THE NATURE OF DARK MATTER EXPLAINED Hi, it's Mike here.Our life is full of
mysteries, unexplained events and problems that we’ll probably never manage to solve.
Usually, strange events and legends are connected with night and darkness, since it’s the period
of the day in which nothing is clear and the sunlight, that lights up everything, is absent.
Historically, darkness is linked with horror stories, with the unknown,
which is maybe the most scaring thing ever. Well, now it may be obvious why scientist
decided to use the definition “dark matter” for a strange substance that permeates the
entire universe but is almost invisible, impossible to detect through usual methods.
This mysterious matter cannot be seen with telescopic observations and it
might explain several things about the rotation of remote galaxies.
But what is the nature of dark matter? When was it firstly discovered? And
how? What are the consequences of its existence? When did dark matter formed?
These are just some of the questions we’ll try to answer, so, if you are interested,
stick with me and prepare to an epic journey through the nature of dark matter.
(roll intro) When talking about dark matter
we are dealing with one of the most important problems modern physicists are trying to solve.
To understand what dark matter is, we firstly need to give a proper definition of matter.
Matter is any substance with a mass. Everything that we can touch around us has its own mass,
since it’s made by molecules, that are composed by atoms. However, in physics we must get deeper into
this definition to understand why dark matter is something that is totally different from what we
are used to know. In the last two centuries, we have discovered that atoms are not indivisible,
but they are composed by smaller particles. Most of you surely knows that they are made of protons,
neutrons and electrons; but physicists have discovered something more. They understood that
protons and neutrons are made of smaller particles called quarks. What a shocking result! And why do
they have a different charge and behavior? This is a brilliant question and here is the
answer. Quarks are characterized by a spin can have six different “flavors” (they differ for
electrical charge): up, down, charm, strange, top and bottom. We have found out that the most
common ones are the first two and they combine in different ways to form the particles of our atoms:
the proton is made of two quarks up and one quark down, whereas the neutron is composed of two
quarks down and one quark up. Quarks and electrons are some of the so-called fundamental particles.
Everything in our world is made out of these tiny objects. More precisely, we define as hadrons all
particles composed of quarks, and if the number of these last ones is odd, we call them baryons.
We’ve reached the answer to the previous question:
what we commonly call as matter is everything that is made of baryons.
So what is dark matter? Probably, dark matter is not baryonic and the reason why it’s really
difficult to detect is that it doesn’t interact with electromagnetic radiation. What does it
mean? Well, differently from baryonic matter, it doesn’t absorb, reflect or emit light or other
types of electromagnetic radiation like X and UV rays or microwaves. You can easily understand
that it’s almost impossible to detect something in the night sky if it doesn’t emit any type of
signal or light detectable from our telescopes. Ok, we have a brief idea of what is dark matter,
but why is it so important for physicists? Why do they consider a problem not knowing this
particular substance? Well, through several studies it has been shown that dark matter
accounts for nearly the 85% of the mass of our Universe. A pretty impressing number! And there’s
more! In the beginning of the 20th centuries Einstein proved that mass can be converted into
energy through the world-wide known law E=mc2. So, if we want to make an estimate of the “energy
density” of our universe, that takes count of both matter and energy in the form of electromagnetic
radiation or other interactions, dark matter is almost a quarter of the total energy density.
“I know that I know nothing” is one of the most famous sentences attributed to the
Greek philosopher Socratis. Well, it perfectly describes the condition of modern scientists:
we have understood that we know almost nothing about the 85% of the matter in our universe!
This is pretty common in Astronomy: the most we study, the more we discover to be ignorant and far
from achieving a complete knowledge of our world. But when dark matter was discovered? And how?
We need to start from the 19th century when Lord Kelvin, the one who enounced the main
principles of thermodynamics, made and estimate of the mass of our galaxy and he found out that
it was lower than the number of stars that can be seen in the night sky. Lord Kelvin
thus concluded "many of our stars, perhaps a great majority of them, may be dark bodies".
In 1906 Henri Poincaré in "The Milky Way and Theory of Gases" used "dark matter", or "matière
obscure" in French, in discussing Kelvin's work. Similar inferences were made in the beginning
of 20th century by Jacobus Kaptein and other astronomers such as Jan Oort and Fritz Zwicky.
They respectively studied the motion of different stars in the local galactic neighborhood and
various galaxy clusters. Unfortunately, the results of their studies turned to be wrong and
incorrect by at least one order of magnitude. Although the numbers they found were totally
useless, they contributed to underline that there must be a hidden mass in our Universe.
But wait a moment, how did they manage to calculate the mass of some parts of our galaxy?
To answer this question, we need to get back to Kepler and Newton in the 17th century.
The first one stated that in their orbit around the Sun planets cover equal areas in equal amounts
of time, that means that when they are closer to the Sun they have a higher velocity. Additionally,
Newton put numbers to these assumptions formulating the classical theory of gravitation,
which states that the force between two objects with mass is inversely proportional to the square
of their distances. By using this principle, he proved that the velocity of an orbiting body
must decrease when the object gets more distant. This is true when talking about
the Solar system, but even when dealing with other objects like galaxy clusters.
