<html>#2887: TwoPunctures does not support Slow-Start Lapse.
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<tr><td style='text-align:right'> Reporter:</td><td>Zach Etienne</td></tr>
<tr><td style='text-align:right'> Status:</td><td>open</td></tr>
<tr><td style='text-align:right'>Milestone:</td><td></td></tr>
<tr><td style='text-align:right'> Version:</td><td></td></tr>
<tr><td style='text-align:right'> Type:</td><td>enhancement</td></tr>
<tr><td style='text-align:right'> Priority:</td><td>major</td></tr>
<tr><td style='text-align:right'>Component:</td><td></td></tr>
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<p>Comment (by Zach Etienne):</p>
<p>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.</p>
<p>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 <a data-is-external-link="true" href="https://arxiv.org/abs/2404.01137" rel="nofollow">https://arxiv.org/abs/2404.01137</a>). 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 <em>spike</em> in Einstein constraints.</p>
<p>--<br/>
Ticket URL: <a href='https://bitbucket.org/einsteintoolkit/tickets/issues/2887/twopunctures-does-not-support-slow-start'>https://bitbucket.org/einsteintoolkit/tickets/issues/2887/twopunctures-does-not-support-slow-start</a></p>
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