The End

0. Forever

The central ideas of Big Bang theory, that is, initial state at high temperature, formation of galaxies, continuous development, expansion and cooling of the universe, have been confirmed by several independent observations including also supernovae, therefore they can no longer be confuted as main characteristics of the universe.

However it's necessary to point out that the core of Big Bang theory refers to the fact that the universe is now expanding and cooling, in other words it describes how it evolves, not how it started. The extrapolation of the expansion of the universe back in time leads indeed to a state of density and temperature so high to tend to infinity; furthermore this condition has been maintained for an infinitesimal quantity of time, so short that it's impossible to study it with the means actually available.

Conclusion, the comprehension of the first instants of life of the cosmos remains one of the greatest unresolved issues of physics.

And it's not the only one.

From the study of some observational evidence, including again supernovae, astrophysicists concluded that the universe is now dominated by a mysterious form of energy, known as dark energy, permeating all the space. The observations suggest that more than 70% of all the current energy density is in this form still to be studied.

Important consequence, the Big Bang wasn't an explosion of matter moving outwards to fill an empty space, because the unexpected news is that the void, the nowhere, the nothing we always imagined with fear, at least in life preceding death, doesn't exist.

When the cosmos was younger, it was equally permeated by the dark energy, but the gravity pull was stronger, slowing down the expansion, as there was less space available and the various astronomical objects were closer. After a few billions years, the growing abundance of dark energy caused an acceleration of the expansion. The composition and the mechanism of such astonishing cosmic element are totally unknown.

Likewise we don't know so much about the dark matter. Numerous observations have shown beyond any reasonable doubt that there's not enough visible matter for explaining the gravitational attraction inside galaxies and among them. This led to the idea that over 90% of cosmic matter is dark; its possible characteristics are the lack of emission of light and of interaction with the standard matter we already know, but everything else remains shrouded in mystery.

Considering all this, it seems to me we haven't yet understood anything about the universe, not just about its beginning, even about its daily routine. And what about its end? Theories are so many and varied to resemble more to performances of dreamers than scientists.

Before observations about dark energy cosmologists considered two possible scenarios for the future of the universe; according to the first hypothesis if the mass density will be higher than the critical density, the universe will reach a maximum size, then it will begin to collapse. At that point it will become denser and hotter, returning to a physical condition somehow similar to the one leading to the Big Bang. This first hypothesis is known as 'Big Crunch'.

The second hypothesis on the other hand assumed that if the mass density will be equal or less than the critical density, the expansion will continue but slowing down, that is, never stopping. Stars formation will end when all the interstellar gas present inside galaxies will be consumed; stars will end their existence transforming themselves into white dwarfs, neutron stars and black holes. Very slowly over time, collisions between these objects will produce the collapse of the matter inside more and more large black holes. The average temperature will reach the absolute zero, a state called the 'Big Freeze'.

The destiny of the universe is in any case linked to its density, this thing is now so clear to the science that I have understood it too. Most of the evidence collected until now, based on measurements of expansion speed and mass density, argue that the universe won't collapse, as pushed by the disruptive power of dark energy, which at least for the moment is more powerful than dark matter and gravity, which instead are attractive forces.

Third possibility, another form of dark energy, known as ghost energy, implies that all the matter will be destroyed by the increasing speed of expansion in a process known as 'Big Rip'.

That's not all, the latest observations based on the concept of dark energy imply that by the time an increasing part of the universe will pass beyond the horizon of events and will be no longer visible.

The final result of this fascinating process is totally unknown.