Here on Earth, we are used to laws that shift according to the unique customs and demands of various regions. You can turn right on a red light while driving in most places, for instance, but not in New York City, where traffic rules are stricter to accommodate busy roads.
The physical laws of the universe, in contrast, do not tolerate any localized deviations of this sort—or so our best theory goes. Scientists operate under the assumption that there are universal laws of physics that affect matter the same way everywhere, from our own solar neighborhood to galaxies billions of light years away. In other words, while there are obviously variations in the density and distributions of matter across space, scientists assume that the universe is statistically homogenous at large scales of hundreds of millions of light years, because the actual hard wiring of the universe is equally applied everywhere.
This notion of universal laws, known as the cosmological principle, has produced centuries of theory and has so far been borne out by astronomical observations. The model of an isotropic universe helps explain crucial phenomena such as the homogeneity of the cosmic microwave background, the oldest light in the universe, as well as the apparent expansion of the universe at a uniform rate.
“The cosmological principle is, in more tangible terms: Is the universe playing fair with us?” explained Robert Caldwell, a professor of physics and astronomy at Dartmouth College, in a call. “Are the laws of physics the same everywhere? Or is there a preferred location in the universe?”
While most evidence suggests the universe is playing fair, there are also many cosmic wildcards that seem to clash with the cosmological principle. Just within the past few months, for instance, two teams of physicists published completely different observations of anomalies in the universe that hint at potential variations in fundamental laws and forces.
Even weirder, this new research bolsters past studies sketching out a “directionality” to these variations. In other words, they conjure up a possible model of the universe where physical laws shift in certain directions as if they are on a mysterious cosmic gradient. These findings don’t match other tests of isotropy, or the homogeneity of the universe, that suggest that the universe has no preferred direction.
Conflicting results don’t mean we have to throw out the cosmological principle, as it requires an enormous amount of evidence to oust established physics. But the new studies document phenomena, at both “local” and extremely distant scales, that are currently unexplained and that challenge our fundamental expectations about the behavior of the universe.
The inconstant constant
There are four known fundamental forces of nature: Gravitation, electromagnetism, and the weak and strong nuclear interactions. The cosmological principle suggests that these forces affect matter equally across the universe, which is why visible objects, like stars and galaxies, typically look and behave the same way wherever you gaze in the sky.
But if you look a little closer, oddities can emerge in physical constants. For instance, the strength of the electromagnetic force is calculated with a value known as the fine-structure constant. This constant is mathematically scaffolded to unchangeable values such as the Planck constant and the speed of light. If the universe is truly isotropic, the fine-structure constant (like all constants) should never change within it.
But over the past decade, scientists have measured this constant in distant pockets of the universe and found evidence that it may fluctuate. This puzzling trend reached a new milestone with “the most distant direct measurements of [the fine-structure constant] to date,” from an ancient “quasar” galaxy 13 billion light years away, which are reported in a Science Advances study published in April.
Though scientists have been using light from cosmic objects to spot-test the fine-structure constant for years, the new paper extends the scope of the experiment into the infant universe, just one billion years after the Big Bang.
“We’ve gone further than ever before,” said co-author John Webb, a cosmologist at the University of New South Wales in Sydney, in a call. “In terms of lookback time, we’ve gone closer to the Big Bang than before. If you prefer in terms of distance, we’ve gone to greater distance than has been done before with any direct measurement of the electromagnetic force in the early universe.”
The team was able to accomplish this feat with a specialized spectrograph called X-SHOOTER on the Very Large Telescope (VLT) in Chile. The instrument’s acuity in the near-infrared part of the spectrum enabled Webb and his colleagues to peer at objects at higher “redshifts,” meaning they are farther away in distance and also further back in time, causing their light to redden.
“Perhaps there is some kind of relationship between these things that we don’t yet fully understand, and it’s interesting to note this alignment"
Using X-SHOOTER, the team studied light from a 13-billion-year-old quasar—a type of super-luminous galactic core—called J1120+0641. On its way to Earth, this ancient light was filtered through four gas clouds at lower redshifts along the line of sight to J1120+0641. Webb and his colleagues used the spectral patterns of the light, as it passed through the clouds, to calculate the value of the fine-structure constant.
Those observations did not reveal variations in the constant over time. But when the researchers compared their study to the larger web of data points collected in past research, they found it matched previous signs of possible variation along a spatial axis: Stronger measurements came from the direction facing toward the Milky Way’s galactic center, and weaker measurements were found in the opposite direction. This conjures up a model of a “dipole” universe, which might have something resembling a North and South pole.
“The fascinating scientific situation is that there are all these strange effects, hints of anisotropy and directionality in the universe, and many of them do line up on the sky,” said Webb. “Perhaps there is some kind of relationship between these things that we don’t yet fully understand, and it’s interesting to note this alignment.”
While the observations are certainly tantalizing, it will take more research to constrain what is causing these apparent fluctuations. They may turn out to be the result of more mundane issues, such as instrumentation that is not yet precise enough to avoid large error margins when making measurements.
“Whether this is just a set of cosmic coincidences or whether it’s telling us something meaningful about fundamental physics and the origin and evolution of the universe really remains to be seen,” Webb said. “At the moment, we just chip away the best we can, making the best measurements that we can, and in particular understanding the uncertainties in the measurements as best we can.”
