Imagine that time is running backwards. People will grow younger, not older, and after a long life of gradual rejuvenation - forgetting everything they know - they die as babies. We observed a similar passage of time in the hero of the novel by Francis Fitzgerald "The Mysterious Story of Benjamin Button". But, surprisingly, the direction of time is also a problem facing cosmologists.
So why does time always move forward? Will it always be that way?
Does time have a beginning?
Any universal concept of time must ultimately be based on the evolution of the cosmos itself. When you look at the universe, you see events that happened in the past - it takes time for the light of the stars to reach us. In fact, even the simplest observation can help us understand cosmological time: for example, the fact that the night sky is dark. If the universe had an infinite past and was infinite in extent, the night sky would be absolutely bright - filled with the light of an infinite number of stars in space, which has always existed.
For a long time, scientists, including Albert Einstein, thought that the universe was static and infinite. Observations since then have shown that it is actually expanding and accelerating. This means that it must come from a more compact state, which we call the Big Bang, implying that time does have a beginning. In fact, if we're looking for light enough old, we might even see the relict radiation from the Big Bang - the cosmic microwave background. This realization was the first step in determining the age of the universe.
But there is a catch, Einstein's special theory of relativity shows that time is relative: the faster you move relative to your friend, the slower time will be for you relative to his perception of time. Thus, in our universe of expanding galaxies, revolving stars and revolving planets, the experience of time is changing: all past, present, and future are relative.
Is there a universal time?
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It turns out that since the universe is on average the same everywhere and on average looks the same in all directions, there is "cosmic time." To measure it, all we have to do is measure the properties of the cosmic microwave background. Cosmologists have used this to determine the age of the universe; his space age. It turns out that the universe is 13.799 billion years old.
Arrow of time
So we know that time most likely began during the Big Bang. But one question remains: what is time?
To find the answer, we need to look at the basic properties of space and time. In the dimension of space, you can move back and forth; passengers experience this every day. But time is different, it has a direction, you always move forward, and not vice versa. So why is the measurement of time irreversible? This is one of the main unsolved problems in physics.
To explain why time itself is irreversible, we need to find processes in nature that are also irreversible. One of the few such concepts in physics (and life!) Is that things become less "neat" over time. We describe this using a physical property called entropy, which stands for something ordered.
Imagine a box of gas in which all the particles were initially placed in one corner (an ordered state). Over time, they will naturally tend to fill the entire box (disordered state), and it takes energy to return the particles to an ordered state. It's irreversible. It's like breaking an egg to make an omelet - once it decomposes and fills the pan, it never reverts to egg shape. It's the same with the universe: as it evolves, the total entropy increases.
It turns out that entropy is a pretty good way to explain the arrow of time. And while the universe may appear to be becoming more orderly, not less - moving from a wild sea of relatively evenly distributed hot gas in its early stages to stars, planets, people, and articles on time - it is still possible that it is growing in disorder. This is because gravity, associated with large masses, can pull matter into seemingly ordered states - with the increasing disorder that we think should have taken place, is somehow hidden in gravitational fields. So the disorder can grow even if we don't see it.
But given nature's tendency to favor disorder, why did the universe begin in such an orderly state? This is still considered a mystery. Some researchers argue that the Big Bang may not even have been the beginning, in fact, there may be “parallel universes” where time flows in different directions.
Will time run out?
Time had a beginning, but whether it will have an end depends on the nature of the dark energy that causes it to expand at an accelerating rate. The speed of this expansion could eventually tear the universe apart, forcing it to end in the Great Rip; alternatively, dark energy could decay, reversing the Big Bang and ending the Universe in a Big Compression; or the universe might just expand forever.
But will any of these future scenarios end in time? Well, according to the strange rules of quantum mechanics, tiny random particles can momentarily fall out of the vacuum - something constantly observed in particle physics experiments. Some argue that dark energy can cause such "quantum fluctuations", creating a new Big Bang, ending our timeline and starting a new one. While this is extremely speculative and highly unlikely, we know that only when we understand dark energy will we know the fate of the universe.
So what's the most likely outcome? Time will tell.
