The Archaeology of the Universe
Paolo de Bernardis
Dipartimento di Fisica, Università La Sapienza, Roma
Today I am going to talk to you about how the most remote past of the universe
is studied with scientific methods: what we can understand in this way and what
enigmas still remain unsolved today.
Cosmic distances are enormous: light takes days, years, thousands of years,
billions of years to reach us after starting from a source, from a star, from a galaxy.
Hence the most far-away sources appear to us as they were a very long time ago,
when the light left. And so we think we can study the remote past of the universe
simply by looking at the most far-away sources available to us. In short, by looking
far away, we are looking back into time. In this sense, our telescopes are marvellous
time machines that enable us to study the universe’s past.
When they do such work, astrophysicists understand that the universe evolves,
that it has gone through phases that are different from today’s, but I will talk about
this later. In order to start at the beginning of astronomy, astrophysics and
cosmology, I have to talk about Galileo Galilei.
Galileo was the one to get the archaeology of the universe started, and for two
reasons. First of all, Galileo was convinced that light takes time to propagate, that is,
it has a finite speed, not infinite; and this is precisely where the possibility of looking
at things in their past states comes from – if they are far enough away. In the second
place, Galileo was the first to point his telescope towards the heavens and analyse
what he saw with modern mathematical, scientific methods, drawing conclusions
about it, even though it might have been contrary to common sense and the common
philosophy at the time.
In “Dialogues Concerning Two New Sciences”, Galileo tells of an experiment
he did to measure the speed of light, and he admitted that the attempt failed, because
he did not succeed in measuring the delay from the departure of light to its arrival.
That does not mean that light spreads instantly, but simply that his means were not
adequate to measure it. He thus concluded that, by using the results of this attempt,
that the speed of light is at least 10 times higher than the speed of sound.
Galileo’s experiment was very simple. He and one of his collaborators stood on
two hills that were about one mile apart from one another; Galileo uncovered his
lantern and started his chronometer; his collaborator, when he saw the light coming
from Galileo’s lantern, uncovered his. When Galileo saw the light come back from
his collaborator’s lantern, he stopped the chronometer. This was simply a recipient
filled up with water from the tap: Galileo, when he uncovered the lantern, also
opened the tap; when he saw the light come back, he closed the tap. From the
quantity of water that came out of the recipient, he could estimate how much time
had gone by. It is clear that, given the very high speed of light and hence the very
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brief time involved, with this chronometer the experiment was destined to fail.
Galileo, however, was convinced that light took time to spread.
Today, students of physics repeat this experiment in laboratories with modern
means, and they manage to measure light taking about 200 billionths of a second to
reach a mirror 30 metres away and come back.
Galileo did not fail, instead, in developing the telescope. He put together
ordinary components, lenses to correct defective vision, something already
widespread 200-300 years before his birth. Galileo’s chief merit, however, is that of
having applied to celestial bodies – which he saw very well with the telescope for the
first time – the same laws of physics that he had experimented with in the laboratory.
Galileo’s telescope was a very simple object, according to modern standards,
but his measurements had enormous conceptual significance. A biconvex lens was
involved (which cures myopia) and a biconcave lens (to cure presbyopia) placed at a
certain distance. They were little lenses, hence an instrument that was extremely dim,
but they had the capacity of enlarging the image observed. These lenses were already
known long before, but Galileo, who studied nature with mathematical methods,
discovered how to put two lenses together in order to obtain the best enlargement
possible and he succeeded in magnifying as many as 20 times. The name “telescope”
was coined by Federico Cesi, Galileo’s friend and founder of the Accademia dei
Lincei, who named it “the instrument that could see far away” in 1611.
Thanks to his telescope, Galileo succeeded in magnifying what we see as far
away, hence the instrument had terrestrial applications that were very important for
commerce and war. He showed it to the Doge in Venice, and received a raise; he thus
got more time to dedicate to his research and finally pointed the telescope towards the
heavens.
He first observed the Moon, and saw that it was not a metaphysical object:
actually, on the Moon, there are valleys and mountains, and these make shadows. It is
impossible to make these observations with the naked eye. With the telescope,
instead, the images were very clear and Galileo was not afraid of saying something
new, because he was sure of his instrument and of his observations. He had repeated
them and knew that whoever might have made the same observations with a similar
instrument would see the same things.
