Symmetry of the Universe
Slide 14 of 24
Having said that, however, I think the ancient astronomers would have been gratified to see the following pictures. These are maps of the sky surrounding the Earth taken by the Cosmic Background Explorer Satellite (COBE) a few years ago. Instead of mapping the location of visible stars and galaxies, it is looking at a form of electromagnetic radiation (microwaves) that is bombarding the Earth from all directions in space. This radiation is the same type of radiation that you use to cook food with in your microwave oven, although at a much lower intensity. The top map indicates fairly accurately the brightness of the radiation at different points in the sky. The faint S-shaped band of light is microwave radiation coming from dust within our solar system. In the second picture this band has been removed by computer, leaving a bright band directly across the center of the map. Once this too is removed (bottom picture) what is left maps is radiation that is coming to us from intergalactic space, very far away. In fact, this light has travelled for billions of years before reaching the Earth and has not interacted with matter since the very early stages of our universe. It shows us how the matter in the Universe was distributed just a few hundred thousand years after the big bang. It has been describe as a ``baby picture'' of the Universe. What is remarkable about this picture is that this so called ``cosmic background radiation'' is incredibly smooth (like a baby's bottom?) and uniform. No matter what direction we look in the sky, this radiation appears to be exactly to same. There are some irregularities, but they represent deviations of only about one part in ten thousand, which is way to small to detect in this picture. This incredible uniformity tells us something amazing about the matter from which the radiation comes: it was distributed around us in an (almost) perfect, uniform sphere. In short we again appear to be at the center of the Universe, as the ancient astronomers had supposed. Of course there is no scientific reason to believe that our location in the Universe should be special in any way. There is a more "mundane" explanation for the observed uniformity of the microwave radiation: the radiation must look perfectly smooth and spherical from every point in the Universe. This in turn is possible only if the radiation (and the matter that emitted it) were perfectly homogeneous throughout the Universe. Think of yourself emersed in a huge vat of homogenized milk: it would look the same in all directions from anywhere in the vat (as long as you stayed away from the sides). So from looking at this picture and applying what is essentially a symmetry argument, we learn that the matter distribution in the Universe 10 billion years ago was completely smooth and homogenous to one part in 10,000.