The more we learn about the world and the cosmos, the more we have had to let go of ideas of our own importance. We discovered that the world is not the center of the universe around which all objects rotate. Then we found that the Earth rotated around the Sun, and believed that that was the center of the universe instead.
Now we know that is not the case either. Abandoning ideas of our own importance, we now use the Copernican principle and its updated astronomical twin, the cosmological principle; that we on Earth are not at the center of the universe, nor occupy a privileged region within it. Though regions of space may differ – for instance, the Great Nothing – viewed at a large enough scale, the universe is isotropic and homogenous, or the same wherever you are within it.
A violation of this principle would be huge. Not as big as if the laws of thermodynamics were broken, but still a big deal.
“The Copernican principle is a cornerstone of most of astronomy, it is assumed without question, and plays an important role in many statistical tests for the viability of cosmological models,” Albert Stebbins of Fermilab explained to Phys.org. “It is also a necessary consequence of the stronger assumption of the Cosmological Principle: namely, that not only do we not live in a special part of the universe, but there are no special parts of the universe – everything is the same everywhere (up to statistical variation).”
“It is a very handy principle, since it implies that here and now is the same as there and now, and here and then is the same as there and then. We do not have to look back in time at our current location to see how the universe was in our past – we can just look very far away, and given the large light travel time, we are looking at a distant part of the universe in the distant past. Given the Cosmological Principle, their past is the same as our past.”
But when studying the cosmic microwave background (CMB) – the leftover radiation from around 400,000 years after the universe began, that is faintly detectable and permeates all of the known universe – several teams have found apparent violations of this principle.
If the principle (and our current understanding of physics) hold, the first light that began its journey as the hot early universe cooled should be roughly the same temperature, bar for minor fluctuations in temperature.
“[NASA’s COBE and WMAP missions] measured the temperature of the CMB to be 2.726 Kelvin (approximately -270 degrees Celsius) almost everywhere on the sky,” the European Space Agency explains. “The ‘almost’ is the most important factor here, because tiny fluctuations in the temperature, by just a fraction of a degree, represent differences in densities of structure, on both small and large scales, that were present right after the Universe formed. They can be imagined as seeds for where galaxies would eventually grow.”
Dividing up the CMB into segments allows you to analyze the distributions of temperatures. When you divide up the CMB into smaller segments (dipole being two hemispheres, quadrupole being divided into fourths, etc) and compare them, the temperature distribution in these regions should appear completely random. Hot and cold regions in the quadrupole should not correspond to hot and cold regions in the octupole. But in this case, they do.
The universe being what it is, we have been sent a few curveballs from its first light. Adding to the infamous cold spot, physicists found a trail of anomalous hot and cold spots stretching out along an axis, dubbed the “Axis of Evil” in a 2005 paper. Even more annoyingly, this axis aligns with our solar plane (the plane on which the planets of the Solar System orbit the Sun), with temperatures “above” the Solar System being slightly cooler than the temperatures from “below”.
That’s a pretty unusual result, and has sent some looking for another possible “preferred axis” in cosmology, speculating that the universe is not homogenous after all. One 2016 paper points to spiral galaxies, which appear to be more inclined to be “left-handed” than right-handed, among other discoveries.
“Several directional anomalies have been reported in various observations: the polarization distribution of the quasars, the velocity flow, the handedness of the spiral galaxies, the anisotropy of the cosmic acceleration, the anisotropic evolution of fine-structure constant, including anomalies in the CMB low multipoles, such as the CMB parity asymmetry,” the paper reads. “Although the confidence level for each individual anomaly is not too high, the directional alignment of all these anomalies is quite significant, which strongly suggests a common origin of these anomalies.”
But as strange as the results are, they have not been replicated by all who have tried. Some have put the unusual results down to statistical errors, while others have suggested that the fact it appears to line up with the solar plane could be because the microwave radiation is dominated by the local foreground. If statistical errors, data collection errors, and other local causes are eliminated and the problem remains, we could need new physics.
Errors and particularly unusual results do happen sometimes in science, and are corrected by further study. Faster-than-light neutrinos turned out to be a faulty cable, and puzzling radio bursts have turned out to be from microwave ovens, so let’s not get too excited just yet.