He then turned the telescope towards the Milky Way, which appeared as a
light, widespread blotch to the naked eye and in the dark sky of that time. Finally,
thanks to his telescope, Galileo discovered that radiated light was really composed of
an infinite number of tiny luminous sources, an infinite number of stars.
He then turned the telescope towards Venus, and discovered that the morning
star has phases like the Moon: that means that Venus revolves around the Sun!
Galileo also observed Jupiter, and discovered the little luminous blotches (the moons
that he then called the Medicei) that revolve around Jupiter. He then realized that
everything does not revolve around the Earth, because something revolved around
Jupiter and Venus revolved around the Sun. From this he was convinced that the
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Earth, too, revolved around the sun: the Earth was no longer the centre of the
universe.
The fact that the satellites of Jupiter rotate around Jupiter implies that any
object with a great mass attracts surrounding masses and makes them orbit around
itself. This consideration is preliminary to Newton’s great discovery of universal
gravitation, which took place in the following century.
Jupiter’s moons also served to measure the speed of light. Many years after
Galileo, by using the delay with which the satellites entered into Jupiter’s shadow
when they are farther away from the Earth, Roemer succeeded in ascertaining that the
speed of light was around 200,000 kilometres a second: an enormous speed –
incorrect, but not too much: the order of magnitude was already the right one in 1675.
The year Galileo died, Newton, another great physicist, was born: his are the
laws of dynamics and universal gravitation. Newton also had a cosmological
conception: he thought, as a physicist, that if gravitation is truly universal, then the
stars must also feel attraction among themselves. But the stars form a limited system,
and in attracting each other, everything would fall into the centre of the system. This
means that the stars must be arranged without limits in an infinite universe. This is
what Newton thought at the end of the seventeenth century.
In the meantime, scholars understood that to look far away, increasingly bigger
lenses and telescopes had to be used. Newton himself made a contribution by
inventing a telescope that used mirrors (the Newtonian telescope), thus making it
possible to construct even bigger telescopes. Cassegrain and Gregory invented even
more compact schemes with telescopes using two mirrors. In the eighteenth century,
Herschel even succeeded in building a telescope that was 1.20 metres in diameter,
thanks to which he ascertained that the galaxy is a limited system of stars, but that
other systems of stars exist that are farther away from our galaxy. In the meantime,
between eighteenth and nineteenth century, the speed of light was measured with
greater and greater precision, thus leading to the conclusion that it was around
300,000 kilometres per second, or about 1 billion kilometres per hour.
In 1887, Camille Flammarion wrote a book entitled “L'univers antérieur” (The
Previous Universe), in which he explicitly described the extraordinary possibility that
astronomers have of looking at the past of the universe by exploiting the fact that
light employs so a long time in order to cross enormous cosmic distances.
Today we know that the light coming from the Sun takes 8 minutes to reach us,
that is, it was emitted 8 minutes earlier: in short, we see the Sun as it was 8 minutes
before the instant in which we observe it. But for the stars, these times are much
longer. The light of the nearest star takes 4 years to get to us, while that of a relatively
close galaxy, Andromeda, left 2 million years before the moment in which we
observe it. The farthest galaxies that we know emitted the light that we receive today
several billion years ago: we say that they are billions of light years away.
A question comes up spontaneously: if we are looking back in time when
always looking far away, can we then look far enough away to see the beginning of
the universe (admitting of course that there was a beginning)?
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Today we can answer that it is possible, but it is not easy.
First of all, we must consider the fact that the universe does not stand still
(Newton, instead, thought that space was rigid and seized up). On the contrary, it is
expanding, and has not always been the same. This means that, in looking at previous
eras, we are observing a universe that is very different from what we see around us.
In order to do this, then, we need special technical means: optical telescopes are not
enough – we need infrared or millimetre telescopes, which have only recently been
developed.
And this is how we arrive at experiments like BOOMERanG, thanks to which
– for the first time – it has been possible to make a map of the universe in the most
ancient phase in which it can be observed. It is not a map of the Big Bang – no one
can do that, because before the era which we have mapped BOOMERanG, the
universe was opaque; it was not transparent. However, we can make a map of the
universe as it was about 300,000 years after the Big Bang, that is, about 13.7 billion
years ago. The BOOMERanG experiment has been confirmed by NASA’s WMAP
satellite and by other independent experiments. Hence, it is a question of physical
measurements, confirmed independently by various observatories: this is why they
have been accepted by the scientific community as physical reality.
