Add translation to transition to ringdown script #6808

AlexCarpenter46 · 2025-08-11T20:44:32Z

Proposed changes

First pr of 3 adding the rest of the transition to ringdown script from SpEC to SpECTRE.

This pr adds the tracking of the geometric center of the common horizon which will then be used to make the translation function of time used in the ringdown. This is necessary for unequal mass simulations to fully ringdown.

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Further comments

markscheel

I may have missed it, but I don't see comments corresponding to lines 1209-1237 of SpEC's ConstructPostMergerDataFromAhC.pl, where the translation map is corrected to account for translation/rotation in the inspiral. This correction is easy to overlook; are you doing the correction?

markscheel · 2025-10-17T20:22:19Z

src/Domain/Python/StrahlkorperTransformations.cpp

 }  // namespace

 void bind_strahlkorper_transformations(py::module& m) {  // NOLINT
  // Only instantiating for Grid->Inertial because the Py functions are


Update dox here?

This binding was not needed and left in by mistake

markscheel · 2025-10-17T21:39:26Z

src/Evolution/Ringdown/StrahlkorperCoefsInRingdownDistortedFrame.cpp

      false,
      std::nullopt,
-      {100.0},
+      {50.0},


Add comments as to why 50 and not 100 (or some other number). Same for other magic numbers here.

I don't understand what "Radial partition used in current ringdowns" means or why it is 50.

markscheel · 2025-10-28T20:21:03Z

support/Pipelines/Bbh/Ringdown.py

+    if "Translation" not in evaluated_fot_dict:
+        evaluated_fot_dict["Translation"] = None


Why is this needed?

I added this so that we don't try to read in the history of the translation map from the inspiral if there wasn't one used.

Ok, pls add comment.

markscheel · 2025-10-28T20:22:00Z

src/Evolution/Ringdown/StrahlkorperCoefsInRingdownDistortedFrame.hpp

 /*!
- * \brief Transform `Strahlkorper` coefs to ringdown distorted frame.
+ * \brief Transforms `Strahlkorper` coefs to ringdown distorted frame and tracks
+ * the center of mass.


Does it track the center of mass, or does it track the geometric center of AhC? I think the latter.
Maybe Reads inertial-frame Strahlkorper coefs from a file, returns the ringdown-distorted-frame coefs and the Strahlkorper's inertial-frame geometric center ?

markscheel · 2025-10-28T20:26:50Z

src/Evolution/Ringdown/StrahlkorperCoefsInRingdownDistortedFrame.hpp

+ * ringdown distorted frame defined by the expansion, rotation, and translation
+ * maps specified by `exp_func_and_2_derivs`,
+ * `exp_outer_bdry_func_and_2_derivs`, `rot_func_and_2_derivs`, and
+ * `trans_func_and_2_derivs`. The expansion and rotation functions of time
+ * correspond to the ringdown frame's expansion and rotation maps at the time
+ * given by `match_time`, and by `settling_timescale`, the timescale for the
+ * maps to settle to constant values. The translation function of time supplied
+ * does not correspond to the ringdown frame's translation function of time. The
+ * ringdown's translation function of time needs to be built by tracking the
+ * position of the center of mass at multiple times. Only Strahlkorpers and
+ * center of mass points within `requested_number_of_times_from_end` times from
+ * the final time are returned. This function is used to transition from
+ * inspiral to ringdown; in this case, the inertial-frame Strahlkorper is the
+ * common apparent horizon from a binary-black-hole inspiral; the
+ * ringdown-distorted-frame coefficients are used to initialize the shape map
+ * for the ringdown domain. The center of mass points are used to initialize the
+ * translation map for the ringdown domain.


What is the translation function of time that is passed into this function? Is it the translation function of time of the inspiral? (You say what it is not, but not what it is).

And what are "center of mass points"? Does the center of mass actually matter here?

Also, should say that the AhC center is returned in the inertial frame.

Yes, the translation function of time is from the inspiral. And no, the center of mass doesn't actually matter here, I originally assumed that the center of mass and geometric center of the strahlkorper were the same.

markscheel · 2025-10-28T20:35:33Z

src/Evolution/Ringdown/StrahlkorperCoefsInRingdownDistortedFrame.cpp

+      tnsr::I<DataVector, 3, ::Frame::Inertial> inertial_center_point{
+          DataVector{3, 0.0}};
+      // The center point is mapped back to the inspiral inertial frame so that
+      // the center of mass is the same at the match time.


center of mass -> center of AhC

markscheel · 2025-10-28T20:40:25Z

src/Evolution/Ringdown/StrahlkorperCoefsInRingdownDistortedFrame.hpp

 */
-std::vector<DataVector> strahlkorper_coefs_in_ringdown_distorted_frame(
+std::pair<std::vector<DataVector>, std::vector<std::array<double, 3>>>
+strahlkorper_coefs_in_ringdown_distorted_frame(


Maybe rename? The function name sounds like it is returning only the coefs.

