galaxies can move with each other across huge distances,
and against the predictions of basic cosmological models.
The reason why could change
everything we think we know about the universe...
Their variety is
stunning: spirals, ring galaxies shaped like star-studded loops, and
ancient galaxies that outshine virtually everything else in the
These dim structures are made of hydrogen gas and dark matter and take the form of filaments, sheets, and knots that link galaxies in a vast network called the cosmic web.
We know these structures
have major implications for the evolution and movements of galaxies,
but we've barely scratched the surface of the root dynamics driving
Earth's long-form cosmic address, for instance, would have to note that the Milky Way is part of the Local Group, a gang of several dozen galaxies.
The Local Group is inside
the Virgo supercluster, containing more than 1,000 galaxies.
But some galaxies show dynamic links across distances too great to be explained by their individual gravitational fields.
For instance, a study (Mysterious Coherence in Several-megaparsec Scales between Galaxy Rotation and Neighbor Motion) published in The Astrophysical Journal in October found that hundreds of galaxies were rotating in sync with the motions of galaxies that were tens of millions of light years away.
Joon Hyeop Lee and his colleagues studied 445 galaxies within 400 million light years of Earth, and noticed that many of the ones rotating in a direction toward Earth had neighbors that were moving toward Earth, while those that were rotating in the opposite direction had neighbors moving away from Earth.
Lee and his colleagues suggest that the synchronized galaxies may be embedded along the same large-scale structure, which is very slowly rotating in a counter-clockwise direction.
That underlying dynamic could cause the kind of coherence between the rotation of the studied galaxies and the motions of their neighbors, though he cautioned that it will take a lot more research to corroborate his team's findings and conclusions.
While this particular iteration of weirdly synced up galaxies is novel, scientists have observed odd coherences between galaxies at even more mind-boggling distances.
In 2014, a team observed curious alignments of supermassive black holes at the cores of quasars, which are ancient ultra-luminous galaxies, that stretch across billions of light years.
Led by Damien Hutsemékers, an astronomer at the University of Ličge in Belgium, the researchers were able to observe this eerie synchronicity by watching the universe when it was only a few billion years old, using the Very Large Telescope (VLT) in Chile.
The observations recorded the polarization of light from nearly 100 quasars, which the team then used to reconstruct the geometry and alignment of the black holes at their cores.
The results showed that the rotation axes of 19 quasars in this group were parallel, despite the fact that they were separated by several billion light years.
The discovery (Alignment of Quasar Polarizations with Large-scale Structures), which was published in the journal Astronomy & Astrophysics, is an indicator that large-scale structures influenced the dynamics of galaxies across vast distances in the early universe.
The truth behind synchronized galaxies could change everything
The secret of these synchronized galaxies may pose a threat to the cosmological principle, one of the basic assumptions about the universe.
This principle states that the universe is basically uniform and homogenous at extremely large scales.
However, Hutsemékers' cautioned that more of these structures would need to be spotted and studied to prove that this is a serious wrinkle in the cosmological principle.
For the moment, the dynamics behind these quasar positions are not well understood because there are few observational techniques to refine them.
Future radio telescopes, such as the Square Kilometer Array, might be able to probe these mysterious alignments in more detail.
Quasar alignments are not the only hurdles that oddly synchronized galaxies have presented to established models of the universe.
In fact, one of the most contentious debates in cosmology these days is centered around the unexpected way in which dwarf galaxies appear to become neatly aligned around larger host galaxies such as the Milky Way.
Simulations of the universe under the ΛCDM model predict that small satellite galaxies will end up in a swarm of random orbits around larger host galaxies.
But over the past decade, new observations have revealed that a huge chunk of the satellite galaxies around the Milky Way are synced up into one tidy orbital plane.
At first, scientists wondered whether that simply meant something weird was going on with our own galaxy, but a similar plane of satellites was then observed around Andromeda.
The alarm bells really started ringing in 2015, when astronomers published observations (Two Planes of Satellites in the Centaurus A Group) of the same phenomenon a third time around Centaurus A, an elliptical galaxy about 10 million light years from the Milky Way.
This discovery "suggests that something is wrong with standard cosmological simulations," according to a subsequent 2018 study (A Whirling Plane of Satellite Galaxies around Centaurus A challenges Cold Dark Matter cosmology) in Science, led by Oliver Müller, an astronomer at the University of Strasbourg in France.
In a 2015 study (Planes of Satellite Galaxies and the Cosmic Web), Noam Libeskind and his colleagues suggested that filaments in the cosmic web might be guiding these organized galaxies, a process that could cohere with the ΛCDM model.
Ultimately, though, there's no conclusive answer to this dilemma yet.
The next generation of galaxy research
This tantalizing uncertainty has motivated astronomers like Marcel Pawlowski, a Schwarzschild Fellow at AIP and co-author on the 2018 Science study (A Whirling Plane of Satellite Galaxies around Centaurus A challenges Cold Dark Matter cosmology), to make this problem a focus of their research.
Pawlowski is looking forward to data from the next generation of huge 30-meter class observatories that could show whether other big galaxies are surrounded by either isotropic or organized patterns of satellite galaxies.
Whether it's the strange motions of dwarf galaxies in our own galactic neighborhood or the eerie alignment of galaxies over millions or billions of light years, it's clear that the dance moves of galaxies are an essential key to unlocking the large-scale structure of the universe.
The galaxies we see captured in static positions in beautiful deep-field shots are actually guided by many complex forces we don't yet fully comprehend, including the cosmic web that undergirds the universe.