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Testing the Concordance model of Cosmology

The concordance model of cosmology is built on several assumptions: 

  • The Universe is homogeneous and isotropic

  • Gravity is described by General Relativity (GR)

Under these assumptions, the metric of the Universe is described by the Friedmann-Lemaître-Robsertson-Walker (FLRW) metric, describing an expanding universe. ​

Is the FLRW the correct metric? Is the Universe isotropic and homogeneous? Is Dark energy a cosmological constant? 

  • Testing the FLRW metric and the Curvature

Combining model-independent reconstruction of the expansion history h(z) = H(z) / H0 from the Joint Likelihood Lightcurve (JLA) to from the Baryon Oscillation Spectroscopic Survey (SDSS III / BOSS), with Arman Shafieloo, we measured in a model-independent way the combination of the Hubble constant H0 and the sound horizon at the drag epoch rd. We then introduced a new litmus test of the Flat-FLRW metric Theta(z) such that the Clarkson test $Ok(z) = Theta^2(z)-1)/D^2(z)$.

For a Flat FLRW universe, Theta = 1 and Ok = 0.

Our results are consistent with a Flat-FLRW Universe, but show some hint of tension in the CMASS subsample (L'Huillier & Shafieloo JCAP01(2017)015). 

  • Model-independant test of GR

With Arman Shafieloo and Hyungjin Kim (U. Waterloo),  we then combined the independent reconstructions to growth measurements from redshift-space distortion, and put model-independent constraints on the matter density Omega_m, the rms fluctuation sigma_8, and the growth factor gamma. 

For GR, gamma = 0.55. Our results are consistent with LCDM+GR (L'Huillier, Shafieloo & Kim 2018, MNRAS 476, 3263)

With Arman Shafieloo and Alexei Starobinsky, we then combined the latest SNIa data (Pantheon) and growth measurements (including eBOSS DR14Q) and obtained more stringent constraints, but are still consistent with LCDM+GR (Shafieloo, L'Huillier & Starobinsky 2018, submitted).