“Are we almost there yet?” : astronomers determine the distance of 56,000 galaxies from Earth

IDetermining how far from Earth a galaxy is is not easy. In the vastness of space, it can be difficult to tell whether a galaxy is far away but very large, or close but small.

Header image, from the survey: a sky map showing the 56,000 galaxies in the Cosmicflows-4 project with distance measurements. Each dot represents a galaxy, and the color of the dot indicates its distance from us, with blue tones near it and reddish tones for galaxies further away. (R. Brent Tully et al./Astrophysics)

But knowing how far away these galaxies are (as well as their speed) can help determine the size of the universe and the time that has passed since its birth.

A team of researchers recently compiled a huge list of distances between galaxies, 55,877 to be exact. These distances were measured using eight different methods, ranging from observing small changes in color, to fluctuations in brightness, to small changes in kinematic (motion) properties.

According to Brent Tully, an astronomer at the University of Hawaii:

Since the galaxies were identified as separate from the Milky Way a hundred years ago, astronomers have attempted to measure their distances.

Now, by combining our more precise and numerous tools, we are able to measure the distances of galaxies, as well as the related expansion rate of the universe and the time that has passed since the birth of the universe with an accuracy of a few percent.

All of these measurements aim to understand a particular aspect of the universe: the rate of expansion of spacetime. This is called the Hubble constant, and unfortunately it is not as constant as astronomers might hope.

Different methods of measuring the universe’s expansion rate have produced conflicting results.

Based on the background radiation left by the early days of the universe, the cosmic microwave background (below), we get a Hubble constant of 67.5 km per second. second per megaparsec, with a small margin of error of plus or minus 1 km per second per megaparsec.

The map below shows the oldest light in our universe as it was detected with the greatest precision by the Planck mission. Ancient light, called the cosmic microwave background, was imprinted on the sky when the universe was 370,000 years old. It shows the small temperature fluctuations that correspond to regions of slightly different densities that represent the seeds of all future structure: the stars and galaxies of today. (ESA/Planck Collaboration)

But we can also measure the Hubble constant by observing what is called standard light, that is, supernova explosions in relatively nearby galaxies. Using these standard luminosities we can work out how far away they are, and combined with the speed at which galaxies are moving away from us, we can also get a Hubble constant. If we exclude this method, we achieve approximately 74 km per hour. second and per megaparsec.

This diagram illustrates two ways of measuring the rate at which the universe is expanding: the “standard candle” method, which involves the explosion of stars in galaxies, and the “standard ruler”, which involves pairs of galaxies. (NASA)

The difference between the measured and predicted values ​​is much larger than one would expect given the statistical uncertainties. This situation has created a kind of tension in the field. But today we may have to say that there is a crisis.

Either there is a fundamental problem in our understanding of the physics of the cosmos, or there is a hidden systematic error in measurements of distances between galaxies.

Unfortunately, looking at the distance between these 55,877 galaxies didn’t do much to solve this problem either. The research team managed to obtain a value of 75 km for the Hubble constant within 3 km.

But despite this, it is important to calculate thousands of distances with so many methods. All this data is called Cosmicflows-4 and is the largest catalog of high-precision galaxy distances ever produced.

The use of so many different methods is also important. Many such studies use only one method, but if the team uses so many different options, this should reduce large errors due to the inaccuracy of one of the methods.

The study published in the journal Astrophysics and available on arXiv: Cosmicflows-4 and featured on the University of Hawaii website: UH astronomers map distances to 56,000 galaxies, largest catalog ever.

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