I just read this: Anapolar Dark Matter
I'm not sure i've heard the term 'anapolar' before, so i google and i found this:
This confuses me, my understanding is that spherical harmonics spanned 'all possible moments' that you could need to reproduce any kind of field. So what really is this 'unipolar/toroidal moment' and what relationship does it hold with the known and true spherical harmonics?
Is this some kind of 4D (3+1 Minkowski) spherical harmonic moment, that is fundamentally and geometrically inequivalent to the usual 3D spherical harmonics?