Basic Idea: theorists are very puzzled. Measurable by
gravity which is not well understood force.
Puzzles
 Don't know how to calculate it
 Size seems unnatural relative to gravitational scales
 Seems many contributing effects
 Size relative to other energy densities changes with time,
why comparable to other scales today?
Methods to try to explain it (courtesy J. Lykken)
 give up
 anthropic
 some unrecognized symmetry
 existing hierarchy
 dynamics
 original counting wrong
 strings:
 some models of the universe
 ways to get lambda small with arguments of type above
Analogy with inflation
 quintessencesame thing happening now?
 scalar field description
 link to inflation using same field
Some references.
