The First Three Minutes: A (more) Modern View Of The Origin Of The Universe
The scenario of the inflationary universe represents, at present, the best model
in our possession to explain the origin of the cosmos. The Latin philosopher Lucretius wrote that
"nothing can come from nothing", and the economist John Keynes stated, more prosaically, that
"There Ain't No Such Thing as a Free Lunch". And yet, the theory of inflation does indeed seem to
be able to explain how, out of a vacuum, a bubble of space-time was formed by quantum fluctuations,
which, inflating frighteningly rapidly, became our universe. As the father of inflation,
Alan Guth, observed, "the universe was the first and last free lunch we were ever served".
Today we are finally able to retrace, like a movie, the story of the origin of the cosmos.
Resuming the idea that Steven Weinberg used in his famous book The First Three Minutes:
A Modern View Of The Origin Of The Universe, we will describe, frame by frame,
the very first moments of the universe, but, unlike Wienberg, who wrote his book in 1976,
we are now fortunate to be able to go back to the very first fractions of a second, which,
before the advent of inflationary theory, was completely impossible.
Before starting it is necessary to have clear an important fact: the big bang theory and the
inflationary scenario are the best models in our possession to describe the origin of the universe,
but they must be considered only as theories, and not as certainties. And
not only that ... the more our gaze goes to the depths of the earliest moments, the more
theories become uncertain and hypothetical, and bordering on the state of pure conjecture.
In his famous essay The Emperor's New Mind, English mathematician Roger Penrose divides
physics theories into three categories, the "Superb", the "Useful", and the "Provisional",
depending on how accurate they are in their interpretation of experimental results:
well, the standard model of the big bang is classified as "Useful" theory,
while the inflationary scenario does not go beyond the qualification of "Provisional".
For the record, the only theories that Penrose elevates to the rank of "Superbe",
are Euclidean geometry, Newtonian mechanics, Maxwell's electromagnetic
theory, the two theories of relativity, quantum mechanics, and electrodynamics.
So, all I am going to tell you, in short, could be only a fable. Take it into account.
Contrary to what might seem logical, our history does not begin in the zero moment
of the life of the universe. We already know that according to quantum theory,
space-time is not continuous, but it has rather a discrete structure, "spongy",
in the sense that there is a "spatial quantum", which is the Planck Length,
equal to 10-35 meters (a billionth of billionth of billionth of billionth of centimeter);
and a "temporal quantum", which is the Planck Time, equal to 10-43 seconds. These are
indivisible quantities, so it does not make sense to consider lengths shorter than Planck Length,
as well as it does not make sense to speak of time intervals less than Planck Time.
So we can say that, in a sense, time has begun not in the zero instant, but after 10-43 seconds...
and to ask what happened before this instant is completely out of reach of current physics.
1st frame. Our first frame shows the universe at Planck Time. Everything that exists is
concentrated in an incredibly small space, the size of the Planck Length (10 million
billion smaller than a hydrogen atom!), the density of matter and energy is 1094 g/cm3,
and the temperature is 1032 K, that is 100,000 billion billion billion degrees! The universe
has just emerged, from a quantum fluctuation like many others, as a tiny bubble of space-time filled
with virtual particles of matter and antimatter. However, it is very difficult for us to imagine
what its true nature is. With all the energy of the universe enclosed in an infinitesimal "cosmic
egg", do the concepts of space and time still have the meaning they have in the current universe?
Is it possible to measure, in this primordial atom, the distance between two points, or to be
sure that time flows from the past to the future? These questions are, at least for now, unanswered.
All we know, or, better, we can hypothesize, is that at Planck Time the four known forces
(gravitational, electromagnetic, strong nuclear, weak nuclear) are unified in a single
"superforce", also called "quantum gravity", even if, at this very moment, the gravitational
force is separating from the "rest" of the superforce, that we will call "electronuclear".
The nature of superforce is for us almost completely mysterious, since its precise
description is the final goal of physicists engaged in the research of the so called "theory
of everything". The theory currently most credited to play this role is the theory of "superstrings",
according to which the primordial universe would have had, in addition to the usual four space-time
dimensions, other dimensions: they would have remained as wrapped on themselves at Planck scale,
and then with the subsequent expansion of the cosmos would have remained invisible.
2° frame. The scene shows us now the universe on the eve of a great and unrepeatable moment:
inflation. 10-35 seconds have passed since "creation", the universe has a size of 10-28
meters, temperature has dropped to 1028 degrees. Another separation of forces is about to take
place: electronuclear force is divided into strong nuclear force and electroweak force.
During inflation, which is now about to start, the universe will increase its size by a billion
billion billion billion times, assuming the size of a grapefruit: it is shocking to observe,
on the other hand, that in all the rest of its history the universe will expand "only" by a
billion times. To have an idea of the magnitude of the expansion during the inflation era,
it can be compared to an atomic nucleus that, within 10-32 seconds,
becomes as big as a sphere that has its center in the Sun and its surface touching Alpha Centauri!
"Hey, guys, just a moment before we continue... BE sure to join the Insane curiosity Channel...
Click on the bell, you will help us to make products of ever-higher quality!"
3rd frame. Inflation is over. Universe is "old" by 10-32 seconds, and temperature is 1025 degrees.
Symmetry breaking between forces has happened, excited vacuum has released its immense energy
in form of radiation, real particles of matter and antimatter have formed,
and in short, universe slowly starts to take a more familiar shape. From now on the universe
will continue to expand, but more and more slowly. Particles that emerged from quantum vacuum are
quarks and antiquarks, and they are immersed in a bath of photons. Quarks are slightly more than
antiquarks, and it is thanks to this asymmetry that the current universe, after annihilation
between matter and antimatter, is formed, at least so it seems, exclusively of matter.
