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.
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