125 mentioned earlier, I believe.The universe offers plenty of infinities, for all that: it might well be infinite in extent, and there might well be an infinite number of universes out there; some popular and untestable theories postulate an underlying universe of infinite extent and very high energy, which is just a uniform quantum field of the most fundamental particles... but the uncertainty of quantum mechanics pushes small regions over the threshold of a phase transition from time to time, and these 'bubbles' condense into a recognizable universe such as our own.The amusing thing is, that since the underlying field is expanding exponentially, and a universe made out of matter expands only as a power law (as time to the 1/2 or 2/3 power, depending on whether matter or energy is dominant), then in the frame of reference of the underlying quantum field all these 'Big Bangs' would look more like little bubbles quickly shrinking out of existence!However, on Earth, at the current time, the volume of space accessible to us through observation and the number of stars therein is decidedly finite.Cheers,Ben</blockquote>

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