According to Doppler effect, it explains the red shift which is the movement or shifting of the distant stars towards the red part of the visible light spectrum. When the two objects move far away from each other, the frequency of the waves decreases which in turn increases the wavelength. Now, as the Universe is expanding, its wavelength increases and the frequency decreases which appears to shift towards the red part of the visible light spectrum. Also, according to Big Bang theory, the Universe is expanding, therefore, The Doppler Effect supports the theory.
