From one serendipitously fortunate system, we gain a new window into the expanding Universe’s biggest conundrum.
We’ve known our Universe is expanding for ~90 years, yet unsolved mysteries persist.
The ‘raisin bread’ model of the expanding Universe, where relative distances increase as the space (dough) expands. The farther away any two raisin are from one another, the greater the observed redshift will be by time the light is received. The redshift-distance relation predicted by the expanding Universe is borne out in observations, and has been consistent with what’s been known all the way back since the 1920s. (NASA / WMAP SCIENCE TEAM)
Theoretically, everything composing the Universe — matter, dark matter, dark energy, radiation and more — determines the expansion rate.
The history of the expanding Universe can be traced back 13.8 billion years, to the very beginning of the hot Big Bang. A matter-filled Universe with initial imperfections underwent gravitational growth over a long period of time, resulting in the intricate cosmic web we see today. In the upper-left corner, a pie chart details the fractional energy density of the Universe today. Yet the other side of the equation, concerning the expansion rate, yields different and inconsistent values dependent on the method used. (ESA AND THE PLANCK COLLABORATION (MAIN), WITH MODIFICATIONS BY E. SIEGEL; NASA / WIKIMEDIA COMMONS USER 老陳 (INSET))
Only direct observations competently measure the actual rate, but different methods disagree.
The large-scale structure of the Universe changes over time, as tiny imperfections grow to form the first stars and galaxies, then merge together to form the large, modern galaxies we see today. Looking to great distances reveals a younger Universe, similar to how our local region was in the past. The temperature fluctuations in the CMB, as well as the clustering properties of galaxies throughout time, provide a unique method of measuring the Universe’s expansion history. (CHRIS BLAKE AND SAM MOORFIELD)
Methods based on early signals imprinted in the cosmic microwave background and on the Universe’s large-scale structure indicate one value: 67 km/s/Mpc.
An illustration of clustering patterns due to Baryon Acoustic Oscillations, where the likelihood of finding a galaxy at a certain distance from any other galaxy is governed by the relationship between dark matter and normal matter. As the Universe expands, this characteristic distance expands as well, allowing us to measure the Hubble constant, the dark matter density, and even the scalar spectral index. The results agree with the CMB data, and a Universe made up of 27% dark matter, as opposed to 5% normal matter. Altering the distance of the sound horizon could alter the expansion rate that this data implicates. (ZOSIA ROSTOMIAN)
However, methods relying on precise measurements to distant objects deliver a conflicting value: 74 km/s/Mpc.
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, but with many independent methods, it’s impossible for any one rung, like parallax or Cepheids or supernova, to cause the entirety of the discrepancy we find. While the inferred expansion rate could be biased towards higher or lower values if we lived in an underdense or overdense region, the amount required to explain this conundrum is ruled out observationally. There are enough independent methods use to construct the cosmic distance ladder that we can no longer reasonably fault one ‘rung’ on the ladder as the cause of our mismatch between different methods. (NASA, ESA, A. FEILD (STSCI), AND A. RIESS (STSCI/JHU))
Modern measurement tensions from the distance ladder (red) with early signal data from the CMB and BAO (blue) shown for contrast. It is plausible that the early signal method is correct and there’s a fundamental flaw with the distance ladder; it’s plausible that there’s a small-scale error biasing the early signal method and the distance ladder is correct, or that both groups are right and some form of new physics (shown at top) is the culprit. But right now, we cannot be sure. (ADAM RIESS (PRIVATE COMMUNICATION))
Each new measurement has the opportunity to either validate or refute this growing tension.
A doubly-lensed quasar, like the one shown here, is caused by a gravitational lens. If the time-delay of the multiple images can be understood, it may be possible to reconstruct an expansion rate for the Universe at the distance of the quasar in question. The H0LiCOW group has the best quasar measurements so far, deriving an expansion rate of 73.3 km/s/Mpc from it. (NASA HUBBLE SPACE TELESCOPE, TOMMASO TREU/UCLA, AND BIRRER ET AL.)
This image, taken in three Hubble filters, show the main foreground galaxies doing the lensing (G1 and G2), as well as multiple images of two distant sources (S2 and S3) that are multiply lensed by the foreground objects. (A.J. SHAJIB ET AL. (2019), ARXIV:1910.06306)
Originally misidentified as a single, quadruply-lensed quasar, it’s actually two independent doubly-lensed systems.
An illustration of gravitational lensing showcases how background galaxies — or any light path — is distorted by the presence of an intervening mass, but it also shows how space itself is bent and distorted by the presence of the foreground mass itself. When multiple background objects are aligned with the same foreground lens, multiple sets of multiple images can be seen by a properly-aligned observer. (NASA/ESA)
The composite image (top) as well as the three individual Hubble filters (below) all match the same parameters for background source and foreground lens configurations. This is consistent with two doubly-lensed objects at two different distances. (A.J. SHAJIB ET AL. (2019), ARXIV:1910.06306)
Through time-delays between features in the multiple images, astronomers derived distances and redshifts for both systems.
The major result for the expansion rate of the Universe, from just this one system, is dependent on the cosmological model chosen to reflect the total matter density. Our best observations place this value at about 0.32, indicating that an expansion rate of 74 km/s/Mpc is just fine, but one of 67 is unacceptable. (A.J. SHAJIB ET AL. (2019), ARXIV:1910.06306)
The resultant expansion rate matches the other distance ladder values: 74.2 km/s/Mpc, with a 3.9% uncertainty.
A series of different groups seeking to measure the expansion rate of the Universe, along with their color-coded results. Note how there’s a large discrepancy between early-time (top two) and late-time (other) results, with the error bars being much larger on each of the late-time options. The only value to come under fire is the CCHP one, which was reanalyzed and found to have a value closer to 72 km/s/Mpc than 69.8; all the distance ladder measurements are consistently higher than the CMB/LSS observations. (L. VERDE, T. TREU, AND A.G. RIESS (2019), ARXIV:1907.10625)
An illustrated timeline of the Universe’s history. If the value of dark energy is small enough to admit the formation of the first stars, then a Universe containing the right ingredients for life is pretty much inevitable. However, if dark energy comes and goes in waves, with an early amount of dark energy decaying away prior to the emission of the CMB, it could resolve this expanding Universe conundrum. (EUROPEAN SOUTHERN OBSERVATORY (ESO))
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