Another pet hate — people who think that the cosmological redshift is a Doppler/velocity effect. It ain’t. But that doesn’t reduce its importance; if anything, the opposite.
Edwin Hubble made the two most important discoveries in cosmology. First he proved that many “nebulae” are other “island universes” beyond the boundaries of the Milky Way. Then, he discovered that the galaxies, as these nebulae are now known, are moving apart from one another – that the Universe is expanding. But he didn’t make the second discovery on his own; the astronomer who actually carried out most of the painstaking observational work was Hubble’s colleague at the Mount Wilson Observatory, Milton Humason. Hubble, the more senior astronomer, chose Humason as his partner because Humason was, quite simply, the best observer in the world in the 1920s, able to push the abilities of what was then the best telescope on Earth, the 100-inch (2.54 m) reflector at Mount Wilson, to the limit.
From the perspective of the 21st century, it’s worth taking stock of what those measurements involved. Humason had to spend long hours in the cold of an unheated telescope dome, open to the sky, on a mountain top at night, keeping the telescope trained on particular patch of the sky while light from a distant galaxy was focussed on to a glass photographic plate. The dome had to be unheated, because convection currents from any heater would disturb the air and distort the image; and observations were best made in winter, when the air was still and the nights were long. Often, the galaxy being studied would be so faint that at the end of one night’s observing the photographic plate would have to be packed away, in the dark, in a light-proof box, then taken out the next night and used to build up a brighter image of the distant galaxy. And then the plate had to be developed by hand before its image could be analysed. All this required enormous patience, a quality Humason had in abundance but Hubble did not, as well as great skill. A far cry from modern techniques, where telescopes can be operated by remote control from air-conditioned rooms, and the light is gathered on photographic chips, with photons (particles of light) being counted by computers.
In the second half of the decade of the 1920s, Hubble was still primarily interested in measuring the distances to galaxies. He was intrigued by a discovery that had been made in the previous decade by Vesto Slipher, an astronomer working at the Lowell Observatory in Flagstaff, Arizona. Slipher had been working with a 24-inch refracting telescope which had a new instrument called a spectrograph attached to it. This could make photographs of the spectra of faint astronomical objects, by adding up the light over several nights if necessary. Among the objects Slipher studied in this way were several of the family still known as nebulae, which Hubble was about to prove were actually external galaxies. By 1925, just when Hubble was beginning to measure distances to galaxies, Slipher had measured 41 of these spectra, and found that just two of them (including the Andromeda Nebula) showed blueshifts, while 39 showed redshifts. This was the limit of what he could do with the 24-inch telescope, but the evidence hinted that the galaxies that looked bigger and brighter had smaller redshifts. The obvious inference was that galaxies that look bigger and brighter galaxies are closer to us – so Hubble guessed that measuring redshifts might be a way of measuring distances to galaxies, and roped Humason in to test the idea with the 100-inch telescope. Humason measured the redshifts, while Hubble estimated the distances to the same galaxies, using other techniques.
By the beginning of the 1930s, Hubble and Humason had made enough observations to show that the relationship between redshift and distance is about as straightforward as it could possibly be: the redshift is proportional to the distance – or, putting it the way round that mattered to Hubble, distance is proportional to redshift. This is now known as Hubble’s law. It means that if one galaxy has twice the redshift of another it is twice as far away, and so on. Once the distances to a few nearby galaxies had been measured by other means, the rule could be calibrated, and distances to other galaxies, much farther away across the Universe, could be measured simply by measuring their redshifts. In fact, this simple law only applies accurately to relatively nearby galaxies, and a more subtle relationship applies farther out across the Universe, but this does not detract from the importance of Hubble’s discovery.
Hubble himself wasn’t interested in why the light from galaxies showed redshifts. All he cared about was how the redshift (whatever its cause) could be used to measure distances. But the natural guess people made at first was that the redshifts are caused by the Doppler Effect. If so, it meant that just two external galaxies (including Andromeda) are moving towards us, and all the rest are moving away – not as individuals, but as members of clusters like the Virgo Cluster. It was soon realised, however, that this recession of the galaxies is not caused by galaxies and clusters moving through space. Albert Einstein’s general theory of relativity, which he had completed in 1915, described how space itself could be bent by the presence of matter, like a stretched rubber sheet with a heavy weight on it. The equations also described how space as a whole could stretch, but in 1915 nebulae hadn’t even been identified as external galaxies, and Einstein had dismissed this as a trick of the mathematics with no physical significance. After the discovery of the redshift-distance relationship, Einstein and other mathematicians realised that this was exactly what his equations described – space itself stretching and carrying clusters of galaxies along with it. This was the beginning of modern cosmology.
The cosmological redshift is not a Doppler Effect. It is not caused by galaxies moving through space, but by the space between the galaxies stretching during the time it takes light to get from one galaxy to another, and stretching the light to longer wavelengths. Galaxies do move through space, producing Doppler Effects in their spectra, but these are simply added to or subtracted from the cosmological redshift – which is why, for example, Andromeda shows a blueshift. It is moving towards us through space faster than the space between us and Andromeda is expanding. But for all except the nearest galaxies, the cosmological redshift dominates.