Destiny of the universe

One of the open-ended questions which arose a long time in Cosmologie is that of becoming Expansion of the universe: does this one go perdurer indefinitely, or to stop and leave room to a phase of contraction, then a Big Crunch? It is commonly allowed today that the first answer is most probably much more probable than the second.

The description of the evolution of the Univers as a physical system is made by the data of a cosmological Modèle and the skew of the General relativity. This one is indeed capable to describe the evolution of a homogeneous universe and isotropic, via the equations of Friedmann. These last indicate the value of growth the Rate of the universe (of which the current value is the Constante of Hubble) according to the Densité of average energy of the universe, as its evolution according to certain properties of the various matter shapes which fill up the universe, in particular them Pression.

Various destinies

Big RIP

If the density of our universe is largely in lower part of the Densité criticizes, its expansion will continue to accelerate until a Big RIP, thinks one. Big RIP is a kind of self-destruction of the universe. This last will have taken volume so much that the force of cohesion of the atoms will not be enough any more to hold them together. ( Big RIP wanting to say Large Tear in English). Currently, the cosmologists lean for this assumption but nothing is verifiable at present.

Slowed down expansion

If our universe is equipped with a density exactly equal to the critical density, its expansion will be slowed down considerably.

Big crunch

Lastly, last case of figure; if the density of the universe exceeds the critical density, its expansion will be reversed, the universe will narrow, until will reach more the conceivable small size, the Length of Planck. This big bang with back , is called the Big Crunch.

The density of matter : at the origin of gravitational attraction

This density of matter, or density of mass, includes at the same time the visible and invisible matter. Indeed, the Universe is made up:

• to 70% of energy sinks (one will return there)
• on the 30% remainders, one counts 20% of protons and neutrons which have a mass, and matter 80% known as “dark”, still unknown factor. The Matière sinks (with the difference of energy sinks) has a mass and thus intervenes in the calculation of the density of matter, in the invisible matter category…

The “threshold of the density criticizes”

The Universe is thus pulled about between a repelling power, the expansion (depend on the Constante of Hubble), and a gravitational attraction, the gravitational force, which depends on the quantity of matter present in the Universe. What does one call threshold of the critical density then?

The critical density is the point of balance between expansion and gravity. But the probability so that this balance is carried out exactly is negligible. Two great types of scenarios are then possible:

• If the density of the Universe were higher than the critical density, gravity would inevitably take again the top: one would attend then a scenario catastrophe, nothing less than one “Big Bang with back”: it is the assumption of the “Big Crunch” (great collapse). The universe would thus collapse on itself, and the entirety of the matter would be found condensed in only one point, as at the time of Big Bang.
• If the density of the Universe were lower than the critical density (assumption currently privileged), nothing could stop or slow down the expansion: one would lead to an infinite dilation called Big Chill (great cooling).

An easy experiment materializes the “duel” expansion/revolved: that of the stone which one launches in the air and which inevitably falls down because its kinetic energy is slowed down by its weight. However calculations show that while launching the stone with more than 11 km/s, it would reach the escape velocity terrestrial which would free it from gravity, and would never go down again. These two situations are respectively the scenario of “Big Crunch” and that of Big Bang. A question remains: if the expansion continues, as the specialists think it, it will accelerate or slow down?

The density of energy : “nudge in the right direction” with the expansion

It is here that dark energy mentioned above intervenes. The Universe in is almost entirely made up (70%).

The true nature of dark energy concerns for the moment the field the speculation: it could be a question of the cosmological Constante of Einstein, but dark energy could also derive from the existence of called unknown particles Quintessence S (fifth element) created at the time of Big Bang. None of these models is excluded for the moment.

What is established, it is that this dark energy contributes largely to an acceleration of the expansion: it behaves indeed as a repelling power which is opposed to gravity and could finally dominate the energy balance of the Universe, determining an exponential expansion.

The problem, it is that it is not known if this dark energy is constant. If it were to increase with time, one would then attend a third scenario, the Big RIP (large tear): the entirety of the matter of the Universe, atoms included/understood, disappears in a racing without end… What to think? Let us leave the word to the specialists (article “dark energy” of Wikipédia): “In May 2004, the publication of work completed with the Chandra space telescope and consisting in measuring the distances from 26 remote galaxy clusters seem to confirm that the expansion started to accelerate there is 6 billion years, and that dark energy seems to remain constant, or then varies very slowly. That is compatible with the existence of a cosmological constant and returns scenarios such as improbable Big Crunch or Big RIP. ”

Perhaps ultimately, the future models of expansion will depend on the density of curve (=forme) of the Universe.

It is the third side of our triangle, and still most dubious of all. The geometry of the Universe can be only postulated under the current conditions: is the Universe closed, flat, is opened…?

• In an “opened” Universe, two parallels would never cease moving away one from the other.
• In a “closed” Universe (one also speaks about “spherical” space), the two parallels would end up meeting.
• Lastly, in a “flat” Universe, our two parallels would never meet.

See too

• Densité criticizes
• space Courbure