Is Dark Energy an Effective Manifestation of Non-equilibrium Thermodynamics? -- Insights from DESI

This paper investigates a non-equilibrium thermodynamics-based cosmological model featuring adiabatic dark matter creation, finding that while one variant mimics the standard $\Lambda$CDM scenario, another variant is statistically favored over $\Lambda$CDM when incorporating recent DESI BAO data, suggesting dark energy may be an effective manifestation of matter creation.

The Gist

The Big Picture: Is the Universe Running on a New Engine?

Imagine the universe as a giant, expanding balloon. For decades, scientists have believed this balloon is being inflated by two invisible forces: Dark Matter (which acts like glue holding galaxies together) and Dark Energy (which acts like a mysterious gas pushing the balloon to expand faster and faster). This standard recipe is called ΛCDM.

However, this paper asks a different question: What if we don't need a mysterious "Dark Energy" gas at all?

Instead, the authors propose that the universe is expanding faster because Dark Matter is being created on the fly. Think of it like a bakery that doesn't just bake bread and sell it; the bakery magically conjures new loaves of bread out of thin air as it expands. This "creation" of new matter creates a kind of internal pressure that pushes the universe apart, mimicking the effect of Dark Energy.

The authors tested this idea using the latest data from the DESI (Dark Energy Spectroscopic Instrument) telescope, which maps millions of galaxies to see how fast the universe is stretching.

The Two Recipes (Models)

The team tested two specific "recipes" for how this matter creation might work:

1. Model I (The Steady Baker):

   • The Idea: The rate at which new Dark Matter is created depends on how much is already there. If there is a lot of Dark Matter, the creation slows down; if there is little, it speeds up.
   • The Result: This model behaves almost exactly like the standard "Dark Energy" recipe. It predicts a universe that starts with a hot, dense fire (radiation), cools down into a matter-filled era, and then speeds up its expansion.
   • The Verdict: It fits the data just as well as the standard model. It's a "twin" of the current theory, just with a different engine.

2. Model II (The Flexible Baker):

   • The Idea: This is a more complex version. The rate of creation doesn't just depend on the amount of matter; it depends on the amount raised to a specific power (a mathematical "knob" called $l$). This allows the creation rate to be more sensitive to how the universe is changing.
   • The Result: This model is more flexible. When the authors crunched the numbers with the latest DESI data, they found that the universe might be behaving slightly differently than the standard model predicts.
   • The Verdict: When they combined the new telescope data with other observations, this "flexible" model actually looked better than the standard Dark Energy model. It suggests the universe's expansion history might be slightly different from what we thought.

How They Tested It (The Detective Work)

The authors didn't just guess; they played detective using three main types of clues:

• Cosmic Chronometers: Measuring the "age" of galaxies at different distances to see how fast the universe was expanding in the past.
• Supernovae (Type Ia): Using exploding stars as "standard candles" to measure cosmic distances.
• DESI BAO: Using the "frozen sound waves" from the early universe (Baryon Acoustic Oscillations) as a giant ruler to measure the scale of the cosmos.

They ran their "creation" models against these clues and compared the results to the standard "Dark Energy" model.

The Findings

1. Matter Creation is Real (Statistically): In both models, the data strongly suggests that matter is being created. The idea that Dark Matter is perfectly conserved (staying the same amount forever) is less likely than the idea that it is being generated.
2. The "Standard" Model is Hard to Beat: The first model (Model I) is so similar to the standard Dark Energy model that it's hard to tell them apart. They are practically twins.
3. The "New" Model Wins with New Data: The second model (Model II) is where things get interesting. When they added the brand-new DESI DR2 data (the latest, most precise measurements), this model started to look better than the standard Dark Energy model. It suggests that the universe's expansion is driven by this matter creation process rather than a mysterious Dark Energy fluid.

The Bottom Line

Imagine you are trying to explain why a car is speeding up.

• The Old Theory: "There is a hidden gas pedal (Dark Energy) pushing it."
• This Paper's Theory: "The engine is actually creating more fuel (Dark Matter) as it runs, which naturally makes it speed up."

The paper concludes that this "fuel creation" idea is a very strong alternative to the standard theory. While the simple version of this idea is just as good as the old theory, the more complex version actually fits the newest, most precise telescope data better than the old theory does.

Important Note: The authors are careful to say this is a "background" study (looking at the big picture of the universe's expansion). They suggest that to be 100% sure, future studies need to look at how galaxies clump together (perturbations) and include data from the Cosmic Microwave Background (the afterglow of the Big Bang). But for now, the idea that the universe is creating its own matter to drive its expansion is looking very promising.

Technical Summary

Technical Summary: Is Dark Energy an Effective Manifestation of Non-equilibrium Thermodynamics? – Insights from DESI

Problem Statement
The standard $\Lambda$CDM cosmological paradigm, while observationally robust, faces theoretical and observational challenges. Alternative approaches typically involve introducing dark energy fluids with negative pressure or modifying General Relativity (GR). This paper investigates a third avenue: cosmological scenarios driven by gravitationally induced adiabatic matter creation. In this framework, the creation of particles generates a negative "creation pressure" ($p_c$) that can drive the observed accelerated expansion of the universe without invoking a cosmological constant ($\Lambda$) or modifying the gravitational sector. The study specifically addresses whether such non-equilibrium thermodynamic processes can mimic the background expansion history of the universe and how they fare against the latest observational data, particularly the recent DESI (Dark Energy Spectroscopic Instrument) Baryon Acoustic Oscillations (BAO) releases (DR1 and DR2).

