[2001.00394] Viable Gauge Choices in Cosmologies with Non-Linear Structures

Authors:  Timothy Clifton, Christopher S. Gallagher, Sophia Goldberg, Karim A. Malik
Abstract:  A variety of gauges are used in cosmological perturbation theory. These are often chosen in order to attribute physical properties to a particular choice of coordinates, or otherwise to simplify the form of the resultant equations. Calculations are then performed with the understanding that they could have been done in any gauge, and that transformations between different gauges can be made at will. We show that this logic can be extended to the domain of large density contrasts, where different types of perturbative expansion are required, but that the way in which gauges can be chosen in the presence of such structures is severely constrained. In particular, most gauges that are commonly considered in the cosmology literature are found to be unviable in the presence of non-linear structures. This includes spatially flat gauge, synchronous gauge, comoving orthogonal gauge, total matter gauge, N-body gauge, and the uniform density gauge. In contrast, we find that the longitudinal gauge and the Newtonian motion gauge are both viable choices in both standard cosmological perturbation theory, and in the post-Newtonian perturbative expansions that are required in order to model non-linear structures.
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Shaun Hotchkiss
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[2001.00394] Viable Gauge Choices in Cosmologies with Non-Linear Structures

Post by Shaun Hotchkiss » March 02 2020

I've decided to start making some technical cosmology videos. The format will be me interviewing authors of papers I found interesting. The first is on this paper and is now online here. The idea is for this to fill the same niche as a departmental seminar or plenary conference talk, so it is technical, but not meant to be only for those already working on the specific topic. Hopefully it will be of some (potential) benefit to all people at the level of a Cosmology PhD student or above, whatever their own research is on.

I found the paper interesting because it had useful things to say about what we can and can't do when making relativistic predictions from Newtonian simulations, although this isn't how the authors framed the paper themselves. This is something fraught with difficulty and some of the claims made are quite strong, so I wanted to hear more.

I found that Kit (Chris) Gallagher, the interviewee, explained the topic really well. I definitely understand the paper better now than I did from just reading it.

The video is unfortunately quite long (80 minutes). I either need to accept this length in the future or try harder to cut the time down during interviews. If there is enough interest I will probably add tables of contents to videos so that people can skip ahead to bits they'll find interesting.

Feedback is definitely welcomed! As are requests to be interviewed and requests for videos on specific papers.

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