An exposition on Friedmann Cosmology with Negative Energy Densities
  How would negative energy density affect a classic Friedmann cosmology?
Although never measured and possibly unphysical, certain realizations of
quantum field theories leaves the door open for such a possibility. In this
paper we analyze the evolution of a universe comprising varying amounts of
negative energy forms. Negative energy components have negative normalized
energy densities, $\Omega < 0$. They include negative phantom energy with an
equation of state parameter $w<-1$, negative cosmological constant: $w=-1$,
negative domain walls: $w=-2/3$, negative cosmic strings: $w=-1/3$, negative
mass: $w=0$, negative radiation: $w=1/3$ and negative ultralight: $w > 1/3$.
Assuming that such energy forms generate pressure like perfect fluids, the
attractive or repulsive nature of negative energy components are reviewed. The
Friedmann equation is satisfied only when negative energy forms are coupled to
a greater magnitude of positive energy forms or positive curvature. We show
that the solutions exhibit cyclic evolution with bounces and turnovers.The
future and fate of such universes in terms of curvature, temperature,
acceleration, and energy density are reviewed. The end states are dubbed Big
Crunch, Big Void, or Big Rip and further qualified as "Warped", "Curved", or
"Flat", "Hot" versus "Cold", "Accelerating" versus "Decelerating" versus
"Coasting". A universe that ends by contracting to zero energy density is
termed "Big Poof." Which contracting universes "bounce" in expansion and which
expanding universes "turnover" into contraction are also reviewed.
