The most terrifying places in the universe: voids and supervoids
Get in your car and reach a place far from the city lights.
Then get out of the car, take a deep breath and look to the sky.
You can clearly see a lot of stars. This is the first astronomical observation.
You don't need any instruments, you just need to open your eyes and be amazed by the universe.
Don't you feel small and almost...empty? What you can see with your own eyes is just
a tiny fraction of all the stars contained in our Galaxy, the Milky Way. The great majority of the
stars belong to it, you simply can't see them. Our site is, of course, limited.
Then imagine you develop a great passion for astronomy, and you end up buying a telescope.
The number of objects you will see will increase exponentially.
Now you can observe further and further objects, and the more powerful your
telescope, the more details you see. As technology increased during the years,
astronomers reached incredible distances, and as they went further and further with
their telescopes, they catalogued and mapped the objects in our universe.
This is what they did: a description of our cosmos.
They soon realized that, on large scale, the universe is
totally different from the one we are used to. It all starts with the Sun and plenty of stars
in our galaxy, and then they could see some grouping of galaxies, which they called groups
and clusters. The distance between two galaxies is on average of the order of Megaparsecs.
The Milky way, for example, is part of the Local Group, and the Virgo Cluster, which contains a lot
of galaxies -about 1500, for example, Andromeda Galaxy and M87. And the whole Virgo Cluster is the
heart of the so-called Virgo Supercluster. This is the so-called “large scale
structure” of our universe. This waste and enormous structures
are called filaments, and they are the largest known structures in the universe.
However, astronomers noticed that, as you go further, you see also a lot of cosmic voids.
Follow me in this video to get to know what they are and why
they exist. (roll intro)
Cosmic voids are vast spaces that exist between the largest-scale
structures in the universe, the filaments. They contain very few or no galaxies.
This doesn't mean, however, no stars, no planets, no comets.
In fact, to call these structures voids is slightly misleading. Voids are not
entirely empty; rather, they are just incredibly under-populated, usually having about 10 per cent
of the matter that you'd expect to find in an average region of the universe.
Astronomers were amazed when they first faced a void.
The first two papers on the topic of voids in the large-scale structure of the universe
were published in 1978. At that time, astronomers were mapping our universe with great precision.
In 1981, they discovered a large void in the Boötes region of the sky.
It seemed to be one of the most shocking discoveries of the 80s.
At first, astronomers thought that maybe voids could be related to dark matter.
They were not sure if a void was containing dark matter, but recent observations show that halos
of dark matter are not present. No matter, no dark matter.
A void is made of nothing. The most creepy thing about
voids and supervoids is that they extend for something like 100 Million parsecs,
i.e. 100 Megaparsecs. Light would take 325 million years to cross a void region of such dimension.
In the Virgo supercluster, we have a void. It is approximately 60 Megaparsec long,
and our Local Cluster of galaxies lies on the edge of that void.
Also, observation shows that galaxies are kind of repelled by the void.
This is the case of the dwarf galaxy Eso 461-36,
which lies between the void but appears to be moving towards the boundary.
Also, the Milky Way seems to be moving away from the local void, at 260 km per second.
Later on in this video, we will talk more about other important
and well-known voids in our universe. But first, what does physics say about them?
Voids are believed to have been formed by some baryon acoustic oscillations in the Big Bang.
These are, in short terms, collapses of mass followed by implosions
of the compressed baryonic matter. The more likely explanation is that
initially small anisotropies in the early universe grew larger in scale over time:
high-density regions collapsed under gravity, eventually resulting in the
large scale foam-like structure of voids and galaxy filaments we see today.
Cosmologists still have far from clear ideas about how these gaps in the universe can exist.
However, computer simulations suggest that smaller voids,
much more common in the universe, are caused by galaxies approaching each other
due to gravitational pull. This causes the neighbouring regions to empty and,
as the process is self-reinforcing, it tends to form larger and larger voids.
So, if we wanted to be more romantic, we can say that voids come from...imperfections.
Not even the Big Bang was perfect! Since its first steps, the universe
was imperfect. For example, it was not perfectly homogeneous in terms of matter.
These small fluctuations in density, these imperfections, led to filaments and voids
structures we see today. In some places, the matter was denser and in others less. That's all.
The real question is why was that. We are still trying to figure it out, but my first answer
would be, well...why not? Why should we expect the big bang to have been perfectly homogeneous?
Scientists are really trying to answer this kind of question.
Since voids appear to be related to the cosmic microwave background,
astronomers made a huge effort in order to study them.
The existence of voids is also significant in providing physical evidence for dark energy.
In fact, the simultaneous existence of the largest-known voids and galaxy clusters
requires about 70% dark energy in the universe today. Voids are like bubbles in the universe,
sensitive bubbles because they change along with the universe and its expansion. Studying their
changes over a period of time can be then used to constrain cosmological models, based on the idea
that the universe is in an accelerate-expansion phase, due to the presence of dark energy.
