More than four decades ago scientists discovered that the universe is suffused with microwave radiation -long wavelength light- that is a cool relic of the sweltering conditions after the big bang. Earlier on, it was stupendously hot, but as the universe evolved and expanded, the radiation steadily diluted and cooled. Today it is just about 2.7 degrees above absolute zero, and its greatest claim of mischief is its contribution of a small fraction of the snow you see on your television set when you disconnect the cable or turn to a station that isn’t broadcasting.
In 1929, Edwin Hubble, using the 100-inch telescope at the Mount Wilson observatory in Pasadena, California, found that the couple of dozen galaxies he could detect were all rushing away. In fact Hubble found that the more distant a galaxy is the faster its recession.
An essential property of cosmic microwave radiation revealed by precision satellite measurements over the last decade is that it is extremely uniform. The temperature of the radiation in one part of the sky differs from that in another part by less than a thousandth of a degree.
So although the universe is evolving since the big bang, on average the evolution must have been nearly identical across the cosmos.
This conclusion is of great consequence because the universe’s uniformity is what allows us to define a concept of time applicable to the universe as a whole. Thus the universe has enough symmetry to allow us to speak of its overall age and its overall evolution through time.
Using two-dimensional analogy for space, there are three types of curvature that are completely symmetric -that is, curvatures in which the view from any location is the same as that from any other. They are (a) positive curvature, which uniformly bloats outward, as on sphere; (b) zero curvature, which does not bloat at all, as on infinite plane; (c) negative curvature, which uniformly shrinks inward, as on a saddle.
Therefore a short list of curvatures -uniformly positive, negative, or zero- exhausts the possible curvatures for space that are consistent with the requirement of symmetry between all locations and in all directions. And that is really stunning. We are talking about the shape of the entire universe. Yet, by invoking the immense power of symmetry, researches have been able to narrow the possibilities sharply.
So if someone was to wake you in the middle of the night from a deep sleep and demand you to tell him the shape of the universe -the overall shape of space- and grant you a mere handful of guesses, you’ll be able to meet his challenge.
- Compiled from 'the fabric of the cosmos' -Brian Greene-