Details Hien PHAN Cosmology & Astrophysics February 29, 2016

Time is a concept that describes the continuous progression of existence and events occurring in an irreversible succession from the past, through the present, and into the future.

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Imagine if time runs back, people will get younger instead of getting old, and after a period of gradual rejuvenation, their life will end in tears of their parents. It was the time depicted in a novel by science fiction writer Philip K Dick. But, surprisingly, the dimension of time is also a problem for cosmologists.

While we admit that time has a definite dimension, physicists say no: Most of the laws of physics have to “turn back in time,” which means they will only work with time. Time is defined to run back into the past.

So why does time always move forward? And will it always be?

Figure 1: The night sky can tell us a bit about time. Photo: Internet.

Does time have a starting point?

Any universal concept of time must ultimately be based on the evolution of the universe itself. When you look at the universe, you are seeing events that took place in the past, because light takes time to reach us. In fact, even the simplest observation can help us understand cosmic time: take the real example of the dark night sky. If the universe had an infinite and boundless past, then the night sky would have to be completely bright because it was filled with light from the countless stars in the universe that have always existed.

For a long time, scientists, including Albert Einstein, thought that the universe was static and infinite. Later observations have shown that the universe is indeed expanding, and is even expanding faster and faster. This means that it must be derived from a compact state we call the Big Bang, implying that time had a beginning. In fact, if we observe light that is old enough, we can even see the residual radiation from the Big Bang – the cosmic microwave background (CMB). This realization is the first step in determining the age of the universe.

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But there’s a catch, Einstein’s special theory of relativity shows that time is… relativistic! The faster you move towards me, the slower your time will pass relative to my perception of time. Thus in the universe of galaxies moving away from each other, stars orbiting each other, and planets rotating, the time experiment is also different: The past, present, and future of everything are relative.

So is there a universal time that we all acknowledge?

Because the universe is essentially the same everywhere, and is generally the same in all directions, there exists a “cosmic time”. To measure it, all we have to do is measure the properties of the cosmic background radiation. Cosmologists have used this to determine the age of the universe and determine that the universe is about 13,799 billion years old.

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Figure 2: Timeline of the universe. Design Alex Mittelmann, Coldcreation/wikimedia, CC BY-SA

Arrow of time

We know the time most likely started with the Big Bang. But one nagging question persists: What exactly is it?

To answer this question, we must look at the fundamental properties of space and time. In the dimension of space, you can move forward or backward; Everyone experiences this move on a daily basis. But time is different, it has only one dimension, you always have to go forward, never go back. So why can’t time go back? This is a big unsolved problem of physics.

To explain why time itself is irreversible, we need to look to processes in nature that are also irreversible. One of the concepts of physics (and life!) is that things tend to get less “tidy” as time goes on. We describe this by a physical property called entropy, which encodes the way things are laid out.

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Imagine a box containing a gas, in which all the particles are initially placed at an angle (an ordered state). Over time they will naturally find a way to fill the box (a state of confusion). To bring these particles back to an ordered state would require energy. This is called irreversible. It’s like breaking an egg to make an omelette, once the egg has spread and spilled into the pan, it will never return to the shape of an egg. The same goes for the universe: as the universe grows, the overall entropy will increase.

It turns out that entropy is a pretty good way to interpret the arrow of time. And while the universe appears to be becoming more orderly – as it goes from disorder in the early stages of formation, for example, If the universe turns into stars, planets to its present seemingly orderly stage – the universe can still be capable of increasing disorder. Because gravity associated with large masses can pull matter into the quasi-ordered state we see, with the increase in disorder that we think must happen somehow. hidden in the gravitational field. Thus disturbances can increase even if we cannot see them.

But given nature’s propensity for disorder, why did the universe start from an initial steady state? This is still considered a mystery. Some researchers suggest that the Big Bang may not have been the beginning, but that in fact there could be “parallel universes” where time runs in different directions.

Figure 3: Unfortunately, things don’t tidy themselves up. Credit: Alex Dinovitser/wikimedia, CC BY-SA

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Does time end or not?

Time has a beginning, but whether it will have an end depends on the nature of the dark energy that is causing the universe to expand faster and faster. This accelerating expansion could eventually tear the universe apart, forcing the universe to end in a Big Rip; another possibility is that dark energy could decay, reversing the universe back into the Big Bang and ending in a Big Crunch; or the universe will simply expand forever.

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But are these scenarios the end of time? According to the strange laws of quantum mechanics, tiny random particles can momentarily pop out of a vacuum – something seen continuously in particle physics experiments. Some have suggested that dark energy could cause these “quantum fluctuations” to create a new Big Bang, ending our timeline and starting a new one. While this is very theoretical and uncertain, what we do know now is this: only when we understand dark energy will we know the fate of the universe.