Examining a possible redshift dependence of ΛCDM parameters in the Pantheon compilation

Sammanfattning

This thesis investigates indications of a redshift dependence in cosmologicalparameters of the ΛCDM model found in the Pantheon compilation of Supernova oftype 1a (SN1a) data. A maximum likelihood fit of the effective absolute magnitudeM (which combines H0 and the SN1a absolute magnitude) and the matter densityparameter Ωm,0 in redshift binned subsamples have been found to oscillate around thefull dataset best fit, particularly at low z [2]. In this thesis we have further examinedthese findings by including systematic uncertainties in the analysis, extending theanalysis by allowing the dark energy equation of state w to vary, investigated whetherthis dynamic dark energy could adsorb the variation in M and Ωm,0, and finallyperforming a discrete Fourier analysis of the best fit parameters to identify anydominant oscillatory modes. Including the systematic error did reduce tensions,however the oscillatory pattern still persists and the amplitude of the deviation is stillover 1σ. The results are still unlikely in the context of ΛCDM, even with the systematicuncertainties included. Including the equation of state for dark energy w as a freeparameter resulted in: large errors when both M and Ωm,0 were free to vary alongsideit, an oscillated pattern with an amplitude of 1σ with w and M as free parameters,lower w favored for lower z with only w as a free parameter. Fitting a model of w tothis data and performing the redshift bin analysis with M and Ωm,0 as free parametersmarginally improves agreement in some bins, but fails to lower the overall tension. Theresults do not favor the alternative model for w over the ΛCDM model. The frequencyanalysis of overlapping subsamples with M and Ωm,0 as free parameters showed thatmode 3 and 4 were the largest ones, constructively interfering at low z to produce theoscillatory pattern and destructively interfering at higher z.