The multiverse hypothesis emerges naturally from several current cosmological theories such as string theory or the theory of eternal inflation. From an ad hoc hypothesis, it has become a direct consequence of these theories and is now seriously considered by the scientific community. But if our u niverse evolves with others within a Multiverse, and they are expanding, how come they never came into contact?
The idea of a multiverse emerged in Greek antiquity from the pen of the philosopher Anaximander. It will never leave scientists, although the concept has taken many forms over the centuries. The multiverse hypothesis became clearer with the rise of quantum mechanics and the search for solutions to the problem of measurement.
In 1950, physicist Hugh Everett proposed a solution to this problem by suggesting that quantum states not chosen by the measurement of a particle do not disappear, but occur in universes underlying ours; these "sub-universes" do not exist beforehand, but appear during the measurement and then become inaccessible once the latter has been carried out. This solution is called "Everett's Many Worlds Theory".
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Scientists say the multiverse could be teeming with life, but also problematic
In 1982, under the impetus of various works concerning cosmic inflation, the Russian-American physicist Andrei Linde proposed the phenomenon of eternal inflation, that is to say a cosmic inflation which would stop in certain areas of the world. 'Universe, and which would continue eternally in others. Where inflation ends, the affected areas see their spatial curvature close in spherical form, thus giving rise to "universe bubbles".
In 1983, the physicist Paul Steinhardt deepened and clarified Linde's hypothesis by showing that where inflation stops, energy is indeed converted into matter and radiation. These works will be reinforced by those of Alexander Vilenkin. In 1986, Linde published a study presenting the final form of eternal inflation. This scenario is now rigorously studied by cosmologists and fits relatively well into the standard cosmological model.
Inflation is a phenomenon that explains the cosmological principle, that is to say why the Universe is homogeneous and isotropic (problem of the horizon), and why it is spatially flat (problem of flatness).
The inflationary model makes several predictions:a power spectrum characteristic of primordial density fluctuations, a critical temperature reached a few moments after the Big Bang, the existence of fluctuations on larger scales than the cosmic horizon, and a specific spectrum in gravitational wave fluctuations. All but the last have been observed.
Inflation is an epoch of the primordial Universe where it is dominated by the energy of spacetime itself. During this process, since expansion is dominated by the inherent energy of spacetime, the rate of expansion was exponential, expanding space extremely rapidly and abruptly in all dimensions.
Thus, on a very small scale, any non-flat region of space containing matter would be indistinguishable from a flat region, and all particles of matter would be moved apart by dilation, without any two of them ever meeting. .
After a few fractions of a second, the inflation stops, and the energy inherent in spacetime is converted into matter and radiation. One way to understand this phenomenon, and to visualize inflation as a ball at the top of a hill. As long as the ball remains at the top, inflation and its exponential expansion continue.
For inflation to end, no matter what quantum field is causing it, it must… (continued on next page)