In order to observe very faraway distances, we must study what is around us in
the universe. First of all, there are the stars, which are relatively close and similar to
the Sun. Our galaxy is formed by hundreds of billions of these stars, but it is not the
entire universe. A galaxy is an enormous cluster of stars about 100,000 light years
large; galaxies are then grouped in even bigger clusters of galaxies (for example,
COMA and Virgo), which form a complex, spongy structure that fills up the universe
as far as the farthest distances that we can imagine.
On map 2dF of the positions of galaxies in space, each blue point indicates a
galaxy. On the map, there are about 200,000 galaxies for which we know the exact
position, but they do not represent the entire universe. In the farthest regions, we
cannot see anything at all: that does not mean that in that point there are no more
galaxies, but that these are so far away that our telescopes cannot observe them well
enough to measure how far away they are.
We might think that the spongy structure made of galaxies fills up the universe,
because – at least as far as we can observe it with very big telescopes – we do not see
any change. The future of such research, then, is in large telescopes. In Arizona the
Italian large binocular telescope was recently inaugurated, composed of two 8-metre
mirrors that will certainly enable us to observe galaxies that are even farther away.
The next generation space telescope (JWST) will have a large mirror that is about 10
metres in diameter.
However, if we really want to look at the remote past of the universe, this is not the
way to do so, because the universe is expanding. In the 1920s, Carl Wirtz and Edwin
Hubble discovered that all of the faraway galaxies that they observed with their
telescopes appeared to be redder than those that were closer. Hubble’s explanation
was simple enough: this phenomenon depended on the fact that the colour of the light
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that we observe – to which our eyes are sensitive – is really a manifestation of the
wavelength of light. Blue light is given off by a wave of about 0.4 millionths of a
metre, red light by waves of 0.7 millionths of a metre of wavelengths. All of these
undulating phenomena depend on the speed with which the source of these waves
moves – the so-called Doppler effect. In everyday life, when an ambulance comes
towards us, we hear a rather high sound that becomes lower as the ambulance passes
us. Actually, the sound of the siren always has the same frequency and wave length,
but the source moves; hence, the sound waves are compressed when the ambulance
comes toward us and they expand when the ambulance goes away. Thus, the length
of the sound wave depends on the state of movement, and we can think that all of the
galaxies that Hubble and Wirtz saw in the sky were moving away from us, making
the radiation observed redder. Moreover, Hubble and Wirtz noted that the farther
away the galaxies were, the redder they were.
A simplistic interpretation of this discovery is that we are at the centre of the
universe, and that all galaxies are moving away from us. There is a more democratic
interpretation: let’s suppose that all of the galaxies are moving away from each other
(not only from our galaxy, but in all distances), that is, that they are expanding in the
same way. The simplest example to imagine this process is to think of the traditional
Italian holiday bread, the panettone, rising. Let’s imagine putting 20 centimetres of
panettone dough with raisins in it in the oven. In 2 hours, it will have doubled in
bulk; that means that a raisin that in the beginning was 5 centimetres from our
reference raisin will be 10 centimetres away after 2 hours. Thus, it moved 5
centimetres in 2 hours (2 and a half centimetres per hour). The raisin moves away
from the reference raisin at a speed of 2 and a half centimetres per hour. But another
raisin, which initially was 10 centimetres away from our reference raisin, is 20
centimetres away after baking, and hence in the same 2 hours, has moved 10
centimetres (5 centimetres per hour): twice the distance, corresponding to twice the
speed. If we substitute the raisins with galaxies, and the panettone dough with space
– you have an idea of what is meant by space that expands uniformly, with all of the
distances expanding by the same factor. In that space, Hubble’s law is valid, and
twice the distance corresponds to twice the speed of moving away. The
farthest
galaxies, which interest us because in observing them we probe the remote past of the
universe, move away very rapidly, and so their wavelengths will be greatly elongated.
Let’s start with normal, visible wavelengths, hence yellow, green or blue, that then
become red; if we look at galaxies that are even farther away, the wavelengths will
always be longer and will end up in the infrared and then in the millimetric
wavebands, and so will no longer be visible to our eyes, and we will have to use
special instruments that are sensitive to infrared wavelengths.