markscheel · 2025-10-28T21:01:21Z

tests/support/Pipelines/Bbh/Test_Ringdown.py

+        expansion_map = ExpansionMapOptions([1.0, 1e-4, 0.0], 100.0, 1e-6)
+        rotation_map = RotationMapOptions([[0.0, 0.0, 0.0, 1.0]], 100.0)
+        translation_map = TranslationMapOptions(
+            [[1.0, -1.0, 0.5], [0.0, 0.0, 0.0], [0.0, 0.0, 0.0]]


Do you want to test with nonzero velocity?

src/Domain/Python/StrahlkorperTransformations.cpp

src/Evolution/Ringdown/StrahlkorperCoefsInRingdownDistortedFrame.cpp

AlexCarpenter46

Yes I believe this correction is being applied to account for the inspirals rotation, expansion, and translation. This is done by transforming the geometric center from the ringdown-distorted-frame EDIT: ~~back to the inspiral-inertial-frame~~ to the ringdown-inertial-frame. Those center points are then used to fit to a cubic polynomial. I added some comments in the details, but let me know if I missed anything or if more should be added :)

AlexCarpenter46 · 2025-10-30T14:08:28Z

src/Evolution/Ringdown/StrahlkorperCoefsAndCenters.cpp

+          make_not_null(&distorted_ahc), 1e-7);
+      ahc_ringdown_distorted_coefs.push_back(distorted_ahc.coefficients());
+
+      tnsr::I<DataVector, 3, ::Frame::Grid> grid_center_point{


Wanted to point out that I changed these lines in this fixup because doing tnsr::I<DataVector, 3, ::Frame::Grid> {distorted_ahc.expansion_center()} was not constructing the tensor correctly. It was producing a tensor with a DataVector with 3 component at each index which is not what I was trying to do there.

AlexCarpenter46 · 2025-10-31T00:42:11Z

So I had assumed that with all the pieces added (automatically choosing an excision radius and rescaling the shape coefficients) that the ringdown would just work. However, I noticed that the translation still looks off even when rescaling. This can be seen in the two images below,
This first image is the domain for the ringdown and common horizon from the inspiral at the match time.

So I thought okay maybe the shape is incorrect, but in paraview I just transformed the domain around a bit and the shape actually looks really good after being rescaled.

So there must be something I've implemented incorrectly or maybe I'm not accounting for the inspiral rotation, expansion and translation correctly or at all. I'm not sure, but just wanted to share, cause I don't think this should be merged in this state.

Edit for more context: The images above show the apparent horizon of the common horizon at the match time in black. I've turned the opacity down so that we can see where the excision is relative to the horizon (the off-white/grayish ring inside the apparent horizon).

AlexCarpenter46 · 2025-11-06T16:29:58Z

src/Evolution/Ringdown/StrahlkorperCoefsAndCenters.cpp

+      // the center of AhC is the same at the match time.
+      coords_to_different_frame(
+          make_not_null(&inertial_center_point), grid_center_point,
+          ringdown_domain, ringdown_functions_of_time, gsl::at(ahc_times, i));


I want to highlight this change, that was causing the center to be placed incorrectly. I misread what the SpEC script was doing and thought it was taking this center point back to the inspiral inertial frame, but it actually maps it to the ringdown inertial frame instead. Doing this placed the excision center correctly for the 2 mass ratio ringdown as can be seen below.

AlexCarpenter46 · 2025-11-11T20:17:40Z

@markscheel and @nilsvu I've pushed a fixup adding some more comments, so I think this is ready for another round of reviews :)

markscheel

In SpEC's ConstructPostMergerDataFromAhC, there's a 28-line detailed comment, including matrix algebra, that explains why all of this works even though translation and rotation don't commute and the center of AhC is not the origin. In particular, it explains why you can use the coefficients of AhC as-is except for a minus sign, and how you compute the coefficients of the translation map. Please add a comment that is at least as detailed as the SpEC comment.

There is also a 32-line detailed comment in ConstructPostMergerFromAhC explaining what the 'ringdown distorted frame' is (in SpEC this is sometimes called the 'intermediate frame'), and how it is constructed, and explains details like "The ringdown translation map is different than the plunge one because it is tracking a different center, so translation is discontinuous in time". I think all of that still applies here (but let me know if I am wrong). Please add a comment that is at least as detailed as the SpEC comment.

markscheel · 2025-12-05T23:06:41Z

src/Evolution/Ringdown/Python/ComputeAhCCoefsInRingdownDistortedFrame.py

+    translation function of time for the ringdown using the functions of time
+    and the AhC coefficients in the Inspiral inertial frame.

