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Exploring the Cosmic Web: The Next Frontier in Astronomy

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Chapter 1: The ELT's Cosmic Journey

The Atacama Desert, known as the most desolate region on Earth, might seem an improbable location for a construction project. Yet, nestled on a remote mountain, an extraordinary structure is emerging from the arid landscape. This is the Extremely Large Telescope (ELT), poised to be the most formidable instrument ever created for observing the cosmos.

Aerial view of the Extremely Large Telescope site in Atacama Desert

Located near the ELT, the Paranal Observatory is surrounded by a vast, barren expanse. Credit: J.L. Dauvergne & G. Hüdepohl (atacamaphoto.com)/ESO

The telescope’s capabilities are astonishing. For the first time, we will have the ability to directly observe planets orbiting other stars and identify individual stars in distant galaxies. Astronomers will be able to scrutinize the edges of the Universe, gazing billions of years into the past to witness the formation of galaxies and the emergence of the first stars. Paradoxically, this powerful instrument will also uncover the largest structures in the Universe, which have remained largely elusive until now.

Historically, astronomers lacked an understanding of the vastness of the cosmos. While the night sky may seem crowded—especially in a remote location like the Atacama Desert—most visible stars are merely a few thousand light years away. If you’re fortunate, you might glimpse Andromeda, a faint blur in the northern sky, marking the furthest object visible to the naked eye.

Despite its distance of 2.5 million light years, which appears unfathomably far, it’s essential to remember that the Universe spans at least 90 billion light years or more. What we perceive with our eyes is only a minuscule fragment of the immense expanse of creation.

Andromeda is not only the farthest celestial body we can see; it is also the largest. While it is likely slightly bigger than our Milky Way, a few dozen other galaxies, including the spectacular Magellanic Clouds in the southern hemisphere, are visible from Earth. However, Andromeda remains the most significant galaxy within a few million light years.

Two main factors hinder our ability to observe larger objects. The first is the structure of the Milky Way itself. Our solar system is positioned on one side of the galaxy, approximately 25,000 light years from the center. The galaxy's core—an area dense with stars and cosmic dust—appears as a silvery band across the night sky, obstructing our view of what lies beyond.

The second factor is our placement within larger cosmic structures. Just as we cannot discern the shape of our own galaxy—though astronomers have determined it possesses a spiral form—we cannot visualize the configuration of the larger galaxy group to which we belong. Nevertheless, these grand structures do exist, even if they remain largely hidden from our view.

All galaxies observable with the naked eye belong to the Local Group. While Andromeda and the Milky Way are its dominant members, the group contains at least 80 smaller galaxies, many of which interact with our own through orbits or even collisions.

In a billion years, astronomers predict that the Milky Way and Andromeda will merge into a single galaxy. Over even more extended periods, the Local Group is expected to coalesce into a massive galaxy. This process of galactic collisions and mergers, while it may sound violent, is generally not as destructive as one might imagine—stars rarely collide, and such interactions often stimulate bursts of star formation across the galaxies.

The Local Group, along with other galaxy groups, isn't easy to visualize, even with advanced telescopes. The initial signs of their existence only emerged in the late 1950s when astronomers mapped the night sky in detail. George Abell, an American astronomer, observed that galaxies did not appear uniformly distributed but instead clustered together, creating groups surrounded by vast voids.

Visualization of galaxy clusters surrounding voids

Some of the galaxy clusters closest to our own. Image credit: ESO.

Further examinations unveiled even larger structures. The groups of galaxies were organized into superclusters, with the Local Group identified as part of the Virgo cluster. This cluster, in turn, is part of an even larger structure—known as the Laniakea Supercluster—which encompasses over 100,000 galaxies and countless trillions of stars.

When astronomers mapped Laniakea in 2004, they discovered that our galaxy resides on the outer edges of this supercluster. They also found that Laniakea is gradually disintegrating, lacking the gravitational strength to maintain its integrity. Over time, the clusters and galaxies will drift apart, eventually reforming into new superclusters.

On a grand scale, these structures may merely be temporary formations visible to astronomers. Their existence suggests the presence of their counterparts—vast voids where matter is scarce. These empty regions stretch for hundreds of millions of light years, seemingly repelling matter.

Galaxies unfortunate enough to reside within a void tend to accelerate away at high speeds. Gravity pulls them toward superclusters, which are rich in matter. This gravitational attraction is relentless—any matter not bound to a cluster, like a galaxy lost in a void, will rush toward the nearest conglomeration of galaxies.

The clusters and superclusters create a network throughout the cosmos, merging into even larger formations. These structures manifest as sheets, walls, and filaments within the Universe. When astronomers map the cosmos on such an extensive scale, these massive formations create a web-like pattern known as the Cosmic Web.

As astronomers recognized the existence of the Cosmic Web, they began identifying the structures that compose it. One notable structure, dubbed the Great Attractor, is exerting a tremendous pull on our supercluster, seemingly momentarily overcoming the Universe's expansion. Other colossal formations—like the Great Wall, spanning 750 million light years, and the Perseus-Pegasus Filament, measuring over a billion light years—are among the largest known entities in existence.

However, the discovery of these structures has raised questions. How did such immense formations arise? Was gravity the sole force shaping the Universe’s structure? What role does dark matter play in this arrangement?

To address these queries, astronomers turned to computer simulations. Some projects, like the Illustris Project, aimed to replicate the entire history of the Universe. These simulations yielded insights into the Universe's formation.

Simulating the entire cosmos necessitates the use of supercomputers. The Illustris Project utilized two powerful machines located in Germany and France, generating hundreds of terabytes of data. In 2014, after sifting through this data, astronomers found that the model accurately reflected crucial aspects of the real Universe. Galaxies and clusters emerged in the simulation in a manner consistent with reality.

The model indicated that dark matter plays a vital role in the formation of galaxies and the Cosmic Web. According to the simulation, shortly after the Big Bang, dark matter began to collect into threads stretching across the Universe. Over time, aided by gravity, these threads coalesced into denser strings.

Where dark matter accumulated, normal matter also gravitated toward these areas. These threads ultimately served as the seeds for galaxies, distributing them along the filaments and walls observable in the Cosmic Web today. The regions between these threads remained largely empty; without dark matter, regular matter would be too sparsely distributed to ignite stars or galaxies. These voids we observe now are the remnants of those empty spaces.

Although the precise nature of dark matter remains a mystery, models like the Illustris Project provide compelling evidence for its existence. The data supporting dark matter is so robust that astronomers are nearly certain it is real. Competing theories attempting to explain the Universe without dark matter have consistently struggled to account for the Cosmic Web.

Astronomers continue their efforts to map the Cosmic Web, uncovering new large structures in our vicinity. In July 2020, researchers at the University of Hawaii announced the discovery of the South Pole Wall, an immense filament stretching over 1.3 billion light years, housing a vast number of galaxies—the largest structure ever found.

The upcoming generation of telescopes, starting with the ELT, will provide even greater insights into these colossal structures. Through these advancements, astronomers hope to deepen their understanding of how the Universe was formed. While the Atacama Desert may seem like an unconventional construction site, it may soon yield even more astonishing discoveries from its barren sands.

Chapter 2: Understanding the Cosmic Web

In this enlightening video, Claire Lamman discusses the intricate Cosmic Web and its significance in understanding the Universe's structure.

This video on Cosmic Vision: Space-Quakes delves into the phenomena that shape our cosmic environment and their implications for the future of astronomy.

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