We employ Coleman-Weinberg and Higgs potentials to implement inflation in non-supersymmetric grand unified theories ( GUTs ) such as SU ( 5 ) and SO ( 10 ) .
To realize a scalar spectral index close to 0.96 , as indicated by the most recent WMAP 5-year anlaysis , the energy scale of observable inflation turns out to be of order 10 ^ { 16 } GeV .
This implies a GUT symmetry breaking scale of similar magnitude , and proton lifetime of order 10 ^ { 34 } - 10 ^ { 38 } years .
In some SO ( 10 ) models with axion dark matter , the scalar leptoquark boson exchange leads to proton decay with a lifetime of order 10 ^ { 34 } - 10 ^ { 35 } years .