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ReadMe for ModeChord -- PolyChord enabled version of ModeCode.

November 2016.


If you use ModeChord in your work, please cite the following papers:

Modecode papers

PolyChord papers

Cosmomc papers

BibTeX can be found in: modechord.bib

In this ReadMe "ModeChord" refers to the PolyChord enabled version of ModeCode.

ModeChord computes the primordial scalar and tensor power spectra for models of inflation. It can be run as a standalone code, or combined with CAMB to compute CMB angular power spectra and CosmoMC+PolyChord to perform likelihood analysis and constrain parameters of inflationary models.

ModeChord is integrated with the nested sampling package PolyChord, and permits both parameter estimation and the computation of Bayesian evidence.

The code's webpage is:

PolyChord can be obtained here:

CosmoMC can be obtained here:

Version history:

The currently available files are:

  • camb/ - sample driver for standalone code (driver_modpk.f90), new files used by ModeChord, and modified CAMB files

  • source/ - modified CosmoMC code

  • Models/model - files for running CosmoMC with the PolyChord sampler for the models included in ModeChord (note that additional models can be easily added as described below), where model is one of:

    • m2phi2 (quadratic potential)
    • lphi4 (quartic)
    • neq1 (n=1),
    • neq2ov3 (n=2/3)
    • natural
    • hilltop
    • higgs (Higgs inflation/Starobinsky model)
    • monodromy (different versions of the model)
    • reconstruction (Hubble slow-roll reconstruction).
  • batch2/ - directory for storing generic input files for ModeChord. these additional files are accessed via test.ini and files in Models/ . In addition, batch2/params_CMB_defaults.ini is modified to have narrower priors on cosmological parameters, and to include default prior widths for ModeCode's additional V(phi) parameters.

  • data/camspec.paramnames - paramnames file for using Planck 2015 likelihoods.

As usual, modechord.tar.gz files can be unzipped and extracted using the command 'tar -xzvf modechord.tar.gz'.

The current version of the code is based on the November 2016 version of CAMB and CosmoMC and the November 2016 PolyChord.

Only new or modified CAMB/CosmoMC files are provided in the downloads, so any applications beyond using the standalone version of ModeCode will require adding the downloaded files to an existing copy of the basic CAMB and CosmoMC codes.

In each of the modified files, changes to the original code are marked by 'MODIFIED P(K) ... END MODIFIED P(K)' or 'MODIFIED POLYCHORD ... END MODIFIED POLYCHORD' so that they can be easily located.

Setting up and compiling ModeChord

This involves three steps

  1. Install cosmomc
  2. Install polychord (cosmochord)
  3. Install modechord

After obtaining a cosmomc install as detailed here:

you should install the polychord plug-in for cosmomc (cosmochord) as detailed here:

After extracting files from modechord.tar.gz, the ModeChord files must be copied into existing versions of CAMB/CosmoMC for use with those codes. One easy way to do this is to start in the cosmomc/ directory for the ModeChord files, and use 'cp -r . cosmomc_target/' where cosmomc_target is the base directory for the unmodified code.

Note that this will overwrite original versions of some of the CAMB/CosmoMC files; if you do not want this to happen, rename the files first before copying (they will also need to be renamed in the Makefiles).

Once the new and modified code files have been copied into the appropriate directories, the ModeChord versions of CAMB and CosmoMC can be compiled with the usual Makefiles, after adjusting the compiler options in the camb/Makefile and source/Makefile supplied with ModeChord to work on your system. You will also need to supply any likelihood codes (e.g. Planck) that do not ship with the standard CAMB/CosmoMC.

Executing the make command in the source directory will produce the executables cosmomc and getdist.

To compile the sample driver program for the standalone version of ModeCode without compiling CAMB or CosmoMC, run 'make powspec' in the camb/ directory.

General description of ModeChord parameters and options

Inflationary models are described in ModeCode using an array of parameters for the potential V(phi), vparams(i) for i = 1, 2, etc., and the parameter N_pivot (not used for HSR models, see below), which accounts for the uncertainty in the post-inflationary epoch between the inflation and reheating. N_pivot is the number of inflationary e-folds remaining when the mode of a given scale infl_pivot_k exits the horizon. The value of infl_pivot_k can be specified in units of Mpc^{-1}.

