In the universe, a giant structure that shouldn't exist was discovered

In 2021, Alexia M. Lopez was pursuing a graduate degree at the University of Central Lancashire in the United Kingdom. When Lopez set her sights on the constellation of Bootes 9.3 billion light-years away, she was surprised to find that a cluster of 45 to 50 gas clouds seemed to be arranged in a nearly symmetrical huge crescent-shaped structure in the universe - the Giant Arc .

In comparison, the currently observable universe is about 94 billion light-years wide, while this giant arc spans 3.3 billion light-years in space, almost 1/15 of the radius of the observable universe .

For Lopez, such a large arc can almost be called "surreal", and its existence seems to violate the cosmological principle that the universe should be isotropic . "I have to pinch myself, this is an unexpected discovery, I can't believe this observation at all," she exclaimed in an interview with the BBC.

Lopez presented the discovery of the giant arc at the American Astronomy Meeting in June 2021, and published the results in the Monthly Notices of the Royal Astronomical Society in October 2022. But Lopez's fate with this strange structure did not stop there.

In January this year, at the 243rd meeting of the American Astronomical Society (AAS), Lopez once again reported a large-scale structure - the Big Ring , which is relatively close to the giant arc discovered previously, and both are located near the constellation Bootes.

The diameter of the giant ring is about 1.3 billion light years. If we can see it with the naked eye from Earth, its size will be about 15 times the size of the full moon in the night sky. At first glance, this ring is an almost perfect circle, but according to Lopez's analysis, it is more like a spiral coil, except that the hole is facing the earth. Although the diameter of the giant ring is much smaller than that of the giant arc, its circumference is comparable to that of the giant arc .

Image artist's concept of how giant arcs and rings would appear in the sky if they were visible from Earth.

Image credit: Stellarium/University of Central Lancashire/PA

However, these spectacular large-scale structures are not only eye-catching in size, but more importantly, their existence seems to challenge astronomers: the standard model of cosmology may need to be rewritten or revised.

The Universe is Uniform... Or Is It?

If we observe the entire universe from a large enough scale, we usually think that the distribution of matter and space-time is uniform and isotropic, or that the universe is smooth and flat. This description is called the cosmological principle , which, together with Einstein's general theory of relativity, forms the theoretical framework of the standard model of cosmology. Uniformity and isotropy mean that no matter where we are in the universe, the universe should look the same when observed from any direction .

Although the cosmological principle describes the universe as uniform, if we observe the universe, "focusing" like a zoom lens and then gradually "zooming out", we will see multiple planets orbiting a star, billions of stars forming a galaxy, thousands of galaxies forming galaxy clusters, and galaxy clusters forming superclusters.

These galaxies are close to each other, forming filaments on a large scale, shuttling through the vast universe, interweaving and connecting with each other, forming a 3D cosmic web that covers interstellar space. Filaments are naturally where matter gathers, and outside of them are empty areas with extremely low matter density. In this way, is the universe still uniform?

Artist's concept of the cosmic web.

Image credit: NASA

To be precise, whether it is uniform depends on the scale of observation . If the universe is compared to the ocean, the sea surface will be rough when viewed from close up (relatively speaking), but it will be smooth as a mirror when viewed from far away. On a large scale, the distribution of matter in the cosmic web is indeed uniform. But on a relatively small scale, the web structure itself shows that the matter is unevenly distributed .

In fact, large-scale structures like the cosmic web are precisely what puzzles scientists: where does the uneven distribution of matter come from? How did the universe evolve into what we observe today?

To this end, astronomers have been trying to more accurately measure the many celestial bodies in the universe, whether they are stars, galaxies or distant galaxy clusters, hoping to create an accurate cosmic map to deepen our understanding of the universe. The Sloan Digital Sky Survey (SDSS) was born for this purpose, and Lopez discovered these unreasonable structures when observing the absorption lines of quasars in the past SDSS data.

The "giant arcs" and "giant rings" discovered by Lopez are like some conspicuous "bumps" on a smooth surface . The two structures are so close that they can even be merged into a super-large structure. "The question is, how can such a large structure be formed?" said Don Pollacco, a professor in the Department of Physics at the University of Warwick in the UK. It is difficult to imagine the mechanism of the formation of this structure specifically, but it must be related to the formation process of large-scale structures in the universe .

According to the standard model of cosmology, when the universe began to expand, tiny disturbances in matter density caused matter to aggregate in some areas. After billions of years of evolution, gravity eventually caused these clumps to condense into stars and galaxies, ultimately forming the large-scale structure of the universe we observe.

Subir Sarkar, professor of theoretical physics at the University of Oxford, said that this process requires a long enough time. The universe is finitely old, so in theory it is not enough to form objects with a diameter of more than 1.2 billion light years. So, how to explain the formation process of giant arcs and giant rings with a scale of more than 3 billion light years may require further discussion.

Filled with giants

However, the large-scale structures discovered by astronomers are not only Lopez's "arc" and "ring". In fact, as early as 1989, two astronomers observed a galaxy "Great Wall" in the universe, which was called CfA2 Great Wall.

This wall of galaxies is about 500 million light-years long, 300 million light-years wide, and 15 million light-years thick. In 2003, astronomers discovered a longer galaxy wall, the Sloan Great Wall, which is about 1.5 billion light-years long.

In the past decade, with the rapid development of astronomical observation technology, astronomers have been discovering these giants at an increasing rate . In 2016, astronomers discovered the BOSS Great Wall, which spans 1 billion light years and is composed of 830 independent galaxies.

However, the largest large-scale structure currently is the Hercules-Corona Borealis Great Wall discovered in 2013. It spans 10 billion light-years, which is more than one-tenth the size of the observable universe.

Image source: Wikipedia

These behemoths are arguably the largest and heaviest structures in the universe. Most of them exceed the theoretical "possible" size limit, forcing the standard model to be constantly revised. (This process happens from time to time, such as two revisions in the last century, one to introduce dark matter and the other to introduce dark energy.) In fact, after discovering large-scale structures such as the Sloan Wall in the early years, astronomers predicted that more in-depth galaxy surveys in the future would allow us to see more similar structures, such as giant arcs and giant rings.

However, some scientists have questioned Lopez's research. When we look up at the sky and see the various forms of clouds, we can see the shapes of the letters "A, B", and even various animals. This is because we are very good at extracting a pattern from random objects, but this is not the real structure. Is the so-called "arc" or "ring" arrangement of galaxies just a random arrangement of celestial bodies ?

In the 2021 giant arc study, Lopez performed a series of statistical tests to prove that the confidence level of his results was 99.9997%. Sacca said that in this case, it can be considered that this is a real supercluster chain. The newly published giant ring study has not yet been peer-reviewed and may need further verification.

References

[1] https://www.uclan.ac.uk/news/big-ring-in-the-sky

[2] https://www.uclan.ac.uk/news/discovery-of-a-giant-arc-in-distant-space-adds-to-challenges-to-basic-assumptions-about-the-universe

[3] http://astronomy.nmsu.edu/cwc/Research/MgIIreview/mgii-over.html#basics

[4] https://www.bbc.com/future/article/20230302-the-giant-arcs-that-may-dwarf-everything-in-the-cosmos

[5] https://www.bbc.com/future/article/20230302-the-giant-arcs-that-may-dwarf-everything-in-the-cosmos

[6] https://www.cas.cn/kx/kpwz/202009/t20200901_4757786.shtml

Planning and production

Source: Global Science (ID: huanqiukexue)

Author: Bu Zhou

Editor: He Tong

Proofread by Xu Lai and Lin Lin