Actually, thanks to Einstein’s theory of general relativity, we now know that
what makes wavelengths longer is not a true Doppler effect, but the so-called cosmic
redshift: Einstein’s equations foresaw that an infinitely extended, homogeneous
means expands or contracts, and that all lengths within it (including the wavelengths
of photons) expand or contract by the same factor: the scale factor.
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Recently, a galaxy in which ultraviolet light was redshifted nearly 10 times was
observed, hence the wavelength was elongated by about 10 times. According to
Einstein, this would mean that when the light left, all of the distances in the universe
were about 10 times smaller than they are today.
This galaxy, which appears to be minuscule in the astronomical image, is
extremely important, because its wavelength is very long and, as the farthest galaxy
that we know, is a record.
Hard as we might try, and using instruments that are sensitive to increasingly
longer wavelengths, at a certain point galaxies that are farther away will not be seen.
In effect, the most distant galaxies that we will manage to see are different from those
that are closer: they are smaller, irregular, and do not have beautiful elliptical or
spiral forms, but rather take on strange forms.
This makes us think that galaxies have not always existed in the universe; in
short, there was a time when they did not exist – they were formed only later. We
can thus say that we do not see galaxies that are farther away than that “record”
galaxy simply because before that, galaxies did not exist yet. But the matter that they
were then made up of must have existed, and so we expect to see not the primordial
universe made of galaxies like the ones we see today, but a much simpler primordial
universe made of homogeneous matter.
At this point the idea that expansion is always accompanied by cooling comes
into play. If you open a cylinder of compressed gas, when it goes out, the gas expands
and cools off, to the point that at the same time you might see that the tap of the
cylinder freezes.
An analogous phenomenon must have happened with the universe. We have
seen that the universe is expanding, and hence must have been hotter in the past. In a
rather remote past, we might expect to see a period in which the universe was hot like
the Sun: that is the limit of our knowledge. When we look at the Sun, we see a sphere
of incandescent gas, with zones that are a little hotter or a little colder, but we cannot
see inside the Sun, because the incandescent gas is opaque, and does not let light
spread inside it because of the free charges it continuously gives off.
We might expect that the primordial universe looks like incandescent gas to us,
with slightly hotter and slightly colder zones. The light departs from this incandescent
gas, as part of the surface of the Sun. The Sun is close to us, that is why its light takes
8 minutes to get to us, while this incandescent gas is very far away, and so the light
takes about 14 billion years to get to us, and in the meantime its wavelength gets
longer until it becomes microwave radiation. This idea was put forward by George
Gamow, a Russian physicist who emigrated to the United States, in the theory of the
hot Big Bang: the primordial universe must have been very hot; thermonuclear fusion
must have taken place, and from it light must have come, electromagnetic radiation.
This light, which is the oldest light that we can observe, was lengthened by
expansion until it became microwaves. In 1965, Arno Penzias and Robert Wilson
discovered that, wherever they aimed their microwave antennas, they observed
microwave radiation coming from the sky. Actually, they could not interpret their
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observations (they even tried to clean the antenna!), but no matter where they aimed
their antenna, this radiation continued to be there.
The physicists of the Princeton group instead were the ones who were looking
for that radiation because they understood the message of George Gamow. Having
learned of the results of Penzias’s and Wilson’s observations, they finally arrived at
the right interpretation: what the antennae perceived were microwaves spread by
matter present in the primordial universe. Thus one of the paradoxes that had
challenged astrophysicists since Kepler’s time was solved. At the time scientists
thought of a universe that was uniformly filled up with stars, and so wherever one
looked, sooner or later he or she would have to see a star and hence the sky must be
luminous at night, too. But that is not how things are: and this causes a paradox,
called Olbers’ paradox. Actually, wherever we look in the sky, we can observe
microwave radiation from the primordial universe, because wherever we turn our
gaze, if we look far enough away, we can see a phase in which the universe was a
incandescent gas. The paradox has been solved.
For 25 years, after the discovery of the cosmic microwave background in 1965,
scientists continued to measure it, until arriving at a definitive measurement in 1992 –
that of NASA’s COBE satellite, for which J. Mather was awarded the Nobel Prize in
2006. Thus this energy depends on frequency in a very precise way: the black body
radiation curve (Planck’s curve), which concerns all emissions of matter in thermal
equilibrium, like the incandescent gas must have been in the primordial universe.
