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## EXPANDING UNIVERSE

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**“The night sky is dark.” This statement is called**Olbers’ paradox, after astronomer who discussed the subject back in 1823. Why is the darkness of the night sky so paradoxical?**If stars were stuck on a celestial sphere or dome, darkness**would not be paradoxical. There are only a finite number of stars on the celestial sphere.**In an infinite universe with infinite number of stars, a**paradox arises. Let’s see how bright we expect the sky to be in such a universe?**Olbers’ Paradox for Trees and Stars:**In a large enough forest, every line of sight ends at a tree. In a large enough universe, every sight line ends in a star. The sky should not be dark.**Infinity times any finite number, no matter how tiny, is**infinity. The number of stars goes up at the same rate that the light from each goes down. Leading to Olbers conclusion that the night sky should be infinitely bright. Something must be wrong with one or more of the assumptions!**HUBBLE LAW**Galaxy spectra show redshifts, where all the spectral features shift to longer wavelengths. The amount of the shift increases with distance: more distant galaxies are moving away faster. This linear relation was discovered by Edwin Hubble back in 1929.**All distant galaxies have redshifts. (They are moving**away from us.)**Geshe Activity**Let’s simplify the situation: the universe is like the curved surface of an expanding balloon. Draw 10 galaxies on the balloon. Pick a home galaxy. Inflate balloon to 2 different sizes andmeasure distances from home to the other 9.**Galaxies are all moving away from each other, so every**galaxy sees the same Hubble expansion, i.e. there’s no center. • The universal expansion results from the unfolding of all of space since the hot big bang, • i.e. there is no edge.**The redshift is not a Doppler shift; it is due to the**expansion of space itself. Photons are stretched.**Hubble’s law in mathematical form:**v = radial velocity of galaxy d = distance to galaxy H0 = the “Hubble constant” (it is the same for all galaxies in all directions)**What’s the numerical value of H0?**What’s the slope of this line? →**Why it’s importantto know H0:**d If two galaxies are separated by a distance d. They are moving apart from each other with speed v = H0 d.**Basis of the “big bang” concept:**At a time in the past (t ≈ 1/H0), the universe began in a very dense state. 1/H0, called the “Hubbletime”, is the approximate age of the universe in the Big Bang Model.**Expansion explains Olbers paradox:**If the universe has a finite age, then the most distant stars haven’t had time to send us the light message “We’re here!”