9. Cosmological Simulations

RAMSES writes the same info-file fields for every run, so Mera reads them for both idealised and cosmological simulations. A non-cosmological run carries sentinel values (aexp = 1, H0 = 1, omega_m = 1, omega_l = 0); a cosmological run carries physical values (e.g. aexp = 0.875, H0 = 70.3, omega_m = 0.276, omega_l = 0.724).

This tutorial shows the cosmology-aware tools Mera builds on top of that:

• detect a cosmological run and get its redshift / full cosmological state
• gettime returning the age of the universe (cosmological info.time is conformal time, not proper time)
• stellar ages, formation redshift and formation time from the particle birth times (RAMSES stores these as conformal time)
• the gas overdensity field
• comoving ↔ proper conversions

All of these are derived from fields that have always existed on InfoType, so they also work on MERA files of any age (see Backward compatibility below).

Test simulation and reference

The example below uses the public yt project sample output output_00080 — a cosmological zoom simulation (gas, dark matter and stars), provided by the yt project (Turk et al. 2011, yt: A Multi-code Analysis Toolkit for Astrophysical Simulation Data, ApJS 192, 9). It is not redistributed with Mera; download it once with:

mkdir -p yt_cosmo && cd yt_cosmo
curl -OL https://yt-project.org/data/output_00080.tar.gz
tar xzf output_00080.tar.gz

Snapshot properties: z ≈ 0.143 (aexp ≈ 0.875), H0 = 70.3, Ωm = 0.276, ΩΛ = 0.724, flat (Ωk = 0).

Reading a cosmological run

getinfo automatically detects a cosmological run and prints a dedicated cosmology block (redshift, scale factor, density parameters, age, lookback and Hubble time). The reported simulation time is the age of the universe at the snapshot, not the raw conformal info.time.

using Mera
info = getinfo(80, "/Volumes/FASTStorage/Simulations/Mera-Tests/yt_cosmo");

[Mera]: 2026-06-01T19:55:47.873
Code: RAMSES
output [80] summary:
mtime: 2012-08-13T16:51:06
ctime: 2026-06-01T07:42:48.561
=======================================================
simulation time: 11.925 [Gyr] (age of universe)
boxlen: 62.14 [Mpc]
ncpu: 16
ndim: 3
-------------------------------------------------------
cosmological:  true
redshift z:    0.1426   (aexp = 0.8752)
H0: 70.30 km/s/Mpc   Ωm: 0.276   ΩΛ: 0.724   Ωk: 0.000   Ωb: 0.045
age: 11.925 Gyr   lookback: 1.798 Gyr   Hubble time: 13.909 Gyr
-------------------------------------------------------
amr:           true
level(s): 6 - 16 --> cellsize(s): 970.86 [kpc] - 948.11 [pc]
-------------------------------------------------------
hydro:         true
hydro-variables:  6
  --> (:rho, :vx, :vy, :vz, :p, :var6)
γ: 1.4
-------------------------------------------------------
gravity:       true
gravity-variables: (:epot, :ax, :ay, :az)
-------------------------------------------------------
particles:     true  (no particle header file)
particle-variables: 5  --> (:vx, :vy, :vz, :mass, :birth)
-------------------------------------------------------
rt:            false
clumps:           false
namelist-file:    false
timer-file:       false
compilation-file: false
makefile:         false
patchfile:        false
=======================================================

Detecting a cosmological run

iscosmological returns true/false; redshift returns z = 1/aexp − 1 (0 for a non-cosmological run).

iscosmological(info), redshift(info)

(true, 0.14255728632206321)

Full cosmological state: cosmology

cosmology(info) returns a NamedTuple with the complete state of the snapshot — redshift, scale factor, the density parameters, and derived quantities: Hubble time, age of the universe, lookback time and the critical density ρ_crit = 3H(z)²/8πG. Everything is computed from the stored info fields.

c = cosmology(info)

(iscosmological = true, redshift = 0.14255728632206321, aexp = 0.875229637910795, H0 = 70.3000030517578, omega_m = 0.276000022888184, omega_l = 0.723999977111816, omega_k = 0.0, omega_b = 0.0450000017881393, hubble_time_Gyr = 13.9088503437748, age_Gyr = 11.92526533595642, lookback_Gyr = 1.7984823184196177, rho_crit_cgs = 1.0542295486333991e-29)

Cosmic time with gettime

In a cosmological run info.time is conformal time (negative), so the naive info.time × scale is meaningless. gettime therefore returns the age of the universe at the snapshot, in the requested unit.

gettime(info, :Gyr)    # age of the universe at this snapshot [Gyr]

11.92526533595642

Mean and critical densities

mean_matter_density and mean_baryon_density give the mean (proper) densities at the snapshot redshift, ρ̄ = Ω · ρ_crit,0 · (1+z)³ — the reference densities for overdensities. The critical density at the snapshot is c.rho_crit_cgs.

mean_matter_density(info), mean_baryon_density(info), c.rho_crit_cgs   # [g/cm³]

(3.821451389512857e-30, 6.230626996398271e-31, 1.0542295486333991e-29)

Stellar ages

RAMSES stores a star particle's birth time as conformal (super-conformal) time, the same variable as info.time. A physical age therefore is not (info.time − birth) × scale; it is the difference of proper cosmic times at the snapshot and at birth. Mera does this conversion via a Friedmann integration table (as in RAMSES' own friedman routine), so getvar(…, :age) returns the correct physical age on a cosmological run.

