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)