The BIrth of Cosmology
discovering the expansion of the universe
In 1912 Vesto Slipher began measuring spectra of the class of objects known as spiral nebulae. To his surprise he discovered that of 14 nebulae analyzed, 12 exhibited spectra shifted towards the red end of the spectrum. This information provided further evidence that the spiral nebulae were not objects associated with our Milky Way, but were in fact separate galaxies external to our own. In the mid-1920s Hubble and his assistant Milton Humason replicated Slipher's work by photographing the spectra of about two dozen galaxies using the 100-inch Hooker reflecting telescope on the summit of Mount Wilson near Pasadena, California.
Presented on the left is a classic image showing the spectral absorption lines of several elliptical galaxies in different galaxy clusters. The lines (called the K and H lines) are known to be produced when electrons in calcium atoms absorb photons of two specific energies. The "rest" wavelength of the K absorption line is marked by the vertical line and the actual redshifted K and H lines are the thick vertical bands to the right of the rest line.
By measuring the K or H line (The rest wavelengths of the K and H lines are 3,933.7Å and 3,968.5Å respectively) from a reference spectral line, such as an emission line from a helium atom's spectrum, the redshifted wavelengths of the K or H line can be calculated. Having already determined the scale of the image and knowing the wavelength of the reference spectral line from laboratory observations, the separation between the reference spectral line and galactic absorption line can easily be measured on the photograph. The measured shift of the K or H line yields the recession velocity of the galaxy from the Doppler equation:
v = c Δλ/Δλo .
Since the K and H lines will give two slightly different recession velocities, the results must be averaged.
Hubble and Humason used a variety of techniques to determine the distances to the galaxies, but the most astounding discovery came in 1929 when Hubble combined his recession velocity data with the distance determinations and found that the majority of galaxies seemed to be receding from our Milky Way at velocities that were proportional to their distances.
This relationship has come to be known as Hubble's law. It can be expressed as
v = Hor
which is simply the equation of a line (y = mx + b) where the y-coordinate v is the galaxy's recession velocity in kilometers per second, the x-coordinate r is the galaxy's distance in megaparsecs, and the slope of the best-fit line is the Hubble constant (expressed in units of km/s/Mpc). That constant is a rough measure of the expansion rate of the universe. Incidentally, the inverse of the Hubble constant gives an estimate of the time since the universe started expanding (i.e., its age).