The Hubble Constant is constantly perplexing

Astronomers looking at ghostly images of the light from distant quasars have deepened one of the biggest mysteries in modern astrophysics, raising the possibility that important parts of our understanding of the expanding Universe are in error.

The problem, says Geoff Chih-Fan Chen, a cosmologist at the University of California, Davis, is that different methods of measuring the rate at which the Universe is expanding have produced conflicting results.

In recent years, he says, this has produced something he calls a “tension” in the field. But now, “maybe we should say there is a crisis”.

The issue, he reported to this week’s meeting of the American Astronomical Society in Hawaii, involves a number known as the Hubble Constant, which measures the rate at which galaxies are flying away from each other as a relic of the Big Bang.

One way of measuring this is by using radio astronomy to study background radiation leftover from the early days of the Universe. 

This radiation can be used to determine the age of the Universe, and from that, how fast it has been expanding. Based on that, Chen says, cosmologists have estimated the Hubble Constant at 67.4 kilometres per second, per megaparsec (3.26 million light years) of separation. 

In other words, galaxies a billion parsecs apart should be flying away from each other at 67,400 kilometres per second.

But that’s not the only way the Hubble Constant can be measured. It can also be measured from supernovae explosions in galaxies close enough for us to be able to see them.

Astronomers have long known that these exploding stars release predictable amounts of light, meaning that they can be used as “standard candles” whose apparent brightness is an indicator of how far they are away.

Combining that with the speed at which their host galaxies are known to be receding (measured via speed-related shifts in their spectra, known as red shifts) it should then be possible to calculate the Hubble Constant.

Only the result doesn’t come out at 67.4. Rather it’s somewhere around 74, suggesting that the Universe’s rate of expansion has been inexplicably speeding up.

“The two kinds of measurements don’t match,” Chen says.

The burning question, he says, is why. Is it unknown physics? Or simply an “unknown unknown” in one (or both) of the measurements has an error nobody has yet been able to suss out? 

To test this, Chen’s team turned to the Hubble Constant’s namesake instrument, the Hubble Space Telescope, and found an entirely different way of measuring the constant – this one involving quasars and gravitational lensing.

Quasars are enormous bursts of radiation, probably associated with black holes at the hearts of distant galaxies — though not so distant that the light from them has been travelling to us since the dawn of time. 

The ones in Chen’s study averaged about 5.5 billion light years away, making them only about 40% the age of the Universe.

His team picked six such quasars whose light passed through intervening galaxies whose gravity bent it in a process known as “gravitational lensing”, which produced multiple images of each of the quasars. 

They then drew on the fact that the brightness of quasars fluctuates as the black holes feeding them flare up and subside, as they gobble up matter in their host galaxies.

That allowed them to compare the time it took their light to reach us by different angles, from which, by simple geometry, they could calculate the distance it had travelled. Comparing that to its red shift allowed them to calculate the Hubble Constant.

There were complications, of course, but ultimately, Chen says, his team was able to calculate the Hubble Constant related to the six quasars to within 2.4%.

They came up with a figure of 73.3, extremely close to what had been found by the supernova studies. 

But more importantly, he says, that’s sufficiently different from the number obtained from the cosmic background radiation studies to mean there is only a one-in-10-million chance the difference is due to measurement error.

The result, he says, makes him think the “crisis” over measurements of the Hubble constant is more likely “real” than the result of measurement error. 

And, he says, others in the cosmological community are starting to agree.

“It may be that we do not yet fully understand how matter and energy evolved over time, particularly at early times,” says his colleague Sherry Suyu, from the Max Planck Institute for Astrophysics, in Germany.

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