Buchdahl-Inspired Spacetime in R² Gravity

• Buchdahl-inspired spacetime is a family of static, spherically symmetric geometries in R² gravity and compact-star interiors that extend classic Schwarzschild solutions.
• It employs a Buchdahl parameter to control deviations, ensuring central regularity and smooth matching to exterior vacua in various gravity theories.
• Observational studies show that variations in the Buchdahl parameter affect horizon size, ISCO location, and photon ring properties, offering practical metrics for testing gravitational models.

Searching arXiv for the papers on arXiv to ground the article in current records. Buchdahl-inspired spacetime denotes a family of constructions derived from Buchdahl’s program for regular static, spherically symmetric geometries, but the term is used in two distinct and technically important senses in the recent literature. In pure $R^{2}$ gravity, it refers to an exhaustive class of static, spherically symmetric vacuum metrics obtained by reformulating and solving Buchdahl’s nonlinear ODE, including asymptotically flat and asymptotically $(\mathrm{A})\mathrm{dS}$ sectors, with a Buchdahl parameter controlling departure from Schwarzschild or Schwarzschild–de Sitter (Nguyen, 2022). In compact-star modeling, it denotes a Buchdahl-type ansatz for the interior radial metric potential, chosen to ensure central regularity, smooth matching to an exterior vacuum, and tractable matter-sector dynamics in settings such as anisotropic stars, charged stars, $f(Q)$ gravity, and $f(R,T)$ gravity (Sokoliuk et al., 2022).

1. Definition and historical placement

Hans Buchdahl’s 1962 program in pure $R^{2}$ gravity sought static, spherically symmetric vacuum solutions beyond the obvious constant-curvature Einstein spaces. In that setting, the vacuum field equations are

$$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$

with trace

$\Box R = 0.$

Buchdahl reduced the problem to a nonlinear second-order ODE but regarded it as intractable. Recent work reformulated that ODE and obtained what is explicitly described as a “novel exhaustive class of metrics” and “Buchdahl-inspired metrics” in pure $R^{2}$ gravity (Nguyen, 2022).

In a separate compact-object literature, “Buchdahl-inspired” or “Buchdahl spacetime type function” refers to choosing the interior $g_{rr}$ potential in a Buchdahl form. Representative examples are

$$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$

used for strange stars in $(\mathrm{A})\mathrm{dS}$0 gravity (Sokoliuk et al., 2022), and

$(\mathrm{A})\mathrm{dS}$1

used for a charged anisotropic Einstein–Maxwell interior (Fuente et al., 2022). A broader anisotropic Buchdahl model employs

$(\mathrm{A})\mathrm{dS}$2

with $(\mathrm{A})\mathrm{dS}$3 a generalized Buchdahl dimensionless parameter (Maurya et al., 2018).

Usage	Geometric role	Typical setting
Buchdahl-inspired metric	Exact vacuum solution family	Pure $(\mathrm{A})\mathrm{dS}$4 gravity
Buchdahl metric potential	Interior ansatz for $(\mathrm{A})\mathrm{dS}$5	Compact stars
Buchdahl transformation	Hidden symmetry transformation	Einstein and Einstein–Scalar theories

This dual use is not contradictory. It reflects two continuities with Buchdahl’s work: solving Buchdahl’s vacuum ODE in higher-derivative gravity, and using Buchdahl-type interior potentials to construct regular stellar spacetimes (Nguyen, 2022).

2. Exhaustive Buchdahl-inspired metrics in pure $(\mathrm{A})\mathrm{dS}$6 gravity

In pure $(\mathrm{A})\mathrm{dS}$7 gravity with action

$(\mathrm{A})\mathrm{dS}$8

the modern Buchdahl-inspired construction starts from the static, spherically symmetric ansatz

$(\mathrm{A})\mathrm{dS}$9

with gauge choice

$f(Q)$0

This makes $f(Q)$1 constant, so the trace equation gives

$f(Q)$2

where $f(Q)$3 is the asymptotic curvature scale and $f(Q)$4 measures departure from constant curvature (Nguyen, 2022).

