Ole Bjælde Actually, that is not the case, Aristotle. It doesn’t make sense speaking of an infinitely old, infinitely large universe. How do we know? One argument is the dark night sky at which you have often gazed. In an infinitely old universe, the night sky shouldn’t be dark but full of starlight due to the endless amount of stars, whose light have had infinite time reaching us (read more). If the universe is not infinitely old, then light from very distant stars has not yet had enough time to reach us, which explains why the sky is not lit by stars every night as shown in this graphic:

Dead Aristotle The dark night sky would also be explained by an infinitely old universe, but with finite size, right?

Ole Bjælde Such a universe would explain it, too. But we have other reasons believing it is not infinitely old. Take, for instance, the redshift of light from Galaxies, which is proportional to distance. Redshifted light is light which wavelength is increased. This lengthening of the wavelength would be explained by an expanding universe. If the universe is expanding, it must have started somewhere.

Ole Bjælde You will be introduced to another line of evidence called the Cosmic Microwave Background Radiation, later.

Photon What?

Electron We’ve been living together now for what... 380,000 years?

Photon And it’s been awesome, right?

Nucleus Yeah… You’ve always been the more energetic of us. At least in terms of kinetic energy… You know…

Photon I knew it… You guys don’t like me! (Crying smiley)

Electron Nooo no no! It’s nothing like that… Ehm Nucleus? [(Nervous smiley) Hvis det findes?]

Nucleus Yeah... uhmmmm... Electron-and-I-have-been-seeing-each-other-behind -your-back-k-thx-bye!

Ole Bjælde it’s meant to be a painting of the Cosmic Microwave Background Radiation. The colors indicate the temperature at given positions in the sky. Although they look as though they vary a lot, the temperature is actually impressively uniform. Look at this graph. The graph shows the distribution of wavelengths emitted by a blackbody object with a temperature of 2.73 K (the blackbody spectrum is explained by Photon's first post!). The dots on the curve is the actual wavelength distribution from the CMB – including their uncertainties!

Sceptic Wow Ole. That's a beautiful interpretation? Arts major?

Ole Bjælde Sort of… ;)

Sceptic Is this related to the Horizon Problem mentioned earlier?

Ole Bjælde It is! The CMB is the farthest we can look back in the universe (it is called the last scattering surface). The photons creating the CMB were scattered 380,000 years after the Big Bang (you might have noticed Photon's post back then?). Before that, the universe was not transparent (it was a blackbody), therefore we cannot look further back. Because the CMB is this far away, two antipodal points would not have been able to interact. How could these points then end up with the same temperature? Inflation is one possible explanation.

Dead Aristotle Was this evidence of a beginning universe, too?

Ole Bjælde Great, you remembered it! Yes, the CMB is explained by a model in which the universe was initially very hot and dense, producing blackbody radiation, but expanded so that the universe became transparent and the blackbody radiation cooled.

Dead Aristotle I think i get it, now..