A two-sheeted, CPT-symmetric universe

<!--HTML--><p>After reviewing some key hints and puzzles from the early universe, I will introduce recent work with Neil Turok suggesting a rigid and predictive new approach to addressing them.</p> <p>Our universe seems to be dominated by radiation at early times, and positive vacuum energy at late times.&nbsp; Taking the symmetry and analyticity properties of such a universe seriously suggests a picture in which spacetime has two sheets, exchanged by a symmetry that, in turn, selects a preferred (CPT-symmetric) vacuum state for the quantum fields that live on the spacetime.&nbsp; In this picture, the Big Bang may be regarded as a kind of mirror.&nbsp;&nbsp;</p> <p>I will explain how this line of thought suggests new explanations for a number of observed properties of the universe, including: its homogeneity, isotropy and flatness; the arrow of time; several properties of the primordial perturbations; the nature of dark matter (which, in this picture, is a right-handed neutrino, radiated from the Big Bang like Hawking radiation from a black hole); and even the existence of three generations of standard model fermions.&nbsp; I will also mention some observational predictions that will be tested in the coming decade, and some key open questions.</p>