Conventions

2.1 Conventions

Throughout this review, Roman letters from the beginning of the alphabet $a,b,c,...$ denote spacetime indices ranging from 0 to 3, while letters near the middle $i,j,k,...$ range from 1 to 3, denoting spatial indices. Unless otherwise stated, we use units such that $ℏ = c = 1$ so that the Planck mass becomes $MPlanck = G −1∕2$ . We also use the signature convention $(− ,+, +,+ )$ for the metric.