The Copernican Principle is said to be a guiding narrative in modern cosmology. We are told that the great Polish astronomer Nicolaus Copernicus started us on a series of demotions with his removal of the Earth from the center of the Solar System. Later, astronomers discovered that we aren’t even at the center of the Milky Way and that the Milky Way is just one of many billions of galaxies in the observable universe. These discoveries proved that our place in the universe is not special, and when we do cosmology we must assume we are typical observers.
Much of this narrative is false, as Jay Richards and I explain in The Privileged Planet and as Michael Keas further elaborates in Unbelievable. We now know that our planetary home, our Solar System, and our location in the Milky Way are not typical. What about the larger scales? Is the Milky Way typical? What about its place in the local universe?
Not a Typical Galaxy
Astronomers have known for several decades that the Milky Way is not a typical galaxy. It is among the approximately 1 percent of most luminous galaxies in the nearby universe. In addition, the SAGA (Satellites Around Galactic Analogs) survey revealed that the satellite galaxies of the Milky Way exhibit a lower rate of star formation than those around Milky Way analogs. The researchers also found that the Milky Way satellites are more centrally concentrated than those around other galaxies like ours.
A just-published study in Monthly Notices of the Royal Astronomical Society (see here and here) reveals the Milky Way to be very atypical in another way. Based on large-scale simulations of the local universe, the researchers found that the Milky Way is more massive than most galaxies imbedded in cold (low velocity dispersion) “walls” similar to ours.
Not Arranged Randomly
Over the last 40 years, surveys of the “nearby” universe have revealed that galaxies are not arranged randomly in space. Rather, they are mostly arranged in a foam-like structure with mostly empty “voids” outlined by “filaments” and “walls” or “sheets.” These structures have been reproduced with massive computer simulations that include ordinary and dark matter.
The lead author, Miguel Aragón, said, “You might have to travel a half a billion light years from the Milky Way, past many, many galaxies, to find another cosmological wall with a galaxy like ours.” They found that only about one in a million galaxies in the simulation are as special as the Milky Way in this way.
They note that the local wall environment can influence the angular momentum and spin alignment of its member galaxies. They suggest that galaxies in our local wall environment may have experienced fewer mergers. We do not yet know why the Milky Way is special in this way. Is this rare condition needed to make the Milky Way more habitable? Does it give us a privileged place to make cosmological observations from? Answers to these questions will have to await further research.