Parameterized Post-Newtonian Parameters

• Parameterized post-Newtonian (PPN) parameters are dimensionless constants that quantify deviations from General Relativity in weak-field, slow-motion conditions.
• The PPN formalism employs a perturbative expansion of the metric to incorporate corrections up to 1PN and 2PN orders, linking gravitational nonlinearity and symmetry properties.
• Experimental tests, such as Cassini measurements and lunar laser ranging, tightly constrain these parameters, bridging theoretical models and observable gravitational phenomena.

A parameterized post-Newtonian (PPN) parameter is a dimensionless coupling constant appearing in the expansion of the metric and field equations in weak-field, slow-motion limits of gravity theories. The PPN formalism augments the Newtonian and Newton–Cartan limit with corrections up to well-defined post-Newtonian (PN) orders, introducing parameters that systematically encode all permissible deviations from @@@@1@@@@ (GR) arising in a broad class of metric theories. The values of PPN parameters are highly constrained by solar-system and binary pulsar experiments, and play a central role in linking gravitational phenomenology to the parameter spaces of fundamental gravity models.

1. The PPN Formalism: Structure and Parameters

The PPN approach expresses the spacetime metric as a perturbation about flat Minkowski space, with the expansion written in terms of powers of typical velocity $v$, the Newtonian potential $U$, and other small quantities. The standard PPN metric up to first post-Newtonian order (1PN) in isotropic coordinates is

$$\begin{aligned} g_{00} &= -1 + 2\,U - 2\,\beta\,U^2 + O(U^3), \ g_{0i} &= -\frac{1}{2}(4\gamma + 3 + \alpha_1 - \alpha_2 + \zeta_1 - 2\xi)V_i + O(U^{3/2}), \ g_{ij} &= (1 + 2\gamma\,U)\,\delta_{ij} + O(U^2). \end{aligned}$$

Here, $U$ is the Newtonian potential, $V_i$ the PPN vector potential, and the $$\{\gamma, \beta, \alpha_1, \alpha_2, \alpha_3, \zeta_1, \zeta_2, \zeta_3, \zeta_4, \xi\}$$ are the canonical PPN parameters. Each parameter is associated with a specific type of gravitational nonlinearity or symmetry breaking:

• $\gamma$: spatial curvature per unit mass.
• $\beta$: nonlinearity in superposition of gravity.
• $\alpha_{1,2,3}$: preferred-frame effects.
• $\zeta_{1,2,3,4}$, $\xi$: violations of momentum conservation and preferred location.

Higher-order or theory-specific frameworks introduce additional parameters at 2PN and beyond (e.g., $\omega, \delta, \delta_2, \varsigma, \eta$, as in the 2PPN formalism) (Wu et al., 2021, Deng, 2015).

2. Physical Interpretation of the Principal Parameters

The most stringently tested and most significant PPN parameters are $\gamma$ and $\beta$:

• $\gamma$ measures how much spacetime curvature is produced by unit mass; in GR, $\gamma=1$.
• $\beta$ quantifies nonlinear self-interaction of gravity, entering at $U^2$ order in $g_{00}$; in GR, $\beta=1$.

Other parameters serve as direct probes of Lorentz invariance (preferred-frame effects), the nature of gravitational energy and its conservation, and possible breaking of the strong equivalence principle. Their vanishing in GR is a hallmark of its minimal symmetry violation structure (Sanghai et al., 2016, Li, 2012).

Table: Principal PPN parameters, their physical meaning, and expected value in GR

Parameter	Physical meaning	Value in GR
$\gamma$	Space curvature per unit mass	1
$\beta$	Nonlinearity in superposition law	1
$\alpha_1$	Preferred-frame (boost) effects	0
$\alpha_2$	Preferred-frame (rotational) effects	0
$\alpha_3$	Preferred-frame, location, self-acceleration	0
$\zeta_1$–$\zeta_4$, $\xi$	Conservation violations, preferred-location	0

3. Parameter Definitions in Alternative Theories

In scalar-tensor, vector-tensor, higher-order, and multimetric gravity, PPN parameters become explicit functions of the underlying coupling constants and background fields. For scalar-tensor models (Jordan-Brans-Dicke type), for a massless scalar one has (Sanghai et al., 2016, Hohmann, 2015, Koivisto, 2011): $$\gamma = \frac{\omega+1}{\omega+2},\quad \beta = 1 + \frac{\omega,_{\phi_0}}{(2\omega+3)(4\omega+6)}$$ with $\omega$ the coupling function. Screened modified gravity models (e.g., chameleon, symmetron, dilaton) further introduce $r$- and $\epsilon$-dependent PPN functions (screening parameter), yielding $G_\mathrm{eff}(r), \gamma(r), \beta(r)$ that interpolate between unscreened (cosmological) and highly screened (solar-system) regimes (Zhang et al., 2016, McManus et al., 2017).

In quadratic and higher-curvature gravity theories, PPN parameters become functions of distance due to finite-range (Yukawa-type) corrections mediated by extra propagating fields: $$\gamma(r) = \frac{3 - e^{-m_R r} - 2e^{-m_W r}}{3 + e^{-m_R r} - 4e^{-m_W r}},\quad \beta(r) = 1 + \mathcal{O}(r\ln r\, e^{-mr})$$ with $m_R$, $m_W$ the masses of the scalar and ghost-tensor modes, and corrections exponentially suppressed for sufficiently large masses (Zhu et al., 9 Jan 2026).

