We still do not know how fast the universe is expanding

Once we discovered that the universe is expanding. After that, the next step was to identify the scientific or speed the pace of expansion. It was more than 80 years, but we still have not agreed on this issue. Looking at the largest scales of space and explore the old signals - the afterglow of the Big Bang, and large-scale correlation of galaxies - we got one number: 67 km / s / Mpc.

We still do not know how fast the universe is expanding

But looking at individual stars, galaxies, supernovae and other direct indicators, we get another number: 74 km / s / Mpc. Uncertainties are very small: ± 1 to ± first number and the second number 2, and is a statistical chance is less than 0, 1%, that these numbers will reconcile with each other. This contradiction was resolved long ago, but stubbornly held since it was first observed expansion of the universe.

In 1923, Edwin Hubble used the world's largest telescope to search for new stars in other galaxies. Probably should not have to say "galaxy", because then humanity was not sure in the heavenly spirals. Studying the largest of them - M31, now known as the Andromeda Galaxy - he saw first, and then the second and third new. But the fourth appeared in the same place as the first, but it was impossible, because the new recharged hundreds of years or more. His new came less than a week. Nervously, Hubble crossed out the first "N", which was written and recorded over the "VAR!". He realized that it was a variable star (variable), and has since appeared physics of variable stars. Hubble was able to calculate the distance to the Andromeda. He showed that it was definitely beyond the Milky Way and is obviously a galaxy. It was the most beautiful observation of a single star in the history of astronomy.

We still do not know how fast the universe is expanding

Original plate Edwin Hubble that reflects the nature of variable stars in Andromeda.

Hubble continued its work, observing variable stars in many spiral galaxies. Along with their shifted spectral lines, he began to notice that the farther away a galaxy is, the faster it is receding from us. He not only discovered the law - known as Hubble's law - he was the first to measure the expansion rate: the Hubble parameter. He has received the number was, however, great. Very large. So great that if it were true, because it would imply that the Big Bang occurred only two billion years ago. Obviously, no one would have believed it, because we have geological evidence that the earth alone for more than four billion years.

We still do not know how fast the universe is expanding

A composite image of the Western Hemisphere of the Earth in the age of more than 4 billion years old.

In 1943, astronomer Walter Baade was watching variable stars outside the Milky Way, and noticed something incredibly important: not all variables Cepheids - the type that is used to determine the Hubble expansion of the Universe - behave the same way. Instead, there were two different classes. And suddenly it turned out that the Hubble constant was not as large as the Hubble decided.

We still do not know how fast the universe is expanding

The measurements of variable stars in Andromeda, Walter Baade was the most important proof of the existence of two distinct populations of Cepheids and allowed to keep the Hubble parameter to a sensible size.

Instead, the universe expanded more slowly, which means that it took longer to reach this state. The universe for the first time surpassed the earth in age, and it was a good sign. Over time, further refinements were growing, and the Hubble declining gradually, while the age of the universe continued to expand. Ultimately, even the age of the oldest stars drowned in the age of the universe.

We still do not know how fast the universe is expanding

How do assess the Hubble parameter over time.

The story does not end there. Do you know why the Hubble Space Telescope was named this way? Not because it was named after Edwin Hubble, who discovered that the universe is expanding. Rather, because it was to measure the primary mission of the Hubble parameter, or the rate at which expanding universe. Prior to the launch of the telescope in 1990, there were two camps, pro completely different universes: one under the leadership of Allan Sandage and expansion of the universe at a speed of 50 km / s / Mpc, and the age of 16 billion years; the other under the leadership of Gerard de Vaucouleurs and the universe to expand at the rate of 100 km / s / Mpc, and under the age of 10 billion years. The two camps were convinced that the opposite camp make systematic errors in their measurements, and that there is no middle ground. The main scientific goal of the Hubble Space Telescope was to measure the rate of expansion once and for all.

And he achieved it. 72 ± 8 km / sec / Mpc was the final result of the project. Today error or errors even less, as well as the voltage between the two different methods. If you look at the universe on the largest scales, fluctuations in 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 the direct measurements of individual stars in our galaxy, and then on the same classes of stars in other galaxies, supernovae and then on top of that, you get more value: 74 km / s / Mpc. But a systematic error in the measurements of nearby stars, even an error of a few percent, would significantly reduce this number even to its lowest level of the proposed. As soon as the mission EKA Gaia continues to measure the parallax with unprecedented accuracy billion stars in our galaxy, this voltage can be resolved by itself. Today we know the rate of expansion of Hubble's pretty accurately, and two different methods of extracting it seems, give conflicting values. Now runs a number of different measurements, each camp is trying to prove his innocence and find the mistakes of others. And if the story we learn something, we can say that we are, first, to learn something new and interesting about the nature of our universe, when the issue is resolved, and secondly, the dispute on the rate of expansion is clearly not latest.