A coherent view?

We conclude by returning to the question we started with: what is our best current picture of the physical universe, and what are its problems and uncertainties?

There is an agreed basic view of the universe, the standard model of cosmology, in which the universe expands from a hot big bang early phase to a late-time cool, accelerating phase driven by a cosmological constant, with structure formation taking place through gravitational instability around the ‘scaffolding’ provided by dominant CDM, acting on seed perturbations generated by inflation. This seems to provide a statistically good fit to all the data up to now, with the same set of parameters.

Given the major physical uncertainties concerning this model, we need to continue subjecting it to further observational tests with ever-improving data – and we need to query its uniqueness, and probe the alternative possibilities. A key feature is that uncertainty about both geometry and physics increases with time in the past, and with distance from our world line. Our cosmological claims should make this feature clear. Use of the FLRW models as a starting point of analysis tends to hide this fact. The perturbative inhomogeneity imposed in these models changes the physics equations from PDEs to ODEs and so hides the nature of causality and associated causal domains.

The visible universe

The visible universe is the part of spacetime within our past light-cone since decoupling. The CMB emitted from where our past light-cone intersects the last scattering surface marks the boundary of this domain – the matter emitting this light represents our visual horizon, the furthest away matter that we can detect by electromagnetic radiation of whatever wavelength.