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)