Did you know that the universe we observe has fairly definite boundaries? We are used to associating the Universe with something infinite and incomprehensible. However, modern science to the question of the "infinity" of the Universe offers a completely different answer to such an "obvious" question.
According to modern concepts, the size of the observable universe is approximately 45.7 billion light years (or 14.6 gigaparsecs). But what do these numbers mean?
The border of the boundless
The first question that comes to mind of an ordinary person is how the Universe cannot be infinite at all? It would seem indisputable that the container of everything that exists around us should not have boundaries. If these boundaries exist, what are they?
Let's say some astronaut flew to the borders of the universe. What will he see in front of him? A solid wall? Fire barrier? And what is behind it - emptiness? Another Universe? But can emptiness or another Universe mean that we are on the border of the universe? After all, this does not mean that there is "nothing". The emptiness and the other Universe are also “something”. But the Universe is something that contains absolutely everything “something”.
We come to an absolute contradiction. It turns out that the border of the Universe should hide from us something that should not be. Or the border of the Universe should fence off “everything” from “something”, but this “something” should also be a part of “everything”. In general, a complete absurdity. Then how can scientists claim the limiting size, mass, and even age of our universe? These values, although unimaginably large, are still finite. Is science arguing with the obvious? To deal with this, let's first trace how people came to the modern understanding of the universe.
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Expanding the boundaries
From time immemorial, man has been interested in what the world around them is. One need not give examples of the three whales and other attempts of the ancients to explain the universe. As a rule, in the end it all came down to the fact that the foundation of all that exists is the earthly firmament. Even in antiquity and the Middle Ages, when astronomers had extensive knowledge of the laws governing the motion of planets along the "stationary" celestial sphere, the Earth remained the center of the Universe.
Naturally, even in Ancient Greece there were those who believed that the Earth revolved around the Sun. There were those who spoke about the many worlds and the infinity of the universe. But constructive substantiation of these theories arose only at the turn of the scientific revolution.
In the 16th century, the Polish astronomer Nicolaus Copernicus made the first major breakthrough in understanding the Universe. He firmly proved that the Earth is only one of the planets orbiting the Sun. Such a system greatly simplified the explanation of such a complex and intricate movement of the planets in the celestial sphere. In the case of a stationary earth, astronomers had to invent all sorts of ingenious theories to explain this behavior of the planets. On the other hand, if the Earth is taken to be mobile, then the explanation for such intricate movements comes naturally. This is how a new paradigm called "heliocentrism" was established in astronomy.
Many Suns
However, even after that, astronomers continued to confine the universe to the "sphere of fixed stars." Until the 19th century, they could not estimate the distance to the stars. For several centuries, astronomers have tried in vain to detect deviations in the position of stars relative to the Earth's orbital motion (annual parallaxes). The instruments of those times did not allow such accurate measurements.
Vega, shot by ESO
Finally, in 1837, the Russian-German astronomer Vasily Struve measured the parallax α of Lyra. This marked a new step in understanding the scale of space. Now scientists could safely say that stars are distant similarities to the Sun. And from now on our luminary is not the center of everything, but an equal "inhabitant" of the endless star cluster.
Astronomers have come even closer to understanding the scale of the universe, because the distances to the stars turned out to be truly monstrous. Even the size of the orbits of the planets seemed insignificant in comparison with this. Then it was necessary to understand how the stars are concentrated in the Universe.
Many Milky Way
The famous philosopher Immanuel Kant anticipated the foundations of the modern understanding of the large-scale structure of the Universe back in 1755. He hypothesized that the Milky Way is a huge rotating cluster of stars. In turn, many of the observed nebulae are also more distant "milky ways" - galaxies. Despite this, until the 20th century, astronomers adhered to the fact that all nebulae are sources of star formation and are part of the Milky Way.
The situation changed when astronomers learned how to measure distances between galaxies using Cepheids. The absolute luminosity of stars of this type is strictly dependent on the period of their variability. By comparing their absolute luminosity with the visible one, it is possible to determine the distance to them with high accuracy. This method was developed in the early 20th century by Einar Herzsrung and Harlow Shelpy. Thanks to him, the Soviet astronomer Ernst Epik in 1922 determined the distance to Andromeda, which turned out to be an order of magnitude larger than the size of the Milky Way.
Edwin Hubble continued Epic's endeavor. By measuring the brightness of Cepheids in other galaxies, he measured the distance to them and compared it with the redshift in their spectra. So in 1929 he developed his famous law. His work definitively refuted the established belief that the Milky Way is the edge of the universe. It was now one of many galaxies that had once been considered an integral part of it. Kant's hypothesis was confirmed almost two centuries after its development.
