If you have ever watched a starry sky at night from a remote location, far away from any
city and not affected by light pollution, you may have remained astonished by the huge
amount of stars that cover the sky.
In fact, you may have been thinking that the Universe is probably a place very dense of
stars and galaxies.
This is somehow true, but… only in some parts of the Universe.
In fact, while galaxies are quite rich of stars, the space between galaxies is generally
empty.
And sometimes, these intergalactic spaces can reach enormous sizes, forming huge regions
of Universe that are almost completely dark and empty!
These regions are known as “voids” and “supervoids”.
You want to hear more about them?
Watch this video and I will tell you more!
While you watch a spectacular starry sky at night, you may think the Universe is quite
uniform, with stars and galaxies scattered all around with no particular differences
between one region and another.
That is not actually true!
In fact, the structure of the Universe is quite weird.
Let’s start our journey from a small scale, and then we will enlarge our vision to see
what happens at bigger scales.
As you know, stars are grouped into galaxies, which is a concept you are probably familiar
with.
Galaxies contain an enormous number of stars: hundred millions, some of them even hundred
trillions!
In fact, all the stars you see in the sky are part of our galaxy: the Milky Way.
While galaxies are very huge objects, the distance separating them is even larger.
For instance, while the Milky Way has a diameter of about 100,000 light years (remember: one
light year is the distance covered by light in one year), the closest galaxy, Andromeda,
is 2.5 million light years away: it’s about 25 times the size of the Milky Way.
But despite these numbers being already very large, galaxies are still quite close to each
other, in astronomical terms.
In fact, galaxies tend to “gather” into groups, due to the effect of the force of
gravity bringing them together; these large groups are called “clusters”.
The Local Group, the cluster where the Milky Way is located, contains about 100 galaxies.
And the story goes on: these clusters are gathered into even larger structures, known
as “superclusters”, containing thousands of galaxies.
Here it’s where things get pretty big.
Gravity is always the driving force that shapes the Universe, and it keeps playing its role
at this level as well.
In fact, even these superclusters attract each other.
However, at this point they do not gather into “megaclusters” or something like
that, rather into longer, thin structures known as “filaments”.
In fact, if we could look at the Universe at this scale, it would look like a gigantic
spider web (but without that regular geometric pattern!), with these filaments corresponding
to the strings of the net.
So, we now know how the Universe looks like at its largest scale, at least as far as we
know.
These filaments are massive structures, spanning over million light years.
But what’s in the space between the strings of this huge spider web?
Is there anything between these filaments?
That’s where things get more intriguing, and that’s where we reached the main topic
of this video: these spaces between filaments are those called “voids” and sometimes
“supervoids”!
What are they?
To understand what voids and supervoids are, we have to talk about density first.
Density is a physical quantity describing the amount of matter in a fixed volume.
We are all very familiar with this concept: in fact, we are well aware that ice is less
dense than water, or that a copper coin is more dense than the air in a balloon, and
so on.
But what does it mean on a more microscopic way?
If we look at a smaller scale, density tells us how close are atoms and molecules to each
other in an object or a substance.
Higher density means that the atoms and molecules are closer to each other, and so there is
less empty space between them.
On the other hand, lower density means that atoms and molecules are far from each other.
Now we can apply this concept to a larger scale: the Universe.
But in this model, the atoms are replaced by galaxies.
Different regions of the Universe have different densities: while most galaxies are concentrated
in the filaments, voids are almost completely empty.
In fact, the density of these voids is less than one tenth of the average density of the
rest of the Universe.
Therefore, these voids are big regions of space relatively empty, containing very few
galaxies compared to clusters and superclusters.
Some of these voids can even “merge” together forming huge ones, which are sometimes referred
to as “supervoids”.
The biggest known supervoid, the Boötes void, has a diameter of about 330 million light
years: it’s more than 3000 times the size of our galaxy!
It’s no coincidence that it is sometimes called “the Great Nothing”.
But how did these objects form, and why are they so important for the Universe?
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To understand the origin of voids and supervoids, we have to do a little step back in time,
when the Universe was still at its early age.
Just after the Big Bang, the Universe was consisting of a “plasma”, which is often
referred to as “the fourth state of matter” (the other three being solid, liquid, and
gas: you should be familiar with them).
In a plasma, the three fundamental particles (protons, neutrons and electrons) are not
arranged into atoms or molecules, like in the other three states, but they are just
free to move, alone.
We can think it as a “chaotic state”, in which all particles are moving randomly,
without any bond with each other.
At the beginning, the Universe was very hot.
This means that all these particles were moving super fast (in fact, the speed of the particles
is related to the temperature: the higher the temperature, the faster particles move).
Too fast, actually, so they were not able to form atoms yet: in order to form an atom,
protons must be able to “capture” the electrons… but if the electrons are moving
too fast, this won’t happen.
