Who first propsed that the universe is expanding?

Answer 1

The astronomer Edwin Hubble was the first to find evidence of the expansion of the Universe, through his observations of distant galaxies.

He observed that light emitted from most of the distant galaxies was more red in color. This is known as the Doppler effect or Doppler shift, where objects that emit a certain wavelength and move away from the observer appear to have a longer wavelength than the true wavelengths to the observer. It's the opposite case when objects emitting waves that are coming closer to the observer. These observations lead Hubble to believe that these surrounding red-shifted galaxies are moving away from us. Since most of the galaxies were moving away from us, and since Hubble found that with increasing distance between the galaxies there is an increasing speed at which they are moving apart, he concluded that the Universe was and still is expanding.