The Einstein field> equations for general relativity look like:

G = k T
^   ^ ^
|   | |
|   |  --- The Stuff in The Universe 
|   ---a constant
-----the curvature of space
G and T are Tensor's and so describe stuff in spacetime quite nicely.

From this some people say mass(T) tells space(G) how to bend, space tells matter how to move.

The problem with this equation is that it is unstable. If you get a slightly higher concentration of mass in one part of the universe it will curve space which will make all the rest of the mass move towards the slight concentration increasing the concentration and adding to the problem. This equation predicts a non-static universe as there is no static solution. Einstein didn't like this as he believed in a static universe so he wrote down another equation

 G = k T - L
 ^   ^ ^   ^ 
 |   | |    --- cosmological constant  
 |   |  --- The Stuff in The Universe 
 |   ---a constant
 -----the curvature of space 
The L counters the attractive force of stuff and allows you to set up a static solution. L stands for Lambda. In 1924 people realised that the universe was not static but was expanding (a solution allowed by the first equation) Einstein went Doh! (not something he did very much in his life) Had he predicted a non-static universe it would have been the greatest prediction of theoretical physics.

Recently some people have discovered that the universe is expanding too quickly. They have re-introduced L to explain this. They look at supernovae to see how their brightness changes with distance. This can give you a measure of the curvature of space. If space is flat then the supernovae dim in proportion to their distance. If the universe is positively curved then the curvature can act like a lens and the supernovae would not dim so quickly. If the universe is negatively curved then they dim faster than one would expect. It has been discovered that the latter is the case, but it might just be something funny with supernovae far away. The reason people do this with supernova is they believe them to be a standard candle. Every supernova of a certain type should be as bright as every other. This is hard to prove as we don't know how supernova work. Thats pretty much the state of the field and the cosmological constant at the moment, oh yeah you need one to make inflation work. (a cosmological constant, not a supernova)

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