THE EXPANDING UNIVERSE

Val Eraser's three questions - May 2002:

1. Are the objects which are 15 billion light years away at the centre of the universe? No. They are seen by the light which left them 15 milliard years ago and are not at the "centre of the universe" -- the universe has no centre.

2. How is it possible to be looking at both the edge and the centre of the universe at the same time? - The objects formed at the distance of 15 milliard light years were not at the "centre".

3. Where exactly are astronomers looking when they say the edge of the universe? They are looking at objects so far away that their speeds of recession are almost equal to the speed of light.

These answers do not do justice to the questions asked by Val. In the late 1920's E P Hubble, H Shapley, M L Humason and V M Slipher discovered the expanding universe in which the galaxies are receding from each other. One can visualise this as a balloon with spots on it to represent the galaxies. When the balloon is inflated all the spots move away from each other as they move outwards. A tiny ant crawling on the surface of the balloon from galaxy to galaxy finds no end to the surface and he thinks his universe is infinite, yet we with our better point of view and superior know- ledge know that the ant's universe is finite. Wherever the ant finds himself he sees himself as being at the centre of the universe and his observable edge is the circle representing his horizon. Similarly, we see ourselves as if we were in the centre, not only of the universe, but also at the centre of our Galaxy which trails right across the sky as the Milky Way and we can see the stars describing their diurnal and annual motions across the sky with us in the middle. But there is no centre to the universe and the observable edge depends on the position from where the universe is viewed.

Objects further and further away are receding faster and faster. When they reach the observable edge of the universe their speeds of recession approach the speed of light and if they were to recede at the speed of light, we would never be able to observe them, even with the mightiest telescopes.

How far away is this edge of the universe?

The redshift z is the percentage by which the spectral lines of an object are shifted towards the red end of the spectrum and it can be measured very accurately. If the speed of recession of a body is v and the speed of light is c, then so that .

The greatest redshift yet measured stands at z = 6. Therefore , 

so that 49c - 49v = c + v and 48c = 50v.

The speed of recession v in terms of the speed of light c is given by or 96% of the speed of light which is 300 000 km. s^-1.

How far would this object having a redshift of 6 be? From the equation of Hubble's law the distance

D = v ÷ H_o where H_o is the Hubble constant. This constant can be anything between 55 and 100 kilometres per second per mega parsec ( km.s^-1 Mpc^-1 ). So if we consider a body having a speed of recession of 96% of the speed of light and using a Hubble constant of say, 64 km.s^-1 Mpc^-1 , we get a distance . To convert this distance into light years we must multiply by 3,26 and to obtain milliards, we must divide by 1000. Therefore = 14.67 milliard light years namely 14,67 x 10⁹ light years.

For a redshift of 8, the same calculation gfves a speed of recession of 96,9% of the speed of light and a distance D of 14,9 milliard light years and for a redshift of 10, a speed of recession of 98,36% of the speed of light and a distance D of 15,03 light years. The greater the speed of recession, the greater is the distance, When the speed of recession becomes 100% of the speed of light, the distance works out to 15,28 milliard light years, BUT THE REDSHIFT IS THEN EQUAL TO infinity. This distance must then be the edge of the observable universe and we can never observe it because bodies at that distance are receding at the speed of light. Such a speed is impossible because Einstein proved that it requires an infinite force to accelerate a material body to the speed of light and there can be no thing like an infinite force. There is, however, nothing to stop the expansion of the universe beyond the observable edge.

The speed of recession of galaxies gets closer and closer to the speed of light: it approaches the speed of light asymptotically, but can never reach the speed of light.

So, the objects at the observable edge of the universe are not at the centre of the universe, but we do see them by the light which left them 15 milliard years ago, when the universe was young, just after the Big Bang (A far better name would be the cosmo-genesis). This event was not limited to a single point. It took place throughout the universe, which at that time consisted only of radiation. Matter came into existence according to Einstein's equation E = mc² where the energy E is in ergs and it was converted into mass m in grams multiplied by the square of the speed of light in centimetres per second, namely (300 000 x 100 000 )²  namely 9 x 10²⁰.

A distance of 15 milliard light years in all directions around us does not fill up all of space. Far, far beyond that limit there must be other universes, some undergoing expansion like ours; others undergoing contraction down to a Big Crunch; others in the beginning stage of the cosmo-genesis and so on ad infinitum. This is the concept of the Multiverse. But all of it forever beyond the edge of the observable universe.

Jan Eben van Zyl