Cosmology is the science that studies the origin and the evolution of
the Universe.
It had a very important role in the history of western and scientific
thought, a role that was somehow bound to philosophy and religion. Up to
a few centuries ago, the known Universe was described by the Ptolemaic
system, according to which the Universe was perfect and immutable and
had its centre in the Earth.
With Copernicus, Galileo and Kepler the geocentrical conception
of the Universe ended, and science moved towards a heliocentric
conception. It was more than a mere change of perspective, but the
start of a real revolution in science, because since then, the dogma has
been replaced by experimentation.
We know today that the Earth is not the centre of the Universe, but
is part of a planetary system, which, in its turn, is part of the Milky
Way, which is one of the many galaxies present in the Universe.
Nevertheless, up to a few decades ago it was thought that our galaxy
constituted the entire Universe and that all the visible stars and nebulae
were part of it.
It was only in the twenties, that the astronomer Edwin Hubble discovered
that some of those stars and nebulae were external to the Milky Way, and
were actually very distant galaxies.
The expansion of the Universe and the Big Bang
In 1929, Hubble discovered that all galaxies seem to move away from
us, in fact the radiation that they emit is shifted towards the red part
of the spectrum, in other words they present the phenomenon called redshift:
in the spectrum of visible light, the colour is a function of the wavelength.
At approximately 4,000 Angstrom
light has a violet colour, which, as the wavelength increases, turns to
green, to yellow, and then to red, at approximately 7,000 Angstrom. When
a source moves towards or away from an observer, the emitted light behaves
as acoustic waves. It is well known that when a train is approaching, its
whistle becomes acute, because the waves reach us at intervals that get
smaller as the source becomes nearer. Vice versa, the tone becomes lower
as the train moves away. This is the so-called "Doppler effect". Likewise,
when a source of light moves towards the observer, we can say that the
number of oscillation of the electromagnetic wave per time unit increases,
so the wavelength decreases and it is said that light shifts towards the
blue ("blueshift").
If, on the contrary, the source of light moves away from the observer,
it seems that the wavelength increases and the shift towards the red ("redshift")
occurs. The shift is proportional to the speed of the light source.
The redshift is calculated through some easily identifiable spectral
lines by measuring the difference between their wavelength and the
one they would have if they had been emitted by a source at rest, the
latter being unknown.
By measuring the speed of the galaxies through their redshift, and their distance, Hubble stated that they are moving away from us with a speed that is proportional to their distance, according to what is known today as "Hubble's law":
where V is the speed at which the galaxy moves, d is its distance and Ho is Hubble's constant. The Universe, therefore, is subject to an expansion motion.
This fact gives the impression that the Earth is the centre of a general recession motion, while in fact it does not have a centre. Think of dots drawn on a balloon that is blown; they move away from each other with a speed that is proportional to their distance: each dot can be considered as the centre of the expansion. Likewise, we are not the centre of the expansion of the Universe, but one of the dots: an other observer, from a point on an other galaxy, would see the same things that we see. A fatal blow to man's pride...
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The deep sky explored by Hubble Space Telescope. We can distinguish hundreds of young galaxies of various types. (HST) |
The ascertainment that the Universe expands raised a new question: that
of its birth. The fact that the galaxies are moving away from each other
implies that, if they moved backwards with the same speed, after a few
billion years they would be reunited, and all the matter that forms the
Universe would form a very dense and hot agglomerate.
This consideration lead to the evolutive theory called the "Big Bang",
that is a huge original explosion which gave rise to the Universe and caused
its expansion that we can still observe today. According to such theory,
the primordial Universe was made of very dense and hot matter, concentrated
in an infinitesimal space. Its physical state was so extreme that it is
hard even to imagine it; only theoretical physics is able to describe it.
Later, it exploded and expanded, thus becoming less hot and dense, until
it gradually achieved the aspect that we know today.
We deduce from Hubble's law that the Universe was born 15-20 billion
years ago; actually, the determination of its age is a lot more complex
and represents one of the main problems modern cosmology has to deal with.
