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The universe is expanding. But how fast? Why is this happening? And what will be the universe’s ultimate fate?
These are big, literally cosmic questions, and astronomers continue to struggle to nail down the answers. Different techniques for measuring what is called the Hubble Constant — the rate at which the universe is expanding — keep coming up with different estimates.
The cosmologists have given a name to this conundrum: the Hubble Tension.
In August, a research paper from University of Chicago astronomers, posted online and not yet peer-reviewed, reported that three observational techniques had produced estimates for the Hubble Constant ranging from 68 to 72 kilometers per second per megaparsec.
Days later, another research team led by cosmologist Adam Riess, a Nobel laureate at Johns Hopkins University, posted online a paper estimating the Hubble Constant at 73.
These numbers may seem, to the average nonscientist, close enough. Not for cosmologists — and certainly not for Riess, who has devoted his career to measuring the cosmic expansion.
“We’re stuck,” Riess said on a recent morning in his Johns Hopkins office, where he keeps a replica of the gold medal he was given in Stockholm.
The Hubble Tension flared in the past decade. One observational technique is based on analysis of the oldest light in the universe, known as the Cosmic Microwave Background radiation, or CMB. That analysis estimated the Hubble Constant at 67.
But Riess and his colleagues used the Hubble Space Telescope to study exploding stars (supernovae) in distant galaxies and came up with the speedier estimate of 73, and more recent observations by the James Webb Space Telescope reached a similar result.
The lead author of the University of Chicago paper, Wendy Freedman, said the Hubble Tension may be no big deal, that the scientific community may merely need to improve the way it estimates the distances between galaxies and thus the speed they recede from one another. If that happens, the Hubble Tension could be resolved without the requirement of revolutionary theories.
“We really will get to the bottom of this,” Freedman vows.
The debate — the tension — is simultaneously esoteric and fundamental. It’s esoteric in that the universe evolves on billion-year time scales. The Hubble Constant has no effect on any human being. Cosmic expansion occurs in intergalactic space, not down here on the ground where we all live. “Who cares?” is a reasonable reaction to this entire discussion.
But physicists and cosmologists care very much, because it concerns the fundamental nature of the universe. If the scientists can’t reconcile their estimates, it could mean there’s something going on in the universe that demands dramatic revisions to cosmological orthodoxy.
The rate of cosmic expansion tells us how old the universe is — about 13.8 billion years according to the current estimate based on the background radiation. And cosmic expansion will ultimately determine the future of everything there is. What will be the fate of the universe?
Scientific questions don’t get any bigger than that.