What is the importance of this relation? If we are able to measure the velocity
of a star and its distance from the main source of gravitational attraction in the surrounding,
we can easily derive the mass of the heavy object that attracts the celestial body.
It seems pretty simple, but there’s something that you must ask me: how is it possible to measure the
velocity of a celestial rotating body? This is a brilliant question and one can easily say that
it’s impossible to measure the speed of a star since they are nearly static in the night sky.
We must wait our entire life to observe directly the movement of star in the sky!
However, physicists have found a trick to solve this problem.
You’ll probably know that light is just one particular type of electromagnetic radiation and
that it differs from other waves like x rays or infrared rays just due to its frequency. Usually
waves have a strange behavior when their source is moving respectively to the observer. Have you
ever listened to the siren of an ambulance? When it’s reaching us the sound gets a sharper tone,
whereas it fades away as the ambulance disappears far from us. In physics this phenomenon is called
Doppler effect. When the source is getting closer to the observer the frequency of the wave becomes
higher and higher. Viceversa, when it goes away the frequency rapidly decreases. The same effect
can be observed with electromagnetic waves. How can we detect this phenomenon? Usually,
scientists analyze the light of celestial body by dividing its colors in a spectrum
(a catalogue of the intensity of light in different frequencies). The spectrum
is characterized by lines corresponding to the elements that compose the star and its atmosphere.
How does the color of these lines change if its frequency grows? It shifts towards
blue. Opposed to that, if the frequency decreases the color results to be more red.
Consequently, if a star is receding from us, we observe a “redshift” in the spectrum.
So, we have understood that we can measure the velocity
of a star thanks to the “redshift” or “blueshift” observed in its spectrum.
This technique was deeply used by Vera Rubin in the 60’s and in the
70’s to study the relation between velocity and distance from the center of spiral galaxies.
What do spiral galaxy are? Like our Milky Way, they are young galaxies that consist of a flat,
rotating disk containing stars, gas and dust, and a central concentration of stars known as
the bulge. These are often surrounded by a much fainter halo of stars. Spiral galaxies are named
by their spiral structures that extend from the center into the galactic disc. The spiral arms
are sites of ongoing star formation and are brighter than the surrounding disc.
Vera Rubin made a plot of the velocity of stars in dependence of their distance from the center and
she found out that, opposed to Newton’s law, their speed increased with the radius and in
the best cases it flattened becoming constant. What an extraordinary result! The only way to
reproduce those plots was to add an invisible and incredibly big amount of matter in the
surroundings of the spiral galaxies. It was estimated that this “dark matter”, that was
not visible through telescopes but strongly contributed to the gravitational attraction,
must be at least six times the visible matter of the galaxy.
The difference between experiment and theory was too high, it was a solid
claim of the existence of dark matter. From that moment, more and more research
works confirmed Rubin’s results strengthening the hypothesis of an invisible matter that contributes
to the gravity of galaxy. But what is dark matter?
And what is it composed of? Physicists found out that dark
matter can’t be made of baryonic matter (protons and neutrons gathered with electrons in atoms).
One possible candidate is a strange particle called axion,
never observed in laboratories, which resembles the behavior of dark matter
since it doesn’t interact with common matter and doesn’t emit radiation.
And how can we verify if this is the solution to the following problem:
what is dark matter? Here’s the technique used by scientists.
They studied the Bullet cluster, which is the result of the collision of other two clusters.
What does it happen when two galaxies hit with each other?
Almost nothing, since stars and objects are two distant from each other and they don’t collide.
But in the case of the Bullet cluster both the galaxies have a lot of gas among stars.
This gas usually interacts in the collision decreasing its velocity and emitting X rays.
As a matter of fact, by observing the area using the Chandra X ray observatory devices
astronomers reconstructed the images of the two collided clusters and they found almost in the
middle of them an interesting cluster of emitting gas. Further measurements have been made on the
gravitational effects of this gas: as a matter of fact, if dark matter is made of axions they
must be hidden in the surrounding of the gas. How is it possible to measure the gravity of these
clusters of gas? We can use the incredible results found by Einstein: he discovered
that gravity alters the geometry of space, like a heavy ball on a towel. This deformation of space
affects even light that goes on a straight line on a curved space. What is the result of this theory?
Sometimes heavy objects deform the images of other bodies around them like a lens. In fact, this
phenomenon is called gravitational microlensing. By observing the images of galaxies and stars,
physicists have found that the gravitational distortion created by the gas cluster in the
Bullet cluster implies the existence of some invisible matter around the gas.
This extraordinary result is another proof of the existence of dark matter.
But in the end what is dark matter? Even if this research work shows that probably axions
are the constituent of dark matter, we can’t state rigorously what the nature of this substance is.
However, we are ready to undergo more and more observations to found out something
else about one of the most fascinating topics of modern physics: the nature of dark matter.
"Are you amazed by the nature of dark matter? Did you already know the
existence of a non-baryonic component of the density of the universe? Drop a line below “