Why do scientists measure the expansion of the universe?
Scientists measure the expansion of the universe to learn more about it. If scientists observe a certain behavior in the expansion of the universe, then they may want to learn more about the reason behind that behavior. If scientists do learn why that behavior occurs, then there is a chance that they can also learn about other such behaviors governing the universe, and they hope that we can get a deeper understanding of the other laws that govern existence.
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One reason is that the rate of expansion provides an indication of the future of the universe.
If the rate of the universe is slowing down then this indicates that the Big Bang will be followed by a Big Crush. If the universe is going to return to its beginnings then it is possible to believe that Matter and Energy are eternal. In the words of Neil DeGrasse. the universe is made up only of protons, electrons, neutrons and morons who do not believe that matter and energy is all that there is. This philosophy is called material realism.
If the rate of the universe is speeding up then this indicates that the Big Band will be followed by a Heat Death of the universe. If the universe is not going to return to its beginnings and matter and Energy are not eternal. If matter and energy are not eternal then the universe is an open system and something outside of the universe.
This also has philosophical implications.
In 1998 a study of supernovae by the Lawrence Berkley National Laboratory it was found that rate of the expansion of is indeed increasing.
The rate of the expansion of the Universe is studied because it gives an indication of what the future of the universe will be.