Bizarre Large Scale Structure of the Universe

Learning about the Large Scale Structure of the Universe is just having a bigger perspective of this mysterious cosmos we thrive in.

While the ‘just’ we just mentioned accompanies a large uncertainty and complexity.

The Universe as we know it is humongous, and it’s really tough to elaborate on its vastness, it expands to around 93 billion light-years across.

How big is that? The galaxy that houses our solar system, i.e., the Milky Way galaxy, has a span of 100,000 light-years.

This vastness causes the Universe to have a wide range of large structures at different levels.

From microscopic to macroscopic, all the way from atoms and molecules to the planets, stars, stellar clusters, galaxies, walls, filaments, and voids, it’s very diverse.

Let us have a broad view of the Universe from God’s eyes, i.e., the large-scale structure of the Universe.

When we look at the Universe’s large-scale structure, we see a uniform sponge-like formation all around.

Although it seems uniform from a bigger perspective, as we try to zoom in, we can see two main features of the structure, which we call galaxy filaments and cosmic voids.

Before we go further on exploring these filaments and voids, let us understand how and why this bigger perspective is required.

History of our Understanding of the Cosmos

As kids, we always had this question: the Universe we live in, what does it look like from the outside?

Is it dark and shiny dots all the way around? We were unable to grasp how big this Universe is.

This question existed from the very since humans started observing the world around them, they were and are very curious about what the big picture of the world looks like.

Journey of Flat to Round Earth

Before 300 BC, people believed that the Universe just comprises a flat Earth at the center sitting on the back of some elephants standing on a turtle’s back with the Sun, moon, and stars revolving around it.

But later, Aristotle understood and realized that the Earth was indeed round.

After a few more centuries, Nicholaus Copernicus came up with a more accurate model, which stated that Earth is not the center of the Universe and that Earth itself revolves around the massive Sun. 

Fact – Greek philosophers were the first to realize and accept the round shape of the Earth and its motion around the Sun. It turns out that the ancient Indian civilizations already knew this fact long ago.

Ancient ‘Sanskrit‘ text known as ‘Rig Veda‘ mentioned that Earth and the other planets were round and revolved around the Sun. ‘Rig Veda’ was written during 1500-1000 BCE by Rishi Ved Vyas.

Galileo was the first to observe outer space with his telescope, it marked the birth of modern astronomy.

Since then, many astronomers have been observing the heavens extensively, which rapidly expanded our understanding of the actual structure of the Universe.

With time, telescopes were improved, and we could now see farther and farther away with the best possible resolution.

In 1610, Galileo found that the streak of lights we usually see across the night sky is actually an extensive collection of stars; It was named the Milkyway Galaxy.

Later in 1755, Immanuel Kant found that the force of gravity binds stars in the Milky Way galaxy in a similar way the Earth is bound to the Sun.

At that time, the Milky Way galaxy was the known scale of the Universe.

Discovery of fellow Galaxies & Universe’s Expansion

In 1923, Edwin Hubble discovered that the spiral nebulas reported by previous astronomers were an entire galaxy.

They were just too far that they looked like small chunks of gases and dust, the actual vastness of the Universe was understood after the discovery of other galaxies.

Later, he analyzed different galaxies and observed their redshift in the spectra to find their respective velocities. 

Eventually, he found that galaxies were drifting apart, which meant that the Universe is expanding.

In 1929, he published his result along with a catalog of 400 different galaxies.

He discovered that Universe was homogeneous, i.e., galaxies were evenly distributed over long distances.

In 1932, the more extensive Shapley-Ames catalog of bright galaxies was published, which concluded in general unevenness in the distribution of galaxies. 

In 1968, Shane and Wirtanen published a new catalog with over a million galaxies.

The map of these galaxies showed that Universe is not entirely uniform but has a foam-like structure containing walls and filaments with long strands of galaxies but space is constituted chiefly of large empty regions with almost no galaxies.

In 1978, Gregory & Thompson mapped the three-dimensional spatial distribution of 238 galaxies around and towards the Coma/Abell 1367 supercluster.

They found the large regions in the foreground at lower redshift (more than 65 million light-years across) with no galaxies, which they termed’ voids.’

What does the Universe look like?

Since the discovery of galaxies and clusters, many astronomers have conducted surveys and improved the map of the Universe. Today it looks like this –

The current structure of the Universe is concerned with the construction of the early Universe, hence, we need to understand why our Universe looks this way to understand its origin.

The large-scale structure formation has mostly to do with forming the galactic cluster and the filaments or tubes.

Initially, astronomers believed that the density of the Universe is pretty constant, and galaxies are distributed evenly but as they gathered more data, this was found to be false.

The simulation below will give you a better idea of how our Universe evolved like this.

Not during any time before have we known such an accurate and compelling view of the cosmos and its constituents.

Our Universe exists in the form of galactic structures, filaments, and voids.

So, this is a clear indication that the early Universe was anisotropic, i.e., its density was not uniform.

It means that there is yet another major factor in the structure of the Universe, and the main culprit for this is suspected to be the unknown Dark Matter.

Why is the Universe the way it is?

Astronomers now believe that initially, there were some small perturbations in the cosmic density field.

Those non-uniformities coupled with a gravitational force led to the accumulation of matter at high-density points and the formation of galaxy clusters.

The formation of the Galaxy filaments cannot be explained by gravity alone, in a Universe with gravity as a dominant factor, galaxy filaments shouldn’t exist.

