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Revisiting the Matter Creation Process: Observational Constraints on Gravitationally Induced Dark Energy and the Hubble Tension

This paper investigates whether gravitationally induced particle creation can act as a dynamical dark energy candidate, finding that such models provide fits comparable to $\Lambda$ CDM and help alleviate the Hubble tension by reducing it to approximately $2.4\text{--}3\sigma$ .

Original authors: Tiziano Schiavone, Mariaveronica De Angelis, Luis A. Escamilla, Giovanni Montani, Eleonora Di Valentino

Published 2026-04-28

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CC BY 4.0

Original authors: Tiziano Schiavone, Mariaveronica De Angelis, Luis A. Escamilla, Giovanni Montani, Eleonora Di Valentino

Original paper licensed under CC BY 4.0 (http://creativecommons.org/licenses/by/4.0/). ✨ This is an AI-generated explanation of the paper below. It is not written or endorsed by the authors. For technical accuracy, refer to the original paper. Read full disclaimer

The Cosmic "Popcorn" Theory: A Simple Guide to the New Paper

Imagine you are watching a movie of the Universe’s history. For decades, scientists have been watching the "credits" roll—the current era where the Universe is expanding faster and faster.

But there’s a problem. Scientists are having a massive argument called the "Hubble Tension." It’s like two different groups of detectives looking at the same crime scene: one group uses high-tech forensic tools (the Cosmic Microwave Background from the early Universe) and says the Universe is expanding at one speed. The other group uses a stopwatch and looks at nearby stars (Supernovae) and says, "No, it’s moving much faster!"

This paper proposes a wild, creative solution: What if the Universe is literally making more "stuff" as it grows?

1. The Concept: The Universe as a Self-Filling Bathtub

In the standard model of cosmology ( $\Lambda$ CDM), the Universe is like a bathtub where the water level is rising because the tub is getting bigger, but the amount of water stays the same.

This paper suggests a different scenario: The Universe is a bathtub with a leaky faucet. As the tub expands, new water (particles) is being created out of the vacuum itself. This isn't just "extra water"; this new water has a special property—it creates a kind of "push" (negative pressure) that helps drive the expansion even faster.

In physics terms, they call this Gravitationally Induced Particle Creation. Instead of needing a mysterious "Dark Energy" constant to push the Universe apart, the very act of the Universe expanding triggers the birth of new particles, which in turn fuels the expansion. It’s a cosmic feedback loop.

2. The "Agnostic" Approach: Not Caring What the Particles Are

Usually, scientists try to guess what these new particles are (like "Dark Matter"). But the authors of this paper take an "agnostic" approach.

Think of it like a chef tasting a soup. Instead of trying to guess exactly which brand of salt or which specific herb was used, the chef simply says, "I don't care what it is; I just know that whatever it is, it's making the soup saltier."

The researchers didn't assume what the new particles were; they just gave them a mathematical "flavor" (an equation-of-state parameter) and let the data tell them how much "salt" was needed to make the math work.

3. The Four Recipes (The PC Models)

The researchers tested four different "recipes" for how fast this particle creation happens:

Model 1 (The Power Player): Creation speeds up or slows down based on a specific mathematical curve.
Model 2 (The Linear Grower): Creation is directly tied to how fast the Universe is currently expanding.
Model 3 (The Steady Stream): Particles are created at a constant, unchanging rate.
Model 4 (The Hybrid): A mix of a steady stream and a changing rate.

4. The Verdict: Does it solve the mystery?

After running these models through massive amounts of data (from Supernovae to the Cosmic Microwave Background), here is what they found:

It works! These "Particle Creation" models fit the observed data just as well as, and sometimes slightly better than, the standard model.
It eases the tension: While it doesn't completely "fix" the argument between the two groups of detectives, it brings them much closer together. It reduces the "tension" (the disagreement) from a massive $4.3\sigma$ (a huge red flag) down to about $2.4\sigma$ (a much more manageable disagreement).
The "Dark Energy" reveal: Most importantly, the data showed that these new particles must behave like Dark Energy. If the particles were just normal, heavy matter, the math wouldn't match what we see in the sky. The Universe seems to require this "pushing" particle to keep expanding the way it does.

Summary

Instead of looking for a mysterious, invisible "Dark Energy" force that has always been there, this paper suggests that the expansion of the Universe itself is a factory. The expansion creates particles, and those particles, in turn, drive the expansion. It’s a self-sustaining cosmic engine that might finally explain why our measurements of the Universe's speed don't seem to add up.

