[ET Trac] #2887: TwoPunctures does not support Slow-Start Lapse.

[Zach Etienne]

#2887: TwoPunctures does not support Slow-Start Lapse.

 Reporter: Zach Etienne
   Status: open
Milestone: 
  Version: 
     Type: enhancement
 Priority: major
Component: 

Comment (by Zach Etienne):

When Slow Start Lapse \(SSL\) is disabled, setting initial\_lapse = psi^n with n = -2 results in evolutions that are nearly indistinguishable from initial\_lapse = W. However, keeping the default as initial\_lapse = W is safer for future compatibility: if SSL is later enabled and the lapse is not initially W, you’ll see a spike in the Einstein constraints.

Here’s why: SSL exponentially relaxes the lapse toward \*W\* very early in the wormhole-to-trumpet transition, smoothing the sharp outgoing-lapse feature that accounts for >99.9% of constraint violations in moving-puncture BBH evolutions \(see [https://arxiv.org/abs/2404.01137](https://arxiv.org/abs/2404.01137)\). If the lapse isn’t initially set to W, then “Slow Start” would be better termed “Fast and Reckless”, as the initial lapse will be exponentially driven to W from whatever it was set to initially – leading to the aforementioned _spike_ in Einstein constraints.

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Ticket URL: https://bitbucket.org/einsteintoolkit/tickets/issues/2887/twopunctures-does-not-support-slow-start
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