/sci/ - Science & Math

By definition, the universe includes everything that there is. Therefore there is only one universe, an existential set that includes everything.
If there is one universe composed of several components, those components are located through the universe's dimensions. Time, being a dimension, is fixed. The future is as determined as the past, and our limited ability at making predictions does not invalidate the fact that the future is fixed, destiny unavoidable.

In the continuous chain of cause and effect we evolved a nervous system through natural selection, this eventually led to consciousness. The ventromedial prefrontal cortex area in the brain subconsciously runs emotional simulations and helps the dorsolateral prefrontal cortex take rational decissions based on what diminishes pain/maximizes dopamine release in the long run. It's a complex survival mechanism, the result of building rationality over our mammalian emotional limbic structures. All our decissions are ultimately emotional.
A consequence of this process is that we compare many imagined emotional possibilities, an illusion of several possible outcomes some of which appear more desirable than others, as if we had free will deciding among various timelines, but the truth is that we are still determined to pick one, our behavior (free "choice") is determined by environment, past experiences and our genes. The other imagined outcomes remain in memory and with them the illusion of different realities. But you were always going to decide the same.
The chain of cause and effect is what makes the universe a single thing, it's what connects elements through the time dimension. The chain is one. If you had free will it would mean that each human for some magic reason has the ability to create new effects beyond the causes that led him to the point of decission. It would imply every human is a God capable of new genesis. It is an illusion, and a typical religious concept.