Time travel is not possible. We can’t go back to the past, nor can we go into the future.
The universe was designed for constant change, and that change is manifested as matter and energy being converted as “time passes”. The universe is evolving right before our eyes, and we see it constantly. Every “new instant” brings about a change that was inevitable. That change will instantly fall into the past, and a new instant and current version of the universe will come into existence.
Time is just a measurement of change. We cannot undo the changes. We are “limited” in our capacity to change the current instance of the universe, even as we are agents of change for our immediate environment. We will “flow” into the future, and it’s the only destiny for humans and the whole universe. The universe will never “flow” backwards, which means that, humans can’t go back to the past.
However, the past will still exist. It will be there, but unreachable. That’s the design.
That unreachable past will continue to affect all the “new instances” of the universe. In fact, that past, and all of its instances of the past universe, are what cosmologist call “dark energy and dark matter”. That past comprises most of the “single universal constant”. The current instance of the universe is being affected by “all” of the past instances of the universe. Only the most basic of particles are able to interact with the past; likewise, only the most basic of particles from the past can interact with the “newer instances” of the universe’s instances.
The farther back an instance of the universe, the less interactions there will be with the “current instance” of the universe. The “most recent” instances of the universe, will have the most interactions with the current instance. So, going back to an older instance of the universe, will get more difficult, and perhaps impossible. Thus, we can’t go back in time, except to the more recent of instances of the universe.
(And, I don’t know what I just said, or where it came from; I just stay awake late at night trying to figure out how the universe began, and why it behaves as it does. When I have finally figured it all out, I will be dead).
BTW, there was no “Big Bang”, other than the TV show.
Did you hear about the insomniac, dyslexic agnostic?
He lays awake at night wondering if there is a dog.................