The study of the evolution and future of the universe is one of the main challenges of today's cosmology. Despite the fact that the riddle of the birth of the universe is not yet solved, this does not prevent scientists from also looking into its fate. In a previous article, Trust My Science described the different parameters that physicists use to try to determine the fate of the universe.
Although these cosmological parameters suffer from imprecise values, cosmologists still use them to form hypotheses about the potential fates of the universe. From the Big Rip to the Big Crunch, what fate awaits our universe?
Before the discovery of the acceleration of the expansion in 1998 (1), for physicists, the fate of the universe was presided over only by its content. Such a postulate offers a simple bipartite classification:when the average density of the universe is less than or equal to the critical density, i.e. when the universe is open (hyperbolic) or flat (Euclidean), the expansion continues forever. Clarification made that in the case of a flat universe, according to Friedmann's equations, the expansion gradually slows down.
Conversely, when the average density is greater than the critical density, i.e. when the universe is closed (spherical), the expansion eventually slows down, stops, and reverses under the effect of gravity. This resulting in the contraction of the universe until it collapsed.
However, with the introduction of dark energy, this classification becomes more complex. For a flat universe, the Friedmann equations must be modified; expansion no longer slows down but, on the contrary, accelerates. Similarly, a closed universe is no longer necessarily synonymous with contraction and collapse; once the acceleration of the expansion is large enough, it can continue indefinitely.
The classification of scenarios cannot therefore be based solely on the type of universe but must also take into account the dynamics of dark energy. Observations carried out since 1998 have confirmed the phenomenon of acceleration of expansion and therefore tend today to rule out any scenario of the "contraction-collapse" type (2).
However, the nature of dark energy still remains unknown and, by extension, it is extremely difficult to make predictions about its dynamics. Pending a solid theoretical framework for dark energy, all scenarios must therefore be considered.
The Big Freeze or Big Chill model predicts that if the expansion maintains its current rate, then, according to the laws of thermodynamics, the temperature of the universe will continue to drop as it is. case since the Big Bang. The temperature will decrease inexorably until it finally reaches the Gibbons-Hawking threshold value, i.e. 10 -29 K, resulting in the thermal death of the universe. This evolution can be broken down into three distinct phases.
Up to 10 11 years after the Big Bang, galaxies still have enough gas to form new stars. After this time, the galactic gas gradually becomes scarce until the galaxies completely exhaust their gas stocks, which are therefore no longer sufficient to cause the formation of new stars (3).
As for the stars, they no longer have the necessary fuel to continue their thermonuclear fusion reactions; stellar nucleosynthesis ceases. Then the stars, depending on their mass, cool down and gradually turn into black dwarfs, neutron stars and black holes (3).
After 10 19 years, these various transformations lead to significant gravitational disturbances in the solar systems which eject the planets, which have become sterile, from their orbits. Some planets will wander indefinitely in the cosmos, others will collide with surrounding bodies and still others will be destroyed by the black holes which are becoming more and more numerous (3).
By identical gravitational disturbances, the remaining stars are in turn ejected from the galaxies. Some are expelled towards the center of the galaxies and gather there in number, gradually increasing the central galactic density until the center collapses on itself, under the effect of the excessive concentration of stellar mass, eventually forming a supermassive black hole. The remaining stars, ejected outside the galaxies, will themselves become stellar black holes or be destroyed by them (3).
Under the effect of the various gravitational imbalances following the transformation of their stars, the galaxies gradually condense and collapse in turn to form galactic black holes. Some of its galactic black holes merge together and then form hypermassive black holes within galactic superclusters. Galaxies that do not collapse merge with other galaxies and form hypermassive black holes the size of a galactic cluster.
Over 10 36 years, the proton half-life threshold being 10 33 years according to theoretical models, the proton decays (4). All atoms break down into electrons, neutrinos and photons. Baryonic matter can no longer structure itself, the few remaining gas clouds disappear, definitively depriving the universe of stars and Life.
Black holes lose their accretion disk. The universe now contains only inert black holes, neutrinos, photons and possibly dark matter. Past 10 49 years, only radiation will travel the universe, as it did in its beginnings.