    Arugments:


Minor: spelling.

ping on this

markscheel · 2025-12-05T23:09:27Z

src/Evolution/Ringdown/Python/ComputeAhCCoefsInRingdownDistortedFrame.py

+    (
+        fit_ahc_translation_coefs,
+        fit_ahc_dt_translation_coefs,
+        fit_ahc_dt2_translation_coefs,
+    ) = fit_to_a_cubic(
+        ahc_times_for_fit,
+        ahc_inertial_centers_for_fit,
+        match_time,
+        zero_coefs_eps,
+    )
+    ahc_translation_fot = [
+        fit_ahc_translation_coefs,
+        fit_ahc_dt_translation_coefs,
+        fit_ahc_dt2_translation_coefs,
+    ]


Just asking: is there a reason this can't be

ahc_translation_fot = fit_to_a_cubic(...)

instead of doing it in two steps?

no reason, I'll swap it to that

markscheel · 2025-12-05T23:21:34Z

src/Evolution/Ringdown/StrahlkorperCoefsAndCenters.cpp

-                                 translation_map_options,
+                                 translation_fot_from_volume,
                                 true};
  const domain::creators::Sphere domain_creator{


This appears to make a fake Domain that is different from the actual Domain that will be used for the final ringdown. Below you used to call it temporary_domain, so I think my interpretation is correct, but now it's no longer called temporary_domain (why)? Please add a (long, at least one paragraph, probably several paragraphs) comment of why you are doing this in the first place, what will this fake domain be used for, and what assumptions are being made about the fake domain (e.g. why is the inner radius so tiny as to make the domain extremely distorted [I don't think SpEC does this]? Why is the outer radius only 200M? Do you assume that AhC in the ringdown must have r < 200M [and if so, in what frame?] Why only 5 grid points?).

markscheel · 2025-12-05T23:30:21Z

src/Evolution/Ringdown/StrahlkorperCoefsInRingdownDistortedFrame.cpp

      false,
      std::nullopt,
-      {100.0},
+      {50.0},


I don't understand what "Radial partition used in current ringdowns" means or why it is 50.

nilsvu · 2025-12-12T19:40:00Z

src/Evolution/Ringdown/Python/ComputeAhCCoefsInRingdownDistortedFrame.py

+    translation function of time for the ringdown using the functions of time
+    and the AhC coefficients in the Inspiral inertial frame.

    Arugments:


ping on this

nilsvu · 2025-12-12T19:41:20Z

src/Evolution/Ringdown/Python/ComputeAhCCoefsInRingdownDistortedFrame.py

-            None,
-        )
+    shape_and_translation_coefs = Ringdown.strahlkorper_coefs_and_centers(
+        str(path_to_volume_data),


Use named arguments. Makes it clearer what each argument means and prevents bugs.

AlexCarpenter46 marked this pull request as draft August 11, 2025 20:45

AlexCarpenter46 force-pushed the ringdown_translation branch 2 times, most recently from cb37705 to 3a24d16 Compare August 11, 2025 23:12

This was referenced Aug 12, 2025

Add auto ringdown excision radius to transition script #6809

Open

Add BCO Python Binding #6815

Merged

AlexCarpenter46 force-pushed the ringdown_translation branch from 3a24d16 to 0b89dc2 Compare August 20, 2025 20:55

AlexCarpenter46 mentioned this pull request Aug 21, 2025

Move CoordsToDifferentFrame and Add Error Tol to Recenter #6820

Merged

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AlexCarpenter46 mentioned this pull request Sep 30, 2025

Transition to ringdown automatically #6838

Open

AlexCarpenter46 force-pushed the ringdown_translation branch 2 times, most recently from eb5ca91 to aa50963 Compare October 8, 2025 19:09

AlexCarpenter46 marked this pull request as ready for review October 8, 2025 21:11

AlexCarpenter46 requested a review from markscheel October 8, 2025 21:11

AlexCarpenter46 force-pushed the ringdown_translation branch from aa50963 to d413b76 Compare October 10, 2025 18:06

markscheel requested changes Oct 28, 2025

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nilsvu requested changes Oct 29, 2025

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AlexCarpenter46 commented Oct 30, 2025

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AlexCarpenter46 force-pushed the ringdown_translation branch 2 times, most recently from bab84f6 to 1978a2d Compare October 30, 2025 15:16

AlexCarpenter46 requested review from markscheel and nilsvu October 30, 2025 16:30

AlexCarpenter46 commented Nov 6, 2025

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AlexCarpenter46 force-pushed the ringdown_translation branch from 6ed71b3 to e9ed405 Compare December 1, 2025 19:20

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AlexCarpenter46 force-pushed the ringdown_translation branch from 632058b to 0d76c3a Compare January 12, 2026 16:41

AlexCarpenter46 added 2 commits January 12, 2026 08:46

Add translation to ringdown script

2694aa3

fixup comments

c79de3c

AlexCarpenter46 added 6 commits January 12, 2026 08:48

fixup translation fixed

d64c505

fixup adding more comments

326335c

fixup smaller excision radius

e2bbb33

fixup rebase

4f24670

fixup adding more comments

0a00386

fixup comments and rebase

0d76c3a

		if "Translation" not in evaluated_fot_dict:
		evaluated_fot_dict["Translation"] = None

Add translation to transition to ringdown script #6808

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Add translation to transition to ringdown script #6808

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