Our universe has an estimated age of almost 13.8 billion years,
and during this time several things that we will tell you about in this video, have happened;
13.8 billion years, however, is such a large number that it is difficult to imagine:
almost certainly not even your grandparents have reached such a remarkable age!
What we can do to give a more concrete idea is to imagine to compress the 13.8 billion years
of our universe in a time span of one year of our calendar: in practice, making the necessary
proportions, when the various evolutionary steps of the universe happened if it were one year old?
This idea was born from the fervid mind of the famous astronomer and science popularizer Carl
Sagan (1934 - 1996), who in this special cosmic calendar imagined placing the Big Bang in the
first second of January 1st and our present in the second preceding midnight on December 31st:
Using this time scale, each second of the calendar corresponds to approximately 444
Earth years and 31 of its days correspond to approximately one billion of its years.
Our universe is currently expanding: in 1929 the American astronomer Edwin Hubble discovered that
galaxies move away with a speed proportional to their distance: more distant they are,
faster they move away. The recession law of galaxies discovered by Edwin Hubble had been
predicted a few years earlier by two theoretical physicists, the Russian Aleksandr Friedman and the
Belgian Georges Lamaitre, who had shown that the equations of general relativity predicted
an expanding universe. If the universe is currently expanding, then it means that,
going far back in time, matter and energy were concentrated in a very small region of space,
with an infinite density. From this singularity the universe originated, 13.8 billion years ago,
with a sort of colossal explosion, the Big Bang: in our model of the universe in one year,
this moment corresponds to 1st January. It is believed that at the time of the Big Bang the
universe was governed by a single force, which united the four fundamental forces known today:
• strong interaction (which holds atomic nuclei together);
• weak interaction (responsible for the beta decay of atomic nuclei associated with radioactivity);
• electromagnetic interaction (the interaction between objects that have an electric charge,
responsible for the electromagnetic field; it propagates in the form of an electromagnetic
wave at the speed of light); • gravitational force.
This situation lasted for a very short time: after 10-43 s (the so-called Planck time),
the universe underwent a phase transition, during which the gravitational force separated from the
other three forces. The temperature was then 1032 degrees, a million billion times higher than the
maximum energy reached so far by accelerating machines; in practice billions of times higher
than there are in Death Valley! Can you imagine them? Write it in the comments! Since we do not
yet have a unitary theory of all forces, we do not know how the universe was made in its
first 10-43 s: the known (or at least imaginable) history of the cosmos begins from that instant.
At 10-36 s from the Big Bang, when the temperature was 1028 degrees, the strong force separated from
the weak force and the electromagnetic force. The universe then underwent in a very rapid time
interval (10-32 s) a very rapid expansion, of an exponential type, called "inflation",
which increased the size of the cosmos by 1025 times, transforming microscopic volumes into
gigantic regions: two points initially at a thousandth of a millimeter from each other,
they finally found themselves at a distance of a thousand light years.
The inflationary expansion produced an enormous amount of gravitational waves,
which today constitute a stochastic background of very large wavelength
(and, consequently, very low frequency). The term "stochastic background" means
the superimposition of waves coming from all directions and randomly distributed over time,
so overlapping each other as to make it very difficult to identify the individual components.
The stochastic background covers all frequency bands and contains, mixed together, the primordial
waves originating from the Big Bang and those of the indefinite and innumerable discrete sources
distributed throughout the entire Universe. The mixture of elementary particles (quarks,
leptons, photons, gluons, etc.) of which the universe was made continued to expand
and gradually cool down to a temperature of 1015 degrees. At this point - 10-12 s
from the Big Bang - the weak force and the electromagnetic force separated.
Just think, all these very early evolutionary stages of the universe took place in a time
much shorter than the blink of an eye! Can you imagine it? Write it in the comments!
One millionth of a second after the Big Bang at a temperature of 1013 degrees,
baryogenesis took place: quarks - which up to that moment made with
gluons an incandescent plasma - began to join together and form baryons (protons,
neutrons and respective antiparticles). In this phase, particle - antiparticle pairs
continually annihilated each other, generating photons, which in turn materialized in pairs.
When the universe had a hundredth of a second of life, the energy of the photons was no longer
sufficient to allow the creation of particle - antiparticle pairs and began to dominate
the reverse process, that of annihilation. Since a slight prevalence of matter had been
determined in the meantime, the antimatter disappeared completely with the annihilation,
and the excess matter ended up completely populating the cosmos.
A second after the Big Bang the neutrinos stopped interacting with matter and began
to roam freely. When they reached us, they formed the cosmic background of cosmological neutrinos,
a dust of very low energy particles that envelop and pass through us.
Nucleosynthesis began a few minutes after the Big Bang (the temperature was 109 degrees):
then the first nuclei of deuterium (heavy isotope of hydrogen consisting of a proton
and a neutron) were formed, and subsequently those of tritium (isotope of hydrogen with
one proton and two neutrons), helium, lithium and beryllium.
At that time the universe was opaque, that is, the matter inside it was immersed in a
cloud of radiation and ionized gas, and we can imagine everything as a sort of fog in
which light and electromagnetic radiation in general could not propagate freely.
