diff --git a/src/PointwiseFunctions/Xcts/CenterOfMass.cpp b/src/PointwiseFunctions/Xcts/CenterOfMass.cpp index 15784cdd2b3a..184bad2f2532 100644 --- a/src/PointwiseFunctions/Xcts/CenterOfMass.cpp +++ b/src/PointwiseFunctions/Xcts/CenterOfMass.cpp @@ -12,7 +12,8 @@ void center_of_mass_surface_integrand( const Scalar& conformal_factor, const tnsr::I& coords) { const auto euclidean_radius = magnitude(coords); - tenex::evaluate(result, 3. / (8. * M_PI) * pow<4>(conformal_factor()) * + tenex::evaluate(result, 3. / (8. * M_PI) * + (pow<4>(conformal_factor()) - 1.) * coords(ti::I) / euclidean_radius()); } diff --git a/src/PointwiseFunctions/Xcts/CenterOfMass.hpp b/src/PointwiseFunctions/Xcts/CenterOfMass.hpp index ed913d725720..116803ec933b 100644 --- a/src/PointwiseFunctions/Xcts/CenterOfMass.hpp +++ b/src/PointwiseFunctions/Xcts/CenterOfMass.hpp @@ -13,16 +13,37 @@ namespace Xcts { /*! * \brief Surface integrand for the center of mass calculation. * - * We define the center of mass integral as (see Eq. 25 in - * \cite Ossokine2015yla): + * We define the center of mass integral as * * \begin{equation} * C_\text{CoM}^i = \frac{3}{8 \pi M_\text{ADM}} - * \oint_{S_\infty} \psi^4 n^i \, dA, + * \oint_{S_\infty} (\psi^4 - 1) n^i \, dA, * \end{equation} * * where $n^i = x^i / r$ and $r = \sqrt{x^2 + y^2 + z^2}$. * + * Analytically, this is identical to the definition in Eq. (25) of + * \cite Ossokine2015yla because + * \begin{equation} + * \oint_{S_\infty} n^i \, dA = 0. + * \end{equation} + * Numerically, we have found that subtracting $1$ from $\psi^4$ results in less + * round-off errors, leading to a more accurate center of mass. + * + * One way to interpret this integral is that we are summing over the unit + * vectors $n^i$, rescaled by $\psi^4$, in all directions. If $\psi(\vec r)$ is + * constant, no rescaling happens and all the unit vectors cancel out. If + * $\psi(\vec r)$ is not constant, then $\vec C_\text{CoM}$ will emerge from the + * difference of large numbers (sum of rescaled $n^i$ in each subdomain). With + * larger and larger numbers being involved in this cancellation (i.e. with + * increasing radius of $S_\infty$), we loose numerical accuracy. In other + * words, we are seeking the subdominant terms. Since $\psi^4 \to 1$ in + * conformal flatness, subtracting $1$ from it in the integrand makes the + * numbers involved in this cancellation smaller, reducing this issue. We have + * also tried different variations of this integrand with the same motivation, + * but $(\psi^4 - 1)$ is the best one when taking simplicity and accuracy gain + * into consideration. + * * \note We don't include the ADM mass $M_{ADM}$ in this integrand. After * integrating the result of this function, you have to divide by $M_{ADM}$. *