4th frame. Arrived at the age of 10-12 seconds, the universe has a temperature of
1015 degrees and has now considerable size: a radius of about 300 million kilometers,
twice the distance between Earth and Sun. The last separation of forces is happening:
the electromagnetic force is detaching from the weak nuclear force.
Finally, the four fundamental forces we know have differentiated and will remain so for the rest of
the history of the cosmos. Moreover, the soup of quarks and antiquarks is being enriched by another
type of lighter particles: leptons, which include, among others, electrons and antielectrons.
5th frame. We are now one millionth of second after the initial instant:
the universe is a sphere more or less as big as our solar system,
that is 10 billion kilometers. The temperature has decreased to 10.000 billion degrees, and at
this thermal level quarks and antiquarks can no longer exist alone, and begin to gather together,
forming more massive particles (protons, neutrons, antiprotons, antineutrons, and mesons).
At the same time, however, a catastrophic event comes to make life difficult for these particles:
a colossal annihilation between quarks and antiquarks. While previously,
when the temperature was higher, the collisions between particles were so energetic to produce
new quark-antiquark pairs, now the thermal level has become too low to support this continuous
generation; the annihilation takes then the upper hand on the production, with the dramatic
effect of converting, in a few seconds, an immense amount of matter and antimatter in electromagnetic
radiation. In the primordial universe, there was a slight prevalence of matter compared
to antimatter, so a relatively small number of quarks can survive, completing their combination
in protons and neutrons, and giving origin to most of the matter that today constitutes our universe.
Frame 6. At the age of one second, the temperature is 10 billion degrees,
roughly a thousand times the temperature we measure today at the center of the Sun.
The universe is experiencing a second great annihilation.
This time, after protons and neutrons, electrons and positons are the ones to pay the price.
Neutrinos and antineutrinos are able to escape annihilation, they are notoriously not
able to interact with each other and with other particles: we should be able to observe them even
today if it was not that they are unfortunately very elusive particles and difficult to detect.
Their discovery would be anyway a good proof of this model of hot primordial universe.
The consequences of the second colossal annihilation are very similar to those of
the first: on one hand the production of a huge amount of energy in the form of electromagnetic
radiation, on the other hand the exit from the scene, now final, of antimatter. From this moment
on the universe is a big luminous mass of burning plasma composed of protons,
neutrons, and free electrons: from this moment begins what physicists call "plasma era".
7th frame. The "creation" movie has arrived at three minutes from the beginning of time.
The temperature has dropped to one billion degrees, approximately the temperature that
exists inside the hottest stars. Protons and neutrons in the plasma no longer have enough
energy to escape the nuclear attraction to each other, and they join forming the first nuclei.
Hydrogen nuclei are already present, being formed simply by single protons; the first compound
nuclei that appear are deuterium nuclei, or heavy hydrogen, consisting of a proton and a neutron;
then appear the first helium nuclei, containing two protons and two neutrons,
and finally make their appearance small amounts of lithium nuclei and beryllium.
These elements represent 99% of the matter present today in the universe: the heavier elements,
including carbon, nitrogen, and oxygen that will play a central role in the appearance of life,
will make their appearance much later, generated inside the stars
as a result of thermonuclear reactions. Accurate simulations have been made of this
phase of production of the first atomic nuclei, lasted a few hours and called by physicists
nucleosynthesis, and resulted, in particular for primordial helium,
an abundance that agrees almost exactly with the proportion observed experimentally:
this is one of the most important evidence of the validity of the idea of the hot big bang.
At this stage in the history of the universe, while protons and neutrons combine, electrons are
still too energetic to join nuclei and continue to roam freely within the primordial plasma.
Frame 8. Let's now take a big jump forward, to about 300,000 years after the big bang,
to the time of "recombination". Actually, between the previous frame and this one,
not much has happened, if we exclude the incessant cosmic expansion, moreover always
slower. Undoubtedly the events occurred in the first three minutes were immensely more numerous
and more important than those that happened in the next 300.000 years. The temperature is now
only 10.000 degrees, approximately the surface temperature of the Sun, and this is a level at
which free electrons are no longer able to escape the electromagnetic attraction of atomic nuclei:
so the first hydrogen and helium atoms are formed. As we have already seen, before this happened,
photons, present in very high number in primordial plasma, were continuously diffused and absorbed
by free electrons. Now, however, with the disappearance of the latter, electrons cease
to interact with matter, and can finally cross the universe without encountering obstacles.
Universe becomes so "transparent", and the radiation that can finally be released
is the one that we observe today in the microwave spectrum and that we call background radiation.
The decoupling between matter and radiation coincides with the transition from a universe
dominated by radiation to a universe dominated by
matter, which is the current one: with a temperature dropped to current levels,
the wavelength of background radiation is now increased a lot, and the energy density
of radiation permeating the universe is consequently lower than that of matter.
The next story is no longer part of our movie, but it is the story of the recent universe.
After a few hundred million years, with the temperature dropped below 4000 degrees,
matter will become largely electrically neutral and its interaction with radiation
less and less frequent. The matter, then, under the action of gravitational force,
will begin to aggregate forming the first protogalaxies: giant clouds of cold gas that
will give rise, within a billion years, galaxies. After about two or three billion years from the
big bang, galaxies will gather in clusters, and at four billion years will form the first stars.
The universe, meanwhile, will continue to expand and cool, while the background
radiation will become less and less energetic, moving to longer and longer wavelengths.
Around some of the stars, finally, will be created planetary systems,
and in one of these - but perhaps in many others - about 14 billion years after creation, will appear
living organisms capable of reconstructing the story that we have tried to tell ...