This measurement has very small margins of error (one part in 10,000): it is one of
the most precise measurements in the whole of astrophysics.
Thus Gamow’s theory was definitively confirmed: the primordial universe was
effectively incandescent and produced the very radiation that we might expect to be
emitted by an incandescent gas.
The next challenge was to understand what image this radiation from the
primordial universe should have. It is very difficult to answer this question, first of all
because these microwaves are in themselves already very weak: it is a black body at 3
degrees Kelvin, equal to one hundredth of the emission of the Earth and objects at
ambient temperature (300 degrees Kelvin).
After Penzias and Wilson, scientists tried to take more and more precise
measurements, but the image remained as uniform as a photograph without contrast –
grey, the same everywhere. They began to establish that it was equal everywhere
within 10 per cent, then within 1 per cent and finally within 1 per thousand; at the end
of the 1980s, uniformity on the order of one part per 10,000 was established.
With another experiment on board the COBE satellite, DMR, scientists
succeeded in visualizing slight variations in intensity of the cosmic background called
anisotropies at a level of about 10 parts per million. An image of this type has
extremely poor contrast, but with resolution and sensitivity enough to manage to see
great blotches that are slightly lighter and darker in the image of the primordial
universe.
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We would like to do more – we would like to obtain greater detail in the image.
In fact, reasoning as Gamow did, we find that the time that passed from the Big Bang
(a state of infinite density and temperature) to the moment when the universe cooled
off by a few thousands of degrees and then became transparent is about 400,000
years. Thus regions that at that time were more distant than 400,000 years did not
have enough time to interact among themselves, and hence did not even have enough
time to equalize their temperatures. And so we should expect that their emission of
microwaves is a little different. But a distance of 400,000 light years, seen from a
distance of 14 billion light years, and taking the expansion of this universe into
account, subtends an angle of about one degree. In order to see this lack of
uniformity, then, a bigger telescope must be built so that better resolution than one
degree can be obtained.
In his painting “Starry Night”, Van Gogh painted the stars as globes with onedegree diameter; these stars, seen with COBE resolution, which is about 7 degrees,
disappear, they are completely invisible. Hence the image of that little NASA
satellite, although so important, was not clear enough to see more important
structures. In order to obtain a clearer image, a true telescope for microwaves is
necessary, and this is what we have tried to construct with the Boomerang
experiment. In creating a true telescope for the cosmic background, we could thus get
better resolution by one degree, and hence it would be possible to observe all of the
details of the primordial universe: this was the challenge at the end of the 1990s.
In order to carry out something of this order, thanks to international
collaboration led by the Università di Roma La Sapienza, with very important
contributions by American, English and Canadian colleagues, BOOMERanG, a true
microwave telescope, was built. There are many parts that make up this telescope: a
mirror of 1.20 metres, constructed in Italy; very sensitive microwave detectors, built
at Caltech, are cooled off at very low temperatures to make them extremely sensitive.
The instrument that cools off the detectors at only 3 tenths of a degree above absolute
zero was built in Rome and Frascati: with this system, the thermal agitation that
prevents detectors from being sensitive is frozen and so photons can be measured.
It was then necessary to move the experiment above the earth’s atmosphere,
because this is not transparent to microwaves. Thus it was decided to launch the
experiment from the Antarctic, with a big helium balloon of about 1 million cubic
metres, taking it to an altitude of 40 kilometres. At that altitude we could collect all of
the necessary measurements without disturbances. Obviously this is an automatic
experiment: on board there are computers that programme the measurements, take
them and then transmit the data back to earth. The Antarctic was chosen because
uninhabited (hence the experiment could fly over it without causing any danger) and
because from there it is possible to observe the areas of the heavens that are less
contaminated by our galaxy: in that zone our galaxy is transparent and does not
interfere with the measurements; moreover, in observing such regions during the
Antarctic summer, the Sun is always behind us, and so we can take these
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measurements without being disturbed by the very strong emissions of microwaves
from the Sun.
