What Lies Outside The Observable Universe?

When we speak of the spatial extent of the Universe, it is necessary to separate two concepts. The first is the size of the observable portion of the Universe or the span of the current horizon. The name speaks for itself. It is the direct equivalent of the horizon as we defined it on Earth, the imaginary border of the visible part of the Earth. And in this case, the Universe. We don't see whatever lies beyond this border. That's not necessarily because there is nothing there. Just as in the case of the Earth's horizon, we don't see what is beyond, because the light from there hasn't reached us. When it comes to the Earth's horizon, it doesn't reach us because the Earth's surface obstructs it. In the case of the cosmological horizon, the light from the photons simply hadn't had enough time to reach us. The unique thing about observing distant objects in space is that the light registered from these objects today has travelled throughout the Universe for a long time and in actual fact was emitted an extremely long time ago. Thus, objects located at cosmologically large distances are objects that have existed since the beginning of time and emitted the light that we register very, very early on. The very earliest signal of which we are aware and have studied well is the extraordinary cosmic microwave background radiation, which appeared in the era of hydrogen formation, when there were no galaxies or stars. At that distant time, the Universe was a billion times denser, and photons were a thousand times hotter than they are today. As these photons spread throughout the Universe, it was expanding. By reviewing microwave research, information can be obtained about the characteristics of the expansion of the Universe, its composition and internal structure. The most distant region that is possible to see and distinguish is the last scattering surface. It is from there that the cosmic microwave background photons emanate. This is the so-called surface of the last scattering. Beyond this region is what is not yet subject to study by our devices. We cannot see the area that is located beyond the surface of the last scattering because it is opaque. After all, it is light that allows us to see distant objects and judge their properties by one means or another. Regardless of the fact that it is impossible to see what is happening beyond the last scattering surface, researchers are able to form an opinion about the expanse beyond it. To do this, they observe what effect it has on existing astrophysical objects. More than that, according to the latest data, galaxies are moving away from each other at an accelerated pace. And the further away the galaxy is, the faster it is moving away from us. This means that at some point the speed of the separating galaxies will exceed the speed of light who will no longer see them. These objects will go beyond the horizon but will not disappear. This fact implies that perhaps out there, beyond the observable universe, lies an additional vast expanse that is hidden from us by the barrier of the speed of light. The point being that CMB maps made with telescopes such as those of NASA's WMAP and the ESA's Planck showed on a vast scale a mystifying lack of perturbations. This jaunty little word means the departure of a celestial body from an orbit because of the influences of forces other than the gravitational attraction of the center of mass of the system, such as other celestial bodies or environmental resistance. In order to find out if these missing perturbations could be caused by a multiply-connected universe, scientists made many computer simulations of what the cosmic microwave background radiation would look like if the universe had the form of a giant three-dimensional donut where it is connected to itself in all three dimensions. The properties of the observed fluctuations, such as the deviation from the mean value of a random magnitude characterizing a system of a large number of chaotically interacting particles of the cosmic microwave background radiation, show insufficient power on a scale exceeding the size of the universe. This lack of power means that fluctuations in the cosmic microwave background radiation are not present on such scales and that our universe is multiply-connected and finite. In other words, it looks like the cosmic microwave background is missing signals which must be present if the universe were truly infinite. One explanation for this suggests that the topology of the universe is curved in such a way that it connects back to itself like a donut or a bagel of intergalactic dimensions. Just as you can roll a sheet of paper into a tube without changing its parallel properties, the universe can be donut-shaped while remaining flat. This is exactly what the researchers have found out with the aid of simulations of the cosmic microwave background. It turns out that compared to the standard cosmological model, which is considered infinite, we've found a much better matching observation of the fluctuations. Such a universe must have an end, and the entirety of the vast expanse is possibly no more than four times larger than the boundaries of the universe that is observable by man, and its size is 47 billion light-years in diameter. The universe can be self-contained in three dimensions and have the shape of a three-dimensional donut. Models of the finite universe may be intimidating to some people, but you will not perceive these boundaries. For all practical purposes, you simply live in an infinite universe, despite it having finite dimensions. But even if you don't necessarily end up finding yourself at the edge of this finite universe, would you be able to circumnavigate it and return to where you started? In theory, yes. After all, light can travel across the entire finite universe. I wonder how our donut-shaped universe looks from the side, presumably situated with others.