The universe shrinks...?

We already know that our universe is expanding. However, the equations of the theory of relativity also allow another possibility - compression. Does it matter that the Universe is expanding rather than shrinking?

Let's imagine that our universe shrinks

Let's imagine that our universe shrinks. What will change in this in the picture of the world around us?

To answer this question, you need to know the answer to another question: why is it dark at night? He entered the history of astronomy under the name of a photometric paradox. The essence of this paradox is as follows.

If stars are scattered everywhere in the Universe, which, on average, emit about the same amount of light, regardless of whether they are grouped in the galaxy or not, they would cover with their disks the entire celestial sphere. After all, the universe is made up of many billions of stars, and wherever we direct our eyes, it will almost certainly sooner or later come across some star.

In other words, each section of the starry sky would have to shine like a portion of the Sun's disk, since in such a situation the apparent surface brightness does not depend on distance. From the sky, we would be struck by a dazzling and hot stream of light corresponding to a temperature of about 6 thousand degrees, almost 200,000 times greater than the sun's light. Meanwhile, the night sky is black and cold. What is the matter?

Only in the theory of the expansion of the universe photometric paradox is automatically eliminated. Since the galaxies scatter, their spectra show a red shift in the spectral lines. As a result, the frequency, and hence the energy of each photon decreases. After all, the red shift is a shift in the electromagnetic radiation of the stars of the galaxy toward longer waves. And the longer the wavelength, the less energy the radiation carries with it, and the further the galaxy, the weaker the energy of each photon coming to us.

In addition, the continuous increase in the distance between the Earth and the retreating galaxy leads to the fact that each subsequent photon is forced to overcome a somewhat larger path than the previous one. Due to this, photons enter the receiver less often than they are emitted by the source. Consequently, the number of photons arriving per unit time also decreases. This also leads to a decrease in the amount of energy arriving per unit time. That's why the night sky remains black.

Therefore, if we imagine that the Universe is shrinking and contracting it lasts billions of years, then the brightness of the sky is not weakened, but, on the contrary, is strengthened. In this case, we would be struck by a dazzling and hot stream of light, corresponding to a very high temperature.

In such conditions on Earth, life, probably, could not exist. Hence, we do not by any chance live in the expanding universe.