4. Constraints from Solar System and Cosmological Experiments

The PPN parameters are tightly bounded by a cross-section of high-precision local experiments:

• Cassini time-delay (Shapiro): $|\gamma - 1| \lesssim 2.3\times10^{-5}$
• Mercury perihelion advance: $|2\gamma - \beta - 1| \lesssim 3\times10^{-3}$
• Lunar laser ranging: $|\beta - 1| \lesssim 10^{-4}$

Cosmological observations, including Planck CMB measurements, have extended constraints on the time-averaged values and possible variations of the parameters $\alpha(t)$ and $\gamma(t)$: $$\bar{\alpha} = 0.89^{+0.08}_{-0.09},\quad \bar{\gamma} = 0.90^{+0.07}_{-0.08} \quad (\textrm{68\% CL})$$ indicating that PPN parameters cannot deviate more than $\sim20\%$ from GR over cosmic history (Thomas et al., 2024).

5. Extensions to 2PN and Multimetric Theories

At second post-Newtonian order (2PN), three additional parameters $(\omega, \delta, \delta_2)$ are required for a complete parameterization in the most general setting with conservation laws (Wu et al., 2021):

• $\omega$: 2PN curvature-nonlinearity in $g_{ij}$
• $\delta$: cubic self-interaction of $U$ in $g_{00}$
• $\delta_2$: mixed Newtonian–self-energy coupling (dominantly in $g_{00}$)

In the 2PPN formalism for astrometric observations, two further parameters $(\varsigma, \eta)$ parameterize isotropic and anisotropic $c^{-4}$ spatial-metric corrections relevant for microarcsecond-level measurements (Deng, 2015).

In multimetric gravity, the parameter space of PPN parameters is systematically extended to a collection $$\gamma^{IJ}, \beta^{IJ}, \sigma_\pm^{IJ}, \psi^{IJK}_A$$ indexed by visible and hidden sectors, enabling cross-sectoral predictions for gravitational interactions and new observable channels (e.g., dark sector deflection) (Hohmann, 2013).

6. PPN Parameters in Cosmology and Large-Scale Structure

The parameterized post-Newtonian cosmology (PPNC) program introduces four time-dependent functions $\alpha(t), \gamma(t), \alpha_c(t), \gamma_c(t)$, entering the modified Friedmann and Newton-Poisson equations for the background and perturbed universe. The PPN slip parameter $\eta \equiv \Phi/\Psi$ becomes

• Small scales: $\eta(k\gg H) = \alpha(t) / \gamma(t)$
• Large scales: $\eta(k\ll H) = 1 - \hat\gamma'/\hat\gamma$

PPN cosmology connects solar-system and laboratory constraints on PPN parameters to cosmological observables (CMB, lensing, BAO) and enables robust, theory-agnostic tests of GR across more than 16 orders of magnitude in scale (Sanghai et al., 2016, Sanghai, 2017, Anton et al., 25 Apr 2025).

7. Applications and Future Directions

The PPN framework underpins a spectrum of phenomenological and experimental applications:

• Precision tracking of planetary and binary pulsar orbits: constraints on $\gamma$, $\beta$ from precession rates and timing residuals (Li, 2012).
• Quantum metrology protocols (e.g., Hong-Ou-Mandel interferometry): direct measurement of $\gamma, \beta$ with quantum-enhanced sensitivity at the $10^{-8}$–$10^{-12}$ level from proper-time delays (Rivera-Tapia et al., 2021).
• Coherent GW phasing analysis in LIGO/Virgo/ET: bounds on PN coefficients identifying effective PPN terms across the inspiral phase (Mishra et al., 2010).
• Next-generation astrometry (LATOR, Gaia): measurement of 2PN parameters $(\varsigma, \eta)$ at the $10^{-6}$–$10^{-8}$ level (Deng, 2015).
• Screening and Vainshtein effects: position-dependent $G_\mathrm{eff}(r)$, $\gamma(r)$, $\beta(r)$ mapping the transition between GR-dominated and alternative-regime dynamics at different scales and densities (McManus et al., 2017, Avilez-Lopez et al., 2015).

Constraints on PPN parameters are projected to improve substantially with advances in timing, interferometry, space-based astrometry, laboratory gravity experiments, and cosmological surveys—testing not just the presence, but the possible running or spatial dependence of fundamental gravitational couplings.

---

References:

• (Zhu et al., 9 Jan 2026) Parameterized Post-Newtonian Analysis of Quadratic Gravity and Solar System Constraints
• (Anton et al., 25 Apr 2025) Gravitational Slip in the Parameterized Post-Newtonian Cosmology
• (Sanghai et al., 2016, Sanghai, 2017, Thomas et al., 2024): PPN Cosmology and CMB constraints
• (Wu et al., 2021, Deng, 2015): 2PN and 2PPN parameter frameworks
• (Hohmann, 2013): Multimetric gravity parameterization
• (Li, 2012): PPN orbital effects
• (McManus et al., 2017, Avilez-Lopez et al., 2015): PPN in screened and Vainshtein-modified gravity
• (Koivisto, 2011, Zhang et al., 2016): C-theories and screened modified gravity
• (Rivera-Tapia et al., 2021, Mishra et al., 2010): Quantum and gravitational-wave PPN parameter estimation
• (Hohmann, 2015, Saito et al., 2024): PPN mapping in Horndeski and DHOST theories