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More than 90 years ago, astronomer Edwin Hubble observed the first hint of the rate at which the universe expands, called the Hubble constant.
Almost immediately, astronomers began arguing about the actual value of this constant, and over time, realized that there was a discrepancy in this number between early universe observations and late universe observations.
Early in the universe's existence, light moved through plasma--there were no stars yet--and from oscillations similar to sound waves created by this, scientists deduced that the Hubble constant was about 67. This means the universe expands about 67 kilometers per second faster every 3.26 million light-years.
But this observation differs when scientists look at the universe's later life, after stars were born and galaxies formed. The gravity of these objects causes what's called gravitational lensing, which distorts light between a distant source and its observer.