Simulation of the cosmic web. Image: Burchett et al., ApJL, 2020
ABSTRACT breaks down mind-bending scientific research, future tech, new discoveries, and major breakthroughs.
When light from the early universe reaches Earth, it presents an eerie snapshot of ancient stars and galaxies that have long since died, or taken on different forms over the course of billions of years.
Though we cannot directly see the future of these objects, scientists have now figured out how to do the next best thing by “fast-forwarding” simulations of the cosmic web, a network of large-scale structures that connects the universe, over the course of 11 billion years to its present state, reports a new study.
In this way, researchers led by Metin Ata, a cosmologist at the University of Tokyo’s Kavli Institute for the Physics and Mathematics of the Universe, were able to unspool the long-term evolution of giant galaxy clusters like a “time machine,” in the words of one author.
The new technique allowed the team “to ‘fast-forward’ the simulation to our present day and study the evolution of observed cosmic structures self-consistently,” revealing that at least one of these ancient “protoclusters” likely collapsed into an enormous cosmic web filament spanning 300 million light years, according to a study published in Nature Astronomy on Thursday.
The results also provide a means to test the standard model of cosmology, alternately known as the Lambda cold dark matter (ΛCDM), which is a well-corroborated framework to explain the weird properties of the universe, including the existence of dark matter, an unexplained substance that is far more abundant than regular “baryonic” matter.
“Understanding the formation of large-scale structures in the Universe, starting from tiny fluctuations in the matter density and subsequently evolving gravitationally into the complex cosmic web seen at the present epoch, is a key ambition of cosmological science,” Ata and his colleagues said in the study.
“As gravitationally evolving objects, protoclusters are ideal observables to study early structure formation and to compare with theoretical predictions,” they continued, adding that they are “excellent laboratories to jointly study the interplay between baryonic physics and dark matter models.”
Most cosmological simulations match the general statistical distribution of matter across the universe, rather than reproducing any specific cosmic structures that we can observe from Earth. However, a subcategory of these models, known as constrained cosmological simulations, do mimic real observations, though the new study notes that they are “mainly focused on the local universe or nearby structures” rather than the distant ancient universe, which is called the “ high-redshift” universe because light waves from this era become stretched into redder bands of the spectrum over time.
By merging constrained cosmological simulations with the Hubble Space Telescope’s Cosmological Evolution Survey (COSMOS), Ata’s team capitalized on what they call “a unique opportunity for studying early structure formation and matching galaxy properties” between the ancient and modern universe, according to the study.
“Observationally, the effort to find and characterize protoclusters is a lively, ongoing field,” the researchers said. “In particular, the COSMOS field is an excellent site for this, as it is covered by deep and coordinated multi-wavelength observations over a wide field” that are “suited to protocluster studies.”
“Up to this point, there has not been a uniform and self-consistent study dedicated to these structures in the COSMOS field,” the team continued. “We address this problem with constrained simulations applied towards the rich legacy of large-scale spectroscopic surveys that have been conducted on the COSMOS field over nearly a decade, achieving a cosmic volume and number density unmatched anywhere else on the sky.”
In other words, the researchers looked at real protoclusters that existed 11 billion years ago and turned the clock forward in their constrained simulations. Of particular interest was the fate of the Hyperion super-protocluster, the largest structure of its kind during cosmic dawn, which the team called “the subject of scientific and public curiosity” in the study. Whereas some scientists have speculated that this immense elongated structure would eventually collapse into a single massive galaxy cluster, Ata and his colleagues suggest that it has evolved into a giant filamentary supercluster within the cosmic web, which is embedded with multiple massive cluster cores.
“We confirm that several previously reported protoclusters will evolve into massive galaxy clusters by our present epoch, including the ‘Hyperion’ structure that we predict will collapse into a giant filamentary supercluster spanning 100 [megaparsecs],” or 300 million light years, the team said in the study. “We also discover previously unknown protoclusters with lower final masses than are typically detectable by other methods that nearly double the number of known protoclusters within this volume.”
The large-scale structures that undergird the cosmic web are mostly made of dark matter, which is only observable due to its gravitational effect on luminous objects made of regular baryonic matter. As a result, the simulations pioneered by the new study can help shed light on the nature of dark matter by charting the course of its cosmic distribution across time, providing a key test of the ΛCDM model.
“Constrained simulations of these upcoming high-redshift galaxy surveys will also allow us to probe early structure formation for consistency with the ΛCDM model with increasing sensitivity to lower-mass galaxy (proto)clusters,” the researchers said in the study.
“Each identified protocluster represents a unique environment to study the morphologies and merger rates of member galaxies in high-density environments” in the early universe, they concluded, “which is so far only possible in theoretical studies or random cosmological simulations.”