ModeCode options:

  • use_modpk (logical): a general flag to turn ModeCode on or off. Setting use_modpk=F recovers the original functionality of CAMB/CosmoMC while retaining the PolyChord sampler. When running the code with use_modpk=F, ParamNamesFile in params_modpk.ini should be changed to params_modpk_F.paramnames.

  • modpk_physical_priors (logical): Excludes models where the scalar spectral amplitude is vastly different from the observed value.

  • modpk_rho_reheat (real): Reheating density in GeV^4 Note that the temperature / energy ~ modpk_rho_reheat^(1/4)

  • modpk_w_primordial_lower (real): Lower limit on effective equation of state parameter during between the end of inflation and rho=rho_reheat

  • modpk_w_primordial_upper (real): Upper limit on effective equation of state parameter during between the end of inflation and rho=rho_reheat

  • potential_choice (integer): specifies which inflationary model to use. The translation between potential_choice and V(phi) is set in camb/modpk_potential.f90. The models corresponding to different values of potential_choice by default are listed below.

  • vparams_num (integer): number of parameters needed to describe V(phi) (for general reheating models, the total number of MCMC parameters is vparams_num+1). The maximum number of parameters is set to 9 by default, but can be increased in camb/modpk_modules.f90 by changing max_vparams.

  • phi_init (real): starting guess for the initial value of phi; for all models provided with the code, this setting is superceded by the 'initialphi' function in camb/modpk_potential.f90.

  • infl_pivot_k (real): ``pivot'' wavenumber for evaluating N_pivot, in Mpc^{-1}. Note that this parameter is named k_pivot, when ModeCode is called with CAMB instead of CosmoMC/PolyChord.

  • N_pivot (real): number of e-folds of inflation remaining after the mode with wavenumber infl_pivot_k leaves the horizon (note that N_pivot is treated as a parameter to be varied rather than a fixed setting).

  • instreheat (logical): whether or not to assume instant reheating, which fixes the value of N_pivot; if instreheat=T, the values chosen for infl_pivot_k and N_pivot are ignored.

  • slowroll_infl_end (logical): whether or not to determine when inflation ends by the breakdown of slow roll conditions, i.e. epsilon_H=1. If slowroll_infl_end=F, the end of inflation is assumed to occur when phi=phi_infl_end.

  • phi_infl_end (real): final value of phi during inflation for models that do not end via slow roll violation (slowroll_infl_end=F). If slowroll_infl_end=T, the value of this parameter is ignored.

  • vnderivs (logical): whether to use numerical derivatives of the potential (vnderivs=T) or analytic forms supplied in camb/modpk_potential.f90 (vnderivs=F). The latter option is STRONGLY recommended for all models for which the derivatives of V(phi) can be computed and expressed analytically, as the use of numerical derivatives may lead to inaccurate results for certain models.

  • action (integer) action = 5 to use PolyChord, action=0 to use the standard MCMC sampler.

Default models in ModeCode:

  • potential_choice = 1: quadratic V(phi) = m^2 phi^2 / 2 vparams(1) = log_10(m^2)
  • potential_choice = 2: natural V(phi) = Lambda^4 [1+cos(phi/f)] vparams(1) = log_10(Lambda), vparams(2) = log_10(f)
  • potential_choice = 3: quartic V(phi) = lambda phi^4 / 4 vparams(1) = log_10(lambda)
  • potential_choice = 4: linear V(phi) = lambda phi vparams(1) = log_10(lambda)
  • potential_choice = 5: exponent n=2/3 V(phi) = (3/2) lambda phi^{2/3} vparams(1) = log_10(lambda)
  • potential_choice = 6: hilltop V(phi) = Lambda^4 - lambda phi^4 / 4 vparams(1) = log_10(Lambda), vparams(2) = log_10(lambda)

Some further models are available (check camb/modpk_potential.f90). Oscillatory models such as axion monodromy (potential_choice=10), generalised axion monodromy (potential_choice=13) and a potential with a step feature (potential_choice=11) require the flag -DWIGGLY to be set in the compilation FFLAGS in order to sufficiently increase the accuracy of CAMB. This will in general significantly slow down the computation of angular power spectra. We caution the user to test the numerical convergence of these settings when adapting the code.

