add luminosity_distance to LensingCosmology

Implements D_L(z) = (1+z)^2 * D_A(0,z) returning Mpc, matching
astropy's convention. Fixes AttributeError in workspace scripts
that call cosmo.luminosity_distance().

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

Author: 2 people

autogalaxy/cosmology/model.py

@@ -62,6 +62,27 @@ def kpc_per_arcsec_from(self, redshift: float, xp=np) -> float:
         """
         return self.kpc_proper_per_arcsec(z=redshift, xp=xp)
 
+    def luminosity_distance(self, z: float, xp=np) -> float:
+        """
+        Luminosity distance to redshift z in Mpc.
+
+        For a flat universe:
+
+            D_L(z) = (1 + z)^2 * D_A(0, z)
+
+        where D_A(0, z) is the angular diameter distance from Earth.
+
+        Returns Mpc, matching the convention of astropy.cosmology.FlatLambdaCDM.luminosity_distance(z).value.
+
+        Parameters
+        ----------
+        z
+            Redshift at which the luminosity distance is calculated.
+        """
+        D_A_kpc = self.angular_diameter_distance_to_earth_in_kpc_from(redshift=z, xp=xp)
+        D_A_Mpc = D_A_kpc / xp.asarray(1.0e3)
+        return (xp.asarray(1.0) + xp.asarray(z)) ** 2 * D_A_Mpc
+
     def angular_diameter_distance_to_earth_in_kpc_from(
         self, redshift: float, xp=np
     ) -> float:

test_autogalaxy/cosmology/test_model.py

@@ -115,6 +115,17 @@ def test__scaling_factor_between_redshifts_from():
     assert scaling_factor == pytest.approx(0.6739452581456, 1e-4)
 
 
+def test__luminosity_distance():
+
+    cosmology = ag.cosmology.Planck15()
+
+    # D_A(0, 0.1) = 392840 kpc = 392.840 Mpc (verified by test__angular_diameter_distances)
+    # D_L(0.1) = (1 + 0.1)^2 * 392.840 = 1.21 * 392.840 = 475.3364 Mpc
+    luminosity_distance = cosmology.luminosity_distance(z=0.1)
+
+    assert luminosity_distance == pytest.approx(475.336, 1e-4)
+
+
 def test__velocity_dispersion_from():
 
     cosmology = ag.cosmology.Planck15()