Question

How do scientists know that the universe is expanding?

Answer 1

Hint: When we discuss the expansion of the universe we mean that after the big bang how much again the universe is expanding. Scientists have various techniques and assumptions to find out whether the universe is getting expanded.

Complete answer:
The universe/space is not stationary but it is always moving. The distance of every other body from each other constantly keeps on changing. The galaxies as such do not move through space but since space is moving, the galaxies also keep on moving further. Therefore, none of the objects in the universe are at a fixed position; they keep on moving farther and farther from each other. The galaxies that are farther away from us i.e. earth tend to move the fastest.
Hubble’s law states that the distance of the galaxies and their distance from the earth are directly proportional to each other. It was believed earlier that because of gravity expansion of the universe may be slowing down but it seems that the universe is expanding and can still be expanded to eternity.
It is well known that a given object may emit or absorb certain wavelengths of light of the electromagnetic spectrum, which is represented as lines. Therefore, if we study these lines for even a star we may find out if it is farther or nearer to us. Even the Doppler Effect is seen in space i.e. as the distance of any celestial object increases from us the wavelength shifts towards red. This is seen in many stars and other objects like galaxies in the universe.
In Hubble’s law, the ratio of the distance of the galaxies and velocities with which they move gives a constant called Hubble’s constant, which represents the expansion of the universe.

Note: Recent research shows that earlier it was predicted that expansion of the universe is constant i.e. it may not increase further but it is not. With the concept of ‘dark energy’ which is increasing the speed of expansion, this concept may change.