Deprived of sources of light, the universe is a dark place, shrouded in inky blackness. The first stellar black holes disappear by evaporation according to calculations made by physicist Stephen Hawking as part of Hawking's radiation theory (5). Then past 10 106 years, it is the turn of supermassive black holes to disappear. After 10 150 years, the last black holes, the hypermassive ones, evaporate.
Each evaporation releases streams of neutrinos and electromagnetic radiation in the form of violent flashes of light briefly lighting up a dark and cold universe. This is the last time the universe is enlightened. Past 10 200 years all black holes have disappeared, expansion continues as the universe has reached its maximum entropy and final energy state (6). Its temperature is then, to within a few decimal places, identical to absolute zero, signifying its thermal death.
Observations carried out over the past twenty years show a universe in accelerated expansion, dark energy behaving like a cosmological constant (constant density over time) and zero spatial curvature. These parameters fit perfectly with the Big Freeze scenario, making it currently the most probable model for the scientific community.
Proposed by physicists R. Caldwell, M. Kamionkowski and N. Weinberg (7), the Big Rip model, "Grand Déchirement" in French, predicts that the expansion will continue to accelerate exponentially and more and more " brutal" until it dislocates all matter in the universe, from galaxies to atoms. This great rift is supposed to unfold within 15 to 20 billion years.
For this, the Big Rip scenario requires dark energy to have enough negative pressure for its density to increase during expansion. An energy possessing this characteristic is called “phantom energy” by its authors (7). An increase in the density of phantom energy during the expansion of the universe would mean that the acceleration of the expansion would increase dramatically over time, to the point of tearing apart all structures in the universe.
In their work, the three physicists perform calculations, using modified Friedmann equations, among other things, to establish a chronology of the effects of the Big Rip. It turns out that gradually, while the density of matter (baryonic and non-baryonic) undergoes a dilution and decreases, the density of phantom energy increases and ends up being the dominant energy density in the universe (7).
It is therefore the large structures, such as superclusters and galactic clusters, which are affected first. Phantom energy disassembles these structures by violently expanding the space between the galaxies, pushing them extremely far apart, thus breaking their gravitational balance. Then, as the phantom energy density continues to increase, it is the galaxies themselves that disassemble, with the solar systems drifting apart. Then, it is the turn of the solar systems to fall apart, the planets and stars being torn from their orbit.
Subsequently, the planets and stars themselves are torn apart, dislocated and disintegrate into atoms. Gas clouds, nebulae, accretion disks suffer the same fate. All arrangements of baryonic matter are reduced to the state of free atoms.
The phantom energy density becomes so great that it eventually overwhelms the intra-atomic bonds; atoms break up into electrons, protons and neutrons. Finally, in turn, the nucleons are literally torn apart and reduced to the state of quarks and gluons. Between the beginning of the dislocation of large structures until the dislocation of atoms, approximately 350 million years pass.
Eventually, the density of phantom energy becomes infinite. The expansion is then so violent that space-time itself is affected. The spatio-temporal fabric tears under the phenomenal tension applied by the phantom energy, in a process identical to that of the flop transition in string theory. The spatial dimensions are dislocated, making it impossible to establish any coordinate system. As for the temporal dimension, depending on its true nature, its dislocation could be synonymous with the stopping of Time.
The Big Rip model requires several elements that make it highly speculative. First of all, from a purely theoretical point of view, physicists wonder about the possibility of an energy like phantom energy. This requires a unique physical mechanism since its density must increase with expansion. However, it is unlikely that such a form of energy exists. Furthermore, from an observational point of view, the latest observations since 1998, notably by WMAP, seem to indicate a constant density for dark energy (8).
However, in June 2016, the H0LiCOW collaboration (H0 Lenses in COSMOGRAIL's Wellspring) published the results of its observations indicating a value of the Hubble constant higher than that measured by the Planck mission in 2013 (9). Hubble's constant, denoted "H0", gives the rate of expansion, that is to say the speed of expansion.
In 2013, the Planck mission gave a value of 67.8 km/s/Mpc for H0, which was the most precise value. But mid-2016, the H0LiCOW collaboration gives for H0 a value of 71.9 km/s/Mpc. Currently, nothing can explain this discrepancy, but for some cosmologists, it could announce a return of the Big Rip among the possible scenarios.
