Astronomers are enhancing their scale of distances to the universe. Sadly, it would not remedy the disaster in cosmology

Measuring the expansion of the universe is difficult. For one, the scale of your distance measurements affects the scale of expansion as the universe expands. And since light from distant galaxies takes time to reach us, you cannot measure what the universe is, but what it was. Then there is the challenge of the cosmic distance ladder.

The ladder of distance arises from the fact that while we have many ways of measuring cosmic distance, none of them work at all levels. For example, the greatest distances are determined by measuring the apparent brightness of supernovae in distant galaxies. This works great over billions of light years, but there aren’t enough supernovae in the Milky Way to measure distances nearby. Perhaps the most accurate distance measurement uses parallax, which measures the apparent shift in a star’s position as the earth orbits the sun. Parallax is a matter of simple geometry, but it is only accurate to a few thousand light years.

Some of the methods of measuring cosmic distances. Photo credit: Tabitha Dillinger

It is for this reason that astronomers often measure the standard by building one method on top of the other. Use parallax for the closest stars, including a type of variable star called Cepheid variables. Cepheids vary in brightness proportionally to their average luminosity, so you can use them to measure distances of up to 100 million light years. Supernovae are constantly occurring in this area, so you can use supernova measurements to determine distances over billions of light years. These are not the only methods used in the cosmic distance ladder, but each method has limited range and accuracy.

Since there is uncertainty with every measurement you take, errors can form in the distance ladder. If your parallax measurements are a little off, your Cepheid measurements will be worse to begin with, and your supernova measurements will be even less accurate. Therefore, when we measure cosmic expansion using different methods, we get results that are easily inconsistent. This is known as cosmic tension. In the past this wasn’t a big problem. While different methods gave different results, the measurement uncertainty was large enough that the results overlapped. However, as our measurements become more accurate, they no longer overlap. You totally disagree.

The new distance ladder dimension does not match the Planck dimension. Photo credit: Riess et al

To solve this problem, a team of astronomers recently focused on making the cosmic distance ladder more accurate. Your focus is on parallax measurements, the ground on which the distance ladder stands. In this case they are using data from the Gaia spaceship. Gaia has measured the parallax and motion of more than a billion stars, including Cepheid variable stars. As a result, the team reduced the uncertainty of the Cepheid distance method to just 1%. With this new result in the cosmic distance ladder you get a measurement for the Hubble constant (the speed of cosmic expansion) between 71.6 and 74.4 km / s / Mpc. This is great, but it contradicts other methods, especially data from the Planck satellite measurement of the cosmic microwave background, which gives a value between 67.2 and 68.1 km / s / Mpc.

The more accurate our measurements, the worse the voltage problem becomes. There is something about cosmic expansion that we clearly do not understand, and we can only hope that more and better data will lead us to a solution.

Reference: Riess, Adam G. et al. “Cosmic distances calibrated to 1% precision using Gaia EDR3 parallaxes and Hubble Space Telescope photometry of 75 Milky Way Cepheids confirm the tension with LambdaCDM.” arXiv preprint arXiv: 2012.08534 (2020).

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