## [CosmoMC + ModeChord] Suspiciously (very) tight posterior distributions

Use of Healpix, camb, CLASS, cosmomc, compilers, etc.
Mason Ng
Posts: 25
Joined: May 17 2017
Affiliation: The University of Auckland

### [CosmoMC + ModeChord] Suspiciously (very) tight posterior di

I am doing a quick test run with ModeChord where I am calculating the Bayesian evidences for the inflationary models with potentials m2φ2 and λφ4. When I run these and plot the chains, I get a very suspicious-looking triangle plot.

As seen above, the parameters related to inflation (ns, r, log(m2)*) have a really tight distribution. This is not seen in tau. I did not put this in the plot I attached, but log(As) does not have a tight distribution (i.e. something more sensible).

I was wondering if anyone has any idea about this? I attach a sample .ini file below. In one of the lines, note that I had "compute_tensors=T". This was set to F for /that/ triangle plot. It was only after I generated that triangle plot did I realize that compute_tensors should probably be set to T. I am also testing right now if putting a prior on Npivot would change things too. I'm running the code again, but I'm just wondering if anyone has any further comments on this, and/or if that was the last thing I missed.

* Note 1: I know that lphi4 uses λ for vpar1 and m2phi2 uses m2. I was just doing a quick plot.

Code: Select allp>

#Use Planck 2015 likelihoods
DEFAULT&#40;./batch2/plik_dx11dr2_HM_v18_TT.ini&#41; #high-l likelihood
DEFAULT&#40;./batch2/lowl.ini&#41; #low-l likelihood; low TT WITHOUT Pol
DEFAULT&#40;./batch2/lensing.ini&#41; #for lensing
#DEFAULT&#40;./batch2/BKPlanck.ini&#41;

#other likelihoods
#DEFAULT&#40;./batch2/BAO.ini&#41;

#general settings
DEFAULT&#40;./batch2/common.ini&#41;

## INFLATION MODEL TO USE
#DEFAULT&#40;Models/m2phi2/m2phi2.ini&#41;
DEFAULT&#40;Models/lphi4/lphi4.ini&#41;

## Reheating settings&#58;
## Instant reheating
DEFAULT&#40;batch2/modecode_inst.ini&#41;

## Defaults
DEFAULT&#40;batch2/modecode_defaults.ini&#41;

# Many model parameters can be 'un-physical'.
# We make use of cosmomc's logzero functionality, but this requires that it
# doesn't stop when it hits an error
stop_on_error=  F

#high for new runs
MPI_Max_R_ProposeUpdate = 30

propose_matrix = ./planck_covmats/base_TT_lensing_lowl_plik.covmat

#folder where files &#40;chains, checkpoints, etc.&#41; are stored
root_dir = ./chains/

#root name for the files produced
file_root = lphi4_tensors_cal

#action = 0 to MCMC, action = 1 to postprocess .data file
#action = 1 for important samples, action = 4 to quickly test likelihoods
#action = 5 for the PolyChord sampler
action = 5
DEFAULT&#40;batch2/polychord.ini&#41;
nlive = 500

#to get theory cl for test point
test_output_root = output_cl_root

start_at_bestfit = F
feedback = 1
use_fast_slow = T

checkpoint = T

#sampling_method = 7 is a new fast-slow scheme good for Planck
sampling_method = 7
dragging_steps = 3
propose_scale = 2

#Set >0 to make data files for importance sampling
#26 August - don't need this yet
indep_sample = 0

#small speedups for testing
get_sigma8 = F

#REMEMBER, to use lensing, turn OFF semi slow parameters.
######## Semi Slow parameters ON #############
# Turn on semi-slow parameters
#use_nonlinear_lensing = F
#block_semi_fast = T

# Set up the fraction of time to spend on each type of parameter
#nest_frac_slow      = 0.75
#nest_frac_semi_slow = 0.20
#nest_frac_semi_fast = 0.00
#nest_frac_fast      = 0.05

