Hubble, Galaxies and Expanding Universe

Hubble,
Galaxies and
    the
 Expanding
  Universe

Bases of modern cosmology
 General relativity (gravity)
 Physics of elementary particles
 Observational results of 20th-century
  astronomy (optical, radio, x-ray, γ-ray,
  ultraviolet)

Task of lecture
   New “tools” of observational astronomy,
    1850-1920
   The “Great Debate” of 1920 on the nature of
    nebulae
   Erwin Hubble’s observational work
     Galaxies are “island universes” outside the Milky
      Way
     Expanding universe of galaxies

   The rise of astrophysics in America

Tools: Photography
   Daguerreotypes (1839) and wet plates (1840):
    sunspots and moon
   Dry plates (1870s): long exposures collect
    more light than eye alone, longer
    “integration” allows seeing fainter objects
     Shapes of nebulae, many new nebulae
     Accurate star maps
     Stellar motions and magnitudes
     Stellar spectra and classification
     Doppler-shifted spectra

Tools: Mountain observatories
    Advantages of reflectors by c. 1910
      Achromatic   optics
      larger mirrors supported at back
      glass quality not so important
      shorter focal lengths mean shorter tubes
       and smaller domes
Lens                                           Mirror

       Refractor
                                   Reflector

The world’s biggest telescopes
   Lord Rosse’s “Leviathan”, 1845
              72” reflector (speculum mirror)
   Lick Observatory (SE of San Francisco), 1880s
              36” refractor
   Mt. Wilson Observatory (above Los Angeles), 1910s
              100” reflector
   Mt. Palomar Observatory (SE of Pasadena), 1940s
              200” reflector
   After 1970, “smarter” replaces “bigger”
              Multiple mirrors, new detectors (charge-coupled devices),
               computer image processing, adaptive optics, space
               telescopes, radio/x-ray/infrared telescopes
   Largest optical telescope: VLT in Chile
              Four mirrors each 8.2m (=320”) or area of one 16m (=630”)

Biggest telescopes, 1850-1950
                           Mount
                           Wilson
               Rosse’s
               Leviathan

                                    Mount Palomar
 Lick
 Observatory

Tools: Cepheid variable stars
   Variable supergiants
     Luminosity cycles of 1-100 days
     Stars pulsate in size, post-Main Sequence

   Henrietta Leavitt at Harvard, 1908
        – 24 cepheids in Small Magellanic Cloud, i.e., all same distance
        – Found period-luminosity relation
   Harlow Shapley, Mt. Wilson, 1918
        – Measures distance to 11 cepheids
        with trigonometric parallax (solar motion)
        – Calibrates cepheids as distance
        indicators, i.e, as “standard candles”

How big is the “universe?”
Jacob Kapteyn’s “stellar system” model, 1910s
      – Based on star counts, proper motions, ,statistics, no absorption

                                                 3,000 pc
Shapley’s globular cluster model, 1918
                                                            Globular
                        Sun
                              8000 pc                       clusters
                                        Center
                                        of MW

                    Overall diameter = 100,000 pc!

Competing models of
nebulae, 1900-1925
                           MW
I. Nebulae =
Island                    Sun
universes           M31

        MW                Sun

                  M31

II. Nebulae = Gas clouds in Milky Way

“Great Debate” on scale of
universe
   National Academy of Sciences, April 1920
   Shapley’s “Large Universe” model (Mt. Wilson)
       – Milky Way large (dia 100,000 pc) and contains all nebulae
              Distance to globular clusters from cephieds
       – M31 close (from “nova” seen in 1885)
       – “Zone of avoidance” (most nebulae observed around poles
         of MW galaxy) means nebulae “know” where MW is and
         are located within it (orbit its center?)
       – van Maanen’s observed rotating spirals must be close or else
         arms would move at v > c if those spirals were size of MW
       – THUS, Milky Way galaxy comprises the entire universe!

Van Maanen’s rotation of M33

     Non-reproducible observations!

The “Great Debate” continued
   Heber Curtis’s “Island Universes” model (Lick)
       Milky Way small (dia=10,000 pc) and nebulae are
        other island universes like MW
         – Distance to globular clusters from average stars
     M31 far (observed “nova” in the nebula)
     “Zone of avoidance” means few nebulae in MW so
      most observed nebulae must be extragalactic
     van Maanen’s spirals? Bad data and must be rejected!
         – Curtis’s own observations showed NO rotation of arms

Hubble confirms island
universes, 1923-25
   – Hubble to Shapley, 1923: “You will be interested to hear
     that I have found a Cepheid in M31. I have followed the
     nebula closely this season and in the last five months
     have netted 9 novae and 2 variables .... The distance
     comes out to something over 300,000 pc.”
   – Shapley replies: “This letter destroyed my universe!”
   – Curtis was right about island universes, wrong on size
     of Milky Way
          MW is 30,000 pc in diameter, 200 pc thick
          M31 is ca. 800,000 pc away from MW

Shapley loses the battle, but
wins the war
   Shapley loses the “great debate” ...
     Shapley’s “nova” were “supernovae,” i.e., he
      placed M31 too close
     van Maanen’s data on rotational rates could not
      be replicated
     Interstellar absorption in MW makes Shapley’s
      cepheids closer (“faintness means farness”
      disrupted by absorption)
   But Shapley succeeds Pickering as Director
    of Harvard Observatory in 1921 (retired 1952)

Hubble’s expanding universe
   Earliest cosmological models by Einstein
    were static
   Vesto Slipher in 1910s measured redshifts of
    22 spiral nebulae, but had no distances
       Lowell Observatory, Flagstaff, AZ
   Other observers sought distance-velocity
    relationship for globular clusters, but data
    were ambiguous
       In an expanding universe would expect a
        distance-velocity relationship

Distance versus velocity in an
expanding universe
Raisin Bread:
                  T=0
                              T = later

Universe:

       T=0
                  T = later

Hubble’s Distance-Vel relation
   How to measure distances to far nebulae with
    various “standard candles”
     Distances to 6 nebulae via cepheids
     Then distances to 14 nebulae via brightest stars
     Then distances to 22 via luminosity of nebula

   Redshifts easier to measure
   Hubble’s Law: V = H0D,
     H0 = slope of line,
                                   Vel
    or Hubble Constant
     Assume constant velocity
     Exanding universe!!                    Distance

Hubble’s 1929 diagram

        From http://antwrp.gsfc.nasa.gov/diamond_jubilee/d_1996/hub_1929.html (6 May 2003)

Implications of Hubble’s Law
 Farther away a galaxy is, the faster it
  recedes (i.e., expanding universe)
 Can use Hubble Law to estimate
  distances from measured redshifts
 Hubble age of expansion
     – V = H0D
     – But V also = D/T, so 1/H0 = T of expansion
     – H0(1929) = 540 km/sec/Mpc, or T = 2 billion yrs
     – H0(2005) ≈ 72 km/sec/Mpc, or T = 14 b yrs

Problems with Hubble Age
   Gravity slows expansion so Hubble Law
    overestimates age
     Went faster earlier
     Has taken less time to reach current state

   Accelerating forces may also affect
    assumption of uniform velocity
       Einstein’s cosmological constant is back!
   Hubble’s “distance ladder” was flawed (not
    all galaxies have same intrinsic brightness)