Big Bang, 4 December 2013 !Opening question: Does the universe have a center of mass?!1.As far as astronomers know, no. The current concept is that the big bang A.included everything, and everything in the universe was a part of the big bang. "Center of mass" would not be an appropriate term in this conception, though conceptions often change. Regardless, any center of mass would probably be unknowable by present means.!Measuring distance in astronomy: !2.For objects within 1 light hour, astronomers can use radar.!A.For objects up to around 200 light years, astronomers can triangulate using B.the diameter of the earth's orbit. !For objects up to a few thousand light years, astronomers can fit stars to C.the main sequence.!This is assuming the main sequence is correct everywhere. !a.For objects within 70 million light years, astronomers use cepheid variables.!D.Beyond that, astronomers can use type 1a supernovae.!E.This is probably not a good means: the Tully-Fisher is very flimsy. !a.Beyond even that, all astronomers have Hubble's law. !F.This is probably not a good means. It is circular reasoning to think that a.the universe is expanding at the rate astronomers see because of Hubble's Laws, because Hubble's laws are based on the rate of expansion that astronomers see. !It is like saying astronomers are seeing red shift and therefore they 1.think they are seeing a red shift. !Hubble's law: the redshift of galaxies receding from earth is proportional to 3.their distance from earth.!The light from everywhere in the universe beyond our local cluster of A.galaxies is red shifted.!To review: redshift occurs when a source of light is moving away from an a.observer, so that the observer witnesses less waves per second than the emitter emitted. This makes the wavelength longer, so the light appears to be farther toward the red end of the spectrum. !This is an indisputable fact. !b.The farther away things are, the faster they are moving away. !B.This is approximately true, but there was a long debate over whether it a.was exactly true. !Over the years, the holes in this theory have been filled in, and a exact b.rate for the expansion of the universe has been confirmed by two independent means.!The universe is expanding at 71 kilometers per second per 1.megaparsec (million parsecs).!A megaparsec is 3.26 lightyears.!A.The parsec unit is only useful for this calculation in this course. !a.At each block of that distance things are moving away from us at an c.added rate of 71 km per second.!The faster things are moving away, the more red shifted they are. !C.When an object is red shifted, the spectral lines of its spectra are shifted a.towards the longer red end of the spectrum and they are spread farther apart.! This is the only means for reckoning distance in the farthest parts of the D.universe. !Hubble's law was first proposed in the 1920s and 30s by Edwin Hubble. E.Understanding it occupied most of the 1900s, but it has been pinned down in the last few years. !The Big Bang Cosmology: !4.A cosmology is a way to describe the origin and evolution of the universe. !A.There are about 100 proposed cosmology, 70 of which are versions of a.the big bang.!The idea of the big bang came from two people who lived during different B.times and had different attitudes. !Abbé Georges Lemaître (1894-1966)!a.Lemaître was a Belgian abbot with a phd in astrophysics who was 1.trying to find a cosmology that was consistent with both Christianity and astrophysics.!This was his motive. Whether he was right or not is separate. !A.When he was doing his work in the 1930s and 40s, Hubble had just B.announced that the universe was expanding. ! If you can see the universe expanding from what you know, you can 2.conclude the universe was smaller before. You can trace this back farther by drawing a straight line extrapolation. !The black is the fundamental graph of cosmology: !1.Time runs to the right.!2.Size increases upwards. !3."NOW" is what the universe looks like now, and 4.the black diagonal line the time covered by the stuff astronomers can see from far away. !The blue line is Lemaître's straight line 5.extrapolation. SIZETIMENOWwhat astronomersknowEXTRAPOLATIONFUNDAMENTAL GRAPH OF COSMOLOGYThis is a straight forward explanation: If the universe was A.expanding now, it was expanding in the past. !This is based on physical principles of conservation of a.momentum. !Making a straight line extrapolation is the first thing any b.theoretician should do. !Lemaître extrapolated that everything in the universe was in one B.place at the earliest knowable time. He called this the primeval egg. After that, the universe started expanding, resulting in the universe today. !This posed a bit of a problem because everything in the universe a.being in the same place means a black hole, and they were not known at the time. !People started theorizing about black holes in 1967, a year 1.after Lemaître died. !Also, how could anything come out of a black hole? !2.People could only extrapolate back to a little bit after the 3.beginning of the universe. !This "straight big bang" was consistent with everything known about 3.cosmology in the 1930s-60s. !In the 1960s, many versions of the theory were hatched. !A.George Gamow (1904-1970)!b.Gamow grew up in Soviet Russia, but escaped to Turkey in the 1930s. 1.He later earned professorships at american universities. !He was a world class physicist and astronomer with a good sense A.of humor. !Gamow came up with his own version of the Big Bang with a similar 2.result, but a different line of reasoning. !He called the "primeval egg" the "ylem."!A.The particle physics of the condense universe was different in his B.version.!The expansion of the Big Bang: !C.The universe must have been hot at the beginning and cooled off as it a.expanded. !It probably took most of a billion years for the universe to cool off 1.enough that ices and tars could get together.!In the mid 1970s, astronomers realized that the expansion of the Big b.Bang is not uniform: !There was a period where it was much stronger, called "inflation."!1.A lot of nuclear physics happened in the first few seconds of the Big 2.Bang to cause this, which we will not have to memorize. !The details of this physics are frequently debated and change all the A.time. !One set of theories holds that the Big Bang caused present day c.expansion, leaving embers behind. !Gamow did not find these embers, but they were discovered by 1.accident