Kalman Kingsley, of Palmerston North, asks :-
Since looking through a super telescope is looking backwards in time, does the universe have a visible "edge" (a kind of medieval stellatum or Primum Mobile) where it is impossible to see any further - because to see further is to see before time? Also does the universe have a center or does it always appear that way from a viewer's perspective?
Tony Signal, a physicist at Massey University, responded.
Light, and all other forms of electromagnetic radiation such as radio waves, infrared and X rays, travels at a constant speed of about 300 000 kilometres per second. So there is always a finite time interval between the emission of light and its detection. It takes about 8 minutes for light from the sun to reach us on Earth, and a few hours for that light to make it to the outer reaches of the solar system. Because astronomical distances are so large, it typically takes many years for light from ‘nearby’ stars to reach us, and millions of years for light from far away galaxies to be detected by our telescopes. Interestingly, the light from these farthest galaxies is ‘stretched out’, that is its wavelength is increased, because of the expansion of the universe that has occurred since its emission. The most distant objects we have yet detected are quasars, very bright objects which are probably powered by massive black holes heating gas clouds, some of which have been found at a distance of 13 billion light years. As the Big Bang occurred about 14 billion years ago, light from these quasars is carrying information about the state of the universe shortly after its formation.
It is possible to ‘see’ further back in time than this though. After the Big Bang, all the matter in the universe was ionized gas, mostly the nuclei of Hydrogen and Helium plus electrons. As all these particles are electrically charged, light cannot travel far in such a gas before it is absorbed. However, as the universe expanded, the ionized gas cooled, and about 300 000 years after the Big Bang, it had cooled enough that the atomic nuclei and the electrons were able to form stable atoms. As atoms are electrically neutral, light could now travel through the universe unimpeded, and some of this light is still reaching us today. We can detect this light on Earth as microwaves and radio waves and it is called the cosmic microwave background - some of the static on your FM radio comes from this background.
The cosmic microwave background was first discovered in 1964, and in recent years it has been investigated in great detail by satellite experiments such as WMAP and Planck. These very sensitive experiments are investigating the very small irregularities in the radiation, which are the earliest evidence of structure in the universe, the seeds of our present day galaxies.
One of the fascinating outcomes of the Big Bang is that all observers in the universe will see the rest of the universe expanding away from them, so everybody thinks they are the centre of the universe. An analogy to this situation is blowing up a spherical balloon: as the balloon is inflated any point on the surface sees all the nearby points on the surface getting stretched away from it. Our universe is like a 3 dimensional balloon ‘surface’, continually being expanded by the power of the Big Bang.