Using ModeCode for HSR reconstruction:

J. Norena, C. Wagner, L. Verde, H. Peiris, and R. Easther, arXiv:1202.0304

In the Hubble slow-roll (HSR) reconstruction the Hubble parameter is modeled by a finite polynomial: H(phi) = H_star (1 + A_1 phi + A_2 phi^2 + ... + A_N phi^N).

The coefficients A_i are related by a one-to-one correspondence to the Hubble slow-roll parameters (epsilon, eta, xi, ...), and H_star sets the overall energy scale.

The corresponding potential is then given by: V(phi) = M_pl^2 H_star^2 [3(1 + A_1 phi + ... + A_N phi^N)^2 - 2(A_1 + ... + N A_N phi^(N-1))]

At the moment, the HSR potential is implemented in ModeCode for the first three HSR parameters, i.e. epsilon, eta and xi. However, the extension to higher orders is straightforward. To allow for uniform and log priors on epsilon, one can choose between:

  • potential_choice = 7: HSR with eps, eta, xi vparams(1) = epsilon vparams(2) = eta vparams(3) = xi vparams(4) = log(10^10 A_SR)


  • potential_choice = 8: HSR with eps, eta, xi vparams(1) = log_10(epsilon) vparams(2) = eta vparams(3) = xi vparams(4) = log(10^10 A_SR)

where the HSR parameters are given at phi=0. A_SR is the amplitude of the curvature power spectrum at the pivot scale (i.e., the mode which exits the horizon at phi=0) computed at second order in the HSR parameters. This relation between the HSR parameters and the amplitude is then used to set the value of H_star.

Note that N_pivot is not used. Instead the parameter reconstruction_Nefold_limit specifies the minimum number of inflationary e-folds after the scale given by infl_min_k exits the horizon. In addition, the smallest scale for which one requires that it exits the horizon still during inflation is given by infl_max_k.

In summary, the parameters needed for the HSR reconstruction are:

  • infl_pivot_k (real): specifies the mode which exits the horizon when phi=0, in Mpc^{-1}. In addition, derived parameters like the spectral tilt or the tensor-to-scalar ratio etc. are evaluated at this scale. Note that this parameter is named k_pivot, when ModeCode is called with CAMB instead of CosmoMC/PolyChord.

  • infl_min_k (real): largest scale for which one requires that it exits the horizon during inflation, in Mpc^{-1}.

  • infl_max_k (real): smallest scale for which one requires that it exits the horizon during inflation, in Mpc^{-1}.

  • reconstruction_Nefold_limit (real): minimum number of inflationary e-folds counted from infl_min_k.

Computing power spectra (standalone code)

The sample driver included with the code (camb/driver_modpk.f90) can be run using the command 'powspec' in the camb/ directory. It is set up to compute the scalar and tensor power spectra at 500 k values between 5x10^{-4} Mpc^{-1} and 5 Mpc^{-1} for a natural inflation model. The code outputs (k, P_s(k), P_t(k)) and also computes the scalar and tensor amplitudes and spectral tilts, as well as the running of the scalar spectral index, at k_pivot = 0.05 Mpc^{-1}. This program can be easily modified to compute the spectra for different models of inflation and/or different values of k.

Warnings that phi_init is inconsistent and the value of phi_init is being rescaled are a normal byproduct of the algorithm that searches for self-consistent initial conditions for inflation. If there are an excessive number of these warnings for each model evaluated, it may help to change the user-supplied phi_init value or the function used in 'initialphi' in camb/modpk_potential.f90 for the inflationary model in question.

Other warnings or errors produced by ModeCode typically indicate that the chosen model parameters do not have a physically acceptable inflationary solution.

