Our own universe may be but one bubble of space-time in a much bigger multiverse.
Credit: Greg Smye-Rumsby
This article is an edited extract of the book From Here to Infinity: The Royal Observatories Greenwich Guide to Astronomy, published by the University of Western Australia Press.
Credit: University of Western Australia Press
Edwin Hubble made the two most important discoveries in cosmology. First, he proved that many nebulae are other "island universes" or galaxies, beyond the boundaries of the Milky Way.
Then, working alongside colleagues at the Mount Wilson Observatory, he discovered that these galaxies are moving apart from one another – in effect that the universe is expanding.
In the second half 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 (61-cm) refracting telescope that 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 then 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 are closer to us – so Hubble guessed that measuring redshifts might be a way of measuring distances to galaxies, and roped in his co-worker Milson Humason to test the idea with the 100-inch (250-cm) telescope. Humason measured the redshifts, while Hubble estimated the distances to the same galaxies using other methods.
THE REDSHIFT
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 was not 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.
The shape of the universe
Regarding the way we see the shape of the universe,"open","closed","flat".Maybe it's possible that there are other geometries,just not avilable to the common eye.Of course this point does spill over into 'religion',but maybe for to long now we have been seeing the universe through scientific blinkers. Maybe there is a need to take a more "holistic" view?
Excellent science writing
Thanks to John Gribbon for laying out all these ideas with exceptional clarity.