Recent work on superstring theories has prompted interest in 'shadow matter', exotic matter which interacts only gravitationally with normal matter. Green and Schwarz1,2 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 phase6-8 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.
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