The value of Hubble's constant accepted at present is between 50
and 100 Km/sec per Megaparsec;
in other words, the galaxies move with a speed that increases by 50-100
Km/sec for each Megaparsec from us.
Abbot G. Lemaitre was the first, in 1927, to propose the scenario of
an original explosion, but it was only in the early forties that the physicist
G. Gamow studied the problem more quantitatively. He postulated that the
light atomic nuclei (Hydrogen, Helium, Deuterium
and Lithium) were formed in the early instants of the life of the Universe.
It was later verified that the quantities of such elements present in the
Universe corresponded to those expected according to the theory, thus confirming
its validity.
An other confirmation came in 1965 with the casual discovery of a weak
radiation that fills the whole Universe, coming from all directions. It
has a maximum of intensity at the wavelength of 2.6 cm and is called cosmic
background radiation. Scientists think it is the remainder of the very
intense and highly energetic radiation produced after the Big Bang.
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A cluster of galaxies at the distance of 9 billion light years. The image covers a distance of two million light years. (HST) |
The history of the Universe from the Big Bang to
the formation of new galaxies
The Big Bang theory allows to explain a number of observations and is
therefore considered a reliable working hypothesis; even though it still
presents a few problems and it was questioned several times, there are
no alternative theories at present.
But what happened in the early phases of the life of the Universe?
According to physicists, the Universe did not begin at the instant
zero, but at a time called "Planck's time", 10-43 seconds
after the Big Bang. Before such instant, it was completely inaccessible,
because all its matter and its energy were so much concentrated that they
constituted a "singularity": an extreme state, in which the space/time
of Relativity has no sense, and that is not part of physics as we know
it.
At the time of Planck, the Universe was very hot (T=1032
degrees) and it had the size of 10-33 cm.
Later the first particles were formed, the quarks,
from which the neutrons and then the protons originated, with the relative antiparticles.
Matter and antimatter, in fact, were both always present in the Universe.
After 10-23 seconds, the Universe was still very small,
having the size of a proton. From this moment to 10-6 seconds
after the Big Bang, protons and antiprotons
annihilated each other, in other words they merged together, transforming
their entire masses (m) into electromagnetic energy (E), according to Einstein's
equation E=mc2. Subsequently, electrons and antielectrons appeared,
and they annihilated each other as well.
These annihilations produced a huge amount of energy, in the form of
electromagnetic radiation. The Universe was dominated by the radiation
and therefore this period is called "radiative age".
At the age of 1 minute the first atomic nuclei were formed (Deuterium,
Helium and Lithium): the temperature of the Universe had gone below 10
billion degrees, so that the left protons and neutrons began to collide
less violently, and to trigger the first nuclear
fusion reactions.
After a few thousand years, the Universe was no longer dominated by
the radiation, but by matter, which was still immersed in a very intense
and energetic radiation. The temperature was still very high and therefore
matter and energy were coupled, in other words they continuously transformed
into the other.
It was only after 300,000 years from the Big Bang that the temperature
further decreased, and they de-coupled. From that moment on the Universe
became transparent to radiation.
In the meantime, the electrons bound to the nuclei to form the atoms.
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Young galaxies. In the first box you can see a small region in the Sculptor constellation. In the second, a picture of the farthest cluster of galaxies revealed by Hubble Space Telescope (its distance is 12 billion light years and it contains 14 galaxies). In the third box, the farthest galaxy ever observed, its distance is 12 billion light years and its redshift is 3.33. Only quasars can be observed at longer distances. (HST) |
After a few hundred million years, the temperature was below 4,000 degrees;
the electrons combined with the nuclei: matter became mostly electrically
neutral and its interaction with the radiation became less frequent. Therefore,
matter began to aggregate and later the first protogalaxies were formed:
gigantic clouds of very cold gas (-220 oC) which gave rise to
the galaxies by gravitational collapse, in the following billion years.
Approximately 2-3 billion years after the Big Bang, the galaxies began
to gather into clusters, and after 4 billion years the first stars were
formed.
In the meantime the Universe had expanded and had cooled down,
the radiation was less energetic, in other words it had shifted to higher
wavelengths: the Universe was beginning to assume the aspect that we know
today.