Methodology
The authors construct a flat Friedmann-Lemaître-Robertson-Walker (FLRW) universe containing three independently evolving components: radiation, baryons, and dark matter (DM). While radiation and baryons follow standard conservation laws, the DM sector is governed by an adiabatic matter creation process, leading to a modified continuity equation. The creation pressure is derived from the Gibbs-Duhem relation under the assumption of adiabaticity, resulting in a creation rate $\Gamma$.

Two specific models for the creation rate $\Gamma$ are analyzed:

1. Model I: $\Gamma = 3\alpha H (\rho_{c,0}/\rho_{dm})$, known as the LJO model (Lima, Jesus, Oliveira), where $\alpha$ is a positive parameter.
2. Model II: $\Gamma = 3\alpha H (\rho_{c,0}/\rho_{dm})^l$, a generalization of Model I introducing a power-law exponent $l$ to allow for non-linear dependence on DM density.

The analysis proceeds in two stages:

• Dynamical System Analysis: The cosmological equations are transformed into an autonomous system using dimensionless variables ($x, y, \xi$). The authors perform a phase-space analysis to identify critical points, their stability, and the corresponding cosmological epochs (radiation, matter, acceleration) to determine if the models can reproduce a $\Lambda$CDM-like thermal history.
• Observational Confrontation: The models are constrained using Markov Chain Monte Carlo (MCMC) analysis against a comprehensive set of datasets:
  • Cosmic Chronometers (CC) for Hubble parameter measurements.
  • Type Ia Supernovae (SNIa) from three samples: Pantheon+ (PP), DES Year 5 (D5), and Union3 (U3).
  • DESI BAO data from Data Release 1 (DR1) and the latest Data Release 2 (DR2).
  • Statistical model comparison is performed using the Akaike Information Criterion (AIC) and Bayesian Information Criterion (BIC) relative to the standard $\Lambda$CDM model.

Key Contributions and Results

• Dynamical Evolution:

  • Model I: The phase-space analysis reveals that Model I possesses a stable attractor (point $A_2$) corresponding to a dark-matter-dominated, de Sitter accelerating phase ($q=-1$). The system naturally evolves from a radiation-dominated decelerated phase to a matter-dominated phase and finally to acceleration, effectively mimicking the $\Lambda$CDM background evolution without an explicit cosmological constant.
  • Model II: The behavior depends on the parameter $l$. For $0 < l < 1$ and $l > 1$, the models exhibit a $\Lambda$CDM-like evolution with a stable de Sitter attractor. However, cases where $l < 0$ or $l = 0$ lead to early acceleration or phantom-like behaviors that are deemed physically uninteresting or ruled out by data.

• Observational Constraints:

  • Evidence for Matter Creation: In both Model I and Model II, the data consistently show evidence of matter creation (non-zero $\alpha$) at many standard deviations across all dataset combinations.
  • Hubble Constant ($H_0$): The inclusion of the DESY5 dataset tends to yield slightly higher $H_0$ values ($\sim 69.7 - 70.5$ km/s/Mpc) compared to Planck 2018 results within $\Lambda$CDM, suggesting these models combined with specific datasets may help alleviate the Hubble tension, though the paper notes this is dataset-driven.
  • DESI Impact: The inclusion of DESI DR2 data significantly tightens constraints on the parameters. For Model II, the inclusion of DESI DR2 with CC+SNIa data causes the parameter $l$ to deviate from unity ($l=1$ corresponds to $\Lambda$CDM-like behavior) at greater than 95% confidence level, indicating a preference for a deviation from standard $\Lambda$CDM cosmology.

• Model Comparison (AIC/BIC):

  • Model I vs. $\Lambda$CDM: Model I is statistically equivalent to the $\Lambda$CDM scenario. Since both models have the same number of free parameters in the context of the datasets used, the preference is driven by $\Delta\chi^2_{min}$. The results show they are practically indistinguishable, with $\Lambda$CDM having a negligible edge in a few specific dataset combinations.
  • Model II vs. $\Lambda$CDM: The comparison yields a mixed picture. For most datasets, $\Lambda$CDM is favored due to the penalty of the extra parameter $l$ in the BIC. However, when DESI data (specifically DR2) is combined with SNIa data, Model II is favored over $\Lambda$CDM by the Likelihood Ratio Test (LRT) and AIC, suggesting that the data supports the more complex matter creation scenario in these specific configurations.

Significance and Claims
The paper claims that cosmological matter creation scenarios offer a viable alternative to both dark energy and modified gravity theories. The primary significance lies in demonstrating that:

1. Adiabatic matter creation can naturally generate the observed late-time acceleration and reproduce the thermal history of the universe.
2. Recent DESI DR2 data, when combined with other probes, provides evidence for matter creation at high statistical significance.
3. Specifically, Model II shows that the data may favor a deviation from the standard $\Lambda$CDM paradigm (where $l \neq 1$) when DESI data is included, aligning with recent indications of dynamical dark energy.

The authors modestly conclude that while the background-level analysis is promising, a full validation requires extending the analysis to perturbations and confronting the models with Cosmic Microwave Background (CMB) data, which is left for future work. They emphasize that the current results suggest matter creation is a competitive scenario that warrants further investigation.