Higher-spin signatures of holography in the early universe.

The early universe is the ultimate experiment in high-energy physics. Through modern telescopes and satellites, we can image a faint glow of light -- the cosmic microwave background -- that is the last surviving trace of the radiation fireball that filled the universe 380,000 years after the big bang. Curiously, the temperature of this fading fire-ball is not quite uniform but varies slightly over different patches of the night sky. These small variations in temperature arise from quantum fluctuations and hold precious clues to the earliest moments of the universe: to correctly predict their statistical properties is a make-or-break test of any new cosmological theory. At the same time, a breathtaking race is now under way to accurately measure similar small variations recently found in the polarisation pattern of this fireball. These small variations in polarisation carry new information about the early universe, and could one day provide evidence for a gravitational wave echo accompanying the primordial fireball.

To understand the earliest moments of the universe requires a new understanding of physics. Einstein's theory of gravity describes spacetime in terms of geometry, and as we go back in time to the big bang, it predicts this geometry should become more and more curved, until at some finite time in the past, the curvature becomes infinite. This moment, the big bang singularity, represents a breakdown of Einstein's theory and is thought to be curable only through a fully quantum theory of gravity. Based on current ideas from black hole physics and string theory, our best guess is that this quantum theory of gravity should be "holographic". This means it should allow the early universe to be described in terms of a second, and at first sight, completely different theory. This alternative "holographic'' description takes the form of an ordinary quantum theory, very similar to the kind routinely used to describe particle collisions at the Large Hadron Collider, but with one very surprising feature: it lives in only three spatial dimensions and is missing the dimension of time.

My work has shown how to construct just such a holographic description of cosmology, revealing a new solution to the problem of the big bang singularity. At early times, when a geometrical description of spacetime breaks down as in Einstein's theory, we can instead turn to the alternative holographic description in terms of an ordinary quantum theory living in one dimension less. This holographic description remains completely well-behaved, even when spacetime can no longer be described in terms of geometry, meaning there is no longer any loss of predictive capability. In past work, I have shown how to reconstruct the detailed patterns we see in the cosmic microwave background starting directly from this holographic description in terms of an ordinary quantum theory. My new research will explore a key prediction made by this scenario: the existence of a special class of particles, known as higher-spin particles, in the early universe. A famous theorem by Higuchi restricts the masses these particles are allowed to have, and further investigation is urgently needed to determine whether the holographic scenario can satisfy both this theorem and all observational data. The presence of these higher-spin particles at early times could also leave direct tell-tale signatures in the temperature variations and polarisation patterns of the cosmic microwave background. My work will predict the form of these tell-tale signatures and explore new strategies for detecting them in future experiments. The results of this investigation could either confirm, or else rule-out, the natural solution holography offers to the problem of the big bang singularity.