Inflation and shadow matter

Recent work on superstring theories has prompted interest in 'shadow matter', exotic matter which interacts only gravitationally with normal matter. Green and Schwarz^1,2 have shown that the only anomaly-free type 1 superstring theories are those with gauge group SO₃₂. Gross et al.³ have constructed a superstring theory resulting in a gauge group E₈×E₈. Such a theory could result, at low energies, in the existence of two sectors: an 'observed' sector associated with all familiar particles and interactions, and another 'hidden' sector, previously discussed in various low-energy super-symmetry models (see, for example, ref. 4 and references therein). The particles of the hidden E₈ sector couple only through gravitational interactions with ordinary matter. Kolb, Seckel and Turner⁵ have explored some of the astrophysical and cosmological implications of the existence of such shadow matter. We demonstrate here that if, in the early Universe, an inflationary phase^6-8 is associated with the breaking of one of the symmetries in the E₈×E ₈ theory, this strongly constrains the physics of both sectors if shadow matter is to be the missing mass in the Universe.