In 1687, Isaac Newton laid the foundation for understanding the universe in his book "Principia," proposing the revolutionary law of universal gravitation that explains how every mass in the Universe is attracted to every other mass. The idea, simple enough in itself, had puzzling implications at the time. Newton unveiled with his laws that if our Universe was finite, the attractive forces between galaxies should have made it collapse in on itself. Since this did not happen, other conclusions had to be drawn; the Universe had to be infinite.
Something, however, prevented the Universe from being considered infinite, a paradox known as "Olbers' paradox." This paradox was described by Wilhelm Olbers in the early 19th century. Olbers argued that the darkness of the night sky conflicted with the conclusion drawn by Newton, so the universe could not be infinite.
We know that the apparent size of a star decreases with increasing distance. But even if distant stars are smaller and less visible, we would see more of them if the universe were infinitely large and existed for an infinitely long time; in short, we should see nothing but starlight in the sky. Since, on the other hand, we observe vast dark spaces, the universe cannot be infinite.
How can we reconcile the ideas of Newton and Olbers?
In 1913, American astronomer Vesto Slipher analyzed the spectral lines of distant galaxies and discovered that the light they emitted shifted toward the red end of the light spectrum. Slipher regarded the redward shift as evidence that the galaxies were moving apart.
Drawing on Slipher's work, astronomer Edwin Hubble measured the spectral lines of the galaxies and then compared them with their relative distance, making an extraordinary discovery: the galaxies were moving away from each other the faster the greater the calculated distance: the universe Hubble discovered was expanding.
If the universe is expanding, one can infer that it must have been smaller at some point in the past. Looking at the expansion like a movie projected in reverse, at some instant all the galaxies will concentrate at a single point we call the Big Bang, the origin of the Universe.
Today we know from models and estimates of the expansion rate that the universe is about 13 billion 799 million years old. Unfortunately, expansion means that we continue to lose parts of the universe that we will no longer be able to observe.
In the last decades of the 20th century, two teams of scientists measured the rate of cosmic deceleration (how much the expansion of the Universe slows down) by calculating the distance and receding speed of type 1a supernovae. The two teams were in for a surprise, finding that, contrary to assumptions, the expansion was not decelerating. The more distant galaxies seemed to be traveling faster and faster as their distance increased.
It is not the edges of the universe that are receding, every portion of space is expanding, matter and energy have a limit speed they can reach, the fabric of space-time does not, it can expand at any speed without breaking Einstein's relativity.
It is not the galaxies that are moving apart, it is space-time that is expanding at an accelerated rate.
Thanks to new calculations, which take accelerated expansion into account, we now know that the observable Universe has a radius of at least 46 billion light-years. The observable Universe is only a part of the entire Universe. And that is where the unobservable Universe comes in.
The observable Universe is a spherical region that includes everything that can currently be observed from our planet. Everything that exists beyond this spherical limit is in the unobservable universe; its light has yet to reach Earth because of the enormous distance it must travel. Since light has a maximum speed, light from objects placed at a sufficient distance may still be traveling. If the expansion of the Universe were not accelerating, given enough time, we would eventually be able to see everything in the cosmos.
Unfortunately, because of the accelerated expansion, regions of space sufficiently distant from Earth are moving away from us faster than light itself. This tells us that light from galaxies in those portions of space can never reach our instruments. Today, if a photon were to leave Earth and begin traveling through the cosmos, it could not reach any area of space more than 15 billion light-years distant, because space beyond that point is expanding faster than the speed of light.
Starting from Earth today and traveling, for argument's sake, at the speed of light we could only reach 3 percent of the total number of galaxies in our observable Universe. The other 97 percent is beyond our reach.
But it's not over, the expansion of the universe is continuously accelerating, and every year more and more galaxies move beyond the cosmic horizon and enter the unobservable universe.
Should the accelerated expansion continue indefinitely our descendants, if there are any in such a distant epoch, will see the space around the Milky Way as an immense dark and cold expanse and, if they have no memory of it, they will be unable to know anything about the universe we observe today.
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