The instrument was powered by great solar panels and protected from radiation
from the earth and the sun by great reflecting panels. It was taken to an altitude of 40
kilometres, where it scanned the sky for 11 days, and in the end created a map of the
primordial universe. On Ross Island, in the Antarctic, there is a laboratory built of
wood, in which we assembled the instrument that was previously “packaged” in Italy:
in 2-3 months of work we managed to assemble it, calibrate it and finally launch it.
The international collaboration really must be emphasized, because we would
never have been able to finish this experiment with our forces alone. A group of
about 40 people, spread over four different nations, decided to work together, and
with relatively scarce funds, managed to build an experiment that yielded important
results.
After the launch, the experiment moved slowly, at the mercy of the jet streams
of the stratosphere, and in 11 days it came back more or less to the area it left from; it
landed with a parachute, and then the data and instruments were retrieved. While we
were working, a nice little penguin came to visit us: it was an unusual episode, but we
were pleased.
With this experiment, we obtained a map of the primordial universe, where
slight variations in temperature, regions that were slightly colder or slightly hotter
could be found. In any event, I am talking about variations of about 100 millionth of
a degree, and so they are truly very small fluctuations, but they do have a very
particular form, such that the results have been published in important international
reviews. The particularity comes from the fact that the dimension of most of these
blotches is of about 1 degree.
Let’s try to understand why this dimension was found. We see incandescent
gas as it was about 300,000 years after the Big Bang: this is how long it takes the
universe to cool off enough to become neutral, after the phase in which it was
incandescent gas. That means that two points that were more than 300,000 light years
apart would not have had a way to interact (among themselves); in short, the force
did not have the time to act from one point to another that was more than 300,000
light years away. This means that zones of the universe that at the time were more
than 300,000 light years apart must be slightly different. We see these zones from a
distance of 14 billion light years: if we set up a ratio between 300,000 and 14 billion
and multiply by a another factor of 1,000 (because in the meantime the universe has
expanded 1,000 times), we will find the very angle subtended by these zones by
about 1 degree.
We have put forward the hypothesis that, from the two extremes of this region
of 300,000 light years, light travelled to us in straight lines, but this is not always true
in the universe, where instead it often happens that light curves its trajectory. This is
one of the predictions and first proof of Einstein’s general relativity: light, in the
presence of great masses, curves its trajectory.
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This has been observed when light, departing from the same source (for
example, a very distant Quasar), takes two or more different routes to get to us,
passing above and below mass (a galaxy or cluster of galaxies) between us and the
source. The mass curved the surrounding space, deflecting the light that passes
nearby. In this case, multiple images from the same source can be observed.
General relativity predicts that even on a large scale (like that of the cosmic
background, which runs through the entire observable universe before arriving at our
microwave telescopes) light can take curved trajectories if space is curved by the total
density of mass and energy present in the Universe.
In short, if we emit two rays of parallel light, they continue to be parallel, or
they converge or diverge (according to whether the space is Euclidean, or if it is
positively or negatively curved). To measure the degree of curvature of the universe
would hence enable us to measure the total mass-energy density. If we consider two
rays of light coming from the opposite extremes of a great blotch of 400,000 light
years, in the primordial universe, travelling on straight lines, they will arrive at the
observer, forming an angle of 1 degree. But if the space is curved positively, the rays
will converge towards the observer, forming an angle that is greater than one degree.
The blotch will seem to be bigger. If the space is negatively curved, the blotch will
seem to be smaller.
Before the Boomerang measurements, we did not know if the universe had Euclidean
geometry or curved geometry because of its mass-energy. Fourier’s analysis (a very
powerful mathematical procedure) of the map of the primordial universe measured by
BOOMERanG made it possible to establish how many blotches are of 1 degree, as
well as how many of them were less than one degree or one third of a degree. The
conclusion was that the majority of the blotches were of dimensions of 1 degree.
Hence the geometry of the universe is really Euclid’s, and the rays of light of the
cosmic background really have spread in straight lines for 14 billion years. Euclid
was right, on a cosmological level, too.
We found a new result, but now we must answer two further, important
questions. First of all, if in the universe we need so much mass-energy (the right
amount for space to be flat, Euclidean), what is it made of? We know the stars,
galaxies, clusters of galaxies, the gas between the stars – but if we count all that mass,
its density is equivalent to about 4 per cent of that which would be necessary to send
out light rays along straight lines. In short, if in the universe there were only mass
that we measure directly, then the rays of light must converge on curved lines
towards the observer.
