How would negative energy density affect a classic Friedmann cosmology? Although never measured and possibly unphysical, the evolution of a universe containing a significant cosmological abundance of any of a number of hypothetical stable negative energy components is explored. These negative energy (Omega < 0) forms include negative phantom energy (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 ultra light (w > 1/3). Assuming that such universe components generate pressures as perfect fluids, the attractive or repulsive nature of each negative energy component is reviewed.
The Friedmann equations can only be balanced when negative energies are coupled to a greater magnitude of positive energy or positive curvature, and minimal cases of both of these are reviewed. The future and fate of such universes in terms of curvature, temperature, acceleration, and energy density are reviewed including endings categorized as a 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 a Big Poof. Which contracting universes “bounce” in expansion and which expanding universes “turnover” and contract are also reviewed.