(Scroll down for original CLASS v3.2.0 README.md contents)
Author: Kevin Croker
This verison forked from v3.2.0. The CLASS team did a bunch of reworks, like improving the Python side of the build chain and removing OpenMP in subsequent versions, I've not ported the features below to that stuff yet.
When CLASS (the C stuff) errors out because of a bad cosmology, it didn't free its allocated memory. This was no problem if just called from the command line because the OS cleans up for you. But if called from the Python wrapper, this became a severe memory leak. If driven from MontePython for MCMC, available memory could be exhausted in seconds if the MCMC got into some squirelly place in cosmological parameter space. This was not a rare bug, it was around for many years and people complained about it on github, and annoyingly worked around it by just restarting MontePython, as many times as necessary.
This version of CLASS uses a very lightweight doubly linked list memory tracker so that, when failure happens, all allocated memory can be freed reliably. In my own use, MontePython can now run indefinitely and other diagnostics show stable memory usage over many hours of operation. (if you want to lol, run valgrind on CLASS ;) )
Under the hood, CLASS integrates physical densities in 1/Mpc2 units. Yet, for accounting, closing, and in various other places convenience, it works with big Ω's and H0. Although this is the standard way of "thinking" about Friedmann models, this is actually really bad for searching model space because it binds a present-day scale (H0) to the code that establishes initial conditions. For example, by writing Ωb H02/ai^3 to get the baryon density at ai, we've just projected backwards assuming nothing squirrely is happening across the intervening 13.8Gyr. Its a bold strategy Cotton, lets see if it pays off for 'em.
Anyway.
In this version of CLASS, the background and thermodynamics modules have been modified to work without specification of an H0 or an h.
Its an option, that can be flagged with
without_h = yes
What then happens is that you are only permitted to specify the little omegas (ω's).
These are already physical quantities, because they have a density scale baked into them.
CLASS then interprets the ω quantities as densities that can be projected backwards in time for initial conditions.
Anywhere in the code that made projection assumptions at intermediate scale factors, presumably for simplicity, has been adjusted to use the actual integrated densities determined by CLASS in the background module.
The motivation for this is that CMB experiments are telling you about early universe physics, before squirrely late-time things could be happening. So the best-fit parameters should be describing the early-universe. Other parameters can describe departures from the post-recombination conditions established by the little ω's. The upshot of all this is that early-time and late-time physical processes determine the resultant H0, so it obviously becomes a derived parameter, as it should be.
TLDR - if without_h is enabled, then the ω's describe the universe that would result today if late-time physical processes do not alter the background expansion history. Hubble can no longer be specified, as it becomes determined by the expansion history from initial conditions set by the ω's projected backwards.
CAVEAT - Only implemented in background and thermodynamics for now.
Also, departures from ΛCDM like dark radiation, interacting dark radiation, CLP and EDE fluids, and warm relics have not been updated because I don't require these in my work.
If you'd like to port these over, fix bugs, or do work on the other CLASS modules, I am super receptive and gracious to PRs!
(Below is the CLASS boilerplate)
Authors: Julien Lesgourgues, Thomas Tram, Nils Schoeneberg
with several major inputs from other people, especially Benjamin Audren, Simon Prunet, Jesus Torrado, Miguel Zumalacarregui, Francesco Montanari, Deanna Hooper, Samuel Brieden, Daniel Meinert, Matteo Lucca, etc.
For download and information, see http://class-code.net
(the information below can also be found on the webpage, just below the download button)
Download the code from the webpage and unpack the archive (tar -zxvf class_vx.y.z.tar.gz), or clone it from https://github.com/lesgourg/class_public. Go to the class directory (cd class/ or class_public/ or class_vx.y.z/) and compile (make clean; make class). You can usually speed up compilation with the option -j: make -j class. If the first compilation attempt fails, you may need to open the Makefile and adapt the name of the compiler (default: gcc), of the optimization flag (default: -O4 -ffast-math) and of the OpenMP flag (default: -fopenmp; this flag is facultative, you are free to compile without OpenMP if you don't want parallel execution; note that you need the version 4.2 or higher of gcc to be able to compile with -fopenmp). Many more details on the CLASS compilation are given on the wiki page
https://github.com/lesgourg/class_public/wiki/Installation
(in particular, for compiling on Mac >= 10.9 despite of the clang incompatibility with OpenMP).
To check that the code runs, type:
./class explanatory.ini
The explanatory.ini file is THE reference input file, containing and explaining the use of all possible input parameters. We recommend to read it, to keep it unchanged (for future reference), and to create for your own purposes some shorter input files, containing only the input lines which are useful for you. Input files must have a *.ini extension. We provide an example of an input file containing a selection of the most used parameters, default.ini, that you may use as a starting point.
If you want to play with the precision/speed of the code, you can use one of the provided precision files (e.g. cl_permille.pre) or modify one of them, and run with two input files, for instance:
./class test.ini cl_permille.pre
The files *.pre are suppposed to specify the precision parameters for which you don't want to keep default values. If you find it more convenient, you can pass these precision parameter values in your *.ini file instead of an additional *.pre file.
The automatically-generated documentation is located in
doc/manual/html/index.html
doc/manual/CLASS_manual.pdf
On top of that, if you wish to modify the code, you will find lots of comments directly in the files.
To use CLASS from python, or ipython notebooks, or from the Monte Python parameter extraction code, you need to compile not only the code, but also its python wrapper. This can be done by typing just 'make' instead of 'make class' (or for speeding up: 'make -j'). More details on the wrapper and its compilation are found on the wiki page
https://github.com/lesgourg/class_public/wiki
Since version 2.3, the package includes an improved plotting script called CPU.py (Class Plotting Utility), written by Benjamin Audren and Jesus Torrado. It can plot the Cl's, the P(k) or any other CLASS output, for one or several models, as well as their ratio or percentage difference. The syntax and list of available options is obtained by typing 'pyhton CPU.py -h'. There is a similar script for MATLAB, written by Thomas Tram. To use it, once in MATLAB, type 'help plot_CLASS_output.m'
If you want to develop the code, we suggest that you download it from the github webpage
https://github.com/lesgourg/class_public
rather than from class-code.net. Then you will enjoy all the feature of git repositories. You can even develop your own branch and get it merged to the public distribution. For related instructions, check
https://github.com/lesgourg/class_public/wiki/Public-Contributing
You can use CLASS freely, provided that in your publications, you cite
at least the paper CLASS II: Approximation schemes <http://arxiv.org/abs/1104.2933>. Feel free to cite more CLASS papers!
To get support, please open a new issue on the
https://github.com/lesgourg/class_public
webpage!