In the 1920s, a debate raged among astronomers about the size of the universe and the nature of nebulae—diffuse objects of which several thousand have been cataloged. Some scientists argue that they are gaseous objects located in our galaxy and that this makes up the entire universe, while others argue that they are actually star systems similar to the Milky Way, “island universes” that appear diffuse in the distance. The argument was resolved by Edwin Hubble, who, using the connection obtained by Henrietta Swan Leavitt, was able to measure the distance to the Andromeda Nebula, the only one visible to the naked eye from the Earth’s northern hemisphere. The value obtained by Hubble is much larger than the size of the Milky Way, proving the existence of other galaxies and dramatically increasing the size of the universe.
Astronomical distances are usually measured in light years. A light year is the distance light travels in one year; approximately nine trillion kilometers. The diameter of the Milky Way is 900 quadrillion kilometers, and the distance to Andromeda is 22.5 quintillion kilometers. These are huge distances, even if Andromeda is still part of the group of galaxies we call the Local Group—that is, our neighborhood. The fact is that the universe is so vast that we cannot see it in its entirety, because after 13.8 billion years of life there are some regions whose light has not yet reached us.
The universe we can see—the known universe—is a sphere whose radius marks the distance between the regions that emit the radiation we observe today as the cosmic microwave background radiation and our planet. If the universe were static, this boundary, which we call the particle horizon, would be 13.8 billion light years away. However, the distance is much longer: 46 billion light years.
The reason is that the universe is expanding, which Hubble also explains in the article Distance-radial velocity relation among extragalactic nebulae, published in 1929. Hubble carefully measured the velocities and distances of a sample of galaxies, showing that they are moving away from us in all directions, gaining speed as they move away. Although Hubble was very cautious in his conclusions, the conclusions were clear. Just five years before, the scientist’s work had dramatically increased the size of the universe; now it has expanded the universe itself.
A raisin muffin is often used as a way to illustrate the expanding universe. When we put the cake in the oven and it starts to rise, each raisin sees the others move away. When it doubles in size, two raisins that were originally an inch apart will be two inches apart, while those that were three inches apart will be six inches apart. This means that in the same time, the distance between the farthest raisins will have increased three times more than the distance between the closest ones, that is, they will have moved away three times faster.
The background radiation was emitted in the early stages of the universe, but its light had to travel through an expanding universe for 13,800 years before it finally reached us. However, everyone
this time, these regions have continued to move away, and the spots we see in the background radiation have evolved into galaxies and galaxy groups similar to those around us. If we could stop the expansion of the universe right now, the light from these galaxies would take another 46 billion years to reach us. But we can’t stop the expansion of the universe, and we’ll never be able to see the galaxies these specks have become, no matter how long we wait. This is because these regions are moving away from us at speeds greater than the speed of light, so light, try as it might, will never be able to cover the distance that separates it from us. In this sense, the particle horizon, the known universe, marks the visible limit of the universe’s past, but not the universe with which we can interact.
Recently, we were able to see in images obtained with the James Webb Space Telescope, galaxies whose light may have been emitted 13.5 billion years ago. Newly formed galaxies inhabiting a baby universe only 300,000 years old. In a way, they are snapshots of ghost galaxies in a region of the universe that we will never be able to interact with. Can we then say that they are still part of our universe?
Let us then define the boundary of the universe that we can interact with. Within this limit – and as long as we have enough time – we can still get the light that galaxies emit now. This is the region of the universe whose speed of expansion is below the speed of light, and its limit is 16 billion light years. This is called the event horizon and marks the boundary of the universe with which we can exchange information.
The sad news is that if the most accepted models of the universe are correct, the number of galaxies we will be able to see in the future will decrease until they all disappear from our view. Well, maybe not everything, because not all regions of the universe are expanding. Like the raisins in our cake, galaxies are not expanding; neither the Earth, nor the trees, nor us. The Local Group we are in is not expanding, and in fact, due to gravity, the Andromeda galaxy is moving towards us. However, this gravity will cause all the galaxies that are not receding to come closer and closer until they merge into one, which will be the only one that the astronomers then inhabiting it will be able to observe. They won’t be able to measure the velocities or distances of other galaxies to know that the universe is expanding, and will probably end up thinking, like 19th century astronomers, that the universe consists of a single galaxy: their own.
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