It follows that in the universe there is other mass or other energy, however, that
we have not experimented with in the laboratory. We have an indirect confirmation
of their presence, because mass and energy are also lacking in the universe to account
for the movements of the stars in the galaxies, and of the galaxies in the clusters of
galaxies. Thus we know that a “dark matter” must exist, and that it has a mass and
hence must act gravitationally, but it does not interact with light, because we do not
see it. Particles of this sort have never been observed in the laboratory. One of the
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great hopes of the LHC experiment at CERN in Geneva, which should be operative in
2008, is precisely that of being able to measure these elusive particles, foreseen in
super-symmetric extensions of the standard model of elementary particles. Hence,
when you hear people talking about dark matter and dark energy (and at this point
they are talked about often in scientific monthlies and in scientific broadcasts), be
aware of the fact that they have never been measured in the laboratory, and that there
is only indirect evidence of their presence.
The new frontier of research in physics and cosmology is to understand what
this dark energy and this dark matter can be. Our standard model of the physics of
elementary particles does not succeed in furnishing an explanation in this sense, and
so it is necessary to take a quality leap, to formulate a new physical theory that can
explain for example how the fluctuations in density that we observe in the
Boomerang map occur, starting with a field of primordial energy, that then is
expanded from microscopic to macroscopic, until it forms the incandescent universe
that we see.
In the meantime, new experiments have been done. The WMAP satellite has
made a map of the entire heavens in microwaves and has fully confirmed the results
that we got with Boomerang. In 2008, a new satellite, Planck, will be launched, and it
will go as far as 1 million and a half kilometres from Earth in order to be able to
make even more precise observations of cosmic background radiation.
Moreover, attempts are being made to study not only how much energy arrives
from the cosmic background, but also what the directions of polarization of
electromagnetic waves are, because these could help us to penetrate the incandescent
gas and get to the first instants after the Big Bang. In fact, what we expect is that the
electromagnetic waves spread throughout the primordial universe are polarized, a
little like blue light of the sun diffused by the earth’s atmosphere is polarized.
In order to do these studies, we have launched Boomerang again, after having
equipped it with new detectors that are sensitive to polarization. In this way, not only
has a map of energy been made, a map of the directions of the polarization of
electromagnetic waves has, too. In this complex design of the directions of the
polarization, the answer to one of the enigmas of today’s cosmology is hidden. In
fact, there is a fascinating theory, the theory of cosmic inflation, that links the
microscopic world with the cosmological world. Supposedly, a few instants after the
Big Bang, the universe went through a phase of very rapid accelerated expansion, and
this led microscopic volume of sub-nuclear dimensions to become bigger than the
entire observable universe today. This event, which happened to energies that we can
never create in our earthly laboratories, make it possible to explain the Euclidean
geometry of the Universe (whatever pre-existing curvature would be “straightened
out” and flattened by the enormous expansion), and the nature of the fluctuations of
primordial densities that we see as anisotropies of the cosmic microwave background
(that would be produced by the quantistic fluctuations present before inflation). In
effect, all of the data brought together so far confirms this theory. A further
prediction of the inflationary theory is the production of gravitational waves during
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the very rapid expansion. These would produce particular symmetries in the
directions of polarization in the cosmic microwave background that can be observed
with extremely precise experiments. New flight of BOOMERanG is only a step in the
long journey dedicated to these measurements, which will probably be complete with
an experiment on a satellite: an international proposal called B-Pol has been
presented by the European Space Agency and to the Italian Space Agency, and it
contains a series of experiments that are supposed to cross technologies necessary for
this very difficult measurement little by little. Observations from the earth will also
contribute to this study; they will be carried out in the coldest and driest places on the
globe – like the BRAIN experiment, from the Antarctic, or the ClOVER experiment,
from the Atacama Desert.
Our group is also preparing other measures for the physical study of the
primordial universe: in observing particular clusters of galaxies and the “shadow”
they cast on the cosmic microwave background, the OLIMPO experiment – again
with a balloon – will make it possible to throw light on the nature of dark matter, by
observing products of decay.
In conclusion, we have discovered much by using physical methods to study
cosmology. But there is still much more to understand by synergetically using
laboratory experiments and cosmological observations on the ultimate nature of our
Universe.
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