One of the most commonsense experiences is the passage of time. One o'clock, two o'clock, three o'clock rock... goes the old Rock n' Roll song. However, it is anything but clear as to why this should be the case. That is, what is it about the world that requires a sequential experience of time?
In the case of the other dimensions that we experience (x,y,z), we at least have some experience of an answer. There exists a 'space' that is 'filled' with quantum objects, each having a unique 'position' that collectively compose the material universe. So far so good. Yet, when you apply this generalization to time, something doesn't quite add up.
Let's assume that instead of 'space' we have 'spacetime'. Instead of "filled with quantum objects" we say "filled with quantum events/objects". Instead of "unique position" we have "unique spacetime address". Now, we have a problem. If the timeline of the universe is a "spacetime filled with quantum events/objects each having a unique spacetime address", then why does each event/object have that particular spacetime address?
In the case of spatial dimensions, this is an easy question. There exists a causal trail from when the universe was a quantum-sized object at or near the big bang all the way until it is the macro object that it is. Each object is at its unique address location as a matter of the spatial evolution that has occurred over the last 13.7 billion years.
If we apply this reasoning to time, then it is very difficult to recover the notion of causality, at least in the manner it is usually considered in terms of a temporal sense. For example, we can explain the position of our galaxy in terms of the early universe position of the energy/matter release at the big bang. Perhaps the matter that was to become our universe was a particular quantum fluctuation as revealed by the cosmic background radiation studies. The position of our galaxy in relation to other galaxies in the universe is directly tied to this early location fluctuation of quantum foam.
However, if we say that our location in time (i.e., as of right now) is tied to the geometry of matter as it existed near the time of the big bang, then on what basis can we say that our 'spacetime address' is causally constructed? That is, if I drop a cup of water on the ground, are we to say that this event is because of a unique geometry at the big bang and not because I just decided to drop the cup of water? That seems preposterous. Am I required to have this thought and follow-through on this action simply because of an infinitestimal geometry at the 'beginning of time'? This is exactly what some quantum cosmologists would have us believe.
A much better solution to this quandry is that our timeline is similar to a building. Each floor of the building represents "all of history" which has floors above it and floors below it. The floors below are timelines similar to our's, except certain events (both past and future) do not occur in our 'floor'. Perhaps, also, some of the events that occur in our 'floor' don't occur in the previous floor.
So, in this illustration, the floors at the 'bottom' are quite basic. Perhaps there is no universe expansion, perhaps it is a big burp of a quick expansion-contraction. The logical reason for the universe (or the reason there is anything at all), doesn't like that answer, so another 'floor' (or timeline) is added. This second floor, if you will, moves ahead with a successful expansion, but no galaxies or stars form, just sub-elementary particles (I probably skipped a few thousand floors, but you get the idea).
As we move up the building, we finally get to our 'floor'. It solves some of the problems of the preceding timelines, but it still doesn't fully answer the purpose for the universe. So, unbeknownst to us, there are a large number of floors 'above' us, perhaps with universes that look like ours, but certain flaws are removed. The top floor is 'as good as it gets' to the reason for the 'building'.
Now, the reason why this structure of time is so adept at explaining the timeline is that causality is restored. As in the case of the spatial expansion providing a consistent and causal explanation of why every object has its own unique spatial address, this account provides a consistent and causal explanation of why every event/object has its own unique temporal address, and it does so without quantum cosmology which introduces Many Histories. The problem with Many Histories is that each history has a particular probability (e.g., path probability of happening), and this excess of histories means that our imaginations can go wild. In most of our experiences, when our imaginations are going wild, it is more likely that our imagination is leading our theories, rather than our theories leading our imagination.
The question someone skeptical might ask is why would I consider this a more parsimonious account of time, versus the more popular competitors that are at least paralleling quantum theory to its natural fruition as a cosmology theory. Well, there is no requirement (that we know of) within natural theory that demands that quantum probabilities are all actualized somewhere. The standard interpretation of quantum theory is that the probabilities are epistemological based, not ontological based (i.e., probabilities based on the uncertainty of the mind, not probabilities demanding other worlds to exist). Demanding the existence of other worlds, much less a near infinite number of them, represents a huge hurdle for a theory - even a philosophical theory.
In addition, a building structure of time conforms to a natural selection process which, granted, has nothing to do with metaphysics, at least like this, but the use of natural selection as a concept can help explain the oddity of cosmic coincidences. That is, if timelines can evolve by having floors added to the top of them, then as you progress up the timeline floors, the universe will appear more and more designed, while leaving the floor with its vestiges of the lower floors where the 'designs' were primitive. So, rather than wish away apparent design as part of scientific theory, the 'floor model' of time (if you don't mind me calling it a name), offers a way to include design in the universe without calling upon a random multiverse to account for it. In this floor model version, 'species' of timelines (i.e., from the beginning to end of time), either become the top floor (i.e., they survive) or they 'evolve' a new floor which takes a try of 'surviving'. The final test for each floor is given at the end of the floor's timeline, and the decision to make it the top floor or add another floor is a question of satifying the fundamental existence requirement of the Universe.
I'm not suggesting that our universe is the one and only building in this Noosphere of timeline buildings. Rather, there might be many - if not infinite - structures existing 'out there'. However, our universe is unique in that it is based on a particular set of requirements that no other structure must satisfy. There may not be anything similar to it, or there may be an infinite number of similar structures like ours.
Of course, by saying that, the parsimony flies out the window and quantum cosmology is competitive in that sense. But, there's a difference in the two competing versions. In this 'floor model' version, the Universe is a Platonic structure which each structure possessing a wide range of characteristics. The main criteria that allows a structure is its logical and/or mathematical beauty. The structure itself might be an iterative attempt in reaching its particular beauty, but as a whole it is purposeful within the scope of the Platonic world.
In the case of quantum cosmology, there is no ryhmn nor reason for the laws of quantum universes. It is just probability and quantum laws existing with this sore thumb multiverse existing in it. It is sloppy and haphazard. Not so, with the floor model universe. Our universe is just one of many beautiful structures in mathematics that exist. It is just that our universe is inaccessible to the mathematician given the complexity of its structure.
So, here we are. Timelines similar to our timeline, immediately below us, timelines similar to our timeline, immediately above us. At the top is the finished timeline, the way things should be. Our building of timelines is just one of many other structures in the Universe, just 'there' as Existence requires them to be. We can simulate some those structures in our mind as mathematical modelling, but we ourselves are just members of this community. All existing because some iteration of existence requires it to be so. We are all identifiable based on our own unique position within the Universe. That's why we are here...