Show how to compute comoving distance

source/lightcones/healpix_shell_redshifts.rst

@@ -32,6 +32,13 @@ Each shell is defined by a minimum and maximum redshift, as shown
 below. All simulations use the same redshift shells except that
 shells beyond the maximum redshift are omitted.
 
+.. note:: Not all of the FLAMINGO simulations use the same
+          cosmological parameters, so the relation between redshift
+          and comoving distance varies. If you need to translate
+          between redshift, distance and time in the cosmology of a
+          specific simulation, we recommend :ref:`using swiftsimio
+          <swiftsimio_cosmology>`.
+
 .. _shell_redshift_table:
 
 .. list-table::

source/lightcones/particle_lightcone_redshifts.rst

@@ -3,7 +3,10 @@ Particle lightcone redshift ranges
 
 The tables below show the maximum redshift at which particles of each
 type are output in each of the FLAMINGO simulations. Where no redshift
-is listed no lightcone particle data are available.
+is listed no lightcone particle data are available. The corresponding
+comoving distances vary depending on the cosmological parameters of
+the simulation and can be :ref:`computed with swiftsimio
+<swiftsimio_cosmology>`.
 
 There are two thresholds for gas particles to be output. All gas
 particles are output up to the redshift specified in the "All gas"

source/snapshots/swiftsimio.rst

@@ -73,16 +73,7 @@ metadata. The simulation box size is available as::
 
   snap.metadata.boxsize
 
-If you need cosmology information, you can get an astropy `cosmology
-object <https://docs.astropy.org/en/stable/cosmology/index.html>`__
-with::
-
-  cosmo = snap.metadata.cosmology
-
-This allows accurate calculation of the age of the universe or the
-comoving distance at a particular redshift in the FLAMINGO cosmology,
-for example. The numbers of particles of each type in the snapshot are
-available as::
+The numbers of particles of each type in the snapshot are available as::
 
   snap.metadata.n_gas
   snap.metadata.n_dark_matter
@@ -111,6 +102,33 @@ See the `swiftsimio documentation
 <https://swiftsimio.readthedocs.io/en/latest/loading_data/index.html#using-metadata>`__
 on snapshot metadata for more information.
 
+.. _swiftsimio_cosmology:
+
+Cosmology
+---------
+
+If you need cosmology information, you can get an astropy `cosmology
+object <https://docs.astropy.org/en/stable/cosmology/index.html>`__ by
+opening a snapshot as shown above, then accessing its cosmology
+field::
+
+  cosmo = snap.metadata.cosmology
+
+This is the recommended way to translate between redshift, comoving
+distance and lookback time in the FLAMINGO simulations. You can
+compute the age of the universe at :math:`z = 0.5`, for example,
+with::
+
+  age_of_universe = cosmo.age(0.5)
+
+or compute the comoving distance to the same redshift with::
+
+  comoving_distance = cosmo.comoving_distance(0.5)
+
+See the `astropy documentation
+<https://docs.astropy.org/en/stable/cosmology/index.html>`__ for more
+details.
+
 Reading particle data
 ---------------------