The discovery of dark energy in 1998 meant that our universe needed to have a positive cosmological constant. The problem is that all of the string theories were built in universes with negative cosmological constants (or a zero value). When work did discover possible ways to incorporate a positive cosmological constant, it resulted in a theory that has a vast number of possibilities!
Dark energy is an energy that seems to fill much of the universe and causes space-time to expand. By current estimates, more than 70 percent of the universe is comprised of dark energy.
Prior to the 1998 discovery, the assumption was that the universe had a zero cosmological constant, so all the work done in string theory was focused on that sort of a universe. With the discovery of dark matter, priorities had to change. The search was on for a universe that had a positive cosmological constant.
Joe Polchinski and Raphael Bousso extended others’ earlier research by experimenting with extra dimensions that had electric flux (a number that represents the intensity of an electric field through a surface) wrapped around them. Branes carried charge, so they could also have flux. This construction had the potential to limit some parameters of the theory in a way that couldn’t vary continuously.
In 2003, a Stanford group including Renata Kallosh, Andrei Linde, Shamit Kachru, and Sandip Trivedi released a paper that showed ways to extend the Polchinski-Bousso thinking to construct string theories with a positive cosmological constant. The trick was to create a universe and then wrap it with branes and anti-branes to contain the electric and magnetic flux. This introduced the potential for two effects:
‘ It allowed a small positive cosmological constant. ‘ It stabilized the extra dimensions in string theory.
On the surface, this would seem to be an excellent outcome, providing two necessary components to string theory. Unfortunately, there was one little problem — there were far too many solutions!
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