Technical Summary: Revisiting the Matter Creation Process

Title: Revisiting the Matter Creation Process: Observational Constraints on Gravitationally Induced Dark Energy and the Hubble Tension
Authors: Tiziano Schiavone, Mariaveronica De Angelis, Luis A. Escamilla, Giovanni Montani, and Eleonora Di Valentino.

1. The Problem: The $\Lambda$ CDM Crisis

The standard cosmological model ( $\Lambda$ CDM) successfully describes the evolution of the Universe but faces two major theoretical and observational challenges:

The Dark Energy Problem: The physical origin of the cosmological constant ( $\Lambda$ ) remains unknown and theoretically problematic.
The Hubble Tension: A significant discrepancy exists between "early-Universe" measurements of the Hubble constant ( $H_0$ ) from the Cosmic Microwave Background (CMB) and "late-time" local measurements from Type Ia Supernovae (SNe Ia) and Cepheids. This tension has reached a statistical significance of $4.3\sigma$ to $7.1\sigma$ depending on the dataset used.

2. Methodology: Gravitationally Induced Particle Creation (PC)

The authors propose an alternative to the cosmological constant by invoking gravitationally induced particle creation.

Theoretical Framework: Based on non-equilibrium thermodynamics in open systems, the authors model a process where the time-dependent gravitational background spontaneously produces particles. This process is incorporated into the continuity equation of the cosmic fluid, resulting in an effective negative pressure ( $\Pi$ ) that can drive cosmic acceleration.
Agnostic Approach: Unlike previous studies that focused on creating Cold Dark Matter (CDM), this paper adopts an "agnostic" stance. They assume the creation of an unknown particle species with an arbitrary equation-of-state parameter ( $w_E$ ), treating it as a free variable to see if it naturally mimics dark energy.
Phenomenological Models: They introduce four specific parametrizations of the particle creation rate ( $\Gamma$ $Γ$ ) to describe the late-time Universe ( $0 < z < 3$ $0 < z < 3$ ):
- PC1: A power-law dependence on the Hubble rate ( $\Gamma \propto H^\alpha$ ).
- PC2: A linear scaling with the Hubble rate ( $\Gamma \propto H$ ).
- PC3: A constant creation rate ( $\Gamma = \text{const}$ ).
- PC4: A mixed model combining constant and linear terms.
Statistical Analysis: The models are tested using a joint analysis of multiple datasets: Cosmic Chronometers (CC), PantheonPlus SNe Ia, DESI DR2 Baryon Acoustic Oscillations (BAO), SH0ES local $H_0$ measurements, and Planck CMB data. They utilize Nested Sampling to estimate posterior distributions and Bayesian Evidence to compare models.

3. Key Contributions

Generalization of PC Models: The study moves beyond the assumption that created particles must be matter ( $w=0$ ), allowing the created species to act as a dynamical dark energy component.
Late-Time Restriction: To preserve the successful $\Lambda$ CDM description of the early Universe, the authors restrict the particle creation mechanism to the redshift range $0 < z < 3$ .
Effective Equation of State: They derive the "effective" dark energy equation of state ( $w_{\text{eff,DE}}$ ), showing how the combination of the intrinsic $w_E$ and the creation rate $\Gamma$ produces dynamical behavior.

4. Results

Nature of the Created Species: The data strongly disfavors the creation of pressureless matter ( $w_E = 0$ ). Instead, the constraints force $w_E < -1/3$ , meaning the created particles must behave like dark energy to be consistent with observations.
Alleviation of Hubble Tension: The PC models successfully reduce the Hubble tension. While $\Lambda$ CDM shows a $4.3\sigma$ discrepancy, the PC models reduce this to approximately $2.4\sigma$ – $3\sigma$ .
Model Comparison:
- The PC models provide a better fit ( $\chi^2_{\text{min}}$ ) than $\Lambda$ CDM.
- The PC2 model (linear scaling) is particularly notable as it behaves like a $w$ CDM model with a constant effective equation of state.
- The PC1 model shows the most interesting dynamical evolution, transitioning from a quintessence-like regime to a phantom-like regime at low redshifts.
- In terms of Bayesian Evidence, the PC models are mildly favored over $\Lambda$ CDM, though the statistical preference is categorized as "weak" or "inconclusive" according to the Jeffreys scale.

5. Significance

This paper demonstrates that gravitationally induced particle creation is a theoretically consistent and observationally viable framework for explaining late-time cosmic acceleration. By treating the created component as a dynamical fluid, the model provides a physical mechanism for dark energy that can simultaneously mitigate the Hubble tension without violating early-Universe constraints. It offers a compelling alternative to the purely phenomenological dark energy models (like $w_0w_a$ CDM) by rooting the acceleration in the thermodynamics of an expanding spacetime.