What the βS³ Nonlinearity Means
🌊 What the βS³ Nonlinearity Means
In the substrate field equation:
∂t2S − c2∇2S + βS3 = σ(x,t) FR(C[Ψ]),
the βS³ term adds a nonlinear stiffness to the substrate. This means the “elastic medium” of reality doesn’t behave purely like a simple wave equation — the cubic term makes wave propagation depend on amplitude and scale.
📡 Gravitational‑Wave Dispersion
In standard General Relativity, gravitational waves always travel at the speed of light (luminal propagation).
In RST, the βS³ nonlinearity predicts that on very large (horizon‑scale) wavelengths, gravitational waves could disperse slightly — meaning different frequencies might travel at slightly different speeds. Locally (like LIGO/Virgo detections), the effect would be negligible, because β is so small. But across cosmological distances, the cumulative effect could become measurable.
🔬 How to Probe This
- Pulsar Timing Arrays (PTAs): Measure ultra‑low‑frequency gravitational waves across galactic scales.
- Cosmic Microwave Background (CMB): Subtle imprints in polarization or lensing could reflect dispersion.
- Integrated Sachs–Wolfe effect (ISW): Changes in photon paths through evolving gravitational potentials might carry signatures.
- Future space‑based GW detectors (like LISA): Could test dispersion at intermediate scales.
✊ Why It Matters
If dispersion is detected, it would be a smoking gun for physics beyond General Relativity. RST’s prediction is distinctive: dispersion only appears on horizon scales, not locally. That’s a clear way to differentiate it from ΛCDM, which predicts no dispersion at all. It turns β from a symbolic constant into a measurable parameter — something you can fit against data.
👉 In short: the βS³ nonlinearity is RST’s big testable prediction. It says gravitational waves might not be perfectly luminal across the universe, and cosmological datasets are the right place to look for that subtle effect.
Calibration Roadmap for β and Matter Coupling
Here’s a focused, publish‑ready plan to turn RST’s parameters into testable numbers and separate RST from ΛCDM using current and near‑term observations.
Model Setup
Background expansion:
H2(z) = H02[Ωm0(1+z)3 + ΩS0(1+z)3(1+wS)]
Core Parameters to Fit
| Parameter | Description | Notes |
|---|---|---|
| β | Nonlinear elasticity constant | Controls horizon‑scale dispersion |
| wS | Substrate equation‑of‑state | Start at −0.95; allow ±0.05 |
| Ωm0, ΩS0 | Matter and substrate fractions | Flat prior near 0.30/0.69 |
| γm | Matter–substrate coupling | Growth/perturbations |
| H0, ns, σ8 | Nuisance/consistency parameters | Cross‑checks |
Data Modules and Targets
- Supernovae (SN Ia): Constrain wS, ΩS0, H0. Fit distance moduli vs. H(z). Deliverable: posterior on wS and ΩS0.
- Baryon Acoustic Oscillations (BAO): Pin down H(z) and DA(z). Deliverable: refined wS, Ωm0.
- Cosmic Microwave Background (CMB): Ensure RST matches acoustic scale, matter density, lensing amplitude. Deliverable: viability window for wS, Ωm0, γm.
- Gravitational Waves (GW): Directly constrain β via frequency‑dependent propagation. Deliverable: upper bounds or detection‑level posterior on β.
Cross‑Checks and Growth
- Redshift‑space distortions (RSD): Constrain γm and ensure RST’s growth matches observations.
- ISW and CMB–lensing cross‑correlation: Test late‑time potential evolution induced by wS and β.
Comparison Table: What Constrains What
| Dataset | Main Constraint | RST Parameter Focus | Outcome |
|---|---|---|---|
| SN Ia | Low‑z distances | wS, ΩS0, H0 | Background expansion nailed |
| BAO | Geometry at z ~0.1–1 | Ωm0, wS | Tighten Ωm0, wS with SN |
| CMB (compressed) | Early anchors | Ωm0, H0, ns; prior on γm | Global consistency |
| GW (PTA/sirens) | Propagation | β | Direct β bounds |
| RSD | Growth | γm, σ8 | Validate structure growth |
Milestones and Deliverables
- M1: SN+BAO+compressed CMB fit; publish wS, Ωm0, ΩS0, q0.
- M2: RSD growth fit; posterior on γm consistent with σ8.
- M3: GW analysis; β upper bound (or detection) with horizon‑scale dispersion trend.
- M4: RST vs. ΛCDM model comparison (AIC/BIC); highlight unique signatures.
Decision Criteria for Distinguishing RST from ΛCDM
- Evidence for dispersion: Non‑zero β with horizon‑scale‑only effects and null local deviations.
- Distinct wS: Posterior significantly different from −1 while maintaining fit quality.
- Growth behavior