麻豆传媒

It鈥檚 not just particle physics that has its standard model; cosmology does, too. While our picture of the very small is rooted in quantum theory, our model of the universe at large is based on general relativity, Einstein鈥檚 theory of gravity.

General relativity鈥檚 equations are notoriously complex. Making a workable cosmological model out of them requires a crucial simplifying assumption: that the universe is pretty much the same on all scales and in all directions. That seems reasonable enough, given everything in the universe started in the same place, in the pinprick of the big bang. But there are more than a few hints it ain鈥檛 necessarily so.

They are most obviously seen in the cosmic microwave background (CMB), radiation sent scudding across space-time some 380,000 years after the big bang as it pinged off the first atoms that formed in the cooling universe. The atoms would have been uniformly distributed, more or less, giving the CMB a uniform temperature 鈥� now a frigid 2.725 kelvin, after more than 13 billion years of cooling. Small patches are a little hotter and cooler, but only by a few hundred-thousandths of a kelvin. These indicate small matter inhomogeneities that you would expect because of random quantum fluctuations in the density of the early universe. These blobs would later become the seeds for structures such as stars and galaxies.

So far, so good. But in 2005, Kate Land and Jo茫o Magueijo of Imperial College London discovered a string of hot and cold spots stretching on an axis across the cosmic microwave background 鈥� something entirely incompatible with a uniform universe.

The most accurate maps of the CMB to date, courtesy of the European Space Agency鈥檚 Planck satellite, confirm the existence of this 鈥渁xis of evil鈥�, as well as a cold spot much larger and much colder than should occur in a uniform bath of radiation. Add that to a handful of other, more technical, but also persistent anomalies, and cosmologists are left scratching their heads. 鈥淔or a long time, part of the community was hoping that this would go away,鈥� says of the University of Bielefeld in Germany.

But it hasn鈥檛. Exotic explanations for the patterns are that they result from collisions with another universe, entanglement with another cosmos or a 鈥減hase change鈥� in the early universe 鈥� something akin to ice melting into water. At the other end of the scale, the anomalies could still be a statistical fluke born out of random variation, says , a cosmologist based at the University of Oxford 鈥� after all, we only have one sky to look at. Efforts are now afoot to measure the polarisation of the CMB. If the polarisation map also contains anomalies, that will strengthen the case for re-examining our cosmic model.

That could mean a long, hard look at the homogeneity assumption. Maybe there is a skew introduced because the universe is different outside our particular patch 鈥� if Earth were sitting in a giant void of lower than average density, for example. Or perhaps something in those quantum fluctuations caused the early universe to generate less structure over the largest scales, altering the CMB鈥檚 properties. 鈥淭hese anomalies could certainly teach us about things that might be going on beyond the scale of our cosmic horizon,鈥� says Dunkley. 鈥淩ight now there isn鈥檛 a good model that fits them, but that doesn鈥檛 mean there isn鈥檛 one out there.鈥�

Future surveys of the large-scale structure of galaxies could give us a wider perspective. Meanwhile there is also everyone鈥檚 least favourite option: that the universe is, for some unknown reason, just not homogenous or isotropic on any scale. If so, there鈥檇 be no exciting new stuff to investigate, just a long hard slog to fully understand what we already know.

Read more: 鈥�The 6 biggest glitches in physics鈥�

This article appeared in print under the headline 鈥淭hat鈥檚 odd鈥� The axis of evil鈥