As the Universe kept expanding, it also cooled down, and the temperature got lower.
At the same time, particles started moving slower and slower, and at some point, protons
became able to “catch” electrons, to form the simplest atom in the Universe: hydrogen.
This very important moment is sometimes called “recombination”.
Later, as the temperature of the Universe continued decreasing, protons and electrons
(and neutrons as well) became able to form atoms of heavier elements as well (helium,
carbon, oxygen, and so on).
However, this process didn’t happen uniformly in the whole Universe.
In fact, as atoms started to form, some regions resulted in having more density than others.
And it’s here that gravity started to play a fundamental role in the destiny of the Universe.
In fact, as the first atoms appeared, they started to attract each other due to the force
of gravity.
And as atoms aggregated, they kept attracting more and more.
As these groups of atom grow larger in size, the force of gravity they exerted on the surrounding
areas became stronger, attracting more atoms and becoming larger and larger: so, regions
with higher density grow faster in size.
On the contrary, regions with lower density could exert less attraction (due to weaker
force of gravity), so they were able to gather less atoms.
As this process went on, the Universe became less uniform: regions with higher density
attracted more and more matter, while regions with less density became emptier.
The former are those that we now call clusters and superclusters, while the latter are those
that we now call voids and supervoids!
So, this is how they formed.
Ok, we now have understood the origin of these enormous regions with very few galaxies inside,
and also that because of their low density, they exert very little gravitational attraction.
So, how do they play a role in the Universe?
In reality, these voids and supervoids exert some force… but a totally different kind.
It is, in fact, a repulsive force.
What is it?
Before answering this new question, we need to do another little step back, and talk about
the composition of the Universe.
In fact, the Universe consists of three main “components”: ordinary matter, dark matter
and dark energy.
What are they?
Let’s start with ordinary matter, which is the easiest one.
Ordinary matter includes all the objects that we are familiar with: planets, stars, galaxies,
even black holes… basically, all the things in the Universe that we are somehow able to
detect with our instruments (even those that are too far away to be seen).
While our common sense would suggest that these objects constitute the majority of the
Universe, it may be surprising to hear that ordinary matter actually makes only…
5% of the Universe!
Yes, ordinary matter is just a small fraction of the whole Universe!
Then, what about the rest?
Let’s now move to dark matter.
Dark matter includes all matter that we are not able to “detect”.
Ok, wait… but if we are not able to see it, how can we be sure it’s there?
The thing is, even if we cannot directly observe it, we are able to see the effects it has
on ordinary matter.
In fact, dark matter exerts gravitational attraction as well, so it produces visible
effects on other familiar objects, such as stars and galaxies.
There are indeed many “proofs” of the existence of dark matter.
For example, it turns out that the velocity of rotation of stars in the outermost region
of galaxies is too fast, and it can be explained only if there is “additional matter” inside
the galaxies that we cannot detect.
Another example is the so-called “gravitational lensing”: the gravitational force exerted
by matter is able to “curve” light from distant objects.
In certain regions of space, the amount of ordinary matter is not enough to get the lensing
effect that we observe for some distant objects, so it can only be explained by assuming there
is additional matter in between, which is indeed the dark matter.
Ok but wait, what does this dark matter contain?
The answer is “we don’t know”.
It must be particles that do not emit light nor radiation (because they are “invisible”
to us), but we have only hypothesis so far.
And what is more surprising is, dark matter constitutes about 27% of the all things in
the Universe, which is an important fraction.
Ok, so we are now left with the third and last constituent of the Universe… dark energy.
What is it?
Well, I know it may sound very disappointing, but the answer is… we have no idea.
Yes, that’s right: this thing represents almost 70% of the whole Universe, and we don’t
even know what it is!
The only thing we know is that it is some form of “unknown” energy that permeates
the whole space, and it “pushes” somehow galaxies apart, so it acts as a repulsive
force, the opposite of gravity.
This effect plays a very important role: in fact, the reason for which the expansion of
the Universe is currently accelerating is indeed the presence of dark energy.
And getting back to our main topic, dark energy is strictly related to voids and supervoids.
In fact, we can think voids and supervoids as huge “reservoirs” of dark energy, like
giant bubbles that keep expanding getting larger and larger in size, pushing galaxies
apart, far away from each other.
In a certain way, we can say that these fascinating objects are somehow responsible for “shaping”
the Universe!
If you think about it, it sounds almost like a paradox: regions that are almost empty are
responsible for determining the faith of the Universe… how weird is it?
But in the end, we shouldn’t be too surprised about that: we already knew that the Universe
was a bizarre place!
"And this video ends here!
Thanks for watching everyone!
Did you find voids and supervoids an interesting topic?
Is there anything more you want to discuss about them?
Let me know in the comments below, be sure to subscribe, and I'll see you next time on
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