Since way back, humankind is looking up into the night sky, observing orbs and wondering about the origin and the look of the cosmos. Is the universe really expanding as we all learn in school? How does the border of the universe (if it exists) look like? Admittedly, a modern scientific approach to this problem is very abstract and not easily explained in layman’s terms.The following explanation therefore spares any detailed mathematical considerations for simplification.
Derived from Einstein’s theory of relativity, there are found different possibilities. In simplified terms, mass warps the space and thus determines its shape. Complex mathematical considerations result in a critical density of the universe. A structure can be assigned from the density parameter, omega, which is the quotient of the average density of the universe and the critical density. Three border cases emerge whose abstract values can be translated into two-dimensional images for a more vivid explanation (Fig. 1):[1,2]
a) The density is bigger than the critical density (omega > 1). The universe is big enough to stop the expansion sometime but after that point it will be shrinking again. This is called “closed universe”.
b) The density is smaller than the critical density (omega < 1). The universe expands forever and its shape is saddle-like. This is called “open universe”.
c) The density has the exact value of the critical density (omega = 1). The expansion rate decelerates over an infinite time-span and the shape is flat and endless.Another discussed model is the “Picard topology” that defines the universe as a horn which is closed at the end. Here very surreal phenomena would occur depending on whether one is situated at the peak or the broad end .
Measurements from the Wilkinson Microwave Anisotropy Probe (WMAP) give hints that the density of the universe equals the critical value. Accordingly, the shape would be flat. Still, with our limited technical possibilities we only can observe a very small area of the universe. No one can yet (or maybe never will?) know with absolute certainty how the universe looks like .