DARK MATTER
Have you ever wondered what exactly is the universe made of?
According to the cosmological model, our universe was born 13.7 billions years ago,
by a huge explosion that gave birth to our space like we know it nowadays:
the Big Bang. Everything that exists was born in that immense explosion.
Science development led us to more and more sophisticated and accurate technology to
investigate the cosmos, and little by little astronomers and physicists have learned a lot
about the universe, discovering stars, stardust, planets, satellites, asteroids, galaxies, nebulae,
clusters of galaxies and so on...Until recently we thought to be well advanced in the comprehension
of matter distribution around the universe, nevertheless, starting from the last century,
observations suggest that there is far more to discover!
As a matter of fact, all that exists in the universe seems to be much more than what we
can detect. The mass we can see and measure is only a minimum fraction of the whole universe.
Yes, because the larger amount of matter is invisible and undetectable by our instruments.
It's like an immense halo that permeates the cosmos without emitting, absorbing and reflecting
nor light neither electromagnetic wave. This is the dark side of the universe.
This is DARK MATTER.
Now physicists assess that only the 15% of the matter in the universe is made of detectable
ordinary mass. The overwhelming majority is made up of something that we can't detect directly,
so scientists called it "dark matter”. The name dark is due to the fact that it doesn't shine,
it doesn't absorb, it doesn't reflect light or any other electromagnetic radiation,
we detect its presence by indirect effects only. It’s like a phantom,
we can't see it, it's completely invisible.
Wait, what? How is that possible?
Curious to know how? Stick with us and we will tell you how are things in this video!
what is dark matter? what exactly is made of?
where can we find it? how can we know that
it exists and how we measure it? How is it arranged in the universe?
How much dark matter?
To answer to all these questions we need to step back
so, let's rewind a bit the tape and go back in time.
STORY OF DARK MATTER: the 1st evidence in ROTATIONAL VELOCITIES
Since the end of 19th to the beginning of 20th century,
many scientists were interested in so-called "obscure stars
and dark gases", little luminous matter that couldn't be identified with telescopes.
In 1877 Italian priest Angelo Secchi, expert in stellar spectroscopy,
in a paper, asked himself questions about the presence of scattered dark masses in space.
In 1933 the swiss physicist Fritz Zwicky was trying to understand the behaviour of clusters of
galaxies. In particular, while observing a cluster of a thousand galaxies in the "Coma Berenices",
150 million light-years far from here, he found some anomalies in their motion. In fact,
they were rotating with a speed about 100 time faster than what the theory expected,
as if the total mass of the cluster was about 400 times higher than it was.
In other words, the gravity effect of the cluster was too low for such fast orbits.
Therefore Zwicky argued that there had to be much more gravity and more mass than what appeared
to keep together these galaxies with those high rotational speeds.
The swiss was the first to use the term " dark matter", to express
that lack of matter: " if these observations were confirmed, we will get the surprising
result that dark matter is present in larger amount than luminous matter", he said.
Unfortunately, Zwicky wasn't very popular within astronomer community, because of his
short-tempered behaviour. Therefore, at his time, his idea was considered absurd and set aside.
6 years later the astronomer Horace Badcock observed other anomalies in the motion of stars
in the Andromeda galaxy: as a matter of fact, their rotation speeds,
while proceeding towards the outskirts, didn't slow down, in contrast to gravitational theory.
Nevertheless, so far, this was considered an issue due to a mistake in the process of measurement,
because of technical limitation. It was essentially believed that the
missing mass was made of faint stars, planets, meteor, scattered gas in interstellar spaces.
Several decades passed and the thing came back to the surface thanks to the work in 1974
of some Princeton cosmologists who claim that there are good reasons
to believe that the mass of galaxies had been underestimated by a factor 10 or more.
The turning point occurred at the end of the '70s, when american astronomer Vera Rubin was working on
the motion of spiral galaxies and assessed the same thing that Zwicky claimed 40 years before.
Rubin and her colleague Ford had studied and measured the rotation curves
of about 20 galaxies and had found that moving away from the center of mass, velocities didn't
decrease. According to those data, galaxies couldn't stay in balance, they should have
flaked off with the outer stars departing for the tangent. There had to be much more mass in there!
To understand this concept and explain all these conclusions, we need to do a short digression
about Newton's gravitational and Kepler's laws. Kepler's laws are deducted directly by Newton's
gravitation. All rotational systems that can be modeled as a heavy central mass with some
smaller and lighter objects orbiting around it, can be investigate by applying Keplero's Laws.
The rotational speed only depends on the distance from the central mass and on the matter amount in
the center, and DO NOT depend on their masses. It is therefore expected that, proceeding
towards the outskirts, the rotational speed decrease. if this doesn't occur,
the obvious way to resolve this discrepancy is to conclude that the mass amount is higher
and postulate the presence of non-luminous matter (dark matter) to balance the system.
Scientific community was still skeptical about dark matter,
so was fundamental to find another method, using another independent data set, to compare results.
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2nd evidence of dark matter: THE GRAVITATIONAL LENSE EFFECT
Nowadays we have another instrument to assess the amount of dark matter in the galaxies.
