"When it comes to the question of the expanding Universe, all observations agree that yes, the Universe is indeed expanding, in agreement with Edwin Hubble's earliest observations dating all the way back to the 1920s. However, the question of "how fast" the Universe is expanding is one that's long been controversial, and remains so even today. In fact, there are two main classes of measurement one can make to determine the cosmic expansion rate, and they yield different, incompatible values."
"You can start with a primordial signature imprinted early on in the hot Big Bang, like the acoustic scale, and evolve it forward in time to explain what you observe today. This "early relic" method yields values for today's expansion rate of 67 km/s/Mpc. Or you can start in the here-and-now and look back to a distant set of signal sources, like stars, supernovae, quasars, etc. This "distance ladder" method yields a different set of values for today's expansion rate of 73 km/s/Mpc."
Observations confirm the Universe is expanding, but the measured expansion rate differs by method. Early-Universe acoustic-scale measurements give about 67 km/s/Mpc, while local distance-ladder methods give about 73 km/s/Mpc. This persistent discrepancy is called the Hubble tension and represents a significant cosmological problem. Proposed independent approaches include gravitationally lensed supernovae, lensed quasars, and neutron-star standard sirens to arbitrate the disagreement. Gravitational lensing techniques are powerful tools, but they are not sufficient by themselves to resolve the Hubble tension, so further methods or insights are needed.
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