Shulou(Shulou.com)11/24 Report--

In the news last month, I don't know if you noticed that scientists found that the speed of the early universe (or the flow rate of time) was five times slower than we are now. Is the universe accelerating all the time?

With regard to the description of "the flow of time is five times slower", many people are surprised and confused: "isn't time used to measure the speed of an object's movement? why does it have its own speed?" Some people say: "I know, relativity, there is no absolute time, only relative time, but …"... who is the relative speed of cosmic time? "

In addition, some people have given some explanations: "at the beginning of the universe, the particles that make up matter moved so slowly that they slowed down, including physical changes and chemical reactions." so it gives the impression that time is getting slower. "

Others explained from the perspective of general relativity: "because the matter in the early universe was very dense, so the gravity was also very strong." at that time, the universe expanded as much time as the space-time around Calgontua in Interstellar, so time was slower than us. "

How to understand the fact that the early universe is slower than it is now, and whose time the universe is relative to? Today let's get to the bottom of these problems.

In order to clarify the speed of time in the universe, let's first figure out the latter: who is the relative to whom the time in the universe is told.

If you have been flying in a near-light-speed spaceship since the birth of the universe, then from your point of view, the age of the universe should be much younger than we think, perhaps only 5 billion years old. But if we take the earth as the frame of reference, the universe is 13.8 billion years old.

Is there no standard answer to how old the universe is?

In fact, due to the different frame of reference, whether 5 billion or 13.8 billion, these two statements are actually correct, and they are not contradictory. So why do we choose 13.8 billion as the age of the universe, just because we live on earth?

If there is anything that can represent the whole universe, it must be the cosmic microwave background, because it is a kind of background radiation that always fills the universe, so its time is the time that best represents the real evolution of the universe.

Background radiation is characterized by homogeneity and isotropy, which is very similar in any direction. But when we move at high speed relative to the earth, you will find that the background radiation will no longer be uniform!

Because of the Doppler effect, its wavelength will be compressed in the direction you are moving, so this part of the background will become more "hot", while in the direction you are away from, its wavelength will be elongated, so this part of the background will become "colder".

Only when we are motionless relative to the earth can the isotropic characteristics of the microwave background be well demonstrated, so it can be considered that the earth and the cosmic microwave background can be put into the same frame of reference.

I know that although the earth, including the solar system and the Milky way, is also in relative motion in the universe, they move very slowly relative to the speed of light, so the relativistic effect here can be ignored.

Well, because the time at rest relative to the earth is the time at rest relative to the background radiation, measuring the age of the universe relative to the earth is to measure the age of the universe relative to the background radiation, which is the "real" age of the universe.

In fact, the "cosmic time" here is a kind of "cosmic time" in the co-motion coordinate system. As the name implies, "co-motion" means "common change", and the "co-motion distance" often referred to in astronomy refers to the distance between celestial bodies that does not increase with the expansion of the universe. You can imagine that the ruler used to measure the length itself will grow as the space expands, so no matter how you measure it, the final value will be exactly the same.

In other words, for anyone in the universe (whether humans on Earth or aliens hundreds of millions of light-years away), as long as they think of themselves as moving with the universe, then today's measurements of the age of the universe will not be much different, all about 13.8 billion years.

Some people will say, "the unit of 'year' is determined according to the revolution of the earth. Aliens don't know how long a year is."

Come on, it's just a matter of unit conversion. You can agree with aliens to define a second in terms of 9192631770 periods of hyperfine structure transitions in the ground state of cesium-133atoms, and multiply it by 31536000 (seconds of a year) to represent a year. That's not a problem.

Cosmological time expansion is good, and when you agree with the universe, you no longer worry about the time benchmark in the universe, and now we begin to think about the slowing down of time in the early universe.

We know that because of the upper limit of the speed of light, the farther the distance, the longer the light needs to run. When a group of photons from the early universe reached us, the picture we saw was actually a long time ago. That's why today we can see all kinds of galaxies at different times in the universe.

As the universe expands, the wavelength of this light is continuously lengthened as it flies to us, that is, a cosmological redshift occurs. But how does it slow down in response to time?

For example, you might think of it this way:

A longer wavelength means a lower frequency, and a lower frequency means a lower transmission rate of information. It also takes 1 second to transmit a set of information, but now it takes 5 seconds. If this group of information is originally playing a 1-second picture, it will take 5 seconds to receive and broadcast it to the receiver. If the receiver doesn't know why, when he sees pictures full of slow motion, he has reason to suspect that the time there may have passed very slowly. But in fact, it was only one second after this scene took place, and there is no such thing as "slowing down" for those who experienced the event.

So the universe is not accelerating, but the images of the past are getting slower and slower, which is called "cosmological time expansion".

In fact, this is nothing new, it has long been predicted by general relativity and standard cosmology. Including astronomical observations, scientists have long had corresponding observational evidence.

For example, Ia supernovae, which are used as standard candlelight for ranging, not only have the same maximum brightness, but also have the same luminosity curve and similar light variation period.

According to what we just said, the early time of the universe expands (that is, it slows down), and the farther away the supernova is, the longer its luminosity should change. Through the actual observation of astronomers, it is true that the passage of time slows down with the increase of distance.

But this evidence is still not strong enough, because due to distance, observations of supernovae do not give us a view of the earlier universe, so it is still uncertain whether the early universe is also in line with theoretical predictions.

However, in July 2023, a paper published in the journal Nature Astronomy said that researchers also found a time-to-distance trend similar to that of supernovae in 190 ancient quasars. This also verifies general relativity and its corresponding cosmological time expansion on a larger scale.

This article comes from the official account of Wechat: Linvo says ID:linvo001, author: Linvo

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