Entia non sunt multiplicanda praeter necessitatem (entities must not be multiplied beyond necessity)

The current theory of the origin of our Universe, which is based on Einstein's general theory of relativity (GR), assumes that our Universe has started from an extremely hot and dense state called the big bang. The big-bang cosmology successfully describes primordial nucleosynthesis (production of the lightest elements in the early Universe) and predicts the cosmic microwave background (CMB) radiation, which we observe coming from all directions in the sky. In order to explain why the Universe that we observe today appears spatially flat, homogeneous and isotropic at the largest scales, the theory of cosmic inflation has been proposed, according to which the very early Universe went through an extremely rapid exponential expansion by an enormous factor in volume. A big success of inflation was also to predict the form of density fluctuations (which seed the structure formation in the Universe) observed in the CMB, although this theory requires the existence of a hypothetical matter field with a specific interaction.

The big bang itself, however, is unphysical: the Universe started from being a point of infinite density, called singularity. In addition, this cosmology does not address four fundamental questions. What caused the big bang and the subsequent rapid expansion of the Universe? What reality existed before? Why does the cosmic time flow in one direction? And what happened to antimatter? The answer to these problems may come from an old extension of general relativity, called the Einstein-Cartan (EC) or Einstein-Cartan-Sciama-Kibble theory of gravity. This theory extends GR by removing its artificial constraint on the affine connection of spacetime, allowing its geometrical part called torsion to be different from zero (like curvature). By doing so, the EC gravity also correctly accounts for the quantum-mechanical, intrinsic angular momentum (spin) of elementary particles called fermions.

At extremely high densities, much larger than the density of nuclear matter, torsion manifests itself as a force that counters gravity. As in GR, very massive stars end up as black holes: regions of space from which nothing, not even light, can escape. Gravitational attraction due to curvature initially overcomes repulsion due to torsion and matter in a black hole collapses, but eventually the coupling between torsion and spin becomes very strong and prevents the matter from compressing indefinitely to a singularity. The matter instead reaches a state of finite, extremely large density, stops collapsing, undergoes a bounce like a compressed spring, and starts rapidly expanding. Extremely strong gravitational forces near this state cause an intense particle production, increasing the mass inside a black hole by many orders of magnitude and strengthening gravitational repulsion that powers the bounce. The rapid recoil after such a big bounce could be what has led to our expanding Universe. It also explains why the Universe that we observe today appears at largest scales flat, homogeneous and isotropic, without needing cosmic inflation.

Torsion in the EC gravity therefore provides a plausible theoretical explanation of a scenario, according to which every black hole produces a new, baby universe inside and becomes an Einstein-Rosen bridge (wormhole) that connects this universe to the parent universe in which the black hole exists. In the new universe, the parent universe appears as the other side of the only white hole, a region of space that cannot be entered from the outside and which can be thought of as the time reverse of a black hole. Accordingly, our own Universe could be the interior of a black hole existing in another universe. The motion of matter through the black hole's boundary called an event horizon can only happen in one direction, providing a past-future asymmetry at the horizon and thus everywhere in the baby universe. The arrow of time in such a universe would therefore be inherited, through torsion, from the parent universe.

The new universe in a black hole is closed: finite but without boundaries. It can be thought of as a three-dimensional analogue of the two-dimensional surface of a sphere. The formation and evolution of such a universe is not visible for external observers in the parent universe, for whom the event horizon's formation and all subsequent processes would occur after an infinite amount of time had elapsed (because of the time dilation by gravity). A baby universe is thus a separate, closed spacetime branch with its own timeline. However, if we were living in a rotating black hole (all stars rotate) then our Universe would have inherited the black hole's axis of rotation as a "preferred direction". Such a preferred axis could explain the observed cosmic parity violation: a preference for spiral galaxies to be left-handed or right-handed spirals in different regions of the sky.

The energy of matter at the big bounce is an order of magnitude higher than the Planck energy. Recent observations of high-energy photons from gamma-ray bursts, however, indicate that spacetime may behave classically even at scales above the Planck energy. The classical spin-torsion mechanism of the bounce is thus viable. Furthermore, the EC theory passes all tests of GR because both theories give significantly different predictions only at extremely high densities that exist in black holes or in the very early Universe.

In addition to astrophysical and cosmological implications, torsion modifies the Dirac theory that describes the quantum-mechanical behavior of fermions (quarks and leptons) that form ordinary matter. These particles must be spatially extended, which may solve the problems in quantum field theory arising from treating them as points. This modification may also be responsible for the observed imbalance of matter and antimatter in the Universe and could relate the apparently missing antimatter to dark matter, a mysterious form of matter that does not interact electromagnetically and accounts for a majority of the matter in the Universe. Finally, it may be the source of dark energy, a mysterious form of energy that permeates all of space and increases the rate of expansion of the Universe, allowing it to grow infinitely large and to last infinitely long.

Spacetime torsion is a geometrical phenomenon, which may naturally solve all fundamental problems of general theory of relativity and quantum field theory. The spatial extension of fermions arising from torsion is at the level of the Cartan length, which for an electron is about one hundred million Planck lengths. Consequently, the existence of spin forbids fermions from being Planck-scale strings and thus invalidates string theories. The coupling between spin and torsion also invalidates other theories of spacetime such as teleparallel gravity. Unlike theories which assume the existence of superfluous hypothetical objects (string theories, supersymmetry, extra dimensions, cosmic inflation, and many others), the Einstein-Cartan theory of gravity is the simplest and most natural theory of gravity, based on the affine connection as the fundamental physical structure.