Examining the Viability of Phantom Dark Energy

Understand

In the standard cosmological framework of the 0th-order FLRW metric and the use of perfect fluids in the stress-energy tensor, dark energy with an equation-of-state parameter $w < -1$ (known as phantom dark energy) implies negative kinetic energy and vacuum instability when modeled as a scalar field.

• However, the accepted values for present-day $w$ from Planck and WMAP9 include a significant range of values less than $-1$.
• We find that it is not as obvious as one might think that phantom dark energy has negative kinetic energy categorically.
• Analogously, we find that field models of quintessence dark energy ($w_\phi>-1$) do not necessarily have positive kinetic energy categorically.