Tidal Deformability in Neutron Stars

Updated 2 December 2025

Tidal deformability in neutron stars is a measure of their ability to develop a mass quadrupole moment under external tidal fields, directly reflecting dense matter properties.
It is computed using the Tolman–Oppenheimer–Volkoff equations and derived from the tidal Love number, with clear dependencies on stellar radius and the equation of state.
Observations from gravitational waves, such as GW170817, constrain tidal deformability values, thereby informing the nuclear EOS and hinting at potential exotic physics.

Tidal deformability in neutron stars quantifies the ability of these compact objects to develop a mass quadrupole moment in response to the external tidal field exerted by a binary companion. This property serves as a direct probe of dense matter physics, notably the supranuclear equation of state (EOS), and leaves a distinctive imprint on the gravitational-wave (GW) phasing during the late inspiral phase of binary neutron star (NS) mergers. Constraints on tidal deformability from GW observations, such as GW170817, have led to major advances in our understanding of both nuclear EOS parameters and potential modifications to general relativity, while also enabling searches for exotic matter and dark sector interactions in NS interiors.

1. Formal Definition and Computation of Tidal Deformability

The dimensionful tidal deformability, $\lambda$ , characterizes the response of a nonrotating neutron star to an external static, quadrupolar tidal field $\mathcal{E}_{ij}$ . By convention,

$Q_{ij} = -\lambda\, \mathcal{E}_{ij},$

where $Q_{ij}$ is the induced mass quadrupole. In geometrized units $(G=c=1)$ , the relation between $\lambda$ , the $\ell=2$ dimensionless tidal Love number $k_2$ , and the circumferential radius $R$ is

$\lambda = \frac{2}{3} k_2 R^5.$

The dimensionless tidal deformability is then

$\Lambda \equiv \frac{\lambda}{M^5} = \frac{2}{3} k_2 \left(\frac{R}{M}\right)^5 = \frac{2}{3} k_2 C^{-5}$

where $M$ is the gravitational mass and $C = M/R$ is the compactness.

To compute $k_2$ for a given EOS, one first determines the hydrostatic background configuration via the Tolman–Oppenheimer–Volkoff (TOV) equations. Next, a static, even-parity $\ell=2$ metric perturbation $H(r)$ is evolved atop the background, with boundary conditions enforcing regularity at the center and asymptotic matching to the Schwarzschild exterior. The Love number is then extracted via the solution’s logarithmic derivative at the surface,

$y_R = \left. \frac{r H'(r)}{H(r)} \right|_{r=R},$

using a closed analytic formula:

$k_2 = \frac{8 C^5}{5}(1-2C)^2 [2 + 2C(y_R-1) - y_R] / D(C, y_R),$

where $D(C, y_R)$ is a known algebraic denominator involving $C$ , $y_R$ , and a logarithmic term (0911.3535).

Typical values for a $1.4\,M_\odot$ star:

Soft EOS: $R\approx9.4$ km, $k_2\approx0.05$ , $\lambda\sim0.4\times10^{36}\,\mathrm{g\,cm}^2\mathrm{s}^2$
Stiff EOS: $R\approx14-15$ km, $k_2\approx0.10-0.11$ , $\lambda\sim8-9\times10^{36}\,\mathrm{g\,cm}^2\mathrm{s}^2$

2. Equation of State Dependence and Physical Interpretation

Tidal deformability is acutely sensitive to the EOS, with $\Lambda(M)$ scaling roughly as $R^5$ to $R^7.5$ for fixed mass. This leads to order-of-magnitude variations between soft and stiff EOS scenarios (0911.3535, Kim et al., 2018, Seif et al., 15 Jul 2025). Increasing the nuclear incompressibility $K$ or the pressure at densities $n\sim2n_0$ (where $n_0$ is saturation density) increases both $R$ and $\Lambda$ . Conversely, exotic degrees of freedom—hyperons, kaon condensates, quark matter, or strong first-order phase transitions—can lead to either monotonic decreases (softer EOS, smaller $R$ ) or discontinuities and kinks in the $\Lambda(M)$ curve (Han et al., 2018, Nobleson et al., 2021).

Importantly, the addition of strange particles or phase transitions can reduce $\Lambda_{1.4}$ (tidal deformability at $1.4\,M_\odot$ ) from values typical of nucleonic EOS ( $\sim400$ ) to below 100. For example, a sharp phase transition at low densities can yield $\Lambda_{1.4}\sim60$ –80 (Han et al., 2018).