Later, the connection between the distance of the galaxy from the observer and the speed of its removal from the observer, discovered by Hubble, made it possible to compose a complete picture of the large-scale structure of the Universe. It turned out that the galaxies were only a tiny part of it. They linked into clusters, clusters into superclusters. In turn, superclusters fold into the largest known structures in the universe - filaments and walls. These structures, adjacent to huge supervoids (voids), make up the large-scale structure of the currently known universe.
Apparent infinity
From the foregoing, it follows that in just a few centuries, science has gradually leapt from geocentrism to a modern understanding of the Universe. However, this does not provide an answer as to why we are limiting the Universe these days. After all, until now, it was only about the scale of the cosmos, and not about its very nature.
Evolution of the universe
The first who decided to justify the infinity of the Universe was Isaac Newton. Having discovered the law of universal gravitation, he believed that if space were finite, all her bodies would sooner or later merge into a single whole. Before him, if someone expressed the idea of the infinity of the Universe, it was exclusively in a philosophical key. Without any scientific justification. An example of this is Giordano Bruno. By the way, like Kant, he was ahead of science by many centuries. He was the first to declare that the stars are distant suns, and planets revolve around them too.
It would seem that the very fact of infinity is quite justified and obvious, but the turning points of science of the 20th century shook this "truth."
Stationary universe
Albert Einstein took the first significant step towards the development of a modern model of the universe. The famous physicist introduced his model of a stationary universe in 1917. This model was based on the general theory of relativity, which he developed the same year earlier. According to his model, the universe is infinite in time and finite in space. But, as noted earlier, according to Newton, a universe of finite size should collapse. To do this, Einstein introduced a cosmological constant, which compensated for the gravitational attraction of distant objects.
As paradoxical as it may sound, Einstein did not limit the very finiteness of the Universe. In his opinion, the Universe is a closed shell of a hypersphere. An analogy is the surface of an ordinary three-dimensional sphere, for example, a globe or the Earth. No matter how much a traveler travels around the Earth, he will never reach its edge. However, this does not mean at all that the Earth is infinite. The traveler will simply return to the place where he started his journey.
On the surface of the hypersphere
Likewise, a space wanderer, overcoming Einstein's universe on a starship, can return back to Earth. Only this time the wanderer will move not along the two-dimensional surface of the sphere, but along the three-dimensional surface of the hypersphere. This means that the Universe has a finite volume, and hence a finite number of stars and mass. However, the Universe has no boundaries or any center.
The future of the universe
Einstein came to such conclusions by linking space, time and gravity in his famous theory. Before him, these concepts were considered separate, which is why the space of the Universe was purely Euclidean. Einstein proved that gravity itself is a curvature of spacetime. This radically changed the early understanding of the nature of the Universe, based on classical Newtonian mechanics and Euclidean geometry.
Expanding Universe
Even the discoverer of the "new Universe" himself was no stranger to delusion. Although Einstein limited the universe in space, he continued to consider it static. According to his model, the Universe was and remains eternal, and its size always remains the same. In 1922, Soviet physicist Alexander Fridman significantly expanded this model. According to his calculations, the universe is not at all static. It can expand or contract over time. It is noteworthy that Friedman came to such a model, based on the same theory of relativity. He was able to more correctly apply this theory, bypassing the cosmological constant.
Albert Einstein did not immediately accept this "amendment". The Hubble discovery mentioned earlier came to the rescue of this new model. The scattering of galaxies indisputably proved the fact of the expansion of the Universe. So Einstein had to admit his mistake. Now the universe had a certain age, which strictly depends on the Hubble constant, which characterizes the rate of its expansion.
Further development of cosmology
As scientists tried to solve this question, many other important components of the universe were discovered and various models were developed. So in 1948, Georgy Gamov introduced the hypothesis "about a hot Universe", which would later turn into the big bang theory. The discovery in 1965 of the relic radiation confirmed his guesses. Astronomers could now observe the light that came from the moment the universe became transparent.
Dark matter, predicted in 1932 by Fritz Zwicky, was confirmed in 1975. Dark matter actually explains the very existence of galaxies, galactic clusters and the Universe itself as a whole. So scientists learned that most of the mass of the Universe is completely invisible.
