The Cosmological Constant Problem, Dark Energy, and the Landscape of String Theory

The spectrum of the cosmological constant (vacuum energy, dark energy) in the string landscape (schematic). Each blue line represents one three-dimensional vacuum. With 10^500 vacua, the spectrum will be very dense, and many vacua will have values of  compatible with observation (red/shaded region).


Raphael Bousso
In this colloquium-level account, I describe the cosmological constant problem: why is the energy of empty space at least 60 orders of magnitude smaller than several known contributions to it from the Standard Model of particle physics?
I explain why the “dark energy” responsible for the accelerated expansion of the universe is almost certainly vacuum energy.
The second half of the paper explores a more speculative subject.
The vacuum landscape of string theory leads to a multiverse in which many different three-dimensional vacua coexist, albeit in widely separated regions.
This can explain both the smallness of the observed vacuum energy and the coincidence that its magnitude is comparable to the present matter density….
Read more: http://arxiv.org/pdf/1203.0307v1.pdf

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