How old is the universe? Answer: 13.7 billion years.
Who brought this number? Quite simply, humans were able to determine the age of the universe in two ways.
First method: In the middle of the last century, we noticed something with the development of telescopes we discovered that stars in distant galaxies tend to turn red, and this is something that cannot happen without a reason. Through our studies of planets near our planet, we knew that most of the planets consist of gaseous hydrogen and helium, and usually these elements cannot emit such colors that we see in distant stars In other galaxies.
So distant stars are also made of hydrogen and helium, as is the case in stars near us.
but why does their color tend to be red? The strongest explanation is the fabrication of color due to the movement of the stars, "Redshift, Blueshift" If we assume that there is a body that radiates light and this body is approaching you, then the light will tend to blue, and if it turns away from you, it will tend to be red.
So the red color indicates that these stars are constantly moving away from us, most stars emit this behavior (moving away from us) Let us think if these stars are moving away from us constantly, this means that they were closer to us in the past and if we followed the path of their distance, we would find that they were in one grouped point "Singularity"
In the past it was believed that everything in the universe of galaxies, planets and stars was combined at one point, and from this point the "Big Bang" explodes, then:
1- The universe begins as a single point that is small in size.
2- The universe is constantly expanding.
3- We have the ability to calculate the rate of expansion of the universe.
With these data, we were able to calculate the age of the universe by way of calculating the Redshift.
The second method: The general public since ancient times is observing and studying the stars, so we have a clear picture of the life cycle of the planets and the stars. As a way to know the age of the universe, we started looking for the oldest stars that our eyes were able to reach.The idea was as follows: If we find a planet with an age = X then the universe is more than the value of X.
So we directed our telescopes to the oldest stars that we could find.We found that the oldest stars we have found are 13.2 billion years old.This confirms the validity of the age of our universe.
How did we know the age of our universe?
1- The stars are constantly moving away.
2- We calculated the time the stars needed to reach their current location, and we reached a result.
3- We calculated the ages of the stars and reached a result.
4- The results are compatible and mutually supportive.