Using ModeCode with CAMB

ModeCode can be used within CAMB by running 'camb params_modpk.ini' in the camb/ directory. The ModeCode options and parameters are set in the section of params_modpk.ini marked 'MODIFIED P(K)', which follows the entries for the initial power spectrum parameters from the unmodified version of CAMB. Note that these original spectral parameter values (e.g. scalar_amp(1), scalar_spectral_index(1), etc.) are ignored if use_modpk=T since the initial power spectra are entirely specified by the values of N_pivot and the vparams array.

Using ModeCode with CosmoMC+PolyChord

The use of the ModeCode version of CosmoMC+PolyChord is largely unchanged from the original version of the code.

ModeCode-specific options have been added to the test.ini and batch2/params_CBM_defaults.ini files. These are marked by 'MODIFIED P(K)' and 'MODIFIED POLYCHORD'.

As a sampler PolyChord is parallelised via a master-slave structure. This means that it runs optimally in pure MPI (i.e. with no openMP parallisation, OMP_NUM_THREADS=1). With MPI it is parallelised effectively up to the number of live points nlive (default 500). Note that this contrasts with the default Metropolis-Hastings sampler of CosmoMC. With 128 MPI processes, runs complete in typically ~12 hours. This is approximately 2-4 times longer than a CosmoMC run with a well tuned covariance matrix.

When CAMB+ModeCode is run as a standalone code, N_pivot is a free parameter specified by the user. Within CosmoMC+PolyChord N_pivot is an independent parameter. If instant reheating is assumed (instreheat=T), then N_pivot should be fixed by setting 'param[Npivot] = 50 50 50 0 0' in params_modpk.ini (the choice of 50 here is unimportant since the code ignores this value for instant reheating models).

N_pivot and the vparams array (named vpar1, vpar2, etc. in CosmoMC) are added as semi-slow parameters immediately after the traditional semi slow parameters, but before fast parameters. Elements in the vparams array that are not used by a particular inflationary model should be fixed in the MCMC analysis. Currently a model can have at most 20 V(phi) parameters, but this maximum may be increased by adjusting the value of max_vparams in camb/modpk_modules.f90, and adding additional lines to the relevant .paramnames in the Models directory.

The chain files output by the ModeChord version of CosmoMC have several additional derived parameters which are listed at the end of params_modpk.paramnames:

  • modpk_Npivot; if instreheat=F, this should always be equal to the chain parameter N_pivot, but if instreheat=T it will be the value of N_pivot computed by the code to satisfy the matching equation for instant reheating models. Note that for HSR reconstruction this value is meaningless.

  • modpk_ns, modpk_nt (scalar and tensor spectral tilt)

  • modpk_nrun (scalar spectral running dn_s/dlnk)

  • modpk_logA (ln(10^{10}A_s), i.e. the usual CosmoMC scalar amplitude parameter)

  • modpk_r (tensor-to-scalar ratio)

  • modpk_w Effective primordial equation of state parameter (see arXiv:1112.0326 for details)

Each of these spectral parameters is computed directly from the primordial power spectra computed by ModeCode at the pivot scale.

Using ModeChord with GetDist

Output files can be processed with standard GetDist tools:

Example input files for getdist can be found in the relevant Models directory as Models//distparams_ .

Adding new inflationary models

Single field inflationary models beyond those provided can be simply added to ModeCode by adding the following functions to camb/modpk_potential.f90:

  • V(phi) in the function pot(phi)

  • the first derivative of V in the function dVdphi(phi)

  • the second derivative of V in the function d2Vdphi2(phi)

  • an approximate expression for phi(N_pivot) (e.g., derived using slow roll relations) in the function initialphi; this is used to compute a reasonable first guess for the initial conditions given the shape of the potential

Although only specification of V(phi) is absolutely necessary (with vnderivs=T the code will attempt to compute numerical derivatives of the potential, and the function initialphi defaults to the set value of phi_init if no phi(N_pivot) relation is given), the results of the code are generally much more reliable if all four of these functions can be provided.

The potential choices provided in the code can be used as templates for each of these functions. For example, a new form of the potential corresponding to potential_choice=15 would require adding a 'case(15)' statement to each of the four functions described above with the V(phi) parameters and functions specified following the examples provided.







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