The paper works out the geometry, flux, and geodesics of the Melvin magnetic universe with a positive cosmological constant, showing its two-sphere section is compact and carries a conical singularity, with a Freund-Rubin flux compactification as a critical limit.
Unbounded entropy in spacetimes with positive cosmological constant
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abstract
In theories of gravity with a positive cosmological constant, we consider product solutions with flux, of the form (A)dS_p x S^q. Most solutions are shown to be perturbatively unstable, including all uncharged dS_p x S^q spacetimes. For dimensions greater than four, the stable class includes universes whose entropy exceeds that of de Sitter space, in violation of the conjectured "N-bound". Hence, if quantum gravity theories with finite-dimensional Hilbert space exist, the specification of a positive cosmological constant will not suffice to characterize the class of spacetimes they describe.
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Properties of the magnetic universe with positive cosmological constant
The paper works out the geometry, flux, and geodesics of the Melvin magnetic universe with a positive cosmological constant, showing its two-sphere section is compact and carries a conical singularity, with a Freund-Rubin flux compactification as a critical limit.