Calculations based on observations of the early universe — namely, the cosmic microwave background (CMB) that is a sort of afterglow of the Big Bang — produce one answer for the Hubble constant. Observations of the “late universe” instead compare the distances of astronomical objects, often standard candles with known distances, to the speed those objects are moving away from us. The two techniques provide different answers, a discrepancy that has become known as the Hubble-constant tension.
The Hubble constant expresses the universe’s present-day rate of expansion. There’s only one current expansion rate of the universe, but different studies are coming up with different answers for what it is.
Calculations based on observations of the early universe — namely, the cosmic microwave background (CMB) that is a sort of afterglow of the Big Bang — produce one answer for the Hubble constant. Observations of the “late universe” instead compare the distances of astronomical objects, often standard candles with known distances, to the speed those objects are moving away from us. The two techniques provide different answers, a discrepancy that has become known as the Hubble-constant tension.The past decade or so has seen dozens of measurements of the Hubble constant, using sources near (in the box labeled "Late") and far (in the box labeled "Early"). There seems to be a discrepancy depending on whether the measurements are based on the early universe or the present-day universe, as seen in the box labeled "Early vs. Late," though the amount of discrepancy depends on which sources are used.
No one knows why the early and late methods give different answers. At first, people thought that more and better measurements would cause the numbers to converge. But instead, in study after study, the error bars have shrunk to the point where the difference has become statistically significant.
The first and most precise measurements of the current expansion rate were made using standard candles, sources with known luminosity. If we know how luminous a source is, then we can reckon its distance according to how bright it appears. The two most widely used standard candles are Cepheid variable stars and Type 1a supernovae, but there are many others.
NEW RESULTS
Two independent groups using data from the Hubble Telescope have just published new studies measuring the Hubble constant in different ways, yet their results match. The results give further credence to the late-universe consensus of an expansion rate around ~73 km/s/Mpc. But it also serves to deepen the tension, as studies published in the last decade calculating the rate from the properties of the CMB give an answer around ~67 km/s/Mpc. This may seem like no big deal, but the difference is big enough that some astrophysicists are calling it a “crisis for cosmology.” READ MORE
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