Once we discovered that the universe was expanding. After that, the next scientific step was to determine the speed or rate of this expansion. More than 80 years have passed, but we still have not agreed on this issue. Looking at the largest cosmic scales and studying the oldest signals - the afterglow of the Big Bang and large-scale correlations of galaxies - we got one number: 67 km / s / Mpc.
But looking at individual stars, galaxies, supernovae and other direct pointers, we get a different number: 74 km / s / Mpc. The uncertainties are very small: ± 1 to the first number and ± 2 to the second number, and there remains a statistical chance of less than 0.1% that these numbers will be reconciled with each other. This contradiction should have been resolved long ago, but has persisted ever since the expansion of the universe was first discovered.
In 1923, Edwin Hubble used the world's largest telescope to look for new stars in other galaxies. Probably, it would not be worthwhile to say "galaxies", because then humanity was not sure of heavenly spirals. While studying the largest of them - M31, now known as the Andromeda Nebula - he saw the first, and then the second and the third new. But the fourth appeared in the same place as the first, and this was impossible, since the new ones take centuries or more to recharge. His new one appeared in less than a week. Excitedly, Hubble crossed out the first "N" that he wrote, and overwritten "VAR!" He realized that it was a variable star, and since then there was physics of variable stars. Hubble was able to calculate the distance to Andromeda. He showed that it was exactly outside the Milky Way and is obviously a galaxy. It was the finest sighting of a single star in the history of astronomy.
Edwin Hubble's original LP revealing the variable nature of a star in Andromeda
Hubble continued his work by observing variable stars in many spiral galaxies. Along with their shifted spectral lines, he began to notice that the further the galaxy is, the faster it moves away from us. Not only did he discover this law - known as Hubble's law - he was the first to measure the rate of expansion: the Hubble parameter. The number he received was, however, large. Very big. So big that if it were true, it would follow that the Big Bang happened just two billion years ago. Obviously, no one would believe this, since we have geological evidence that the Earth alone is more than four billion years old.
Composite image of the western hemisphere of the Earth more than 4 billion years old
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In 1943, astronomer Walter Baade closely observed variable stars outside the Milky Way and noticed something incredibly important: not all variable Cepheids - the type that Hubble used to determine the expansion of the universe - behave the same. Instead, there were two different classes. And suddenly it turned out that the Hubble constant was not at all as large as Hubble had decided.
Measurements of variable stars by Walter Baade in Andromeda were the most important evidence of the existence of two separate populations of Cepheids and allowed the Hubble parameter to be reduced to a more meaningful value
Instead, the universe expanded more slowly, which means it took longer for it to reach its current state. For the first time, the Universe surpassed Earth in age, and that was a good sign. Over time, further refinements increased and the Hubble exponent gradually decreased, while the age of the universe continued to increase. Ultimately, the age of even the oldest stars sank with the age of the universe.
How the estimates of the Hubble parameter changed over time
The story doesn't end there. Do you know why the Hubble Space Telescope was named that way? Not because it was named after Edwin Hubble, who discovered that the universe was expanding. Rather, because its main mission was to measure the Hubble parameter, or the rate at which the universe is expanding. Prior to the launch of the telescope in 1990, there were two camps advocating completely different universes: one led by Allan Sandage and a universe with an expansion rate of 50 km / s / Mpc and an age of 16 billion years; the other is under the leadership of Gerard de Vaucouleur and a universe with an expansion rate of 100 km / s / Mpc and an age of under 10 billion years. These two camps were convinced that the opposing camps were making systematic errors in their measurements and that there was no middle ground. The main scientific goal of the Hubble Space Telescope was to measure the expansion rate once and for all.
And he achieved it. 72 ± 8 km / s / Mpc was the final result of the project. Today there are even fewer errors or inaccuracies, and so is the tension between the two different methods. If you look at the Universe on the largest scales, the fluctuations of the cosmic microwave background and baryon acoustic oscillations in the clustering of galaxies, you get a smaller number: 67 km / s / Mpc. This is not the most favorable result, but higher values are quite possible.
If you look at direct measurements of individual stars in our galaxy, and then at the same classes of stars in other galaxies, and then at supernovae beyond that, you get a higher value: 74 km / s / Mpc. But a systematic error in measurements of nearby stars, even an error of several percent, could significantly reduce this number even to the lowest value proposed. As the ESA Gaia mission continues to measure parallax with unprecedented accuracy of a billion stars in our galaxy, this tension may resolve on its own.
Today we know the Hubble expansion rate quite accurately, and two different methods of extracting it seem to give conflicting values. There are many different dimensions going on right now, each camp trying to prove its case and find the mistakes of the other. And if history has taught us anything, we can say that, firstly, we will learn something new and interesting about the nature of our Universe when this issue is resolved, and secondly, this dispute on the expansion rate will clearly not last.
ILYA KHEL