According to Einstein's general relativity, if we interpose a mass between a distant
electromagnetic source and an object that we're observing,
since the mass curves space-time, this mass will act as a lens to bend the light from this source.
The more massive an object, the more lensing is observed. By measuring the distortion geometry,
the mass of the object can be deduced. If the gravitational lens mass is a galaxy, so we
can get its mass by studying images with the gravitational lens effect. The mass of galaxies
calculated with gravitational lense method, is in reasonable agreement with Galaxy rotation curves:
the amount of dark matter assessed was 10 even 100 time the amount of ordinary matter.
Therefore, the universe appeared made up of dark matter with some little amounts of ordinary
matter. This was the outstanding conclusions that originally led scientists to deny its presence.
Today we also know that the distribution of dark matter is quite bizzare:
it isn't uniform as well. What most intrigues is the "halo arrangement" around galaxies. If we move
from the center towards the outskirts, ordinary matter decreases whereas dark matter increases.
the same goes to clusters of galaxies. for example in the milky way, the amount
of ordinary and dark matter are comparable, by contrast outside dark becomes dominant.
Universe is like made up of bubbles of dark matter in which galaxies swim like ducklings.
3rd evidence of the presence of dark matter: THE COSMIC BACKGROUND RADIATION (CMB)
According to cosmology we all know that the universe is born 13.7 billion years ago
from a huge explosion: the “Big Bang”. Nowadays we have detected a landmark evidence of the Big
Bang origin of the universe: it's about the cosmic background radiation (CMB). This was discovered
for the first time in 1964, by physicists Arno Penzias and Robert Wilson during radio astronomy
and satellite communication experiments. This is an electromagnetic radiation permeating uniformly
the universe, as a remnant from an early stage of the universe. It is the oldest electromagnetic
radiation in the universe that's why it's an important source of data. According to COBE
satellite , sent into orbit from 1988 to 1993 to measure CMB around the universe, cosmic background
radiation is not constant throughout universe, but fluctuates slightly, in fact there are colder
and warmer areas. These differences are called anisotropies. But why are we talking about CMB?
How does CMB is related with dark matter? It's easy to say. According to the analysis of this
radiation it was possible to have a precise reconstruction of the mass composition of
the universe. In fact, if we draw a graph of the fluctuation of cosmic radiation,
this provides us a data set on dark matter. From the curve we obtain, in fact,
we deduce that the mass of the universe cannot be made of ordinary matter only, otherwise we
would find a different graph. The result of the fluctuation of the background radiation
tells us that there is a huge amount of matter that doesn't interact with electromagnetic rays
and therefore we can neither see, nor detect it directly. According to the numbers. after
gravitational lense effects and the spiral galaxies rotational curves, also this alternative
approach, shows that dark matter represents about the 80 % of the total mass of the universe.
What is dark matter indeed? what is it made of?
Over the course of time many hypotheses have been formulated.
At first, this missing mass was thought to belong to invisible stars and gases that could, however,
emit radiation. As a matter of fact, there is a lot of mass of this type around the universe,
but its amount is far lower the missing mass and is not enough to explain dark matter.
Then the focus was on MACHO (MAssive, Compact, Halo, Objects) PARTICLES,
non emitting heavy objects like black holes, neutron stars, white and brown dwarfs. After
years of observations astronomers assessed their effective presence but their total mass,
deducted using their gravitational lense effects, is lower again,
because would represent 15% at most, of what we would expect as dark matter.
All these failure led to the formulation of non-ordinary matter theories to justify
dark matter. The most popular is WIMP (Weakly Interactive Massive Particles), theory.
WIMP particle are massive particle that would interact very weakly.
According to the theory, WIMP, have still been found in the early stages of formation of the
universe, would interact with ordinary mass annihilating, mantaining a kind
of balance. At a certain moment, however, because of the rapid expansion of space,
these interactions would stop, and by that time, dark matter particles
has remained unchanged until today
To detect WIMP, if they exist, we have different ways:
-we can try to reveal them in our particles accelerators like LHC of Cern in Switzerland,
hoping to observe the energy released when dark matter, in particular conditions
interacts with ordinary matter -alternatively we can build "ad hoc" detectors,
sheltered from energy interferences and cosmic particles, like the “Xenon 1ton” experiment at
the Gran Sasso Laboratory in Italy or the Superkamiokande detector in Japan
-eventually, we can always hope to detect some WIMPS interaction events
with ordinary matter, directly in the cosmos, by means of our radio telescopes.
So far, it should be noted that no WIMP has been detected by any of these huge experiments.
CONCLUSIONS Physicists now believe
that dark matter is approximately the 85 % of the whole mass in the universe and represents about a
quarter of its total mass–energy density. These proportions are quite outstanding
and flip our point of view on the universe: Matter, as we all know it, represents a negligible
and almost ridiculous quantity, of the whole mass in the universe. On the one hand more and
more exciting discoveries are yet to come but on the other hand it's so shocking and
upsetting that all we can see is just a midst in a immensity that we still cannot understand.
You can say everything about dark matter, except that it goes unnoticed!
"This video ends here! Thanks for watching everyone! Did you find dark matter exciting?
Is there something more you want to know about dark matter and its relationship with
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