In the Standard Model of the evolution of the Universe – Lambda ‘λ’ CDM model – Dark Matter accounts for nearly 27% of the mass-energy content of the Universe.

We don’t know what it is exactly, but somehow it dominates the structure of the Universe.

It forms a web-like structure in the Universe along which galaxies form filaments.

We can calculate the amount of Dark matter present at a certain point by measuring the magnitude of the gravitational lensing and comparing it with the total amount of ordinary matter (matter that we can see and interact with atoms).

It turns out that there is more Dark Matter in the Universe than ordinary matter (ordinary matter constitutes only 5% of our Universe).

Large Scale Structures

Galaxy Filaments

Galaxy filaments are massive structures, in fact, they’re the largest known structures in the Universe, consisting of walls of galaxy superclusters that are gravitationally bound together.

These look like massive thread-like formations forming boundaries between large voids and marking galaxy clusters at their intersections in the bigger picture, thus, most of the matter is located here.

Even though the galaxy filaments hold most of the mass of the Universe, they only account for less than 5% of its total volume.

These galaxy filaments further have some sub-types – supercluster complexes, galaxy walls, and galaxy sheets.

Cosmic Voids

Voids may be empty regions of the Universe, but they occupy 95% of the total volume.

These regions can be enormously huge, measuring up to 300 million light-years across (Boötes Void is currently the largest known void), not to mention that matter itself is 99% of empty space. 

The density of the void is always around one atom per cubic meter.

For any region to be accepted as a void, it needs to have 1/10th of the matter density compared to the average distribution.

This nothingness is essential for cosmologists to study because they hold critical information about the baby Universe.

Today, the empty regions we see are believed to be formed due to baryon acoustic oscillations in the Big Bang.

Baryonic acoustic oscillations (BAO) are the vibrations or fluctuations in high-density Baryonic matter (Baryonic matter is “ordinary” and made up of protons, neutrons, and other hadrons).

These fluctuations later expanded into empty regions during the inflation period.

Voids not only explain the properties of the early Universe but also explains the existence of Dark Energy.

Voids are sensitive to cosmological effects, and their evolution results from the expansion of the Universe. Since dark energy is the reason behind this expansion, voids can tell us about its precise amount.

In the Standard Lambda CDM Model of the evolution of the Universe, Dark matter constitutes about 68% of the energy content of the Universe, the abundance of voids already says a lot about the voids. 

Now, we may look at some mysterious structures within as well.

We will see some significant large structures which are the subject of debate for many astronomers, they are – Bootes void, CMB Cold Spot, and the Great Attractor.

Boötes Void Explained

Boötes Void is the largest known empty region in the Universe. It happens to inherit its name from the constellation it is located in – the Boötes constellation.

Its diameter is around 250 million light-years which is just around 0.27% of the diameter of the Universe.

It is so big that if we were at its center, we wouldn’t even know about other galaxies before the 1960s.

Why? Because we didn’t have telescopes powerful enough to look that far away. 

Though the region is not entirely empty as 60 galaxies are found in a tube-like structure passing right through the middle.

A strange fact about these galaxies is that they are 25% more luminous than the Universal average.

It is pretty bizarre and needs to be explained.

Its enormous size is also very questionable because the Standard Cosmological Model of the Universe tells us that the void shouldn’t be this large.

This anomaly is why cosmologists may have to consider a new model of the evolution of the Universe.

Fun fact but not so funny

In a novel by Martin Amis, “Night Train” goes around an astrophysicist’s mysterious death. It is claimed that she committed suicide just because she concluded the meaninglessness of life after realizing the immense size of the Boötes Void.

Cosmic Microwave Background (CMB) Cold Spot

The Cosmic Microwave Background (CMB) Cold Spot is a small(not really but yes) patch of blue spot that can be seen on the Cosmic microwave background map (CMB) but this mere blue spot has more to it.

It’s the coldest region in the Universe we have ever found. Its temperature is about 70 to 140 micro Kelvin cooler than the average CMB temperature, while it fluctuates no more than 18 micro Kelvin anywhere else. 

Some temperature fluctuations in the early Universe are thought to be the culprit of this, many Scientists suggest that the region is a Supervoid.

But if it’s a supervoid, its size would be more than a billion light-years across. If this is the case, then the current model of the Universe would fail. 

In a study conducted in 2015, researchers claimed that there is a void in the direction of the CMB Cold Spot with a diameter of around 1.8 light-years away.

Though in a similar study conducted in 2017, they dismissed the claim.

Great Attractor

Great Attractor is yet another mysterious structure located around 250 million light-years away.

It is at the center of the Laniakea Supercluster and is the biggest concentration of mass within a range of millions of light-years.

It’s so massive that even the Milky Way galaxy is drifting towards it.

The problem with studying the region is that it is hidden behind our galactic disk. Astronomers tried X-Rays to observe the region, but they didn’t find anything that massive.

Even the total number of galaxies observed couldn’t add up to its estimated mass. 

But later, it was found that the Laniakea supercluster itself is moving towards another supercluster behind it. Shapely Supercluster is located 650 million light-years away.

It is so massive that everything inside the sphere of a radius of 1 billion light-years around it is being pulled towards it.

So, there’s a lot for us to uncover.

Recommendations

Alison L. Coil, ‘Large Scale Structure of the Universe‘, Arxiv, 21 June 2012

R. Brent Trully et al., ‘The Laniakea supercluster of galaxies‘, Nature, 4 September 2014