The scientific explanation that suggests the universe is expanding relies on two main concepts

The correct answer is red shift and Doppler effect.

Explanation:

The Doppler Effect was used by Hubble to demonstrate that the universe was expanding. According to the effect, the light from the far-off galaxies was shifted towards the lower frequencies, that is, towards the red end of the spectrum. This is known as the Red-shift or the Doppler shift.  

It was in 1925, Edwin Hubble witnessed Red-shift. He observed that the redshift of galaxies was directly equivalent to the distance of the galaxy from the Earth. This signifies that the things distant away from the planet Earth was moving away at a quicker rate, and thus, it can be concluded that the universe is expanding.  

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