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farr / AlignedVersusIsoSpin

A calculation and paper comparing aligned and isotropic black hole spin models.
MIT License
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Aligned Spins Don't Stay Perfectly Aligned---but chi_eff conserved at 2pN #8

Closed farr closed 7 years ago

farr commented 7 years ago

Should cite and mention https://arxiv.org/pdf/1506.03492.pdf (Gerosa paper) noting that misalignment can increase substantially through inspiral where we talk about Cole's paper and Jeremy Schnittman's paper. However, chi_eff is constant of the motion at 2 pN, so mis-alignment doesn't change chi_eff much.

This comment is courtesy of D. Gerosa, after I discussed our paper with him.

ilyamandel commented 7 years ago

Does Gerona mean the same thing by misalignment as us (angle between spins and orbital angular momentum)? Or does he mean angle between the two spin vectors (which can change because of different rates of (azimuthal) precession in phi, even when the theta angle stays the same?

ilyamandel commented 7 years ago

BTW, since we are considering just two populations -- perfectly aligned and perfectly isotropic -- these two populations will remain perfectly aligned and perfectly isotropic through all pN effects.

SimonStevenson commented 7 years ago

Agree with mentioning chi_eff constant at 2pn (it's one of the qualities that make it a desirable quantity to model and compare observations in)

farr commented 7 years ago

@ilyamandel , @SimonStevenson : Davide was wondering what would happen if there was a (small) mis-alignment, as one would expect in nature. At first he mis-remembered one of his papers, and claimed that subsequent dynamics could increase the spin-orbit misalignment up to 90 degrees (this freaked me out)! But after we looked closely, it seems that the increase is more modest (the lighter component can get a few tens of degrees misalignment even if initially things are < 10 degrees), largely consistent with what you would naively estimate from (a) chi_eff = const and (b) setting the more massive spin to perfectly aligned. But given that a small misalignment is almost inevitable, and that can grow (a bit) through dynamics, it seems worth mentioning that chi_eff is conserved and so we don't have to worry about this.