Well done. Know we know a lot about voids. Let's see which of them are the best-known ones.
First one. The Boötes void. Discovered by astronomer
Robert Kirshner and his team back in 1981, the Boötes void, sometimes called the Great Void,
is a huge, spherical region of space that contains very few galaxies. It's approximately 700 million
light-years from Earth and is located near the constellation Boötes, which is how it got its
name. The researchers originally published their surprising discovery in their paper,
"A million cubic megaparsec void in Bootes."
It is the largest void known in the universe so far.
Soon after its discovery, astronomers began to notice just how incredibly sparse the area
really was. At first, they were only able to find eight galaxies across the expanse,
but further observations revealed a total of 60 galaxies. Now, while that might still
seem like a lot, it would be like stumbling upon 60 mile-long objects across a region
larger than the continental United States (and that's just in two dimensions). According to
astronomer Greg Aldering, the scale of the void is such that, "If the Milky Way had been in
the centre of the Boötes void, we wouldn't have known there were other galaxies until the 1960s."
Another famous void is the one astronomers found in the Taurus constellation.
The Taurus Void is circular, about 30 Mpc across, and adjacent to the Perseus-Pisces Supercluster.
A few galaxies have been found inside it including UGC 2627 and UGC 2629 (left) which
are approximately 185 million light-years away; see image to the left (no larger version)
In April 2015, a team of cosmologists at the University of Hawaii found strong evidence
in support of the existence of the largest structure ever observed in the universe,
which turns out to be nothing at all. The results of the cosmological survey,
published in Monthly Notices of the Royal Astronomical Society, pointed to the existence
of the Eridanus supervoid, a span of nothingness 1.8 billion light-years across located about
3 billion light-years from the Milky Way. But here comes the question: we know that
general relativity works amazingly in our universe, at small scales,
and also at larger scales. But...could it be that it fails at very large scales?
"Before finding out the answer to this question, be sure to like or dislike the video so that we
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Well, the answer is: we'll see!
Cosmologists describe the universe by means of some crucial parameters, such as the
Hubble constant or the energy density of matter. The Hubble constant, for example, is defined as
the ratio between the expansion rate over time of the scalar radius of curvature of our universe.
These parameters are strictly bounded with the geometry of our universe,
so its curvature and everything you can think about, as Einstein taught us.
So, in a sense, different results for the Hubble constant evaluated at our time, imply a different
rate of expansion of our universe. It also affects our concept of distance.
A change in the Hubble constant will squeeze or enlarge our universe!
Instead, the acceleration parameter is telling us something about the acceleration of our universe.
It is strictly correlated with the mass density parameter of our universe, because, intuitively, a
universe with more mass “would accelerate less”. If it can help, imagine having a small rock
and a big rock. You want to push
them away from you at a distance of one meter. You'll need to exert a stronger force in order to
move the bigger one because it is more massive. These parameters are really really fragile because
they relate dynamical properties of the universe with geometrical ones. A higher Hubble constant
not only means the expansion rate is higher, but it also has some effects on distances.
And when it comes to voids, we have to face a serious problem!
Because inside of voids, these parameters seem to change drastically. Our understanding of the
universe stops for a while, and we are forced to re-think not only the physic of our universe
but also its geometry, its own structure. Voids (and also supervoids) really seem to
adhere to some cosmological parameters which differ from those of the known universe.
This is why testing models for voids can be performed with very high accuracy, and thus
we can expect some new exciting news coming from the study of such creepy places in our universe.
Another important thing to note is that some theories relate the presence
of cosmic supervoid with the cosmic background radiation map.
In the cosmic background radiation map, some areas appear warmer, while others are colder.
(
The largest and coldest areas are also called cold spots.
The CMB map is the light coming from the edges of the observable Universe, about 13 billion
light-years away, and allows us to see what the cosmos was like a few years after the Big Bang.
When light passes through a void, it takes millions of years to escape,
since the void is enormous in length. Furthermore, the expansion of the universe
over time makes this passage even longer. Since voids are cold zones, the light exists
in the vacuum colder and less charged with energy. Voids almost act as refrigerators.
You could maybe store some beer in a void and see what happens!
According to these hypotheses, some areas of the map of the cosmic background appear colder to us.
But it is just because photons are passing through these voids, and
not because the CMB itself gets suddenly colder. Those are not the coldest areas
of the cosmic background, but the coldest light that reaches Earth!
"This video ends here! Thanks for watching everyone! What do you think about voids
and supervoids? Do you think they will play a crucial role in our understanding of the universe?
Let us know in the comments below! See you next time on the channel!
C