Matter Creation Cosmology

• Matter Creation Cosmology is a framework where the universe’s particle content is continuously produced through non-conservative processes like vacuum decay, fundamentally altering the cosmic fluid dynamics.
• It harnesses non-equilibrium thermodynamics to introduce creation pressure and entropy production, providing mechanisms for late-time acceleration and possible alleviation of current cosmological tensions.
• Observationally, these models can mimic ΛCDM by matching CMB, large-scale structure, and SNe data while offering distinctive signatures that may resolve discrepancies in the Hubble and S8 measurements.

Matter creation cosmology encompasses a broad class of models in which the particle content of the Universe is non-conserved, with cosmological evolution governed by the continuous production of matter—typically dark matter—from gravity or the decay of vacuum energy. This departure from the standard assumption of particle conservation in cosmology has significant consequences for cosmic expansion, structure formation, and the interpretation of dark energy. Matter creation models are formulated within the general-relativistic or extended gravity frameworks, often invoking non-equilibrium thermodynamics, nonminimal couplings, or quantum field theory in curved spacetime to motivate the underlying mechanism. Observationally, these models can mimic or compete with ΛCDM and, in some cases, may relieve current tensions in cosmological data.

1. Theoretical Framework and Dynamical Equations

Matter creation cosmologies modify the energy-momentum conservation law by allowing for a source term in the cosmic fluid equations. In a spatially flat FLRW universe, the total cosmic fluid is split into pressureless dark matter with density $\rho_m$ and vacuum (or dark energy) with density $\Lambda$. The full conservation law is enforced, but energy exchange between components is permitted, leading to the system (Pigozzo et al., 2015):

$\dot{\rho}_m + 3H \rho_m = \Gamma \rho_m,$

$\dot{\Lambda} = -\Gamma \rho_m,$

$3H^2 = \rho_m + \Lambda,$

where $H = \dot{a}/a$ is the Hubble parameter and $\Gamma(t) \ge 0$ is the matter-creation rate, i.e., the rate at which the vacuum decays into dark matter.

A general class of phenomenological ansätze for the vacuum density is:

$$\Lambda = \sigma H^{-2\alpha}, \quad \sigma = 3(1-\Omega_{m0})H_0^{2(\alpha+1)},$$

which leads to:

$\Gamma = -\alpha \sigma H^{-(2\alpha+1)}.$

Key limiting cases:

• $\alpha=0$: $\Lambda$ is constant, $\Gamma=0$ (standard ΛCDM).
• $\alpha=-1/2$: $\Lambda \propto H$, $\Gamma$ is constant.

In single-fluid scenarios with a constant equation-of-state parameter $\omega$, the energy conservation law with matter creation reads (Cárdenas et al., 24 Jan 2025):

$\dot{\rho} + 3 H(1 + \omega)\rho = \rho \Gamma,$

with the creation pressure

$P_c = - (1+\omega)\rho \frac{\Gamma}{3H},$

so that effective pressure becomes $p_\mathrm{eff} = p + P_c$.

2. Thermodynamics, Creation Pressure, and Entropy Production

Matter creation is thermodynamically modeled as an open system, with the entropy production tied to particle production. In the adiabatic creation process (constant specific entropy), the Gibbs relation leads to a negative creation pressure (Ivanov et al., 2019):

$$P = - \frac{1}{3H}\left( \chi \Psi + n T \dot{\sigma} \right),$$

$\Pi = - (\rho + p) \frac{\Gamma}{3H},$

where $\Psi = n\Gamma$, $n$ is particle number density, and $\chi$ the specific enthalpy. For adiabaticity ($\dot{\sigma}=0$):

$P = \Pi = -(\rho + p)\frac{\Gamma}{3H}.$

Irreversible entropy production occurs; the comoving entropy increases as $dS/S = \Gamma dt$ (Lobo et al., 2015). Consistency with the generalized second law and the attainment of a final thermodynamic equilibrium (typically a de Sitter phase) impose nontrivial constraints on allowed creation rates (Lobo et al., 2015, Cárdenas et al., 2023, Cárdenas et al., 24 Jan 2025).

In scenarios that include the entropy of the apparent cosmological horizon, adiabatic expansion conditions can determine the functional form of $\Gamma$ uniquely, yielding regimes where the effective equation of state $w_\mathrm{eff}$ can cross the phantom divide ($w_\mathrm{eff}<-1$), with late-time acceleration naturally emerging from the creation dynamics (Cárdenas et al., 24 Jan 2025, Cárdenas et al., 2023).

3. Observational Constraints and Phenomenology

Recent joint analyses of the linear matter power spectrum (2dFGRS), SNe Ia (JLA), and CMB peak position reveal strong evidence for late-time dark matter creation at high confidence ($\alpha < 0$ at 95% CL), with the best-fit present-day creation rate $$\Gamma_0 \sim \mathcal{O}(10^{-10}\, \mathrm{yr}^{-1})$$ for $-0.4 \lesssim \alpha \lesssim -0.1$, and cosmological parameters $0.24 < \Omega_{m0} < 0.41$, $0.66 < h < 0.74$ (Pigozzo et al., 2015).

Key observational consequences and signatures include:

• Reduced early-time effective matter density for $\alpha<0$, shifting matter–radiation equality to higher redshift and modifying $P(k)$ turnover (Pigozzo et al., 2015).
• Standard linear perturbation theory for growth factor $D(z)$, with no unphysical sound speed in the vacuum component; only moderate deviations in growth index $\gamma$ and RSDs compared to ΛCDM.
• Consistency with large-scale structure and CMB constraints, but with a higher $\Omega_{m0}$ than standard ΛCDM (Carneiro, 2014, Pigozzo et al., 2015).
• The "Om diagnostic" ($Om(z) = \frac{H^2(z)/H_0^2 - 1}{(1+z)^3 - 1}$) shows that matter creation models can mimic quintessence-like behavior ($-1 < w < -1/3$) with a monotonically decreasing $Om(z)$ (Ivanov et al., 2019, Nunes et al., 2016).

