Obituary of John Barrow
Just a quick word to let you know that my obituary of John Barrow (partly based on my blog post here) has now been published in The Observatory Vol. 141 No. 1281 (2021 April) pp. 93-96. The Observatory Magazine isn’t available online so, with the permission of the Editors, I’ve included a link to a PDF of the published version here:
John Barrow Obituary in Observatory by Peter Coles
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In the Dark 
April 2, 2021 at 5:29 pm
When you say he studied inflation in non-GR scenarios do you mean with a nonzero cosmological cnostant or something more radical (and if so then what, please?)
April 2, 2021 at 6:38 pm
He worked a bit on Brans-Dicke theory, and later, on theories derived from extensions of the Einstein-Hilbert action such as f(R) theory.
April 2, 2021 at 10:09 pm
That’s interesting. Were it not for the square root of the determinant of the metric tensor in the integrand in the action, every f(R) would give the same vacuum field equations – although the interactions with matter would vary with f.
Anthony Lasenby has been studying quadratic Poincaré gauge theory, in which you have not only an R^2 term but also terms like R^{ijkl}R_{ijkl} and other contractions; and also throws in terms quadratic in the torsion. The hope – not unrealistic – is to find a theory consistent with observation that is renormalisable without need for exotica at the Planck energy. See arXiv:2101.02645
April 2, 2021 at 10:13 pm
Yes, these theories can be cast into a form in which they look like GR plus some effective energy-momentum tensor…
April 2, 2021 at 10:17 pm
John’s motivation for studying e.g. R^2 theories is that you could perhaps think of these as like a series expansion of GR (which is linear in R) with higher-order terms kicking in at high energies.
April 2, 2021 at 10:50 pm
Higher powers than R^2 render the spectrum of the propagator unphysical by Ostrogradsky’s instability.
April 2, 2021 at 11:05 pm
Indeed but the Ostrogradsky “ghosts” can be cancelled if you introduce appropriate cancelling terms; the Palatini formalism does this.
You might think this is all a bit contrived. I couldn’t possibly comment.
April 3, 2021 at 9:51 am
But not more contrived than the most popular alternative at the Planck energy…
April 4, 2021 at 10:11 pm
Did he trace descent from Isaac Barrow, who was also Professor of Geometry at Gresham College and who also held a Cambridge chair (which he resigned in favour of Newton)?
April 4, 2021 at 10:47 pm
Not as far as I am aware.