######## Semi Slow parameters OFF #############
# Remove the equivalent lines directly above, and uncomment
# the lines below in order to turn on non-linear lensing
# &#40;and turn off semi slow parameters&#41;

# # Turn off semi-slow parameters
use_nonlinear_lensing = T
block_semi_fast = F
#
# # Set up the fraction of time to spend on each parameter
nest_frac_slow      = 0.95
nest_frac_semi_slow = 0.00
nest_frac_semi_fast = 0.00
nest_frac_fast      = 0.05

## priors for physical parameters
H0_min = 20
H0_max = 100

#to vary parameters, set param&#91;name&#93; = center, min, max, start width, propose_width
#for fixed, can just fix fixed value

## ModeCode-modified prior ranges
#param&#91;omegach2&#93; = 0.12 0.08 0.16 0.001 0.002
#param&#91;omegabh2&#93; = 0.02225 0.02 0.025 0.0001 0.0002
#param&#91;theta&#93; = 1.0411 1.00 1.08 0.0004 0.0004
#param&#91;tau&#93; = 0.09 0.05 0.13 0.001 0.001

#neutrino_hierarchy = degenerate
#num_massive_neutrinos = 1
#param&#91;mnu&#93; = 0.06

#param&#91;omegak&#93; = 0
#param&#91;w&#93; = 0
#param&#91;r&#93; = 0
#param&#91;nrun&#93; = 0
#param&#91;nrunrun&#93; = 0

#MODIFIED P&#40;K&#41;
#inflationary potential parameters
#Npivot
param&#91;Npivot&#93; = 50
#vparams array
param&#91;vpar1&#93; = −13.4 −16.0 −10.0 0.1 0.1
param&#91;vpar2&#93; = 0
param&#91;vpar3&#93; = 0
param&#91;vpar4&#93; = 0
param&#91;vpar5&#93; = 0
param&#91;vpar6&#93; = 0
param&#91;vpar7&#93; = 0
param&#91;vpar8&#93; = 0
param&#91;vpar9&#93; = 0
param&#91;vpar10&#93; = 0
param&#91;vpar11&#93; = 0
param&#91;vpar12&#93; = 0
param&#91;vpar13&#93; = 0
param&#91;vpar14&#93; = 0
param&#91;vpar15&#93; = 0
param&#91;vpar16&#93; = 0
param&#91;vpar17&#93; = 0
param&#91;vpar18&#93; = 0
param&#91;vpar19&#93; = 0
param&#91;vpar20&#93; = 0
# END MODIFIED P&#40;K&#41;

param&#91;omegabh2&#93; = 0.02225 0.02 0.025 0.0001 0.0001
param&#91;omegach2&#93; = 0.12 0.08 0.16 0.001 0.0005
param&#91;theta&#93; = 1.0411 1.00 1.08 0.0004 0.0004
param&#91;tau&#93; = 0.09 0.01 0.20 0.001 0.001

param&#91;logA&#93; = 3.1 2.5 3.5 0.001 0.001
#log&#91;1010 As&#93;
param&#91;ns&#93; = 0.96 0.8 1.2 0.004 0.002
inflation_consistency = T

param&#91;omegak&#93; = 0
param&#91;w&#93; = −1

#altered on 7 Sept
param&#91;r&#93; = 0.03 0 2 0.001 0.001
compute_tensors = T
#

param&#91;nrun&#93; = 0
param&#91;nrunrun&#93; = 0

neutrino_hierachy = degenerate
num_massive_neutrinos = 1
param&#91;mnu&#93; = 0.06

Mason Ng
Posts: 25
Joined: May 17 2017
Affiliation: The University of Auckland

### [CosmoMC + ModeChord] Suspiciously (very) tight posterior di

The cause is about the code somehow fixing Npivot somewhere - I am testing this at the moment I'll update on this, though I'm getting an error following on from the above. I'll put this on a new thread.