The Universe is incredibly regular. The variation of the cosmos' temperature across the entire sky is tiny: a few millionths of a degree, no matter which direction you look. Yet the same light from the very early cosmos that reveals the Universe's evenness also tells astronomers a great deal about the conditions that gave rise to irregularities like stars, galaxies, and (incidentally) us.
That light is the cosmic microwave background, and it provides some of the best knowledge we have about the structure, content, and history of the Universe. But it also contains a few mysteries: on very large scales, the cosmos seems to have a certain lopsidedness. That slight asymmetry is reflected in temperature fluctuations much larger than any galaxy, aligned on the sky in a pattern facetiously dubbed "the axis of evil.”
The lopsidedness is real, but cosmologists are divided over whether it reveals anything meaningful about the fundamental laws of physics. The fluctuations are sufficiently small that they could arise from random chance. We have just one observable Universe, but nobody sensible believes we can see all of it. With a sufficiently large cosmos beyond the reach of our telescopes, the rest of the Universe may balance the oddity that we can see, making it a minor, local variation.
However, if the asymmetry can't be explained away so simply, it could indicate that some new physical mechanisms were at work in the early history of the Universe. As Amanda Yoho, a graduate student in cosmology at Case Western Reserve University, told Ars, "I think the alignments, in conjunction with all of the other large angle anomalies, must point to something we don't know, whether that be new fundamental physics, unknown astrophysical or cosmological sources, or something else.”
The cosmic microwave background
Over the centuries, astronomers have provided increasing evidence that Earth, the Solar System, and the Milky Way don't occupy a special position in the cosmos. Not only are we not at the center of existence—much less the corrupt sinkhole surrounded by the pure crystal heavens, as in early geocentric Christian theology—the Universe has no center and no edge.
In cosmology, that's elevated to a principle. The Universe is isotropic, meaning it's (roughly) the same in every direction. The cosmic microwave background (CMB) is the strongest evidence for the isotropic principle: the spectrum of the light reaching Earth from every direction indicates that it was emitted by matter at almost exactly the same temperature.
The Big Bang model explains why. In the early years of the Universe's history, matter was very dense and hot, forming an opaque plasma of electrons, protons, and helium nuclei. The expansion of space-time thinned out until the plasma cooled enough that stable atoms could form. That event, which ended roughly 380,000 years after the Big Bang, is known as recombination. The immediate side effect was to make the Universe transparent and liberate vast numbers of photons, most of which have traveled through space unmolested ever since.
We observe the relics of recombination in the form of the CMB. The temperature of the Universe today is about 2.73 degrees above absolute zero in every part of the sky. The lack of variation makes the cosmos nearly as close to a perfect thermal body as possible. However, measurements show anisotropies—tiny fluctuations in temperature, roughly 10 millionths of a degree or less. These irregularities later gave rise to areas where mass gathered. A perfectly featureless, isotropic cosmos would have no stars, galaxies, or planets full of humans.
To measure the physical size of these anisotropies, researchers turn the whole-sky map of temperature fluctuations into something called a power spectrum. That's akin to the process of taking light from a galaxy and finding the component wavelengths (colors) that make it up. The power spectrum encompasses fluctuations over the whole sky down to very small variations in temperature. (For those with some higher mathematics knowledge, this process involves decomposing the temperature fluctuations in spherical harmonics.)
Smaller details in the fluctuations tell cosmologists the relative amounts of ordinary matter, dark matter, and dark energy. However, some of the largest fluctuations—covering one-fourth, one-eighth, and one-sixteenth of the sky—are bigger than any structure in the Universe, therefore representing temperature variations across the whole sky.
The “axis of evil”
Those large-scale fluctuations in the power spectrum are where something weird happens. The temperature variations are both larger than expected and aligned with each other to a high degree. That's at odds with theoretical expectations: the CMB anisotropies should be randomly oriented, not aligned. In fact, the smaller-scale variations are random, which makes the deviation at larger scales that much stranger.
Kate Land and Joao Magueijo jokingly dubbed the strange alignment “the axis of evil” in a 2005 paper (freely available on the ArXiv), riffing on an infamous statement by then-US President George W. Bush. Their findings were based on data from an earlier observatory, the Wilkinson Microwave Anisotropy Probe (WMAP), but the follow-up Planck mission found similar results. There's no question that the “axis of evil” is there; cosmologists just have to figure out what to think about it.
The task of interpretation is complicated by what's called “cosmic variance,” or the fact that our observable Universe is just one region in a larger Universe. Random chance dictates that some pockets of the whole Universe will have larger or smaller fluctuations than others, and those fluctuations might even be aligned entirely by coincidence.
In other words, the “axis of evil” could very well be an illusion, a pattern that wouldn't seem amiss if we could see more of the Universe. However, cosmic variance also predicts how big those local, random deviations should be—and the fluctuations in the CMB data are larger. They're not so large as to rule out the possibility of a local variation entirely—they're above-average height—but cosmologists can't easily dismiss the possibility that something else is going on.
One of those possibilities is that these fluctuations are due to something in the “foreground,” meaning anything that might have interfered with the CMB photons between their emission and our detectors. Those who favor this solution point out the odd coincidence that the “axis of evil” is somewhat aligned with the ecliptic in the Solar System, which is the plane containing the orbits of the planets and many other bodies. What sort of absorber that might be is puzzling, however, since it must closely mimic the power spectrum in most respects or we wouldn't see the expected results at smaller scales.
