The History and Philosophyof Astronomy (Lecture 24: Hubble II)Instructor: Volker BrommTA: Amanda BauerThe University of Texas at AustinAstronomy 350L (Spring 2005)Edwin P. Hubble: Mariner of the Nebulae • 1889 (Missouri) – 1953 (Pasadena)• leading observationalastronomer of 20thcentury:- discovers galaxies (1924): Milky Way but one ofinnumerable “island universe”- expansion of the universe (1929): Hubble’s Law:v = H0 x dMount Wilson Observatory• 1919 onwards: world’s largest telescopeMount Wilson ObservatoryGeorge Hale (1868-1938)100-inch Hooker TelescopeClassification of Nebulae (=galaxies)• 1923: Tuning-fork diagramBig Q: How do the Spiral Nebulae Move? • figure out radial velocities by taking spectra,and measure the shift of spectral lines  Doppler effect!The Doppler Effect (1842) blueshiftredshiftChristian Doppler (1803-53)The Doppler Effect for Spectral Lines • Calculate radial speed:• By measuring the shift in wavelength of spectrallines, one can figure out radial velocity of sourcewith high precision!The Flight of the Spiral Nebulae (1912) Vesto M. Slipher (1875-1969)• Important discovery: mostspiral nebulae displayredshifts in their spectra!• From Doppler effect: Do spiral nebulae move away from us???Solving Einstein’s Equations of GR• 1917: Einstein constructs model of the universethat is eternal and static • finite but without boundary (spatially closed)• balance betweenattractive gravityand repulsive cosmological constant(“anti-gravity”)Solving Einstein’s Equations of GR• 1917: de Sitter constructs a model of the universethat contains no matter, but predicts motion! • an empty universe!Willem de Sitter (1872-1934)Solving Einstein’s Equations of GR• 1917: de Sitter constructs a model of the universethat contains no matter, but predicts motion! origin• particles are “scattered away” from origin (“de Sitter effect”)-- the larger the distance, the larger the apparent speed!Hubble and the Distance to Andromeda (M31) • Hubble discovers a Cepheid variable in Andromeda!• October 1923: He obtains photograph of M31Brightness vs timeWhat are the Cepheids? • A: Pulsating stars (periods of ~ few days)Cepheidsas Standard CandlesCepheidsas Standard Candles 1) Measure Period3) Calculate distance2) Calculate luminosity • inverse-square law:flux=luminosity/distance2measureHubble teams up with Humason•Hubble and Humason become partners in nebular reserachMilton L. Humason(1891-1972)Discovery of the Hubble Law (1929)• Great Discovery: Redshift is proportional to distance(“Hubble Law”)Discovery of the Hubble Law • greatly improve accuracy by reaching larger distances!Meaning of the Hubble Law • Hubble initially did not interpret his law as implyingan expanding universe! (He only referred to “de Sitter effect”)• recession speed = (Hubble) constant x distance• v = H0x d• (H0=500 km s-1Mpc-1)Solving Einstein’s Equations of GR• 1922: an expanding universe (GR without cosmological constant)Alexander Friedmann(1888-1925)Solving Einstein’s Equations of GR• 1927: Lemaitre independently (re-) discovers theexpanding-universe solutions of GRGeorges Lemaitre(1894-1966)The Expanding Universe (early 1930s)• Lemaitre interprets Hubble’s Law as indicatingthat space itself expands!• redshifts due to stretching of expanding space!The Expanding Universe (early 1930s)• Lemaitre interprets Hubble’s Law as indicatingthat space itself expands!• redshifts due to stretching of expanding space!The Age of the Expanding Universe• Estimate expansion age of the universe:History of H00200400600800100012001920 1930 1940 1950 1960 1970 1980 1990 2000DateH0 (km/s/Mpc)Compilation by John HuchraBaade identifies Pop. I and II Cepheids“Brightest stars” identified as H II regionsJan OortHubble Wars0204060801001201401970 1980 1990 2000DateH0 (km/s/Mpc)generalcosmology dependentKey projectSandage campde Vaucouleurs campreasonable convergence only in last decade – see laterBig Q: What happened in the past, when allthe galaxies where close together???• an initial “Big Bang”???Hubble (part 2)• Discovery of Recession of Spiral Nebulae (1929-31)- already known: most spirals show redshifts in their spectra- Hubble and Humason use Cepheids and other standard candles to determine distances to spiral nebulae- “Hubble’s Law”: v = H0x d- Initial estimate of “Hubble’s constant”: H0= 500 km s-1Mpc-1• Interpretation of Hubble’s Law (1930s)- Lemaitre (rediscovering Friedmann’s work) proposessolutions to Einstein’s General Relativity for anexpanding universe!- Naturally explains “Hubble’s Law” (v = H0x d) with expansionof space