The concept of early Universe inflation resolves several problems of hot Big
Bang theory and quantitatively explains the origin of the inhomogeneities in
the present Universe. However, it is not possible to arrange inflation in a
scalar field model with renormalizable potential, such that it would not
contradict the recent Planck data. For this reason, inflaton must have also
higher derivative couplings suppressed at least by the Planck scale. We show
that these couplings may be relevant during reheating and lead to
non-negligible production of gravitons. We consider the possibility that the
unitarity breaking scale for the model of inflation is lower than the Planck
scale and compute production of gravitons during reheating, due to the inflaton
decay to two gravitons and graviton bremsstrahlung process. The spectrum of
produced gravitons is crucially dependent on reheating temperature and inflaton
mass. We find that for low reheating temperature decay to gravitons lead to
significant amount of dark radiation. Confronting this result with CMB
constraints, we find reheating dependent bounds on the unitarity breaking
scale. We also compare the obtained gravitational wave signals with the
projected limits of future high frequency gravitational wave experiments.
Conclusions:
- The concept of early Universe inflation addresses issues with the hot Big Bang theory and explains the origin of inhomogeneities in the Universe.
- In order to avoid contradictions with recent Planck data, inflation must involve higher derivative couplings suppressed by the Planck scale.
- These couplings may be relevant during reheating and lead to significant production of gravitons.
- The spectrum of produced gravitons depends on reheating temperature and inflaton mass.
- Low reheating temperatures result in a significant amount of dark radiation.
- CMB constraints place bounds on the unitarity breaking scale for the model of inflation.
- Future high frequency gravitational wave experiments can help further constrain the gravitational wave signals.
Future Roadmap:
While early Universe inflation has provided valuable insights into the origin of our Universe, there are still challenges and opportunities on the horizon:
Challenges:
- Resolving the contradiction between inflation and recent Planck data, as the current scalar field model with renormalizable potential is not consistent.
- Determining the relevance of higher derivative couplings during reheating and understanding their impact on graviton production.
- Exploring the consequences of different reheating temperatures and inflaton masses on the spectrum of produced gravitons.
- Understanding the implications of significant dark radiation resulting from decay to gravitons at low reheating temperatures.
- Addressing the boundaries placed on the unitarity breaking scale by CMB constraints and refining the model accordingly.
Opportunities:
- Further advancing our understanding of early Universe inflation and its role in resolving cosmological problems.
- Using future high frequency gravitational wave experiments to validate and refine the model by comparing the obtained gravitational wave signals with projected limits.
- Investigating alternative models or modifications that could address the challenges and limitations identified in the current model.
- Collaborating with researchers in fields such as particle physics and cosmology to gain a more comprehensive understanding of the underlying mechanisms.
By addressing these challenges and exploring the opportunities, we can continue to expand our knowledge of the early Universe and its inflationary phase, ultimately leading to a more complete understanding of the origins of our Universe and its structure.