As Johns Hopkins University cosmologist Marc Kamionkowski phrased it, "These CMB anomalies, if real, will pose similarly big questions for fundamental physics and for the prevailing inflationary paradigm for the origin of the Universe. The case that these anomalies are real is, however, nowhere nearly as well established. They may be there, but they may just as well not."
A few modest proposals
Kamionkowski has coauthored a number of papers on the lopsidedness problem, approaching it from several angles. However, he pointed out that it's not good enough for us to try to explain the “axis of evil” in isolation. “Given the limitations imposed by cosmic variance, my approach has been to think of physical models to explain the anomaly and then see whether these models make any other predictions that can be tested." The more predictions a theory makes, the better: it's possible to distinguish them from each other and to rule out possibilities based on existing or future data.
One way to make the Universe slightly lopsided involves tweaking the process that most cosmologists think created the near-perfect power spectrum we see—inflation. Within the first tiny fraction of a second after the Big Bang, inflation caused an extremely rapid expansion. That explains why the different sides of the sky are almost exactly the same temperature, even though they aren't exchanging energy with each other and haven't been for 13.8 billion years. Kamionkowski and collaborators Donghui Jeong, Jens Chluba, and Liang Dai proposed that if inflation acted in a slightly different way, it could make the cosmos lopsided. This model makes very little alteration to physical laws, but if true, its effects could be measured in the CMB and distribution of galaxies.
Another model that would do minimum violence to physics as we know it was proposed by Andrew Liddle and Marina Cortês. This theory describes a Universe that is very slightly “open," meaning that light rays that begin in parallel diverge a tiny bit over time. Astronomers have shown that the Universe is nearly “flat”—parallel light rays stay parallel—using a variety of observations. Inflation provides a simple explanation for why it should be almost but not quite flat, meaning there's still room for a very small deviation from flatness, both on theoretical grounds and within the experimental errors of our observations. As with the proposal from Kamionkowski and collaborators, a slightly different behavior in inflation could generate this effect.
Many have focused on inflation as a means for explaining the "axis of evil" because we still don't have a lot of data for precisely how it works. Inflation is a general idea rather than a specific theory, leaving a lot of wiggle room for different ways to achieve a nearly flat cosmos. WMAP and Planck have helped rule out some of the more elaborate theories of inflation, but that leaves a number of options that fit the data equally well at present.
Kamionkowski described another possible explanation: the simplest model of inflation contains one type of quantum fluctuation, which makes sure both sides of the sky are the same. If there are two or more types, however, their different effects could produce slightly different large-scale fluctuations depending on which way astronomers look. However, he stressed that this is a preliminary idea: "Although we have yet to write down a physical model that gives rise to such variation, the required variation is sufficiently innocuous to suggest that a fairly minor modification of inflation may work. But we've still got some work left to verify that."
Yoho takes the “axis of evil” problem seriously but doesn't see an obvious resolution in any of the proposals. She told Ars, “I enjoy that people play around with it and try to come up with ideas, even if they seem a little contrived, because that's what us theorists do. But so far nothing I've seen makes me think 'whoa, that's promising...'” She mentioned a particular model that proposed a radical alteration to the consensus on space-time geometry. (Interestingly, this is a similar idea to one bandied about in the days before precision CMB data were available.) However, she cautioned that while it solves the lopsidedness problem, it doesn't fit the other cosmological data, so she can't have any confidence in it—at least not yet.
Our trickster Universe
Models to explain the “axis of evil” have proliferated, which is likely a sign we need more data: theory so far seems unlikely to show us the way out. Unfortunately, CMB temperature data alone is probably insufficient to resolve the issue. Between WMAP and Planck, we have temperature maps of the whole sky that are as close to perfect as we can obtain. (Smaller-scale fluctuations are best studied by ground-based observatories like the Atacama Cosmology Telescope and the South Pole Telescope, which can do a better job for less money than orbiting probes.)
In other words, we need to look at something other than the power spectrum of the CMB. Some aspects of inflation could be measured from a particular polarization mode of the CMB. That mode is a very small signal, one that nobody has succeeded in isolating yet. High-precision measurements of the distribution of galaxies and galaxy clusters could also provide some hints about what's going on.
The first step, of course, is establishing whether the “axis of evil” is a sign of some new physics, interference from a foreground object, or just a statistical fluke. If the Universe is truly lopsided, then the problem is as significant as understanding dark energy. Unlike the dark energy problem, however, we're not yet certain there's a real problem. Yoho told Ars, "The general consensus in the field I'm sure would be that dark matter and dark energy are more pressing issues. But if it turned out that these large angle anomalies were due to primordial physics and not a statistical fluke or systematic errors (two big 'ifs', granted), the impact of the results would huge since there is no way for [the standard cosmological model] to account for that behavior. And that's the kind of thing that us theorists have a field day with."
Yoho continued, "It's common for people to use the statement 'if you look for problems in your data you will find them.' I personally find the existence of several different large angle anomalies to be a compelling reason to warrant further investigation. At this point, though, temperature data alone isn't going to give us any further insight…we need to look into other cosmological probes."
For good or evil, this observable Universe is all we have to study. Despite the paucity of data on the cosmos beyond our field of view, cosmologists are a crafty and resourceful bunch. We may need all that ingenuity in the face of a subtle Universe.
Source: ARS TECHNICA
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