An international team’s project using cosmic microwave background data inferred a Hubble constant of 67, substantially less than the 73 or 74 based on actually measuring the expansion (by analyzing how the light from distant supernova explosions has dimmed over time).
When this discrepancy
first showed up a few years ago, many experts believed it was just a mirage that would fade with more precise measurement. But it hasn’t.
“This starts to get pretty serious,” Adam Riess said at the astronomy meeting. “In both cases these are very mature measurements. This is not the first time around for either of these projects.”
One commonly proposed explanation contends that the supernova studies are measuring the local value of the Hubble constant. Perhaps we live in a bubble, with much less matter than average, skewing expansion measurements. In that case, the cosmic microwave background data might provide a better picture of the “global” expansion rate for the whole universe. But supernovas observed by the Hubble telescope extend far enough out to refute that possibility, Riess said.
“Even if you thought we lived in a void…, you still are basically stuck with the same problem.”
Consequently it seems most likely that something is wrong with the matter-energy recipe for the universe (technically, the cosmological standard model) used in making the expansion rate prediction. Maybe the vacuum energy driving cosmic acceleration is not a cosmological constant after all, but some other sort of field filling space. Such a field could vary in strength over time and throw off the calculations based on a constant vacuum energy. But Riess pointed out that the evidence is growing stronger and stronger that the vacuum energy is just the cosmological constant. “I would say there we have less and less wiggle room.”
Another possibility, appealing to many theorists, is the existence of a new particle, perhaps a fourth neutrino or some other relativistic (moving very rapidly) particle zipping around in the early universe.
“Relativistic particles — theorists have no trouble inventing new ones, ones that don’t violate anything else,” Riess said. “Many of them are quite giddy about the prospect of some evidence for that. So that would not be a long reach.”
Other assumptions built into the current cosmological standard model might also need to be revised. Dark matter, for example, is presumed to be very aloof from other forms of matter and energy. But if it interacted with radiation in the early universe, it could have an effect similar to that of relativistic particles, changing how the energy in the early universe is divided up among its components. Such a change in energy balance would alter how much the universe expands at early times, corrupting the calibrations needed to infer the current expansion rate.
It’s not the first time that determining the Hubble constant has provoked controversy. Edwin Hubble himself initially (in the 1930s) vastly overestimated the expansion rate. Using his rate, calculations indicated that the universe was much younger than the Earth, an obvious contradiction. Even by the 1990s, some Hubble constant estimates suggested an age for the universe of under 10 billion years, whereas many stars appeared to be several billion years older than that.
Hubble’s original error could be traced to lack of astronomical knowledge. His early overestimates turned out to be signals of a previously unknown distinction between different generations of stars, some younger and some older, Riess pointed out. That threw off distance estimates to some stars that Hubble used to estimate the expansion rate. Similarly, in the 1990s the expansion rate implied too young a universe because dark energy was not then known to exist and therefore was not taken into account when calculating the universe’s age.
So the current discrepancy, Riess suggested, might also be a signal of some astronomical unknown, whether a new particle, new interactions of matter and radiation, or a phenomenon even more surprising — something that would really astound a visitor from another universe.
See:
https://www.sciencenews.org/blog/context/speed-universe-expansion-remains-elusive
Recent efforts to measure the Universe further from Earth, like the SH0ES project led by Nobel laureate Adam Riess, have used Cepheids alongside Type Ia supernovae, which was used as a standard candle by Nobel Prize winning Saul Permutter's team when they extrapolated the ever faster expansion rate.. There are also other methods to measure Hubble's constant, such as one that uses the cosmic microwave background - relic light or radiation that began to travel through the Universe shortly after the Big Bang. The problem is that these two measurements, one nearby using supernovae and Cepheids, and one much farther away using the microwave background, differ by nearly 10%. Astronomers call this difference the Hubble tension, and have been looking for new measurement techniques to resolve it. But, the data from a magnified, multiply imaged supernova, which was discovered by a team of astronomers, including Dr Or Graur at the University of Portsmouth, provides insight into a longstanding debate in the field and could help scientists more accurately determine the Universe's age and better understand the cosmos.
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