The cosmic web is the large-scale structure of the universe. If you could watch our cosmos unfold from the Big Bang to the present day, you would see how these filaments (and the voids between them) form over time. Now, using JWST, astronomers have found ten galaxies that form a very early version of this structure, just 830 million years after the universe began.
The “cosmic web” began as density fluctuations in the very early universe. A few hundred million years after the Big Bang, matter (in the form of primordial gas) had condensed into knots at the intersections of gas layers and filaments in the early web. These nodes and filaments hosted the first stars and galaxies. When astronomers look back into the past, it’s only natural that they look for the early versions of the cosmic web. JWST allowed them to look back at very faint, dark objects that existed just after the Big Bang.
The ten galaxies discovered by the team are arranged in a tenuous thread three million light-years long, anchored by a bright quasar. Its appearance surprised the team both for its size and its place in cosmic history. “This is one of the earliest filament structures humans have ever found associated with a distant quasar,” added Feige Wang of the University of Arizona at Tucson, the principal investigator on this program.
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Striving to understand the early universe and the cosmic web
The JWST observations are part of an observing program called ASPIRE: A Spectroscopic Study of Biased Halos in the Age of Reionization. It uses both images and spectra from 25 quasars that existed when the universe began to glow after the “dark ages.” The idea is to study the formation of galaxies as early as possible and the birth of the first black holes. In addition, the team hopes to understand how the early Universe became enriched with heavier elements (the metals) and how it all happened during the epoch of reionization.
This is an artistic illustration showing the timeline of the early universe, showing some important time periods. On the left is the early day of the universe when intense heat prevented much. After that, the CMB will be released once the universe has cooled down a bit. After that, the neutral universe is shown in yellow, the time before star formation. The hydrogen atoms in the neutral universe should have been emitting radio waves that we can see here on Earth. Image source: ESA – C. Carreau
The ASPIRE goals are an important part of understanding the origin and evolution of the universe. “The last two decades of cosmological research have given us a solid understanding of how the cosmic web forms and evolves. “ASPIRE aims to understand how we can incorporate the formation of the earliest massive black holes into our current history of the formation of the cosmic structure,” explained team member Joseph Hennawi of the University of California, Santa Barbara.
Focus on the early black holes
Quasars lure through time and space. They are powered by supermassive black holes that produce incredible amounts of light and other emissions, as well as powerful jets. Astronomers use them as standard candles for measuring distances and studying the vast regions of space their light traverses.
Artist’s impression of a quasar. At least one is involved in an early filament in the cosmic web. Credit: NOIRLab/NSF/AURA/J. because Silva
At least eight of the quasars in the ASPIRE study have black holes that formed less than a billion years after the Big Bang. These black holes have masses between 600 million and 2 billion solar masses. This is actually pretty massive and raises a lot of questions about her rapid growth. “To form these supermassive black holes in such a short time, two criteria must be met. First you need to start growing from a massive “seed” black hole. Second, even if this seed starts with a mass equivalent to a thousand suns, it still has to accumulate a million times more matter at the maximum possible rate over its lifetime,” Wang explained.
For these black holes to grow like this, they needed a lot of fuel. Their galaxies were also quite massive, which could explain the giant black holes at their hearts. Not only did these black holes suck in a lot of material, but their outflow also has implications for star formation. “Strong winds from black holes can suppress star formation in the host galaxy. “Such winds have been observed in the nearby Universe, but never directly in the epoch of reionization,” Yang said. “The magnitude of the wind is related to the structure of the quasar. In the Webb observations we see that such winds existed in the early universe.”
Why the epoch?
We often hear from astronomers who want to look back to the epoch of reionization. Why is it such an enticing destination? It offers a glimpse into the time when the first stars and galaxies formed. After the Big Bang, the young universe was in a hot, dense state. Sometimes we hear that it is the primordial soup of the cosmos. Then the expansion started and the situation began to cool down. This allowed electrons and protons to combine and form the first neutral gas atoms. It also allowed the thermal energy of the Big Bang to propagate. Astronomers discover this radiation. It is redshifted to the microwave region of the electromagnetic spectrum. Astronomers call them the “Cosmic Microwave Background Radiation” (CMB).
A visualization of what the universe looked like when it went through its last major era of transformation: the epoch of reionization. Photo credits: Paul Geil and Simon Mutch/University of Melbourne
This aspect of the early Universe showed minute density fluctuations in its expanding material. This material was neutral hydrogen. There were no stars or galaxies yet. But eventually, these higher-density areas began to clump together under gravity, causing the neutral matter to start clumping as well. This led to the further collapse of the high-density regions, eventually leading to the birth of the first stars. They heated the surrounding material, punching holes in the neutral areas – and this allowed the light to spread. Essentially, these holes (or bubbles) in the neutral gas allowed the ionizing radiation to continue traveling through space. It was the beginning of the epoch of reionization. A billion years after the Big Bang, the universe was completely ionized.
So how do you explain the early supermassive black holes?
It is interesting that the early galaxies found by JWST were already complete, along with their quasars, and had supermassive black holes at their cores. The crucial question remains: How did they grow so big so quickly? Their existence could tell astronomers something about the “overdensities” in the early cosmos. First, the black hole “seed” requires an overdense region full of galaxies to form.
So far, however, observations prior to the JWST discovery have found few overdensities of galaxies around the earliest supermassive black holes. Astronomers need to make more observations during this epoch to explain why this might be the case. The ASPIRE program aims to help clarify questions about the feedback between galaxy formation and black hole formation in this very early epoch of the universe. Along the way, they should also see other fragments of the large-scale structure of the cosmic web of the universe forming.
For more informations
NASA’s Webb identifies the earliest strands of the cosmic web
A Spectroscopic Study of Biased Halos in the Age of Reionization (ASPIRE): JWST reveals a filament structure around az = 6.61 quasar