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MSU AST 207 - Hubble’s Law

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1Hubble’s Law—17 Oct• Hubble’s Law• Universe is expanding• Universe started with a Big Bang• Age of the universe• Intro; What are galaxies?Hoag’s GalaxyD=900MLyv=18,000km/sComa ClusterD=300MLyv=6000km/sFirst 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? (3min)Edwin Hubble 1889-1953First 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 1889-1953Hoag’s Galaxy2NGC4881, central galaxy in Coma ClusterMilky Way GalaxyHubble’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 m300Mpc18,000 km/sHoag’s Object100Mpc6,000 km/sComa0 Mpc0 km/sMilky WayDistSpeedHubble’s Law• Velocity V is proportional to distance D•V = H × D2. 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. NN300Mpc18,000 km/sHoag’s Object100Mpc6,000 km/sComa0 Mpc0 km/sMilky WayDistSpeed3Hubble’s Law•V = H × D2. Hoag is 3 times as far as Coma. Is this still true in the future? Was this true in the past? YY. • 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.• Current physics can explain universe 10-10s after Big Bang, when proto-Coma was 1 mm from proto-us.300Mpc18,000 km/sHoag’s Object100Mpc6,000 km/sComa0 Mpc0 km/sMilky WayDistSpeedHubble’s Law•V = H × D3. If we are in Coma, would H’s Law apply?a. Yb. N300Mpc18,000 km/sHoag’s Object100Mpc6,000 km/sComa0 Mpc0 km/sMilky WayDistSpeedWhat does value of H imply?•V = H × D4. What is the value of Hubble’s constant? Express your answer in km/s/Mpc•H=V/D= 30000km/s/(470Mpc)= 64km/s/Mpc• Change Mpc to kmH = 64km/s/(3×1019km)= 1/(15Byr)Hubble Diagram 2003Galaxy that moves at 30000km/s is 470Mpc from usWhat does value of H imply?• Write H’s law in more familiar formD = V × (1/H)1/H is the time for an object moving at speed V to move distance D.•1/H = D/V= (470Mpc) / 30000km/s= 15Byr5. Some matter that was very near us soon after the Big Bang was moving at 30,000km/s. How far has it moved in 1.5 Byr? In 15 Byr?Hubble Diagram 2003Galaxy that moves at 30000km/s is 470Mpc from us4Value of H implies age of universe• Write H’s law in more familiar formD = V × (1/H)•1/H = D/V= (470Mpc) / 30000km/s= 15Byr• 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 Mpc and become part of a galaxy. We became MSU students and part of the solar system.• Be aware: We assumed matter does not speed up or slow down.Hubble Diagram 2003Galaxy that moves at 30000km/s is 470Mpc from usSummarizing questions• Why does Hubble’s Law imply a Big Bang?• Do aliens on another galaxy also observe galaxies to move according to H’s Law?• If the motion of matter slows down, is the age of the universe longer or shorter than


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MSU AST 207 - Hubble’s Law

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