Some selected highlights of the dramatic advances in observational cosmology this century.
At the beginning of the twentieth century, it was generally accepted that our galaxy was disk-shaped and isolated. But what were the spiral nebulae like M31 (Andromeda) - were they inside or outside the Milky Way? Immanuel Kant had speculated that they were other `island' universes.
Slipher measured spectra from the nebulae, showing that many were Doppler-shifted, that is, the frequency of light was affected by speed of the source (just as the frequency of sound alters for a passing train). By 1924, 41 nebulae were measured, and 36 of these were found to be receding.
A key advance in cosmology came with the development of means to measure the distance to these nebulae. Shapley used Cepheids, bright stars which pulsate at regular intervals between a few days and a month. The period of their variability is correlated with their absolute luminosity, which he calibrated in the nearby Large Magellenic Cloud.
Hubble was able to resolve Cepheids in M31 (the Andromeda galaxy) with the 100" telescope at Mt Wilson. He developed a new distance measure using the brightest star for more distant galaxies. He correlated these measurements with Slipher's nebulae to discover a proportionality between velocity v vand distance d, that is, Hubbles law v=Hd. The constant of proportionality H is called Hubble's constant (it was significantly over-estimated by Hubble himself).
Working with a horn antenna (7.35cm) at Bell Labs, Penzias & Wilson fortuitously discovered an isotropic radio background, a relic left-over from the primordial fireball. This cosmic microwave background radiation is key evidence for the Hot Big Bang model. The temperature of this blackbody radiation is today measured to be T = 2.73K (that is, a rather cold -270C).
Deep redshift galaxy surveys demonstrated the existence of huge bubbles, filaments and sheets on scales from 25Mpc to over 100Mpc. Subsequent galaxy surveys are providing detailed information about the distribution of large-scale structure. Radio galaxy and quasar surveys indicate homogeneity (or uniformity) is approached on scales of several hundred Mpc (that is, nearly a billion light years).
In April 1992, the COBE satellite team announced the discovery of anisotropies in the cosmic microwave background radiation at the level of one part in 100,000. These are thought to be a snapshot at t=400,000 years of the primodial fluctuations that led to galaxy formation. This map of the sky is also the best evidence for the isotropy (or spherical symmetry) of the Universe.
The Hubble Space Telescope was able to resolve Cepheid variable stars in galaxies in the Virgo cluster, ensuring a much better calibration of distance measures. This has allowed more accurate estimates to be made of Hubble's constant H. Early galaxies and quasars have also been observed by the HST raising serious doubts about current structure formation models.
The Hubble Space Telescope (click for a larger view).
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