We continue our series on the outstanding survey paper by George F. R. Ellis, Issues in the Philosophy of Cosmology.
The particle horizon is the distance beyond which light would have not yet had time to reach us in all the time since the Big Bang. The visual horizon is the distance beyond which the universe was still opaque to photons due to high temperature and density. The visual horizon, therefore, is not as far away as the particle horizon. FL stands for Friedmann-Lemaître, the standard models of a flat, open, or closed universe.
What is inflation? At the moment of the Big Bang, the expansion of the universe accelerated exponentially for a very short period of time. This caused portions of space that had been close enough together to be causally connected to become causally disconnected. While inflation does a very good job of explaining many observed features of our universe, such as its uniformity in all directions, at this point it is an untestable hypothesis (unlike special and general relativity), and the underlying physical principles are completely unknown.
We currently live in a universe where the density may be too low to observe how gravity behaves at the quantum level. Though we may never be able to build a particle accelerator with energies high enough to explore quantum gravity, quantum gravity might possibly play a detectable role in high-density stars such as white dwarfs, neutron stars, or black holes. At the time of the Big Bang, however, the density of the universe was so high that quantum gravity certainly must have played a role in the subsequent development of our universe.
Do we live in the universe that had no beginning and will have no end? A universe that is supratemporal—existing outside of time—because it has always existed and always will exist? Admittedly, this is an idea that appeals to me, but at present it is little more than conjecture, or, perhaps, even wishful thinking.
Will science ever be able to answer this question? I sincerely hope so.
Thus, there are (many) places in our universe that are receding from us so fast that light will never have a chance to reach us from there. Indeed, the cosmic background radiation that pervades our universe today was emitted by matter that was receding from us at 61 times the speed of light at that time. That matter never was nor ever will be visible to us, but the electromagnetic radiation it emitted then, at the time of decoupling, is everywhere around us. Think of it as an afterglow.
Ellis, G. F. R. 2006, Issues in the Philosophy of Cosmology, Philosophy of Physics (Handbook of the Philosophy of Science), Ed. J. Butterfield and J. Earman (Elsevier, 2006), 1183-1285.