The expanding Universe, full of galaxies and the complex structure we observe today, arose from a… [+] smaller, hotter, denser, more uniform state. It took thousands of scientists working for hundreds of years for us to arrive at this picture, and yet the lack of a consensus on what the expansion rate actually is tells us that either something is dreadfully wrong, we have an unidentified error somewhere, or there’s a new scientific revolution just on the horizon. The expanding Universe, full of galaxies and the complex structure we observe today, arose from a smaller, hotter, denser, more uniform state. It took thousands of scientists working for hundreds of years for us to arrive at this picture, and yet the lack of a consensus on what the expansion rate actually is tells us that either something is dreadfully wrong, we have an unidentified error somewhere, or there’s a new scientific revolution just on the horizon. (C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47))

The Expanding Universe Might Not Depend On How You Measure It, But When

It’s been called the biggest conundrum in all of cosmology, and recent measurements just add to the confusion.

One of the most puzzling facts about the Universe is that different ways of measuring how fast it’s expanding yield different results. It’s not that there are two ways to measure it and they don’t agree; it’s that there are perhaps a dozen different ways of measuring it, and they yield two different sets of results. Both of them require a Universe filled with normal matter, dark matter, and dark energy, but their preferred values differ by about 9%: much greater than the uncertainties involved.

No sources of error have been identified that could explain the discrepancy, with multiple independent lines of evidence existing for both sets of results. Recently, however, a very clever new test of the Universe’s expansion rate has been devised and leveraged, and it appears to offer a clue like none before: the same test favors different values at late versus early times. Perhaps expanding Universe depends on when, rather than how, you measure it.