What the universe is made of
Finally, in 1998, during a study of the distance to type Ia supernovae, it was discovered that the universe is expanding with acceleration. This next turning point in science gave rise to the modern understanding of the nature of the universe. The cosmological coefficient introduced by Einstein and refuted by Friedman again found its place in the model of the Universe. The presence of the cosmological coefficient (cosmological constant) explains its accelerated expansion. To explain the presence of a cosmological constant, the concept of dark energy was introduced - a hypothetical field containing most of the mass of the Universe.
The current model of the universe is also called the ΛCDM model. The letter "Λ" denotes the presence of a cosmological constant that explains the accelerated expansion of the universe. CDM means the universe is filled with cold dark matter. Recent studies indicate that the Hubble constant is about 71 (km / s) / Mpc, which corresponds to the age of the Universe 13.75 billion years. Knowing the age of the universe, one can estimate the size of its observable area.
Evolution of the universe
According to the theory of relativity, information about any object cannot reach the observer with a speed greater than the speed of light (299792458 km / s). It turns out that the observer sees not just an object, but its past. The further the object is from it, the more distant past it looks. For example, looking at the Moon, we see what it was a little over a second ago, the Sun - more than eight minutes ago, the nearest stars - years, galaxies - millions of years ago, etc. In Einstein's stationary model, the Universe has no age limit, which means that its observable region is also not limited by anything. The observer, armed with more and more advanced astronomical instruments, will observe more and more distant and ancient objects.
We have a different picture with the modern model of the Universe. According to her, the Universe has an age, and therefore a limit of observation. That is, since the birth of the Universe, no photon would have had time to travel a distance greater than 13.75 billion light years. It turns out that we can state that the observable Universe is limited from the observer by a spherical region with a radius of 13.75 billion light years. However, this is not quite true. Do not forget about the expansion of the space of the Universe. Until the photon reaches the observer, the object that emitted it will be 45.7 billion sv from us. years old. This size is the horizon of particles, and it is the boundary of the observable Universe.
So, the size of the observable Universe is divided into two types. Visible size, also called the Hubble radius (13.75 billion light years). And the real size, called the particle horizon (45.7 billion light years). Essentially, both of these horizons do not at all characterize the real size of the Universe. First, they depend on the position of the observer in space. Second, they change over time. In the case of the ΛCDM model, the particle horizon expands at a speed greater than the Hubble horizon. The question of whether this trend will change in the future, modern science does not give an answer. But if we assume that the Universe continues to expand with acceleration, then all those objects that we see now, sooner or later, will disappear from our "field of view."
At the moment, the most distant light observed by astronomers is the microwave background radiation. Peering into it, scientists see the Universe as it was 380 thousand years after the Big Bang. At this moment, the Universe has cooled down so much that it was able to emit free photons, which are captured today with the help of radio telescopes. In those days, there were no stars or galaxies in the Universe, but only a solid cloud of hydrogen, helium and an insignificant amount of other elements. From the inhomogeneities observed in this cloud, galactic clusters will subsequently form. It turns out that exactly those objects that are formed from the inhomogeneities of the relict radiation are located closest to the particle horizon.
True boundaries
Whether the universe has true, unobservable boundaries is still the subject of pseudoscientific conjectures. One way or another, everyone converges at the infinity of the Universe, but they interpret this infinity in completely different ways. Some consider the Universe to be multidimensional, where our “local” three-dimensional Universe is just one of its layers. Others say that the universe is fractal - which means that our local universe may be a particle of another. Do not forget about the various models of the Multiverse with its closed, open, parallel Universes, wormholes. And there are many, many different versions, the number of which is limited only by human imagination.
But if we turn on cold realism or simply move away from all these hypotheses, then we can assume that our Universe is an infinite homogeneous repository of all stars and galaxies. Moreover, at any very distant point, be it billions of gigaparsecs from us, all conditions will be exactly the same. At this point, there will be exactly the same horizon of particles and the Hubble sphere with the same relic radiation at their edge. There will be the same stars and galaxies around. Interestingly, this does not contradict the expansion of the universe. After all, it is not just the Universe that is expanding, but its very space. The fact that at the moment of the big bang the Universe arose from one point only indicates that the infinitely small (practically zero) dimensions that were then have now turned into unimaginably large. In what follows, we will use just this hypothesis to ensure thatthat clearly understand the scale of the observable universe.
Visual representation
Various sources provide all kinds of visual models that allow people to understand the scale of the universe. However, it is not enough for us to realize how big the cosmos is. It is important to understand how concepts such as the Hubble horizon and the particle horizon actually manifest. To do this, let's imagine our model step by step.