Star particles have birth < 0; non-star particles (dark matter) carry the sentinel birth = 0 and are reported with age 0 — select stars with birth .< 0.

particles = getparticles(info);
birth = getvar(particles, :birth)
stars = birth .< 0.0
ages  = getvar(particles, :age, :Gyr)
(n_stars = count(stars), age_min = minimum(ages[stars]), age_max = maximum(ages[stars]))

[Mera]: Get particle data: 2026-06-01T19:55:52.461
Using threaded processing with 4 threads
Key vars=(:level, :x, :y, :z, :id)
Using var(s)=(1, 2, 3, 4, 5) = (:vx, :vy, :vz, :mass, :birth)
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [Mpc] :: 62.135 [Mpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [Mpc] :: 62.135 [Mpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [Mpc] :: 62.135 [Mpc]
Processing 16 CPU files using 4 threads
Mode: Threaded processing
Combining results from 4 thread(s)...
Found 1.090895e+06 particles
Memory used for data table :
74.9068775177002 MB
-------------------------------------------------------

(n_stars = 31990, age_min = 5.0186259167604e-15, age_max = 11.225102965242586)

using PyPlot
rc("figure", dpi=300); rc("savefig", dpi=300)
figure(figsize=(6,4))
hist(ages[stars], bins=40, color="steelblue")
xlabel("stellar age  [Gyr]"); ylabel("number of star particles")
title("Stellar ages (cosmological run)"); tight_layout();

Formation redshift and formation time

:zform (alias :formation_redshift) gives the redshift at which each star formed, :formation_time the corresponding age of the universe. They satisfy formation_time + age = age of the universe at the snapshot.

Non-star particles (birth ≥ 0) return NaN for both :zform and :formation_time (the underlying formation_redshift / formation_time are undefined for the birth = 0 sentinel) — so select stars with birth .< 0 or filter !isnan, not == 0. (Only :age returns 0 for non-stars.)

zform = getvar(particles, :zform)
ftime = getvar(particles, :formation_time, :Gyr)
(zform_max = maximum(zform[stars]), ftime_min = minimum(ftime[stars]), ftime_max = maximum(ftime[stars]))

(zform_max = 7.593387894486433, ftime_min = 0.7001634695003264, ftime_max = 11.925247959877392)

figure(figsize=(6,4))
hist(zform[stars], bins=40, color="darkorange")
xlabel("formation redshift  z_form"); ylabel("number of star particles")
title("Star formation vs. redshift"); tight_layout();

Gas overdensity

:overdensity (alias :delta) is the gas density contrast relative to the mean baryon density, δ = ρ/ρ̄_b − 1. By construction δ ≥ −1; it is ≈ 0 in the mean field and ≫ 1 in collapsed structures.

gas = gethydro(info);
delta = getvar(gas, :overdensity)
(delta_min = minimum(delta), delta_max = maximum(delta))

[Mera]: Get hydro data: 2026-06-01T19:56:00.568
Key vars=(:level, :cx, :cy, :cz)
Using var(s)=(1, 2, 3, 4, 5, 6) = (:rho, :vx, :vy, :vz, :p, :var6)
domain:
xmin::xmax: 0.0 :: 1.0  	==> 0.0 [Mpc] :: 62.135 [Mpc]
ymin::ymax: 0.0 :: 1.0  	==> 0.0 [Mpc] :: 62.135 [Mpc]
zmin::zmax: 0.0 :: 1.0  	==> 0.0 [Mpc] :: 62.135 [Mpc]
📊 Processing Configuration:
   Total CPU files available: 16
   Files to be processed: 16
   Compute threads: 4
   GC threads: 4
Processing files: 100%|██████████████████████████████████████████████████| Time: 0:00:01 (75.31 ms/it)
✓ File processing complete! Combining results...
✓ Data combination complete!
Final data size: 1749455 cells, 6 variables
Creating Table from 1749455 cells with max 4 threads...
  Threading: 4 threads for 10 columns
  Max threads requested: 4
  Available threads: 4
  Using parallel processing with 4 threads
  Creating IndexedTable with 10 columns...
  0.846090 seconds (3.28 M allocations: 478.784 MiB, 0.80% gc time, 88.93% compilation time: 1% of which was recompilation)
✓ Table created in 1.142 seconds
Memory used for data table :133.47387886047363 MB
-------------------------------------------------------

(delta_min = -0.9852022034691776, delta_max = 1.2811226613578297e7)

figure(figsize=(6,4))
hist(log10.(1.0 .+ delta), bins=60, color="seagreen")
xlabel("log10(1 + δ)"); ylabel("number of cells")
title("Gas overdensity distribution"); tight_layout();

Comoving ↔ proper

RAMSES unit_l/unit_d (hence all of Mera's length/density scales) are the proper (physical) values at the snapshot's scale factor, so positions, densities, projections and surface densities returned by Mera are in the proper frame. Convert to/from comoving with the helpers (identities for a non-cosmological run, where aexp = 1):

l_proper   = comoving_to_proper_length(info, 1.0)    # comoving → proper  (× aexp)
l_comoving = proper_to_comoving_length(info, 1.0)    # proper → comoving  (÷ aexp)
ρ_proper   = comoving_to_proper_density(info, 1.0)   # × aexp⁻³
(l_proper, l_comoving, ρ_proper)

(0.875229637910795, 1.1425572863220632, 1.4915367304559788)

Backward compatibility

None of this required new stored fields: the accessors read only aexp, H0 and the omega_* parameters, which every InfoType — and therefore every MERA file ever written — already contains. Consequently:

• old MERA/JLD2 files load unchanged, and infodata prints the same cosmology block as getinfo;
• on a non-cosmological run iscosmological is false, redshift is 0, and the cosmology-only getvar variables (:zform, :formation_time, :overdensity) raise a clear error.

Summary of the cosmology API