Chapter 26 - Physics 1902 - Cosmology Lecture 11a,b Chapter...

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Unformatted text preview: Physics 1902 - Cosmology Lecture 11a,b Chapter 26/27 a) There will be one more WebCT quiz before the end of term: deadline will be the last day of classes. b) the final exam (13th April) will be the same format as the midterm. 30 (instead of 20) multiple choice questions and 5 out of 8 short answers. It will concentrate on the material from Chap 21 onwards: however you should review all of the material on stars, since it is required to make sense of the later material. c c) Next week the course will be given by Dr. David Sinclair, who taught the course last year. I will be available by Email during this time and I will have office hours on the evening of Monday 12th, to answer any questions you may have. Adam will be available during this time. d) Midterms can be collected from my office Monday and Wednesday afternoon. Olbers Paradox Why is the sky dark at night? One of the most profound observations in Cosmology is known as Olbers Paradox If universe is static and uniform then if we look far enough in any direction we see a star The night sky should be as bright as the surface of a star But it isnt! Apparent Ways out: Obviously universe is not uniform for stars But it is for galaxies! Light from stars falls off as inverse square law I ~ 1/r 2 But the number of stars increases as r 2 so the effects cancel Absorption by interstellar matter dims distant stars But the matter would by now be hot and radiating Light is redshifted to very long wavelengths so objects within ~ 3000 Mpc will have any visible radiation at all But this implies that the universe must be expanding Very distant objects would correspond to an age of more than 10 billion years No reason why the universe should be the same then Hence an expanding universe with a beginning is almost required by Olber's Paradox: it cannot be infinite in both space and time. What can we say about large scale matter distributions On scales of >~200 Mpc we see uniform galaxy distributions On scales >~200 Mpc we see uniform hydrogen from Lyman Thus space is homogeneous at this scale- i.e. it is smooth Space is isotropic at this scale i.e. there is no preferred direction Note homogeneous and isotropic are different SDSS Narrow field red shift distribution Found in 1920's (Hubble, Humason, Slipher) that faint galaxies are receding from us: fainter the galaxy, faster the recession. The Age of the Universe Hubbles Law states that recessional velocity V = H D H =70 km/s/Mpc Big Bang (once over lightly) Note although all galaxies are receding from us, does not imply we are at the centre: in the currant cake model all currants see all the others as receding. Big Bang (once over lightly) RULE 1 in Physics 100: Never mix your units!...
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Chapter 26 - Physics 1902 - Cosmology Lecture 11a,b Chapter...

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