Hubble,Galaxies and
theExpandingUniverse
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 MilkyWay
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 backglass quality not so importantshorter focal lengths mean shorter tubes
and smaller domes
Refractor
Reflector
Lens Mirror
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, spacetelescopes, 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
Rosse’sLeviathan
LickObservatory
MountWilson
Mount Palomar
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 cepheidswith trigonometric parallax (solar motion)– Calibrates cepheids as distanceindicators, 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
Shapley’s globular cluster model, 19183,000 pc
Centerof MW
Sun
Overall diameter = 100,000 pc!
8000 pcGlobularclusters
Competing models ofnebulae, 1900-1925
SunMW
I. Nebulae =Islanduniverses
SunMW
II. Nebulae = Gas clouds in Milky Way
M31
M31
“Great Debate” on scale ofuniverse 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 andare located within it (orbit its center?)
– van Maanen’s observed rotating spirals must be close or elsearms 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 areother 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 islanduniverses, 1923-25
– Hubble to Shapley, 1923: “You will be interested to hearthat I have found a Cepheid in M31. I have followed thenebula closely this season and in the last five monthshave netted 9 novae and 2 variables .... The distancecomes 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, butwins the war Shapley loses the “great debate” ...
Shapley’s “nova” were “supernovae,” i.e., heplaced M31 too close
van Maanen’s data on rotational rates could notbe replicated
Interstellar absorption in MW makes Shapley’scepheids closer (“faintness means farness”disrupted by absorption)
But Shapley succeeds Pickering as Directorof 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-velocityrelationship for globular clusters, but datawere ambiguous In an expanding universe would expect a
distance-velocity relationship
Distance versus velocity in anexpanding universeRaisin Bread:
T = 0T = later
Universe:
T = 0T = 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,or Hubble Constant Assume constant velocity Exanding universe!! Distance
Vel
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 affectassumption of uniform velocity Einstein’s cosmological constant is back!
Hubble’s “distance ladder” was flawed (notall galaxies have same intrinsic brightness)