The Rule Of Anomalies

These days, physics seems to find itself in a situation similar to that faced by physicists at the turn of the 19th century, just before the dawn of quantum mechanics and general relativity. In 1894, the co-discoverer of the constancy of the speed of light and first American Nobel laureate in physics, Albert Michelson, stated, “It seems probable that most of the grand underlying principles of physical science have been firmly established.” Many, if not most, physicists at that time believed that the handful of experimental anomalies that seemed to violate those principles were minor details that would eventually be explained by the paradigm of classical physics. Within a generation, history showed that quantum mechanics and Einstein’s theory of relativity had to be invented, and classical physics had to be overthrown, in order to explain those minor experimental details.

In 2012, I spent a year on sabbatical at Princeton due to an invitation by David Spergel, one of the lead scientists of the WMAP space satellite. The satellite was designed to make, at the time, the most precise measurements of the afterglow of the Big Bang, the ripples in the cosmic microwave background radiation (CMB), which according our standard model of cosmology would later develop into the vast structures in our universe, to which there is remarkable agreement. Our best theory of the early universe, cosmic inflation, is armed with the most precise physics known to us, general relativity and quantum field theory. Both theories have been independently tested with beyond hair-thin accuracy.

Despite the success of inflation in predicting a handful of features that was observed by WMAP, similar to the end of the 19th century, some nagging anomalies persist. At Princeton, my colleagues and I spent that year wrestling with those anomalies to no avail. But I forgot about them after a discussion with David who comforted me with the sentiment that the WMAP anomalies are probably due to some unaccounted experimental systematic rather than some strange phenomenon in the sky. David also warned, “if the anomalies persist in the PLANCK satellite, then we will have to take them more seriously.” Well, in 2014 the Planck satellite made an even more precise measurement of the CMB anisotropies and came back with the arguably the most nagging in the suite of anomalies-the hemispherical anomaly. What’s that?

The undulations in the CMB reflect a prediction from cosmic inflation that, aside from those tiny waves, on the largest distance scales the universe looks the same in every direction and at every vantage point in space. This prediction is consistent with one of the pillars of modern cosmology, the Cosmological Copernican Principle. During the epoch that the CMB anisotropies were formed, they too are supposed to, on average look the same in every direction. The theory of cosmic inflation generically predicts this feature. This means that if one divides the sky into two arbitrary hemispheres, we should see the same statistical features of the anisotropies in both hemispheres.

However, both WMAP and Planck see a difference in the amount of anisotropies in different hemispheres in the sky. This feature is in tension one of the most powerful attributes of inflation, whose rapid expansion of space-time smooths out any large-scale directional preference, while democratically sprinkling the space-time fabric with the same amount of ripples in every direction. With some decorative tweaking, it is possible to modify inflation to account for the anomaly, but this seems to be at odds with what inflation was invented for-to make the early universe smooth enough and see the tiny anisotropies that later become galaxies. One might think that this would be a great opportunity for alternative theories of the early universe, such as bouncing/cyclic cosmologies to rise to the occasion and explain the anomalies, but so far, there is no compelling alternative.

Recently, on planet Earth at the Large Hadron Collider in Geneva, the ATLAS, and CMS experiments both reported an “anomaly” when protons collide on energy scales close to 1 trillion electron volts. The experiments see the predicted production of elementary particles that standard quantum field theory of elementary particle interactions predict, with the exception of an excess production of light. Unfortunately, if this observation persists with statistical significance, we will need physics beyond our standard model to explain.

Both of these anomalies, to me and my theoretical colleagues, is a good thing. Will it mean simple, yet less than pretty fix ups of our current “standard models”? Might it be one of our super theories, such as, supersymmetry, strings, loop quantum gravity or GUTS to come to the rescue? Could it be that both anomalies are connected in some yet unseen way? Maybe the anomalies will point us in a direction that our current mode of thinking has yet to do. Whatever it may be, if the anomalies persist, this is exciting times to be a theorist.