The Far Reaches of the Cosmic Web

Galaxies seem to be communicating with each other across vast distances never thought possible before, putting the cosmological principle into question again.

These gravitationally bound structures consisting of gas, dust and trillions of stars exist in the trillions. Most observed galaxies are spiral galaxies like our very own Milky Way, with others being elliptical, lenticular or irregular. The formation and evolution of a galaxy is generally revealed in galactic kinematics, particularly the rotation which is constrained by the conservation of angular momentum. Through studying the rotation of galaxies, scientists can thus infer how the galaxy was formed and how it evolved. Did it form from a rotating dust cloud? Did it evolve as a merger?

As would be expected, galactic behaviour – including its rotation – is influenced by that of its neighbours. However, in a recent report, Korean scientists Joon Hyeop Lee and colleagues presented observational evidence that the rotational direction of a galaxy tends to be coherent with the average motion of its nearby neighbours within 1 mega parsec (Mpc) – that’s 3 million light years or 20 million trillion miles. This is way beyond the expected effects of gravity.

Intrigued, Lee and his colleagues wanted to see just how far this coherency stretched. Utilizing data from the Calar Alto Legacy Integral Field Area survey (CALIFA) – an astronomical project utilizing the Calar Alto observatory in Spain to map 600 galaxies – they mapped the velocity distribution of ‘neighbour’ galaxies within 15 Mpc from the CALIFA galaxies.

A composite of panels depicting maps of some of the properties of galaxies obtained from CALIFA data.

Remarkably, they found that within distances of several mega parsecs the velocity profiles showed unexpectedly strong evidence of the dynamical coherence between the rotation of the CALIFA galaxies and their neighbours. Small scale coherences have been explained in terms of an entropic force – more on that can be read here – but what could be causing this large-scale coherence?

In a recent paper presenting these results, Lee and his team contemplated that question and suggested that a possible relationship exists between the long-term motion of a large-scale structure and the rotations of galaxies in it.

Axis alignments with large-scale structures such as cosmic filaments have been observed before. For example, in 2014 the axis alignments of a significant number of quasars were found to be parallel. Read more here.

However, any correlation in axis alignment over such large distance violates the cosmological principle, which states that we live in a homogeneous and isotropic universe.

To add fuel to the fire, in 2018 a team of scientists found that a significant number of dwarf galaxies followed a coherent velocity pattern aligned with the long axis of their host galaxy, as opposed to orbiting randomly as predicted by the cosmological model. Read more here.

More and more of these mysterious coherences are being observed, putting the cosmological principle into question.

RSF in perspective

The unified perspective looks at the interconnectedness of all things, which can be explained in terms of the granular structure of spacetime at the Planck scale. These coherently moving structures form larger and larger structures … structures within structures all the way up to the universe and beyond.

Voyager 2 reaches interstellar space

This artist's concept shows the locations of NASA's Voyager 1 and Voyager 2 spacecraft relative to the heliosphere, or the protective bubble of particles and magnetic fields created by our Sun. Both Voyagers are now outside the heliosphere, in a region known as interstellar space, or the space between stars.

Voyager 1 has a companion in the realm of the stars.

Researchers at the University of Iowa report that the spacecraft Voyager 2 has entered the interstellar medium (ISM), the region of space outside the bubble-shaped boundary produced by wind streaming outward from the sun. Voyager 2, thus, becomes the second human-made object to journey out of our sun's influence, following Voyager 1's solar exit in 2012.

In a new study, the researchers confirm Voyager 2's passage on Nov. 5, 2018, into the ISM by noting a definitive jump in plasma density detected by an Iowa-led plasma wave instrument on the spacecraft. The marked increase in plasma density is evidence of Voyager 2 journeying from the hot, lower-density plasma characteristic of the solar wind to the cool, higher-density plasma of interstellar space. It's also similar to the plasma density jump experienced by Voyager 1 when it crossed into interstellar space.

"In a historical sense, the old idea that the solar wind will just be gradually whittled away as you go further into interstellar space is simply not true," says Iowa's Don Gurnett, corresponding author on the study, published in the journal Nature Astronomy. "We show with Voyager 2 -- and previously with Voyager 1 -- that there's a distinct boundary out there. It's just astonishing how fluids, including plasmas, form boundaries."

Gurnett, professor emeritus in the UI Department of Physics and Astronomy, is the principal investigator on the plasma wave instrument aboard Voyager 2. He is also the principal investigator on the plasma wave instrument aboard Voyager 1 and authored the 2013 study published in Science that confirmed Voyager 1 had entered the ISM.

Voyager 2's entry into the ISM occurred at 119.7 astronomical units (AU), or more than 11 billion miles from the sun. Voyager 1 passed into the ISM at 122.6 AU. The spacecraft were launched within weeks of each other in 1977, with different mission goals and trajectories through space. Yet they crossed into the ISM at basically the same distances from the sun.

That gives valuable clues to the structure of the heliosphere -- the bubble, shaped much like a wind sock, created by the sun's wind as it extends to the boundary of the solar system.

"It implies that the heliosphere is symmetric, at least at the two points where the Voyager spacecraft crossed," says Bill Kurth, University of Iowa research scientist and a co-author on the study. "That says that these two points on the surface are almost at the same distance."

"There's almost a spherical front to this," adds Gurnett. "It's like a blunt bullet."

Data from the Iowa instrument on Voyager 2 also gives additional clues to the thickness of the heliosheath, the outer region of the heliosphere and the point where the solar wind piles up against the approaching wind in interstellar space, which Gurnett likens to the effect of a snowplow on a city street.

The Iowa researchers say the heliosheath has varied thickness, based on data showing Voyager 1 sailed 10 AU farther than its twin to reach the heliopause, a boundary where the solar wind and the interstellar wind are in balance and considered the crossing point to interstellar space. Some had thought Voyager 2 would make that crossing first, based on models of the heliosphere.

"It's kind of like looking at an elephant with a microscope," Kurth says. "Two people go up to an elephant with a microscope, and they come up with two different measurements. You have no idea what's going on in between. What the models do is try to take information that we have from those two points and what we've learned through the flight and put together a global model of the heliosphere that matches those observations."

The last measurement obtained from Voyager 1 was when the spacecraft was at 146 AU, or more than 13.5 billion miles from the sun. The plasma wave instrument is recording that the plasma density is rising, in data feeds from a spacecraft now so far away that it takes more than 19 hours for information to travel from the spacecraft to Earth.

"The two Voyagers will outlast Earth," Kurth says. "They're in their own orbits around the galaxy for five billion years or longer. And the probability of them running into anything is almost zero."

"They might look a little worn by then," Gurnett adds with a smile.

The Iowa study is one of five papers on Voyager 2 published in Nature Astronomy. These papers confirm the passage of Voyager 2 to interstellar space and provide details on the characteristics of the heliopause.

Gurnett and Kurth are the study's sole authors. Their research was funded by NASA, through a contract with the Jet Propulsion Laboratory.

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Materials provided by University of Iowa. Original written by Richard C. Lewis. Note: Content may be edited for style and length.