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The form of the metric of the Universe, the FRW metric, is obtained from the principles of spatial homogeneity and isotropy which are supported by observation.

If we probe the present Universe, it appears to be spatially homogeneous and isotropic on scale larger than $100$ Mpc or so. However, the CMB observations suggest that at much earlier times, when there were no large scale structures such as clusters and superclusters of galaxies, the Universe was spatially homogeneous and isotropic when probed on a much smaller scale. For example, at the time of last scattering, when the CMB was formed, the homogeneity and isotropy existed above a mustmuch smaller length scalesscale.

Question What does it imply about the applicability of the FRW metric in the present epoch versus the time of last scattering? In short, what difference does this make?

I would guess that any conclusion drawn from the FRW metric of the present Universe have to be attributed on scales larger than $100$ Mpc while any conclusion derived from the FRW metric at earlier times could be attributed to much smaller length scales.

The form of the metric of the Universe, the FRW metric, is obtained from the principles of spatial homogeneity and isotropy which are supported by observation.

If we probe the present Universe, it appears to be spatially homogeneous and isotropic on scale larger than $100$ Mpc or so. However, the CMB observations suggest that at much earlier times, when there were no large scale structures such as clusters and superclusters of galaxies, the Universe was spatially homogeneous and isotropic when probed on a much smaller scale. For example, at the time of last scattering, when the CMB was formed, the homogeneity and isotropy existed above a must smaller length scales.

Question What does it imply about the applicability of the FRW metric in the present epoch versus the time of last scattering? In short, what difference does this make?

I would guess that any conclusion drawn from the FRW metric of the present Universe have to be attributed on scales larger than $100$ Mpc while any conclusion derived from the FRW metric at earlier times could be attributed to much smaller length scales.

The form of the metric of the Universe, the FRW metric, is obtained from the principles of spatial homogeneity and isotropy which are supported by observation.

If we probe the present Universe, it appears to be spatially homogeneous and isotropic on scale larger than $100$ Mpc or so. However, the CMB observations suggest that at much earlier times, when there were no large scale structures such as clusters and superclusters of galaxies, the Universe was spatially homogeneous and isotropic when probed on a much smaller scale. For example, at the time of last scattering, when the CMB was formed, the homogeneity and isotropy existed above a much smaller length scale.

Question What does it imply about the applicability of the FRW metric in the present epoch versus the time of last scattering? In short, what difference does this make?

I would guess that any conclusion drawn from the FRW metric of the present Universe have to be attributed on scales larger than $100$ Mpc while any conclusion derived from the FRW metric at earlier times could be attributed to much smaller length scales.

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About the applicability of the FRW metric in the present epoch vs the time of last scattering

The form of the metric of the Universe, the FRW metric, is obtained from the principles of spatial homogeneity and isotropy which are supported by observation.

If we probe the present Universe, it appears to be spatially homogeneous and isotropic on scale larger than $100$ Mpc or so. However, the CMB observations suggest that at much earlier times, when there were no large scale structures such as clusters and superclusters of galaxies, the Universe was spatially homogeneous and isotropic when probed on a much smaller scale. For example, at the time of last scattering, when the CMB was formed, the homogeneity and isotropy existed above a must smaller length scales.

Question What does it imply about the applicability of the FRW metric in the present epoch versus the time of last scattering? In short, what difference does this make?

I would guess that any conclusion drawn from the FRW metric of the present Universe have to be attributed on scales larger than $100$ Mpc while any conclusion derived from the FRW metric at earlier times could be attributed to much smaller length scales.