The idea of an expanding universe was proposed by Edwin Hubble. Several scientists--Saul Perlmutter, Brian P. Schmidt and others--set out to measure the rate of expansion. They expected to find that it was slowing due to the mass/gravity effect of the universe matter.
The accepted method is to measure the discrepancy between the expected brightness of distant stars, supernovas, to their actual brightness. The redshift of its spectrum is measured to determine how fast the star and its home galaxy are moving away. Instead slowing down, the expansion of the universe was accelerating. Something was counteracting the expected effects of gravity.
The accelerating expansion can not be explained by any type of missing mass, because mass implies gravity and more gravity should put the brakes on expansion. So astronomers proposed a new type of energy to explain the observations. They call it “dark energy”, though it may or may not be energy as we understand it. Its density remains constant even as the universe expands. The more space expands, the greater the effect of dark energy.
Very strange.
Saul Perlmutter, Brian P. Schmidt, and Adam Reiss, won the Nobel Prize for Physics in 2011 for an observation the opposite of the one they expected, an observation that raised a lot more questions than it answered.
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