In physics, time gets even more complicated, incorporated into theories of relativity — the experience of the clock's ticking being relative to the observer and their place in space — and entropy, which gives time its unidirectional flow. Meanwhile quantum mechanics indicates time can move both forwards and backwards, at least inside quantum systems.
There is an alternative cosmological theory to the one that says the Big Bang marked the beginning of time itself, which was then followed by a brief period of rapid inflation of space. The alternative is known as eternal inflation. In this scenario, our universe was born as a small bubble in an infinite space that has been undergoing inflation forever.
So instead of a brief period of inflation happening just after the Big Bang, in the eternal inflation theory it is the other way around, with the Big Bang marking the end of inflation in our part of space, creating our bubble universe outside of which inflation continues. The theory predicts there will be other big bangs happening elsewhere that give rise to other universes — possibly an infinite number of them — all forever separated from each other, and all being driven apart by the ever-expanding, rapidly inflating space outside of them. The totality of this infinite space is referred to as the multiverse, to distinguish it from the individual bubble universes.
This idea, proposed by Roger Penrose, suggests that our universe is going through an infinite series of lives, or epochs, each of which starts with a big bang and ends with a universe in thermal equilibrium, filled only with evenly spread-out thermal radiation. The idea here is not, as you might expect, that the universe reaches some maximum expansion before collapsing down to a 'Big Crunch' that then marks the Big Bang of the next epoch, but rather because of a subtle geometrical idea called 'conformal mapping' that connects up the very different end of one universe (cold and spread-out) with the birth of the next (hot and dense).
Other suggestions for a pre-Big Bang time include a mirror universe, and one that cycles endlessly through expansion and contraction. (Image credit: Curly_photo/Getty Images)A related hypothesis, which I admit I find aesthetically rather appealing, is known as the ekpyrotic universe model. The name is based on the Greek word ekpyrosis (ἐκπύρωσι), meaning 'conflagration', which refers to a cosmological model proposed by the Stoic philosophers of ancient Greece in which the universe cycles endlessly through fiery birth, expansion, and cooling, followed by contraction, reheating, and rebirth.
At the same time, it is drifting away from the other brane that it collided with, but will eventually reach maximum separation from it before they are pulled back together, colliding again in a new Big Bang. Here I simply wish to stress that if this theory is right then there may not have been a beginning of time, and certainly not at the Big Bang.
Another cosmological model, which also does away with the need for inflation, as well as suggesting that the Big Bang was not the beginning of time, has been proposed by another of the originators of the ekpyrotic universe idea, Neil Turok. But this idea, known as the 'mirror universe' theory, is rather different. It's a nice idea, and it certainly solves Loschmidt's paradox regarding the conflict between time symmetry and the second law of thermodynamics. Due to the time-symmetric underlying fundamental equations of physics, starting a system off at any arbitrary moment means that its entropy should increase equally into the past as it does into the future.
Exclusive to Live Science ProBut this just doesn't make sense. If the system at all times before our chosen arbitrary moment has higher entropy, then starting from any of those earlier times should see the system decrease in entropy in order to reach its state at the original arbitrary moment. Therefore, it cannot be arbitrary; the system had to have been in a specially prepared state at that moment, the moment of the Big Bang. There would be no increase in entropy going backwards in time if that is when time started.
How seriously should we take these cosmological theories? Are they even proper science? Their proponents know they require some sort of empirical evidence to support their hypotheses, such as some imprint of a reality before the Big Bang frozen in our cosmic microwave background. For without observational evidence, these ideas will remain just theoretical curiosities.
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Princeton University Press On Time: the Physics That Makes the Universe TickIn this mind-altering book, Jim Al-Khalili tackles the enduring problems of time head-on, carefully examining them across the three central pillars of physics—quantum mechanics, general relativity, and thermodynamics.
Al-Khalili makes the persuasive case that time is real, not illusory, and that a master arrow of time is not merely a trick of our senses but is rather a fundamental feature of reality that was baked into the structure of our universe from the very beginning.
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