Gravitational portals in the early Universe

We consider the production of matter and radiation during reheating after inflation, restricting our attention solely to gravitational interactions. Processes considered are the exchange of a graviton, <math display="inline"><mrow><msub><mrow><mi>h</mi></mrow><mrow><mi>μ</mi><mi>ν</mi></mrow></msub></mrow></math>, involved in the scattering of the inflaton or particles in the newly created radiation bath. In particular, we consider the gravitational production of dark matter (scalar or fermionic) from the thermal bath as well as from scattering of the inflaton condensate. We also consider the gravitational production of radiation from inflaton scattering. In the latter case, we also derive a lower bound on the maximal temperature of order of <math display="inline"><msup><mn>10</mn><mn>12</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math> for a typical <math display="inline"><mi>α</mi></math>-attractor scenario from <math display="inline"><mi>ϕ</mi><mi>ϕ</mi><mo stretchy="false">→</mo><msub><mi>h</mi><mrow><mi>μ</mi><mi>ν</mi></mrow></msub><mo stretchy="false">→</mo></math> Standard Model fields (dominated by the production of Higgs bosons). This lower gravitational bound becomes the effective maximal temperature for reheating temperatures, <math display="inline"><msub><mi>T</mi><mrow><mi>RH</mi></mrow></msub><mo>≲</mo><msup><mn>10</mn><mn>9</mn></msup><mtext> </mtext><mtext> </mtext><mi>GeV</mi></math>. The processes we consider are all minimal in the sense that they are present in any nonminimal extension of the Standard Model theory based on Einstein gravity and cannot be neglected. We compare each of these processes to determine their relative importance in the production of both radiation and dark matter.