3. Tidal Effects and Gravitational Wave Signatures

The leading EOS-dependent matter effect on the GW signal is encoded in the phase shift during the inspiral,

$\Delta\Psi_{\rm tidal}(f) = -\frac{39}{2}\,\tilde\Lambda\,(\pi M f)^{5/3},$

where the mass-weighted effective tidal deformability for a binary is

$\tilde\Lambda = \frac{16}{13} \frac{(m_1 + 12 m_2) m_1^4 \Lambda_1 + (m_2 + 12 m_1) m_2^4 \Lambda_2}{(m_1 + m_2)^5}.$

For near-equal mass binaries, $\tilde\Lambda \approx \Lambda_1 \approx \Lambda_2$ .

Measurements of $\tilde\Lambda$ from events such as GW170817 have provided $90\%$ CL bounds $\tilde\Lambda \lesssim700$ , which in turn constrains $R_{1.4} \lesssim 13$ km, excluding very stiff equations of state (Raithel et al., 2018, Sammarruca et al., 29 Nov 2025, Kanakis-Pegios et al., 2020, Lim et al., 2018, Chatziioannou, 2020). The degeneracy between $R$ and $\Lambda$ is so tight that, once chirp mass is fixed, $\tilde\Lambda$ becomes nearly a function of radius alone, independent of precise mass ratio (Raithel et al., 2018).

The mass-weighted $\tilde\Lambda$ is the only combination currently measurable with practical accuracy in GW signals, so constraints on nuclear EOS from GW data essentially pass through this channel.

4. Impact of Exotic Matter, Phase Transitions, and Dark Sector Admixture

The addition of exotic particles such as hyperons, $K^-$ condensates, or quark matter, as well as the presence of first-order phase transitions or new low-density phases, can dramatically alter $\Lambda(M)$ :

Sharp phase transitions at or just above nuclear saturation density produce discontinuous drops in $\Lambda$ , potentially violating universal $\Lambda$ – $C$ (compactness) relations by up to $20\%-30\%$ (Han et al., 2018, Raithel et al., 2022).
Deconfined quark cores and strange quark stars yield the smallest possible deformabilities (Han et al., 2018).
Dark matter admixtures—whether as ultralight bosonic clouds or as massive DM cores—can modify both radius and $\Lambda$ , with cloud regimes enhancing $\Lambda$ (sometimes $>2000$ ), while cores suppress it (Diedrichs et al., 2023, Karkevandi et al., 2021, Leung et al., 2022).

A sharp transition to a stiff quark phase, for instance, may be required to reach the lowest $\Lambda(M)$ . However, these low values may conflict with lower bounds on $\tilde\Lambda$ inferred from electromagnetic counterparts and post-merger ejecta (Han et al., 2018).

5. Constraints from Observations and Sensitivity to EOS Parameters

Current and past GW detections have placed stringent constraints on tidal deformability:

GW170817 yielded $\tilde\Lambda_{1.4}\lesssim700-800$ (varies by analysis), excluding EOS that would predict $R_{1.4}>13$ km or $\Lambda_{1.4}>800$ (Chatziioannou, 2020, Sammarruca et al., 29 Nov 2025).
Bayesian and frequentist analyses incorporating nuclear-theory priors, laboratory data, and multi-messenger signals narrow $\Lambda_{1.4}$ to $140-400$ for most viable EOS (Lim et al., 2018, Chatziioannou, 2020, Seif et al., 15 Jul 2025).
The correlation between $\Lambda_{1.4}$ and the pressure at $2 n_0$ is nearly linear, so tidal deformability is a direct probe of the symmetry-energy slope and the stiffness of the EOS at twice saturation density (Lim et al., 2018).
Future high-SNR GW events and next-generation detectors (e.g., Einstein Telescope, Cosmic Explorer) are expected to reduce uncertainties on $\Lambda$ to the $\mathcal{O}(10)$ level, enabling discrimination among EOSs that are currently observationally degenerate ("tidal deformability doppelgängers") (Raithel et al., 2022).
Lower bounds on $\tilde\Lambda$ or strong EM/GW multimessenger constraints may exclude very soft EOSs or those with strong phase transitions, narrowing symmetry-energy slope $L$ to $45$–$65$ MeV (Kim et al., 2018).