Recent dynamical system and Bayesian analyses using DESI DR2 BAO, cosmic chronometers, Pantheon+ and other SNIa data strongly support nonzero matter creation rates (both $\Gamma \propto H_0^2/H$ and $\Gamma \propto H$ forms), with the best-fit models statistically at least equivalent to ΛCDM, and possible preference for creation models when using the Akaike information criterion (Bhattacharjee et al., 21 Jul 2025).

4. Microphysical Mechanisms and Theoretical Interpretations

The microphysics of matter creation remains an open question. Several mechanisms are theoretically explored:

• Vacuum decay: Semiclassical quantum field theory in curved spacetime allows for particle creation from the vacuum, with decay rates linked to nonadiabatic gravitational backgrounds (Pigozzo et al., 2015, Carneiro, 2011). In particular, the decay of a dynamical vacuum energy density motivates the scaling $\Lambda \propto H$ (QCD condensate arguments and Gibbons–Hawking temperature support this scaling for late times).
• Bulk viscous stress: The creation pressure is formally analogous to bulk viscosity arising from particle production in non-equilibrium thermodynamics (Ivanov et al., 2019, Cárdenas et al., 2020). In causal formulations (Israel–Stewart), creation rates remain bounded and consistent with quintessence-like expansion; in noncausal (Eckart) theory, big-rip singularities are generic.
• Modified gravity: Nonminimal curvature–matter couplings in the action can induce effective matter creation terms at the level of field equations. The non-conservation of the energy-momentum tensor encodes irreversible energy flow from spacetime geometry into matter (Lobo et al., 2015). Scalar–tensor equivalents make this mapping explicit.
• Analogue models and QFT: Cosmological particle creation is also modeled as the result of mode mixing in an expanding universe—quantified by Bogoliubov coefficients—producing particles from vacuum fluctuations, providing a direct link to laboratory analogues in time-dependent quantum media (Schützhold et al., 2012, Quintin et al., 2014).

5. Structure Formation, Degeneracy with ΛCDM, and Limitations

Matter creation cosmologies can be exactly degenerate with ΛCDM at both the background and linear perturbation level under specific conditions, notably when the effective creation rate is parameterized as $\Gamma = 3\beta H_0^2/H$ (CCDM scenario) and only the clustered contrast is used to match observables (peculiar velocities, $f\sigma_8$, etc.) (Ramos et al., 2014, Trevisani et al., 2023). This observational degeneracy—termed "dark degeneracy"—implies that only higher-order effects, nontrivial multi-component evolution, or specific signatures (e.g. CMB spectral distortions, Sunyaev–Zeldovich effect evolution) can break it.

Certain phenomenological realizations—such as the continuous matter creation model affecting only dark matter (Fabris et al., 2014)—face challenges:

• Overprediction of present-day dark matter-to-baryon ratios relative to cluster observations.
• Negative growth factors for the total density contrast at late times, inconsistent with peculiar velocity and redshift-space distortion data.
• The lack of a robust microscopic derivation for the creation rate, as well as fine-tuning problems analogous to the standard cosmological constant problem.

6. Extensions: Multi-Fluid, Modified Gravity, and Laboratory Analogues

Modern treatments extend the matter creation paradigm to multi-fluid cosmologies, including the gravitational particle production of all components (dark matter, baryons, photons, neutrinos). The background expansion can perfectly mimic ΛCDM if only non-relativistic species are created, but inclusion of photon creation modifies the temperature–redshift relation, recombination, the sound horizon, and potentially alleviates the Hubble and $S_8$ tensions (Trevisani et al., 2023, Montani et al., 2024).

In scalar–tensor or $f(R)$-gravity models, matter creation is tied to the dynamics of the nonminimally coupled scalar field, with the vacuum energy density determined by the scalar potential. Viable scenarios require stability (no tachyonic modes) and agreement with high-redshift Planck evolution, offering dynamical solutions to the Hubble tension and a consistent thermodynamic picture (Montani et al., 2024).

Laboratory analogues (BECs, ion traps) can provide experimental demonstrations of the mode mixing that underlies cosmological creation processes, via engineered time dependence of effective metrics, reinforcing the universality of the particle creation mechanism (Schützhold et al., 2012).

7. Outlook and Future Directions

Matter creation cosmology forms a robust and versatile framework encompassing a range of phenomenologies, from minimal extensions of ΛCDM to fundamentally new approaches to the origin of the cosmic inventory. The central open questions include:

• First-principles derivation of $\Gamma(H)$ from QFT in curved spacetime, including full backreaction and multi-component interactions (Trevisani et al., 2023, Lobo et al., 2015).
• Quantitative CMB and LSS predictions, especially for models with nonstandard perturbation evolution (Pigozzo et al., 2015, Nunes et al., 2016).
• Model selection as large-scale structure, CMB anisotropy, lensing, and cross-correlation data reach percent-level precision.
• Laboratory tests of cosmological particle creation phenomena.

Matter creation remains a viable and observationally testable alternative to dark energy, with the potential to address several outstanding puzzles in contemporary cosmology and to provide new theoretical links between gravity, quantum field theory, and nonequilibrium thermodynamics (Pigozzo et al., 2015, Carneiro, 2014, Trevisani et al., 2023, Cárdenas et al., 24 Jan 2025).