Outlined in light blue, giant collections of galaxies can be divided up into superclusters. But our supercluster, along with many nearby ones, might still reside in an even larger cosmic void. Image credit: R. Brent Tully, Hélène Courtois, Yehuda Hoffman & Daniel Pomarède, Nature 513, 71–73 (04 September 2014).
“No matter what technique you use, you should get the same value for the expansion rate of the universe today.” -Ben Hoscheit
When you think of the Universe on the largest scales, you likely think of galaxies grouped and clustered together in huge, massive collections, separated by enormous cosmic voids. But there’s another kind of cluster-and-void out there: a very large volume of space that has its own galaxies, clusters and voids, but is simply higher or lower in density than average.
The construction of the cosmic distance ladder involves going from our Solar System to the stars to nearby galaxies to distant ones. Each “step” carries along its own uncertainties; it also would be biased towards higher or lower values if we lived in an underdense or overdense region. Image credit: NASA,ESA, A. Feild (STScI), and A. Riess (STScI/JHU).
If our galaxy resided near the center of one such region, we’d measure the expansion rate of the Universe to be higher-or-lower than average when we used nearby techniques. But if we measured the global expansion rate, such as via baryon acoustic oscillations or the fluctuations in the cosmic microwave background, we’d actually arrive at the true, average rate.
Three different types of measurements, distant stars and galaxies, the large scale structure of the Universe, and the fluctuations in the CMB, tell us the expansion history of the Universe. Image credit: NASA/ESA Hubble (top L), SDSS (top R), ESA and the Planck Collaboration (bottom).