“That’s where the main effort goes—to try and make sure we’re not fooling ourselves with something here, and just keep publishing the results and see what eventually emerges,” he noted.
The fine-structure constant is far from the only cosmic bread crumb that could lead to a model where laws and constants vary across the universe. Another study published in April, this time in the journal Astronomy & Astrophysics, also reported eerie anomalies in the X-ray light emitted by galaxy clusters.
Physicists led by Konstantinos Migkas, a PhD researcher at the University of Bonn in Germany, developed a new technique to “investigate the directional behavior” of X-rays emitted by the hot gas surrounding galaxy clusters, according to the study. Their findings line up with some of the results from other teams, pointing to more possible kinks in the cosmological principle.
“Galaxy clusters have not been used before for such a study,” Migkas said in a call, making them “a nice new tool to study the cosmological principle."
“We came up with this idea, an independent method, to test what other people have been testing for a while now,” he continued. “It turned out that it gave us very surprising results with very strong evidence because of these observations.”
Galaxy clusters are the largest gravitationally bound structures in the universe, and contain hundreds or even thousands of individual galaxies. As light from these clusters makes its way to Earth, it is stretched out by the expansion of the universe, so that more distant clusters appear redshifted.
Migkas and his colleagues calculated the X-ray brightness of gas in these clusters using two methods: One derived from the estimated temperature of the gas, a value that is not affected by the universe’s expansion, and another method that does account for the universe’s expansion rate. Tantalizingly, the results from these two tests didn’t always match: Clusters in one specific direction were systematically fainter than expected, and clusters in another direction were systematically brighter than expected.
Even weirder, the directions of these X-ray luminosities roughly match patterns identified by another team that has been hunting for potential cosmic anisotropies, which were published in Astronomy & Astrophysics in 2019. However, the clusters do not appear brighter or fainter along the same 180-degree dipolar axis that was described by Webb’s team: Instead, the angle appears to be closer to 120 degrees.
In this way, various models of potential directionality or anisotropies to the universe, based on observational data, both overlap and conflict with each other—in addition to clashing with other studies that support the model of cosmic isotropy at large scales. The universe is an extremely complicated entity, after all, and humans are constantly developing emerging technologies that reveal new layers of its bizarre intricacies.
To that point, Migkas and his colleagues presented several other explanations for their odd results. They suggested that gravitational forces near the galaxy clusters might be warping light, or that light could get distorted by gas clouds within our own Milky Way (or a combination of these factors).
“The brighter direction is suspiciously close to the galactic center,” Migkas said. “If I had to bet, I would say that the bright region is a result of some unknown X-ray issues that we haven’t yet discovered in our galaxy.”
“The other direction, the faint one, actually corresponds to the direction that other people found in the past, using totally independent methods,” he noted.
Dark matter and new physics
Of course, it’s also possible that these observations really do represent “new physics” that overturns the cosmological principle. One speculative explanation along those lines is that dark energy, the mysterious force propelling the expansion of the universe, might be unevenly applying its powers throughout space.
“Dark energy might form, for example, clumps, like normal matter or dark matter,” Migkas said. “Up until now, we have it in our minds like this is a constant uniform energy field, but it might very well be a material that forms clusters or structures.”
“An uneven distribution of this material to one side of the universe or the other side would cause such an anisotropy,” he added.
Whereas Webb’s team captured potential anomalies at huge distances and lookback times, the observations logged by Migkas and his colleagues come from galaxy clusters within about four billion light years of Earth. That’s still an enormous distance, to be sure, but it represents a more modern era in cosmic history—one in which dark energy has had more of an impact than the universe’s early years.
“If this is mostly happening at low distances, it could have something to do with dark energy because dark energy doesn’t play a very strong role at higher distances,” Migkas said. “And we don’t know anything about dark energy, right? We don’t know it’s nature, we don't know its behavior, so it’s just assumptions that we make about dark energy. Nobody forces dark energy to be isotropic, so it might have something to do with that, if it is cosmological.”
As with the other findings, this new report of weird anisotropies and a potential direction in the universe will have to be evaluated as more data is collected.
“People will try to focus on the alternative explanations before changing the cosmological model, and that’s the healthy thing to do"
“People try to preserve the standard model of whatever they have,” Migkas said. “As more and more evidence starts flowing, we’ll try to find extensions to our current model. Only if the need becomes an absolute necessity will we change our model.”
“People will try to focus on the alternative explanations before changing the cosmological model,” he concluded, “and that’s the healthy thing to do.”
These two new studies are only the latest in a long tradition of testing cosmic isotropy. For instance, scientists have spent years collecting cosmic clues about directionality and anisotropy in the universe by studying distant supernovae, or the explosion of stars.
As our observational tools become ever more sophisticated, a complex array of evidence is likely to emerge from diverse sources that will support models of cosmic isotropy, cosmic anisotropy, or perhaps even weirder permutations of the universe.
“We have this standard model and we’re looking for little cracks in it that might, if we pick at them, might reveal a richer structure or a more reliable theory,” said Caldwell, who was not involved in either study.
“It’s too easy to accept things like the cosmological principle and the validity of the laws of physics, because really those are things that you need to experimentally determine,” he added. “Given that there are huge mysteries that we are trying to figure out, it behooves us to check these fundamental assumptions—especially the cosmological principle.”