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This sequence of pictures of distant galaxies provides a view of the possible evolution of elliptic and spiral galaxies. (HST) |
The cosmological horizon and the inflationary model
Since the speed of light is finite, even though it is very high, what
we receive today from very distant galaxies actually left millions or billions
years ago, and therefore gives us an image of how these galaxies were millions
of billions years before, during the early phases of their lives. The more
distant an object in space, the "younger" we see it.
The nearest galaxy to us, Andromeda, is "only" two million light years
far, but with today's instruments it is possible to observe galaxies and
quasars that are even 13 billion light years far, that is very young.
As we already know, the longer the distance of a galaxy, the faster
it moves away from us. Since the speed of the movement of a galaxy is measured
through the redshift
(shift towards the red) of its spectrum, the very far galaxies are called
"high redshift galaxies".
Their observation is extremely interesting to the Cosmologists, since
it can provide information on the early billion years of the Universe after
the Big Bang. Instruments such as Hubble Space Telescope and the giant
telescopes on the Earth were built for this reason. The latter have diameters
that measure 8-10 metres and are equipped with particular optics than
can correct the deformations of the pictures due to the atmospheric disturbance.
These instruments will be able to carry out deeper and deeper observations
of space, that is back in the past.
Not all the Universe, anyhow, is accessible to our observations, independently of the power of the astronomical instruments: for example, if we observe a galaxy at the distance of 10 billion light years, we can only see it as it was 10 billion years ago, but not as it was, let's say, 8 billion years ago: the light emitted in that moment will reach us in 2 billion year's time. In other words, for each instant there are sectors in space and time (or space-time) that are inaccessible to us, just as part of our past is inaccessible to distant galaxies. This is the so-called "cosmological horizon", that is the sector of space-time that is accessible to us. We cannot have any information on whatever is outside such horizon.
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These objects, at a distance of 11 billion light years from us, would be the "buds" of the actual galaxies. Each one of them is an agglomerate of a few thousand stars. Scientists think that galaxies were formed from the collision and coalescence of objects of this kind. (HST) |
The cosmological horizon constituted a problem for the theory of Big
Bang.
If two objects in space are able to communicate through a "signal"
(mechanical or luminous), we say that they are in casual contact, meaning
that one can cause an effect on the other, as a consequence of the signal
it sends to it (for example a mechanical perturbation, or a luminous irradiation).
The signals travel in space with a finite speed, therefore the effects
of the signal emitted by one object on the other, will be felt after some
time, which is proportional to the distance. The region of space-time
in which a body can have a cause-effect relationship, is called "casual
horizon".
Where is the problem? Even if there are condensations of galaxies and
clusters, and relatively "empty" regions, the Universe appears homogenous
and isotrope (that is it has the same properties in the various points
of space and in the various directions). Even regions of the Universe very
far from each other, each being outside the casual horizon of the other,
seem to have similar properties. Not even light, the signal that travel
faster, could have put them into a cause-effect contact. Then, how could
they exchange the information that allowed them to "agree" on similar properties?
In the early eighties, Alan Guth proposed a modification of the classical
Big Bang model, the so-called "inflationary model". According to the
model, in the first instants after the Big Bang, and precisely after 10-35
seconds, the Universe underwent a very rapid expansion, called "inflation",
which, in 10-32 seconds increased its size by a 1050 factor.
After this phase, evolution carried on according to the classical Big Bang
theory.
Before the inflationary phase, the Universe was so small that the
galaxies that now are out of their respective casual horizons could have
a cause-effect contact. The problem of the horizon is thus resolved, as
other problems of the classical Big Bang theory.
Which was the cause of the inflationary phenomenon? According to Guth,
it could be found among the recent theories of physics, which try to unify
the four fundamental
interactions: the electromagnetic, gravitational, weak and strong nuclear
forces. These four forces should be different manifestations of the same
interaction.
At the very high temperatures and densities of the first instants of
the life of the Universe, they were one thing; in time they would have
differed, as the Universe expanded and cooled down. It is during this diversification
process that inflation occurred.
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