A new estimate deepens the clash over the universe’s expansion rate


When it comes to the expansion rate of the universe, physicists have apparently agreed to disagree.

Two types of measurements clash over how fast the cosmos is expanding (SN: 7/30/19). Now, a new estimate from the Atacama Cosmology Telescope, or ACT, further entrenches this disagreement.

To tease out the properties of the universe, ACT observes light emitted shortly after the Big Bang, known as the cosmic microwave background. Those observations reveal that the universe is expanding a rate of about 67.9 kilometers per second for each megaparsec (about 3 million light-years), physicists report in two papers posted online and submitted to arXiv.org. The number aligns with that of an earlier cosmic microwave background experiment called Planck (SN: 7/24/18).

“As an independent experiment, we see the same thing,” says cosmologist Simone Aiola of the Flatiron Institute in New York City. Located in the Atacama Desert in Chile, ACT observes the cosmic microwave background with a higher resolution than Planck did.

cosmic microwave background
To measure the expansion of the universe, the Atacama Cosmology Telescope mapped out the cosmic microwave background (one portion shown). Colors represent differences in the polarization, the orientation of the light’s electromagnetic waves.ACT Collaboration

Both ACT and Planck disagree with most estimates from objects that emitted their light more recently, such as exploding stars called supernovas and bright hearts of galaxies known as quasars. Those studies tend to indicate a faster expansion rate of around 74 kilometers per second per megaparsec.

If no simple explanation can be found for the discrepancy, it could dramatically alter physicists’ understanding of the contents of the universe and how the cosmos changes over time. For example, dark energy, the shadowy stuff that causes the universe to expand at an accelerating rate, might behave differently than scientists thought.

Some researchers had speculated that an unidentified source of experimental error in the Planck data could have accounted for the mismatch. But with the independent measurement from ACT, that explanation has gone out the window. That frees physicists to focus on other explanations, like potential issues with the supernova or quasar measurements, or the possibility of unexplained new physics phenomena.

Now, says cosmologist Adam Riess of Johns Hopkins University and the Space Telescope Science Institute in Baltimore, “we can proceed without the niggling worries.”

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