The average temperature of the universe is only -270.42°C

There are a huge number of stars in the universe, each of which can be considered a hot "ball of fire" that emits a lot of light and heat into the cosmic space every moment. It is easy to think that the universe as a whole should be warm, but this is not the case, because according to scientists' estimates, the average temperature of the universe is only -270.42°C.

Theoretically speaking, the low-temperature limit in the universe is -273.15°C, which is also known as absolute zero. The temperature of any object can only be infinitely close to absolute zero at most, but can never reach or fall below it, which means that the average temperature of the universe is only 2.73°C higher than the low-temperature limit in the universe, which can be said to be extremely cold.

Why is the universe, with its many stars, so cold? To answer this question, we need to see how the "average temperature of the universe" should be calculated.

According to the definition of classical thermodynamics, temperature refers to the intensity of the thermal motion of a large number of microscopic particles inside the object, but we can not use this to calculate the average temperature of the universe, why?

Because the universe is too empty. Although there are many stars and other celestial bodies in the universe, they are so sparsely distributed that there is an almost empty vacuum between them, and the density of matter in the universe as a whole is surprisingly low, with an average of only about 6 protons per cubic meter.

With such a sparse density of matter, it is inappropriate to use the classical thermodynamic definition of temperature to calculate the average temperature of all matter in the universe, so what should be done?

In our daily life, we often encounter a phenomenon: if we put an object into a specific environment, then after some time, the temperature of the object will be equal to the temperature of the environment, in other words, the temperature of the object can be considered as the temperature of the specific environment.

The reason for this phenomenon is that the object has reached thermal equilibrium, which simply means that the heat it releases to the outside world is exactly equal to the heat it absorbs from the outside world.

By the same token, if we place an object in a certain region of the universe, then when the object reaches thermal equilibrium, its temperature can be considered the temperature of the region.

We know that there are three ways to transfer heat, namely conduction, convection, and radiation, and in the vacuum of space, heat can only be transferred by radiation, and radiation is a phenomenon of spreading energy by electromagnetic waves, and the object can get heat by absorbing the external electromagnetic waves, and at the same time, the object will also release heat outward by releasing electromagnetic waves.

So we can simply think that an object placed in the vacuum of the universe, will emit heat by releasing electromagnetic waves, and it will also gain heat by absorbing electromagnetic waves, and when the heat emitted is equal to the heat absorbed, the temperature of the object is considered to be the temperature of the vacuum in which it is located.

Although the universe has a large number of stars, the universe has a much larger spatial extent, which causes the distance between stars but very far, for example, in our galaxy, the average distance between stars is up to about 5.5 light years, what is the concept?

Let's say there are two stars 5.5 light-years apart, and then assume that they are the same size as the Sun, now we will shrink the two stars into soccer so big, then after the proportional reduction, the distance between the two "soccer" is about 8200 km.

According to the inverse square law, the radiation intensity of the star will decrease sharply with the increase of distance, for example, in our solar system, only 5.9 billion kilometers away from the Sun, the surface temperature of Pluto can be as low as -229 ℃, and the reason why this is so, in fact, because it can receive the solar radiation has been very weak.

5.9 billion kilometers is about 0.000623 light years, compared with the distance between the stars in the universe, it is not worth mentioning, but it is at such a distance, that stellar radiation has been so weak, not to mention the distance between the stars is often counted in light years.

More importantly, the vast majority of stars in the universe are located in galaxies, and the distance between galaxies is hundreds of thousands, or even millions of light years, between them is a void, which undoubtedly further reduces the impact of stars on the average temperature of the universe, so that, on the whole, the average temperature of stars on the universe can be completely negligible.

So what determines the average temperature of the universe? The answer is "microwave background radiation".

"Microwave background radiation" is called "the earliest light in the universe", because the speed of light is finite, the light generated at the beginning of the universe is still in space, but after a long time, they have long been due to the expansion of the universe and They have become microwaves.

"Microwave background radiation" has an important characteristic is "isotropic", no matter in which direction, we receive the "microwave background radiation" is the same, and theoretically, In other words, the "microwave background radiation" fills the entire universe and is very evenly distributed.

In essence, "microwave background radiation" is electromagnetic radiation, in the universe, there is an average of 411 million "microwave background radiation" photons per cubic meter.

So at any point in the cosmic space of the matter can therefore get heat, in the absence of other heat sources, when the matter reaches thermal equilibrium, its temperature is the "microwave background radiation" equivalent temperature, and according to scientists' calculations, this temperature is 2.73K, converted into the Celsius temperature scale is - 270.42℃.

In summary, although the universe has many stars, the universe is so empty that the heat released by these stars can only affect the temperature of a very small part of the matter in space.

On the whole, the part of the universe affected by the stars is negligible, while in the majority of the universe, the temperature of the matter in the universe is determined by the "microwave background radiation", which is why we believe that the average temperature of the universe is the "microwave background radiation "equivalent temperature of the universe, that is, -270.42 ℃.