Working out how far away a galaxy is from Earth is not easy. In the vastness of space, it can be tricky to tell if a galaxy is really far away but really big, or close but small.
But understanding how far away these galaxies are (combined with their velocity) can help tell us how big the Universe is too, and how long has elapsed since its birth.
A team of researchers has recently compiled a huge list of galaxy distances – 55,877 of them to be exact. These were measured with eight different methods, from looking at tiny colour changes to brightness fluctuations and tiny tweaks in kinematic (motion) properties.
“Since galaxies were identified as separate from the Milky Way a hundred years ago, astronomers have been trying to measure their distances,” said University of Hawaiʻi astronomer Brent Tully.
“Now by combining our more accurate and abundant tools, we are able to measure distances of galaxies, and the related expansion rate of the Universe and the time since the Universe was born with a precision of a few percent.”
All of this measuring is to understand a particular thing about the Universe – how fast spacetime is expanding. This is called the Hubble Constant and it’s unfortunately not as constant as astronomers might hope.
Different methods of measuring the rate at which the Universe is expanding have produced conflicting results.
If you measure based on the background radiation leftover from the early days of the Universe, you get a Hubble Constant of 67.5 km per second per megaparsec, with just a small margin of error – give or take 1 km per second per megaparsec.
But we can also measure the Hubble Constant looking at something called ‘standard candles’, these are supernovae explosions inside relatively close galaxies. Using these standard candles we can tell how far away they are, and combining that with how fast the galaxies are moving apart from us we can get a Hubble Constant too. Except via this method, you end up with around 74 km per second per megaparsec.
The difference between the measured and predicted values are MUCH more than can be expected given the statistical uncertainties.
As University of California cosmologist Geoff Chih-Fan Chen told Cosmos back in 2020 this has produced something he calls a “tension” in the field. But now, “maybe we should say there is a crisis”.
Either there is a fundamental problem with our understanding of the physics of the cosmos, or there is a hidden systematic error in the measurements of galaxy distances.
Unfortunately looking at the distance of 55,877 galaxies hasn’t helped this problem much either. The team of researchers managed to get a value of 75 km for the Hubble constant, give or take up to 3 km.
But despite this, working out the thousands of distances and with so many methods is important. Together this data is called Cosmicflows-4 and they make up the largest ever catalogue of high-precision galaxy distances.
Using so many different methods is also important. Many surveys like this only use one method, but with the team using so many different options it should lessen any large errors due to one of the methods being inaccurate.
The research has been published in the Astrophysics Journal and you can read the whole thing at arXiv.
Jacinta Bowler is a science journalist at Cosmos. They have a undergraduate degree in genetics and journalism from the University of Queensland and have been published in the Best Australian Science Writing 2022.
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