The Simplest Solution To The Expanding Universe’s Biggest Controversy -Ethan Siegel

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In 1915, Einstein’s theory of General Relativity gave us a brand new theory of gravity, based on the geometrical concept of curved spacetime. Matter and energy told space how to curve; curved space told matter and energy how to move. By 1922, scientists had discovered that if you fill the Universe uniformly with matter and energy, it won’t remain static, but will either expand or contract. By the end of the 1920s, led by the observations of Edwin Hubble, we had discovered our Universe was expanding, and had our first measurement of the expansion rate.

The journey to pin down exactly what that rate is has now hit a snag, with two different measurement techniques yielding inconsistent results. It could be an indicator of new physics. But there could be an even simpler solution, and nobody wants to talk about it.

Standard candles (L) and standard rulers (R) are two different techniques astronomers use to measure the expansion of space at various times/distances in the past. Based on how quantities like luminosity or angular size change with distance, we can infer the expansion history of the Universe.NASA / JPL-Caltech

The controversy is as follows: when we see a distant galaxy, we’re seeing it as it was in the past. But it isn’t simply that you look at light that took a billion years to arrive and conclude that the galaxy is a billion light years away. Instead, the galaxy will actually be more distant than that.

Why’s that? Because the space that makes up our Universe itself is expanding. This prediction of Einstein’s General Relativity, first recognized in the 1920s and then observationally validated by Edwin Hubble several years later, has been one of the cornerstones of modern cosmology.

A plot of the apparent expansion rate (y-axis) vs. distance (x-axis) is consistent with a Universe that expanded faster in the past, but where distant galaxies are accelerating in their recession today. This is a modern version of, extending thousands of times farther than, Hubble’s original work. Note the fact that the points do not form a straight line, indicating the expansion rate’s change over time.Ned Wright, based on the latest data from Betoule et al. (2014)