Added the primoridal abundance of Helium to the list of physical constants.

src/physical_constants.c

@@ -126,6 +126,11 @@ void phys_const_init(const struct unit_system *us, struct swift_params *params,
   internal_const->const_T_CMB_0 =
       const_T_CMB_0_cgs /
       units_general_cgs_conversion_factor(us, dimension_temperature);
+
+  const float dimension_Yp[5] = {0, 0, 0, 0, 0}; /* [ - ] */
+  internal_const->const_primordial_He_fraction =
+      const_primordial_He_fraction_cgs /
+      units_general_cgs_conversion_factor(us, dimension_Yp);
 }
 
 /**

src/physical_constants.h

@@ -93,6 +93,9 @@ struct phys_const {
 
   /*! Temperature of the CMB at present day */
   double const_T_CMB_0;
+
+  /*! Primordial Helium fraction */
+  double const_primordial_He_fraction;
 };
 
 void phys_const_init(const struct unit_system* us, struct swift_params* params,

src/physical_constants_cgs.h

@@ -95,4 +95,7 @@ const double const_earth_mass_cgs = 5.9724e27;
 /*! Temperature of the CMB at present day [K] */
 const double const_T_CMB_0_cgs = 2.7255;
 
+/*! Primordial Helium fraction */
+const double const_primordial_He_fraction_cgs = 0.245;
+
 #endif /* SWIFT_PHYSICAL_CONSTANTS_CGS_H */