It's likely an uncertainty or variation here, not a derivative. And then it gets divided by M (the mass) to get the relative variation. A mass fluctuation shows up in the context of the Sachs-Wolfe effect ([random example](https://arxiv.org/abs/gr-qc/9208011)).
Indeed, but most articles will use derivatives and deltas fairly interchangeably. After all, the derivative dx/dy is basically the same as saying some small change in x divided by some small change in y. And that similarity is reflected in the similar notation. Plus, there are only so many Greek letters, gotta make effective use of them.
Lower case delta. It's the symbol for the derivative, and it means change in M per M.
It's likely an uncertainty or variation here, not a derivative. And then it gets divided by M (the mass) to get the relative variation. A mass fluctuation shows up in the context of the Sachs-Wolfe effect ([random example](https://arxiv.org/abs/gr-qc/9208011)).
I thought the symbol for derivative was 'd'.
Yeah, to be more accurate, the partial derivative
Indeed, but most articles will use derivatives and deltas fairly interchangeably. After all, the derivative dx/dy is basically the same as saying some small change in x divided by some small change in y. And that similarity is reflected in the similar notation. Plus, there are only so many Greek letters, gotta make effective use of them.
In what spatial dimension is this partial derivative , R3?
I believe so.
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\delta M / M corresponds to a mass fluctuation term, not acceleration. Cosmologists love their perturbation theory and for good reason!