Instead, 380000 years had to be waited for the formation of atoms, corresponding to the first
half hour of January 1st in our calendar, a time that appears long on a human scale,
but is only 0.003% of the current age of the universe (comparable to the first hours of a
person's life); in our cosmic calendar this time is roughly equivalent to the
first half hour after the Big Bang. The temperature was then around 3000 degrees.
Protons and neutrons began to capture electrons (which no longer had enough energy to escape),
forming neutral and stable atoms: this is why we speak of the era of recombination.
With the reduction of the number of free electrons and protons capable of absorbing
photons, the universe became transparent to electromagnetic radiation. It is a crucial
moment in the life of the cosmos, because it is only from then on that we can "observe" it.
The subsequent events of the universe are marked by much more extended times,
hundreds of millions of years. At sufficiently low temperatures, thermal agitation stops
opposing the weakest of forces, the gravitational force, and this begins to aggregate matter and
form the first structures. Current numerical calculations with supercomputers allow us to
simulate the evolution of the universe on a large scale in great detail, reconstructing the very
complicated dynamics of billions of galaxies and gravitationally attracting each other.
And the stars? They began to form only starting from 400 million - a billion years
after the Big Bang, i.e. January 10th in our calendar: it is thanks to their thermonuclear
fusion reactions that the synthesis of all the heaviest atomic nuclei was possible!
10.1 billion years ago, so about 4 billion years after the Big Bang, corresponding to April 11th,
our galaxy was born, the Milky Way, a typical spiral galaxy made up of about 200 billion stars,
dust and gas interstellar. The name Milky Way comes from its appearance in our night sky:
a faint luminous and milky lane, whose diffuse glow is due to the light of the numerous stars
that compose it, too far away to be individually distinguished with the naked eye. In reality,
to an observer looking at it from outside it would appear as a flattened disk with a
bulge in the center. The disk - with a diameter of 100,000 light years and a thickness of 2,000 light
years - is largely formed by clouds of dust, gas and young stars concentrated in spiral-shaped arms
and rotating around the core, which is instead composed of stars and star clusters whose birth
seems to date back to the period of formation of the same galaxy. Around the nucleus and the
disk there is a halo with a diameter of 150,000 light years and populated by globular clusters.
Our Solar System is in a fairly peripheral position in the Milky Way, at about 28.000
light years away from its center, around which it rotates with a speed of about 250 km/s. If
you had a time machine, would you have appreciated to see our galaxy form? Write it in the comments!
From this point on we will focus on what happened
in the area of space where our Solar System is located, and therefore also the Earth.
After the formation of the Milky Way, nothing relevant happens until the end of July;
we have to wait until August 25th, corresponding to 4.9 billion years
ago, to see the origin of the Solar System. Inside the Milky Way a very large cold cloud
of interstellar gas - composed of hydrogen, helium and a small part of dust - begins to contract due
to its own gravitational force. During this phase the cloud begins to rotate more and more quickly,
assuming the flattened shape of a disk with a diameter of about 10 billion kilometers and a
thickness of over 100 million kilometers. Most of the gas accumulates in the center of the cloud,
where it forms a globe with an average temperature of 2,000 degrees Celsius. The
accretion process continues until the globe reaches a sufficient internal temperature
and pressure to trigger the nuclear fusion processes typical of a star.
The Sun is born. The young star begins to produce heat, while in its outer and coldest regions of
the cloud the gas and dust condense, thanks to collisions, due to the reciprocal gravitational
attraction, in increasingly larger blocks of matter up to form protoplanets whose dimensions
and chemical characteristics depend on their distance from the Sun and on the composition
and density of the primordial cloud. The formation process as a whole takes 100 million years. From
this moment the evolution of the solar system goes through a period characterized by numerous impacts
between the bodies that compose it. Only after a few tens of millions of years does the solar
system become dynamically stable. From here on we will focus exclusively on our planet:
On September 25th, 3.7 billion years ago, life appears on Earth.
December 1st, 1.2 billion years ago: Earth's atmosphere is enriched with oxygen.
December 13th, 658 million years ago: the first multicellular animal organisms appear in the sea.
December 25th, 195 million years ago: the dinosaurs appeared. Fans of dinosaurs and the
various Jurassic Park movies, would you have liked to be there? Write it in the comments!
December 29th, 65 million years ago: extinction of the dinosaurs
due to an asteroid; resulting in a large spread of birds and mammals.
December 31st at 9:30 pm (4.00 million years ago) Australophitecus,
one of the ancestors of the current Homo Sapiens,
appeared in Africa: in our calendar, we are at the beginning of the New Year celebrations!
December 31st at 23:52:30 (200,000 years ago) Homo Sapiens appears,
that is the human species still present today.
December 31st 23:59:59 (500 years ago) The Renaissance.
Copernicus formulates the theory of the heliocentric universe.
Our cosmic calendar ends at the stroke of midnight on December 31st,
with man on the moon and everything that happened after until today.
How do you feel to see that the history of the human being occupies only a very small
fraction of the history of the entire Universe? Write it in the comments!
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