Let's forget that modern science does not know about the "foreign" region of the Universe. Discarding the versions about the multiverse, the fractal Universe and its other "varieties", imagine that it is simply infinite. As noted earlier, this does not contradict the expansion of her space. Of course, let's take into account the fact that its Hubble sphere and the sphere of particles are respectively equal to 13.75 and 45.7 billion light years.
The scale of the universe
To begin with, let's try to realize how large the universal scale is. If you have traveled around our planet, then you can well imagine how big the Earth is for us. Now let's imagine our planet as a buckwheat grain that orbits around a watermelon-Sun half the size of a football field. In this case, the orbit of Neptune will correspond to the size of a small city, the region of the Oort cloud to the Moon, the region of the boundary of the influence of the Sun to Mars. It turns out that our Solar System is as much larger than the Earth as Mars is larger than buckwheat! But this is just the beginning.
Now let's imagine that this buckwheat will be our system, the size of which is approximately equal to one parsec. Then the Milky Way will be the size of two football stadiums. However, even this will not be enough for us. We'll have to reduce the Milky Way to a centimeter size. It will somewhat resemble coffee foam wrapped in a whirlpool in the middle of the coffee-black intergalactic space. Twenty centimeters from it there is the same spiral "crumb" - the Andromeda Nebula. Around them will be a swarm of small galaxies from our Local Cluster. The apparent size of our Universe will be 9.2 kilometers. We have come to an understanding of the universal dimensions. Inside the universal bubble
However, it is not enough for us to understand the scale itself. It is important to understand the dynamics of the universe. Let's imagine ourselves as giants for which the Milky Way has a centimeter diameter. As noted just now, we find ourselves inside a sphere with a radius of 4.57 and a diameter of 9.24 kilometers. Let's imagine that we are able to hover inside this sphere, travel, overcoming entire megaparsecs in a second. What will we see if our universe is infinite?
Of course, before us there will be an infinite number of all kinds of galaxies. Elliptical, spiral, irregular. Some areas will be teeming with them, others will be empty. The main feature will be that visually they will all be motionless while we are motionless. But as soon as we take a step, the galaxies themselves will begin to move. For example, if we are able to see the microscopic Solar System in the centimeter Milky Way, we can observe its development. Moving 600 meters away from our galaxy, we will see the protostar Sun and the protoplanetary disk at the time of formation. Approaching it, we will see how the Earth appears, life arises and man appears. Likewise, we will see how galaxies change and move as we move away or approach them.
Consequently, the more distant galaxies we look, the more ancient they will be for us. So the most distant galaxies will be located further than 1300 meters from us, and at the turn of 1380 meters we will see the relic radiation. True, this distance will be imaginary for us. However, as we get closer to the relic radiation, we will see an interesting picture. Naturally, we will observe how galaxies will form and develop from the original cloud of hydrogen. When we reach one of these formed galaxies, we will understand that we have overcome not 1.375 kilometers at all, but all 4.57.
Downsizing
As a result, we will grow even more in size. Now we can place whole voids and walls in the fist. So we find ourselves in a rather small bubble, from which it is impossible to get out. Not only will the distance to objects at the edge of the bubble increase as they get closer, but the edge itself will drift infinitely. This is the whole point of the size of the observable universe.
No matter how big the Universe is, for the observer it will always remain a limited bubble. The observer will always be in the center of this bubble, in fact, he is its center. Trying to get to any object at the edge of the bubble, the observer will shift its center. As it approaches the object, this object will move further and further from the edge of the bubble and at the same time will change. For example, from a shapeless hydrogen cloud it will turn into a full-fledged galaxy or further into a galaxy cluster. In addition, the path to this object will increase as you approach it, as the surrounding space itself will change. Once we get to this object, we just move it from the edge of the bubble to its center. At the edge of the universe, the relic radiation will also flicker.
If we assume that the Universe will continue to expand at an accelerated rate, then being in the center of the bubble and winding time for billions, trillions and even higher orders of years ahead, we will notice an even more interesting picture. Although our bubble will also grow in size, its mutating components will move away from us even faster, leaving the edge of this bubble, until each particle of the Universe wanders scattered in its lone bubble without the ability to interact with other particles.
So, modern science does not have information about what the real dimensions of the Universe are and whether it has boundaries. But we know for sure that the observable Universe has a visible and true boundary, called the Hubble radius (13.75 billion light years) and the radius of particles (45.7 billion light years), respectively. These boundaries are completely dependent on the position of the observer in space and expand over time. If the Hubble radius expands strictly at the speed of light, then the expansion of the particle horizon is accelerated. The question of whether its acceleration of the particle horizon will continue further and whether it will not change to compression remains open.