Cosmic inflation
Starobinsky inflation

In the Soviet Union, Alexei Starobinsky noted that quantum corrections to general relativity should be important for the early universe. These generically lead to curvature-squared corrections to the Einstein–Hilbert action and a form of f (R) modified gravity. The solution to Einstein's equations in the presence of curvature squared terms, when the curvatures are large, leads to an effective cosmological constant. Therefore, he proposed that the early universe went through an inflationary de Sitter era. This resolved the cosmology problems and led to specific predictions for the corrections to the microwave background radiation, corrections that were then calculated in detail. Starobinsky used the action

which corresponds to the potential

in the Einstein frame. This results in the observables: n s = 1 − 2 N, r = 12 N 2. {\displaystyle n_{s}=1-{\frac {2}{N}},\quad \quad r={\frac {12}{N^{2}}}.}
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