If we know something exactly about our universe, it is that it is non-static, changing over time. What does the future hold for her?
Today we have a standard cosmological model that describes well the history of the universe almost from the moment of its birth all the way to our time. Moreover, now there is no serious reason to believe that this model cannot serve as a basis for predicting the subsequent evolution of our world. True, it has competitors who offer completely different scenarios for future events. However, we do not yet have observational data that would indicate a real need not only to revise the standard model, but even to seriously correct it.
Emptiness or shreds
Now about the future. From the standard model it follows that in the very distant future, the role of gravity will practically disappear and the expansion rate of the Universe will begin to increase exponentially. Outer space will become empty, and faster and faster. However, this speed will always increase monotonously, from the present era to the end of time. The Standard Model excludes scenarios in which the vacuum loses stability and its energy density jumps to infinity in a finite time. In this case, the expansion rate of the Universe will also tend to infinity, which will lead to the rupture and disappearance of all material objects - from galaxies and stars to atoms and atomic nuclei. Some competitors to the Standard Model predict this outcome, but astronomers have no data to support these theories. Honestly,I myself do not take them seriously, they are based on very unusual physics. The Standard Model is in excellent agreement with observations, and there is no point in abandoning it.
The accelerating expansion of the Universe will only mean an increase in the rate of expansion of galaxies. Since the density of dark energy will not change, it will not be able to destroy galaxies and other gravitationally stable structures that it does not prevent from existing in the current era. Of course, this does not mean that the galaxies themselves will remain in the form in which they exist today. Over time, all stars will burn fusion fuel and turn into white dwarfs, neutron stars, or black holes. The holes will grow, merging with each other and consuming stellar remnants and interstellar gas. However, these and other destructive processes will take place without the participation of dark energy.
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So what awaits our own Galaxy, the Milky Way? It is approaching the neighboring large spiral galaxy Andromeda - now at a speed of 110 km / s. In 6 billion years, both galaxies will merge and form a new star cluster, Milcomedou. The Sun will remain inside Milcomed, only to move to its periphery compared to its current position in the Milky Way. By an interesting coincidence, just then it will burn hydrogen fuel and embark on the path of cataclysmic changes, which will end in its transformation into a white dwarf.
So far, we've talked about a fairly near future. After stabilization, Milcomed will maintain gravitational stability for gigantic periods of time, at least a thousand times the current age of the Universe. But she will be alone much earlier. In about 100 billion years or a little later, all distant galaxies that we can observe today will disappear from its firmament. By that time, the speed of their expansion, caused by the expansion of the Universe, will exceed the speed of light, so that the photons they emit will never reach Milcomed. In the language of cosmology, galaxies will irreversibly go beyond its event horizon. Their apparent brightness will drop, and eventually they will all fade and go out. So observers in Milcomed will see only her own stars - of course, only those that will still be emitting light by then. The lightest red dwarfs will remain active for the longest time, but in a maximum of 10 trillion years they will begin to die too.
Standard Universe
The Standard Model states that in our time the Universe is changing under the influence of two main factors: the gravity of ordinary and dark matter and the anti-gravitating effect of non-zero vacuum energy, which is commonly called dark energy.
In the early youth of the Universe, the energy of electromagnetic radiation and neutrino fluxes also made a significant contribution to its evolution. Now its role is very small, since the density of radiant energy is extremely low and, moreover, is constantly falling due to the expansion of outer space. At the same time, the density of dark energy, as it appears in the standard model, remains constant. It does not decrease with the expansion of the Universe and is already three times higher than the monotonically falling density of ordinary and dark matter. Therefore, dark energy is causing an accelerating expansion of the universe, which cannot be contained by the weakening gravity of galaxies and the intergalactic medium.
Strategic plans
When the age of the universe reaches a trillion years, the wavelength of the CMB will be equal to its size. Then, and even more so later, no detectors will be able to register these ultracold photons. Therefore, any observers, no matter how perfect their instruments are, will not be able to use the relic radiation as a source of astronomical information.
Now the peak of the spectrum of these photons lies in the microwave range, and they are easily detected by our equipment, providing the most important information about the early history of the Universe. The far future is far beyond the standard cosmological model. We can reasonably assume that growing black holes will absorb a significant part of both baryonic and dark matter, but what will happen to its remnant, scattered across the vast expanses of space?
Physics claims that electrons are not subject to any form of decay, but there is no such certainty about protons. According to modern data, the half-life of a proton cannot be less than 1034 years - this is a lot, but still not eternity. We also do not know the long-term fate of dark matter particles, which have not yet been discovered at all. The most probable prediction of the ultra-distant future boils down to the fact that the Universe will become extremely empty and cool to almost absolute zero.
How exactly this will happen is still unknown, here it is up to fundamental physics. However, the future on a trillion-year scale is quite predictable based on the standard model. Of course, if some new properties are discovered in the vacuum, this scenario will have to be revised, but this is already out of speculation.
Avi Loeb, Professor, Head of the Department of Astronomy at Harvard University, Director of the Institute for Theory and Computer Modeling, Harvard-Smithsonian Center for Astrophysics.
Interviewed by: Alexey Levin, Oleg Makarov, Dmitry Mamontov