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A little over a hundred years ago, no one on our planet knew that the Universe was expanding. But despite all the troubles and misfortunes that the twentieth century brought to mankind, it was this century that was marked by scientific and technological progress. In an incredibly short period of time, we have learned more about the world and the Universe than ever before.
The idea that our universe has been expanding over the past 13, 8 billion years was first proposed by the Belgian physicist Georges Lemaitre in 1927. Two years later, the American astronomer Edwin Hubble was able to confirm this hypothesis. He found that each galaxy is moving away from us and the further it is, the faster it happens. Today, there are many ways in which scientists can understand how quickly our universe is expanding in size. Here are just the numbers that researchers get in the process of measuring, each time they turn out to be different. But why?
The biggest mystery of the universe
As we know today, there is a close relationship between the distance to a galaxy and how quickly it is receding. So, say, a galaxy at a distance of 1 megaparsec from our planet (one megaparsec is approximately equal to 3.3 million light years) is moving away at a speed of 70 kilometers per second. And the galaxy that is a little further away, at a distance of two megaparsecs, moves twice as fast (140 km / s).
It is also interesting that today there are two main approaches to determining the age of the Universe, or, scientifically, the Hubble Constant. The difference between the two groups is that one set of methods looks at relatively close objects in the universe, while the other looks at very distant ones. However, no matter what method the scientists use, the results are different each time. It turns out that either we are doing something wrong, or somewhere far away in the Universe, something absolutely unknown is happening.
In a study recently published on the airxiv.org preprint server, astronomers studying nearby galaxies used a clever method to measure the expansion of the universe called surface brightness fluctuations. It's a fancy name, but it includes an idea that's actually intuitive.
Imagine that you are standing at the edge of a forest, right in front of a tree. Because you are very close, you only see one tree in your field of vision. But if you step back a little, you will see more trees. And the further you go, the more trees will appear before your eyes. Much the same thing happens with the galaxies that scientists observe with telescopes, but much more complicated.
How do you know the rate of expansion of the Universe?
To get good statistics, astronomers observe galaxies that are fairly close to Earth, about 300 million light years or closer. However, when observing galaxies, it is necessary to take into account dust, background galaxies and star clusters that can be seen in images taken with a telescope.
The universe, however, is cunning. Since the 1990s, astronomers have seen that very distant exploding stars have always been farther away than simple measurements would indicate. This led them to believe that the universe is now expanding faster than before, which, in turn, led to the discovery of dark energy - a mysterious force that accelerates the universal expansion.
As the authors of the scientific work write, when we look at very distant objects, we see them as they were in the past, when the universe was younger.If the expansion rate of the Universe was then different (say, 12-13.8 billion years ago) than it is now (less than a billion years ago), we can get two different values for the Hubble Constant. Or maybe different parts of the universe are expanding at different rates?
But if the expansion rate has changed, then the age of our universe is not at all what we think (scientists use the expansion rate of the universe to trace its age). This, in turn, means that the universe has a different size, which means the time it takes for something to happen will also be different.
If you follow this chain of reasoning, then in the end it turns out that the physical processes that took place in the early Universe took place at different times. It is also possible that other processes were involved that affect the expansion rate. In general, there is some kind of mess. "From which it follows that either we do not understand well enough how the universe behaves, or we measure it incorrectly," the authors of the study note.
In any case, the Hubble Constant is a hotly debated topic in the astronomical community. The new study, however, has added even more questions, so the fight against uncertainty will be long. Someday, of course, our understanding of the cosmos will change. But when that happens, cosmologists will have to look for something else to argue about. What they will definitely do.