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))

How Fast Is The Universe Expanding? Incompatible Answers Point To New Physics

As more data comes in, the puzzle gets deeper and deeper.

Whenever you set out to solve a problem, there are a series of steps you have to take in order to arrive at the answer. Assuming your methods are sound and you don’t make any major errors, the answer you get should be correct. It might be a little higher or a little lower that the “true” value, as measurement (and other) uncertainties are real and cannot be eliminated, but the answer you obtain should be independent of the method you use.

For more than a decade, a conundrum has been building in the field of astrophysics: although there are many different ways of measuring the rate at which the Universe is expanding, they fall into two different classes.

• One class relies on an early signal (from the Big Bang) that can be observed today, and those measurements cluster around 67 km/s/Mpc.
• The other class uses astrophysical objects to measure distance and redshift simultaneously, building up a suite of evidence to infer the expansion rate, where those measurements cluster around 74 km/s/Mpc.

A slew of new studies show that the mystery is now deepening even further.