The resulting Buchdahl-inspired spacetime is presented in compact form as

$f(Q)$5

with evolution equations

$f(Q)$6

and scalar curvature

$f(Q)$7

The class is called exhaustive because the Buchdahl equation is described as encoding all static, spherically symmetric vacuum solutions of pure $f(Q)$8 gravity that are not trivially Einstein spaces or $f(Q)$9 spaces (Nguyen, 2022).

The parameters are a quartet $f(R,T)$0, although a degeneracy

$f(R,T)$1

reduces the number of independent dimensionless combinations. The case $f(R,T)$2 gives the constant-curvature Schwarzschild–de Sitter sector. Then $f(R,T)$3 is constant, $f(R,T)$4, and the metric reduces to

$f(R,T)$5

namely Schwarzschild–de Sitter for $f(R,T)$6, Schwarzschild–anti–de Sitter for $f(R,T)$7, or Schwarzschild for $f(R,T)$8 (Nguyen, 2022).

A central conceptual result is that the Buchdahl-inspired metrics evade a Lichnerowicz-type no-go theorem for pure $f(R,T)$9 gravity. The reason given is that the theorem assumes too-rapid asymptotic falloff of $R^{2}$0, whereas for Buchdahl-inspired solutions the boundary term

$R^{2}$1

does not vanish. This leaves room for static vacuum geometries with non-constant scalar curvature (Nguyen, 2022).

3. The special asymptotically flat member and its deformations

A particularly important member of the pure $R^{2}$2 family is the asymptotically flat $R^{2}$3 solution. In exact form, the special Buchdahl-inspired metric can be written as

$R^{2}$4

with

$R^{2}$5

$R^{2}$6

and $R^{2}$7 the sign of $R^{2}$8. The special solution has $R^{2}$9 and thus $$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$0, while $$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$1. The Schwarzschild limit is $$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$2, for which $$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$3, $$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$4, and the metric becomes Schwarzschild (Alloqulov et al., 14 Jul 2025).

The exact asymptotically flat solution is also expressed in a coordinate $$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$5 by

$$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$6

with

$$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$7

This exact form makes it possible to construct a Kruskal–Szekeres diagram for pure $$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$8 spacetime and to show that the Buchdahl parameter fundamentally modifies the global structure (Nguyen, 2022).

The same asymptotically flat sector admits a weak-field parametrization useful for phenomenology. In areal radius $$R\left(R_{\mu\nu} - \tfrac{1}{4}g_{\mu\nu}R\right) + \left(g_{\mu\nu}\Box - \nabla_\mu\nabla_\nu\right)R = 0,$$9, one writes

$\Box R = 0.$0

where

$\Box R = 0.$1

Here $\Box R = 0.$2 corresponds to the Schwarzschild metric of GR. This weak-field Buchdahl-inspired spacetime is used as a one-parameter deformation of Schwarzschild in observational studies (Yan et al., 15 Apr 2025).

Several non-Schwarzschild geometric behaviors follow. In the exact special solution, the horizon area can be zero, finite, or divergent depending on $\Box R = 0.$3. In the weak-field model, the horizon radius follows from

$\Box R = 0.$4

and the ISCO is

$\Box R = 0.$5

This suggests a smaller strong-field region for positive $\Box R = 0.$6 in the approximate metric, while the exact $\Box R = 0.$7-description shows that the geometry can represent a black hole, a traversable wormhole, or a naked singularity depending on the parameter range (Yan et al., 15 Apr 2025).

4. Interior Buchdahl-inspired spacetimes for compact stars

In compact-star applications, a Buchdahl-inspired spacetime is an interior line element

$\Box R = 0.$8

with a prescribed Buchdahl radial potential and a temporally determined $\Box R = 0.$9. For strange stars in $R^{2}$0 gravity, the adopted choice is

$R^{2}$1

with central regularity

$R^{2}$2

and matching condition

$R^{2}$3

The matter sector is anisotropic,

$R^{2}$4

with anisotropy

$R^{2}$5

and the MIT Bag equation of state

$R^{2}$6

The Buchdahl potential is said to ensure central regularity and a smooth match to Schwarzschild, while modulating $R^{2}$7, $R^{2}$8, and the profiles of $R^{2}$9, $g_{rr}$0, and $g_{rr}$1 (Sokoliuk et al., 2022).

