This post contains new content about how the homogeneity and isotropy of the universe equally support BBT and the ISU.
Homogeneity and Isotropy of the ISU
The cosmological principle is a cornerstone of cosmology. For the principle to apply, the whole universe described by a cosmology must be considered homogeneous and isotropic. The expanding curved space-time of BBT is the best known example of a whole universe that is homogeneous, isotropic, that has no edge and where every point can be considered the center. As the expansion proceeds, the volume of curved space-time increases. Expansion causes every point to move away from every other point at the exact same rate. Since that is roughly consistent with what we observe, and there is no reason the think we are at the center ourselves, it seems to support BBT.
However, the apparent movement observed from any random location is not proof of center-less space; it is only evidence of expansion as opposed to explosion. It is not proof that there is no edge; it is evidence that supports the theory that the edge is beyond the observable portion of the universe just as well as it supports BBT.
So a pervasive consistent rate of recession from all points could mean that BBT is right, or it could just mean the edge is out of sight. If the edge is advanced far enough so that it is beyond the observable universe then the lack of an orientation to the center from any random point would be entirely compatible with expansion from a center point.
The ISU is homogeneous and isotropic, is infinite spatially, filled with energy, and our universe is expanding within a tiny arena of that infinite space.
The ISU is not only supported by the evidence just like BBT, but unlike BBT the ISU doesn’t have to track back to a singularity because it predicts a big crunch origin of expansion.
It is argued that though expansion can support both BBT and the ISU, there is the fact that not only is there the pervasive consistent rate of recession but there is the CMBR too which supports BBT.
CMBR in the ISU
The background temperature of the universe is very low, only a few degrees Kelvin. This temperature background is the same in any direction with only slight anisotropy consistent with a violent high energy origin of the universe. Polarization exists that is consistent with the natural formation and interaction of known particles in space. Calculations show that these characteristics of the background are compatible with and were even predicted by BBT and the early exponential expansion scenario of inflation.
However, though rejected by BBT, there is still the consideration that the expansion that we are experiencing started in an environment that was at only a fraction of a degree Kelvin. If matter formed from such an environment and was characterized by subsequent photon generation as a natural consequence of the existence of matter, then the temperature that we detect in the background today would likely be elevated from the cold start by a few degrees as a result.
The ISU is characterized by a near zero degree Kelvin origin of a fully negated big crunch that started as an extreme radiation event, eventually consumed its own heat and radiation, had its matter negated to extremely dense energy, lost its mass and gravity as matter became energy, and burst free as the potential energy of extreme energy density was converted to expansion of energy expanding in space. It expands as a high energy density environment released within a low energy density environment. Matter formed from the high energy density as the density diminished and the resulting photons which form concurrent with matter formation account for the introduction of heat. The two or three degree elevation in temperature over the ~14 billion years of expansion is the sum effort of all of the photons whose population starts at near zero and never stops increasing until negation occurs in distant and future crunches.
The slight anisotropy is caused by the thermalized remnants of the death of an epoch of hydrogen stars that are the natural consequences of the earliest stable particles, protons, surrounded by their electrons. The preponderance of hydrogen supports the hydrogen epoch and the relatively high level of helium supports the concept that the hydrogen epoch was characterized by a homogeneous and isotropic universe filled with huge helium factories in the form of hot, fast burning hydrogen stars.
The normal life of those hydrogen giants was a short but productive period of neutron generation from particle chaos: a high energy, gamma radiation, and matter/antimatter interaction period causing nucleosynthesis followed by explosions and collapses, leaving a temperature gradient signature in the debris. Photons that were emitted from different layers within those hydrogen stars gave rise to the slight anisotropy of the background radiation. The thermalization period that followed the hydrogen epoch accounted for the near perfect black body radiation of the background.
Surviving the thermalization period, the remnants in the form of dust, light elements and remnant black holes left over by the now extinct hydrogen stars were the seeds of subsequent galactic formation and the resulting metallization of the universe.
At least the evidence supports the ISU scenario as well as BBT, and with the ISU the singularity is eliminated and exponential expansion is not necessary.