Page 25 - GALIET PHYSICS BLOSSOMS III

(2) The Relativistic Cosmos
(i) Explain two key pieces of early evidence we have for the Big Bang and the expansion of the universe, viz., (a) the motion of galaxies as seen from the earth, and (b) the microwave background. You need to explain what is seen, how to describe what is seen in physical terms, and why this is evidence for the Big Bang.
BIG BANG EVIDENCE – EXPANSION OF THE UNIVERSE
One of the instrumental evidences for the Big Bang is the motion of galaxies. Slipher in 1917 discovered that every spiral galaxy was receding from our galaxy by observing the receding redshift spectrum.
In time, astronomers have come to know that every galaxy steadily recedes away from us in every direction except for those galaxies that are moving towards us (Andromeda and a few galaxies in the Virgo cluster). Galaxy clusters too recede, though each of the galaxies within the cluster moves randomly in relation to the other. However, it was Hubble’s plotting of the recession of galaxies in his Hubble diagrams in the 1920’s that showed how a galaxy’s recession rate is directly proportional to its distance from us (Hubble’s Law). These recessional galactic motions prove that the universe at the grand scale is not unchanging, but that it is expanding. Hubble’s law does not suggest that the size of our solar system, or galaxies, or galaxy clusters is increasing, but only that the “largest framework of the universe, the vast distances that separate galactic clusters” is expanding.26
Hubble’s law had astonishing implications. It suggested that most recessional velocity galaxies started their journey from a single point, and that this single point may have been the site of some violent event, or what came to be known as the Big Bang.
Now, at the time of the Big Bang, according to Hubble, this point was not different from the rest of the universe, but it involved the entire universe itself, that is, the galaxies did not dwell at a specific point at a specific place within the universe, but the whole universe was that point and that point was the universe. Consequently, given that the Big Bang involved the whole universe, it occurred everywhere at once, and not at any one point. From this, Hubble concluded that the galaxies are not just flying apart into the rest of the universe, but that the universe itself or space itself is expanding. Indeed, to Hubble, the expanding universe stays homogeneous all the time, and there is no empty space beyond the galaxies. If the Big Bang had indeed been a giant explosion spewing matter everywhere in space to form galaxies, there would have been a centre and a boundary, but it was not an explosion in an empty universe, for the universe is the same everywhere, hence, there is no centre to the expansion.
26 Bennet, Donahue, Schneider, Voit. The Cosmic Perspective. 6th Edition. Volume II. Stars and Galaxies. 614-615. Information also gathered and reworded from studying pages 664-667.
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