During the 1920s, a debate about the size of the universe and the nature of nebulae, fuzzy objects of which several thousand were cataloged, raged among astronomers. Some scientists argued that they were gaseous objects located in our galaxy and that this constituted the entire universe, while others claimed that they were actually star systems, similar to the Milky Way, “island universes” that appeared diffuse in the distance The argument was resolved by Edwin Hubble, who, using the relationship 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 northern hemisphere of the land The value obtained by Hubble was much larger than the size of the Milky Way, which proved the existence of other galaxies and dramatically increased the size of the universe.
Astronomical distances are usually determined in light years. A light year is the distance that 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 enormous distances, although Andromeda is still part of the group of galaxies we refer to as 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 the 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 emitted the radiation we observe today as cosmic microwave background radiation and our planet. If the universe were static, this limit, what 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 explained in the paper A relation between distance and radial velocity between extragalactic nebulae, published in 1929. Hubble carefully measured the speeds 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 implications were clear. Just five years earlier, the scientist’s work had dramatically expanded the size of the universe; now, he expanded the universe itself.
A raisin cake 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 watches the rest fall away. When it doubles in size, two raisins that were initially one centimeter apart will be two centimeters apart, while those that were three centimeters apart will be six. This means that during the same time, the distance between the furthest 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, all
this time, these regions have continued to move away, and the blobs we see in the background radiation have evolved into galaxies and galaxy clusters 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 blobs have become, no matter how long we wait. That’s because these regions are moving away from us at speeds greater than the speed of light, so light, no matter how hard it tries, can never 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.
We have recently been able to see, in images obtained with the James Webb Space Telescope, galaxies whose light could have been emitted 13.5 billion years ago. Newly formed galaxies inhabiting a newborn universe, barely 300,000 years old. They are, in a sense, images of ghost galaxies in a region of the universe that we will never be able to interact with. Can we say, then, that they are still part of our universe?
We then define the limit of the universe with which we can interact. Within this limit, and as long as we have enough time, we can still receive the light that galaxies now emit. 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 away. This is called the event horizon and it marks the limit 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 diminish until they all disappear from our view. Well, maybe not all, because not all regions of the universe are expanding. Like the icing on our cake, galaxies don’t expand; 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 closing in on us. However, this gravity will cause all the galaxies that are not moving away to merge into one, which will be the only one that the astronomers who inhabit it will then 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 is made up of a single galaxy: their own.