The same general strategy appears in Einstein–Maxwell models. In a charged anisotropic compact star, after introducing $g_{rr}$2, $g_{rr}$3, and $g_{rr}$4, the “Buchdahl spacetime type function” is

$g_{rr}$5

accompanied by an electromagnetic ansatz

$g_{rr}$6

This yields explicit closed forms for $g_{rr}$7, $g_{rr}$8, $g_{rr}$9, $$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$0, and the mass function, and the interior is matched to Reissner–Nordström (Fuente et al., 2022).

A related charged Buchdahl–Vaidya–Tikekar construction in $$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$1 gravity takes

$$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$2

inside

$$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$3

and matches to a Reissner–Nordström exterior. In that setting, the charged Buchdahl-type compactness bound in $$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$4 gravity is written approximately as

$$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$5

reducing to the charged GR Buchdahl bound when $$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$6 (Bhattacharya et al., 2023).

In anisotropic GR models, the general Buchdahl ansatz

$$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$7

together with

$$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$8

leads to eight exact classes distinguished by the sign and value of $$e^{\lambda(r)}=\frac{K(C r^2+1)}{K+C r^2},\quad 0<K<1,$$9. The resulting models satisfy energy conditions, causality, and the anisotropic TOV equation for suitable parameters, and they approximate a linear equation of state $(\mathrm{A})\mathrm{dS}$00 (Maurya et al., 2018). A Chaplygin extension replaces the radial fluid law by

$(\mathrm{A})\mathrm{dS}$01

while retaining the Buchdahl $(\mathrm{A})\mathrm{dS}$02, and produces exact anisotropic compact-star interiors representing sources such as PSR B0943+10, Her X-1 and SAX J1808.4-3658 (Prasad et al., 2021).

5. Observational and astrophysical tests

The asymptotically flat Buchdahl-inspired $(\mathrm{A})\mathrm{dS}$03 spacetime has been developed into an observational framework by treating it as a parametrized deformation of Schwarzschild. In the weak-field form,

$(\mathrm{A})\mathrm{dS}$04

null geodesics satisfy the standard photon-sphere condition

$(\mathrm{A})\mathrm{dS}$05

Numerical ray tracing shows that the photon sphere radius and critical impact parameter both decrease monotonically as $(\mathrm{A})\mathrm{dS}$06 increases, and hence the shadow size and photon ring diameter also decrease monotonically with increasing $(\mathrm{A})\mathrm{dS}$07 (Yan et al., 15 Apr 2025).

Applied to Sagittarius A* with \textit{ipole}, a radiatively inefficient accretion flow, and $(\mathrm{A})\mathrm{dS}$08 GHz imaging, the Buchdahl-inspired deformation produces ring-diameter constraints

$(\mathrm{A})\mathrm{dS}$09

$(\mathrm{A})\mathrm{dS}$10

$(\mathrm{A})\mathrm{dS}$11

which are stated to be tighter than the earlier S2-star bound

$(\mathrm{A})\mathrm{dS}$12

The same study finds that polarization observables are only weakly sensitive to $(\mathrm{A})\mathrm{dS}$13, while inclination has a much stronger effect (Yan et al., 15 Apr 2025).

A broader observational analysis of asymptotically flat $(\mathrm{A})\mathrm{dS}$14 spacetimes uses light deflection, Shapiro delay, perihelion advance, black-hole shadow, and geodetic precession. In that paper, the special Buchdahl-inspired metric reduces exactly to Schwarzschild at $(\mathrm{A})\mathrm{dS}$15, and the strongest bound on the special Buchdahl parameter comes from Cassini: $(\mathrm{A})\mathrm{dS}$16 A Mercury perihelion bound gives

$(\mathrm{A})\mathrm{dS}$17

while the M87* shadow bound from the rotating solution is

$(\mathrm{A})\mathrm{dS}$18

The paper emphasizes that the Cassini result is the tightest Solar System constraint and that $(\mathrm{A})\mathrm{dS}$19 is expected to be system-dependent (Zhu et al., 2024).

