HISTORY 135C

Department of History
University of California, Irvine
 Instructor:    Dr. Barbara J. Becker

Lecture 18.  The Expanding Universe.

Edwin Hubble and the Galactic Redshift/Distance Relationship
1909
 Vesto Slipher (1875-1969), a member of Percival Lowell's observing team at the Lowell Observatory in Flagstaff, began studying the spectrum of the Andromeda nebula (M31).
1913 Slipher found that M31's spectrum exhibits a significant shift in its position toward the blue.  According to Doppler's principle, this shift indicates that M31 is moving toward us at the high rate of speed of 300 km/sec.
1914 Slipher reported the radial velocities of 14 additional spirals, some of which, like M31, were moving towards us, others moving away.  The randomness of their motion encouraged even skeptical astronomers to accept the findings.
1916-7

Dutch astronomer, Willem de Sitter (1872-1935) developed a model of the cosmos based on Albert Einstein's new theory of general relativity.  Based on this model, de Sitter predicted that the spectra of distant star systems should exhibit redshifts.
1917

After measuring the radial velocities of more nebulae, Slipher finds that nebula with high redshifts appear smaller.  Are high redshift nebula really farther away?  Is there a physical relationship between a galaxy's radial velocity and its distance?


Edwin Hubble (1899-1953)
1923 Hubble identified a cepheid variable in the Andromeda nebula which led him to conclude that it is not a nearby forming solar system after all, but rather a large star system like our own Milky Way, located 1 million ly away.
1924 Swedish astronomer, Knut Lundmark (1889-1958) plotted the radial velocity of 44 galaxies against their estimated distances.  He assumed that M31 is 200,000 pc (650,000 ly) away, then made rough determinations of the distances to other galaxies by comparing their sizes and brightnesses to that of M31.  Lundmark concluded that there may be a relationship between galactic redshift and distance, but "not a very definite one."

Galactic redshift vs. distance, plotted by Knut Lundmark (1924)
1929 Hubble published his evidence that galactic redshifts increase with distance based on data he had gathered with some certainty on 24 galaxies.  He assumed that M31 was about a million ly (300,000 pc) away.  [Note:  Because of differences in scale represented, Hubble's diagram (seen below) would occupy only a small fraction of Lundmark's (shown above).]

Galactic redshift vs. distance, plotted by Edwin Hubble (1929)

Galactic redshift vs. distance, plotted by Hubble and Humason (1931); red rectangle in lower left corner encloses data points plotted in 1929 graph above.

Why was Hubble's redshift-distance relationship so readily accepted by members of the astronomical community?
  • used distance-finding techniques generally accepted by colleagues as reliable and accurate
  • used world's most powerful telescope -- Mt. Wilson's 100-inch reflector
  • viewed by colleagues as authority on extra-galactic astronomy
  • findings had been predicted, and were further corroborated, by noted theorists
  • findings were seen as supporting fruitful avenue of future research
What is the real physical cause of galactic redshifts?
Real physical motion?
  • are galaxies truly moving away from us at varying speeds?

Expanding universe?
  • is galactic motion simply the natural result of the expansion of space?
  • as space expands so does wavelength of light

 Gravitational effect?
  • photons moving in a strong gravitational field will lose energy -- result is longer wavelength

Change in "universal constants" over time?
  • gravitational constant?
  • mass of electron?

Some unknown process???

 
Go to:
  • "The Age of the Sun's Heat," Macmillan's Magazine (March 5, 1862), pp. 288-293, by William Thomson, Lord Kelvin (1824-1907)
  • "On the Secular Cooling of the Earth," Transactions of the Royal Society of Edinburgh (1864) Vol. XXIII, pp. 167-169, by William Thomson, Lord Kelvin
Weekly Readings
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Lecture Notes
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