What are the limits of our universe?

During the 1920s there was a heated debate among astronomers about the size of the universe and the nature of the so-called nebulae, diffuse objects of which there were several thousand catalogued.

Some scientists argued that they were gaseous objects located within our galaxy and that this was the entire universe, while others argued that they were star systems similar to the Milky Way, "island universes", which were diffuse due to their distance.

The debate was settled thanks to Edwin Hubble who, using the relationship obtained by Henrietta Swan Leavitt, measured the distance to the Andromeda nebula, the only one visible to the naked eye from the northern hemisphere.

The value obtained by Hubble was much larger than the size of the Milky Way, which showed the existence of other galaxies and increased, dramatically,

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The telescope that observes how light was born in the universe

It is common to refer to astronomical distances in light years.

A light year is the distance that light travels in one year, approximately 9,000,000 million 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 huge, even though Andromeda is part of the group of galaxies we call the local group, that is, 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 that we can see, the known universe, is a sphere whose radius marks the distance between the regions that emitted the radiation that we observe today as cosmic microwave background radiation and our planet.

If the universe were static, this border, 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, something that Hubble also showed us in the article

, published in 1929. The title, of course, is not at all suggestive for the cosmological implications of the result but it is informative.

Hubble had carefully measured the speeds and distances of a sample of galaxies, showing that they are moving away from us in all directions, faster the farther away they are.

Hubble was very cautious in his conclusions, but the implications were clear.

Only five years ago, this scientist's work had dramatically 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 walnut cake is often used.

When we put it in the oven and it starts to grow, all the nuts see the rest walk away.

When the biscuit doubles in size, two walnuts initially one centimeter apart will be two apart, while those that were three apart will be six apart.

That is, during the same time, the distance between the farthest nuts 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 has been traveling through an expanding universe for some 13,800 years before finally reaching us.

However, those regions have continued to recede all this time, 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, 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 these specks that we see in the background radiation 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, no matter how hard it tries, can never cover the distance that separates them 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.

Just a few days ago we saw, 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 baby universe barely 300,000 years old.

These images are, in a way, images of ghost galaxies, which right now are in a region of the universe with which we will never be able to interact, can we then say that they are 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 emit now.

This is the region of the universe whose expansion rate is below the speed of light and its border is 16 billion light years away.

This border is called the event horizon, by 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 that we will be able to see in the future will be reduced, until a moment arrives when everything disappears from our sight.

Well, maybe not all, because not all regions of the universe are expanding.

Like the nuts in our cake, galaxies don't expand, neither does the earth, nor do the trees, nor do we.

The local group we are in is not expanding and, in fact, the Andromeda galaxy is drawing closer to us, pulled by gravity.

However, this gravity will cause all the galaxies that do not move away, to end up getting closer and closer, until they merge into a single one, which will be the only one that the astronomers that inhabit it will be able to observe.

Patricia Sánchez Blázquez

Cosmic Void

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