What are the limits of our universe? | cosmic void

During the 1920s there was heated debate among astronomers about the size of the universe and the nature of so-called nebulae, diffuse objects of which several thousand have been recorded. Some scientists have argued that they are gaseous objects located in our galaxy and that they are the entire universe, while others have argued that they are star systems similar to the Milky Way, “island universes”, which were diffuse due to their distance. The debate was settled thanks to Edwin Hubble that, using the relation obtained by Henrietta Swan Leavittmeasured the distance to the nebula from Andromeda, the only one visible to the naked eye from the northern hemisphere. Hubble’s value was much larger than the size of the Milky Way, proving the existence of other galaxies and significantly increasing the size of the universe.

It is common to refer to astronomical distances in light years. A light year is the distance traveled by light in one year, approximately 9,000,000 kilometers. The diameter of the Milky Way is 900,000 million kilometers, and the distance to Andromeda is 22,500,000 billion kilometers. These distances are enormous, even though Andromeda is part of the group of galaxies we call the local group, that is to say our neighborhood. The truth is that the universe is so big that we cannot see it in its entirety, because in its 13.8 billion years of life there are regions whose light has not had time to reach 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 microwave cosmic background radiation and our planet. If the universe were static, this boundary, what we call the particle horizon, would be 13.8 billion light-years away. However, it is at a much greater distance, 46,000 million light years.

The reason is that the universe is expanding, which Hubble also showed us in the article Relationship between distance and radial velocity in extragalactic nebulae, published in 1929. The title, of course, is not at all suggestive of the cosmological implications of the result, but it is informative. Hubble had painstakingly measured the velocities and distances of a sample of galaxies, showing that they are receding from us in all directions, faster the farther away they are. Hubble was very cautious in its conclusions, but the implications were clear. Only five years ago, the work of this scientist had greatly expanded the size of the universe and was now expanding the universe itself.

To illustrate how the result implies an expanding universe, the example of a nut cake is often used. When you put it in the oven and it starts to grow, all the nuts see the rest go away. When the cake doubles in size, two nuts initially one centimeter apart will be two apart, while those that were three apart will be six apart. That is, at the same time, the distance between the farthest nuts will have increased three times more than the distance between the nearest, that is, they will have moved away three times more quick.

Background radiation was emitted in the early stages of the universe, but its light passed through an expanding universe for some 13,800 years before finally reaching us. However, these regions have continued to recede throughout this time, and the spots we see in the background radiation have evolved into galaxies and clusters of galaxies similar to those around us. If we could stop the expansion of the universe right now, it would take another 46 billion years for the light from these galaxies to reach us. However, we cannot stop the expansion of the universe, and we will never be able to see the galaxies that have become these specks we see in the background radiation, 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, no matter how hard it tries, can never bridge the distance from them to us. In this sense, the particle horizonthe known universe, marks the visible limit of the past of the universe, but not the universe with which we can interact.

Only a few days ago we saw, on images obtained with the James Webb Space Telescopegalaxies whose light may have been emitted a long time ago 13.5 billion years old. Newly formed galaxies, inhabiting a baby universe barely 300,000 years old. These images are sort of images of phantom galaxies, which right now are in a region of the universe that we will never be able to interact with, so can they be said to be part of our universe?

Let us 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 the galaxies are currently emitting. It is the region of the universe whose rate of expansion is less than the speed of light and whose boundary is 16 billion light-years away. This boundary is called event horizonby analogy with the event horizon of a black hole and 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 can see in the future will be reduced, until a time comes when everything disappears from our sight. Well, maybe not all of them, because not all regions of the universe are expanding. Like the nuts in our cake, galaxies don’t expand, neither does the earth, nor the trees, nor us. The Local Group we are in is not expanding, and in fact the Andromeda Galaxy is moving closer to us due to gravity. However, this gravity will cause all the galaxies that don’t move away to eventually come closer and closer, until they merge into only one, which will be the only one that the astronomers who inhabit it will be able to observe. They won’t be able to measure the speeds or distances of other galaxies to know that the universe is expanding, and they will most likely go back to thinking, like 19th century astronomers, that the universe is made up of a single galaxy, own them.

Patricia Sanchez Blazquez She is full professor at the Complutense University of Madrid (UCM)

cosmic void it is a section in which our knowledge of the universe is presented qualitatively and quantitatively. It aims to explain the importance of understanding the cosmos not only from a scientific point of view but also from a philosophical, social and economic point of view. The name “cosmic vacuum” refers to the fact that the universe is and is, for the most part, empty, with less than 1 atom per cubic meter, whereas in our environment, paradoxically, there are quintillion atoms per cubic meter, which invites us to reflect on our existence and the presence of life in the universe.

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