Has the Universe Always Existed?
We have at the current juncture, two effective theories of the universe:
1. Quantum physics, which is the framework underlying elementary particle physics, describing all known interactions of matter; that is, the electromagnetic, weak and strong force, together with all elementary particles. Together these particles are embedded into the standard model of all matter. The only interaction that is not described by quantum physics is the gravitational interaction.
2. General relativity, on the other hand, is the most complete description of gravity arising from Einstein’s theory of gravitational force. But, for all its strengths, general relativity is incomplete. In at least two specific places in the universe the maths of general relativity breaks down, and fails to produce reliable results i.e. In the centers of black holes and at the beginning of the universe. These regions are usually referred to as "singularities," which are spots in space-time where the current laws of physics crumble.
There is however now another approach that has completely rewritten our understanding of space and time. In all current theories of physics, space and time are continuous. They form a kind of smooth fabric that underlies all of reality. In such a continuous space-time, two points can be as close to each other in space as possible, and two events can occur as close in time to each other as possible.
But another approach, called the “causal set theory”, reimagines space-time as a series of discrete chunks, or space-time "atoms." This theory would place strict limits on how close events can be in space and time, since they can't be any closer than the size of the "atom," i.e., there is a fundamental unit of space-time. It is somewhat akin to placing a microscope on your computer screen, in that you cannot get spacing smaller than one pixel.
This has by definition, important implications for the nature of time and the beginning of time. For in this causal set philosophy, the passage of time is something physical, and should not be attributed to some emergent sort of illusion, or to something that happens inside our brains that makes us think time passes.
The causal set approach also removes the problem of the Big Bang singularity because, in the theory, singularities can't exist. It's impossible for matter to compress down to infinitely tiny points. They can get no smaller than the size of a space-time atom.
So, without a Big Bang singularity, what does the beginning of our universe look like?
In the original causal set formulation and dynamics, a causal set grows from nothing into the universe we see today. Instead, with this new approach, there would be no Big Bang as a beginning, as the causal set would be infinite to the past, and so there's always something before.
The implication is thus, that the universe may have had no beginning; that it has simply always existed. What we perceive as the Big Bang may have been just a particular moment in the evolution of this always-existing causal set, not a true beginning.
What is so fascinating in this theory is in the rethinking of the notion of space-time itself, giving as it does a central role to time. It begs the question as to what it physically means for time to pass, how physical the past really is and whether the future exists already or not?