The Big Bang, and the Age of the Universe

How do we know
  1. the age of the Universe?
  2. the speed of light?
  3. the size of the Universe?

The Big Bang

is an undignified name given to the event that is the beginning of space-time, and the Universe in which we live.

It was intended to be undignified, because the name was coined by the astrophysicist Fred Hoyle, who had a more elegant hypothesis to account for the Red Shift of distant galaxies. The Red Shift is a change in the light travelling from a star or a galaxy. It indicates that the object which emitted the light is travelling away from us, in the same way that a policeman's radar gun can measure the speed of a speeding vehicle.

If nearly all the galaxies are travelling away from us, and from each other, the simple explanation would be that they were all close together at some time in the distant past, and will be widely scattered in the distant future, perhaps even beyond visibility.

The odious thing about this explanation is that it implies a point in time which is the very first point in time, a point for which the idea "before the Big Bang" is as impossible as "colder than absolute zero" or "bigger than infinity" or "smaller than nothing".

Hoyle computed that the continuous production, thoughout all of space, of a few atoms of hydrogen per year per cubic kilometre, would suffice to balance the apparent expansion of the universe well enough that it could be infinitely old forever, and stay the same average density. Such a slow rate would be completely undetected by the experiments upon which the "law of conservation of matter" is based.
It would give us a Steady State universe.
The nature of the Universe would seem the same for all observers, no matter where or when they experimented.

Nevertheless, the experimental evidence seems to refute Hoyle's elegant solution. His hypothesis predicts that the apparent average density of galaxies in the Universe will be the same, no matter how long ago the sample being observed existed. The most distant galaxies are of course being observed by the light (or radio waves) that they sent out very long ago, and it has been found that they seem to have been packed together more closely back then.

1. We know that the Universe is at least as old as the time it takes light to travel from the most distant visible star or galaxy. The age of the Universe is also discussed at UCLA's and this NASA web page Note that their billion is the US billion, a thousand million.

2. The first estimate of the speed of light was made by measuring the longest time it took light to travel from Jupiter to Earth, compared with the shortest. The difference is the time it takes light to travel across the diameter of the Earth's orbit.
But a better way is to have a beam of light hit a rotating mirror, travel a distance to a mirror, and be reflected back. It is then reflected off the first mirror again, which sends it to a spot which is at an angle from the return path. That angle is the rotational speed of the mirror, times the interval required for the two way trip of the light.

3. The size of the universe is deduced first from the diameter of Earth's orbit, using that to measure the distance to various nearby stars, whose apparent position in the more distant sky background shifts as the Earth travels round the sun.
Then it was found that there are stars called "variable stars" whose brightness follows a cyclic pattern, the length of which varies with the size of the star. So the intrinsic brightness of such a star can be deduced from the period of the cycle. Thus the apparent brightness of a distant variable star gives a measure of its distance. Such stars even can be distinguished in nearby galaxies, giving us a measure of these much vaster distances.
But by assuming that some galaxies are about as bright as our own, or the nearby galaxy M31 in the Andromeda constellation (a constellation is merely a region of the sky, not an actual collection of stars) we can deduce its distance from its lack of apparent brightness.
It turns out that even the nearest galaxy, M31, is so far away that light takes at least 2.5 million years to travel from it to us.
Even this is far longer than Archbishop Ussher's estimate of 6000 years for the age of the Earth. Lord Kelvin's estimate, based upon the cooling of molten rock and iron, was 40 million years. It was probably too short for the formation and uplifting of the limestone of the Himalaya mountains.
But the energy and heat of radioactivity was unknown to that brilliant man, and the Earth's age is now reckoned to be about 4,500 million years.

The age of the Universe is discussed at this NASA web page
Note that their billion is the US billion, a thousand million.

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