Scientists Can’t Agree On The Expanding Universe

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, or we have an unidentified error somewhere.C. FAUCHER-GIGUÈRE, A. LIDZ, AND L. HERNQUIST, SCIENCE 319, 5859 (47)

The Universe is expanding, and every scientist in the field agrees with that. The observations overwhelmingly support that straightforward conclusion, and every alternative has failed to match its successes since the late 1920s. But in scientific endeavors, success cannot simply be qualitative; we need to understand, measure, and quantify the Universe’s expansion. We need to know how much the Universe is expanding by.

For generations, astronomers, astrophysicists and cosmologists attempted to refine our measurements of the rate of the Universe’s expansion: the Hubble constant. After many decades of debates, the Hubble Space Telescope key project appeared to solve the issue: 72 km/s/Mpc, with just a 10% uncertainty. But now, 17 years later, scientists can’t agree. One claims ~67 km/s/Mpc; the other claims ~73 km/s/Mpc, and the errors do not overlap. Something, or someone, is wrong, and we cannot figure out where.

The farther a galaxy is, the faster it expands away from us, and the more its light appears redshifted. A galaxy moving with the expanding Universe will be even a greater number of light years away, today, than the number of years (multiplied by the speed of light) that it took the light emitted from it to reach us. But how fast the Universe is expanding is something that astronomers using different techniques cannot agree on.LARRY MCNISH OF RASC CALGARY CENTER

The reason this is such a problem is because we have two major ways of measuring the expansion rate of the Universe: through the cosmic distance ladder and through looking at the signals originating from the earliest moments of the Big Bang. The two methods are extremely different.

  • For the distance ladder, we look at nearby, well-understood objects, then observe those same types of objects in more distant locations, then infer their distances, then use properties we observe at those distances to go even farther, etc. By building up redshift and distance measurements, we can reconstruct the expansion rate of the Universe.
  • For the early signals method, we can use either the leftover light from the Big Bang (the Cosmic Microwave Background) or the correlation distances between distant galaxies (from Baryon Acoustic Oscillations) and see how those signals evolve over time as the Universe expands.

The first method seems to be giving the higher figure of ~73 km/s/Mpc, consistently, while the second gives ~67 km/s/Mpc.

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. Using the candle method is part of the distance ladder, yielding 73 km/s/Mpc. Using the ruler is part of the early signal method, yielding 67 km/s/Mpc. These values are inconsistent.NASA / JPL-CALTECH

This should trouble you deeply. If we understand the way the Universe works correctly, then every method we use to measure it should deliver the same properties and the same story about the cosmos we inhabit. Whether we use red giant stars or blue variable stars, rotating spiral galaxies or face-on spirals with fluctuating brightness, swarming elliptical galaxies or Type Ia supernovae, or the Cosmic Microwave Background or galaxy correlations, we should get an answer that’s consistent with a Universe having the same properties.

But that’s not what happens. The distance ladder method systematically gives a higher value by about 10% than the early signals method, regardless of how we measure the distance ladder or which early signal we use. Here’s the most accurate method for each one.

The parallax method, employed since telescopes became good enough in the 1800s, involves noting the apparent change in position of a nearby star relative to the more distant, background ones. There may be biases in this method due to the presence of masses we have not appropriately accounted for.ESA/ATG MEDIALAB