How do scientists know the universe is expanding?
BY studying the spectrum of galaxies we can observe the red shift.
Red shift is an indication that the galaxies are going away from us and the more far they are they go at more speed.When the galaxies are goin g away the light wanes get stretched and this gives us the red shift. picture schoolphysics.UK.
In 1997 and 1998 three studies of supernova provided evidence that the rate of the expansion of the universe is increasing. These studies the Superova Cosmology Project, Supernova Legacy Survey, Superova Search Team all found evidence of an increasing rate of expansion.
These studies looked at supernovas as a standard candle. The supernova is assumed to give a standard total luminosity. The further away the light source the less the luminosity according to the inverse square relationship. By comparing the red shift of the light from a supernova to the luminosity calculations could be made as to the velocity of the object relative to the observer.
All three of these studied found empirical evidence that the rate of expansion of the universe is increasing. This was significant because the hypothesis was that the studies would find evidence that the universe was decreasing its rate of expansion.
The prevailing theory of the universe was that it was eternal, self existent and was oscillating between periods of expansion and contrition. The supernova studies disproved the theory of an oscillating universe. The evidence of an increasing rate of expansion of the universe indicates that the universe is not eternal, self existent or a closed system.