CPT-Symmetric Universe.

We propose that the state of the Universe does not spontaneously violate CPT. Instead, the Universe after the big bang is the CPT image of the Universe before it, both classically and quantum mechanically. The pre- and postbang epochs comprise a universe-antiuniverse pair, emerging from nothing directly into a hot, radiation-dominated era. CPT symmetry selects a unique QFT vacuum state on such a spacetime, providing a new interpretation of the cosmological baryon asymmetry, as well as a remarkably economical explanation for the cosmological dark matter. Requiring only the standard three-generation model of particle physics (with right-handed neutrinos), a Z_{2} symmetry suffices to render one of the right-handed neutrinos stable. We calculate its abundance from first principles: matching the observed dark matter density requires its mass to be 4.8×10^{8} GeV. Several other testable predictions follow: (i) the three light neutrinos are Majorana particles and allow neutrinoless double ß decay; (ii) the lightest neutrino is massless; and (iii) there are no primordial long-wavelength gravitational waves. We mention connections to the strong CP problem and the arrow of time.

Symmetric Satellite Swarms and Choreographic Crystals.

In this Letter, we introduce a natural dynamical analogue of crystalline order, which we call choreographic order. In an ordinary (static) crystal, a high degree of symmetry may be achieved through a careful arrangement of the fundamental repeated elements. In the dynamical analogue, a high degree of symmetry may be achieved by having the fundamental elements perform a carefully choreographed dance. For starters, we show how to construct and classify all symmetric satellite constellations. Then we explain how to generalize these ideas to construct and classify choreographic crystals more broadly. We introduce a quantity, called the "choreography" of a given configuration. We discuss the possibility that some (naturally occurring or artificial) many-body or condensed-matter systems may exhibit choreographic order, and suggest natural experimental signatures that could be used to identify and characterize such systems.

Quantifying the BICEP2-Planck tension over gravitational waves.

The recent BICEP2 measurement of B-mode polarization in the cosmic microwave background (r = 0.2(-0.05)(+0.07)), a possible indication of primordial gravity waves, appears to be in tension with the upper limit from WMAP (r < 0.13 at 95% C.L.) and Planck (r < 0.11 at 95% C.L.). We carefully quantify the level of tension and show that it is very significant (around 0.1% unlikely) when the observed deficit of large-scale temperature power is taken into account. We show that measurements of TE and EE power spectra in the near future will discriminate between the hypotheses that this tension is either a statistical fluke or a sign of new physics. We also discuss extensions of the standard cosmological model that relieve the tension and some novel ways to constrain them.

Testing inflation: a bootstrap approach.

We note that the essential idea of inflation, that the Universe underwent a brief period of accelerated expansion followed by a long period of decelerated expansion, can be encapsulated in a "closure condition" which relates the amount of accelerated expansion during inflation to the amount of decelerated expansion afterward. We present a protocol for systematically testing the validity of this condition observationally.

Binary-black-hole merger: symmetry and the spin expansion.

We regard binary-black-hole (BBH) merger as a map from a simple initial state (two Kerr black holes, with dimensionless spins a and b) to a simple final state (a Kerr black hole with mass m, dimensionless spin s, and kick velocity k). By expanding this map around a=b=0 and applying symmetry constraints, we obtain a simple formalism that is remarkably successful at explaining existing BBH simulations. It also makes detailed predictions and suggests a more efficient way of mapping the parameter space of binary black-hole merger. Since we rely on symmetry rather than dynamics, our expansion complements previous analytical techniques.

Inflationary predictions for scalar and tensor fluctuations reconsidered.

We reconsider the predictions of inflation for the spectral index of scalar (energy density) fluctuations (ns) and the tensor/scalar ratio (r) using a discrete, model-independent measure of the degree of fine-tuning required to obtain a given combination of (ns, r ). We find that, except for cases with numerous unnecessary degrees of fine-tuning, ns is less than 0.98, measurably different from exact Harrison-Zel'dovich. Furthermore, if ns >or= 0.95, in accord with current measurements, the tensor/scalar ratio satisfies r >or= 10(-2), a range that should be detectable in proposed cosmic microwave background polarization experiments and direct gravitational wave searches.