The Boltzmann brain paradox - Fabio Pacucci
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Nowadays, there is an overwhelming amount of independent data suggesting that the Universe is not infinite in time, rather it started some 13.8 billion years ago. But wait… how do we even know that? How can we measure the age of something as big as the Universe?
Well, it’s complicated. Many methods to estimate the age of the Universe have been proposed in the last one hundred years or so. Some of them rely on calculating the age of the oldest objects in the Universe. After all, the Universe itself cannot be older than any of its constituent objects, right? These objects include globular clusters, white dwarfs, and even black holes.
Here, we briefly describe the story of what can be regarded as The Discovery that opened the way to understanding that the Universe has a finite age: Edwin P. Hubble’s discovery of the expansion of the Universe, one of the most important and astounding scientific achievements of all times.
How did he reach this astonishing conclusion? Edwin Hubble, an American astronomer, spent many of his college years studying law, to follow his father’s wish, despite his passion for astronomy. Eventually, he started his studies of astronomy at the University of Chicago, receiving a Ph.D. in 1917, during World War I. After the war, in 1919 he started to work at the Mount Wilson observatory of the Carnegie Institution for Science. Here, he used the 100-inch Hooker telescope, at the time the most powerful in the world.
He was then ready for his first mind-bending discovery. In 1919, most of the people believed that the Milky Way, our own Galaxy, was the entire cosmos. He started to observe with his giant telescope some nebulas, or cloudy, fuzzy objects that astronomers regarded as being nearby, giant clouds of gas. Hubble was able to detect some remarkable stars in these nebulas, namely Cepheid stars. These variable stars are very powerful “standard candles”, as astronomers name objects whose distance from us can be determined. This important characteristic of Cepheids was discovered by Henrietta Swan Leavitt of the Harvard College Observatory just a few years earlier.
Using this relation, Hubble determined that these fuzzy nebulas were extremely far, outside our Galaxy. This discovery made instantly the Universe much more immense than previously thought. Our Galaxy is not the only one: the cosmos contains an undetermined number of them. On November 23, 1924 the New York Time reported the discovery, calling these galaxies “island Universes”.
Hubble was not done yet. He cataloged some dozens of these external galaxies and, using a property of their spectrum called “redshift”, he was also able to measure their velocity with respect to us. Hubble soon realized that the galaxies are receding from us, and their receding velocity increases with their distance: the Hubble’s law. In other words, the Universe was expanding. By measuring the rate of expansion, it is then possible to calculate how much time elapsed since all the galaxies were so close to each other to occupy the same point in space: the Big Bang.
Edwin Hubble, one of the greatest astronomers of all times, in a few years not only immensely expanded our idea of the Universe, but also discovered that it did not exist for an infinite amount of time. There is no doubt that NASA’s most famous and successful observatory of all times, the Hubble Space Telescope, was named after someone who wholly revolutionized our idea of the Universe.
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