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The Universe has always aroused great fascination, for many reasons; one of the main ones being our difficulty in conceiving its true immensity. Milan Kundera wrote in The Book of Laughter and Oblivion in 1979: “ L& ;#8217;man, although he himself is mortal, can imagine neither the end of space nor the end of time [& #8230;] he still lives in an illusory infinity ”. A literary insight aptly capturing our complex relationship with cosmic infinity. The question of the infinity of the Universe still remains open today, but all advances in cosmology tend to bring us closer to a understanding deeper of its structure and its limits.
From the Moon to the edge of the Solar System
The Moon is our closest star and Earth's only satellite. To imagine the size of our Universe, this is an excellent starting point. It is located 384 400 km from us. This is equivalent to 80 times the width of the United States, already a considerable distance, but ridiculously small on a cosmic scale.
Let's stick to what is easily observed with the naked eye. The Sun, the only star in the Solar System, is located approximately 149.6 million km from Earth. Hypothetically, if we were to build several Great Walls of China from our planet to it, we would have to build more than 7,000.
Still in the hypothetical register, let's imagine that this journey could be made by car. This journey would extend for 170 years considering that it would be carried out at a constant speed of 100 km/h.
It's far away, but it's still just a grain of sand. The Sun, despite its massive size (1.392 million km in diameter and 1.989×1027 tonnes) could be swallowed without problem by this gigantic black hole.< /p>
From there, the distances of the Universe are so great that using km as a unit of measurement is no longer relevant. This is why the astronomical unit (AU)was invented by the International Astronomical Union. One AU is approximately equivalent to the distance between the Earth and the Sun, and was set very precisely in 2012 at 149 597 870.7 km.
Thus, it becomes easier to calculate the even more distant bodies of our Solar System, such as Neptune, located 30 AU from the Sun. Pluto, even further away, is located on average 39.5 AU from the Sun, but its orbit is elliptical, so this distance varies enormously. From 29.7 AU to 49.3 AU. To take the example of the car journey, it would take 6,735 years to reach it from Earth.
The video below, from the YouTube channel Daily random Life, offers a very relevant visual simulation to raise awareness of immensity of our Universe.
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L’stellar scale, from stars to galaxies
Beyond the Solar System, it becomes much easier to calculate distances in light years (al). This is the distance traveled in a vacuum by light in one year and one light year is equivalent to 63 242 AU or 9.431 billion kilometers.
Take Alpha Centauri, for example, the closest star system to our Solar System. This one is located at approximately 4.3 light years from us. This means that when we observe Alpha Centauri, we are observing light that left its stars more than four years ago. In reality, we see her as she was four years ago.
To return to the example of the car journey, we would need more than 47 million years to get there achieve. Since dinosaurs became extinct 66 million years ago (end of the Cretaceous), this hypothetical journey would cover almost the period since this event.
Let's look even further: the Orion Nebula; of which the James Webb telescope brought us sumptuous images in 2022; is located 1 250 light years from us. The center of our galaxy, the Milky Way, is located on average 26 000 light years from Earth.
L’Observable Universe: measuring the incommensurable
What we call Observable Universe is a concept that pushes the limits of our understanding a little further. The Universe, as we know it today, is an old man, being 13.8 billion years old . Naively, we could then think that the most distant observable objects would thus be located 13.8 billion light years away. Well no!
In reality, the Universe is expanding; it is even one of its fundamental characteristics resulting from Einstein's theories of general relativity. Space itself expands and takes galaxies with it, increasing intergalactic distances over time. This means that the observable Universe instead extends 100 billion light-years in the form of a planet. a sphere. With the best theoretical elements available today, this is what we can affirm.
This expansion will not be not done in a chaotic manner. The different galaxies that compose it form a well-organized cosmic web, bringing together groups, clusters and superclusters. For example, the Milky Way is part of the Laniaeka supercluster. This structure extends approximately 500 million light yearsand contains more than 100,000 galaxies.
However, the true scale of the Universe still remains difficult to fully grasp. It is likely that this observable fraction of 100 billion light years is in reality only a tiny portion of the real Universe. Faced with this observation, it is very easy to feel completely insignificant. However, let's set the record straight: the human capacity to understand and quantify this infinity is indeed witness to the genius of our species. Which ultimately makes us creatures great in spirit, although our bodies are only tiny dust.
- The Moon and the Sun, located respectively 384,400 km and 149.6 million km from Earth, are excellent starting points for becoming aware of the immensity of the cosmos.
- Beyond that, distances must be measured in Astronomical Units (AU) and light years (al).
< li>The observable Universe is not a fixed entity and is constantly expanding, with an estimated size of around 100 billion light years.
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