A Gravitational Wave Background from Reheating after Hybrid Inflation (0707.0839v2)

Abstract: The reheating of the universe after hybrid inflation proceeds through the nucleation and subsequent collision of large concentrations of energy density in the form of bubble-like structures moving at relativistic speeds. This generates a significant fraction of energy in the form of a stochastic background of gravitational waves, whose time evolution is determined by the successive stages of reheating: First, tachyonic preheating makes the amplitude of gravity waves grow exponentially fast. Second, bubble collisions add a new burst of gravitational radiation. Third, turbulent motions finally sets the end of gravitational waves production. From then on, these waves propagate unimpeded to us. We find that the fraction of energy density today in these primordial gravitational waves could be significant for GUT-scale models of inflation, although well beyond the frequency range sensitivity of gravitational wave observatories like LIGO, LISA or BBO. However, low-scale models could still produce a detectable signal at frequencies accessible to BBO or DECIGO. For comparison, we have also computed the analogous gravitational wave background from some chaotic inflation models and obtained results similar to those found by other groups. The discovery of such a background would open a new observational window into the very early universe, where the details of the process of reheating, i.e. the Big Bang, could be explored. Moreover, it could also serve in the future as a new experimental tool for testing the Inflationary Paradigm.

• The paper demonstrates that reheating after hybrid inflation produces significant gravitational waves via tachyonic preheating, bubble collisions, and turbulence.
• It employs detailed lattice simulations to capture non-linear field dynamics and contrast gravitational signatures in hybrid and chaotic inflation models.
• The findings suggest that future detectors like BBO or DECIGO could observe these waves, opening new avenues for probing early universe cosmology.

Analyzing the Gravitational Wave Background from Reheating after Hybrid Inflation

The paper investigates the generation of a gravitational wave background (GWB) during the reheating period following hybrid inflation. This period is characterized by the nucleation and collision of bubble-like structures and subsequent turbulence, significantly contributing to the stochastic background of gravitational waves (GW). The paper provides theoretical insights and numerical simulations into the dynamics of the hybrid inflation model, exploring the implications for early universe cosmology and the inflationary paradigm.

The authors focus on hybrid inflation models, wherein the potential involves symmetry-breaking fields coupled to flat directions. These models are pivotal in many extensions of the Standard Model, including supersymmetric and string theories. The proposed scenario proceeds through several stages: tachyonic preheating, bubble collisions, and turbulence, each contributing to GW production.

Key Findings

1. Stages of Gravitational Wave Production:
   • Tachyonic Preheating: Initially, the symmetry-breaking field experiences a spinodal instability, causing fast exponential growth. These dynamics lead to significant GW radiation during the early moments post-inflation.
   • Bubble Collisions: As the universe reheats, bubbles of Higgs fields collide at relativistic speeds, generating bursts of GW. The energy density fraction for GWs during these collisions is significant, although the resulting frequencies may not fall within current detector ranges.
   • Turbulence: Post-collision, the universe enters a turbulent regime. The role of turbulence in GW production remains complex and requires further analytical exploration to be fully understood.
2. Frequency Range and Detectability: The paper evaluates the energy density of produced GWs today, indicating that GWs from certain models of hybrid inflation could be detectable by future observations like BBO or DECIGO, especially in low-scale scenarios. High-scale models, however, might remain inaccessible due to their placement in frequency ranges not detectable by current or near-future instruments.
3. Numerical Simulations & Methodology:
   • The paper utilizes lattice simulations to probe the evolution of fields and GWs, offering a robust avenue for understanding non-linear dynamics during reheating.
   • By comparing chaotic and hybrid inflation models, the authors assess different sources of GWs, noting that chaotic models produce coherent oscillations leading to specific GW spectra.

Implications and Future Directions

The paper's findings are significant for understanding the early universe's dynamics and provide a novel observational window into the reheating process—a cornerstone of the Big Bang theory. Gravitational waves from reheating thus serve as a tool to probe the inflationary epoch, allowing researchers to distinguish between different inflationary models based on their GW signatures.

Future research should focus on refining the theoretical modeling of turbulence's contribution to GW production and improving numerical simulations to encompass gauge fields and quantum backreactions more comprehensively. This could enhance the predictive ability concerning GW signals and inform the design of future GW observatories.

In conclusion, the paper offers a comprehensive examination of gravitational wave production in hybrid inflation scenarios, underlying their potential as experimental probes for early universe cosmology and the inflationary paradigm. While challenges remain in detecting such cosmological signals, advances in detector technology and theoretical modeling hold promise for significant discoveries in the coming decades.