Although the night sky changes little on human timescales, the Universe isn’t a static place. We wheel about in motion round the galactic center. Stars are born, and die in violent explosions. Galaxies collide. And, for the 1st time, astronomers have just found evidence that a number of the largest structures within the cosmos rotate, on a scale of many many hundreds of millions light-years. If validated, it might represent the largest rotating structure ever seen – suggesting that angular-momentum are often generated on absolutely mind-blowing scales.
The structure in question may be a cosmic filament, a long, cylindrical structure of dark-matter, spanning inter-galactic space region as a kind of bridge between galaxy clusters. These filaments are strands of a huge cosmic web, via which galaxies and star-forming material are channeled into the cluster nodes.
This means galaxies are often found along the filament, too, not just within the clusters. this gives scientists a tool for identifying rotational motion within the filament itself.
“By mapping the motion of galaxies in these huge cosmic superhighways using the Sloan Digital Sky survey – a survey of many thousands of galaxies – we found an interesting property of those filaments: they spin,” said astrophysicist Peng Wang of the Leibniz Institute for Astrophysics Potsdam (AIP) in Germany.
The filaments are many many hundreds millions light-years long , but just a couple of million light-years in diameter. On such large scales, we cannot be able to see the galaxies actually moving, but luckily for us, the light of a moving object still gives it away.
It’s called Doppler shifting, changes within the wavelength of light directly depend on whether it’s moving towards or faraway from the viewer. Wavelengths of light from an approaching object will appear to shorten slightly towards the blue end of the spectrum, or blueshift; wavelengths from receding objects will lengthen, or redshift.
By carefully studying light from galaxies on cosmic filaments and comparing them to every other, astronomers found that galaxies on one side of the filament were redshifted as compared to the opposite side. This is often exactly what you’d expect to ascertain if the galaxies were in vortical motion perpendicular to the filament’s spine.
“On these scales the galaxies within them are themselves just specks of dust,” explained cosmographer Noam Libeskind of AIP.
“They move-on helices or corkscrew-like orbits, circling round the middle of the filament while travelling along it. Such a spin has never been seen before on such enormous scales, and therefore the implication is that there must be an so far unknown physical mechanism liable for torquing these objects.”
Figuring out what that mechanism is could help astronomers find out how angular-momentum is generated within the cosmos. Currently, it is a mystery; within the early Universe, consistent with our cosmological models, there was no rotation – matter moved from less dense to more dense regions.
One theory, described as tidal torque, suggests the presence of a shearing force may need added a touch of a twist, but we simply do not know enough to even begin to-take it seriously in models of cosmic evolution.
Because galaxies are connected and fed by cosmic filaments, these structures play an intimate role within the formation and evolution of galaxies, including their rotation. However, whether the filaments themselves spin had previously only been theorized.
The discovery that they are doing will help us better understand the emergence of angular-momentum within the Universe, and therefore the role the cosmic web plays in regulating it.
“It’s fantastic to ascertain this confirmation that intergalactic filaments rotate in real Universe, also as in computer simulation ,” Libeskind said.
The research has been published in Nature Astronomy.