The most important rotating objects within the universe: Galactic filaments lots of of hundreds of thousands of sunshine years lengthy
We have known about the large-scale structure of the universe for some time. Galaxies reside in filaments hundreds of millions of light years in length on a backbone of dark matter. And where these filaments meet, there are clusters of galaxies. In between there are massive cavities in which galaxies are sparse. Now a team of astronomers in Germany and their colleagues in China and Estonia have made a fascinating discovery.
These massive filaments rotate, and this kind of rotation on such a massive scale has never been seen before.
Obviously there is no way to get an actual picture of the large scale structure of the universe. But there are some almost famous images that came from the Millennium Simulation Program. The Millennium Simulation was a supercomputer simulation of a cubic part of the universe over 2 billion light years on each side. The image contains about 20 million individual galaxies organized into filaments and clumps, and it was our first real look at the universe’s LSS.
It is remarkable to look at this picture now and imagine how these filaments spin.
Image of the large scale structure of the universe showing filaments and voids within the cosmic structure. Credit: Millennium Simulation Project
The team of astronomers behind this discovery worked with data from the Sloan Digital Sky Survey (SDSS). The SDSS created a very detailed 3D map of the universe, so SDSS data was critical to the team’s discovery.
“By mapping the movement of galaxies in these giant cosmic superhighways with the Sloan Digital Sky survey – a survey of hundreds of thousands of galaxies – we found a remarkable property of these filaments: they spin,” says Peng Wang, lead author of the now published Study and astronomer at the AIP (Institute for Astrophysics Potsdam).
Each of the galaxies in the filaments is nothing more than a speck of dust on a large scale, and they not only rotate but move along the tendrils like in pipelines.
“They move on spirals or corkscrew-like tracks and circle the center of the filament as they move along it.”
Noam Libeskind, study co-author, AIP.
“Even though they are thin cylinders – similar to pencils – hundreds of millions of light years long, but only a few million light years in diameter, these fantastic lines of matter rotate,” adds Noam Libeskind, initiator of the project at AIP. “On these scales, the galaxies in themselves are just dust particles. They move on spirals or corkscrew-like tracks and orbit the center of the filament as they move along it. Such rotation has never been seen on such enormous scale before, and the implication is that some as yet unknown physical mechanism must be responsible for twisting these objects. “
The fact that these filaments are twisting is hard to imagine and fascinating when one is successful. But discovery is about more than our own fascination. These are the largest objects we have ever seen, and that means that angular momentum can take place on a large scale. One of the secrets of cosmology is how this angular momentum is generated on such a massive scale since there was no primordial rotation in the early universe.
The discovery is based on observations of individual galaxies in the filaments and their Doppler shift. In this study, the redshift is a proxy for the rotation, redshifted galaxies receding and blueshifted galaxies approaching.
This figure from the paper shows the filament rotation speed as a function of the distance between galaxies and the filament spine. The distance from galaxies to the filament spine in the retreat area is shown in red and given positive values, while the distance from galaxies in the approach area is marked in blue and assigned negative values. Error bars represent the standard deviation from the mean. Image source: Wang et al. 2021.
In the current working model of the structure formation of the universe, overdensities grow through gravitational instability. Material from underdense regions flows into overdense regions. But this flow of material has no twist or curvature. This is why cosmologists say that there was no rotation in the early universe. And this is where this discovery becomes more interesting.
The rotation visible in these filaments of galaxies must be generated when the structures are formed. And these filaments and the rest of the cosmic web are associated with the formation and evolution of the galaxies themselves. They also have a strong influence on the spin of individual galaxies and can regulate how a galaxy and its halo of dark matter rotate. There is an unknown in all of this: Scientists do not yet know how our current understanding can predict that the filaments themselves rotate.
“Such rotation has never been seen on such enormous scale before, and the implication is that there must be some as-yet unknown physical mechanism responsible for attracting these objects.”
Noam Libeskind, study co-author, AIP.
Prior to this study, other scientists theorized that these filaments rotate. Dr. For example, Mark Neyrinck, Fellow in the Department of Theoretical Physics at the University of the Basque Country, Spain, is known for the theories on the subject. He is also known for developing the “origami” description of cosmic structure formation. In a 2016 article in The Paper, he said, “… when galaxies rotate (and do), filaments must protrude from them too. In addition, galaxies connected by filament should mainly rotate together, like objects attached to the ends of a rod. In fact, this agrees with astronomical observations; nearby galaxies tend to rotate in the same direction. “
The work of Dr. Neyrinck was an important starting point for the team behind this paper.
“Spurred on by the suggestion of theoretician Dr. Mark Neyrinck that filaments can rotate, we examined the observed galaxy distribution and looked for the filament rotation, ”says co-author Noam Libeskind. “It’s fantastic to see this confirmation that intergalactic filaments rotate in both the real universe and in computer simulation.”
The team used an ingenious mapping method that divided the observed galaxy distribution into segments. Then each of the filaments was approximated by a cylinder. The galaxies in the filament were then divided into two regions on either side of the filament’s spine. Then they carefully measured the mean redshift difference between the two regions. “The mean redshift difference is a proxy for the speed difference (the Doppler shift) between galaxies on the retreating and the approaching side of the filament tube,” the authors write. This is how they measured the rotation of the filaments.
In their paper, the team writes that what they found cannot be accidental. “What is measured and displayed here is the redshift difference between two regions on either side of a hypothetical spin axis that coincides with the filament spine. The complete distribution of this amount, regardless of the angle formed with the line of sight, is incompatible with chance … “
However, the researchers caution their results do not imply that every filament in the universe is spinning. That would be excessive. “This work does not predict that every single filament in the universe will rotate,” they write, “but that there are subsamples – closely related to the mass of the viewing angle endpoint – that show a clear signal that corresponds to the Rotation matches. That is the most important finding of this work. “
“Taken together,” the team writes in its conclusion, “the current study and
The main author of this work is Peng Wang, astronomer at the Institute for Astrophysics Potsdam (AIP). The title of the article is “Possible Evidence of Observation for Cosmic Filament Spin”. It’s published in Nature Astronomy.