The Birth and Death of the Universe 

About 10 years before the expanding universe was discovered, Albert Einstein provided the theoretical basis for Cosmology with his General Theory of Relativity or "GR". According to GR, Space and Time cannot be separated from each other and the two need to be thought of as intricately linked into a single Space-Time. Even more surprising, Space-Time is not merely the stage on which the drama of physics occurs, it is an actor in that drama. There is a "fabric" of Space-Time that can be bent and stretched. Einstein found that this stretching of Space-Time was related to gravity. Matter and energy gave Space-Time their shape and that shape determined how matter and energy would move. 

Einstein's theory also allowed physicists and astronomers to create mathematical models of the Universe as a whole including the shape of the entirety of Space-Time. Because GR and the cosmological models it spawned deal with all the Space and Time possible for the Universe, it is a mistake to think of the Big Bang as happening at some point IN space and time. The Big Bang was the expansion of all Space and Time itself. It's a very strange and wonderful idea.  

With GR, physicists had equations that could describe how the Universe emerged from nearly infinite compression to expand into what we see and live in today. The initial "explosion" came to be called the Big Bang. These equations could describe the evolution of the Universe from the Big Bang all the way into the far future.  

What will be the fate of the Universe? Will it go on expanding forever? Will the gravitational attraction of all the matter (and energy) in the Universe eventually slow the Universe’s expansion down so much that it will stop expanding and re-collapse in a "Big Crunch"? The answers to these questions depend on the amount of matter (and energy) the Universe contains. Cosmologists express this quantity in terms of the "density parameter" (called Omega). The density parameter is the ratio of two numbers: the actual density of matter and energy in the Universe right now, and the so called critical density of matter and energy where the Universe’s expansion and deceleration are just balanced. Low values of the density parameter (Omega < 1) mean there is not enough matter and energy to slow the Universe down. High values of the density parameter (Omega > 1) mean the gravitational pull of all the matter and energy in Universe will make it re-collapse. A Universe with Omega = 1 will stop expanding but only when time goes to infinity! 

All this tells us there are many kinds of Universes possible depending on the density of matter and energy (Omega) and the rate of expansion (the Hubble Constant H₀). With this cosmology interactive you can see how changing these quantities changes the fate of the Universe.