10/29/2010AST210, Fall 2010 1Hubble’s Law29 Oct• Hwk6, Q2. 1mo=.08yr.• Hubble’s Law describes how galaxies move and how the universe expands.• Objectives: To answer and give evidence for these questions.– Was there a Big Bang?– Galaxies move away from us in all directions. Are we at the center of the Big Bang?– Why are galaxies moving?– How long ago did the big bang occur?Hoag’s GalaxyD=900MLyv=18,000km/sComa ClusterD=300MLyv=6000km/s10/29/2010 Ast 207, F2010First Hubble Diagram• V M Slipher, Lowell Observatory, pioneered the measurement of the Doppler velocities of galaxies.– Some spectra took several nights.• Hubble measured / estimated distances of galaxies.– Not measurements by parallax. Indirect. Very complicated. • Hubble 1929, Proc. Nat. Acad. Sci. 15, 168• Summarize Hubble’s plot. What is the main finding?Edwin Hubble 1889-195310/29/2010 Ast 207, F201010/29/2010AST210, Fall 2010 2How do galaxies move?• They move according to Hubble’s Law.• Answer these questions by analyzing the motion.– Was there a Big Bang?– Galaxies move awa y from us in all directions. Are we at the center of the Big Bang?– Why are galaxies moving?– How long ago did the big bang occur?• Analyze the motion of 3 galaxies.10/29/2010 Ast 207, F2010First Hubble Diagram• Hubble 1929, Proc. Nat. Acad. Sci. 15, 168• Summarize Hubble’s plot. What is the main finding?• The speed of a galaxy is proportional to its distance. Almost every galaxy is moving away from us.– V = H D– H is called Hubble’s constantEdwin Hubble 1889-195310/29/2010 Ast 207, F201010/29/2010AST210, Fall 2010 3Milky Way Galaxy10/29/2010 Ast 207, F2010NGC4881, central galaxy in Coma Cluster10/29/2010 Ast 207, F201010/29/2010AST210, Fall 2010 4Hoag’s Galaxy10/29/2010 Ast 207, F2010Motion according to Hubble’s Law• Hubble’s Law: Velocity V is proportional to distance D • Demo: Let Coma & Hoag’s Galaxy move according to Hubble’s Law.– Move forward in time. Note relative spacing.– Move backward in time. Note relative spacing.1. If Coma moves one meter, how much should Hoag move?a. 1 mb. 3 mc. 1/3 md. 9 me. 1/9 mSpeed DistMilky Way 0 km/s 0 MpcComa 6,000 km/s 100MpcHoag’s Object 18,000 km/s 300Mpc10/29/2010 Ast 207, F201010/29/2010AST210, Fall 2010 5Hubble’s Law• Velocity V is proportional to distance D– V = H D• Demo: Let Coma & Hoag’s Galaxy move according to Hubble’s Law.– Move forward in time. Note relative spacing.– Move backward in time. Note relative spacing.– Move backward so that Coma and MW are coincident.1. If Coma moves one meter, how much should Hoag move?a. 1 mb. 3 mc. 1/3 md. 9 me. 1/9 mSpeed DistMilky Way 0 km/s 0 MpcComa 6,000 km/s 100MpcHoag’s Object 18,000 km/s 300Mpc10/29/2010 Ast 207, F2010Hubble’s Law• Hubble’s Law • What form is the expansion?2. Hoag is 3 times as far as Coma. Is this still true in the future? Was this true in the past? a. YYb. YNc. NYd. NNSpeed DistMilky Way 0 km/s 0 MpcComa 6,000 km/s 100MpcHoag’s Object 18,000 km/s 300Mpc10/29/2010 Ast 207, F201010/29/2010AST210, Fall 2010 6Self similar expansion• Hubble’s Law 2. Hoag is 3 times as far as Coma. Is this still true in the future? Was this true in the past? a. YYb. YNc. NYd. NN• Motion according to Hubble’s Law is self‐similar. Relative distances are preserved. A special type of expansion.• Is the “expansion” of cars leaving a football game self similar? Consider Joe, Laura, & Nancy. Joe parked near S Stadium. Laura parked at lot on Farm La. Nancy parked on Grand River.Speed DistMilky Way 0 km/s 0 MpcComa 6,000 km/s 100MpcHoag’s Object 18,000 km/s 300Mpc10/29/2010 Ast 207, F2010• Now LaterEvidence of Big Bang• Hubble’s Law • Move backward so that Coma and MW are coincident.• Where is Hoag’s object?• All three galaxies were close at the same time.• Since these three are not unique, we have shown this is true for every galaxy .• Everything was very close at the same instant.• H’s Law Universe began in a Big Bang– Universe was very dense– What became Milky Way was very close to what became Coma & Hoag’s Galaxy & everything else.Speed DistMilky Way 0 km/s 0 MpcComa 6,000 km/s 100MpcHoag’s Object 18,000 km/s 300Mpc10/29/2010 Ast 207, F201010/29/2010AST210, Fall 2010 7Is Hubble’s Law’s valid for Coma?3. If we are astronomers on some planet in Coma, would H’s Law apply? (All guesses are OK.)a. Yb. N• Do the demo.3. If we are in Coma, would H’s Law apply?a. Yb. N• Hubble’s Law applies everywhere.Speed DistMilky Way 0 km/s 0 MpcComa 6,000 km/s 100MpcHoag’s Object 18,000 km/s 300Mpc10/29/2010 Ast 207, F2010Value of H implies age of unive rse• Write H’s law in more familiar form 1/which is the same idea as ___.• 1/H = D/V= (470Mpc) / 30000km/s (1pc=3e13km)= 15Byr (1yr=3e7s)Hubble Diagram 2003Galaxy that moves at 30000km/s is 470Mpc from us10/29/2010 Ast 207, F201010/29/2010AST210, Fall 2010 8Why do galaxies move?• Write H’s law in more familiar form 1/which is the same idea asdistance = speed ×time.• Some matter that was very near us soon after the Big Bang was moving at 30,000km/s.• The age of the universe is 15 Byr.– In 1.5 Byr, that matter moved 47 Mpc from our primordial location.– In 15 Byr, that matter has moved 470 Mpcand become part of a galaxy . We became MSU students and part of the solar system.• Be aware: V is the current velocity. We assumed matter does not speed up or slow down.Hubble Diagram 2003Galaxy that moves at 30000km/s is 470Mpc from us10/29/2010 Ast 207,
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