The Universe originated 13.8 billion years ago with the big Bangand took approx 10 billion years to get to the first life forms on and around Earth 14 billion years (i.e. approximately his age today) to “cook” us humans: well, these times are not as random as it might seem. The one we live in is actually one “cosmological window”as the period that goes is sometimes called from about 2 to about 20 billion years after the Big Bang which is considered the only one in which the cosmos fully presents all the characteristics suitable to support the emergence and development of life.
Because life on Earth was born when it was born: the cosmological window
In the first minutes after the Big Bang the universe was composed almost exclusively of hydrogen, helium and traces of lithium. The heavier chemical elements, necessary for the formation of rocky planets and biological molecules, arrived only after the birth of the first stars, between 150 and 300 million years after the Big Bang.
These were very massive stars with a short life cycle. Nuclear fusion within them and their death as supernovae led to the synthesis of heavier elements, which were then dispersed into space through the explosions themselves. Subsequent generations of stars continued this process, progressively enriching the interstellar environment.
After about two billion years, the universe now contained a significant quantity of elements such as carbon, oxygen, iron and silicon and so on, i.e. chemical ingredients necessary for the formation of rocky planets, atmospheres, oceans and biomolecules.
Yet, for life to develop, a particular type of environment is needed: a stable star for several billion yearslike our Sun, capable of providing constant energy and a peaceful environment for its planets. The very massive stars of the young universe offered neither stability nor sufficient time for biological evolution.
It is only after the first generation of stars that stars of intermediate mass were formed, such as the Sun. It is in this period that the universe becomes physically fit to life. This time interval, in which the abundance of heavy elements is high, the formation of solar-type stars is significant, and the stellar environment is relatively stable, is called cosmological window. A long interval of time, but one that will not last forever.
Because the cosmological window will not always last
Star formation in the universe has reached its maximum between 3 and 7 billion years after the Big Bang. Since then the rate of formation of new stars has progressively decreased. Interstellar gas clouds, i.e. the “raw material” to create new stars, are progressively running out.
In the cosmic future, Sun-like stars will therefore be increasingly rarer as they die. They will mostly stay red dwarfsvery long-lived stars but subject to intense eruptions and variations in activity which can make nearby planets unstable. Their habitable zone is in fact very close to the star itself since they are relatively cold stars.
This means that the universe it will not remain habitable forever.
The cosmological window is a period temporary in which the conditions necessary for complex life coincide.
The origins of life on Earth and the “window” of the Solar System
The history of the Sun also presents a favorable window. We are currently about halfway through its stability phase, which lasts more or less 10 billion years. In this phase, luminosity and energy are relatively stable and consequently life has had time to select and evolve into complex forms.
In the future the brightness of the Sun will gradually increase. This process will lead, in approximately 1 billion yearsunder conditions that make the Earth unsuitable for complex life because it will make the Earth an arid and uninhabitable planet. This window compatible with life, therefore, is also limited.
The exceptionality of our cosmic era
The relevant aspect is not simply that we are within these favorable time windows, but the fact that such windows exist. The universe could last for an extremely long time, and for most of its past and future history it will be inhospitable to complex life.
Our cosmic epoch instead coincides with a relatively limited time frame in which the abundance of chemical elements, the presence of stable stars and the availability of extended evolutionary times converge favorably. This makes it possible for complex ecosystems to appear and, in some cases, species with self-awareness like ours.
Complex life on Earth therefore appears as the result of specific and non-permanent physical conditions, not as an inevitable outcome of cosmic evolution.
