# What is the temperature of the universe? After listening to the explanation of the scientists, I suddenly realized

We are familiar with temperature. We deal with temperature every day and every moment. So how was temperature born? In fact, from the moment of the big bang, the temperature also appeared. According to human cognition, the average temperature of the earth’s surface is 15 degrees Celsius, that of Mars is minus 63 degrees Celsius, that of Venus is 460 degrees Celsius, and that of the sun is 5500 degrees Celsius.

With so many different temperatures, we realize that the world, the universe is a world of temperature. Everywhere, any matter has its own temperature. Including intelligent life, human beings also have a temperature of about 37 ℃. When the temperature of the outside world is far lower or higher than this temperature, the human body will feel uncomfortable and even endanger life.

Since every celestial body and material in the universe has its own temperature, how many degrees has the temperature reached in the vast universe? Maybe many people think that there are a lot of stars in the universe, and stars are a powerful source of thermal radiation energy, so the temperature of the universe should be relatively high with so many stars. Is that really the case?

To understand this problem, we need to have a simple understanding of the nature of temperature. Modern scientific research has found that the essence of temperature is actually the thermal motion of particles. It’s very difficult for us to understand how the temperature changes from the macro level? What is the essence of its change? But when we go into the micro world, you can clearly see the molecular thermal motion of matter.

When the thermal motion of molecules is increasing, the temperature is also increasing. On the contrary, when the thermal motion of molecules is decreasing, the temperature is also decreasing. And there’s a limit to how much molecules weaken, and that’s static. When the molecules stop moving, the temperature reaches the lowest limit, which we call absolute zero, that is, 273.15 degrees below zero.

However, according to the uncertainty principle of quantum mechanics, the molecular motion can not be completely static, it will always be in motion. Therefore, absolute zero is only a theoretical limit that can not be reached. Now we understand that a large number of particles are needed to truly reflect the temperature. If there are no particles in a space, then the temperature will not appear. It is meaningless to discuss the height of temperature in such a space.

At this time, many people may think of the vacuum of the universe. In many people’s cognition, the universe is a space where there is a vacuum, that is, there is no particle, so the universe has no temperature. So is that the case? In fact, this kind of cognition is wrong. Although we often call the universe a vacuum state, the universe is not completely free of matter. The existence of particles is very sparse.

Although there is a lack of material in space, there is no lack of energy radiation, that is, light. Since there are material particles and thermal radiation in the universe, how can there be temperature in space. We all know that there are countless stars in the universe, but the average distance between stars is more than a few light years compared with the vaster space.

It can be seen that the universe is really empty. Although there are countless star light sources, the background of the whole universe is still dark. Because of the emptiness of the universe, even though there are countless stars glowing, it is still not enough to make the whole universe hot. Even in a stellar system, due to the extremely small density of space particles, the thermal motion of molecules is very weak, so the space temperature in the stellar system is also very low.

In the vast universe, the Galactic space is the main place. There is interstellar space between stars. After leaving the galaxy, there is a more spacious interstellar space, which is extremely cold and dark. Maybe you can walk millions of light years without seeing any light source. This is the terrible interstellar space.

Now we know that the universe is a cold space because the density of matter is very small and there may only be one or several particles in the same unit space. So how low is the temperature of the universe? To understand this problem, we also need to understand what dominates the temperature of the universe? Maybe many people think it’s a star. In fact, this kind of cognition is not completely correct.

As we all know, the universe originated from the singularity explosion 13.8 billion years ago. After the big bang, a very old light was born, which is what we now call cosmic microwave background radiation. And it’s this light that determines the average temperature of the universe.

We all know that the universe was just a singularity with infinite temperature and density at the beginning. After the big bang, it began to expand rapidly. At the same time, time, space and temperature were born. The universe just after the big bang was very small, and the matter and energy were concentrated in a very small range, so the temperature of the universe was very high, reaching 100 million degrees Celsius.

With the rapid expansion of space, matter began to spread outward, the density was decreasing, and the universe began to decline rapidly. One second after the big bang, the average temperature of the universe dropped to 10 billion degrees. The original universe was opaque because photons constantly collided with charged particles and could not propagate freely. It was 380000 years after the big bang that the space expanded to a large enough range, and the temperature of the universe dropped to 2700 degrees before photons became free and spread to every corner of the universe.

13.8 billion years later, the universe has expanded from the original singularity to the extent of light years. At the beginning of the big bang, those photons also spread to every corner of the universe. These photons have become invisible light and spread to the whole space in another form, which is now known as microwave background radiation.

According to the observation results of Wilkinson Microwave Anisotropy probe (WMAP) in nine years, the temperature of cosmic microwave background radiation is about 2.73 degrees higher than absolute zero, which is equivalent to – 270.42 degrees Celsius, which is the average temperature of the whole universe. The discovery of cosmic microwave background radiation is very important. It is a great evidence that the universe experienced ultra-high temperature in that year, which strongly supports the big bang theory.

Of course, human science and technology is still very backward compared with the universe, so our understanding of the universe is also very limited. Moreover, many theories about the universe are not supported by enough evidence. Many universe theories are just a kind of conjecture. If we want to make the conjecture come true, we need to have more powerful scientific and technological strength to constantly look for evidence.

In order to achieve this, human beings have to become a real interstellar civilization. At that time, our research and exploration of the universe was not only through the fuzzy observation of the astronomical telescope, but also through the interstellar shuttle, we could quickly reach the target position and get the real observation data from a short distance. We are looking forward to that day.