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Unformatted text preview: HOT Big Bang HOT Big Bang Tuesday, January 22 Hubble’s law: Hubble’s law: Galaxies have a radial velocity (v) proportional to their distance (d). Hubble’s law Hubble’s law in mathematical form: in mathematical form: d H v = v = radial velocity of galaxy d = distance to galaxy H = the “Hubble constant” (same for all galaxies in all directions) What’s the numerical value of H ? What’s the slope of this line? → H = 70 kilometers per second per megaparsec (million parsecs) H = 70 km / sec / Mpc Or, more concisely… Why it’s useful useful to know H : Measure redshift of galaxy: (λλ )/λ Compute radial velocity: v = c (λλ )/λ Compute distance: d = v / H Cheap, fast way to find distance! violet red galaxy spectra galaxy images With modern telescopes and spectrographs, astronomers have measured millions of spectra. Kilometers per second per megaparsec?? What bizarre bizarre units! 1 megaparsec = 3.1 × 10 19 kilometers sec / 10 3 . 2 km/M pc 10 3.1 km/sec/M pc 70 H 18 19 × = × = sec 10 4.4 1 H 17 × = Why it’s intriguing intriguing to know H : Two galaxies are separated by a distance d . They are moving apart from each other with speed v = H d . d sec 10 4 . 4 H 1 d H d t 17 × = = = How long has it been since the galaxies were touching? speed distance time travel = PLEASE NOTE: PLEASE NOTE: This length of time (t = 1/H ) is independent of independent of the distance between galaxies!!...
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 Winter '08
 RYDEN
 Galaxies, Big Bang, Redshift, Hubble

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