A summary of canonical tidal deformability values and observational bounds:

Observable	Soft EOS	Stiff EOS	GW170817 Bound	Multi-messenger Preferred
$R_{1.4}$ (km)	9.4–11.7	12–15	$11.0$–$13.0$	$11.5$–$12.6$
$k_2$	0.05–0.08	0.09–0.11	---	---
$\Lambda_{1.4}$	$<200$	$>800$	$190^{+390}_{-120}$	$140$–$400$

6. Tidal Deformability as a Test of Modified Gravity and Microphysics

Tidal deformability is highly sensitive to modifications of general relativity and nonstandard degrees of freedom:

$f(R)$ gravity ( $f(R)=R+aR^2$ ) increases $k_2$ and $\Lambda$ at fixed compactness, with LIGO/Virgo data from GW170817 restricting $a\lesssim 10r_g^2$ (Nobleson et al., 2021).
Scalar-tensor and scalar-Gauss-Bonnet gravity models can alter $\Lambda$ and $k_2$ by $15$– $200\%$ for large scalar couplings, especially for compact ( $C\gtrsim0.2$ ) stars (Saffer et al., 2021, Brown, 2022).
In unimodular gravity, a small negative nonconservation parameter is favored by tidal deformability data, due to its impact on the structure and mass–radius relation (Yang et al., 2022).
Elastic properties of the neutron star crust induce only minute ( $<10^{-7}$ ) corrections to $\Lambda$ , irrelevant at current or future detector precisions (Gittins et al., 2020).
The presence of "frozen composition" effects—arising from slow weak-interaction processes—induces only $\lesssim$ 5% corrections to tidal Love numbers (Andersson et al., 2019).

7. Universal Relations, Degeneracies, and Future Prospects

Empirical and quasi-universal relations between $\Lambda$ , compactness $C$ , and moment of inertia $I$ hold for a broad class of barotropic EOSs, with $\Lambda \propto C^{-6}$ to within $10-20\%$ except in scenarios involving sharp first-order phase transitions or strong microphysical modifications (Han et al., 2018, Raithel et al., 2022). However, "tidal deformability doppelgängers"—EOSs with very different internal properties but nearly identical $\Lambda(M)$ profiles—arise generically when allowing low-density phase transitions or softening (Raithel et al., 2022). These degeneracies can only be lifted through joint GW/X-ray/nuclear data or next-generation detector sensitivity.

Combining constraints from GW phasing, pulsar mass/radius measurements (e.g., by NICER), and fundamental nuclear physics provides a path to mapping the EOS at several times nuclear saturation density, refining the microphysical properties of dense matter, and, potentially, discovering or ruling out exotic new physics.

References:

(0911.3535) Hinderer et al., "Tidal deformability of neutron stars with realistic equations of state and their gravitational wave signatures in binary inspiral".
(Raithel et al., 2018) Raithel, Özel, & Psaltis, "Tidal deformability from GW170817 as a direct probe of the neutron star radius".
(Chatziioannou, 2020) Margalit & Metzger, "Neutron star tidal deformability and equation of state constraints".
(Seif et al., 15 Jul 2025) Seif & Hashem, "Tidal deformability and compactness of neutron stars and massive pulsars from semi-microscopic equations of state".
(Kim et al., 2018) Lim & Holt, "Tidal Deformability of Neutron Stars with Realistic Nuclear Energy Density Functionals".
(Lim et al., 2018) Lim & Holt, "Neutron star tidal deformabilities constrained by nuclear theory and experiment".
(Han et al., 2018) Han & Steiner, "Tidal deformability with sharp phase transitions in (binary) neutron stars".
(Raithel et al., 2022) Tan et al., "Tidal Deformability Doppelgangers: Implications of a low-density phase transition in the neutron star equation of state".
(Saffer et al., 2021) Silva et al., "Tidal Deformabilities of Neutron Stars in scalar-Gauss-Bonnet Gravity".
(Brown, 2022) Brown, "Tidal Deformability of Neutron Stars in Scalar-Tensor Theories of Gravity".
(Nobleson et al., 2021) Nobleson, Malik, & Banik, "Tidal deformability of neutron stars with exotic particles within a density dependent relativistic mean field model in R-squared gravity".
(Yang et al., 2022) Yang et al., "Tidal Deformability of Neutron Stars in Unimodular Gravity".
(Diedrichs et al., 2023) Cruz et al., "Tidal Deformability of Fermion-Boson Stars: Neutron Stars Admixed with Ultra-Light Dark Matter".
(Karkevandi et al., 2021, Leung et al., 2022) Studies on dark matter admixture and tidal deformability.
(Andersson et al., 2019) Andersson & Pnigouras, "The sum of Love: Exploring the effective tidal deformability of neutron stars".
(Gittins et al., 2020) Gittins, Andersson, & Pnigouras, "Tidal deformations of neutron stars with elastic crusts".