The exact special Buchdahl-inspired spacetime has also been studied through geodesics, epicyclic oscillations, QPO modeling, thin-disk spectra, and ray-traced accretion-disk images. For the special asymptotically flat metric with parameter $(\mathrm{A})\mathrm{dS}$20, the numerical ISCO values satisfy

$(\mathrm{A})\mathrm{dS}$21

so the ISCO moves inward as $(\mathrm{A})\mathrm{dS}$22 increases. QPO fits to XTE J1550–564 and GRO J1655–40 yield the range

$(\mathrm{A})\mathrm{dS}$23

although this is explicitly described as being in tension with an independent shadow-based constraint $(\mathrm{A})\mathrm{dS}$24 from M87* (Alloqulov et al., 14 Jul 2025).

6. Related variants, bounds, and open problems

One major variant of the special Buchdahl-inspired metric is the wormhole sector in pure $(\mathrm{A})\mathrm{dS}$25 gravity. For the asymptotically flat special metric, it is shown that $(\mathrm{A})\mathrm{dS}$26 supports a two-way traversable Morris–Thorne wormhole, $(\mathrm{A})\mathrm{dS}$27 gives a naked singularity, and $(\mathrm{A})\mathrm{dS}$28 yields a non-Schwarzschild structure. In the wormhole sector, the effective energy density derived from the Einstein tensor is negative and the Weak Energy Condition is formally violated, but the exoticity is attributed to higher-derivative geometry rather than to explicit exotic matter (Nguyen et al., 2023).

A different Buchdahl line of work concerns transformations rather than vacuum $(\mathrm{A})\mathrm{dS}$29 solutions. Buchdahl’s first-kind transformation maps a static vacuum seed

$(\mathrm{A})\mathrm{dS}$30

to

$(\mathrm{A})\mathrm{dS}$31

while the second kind generates an Einstein–Scalar configuration. Applied along a spacelike Killing vector, this produces the Schwarzschild–Levi-Civita spacetime and higher-dimensional Levi-Civita extensions of Myers–Perry geometries, providing new static and rotating vacuum black holes and hairy extensions (Barrientos et al., 2024).

The Buchdahl name also persists in compactness limits. In pure Lovelock gravity, the Buchdahl compactness limit is

$(\mathrm{A})\mathrm{dS}$32

with $(\mathrm{A})\mathrm{dS}$33, and in the special dimension $(\mathrm{A})\mathrm{dS}$34 this becomes exactly

$(\mathrm{A})\mathrm{dS}$35

That result generalizes the original Einstein-gravity Buchdahl bound and connects compactness directly to the exterior geometry in certain dimensions (Dadhich et al., 2016).

A recent interacting-vacuum analysis extends the TOV equation by a covariant energy exchange $(\mathrm{A})\mathrm{dS}$36 between matter and vacuum sectors. The generalized equilibrium equation is

$(\mathrm{A})\mathrm{dS}$37

and numerical integration is reported to keep the central pressure finite in regimes where standard GR approaches the Buchdahl threshold. This suggests a mechanism to bypass classical geometric bounds in ultra-compact configurations (Maier, 14 Apr 2026).

Across these usages, two themes remain stable. First, Buchdahl-inspired spacetimes are built to preserve exact solvability or compact representation under strong symmetry assumptions. Second, they are repeatedly used to probe what happens when the classical Schwarzschild or Buchdahl picture is deformed by higher derivatives, anisotropy, charge, non-metricity, scalar structure, or vacuum interaction. A plausible implication is that “Buchdahl-inspired spacetime” is best understood not as a single metric, but as a technically coherent research program centered on Buchdahl-type radial structure, exact reduction methods, and strong-field departures from standard GR (Nguyen, 2022).