For many years, scientists have used a model called ΛCDM to explain how the Universe has changed over time. It describes a Universe containing normal matter, dark matter and a mysterious component called the cosmological constant, Λ.
The cosmological constant is usually connected with dark energy, which is thought to be responsible for the accelerating expansion of the Universe.
But there is a major problem. Scientists still do not know what dark energy actually is. The amount of vacuum energy predicted by quantum physics is also very different from the amount needed to explain the Universe.
At the same time, measurements of the Universe have revealed disagreements between different observations, especially measurements of the expansion rate known as H₀, or the Hubble constant.
Now, a team led by Lee has tested an unusual idea called thermogravity. Instead of using a cosmological constant or traditional dark energy, the theory asks whether the Universe could accelerate because energy is not perfectly conserved on cosmic scales.
A New Way to Look at Gravity
Thermogravity comes from an interesting connection between gravity and thermodynamics.
We already know that black holes have properties such as temperature and entropy. This has led physicists to investigate whether gravity itself could have a thermodynamic origin.
Earlier theoretical work showed that Einstein's equations can be obtained by treating gravity as an equation describing thermodynamic behavior around tiny regions of spacetime.
The new theory takes this idea one step further.
Researchers imagine a small region of spacetime called a causal diamond. In the usual picture, the thermodynamic process is mainly related to heat moving across the boundaries of this region.
Thermogravity adds another process that can be compared with work done by a heat engine.
This extra process changes the behavior of gravity.
One important result is that the theory uses a trace-free version of Einstein's equations. In simple terms, one part of the usual equations is removed.
This is important because vacuum energy, which is normally associated with the cosmological constant, appears in this removed part.
As a result, the cosmological constant does not behave as a normal source of gravity in this theory.
Could Energy Non-Conservation Cause Acceleration?
If the cosmological constant does not drive the expansion, another explanation is needed for why the Universe is accelerating today.
Thermogravity provides one possibility.
The theory allows a controlled violation of the usual conservation of energy and momentum.
In the simplest cosmological picture, energy density can be created without creating the corresponding momentum.
This can change how the Universe expands.
At early times, the Universe can behave much like an ordinary matter-dominated Universe. But as time passes, the energy-creation process can become important and produce late-time acceleration.
This is interesting because the model can generate acceleration without adding a conventional dark-energy component.
The researchers describe the new process using a characteristic timescale. This scale plays a role similar to the role normally played by the cosmological constant in ΛCDM.
The Scientists Tested Two Versions
The study examined two different ways in which this energy non-conservation could operate.
The first version assumes that the effect is universal. In other words, everything—including ordinary matter, photons and dark matter—would be affected.
The second version assumes that the effect happens only in the cold dark matter (CDM) sector.
The difference between these two models is extremely important.
If energy non-conservation affects everything, it could also change the way scientists measure the Universe.
For example, it could affect how we interpret:
Redshifts
Supernova brightness
Photon temperatures
Matter densities
Standard rulers
BAO measurements
This creates a serious problem. Scientists normally use standard formulas to convert observations into information about the expansion of the Universe.
But if the measuring systems themselves are affected by the new physics, those formulas may no longer be correct.
The Universal Version Performs Poorly
The researchers first tested the universal version using observations of supernovae and baryon acoustic oscillations (BAO).
Supernovae can be used as distance indicators, while BAO provide a kind of cosmic measuring stick.
The supernova data alone could be described reasonably well by the model.
However, when measurements from the Dark Energy Spectroscopic Instrument (DESI) were added, the model performed much worse than standard ΛCDM.
The difference in the statistical fit was about:
Δχ² ≈ +51
The main reason is that the model connects today's acceleration too strongly with the expansion rate at earlier times.
In other words, changing the amount of matter creation to improve one part of the observations automatically changes another part of the expansion history.
This makes it difficult for the model to satisfy both the supernova and BAO measurements at the same time.
However, the researchers warn that this result should not be treated as a complete rejection of every possible universal version of thermogravity.
Why?
Because a truly universal energy non-conservation process would also change the way observations themselves are interpreted.
So, applying the normal ΛCDM observational formulas to such a model may not be completely self-consistent.
A Different Idea: Change Only Dark Matter
The second model takes a more limited approach.
Instead of allowing all forms of matter and radiation to experience energy non-conservation, the researchers allow the effect to act only on cold dark matter.
This model is called ξCDM.
This makes the situation much cleaner.
Ordinary matter, photons and neutrinos can continue to behave normally. That means researchers can still use standard interpretations of supernova observations, Big Bang nucleosynthesis and BAO at the background level.
Interestingly, ξCDM provides a better fit than ΛCDM for several combinations of background observations.
The largest improvement reported in the study is:
Δχ² = −4.85
The analysis also found a maximum Bayesian preference of:
ln ℬ = 2.77
This suggests that allowing the dark matter sector to behave differently can improve the description of some observations.
But there is an important limitation.
The Model Does Not Solve the Hubble Tension
Scientists have been dealing with a long-standing disagreement over the value of H₀, the current expansion rate of the Universe.
Measurements based on the early Universe and measurements based on the nearby Universe do not perfectly agree.
The researchers tested whether ξCDM could help with this problem.
The answer from this background analysis is no.
When BAO measurements are calibrated using information from the CMB or Big Bang nucleosynthesis, the model changes the conditions in the early Universe.
In particular, the model reduces the effective early-time amount of cold dark matter.
This increases the sound horizon, the distance that sound waves could travel through the early Universe.
Because BAO measurements are strongly connected to this scale, the larger sound horizon pushes the inferred value of H₀ lower.
That means the model does not remove the disagreement between early- and late-Universe measurements. In this situation, it can actually increase the difference.
The Biggest Test Is Still Coming
The current study focuses only on the background expansion of the Universe.
Scientists still need to test what thermogravity predicts for galaxies, cosmic structures, gravitational lensing and the cosmic microwave background.
This is especially challenging because the theory introduces a preferred frame and changes some of the symmetries normally used in General Relativity.
Researchers therefore cannot simply take the standard calculations used for ΛCDM and apply them to thermogravity.
A new theory of cosmic perturbations will be needed.
What Does This Mean?
The study does not show that ΛCDM is wrong or that thermogravity has replaced dark energy.
Instead, it provides an important first observational test of a completely different idea.
The universal version of the theory struggles strongly when standard supernova and DESI BAO measurements are used. The dark-matter-only ξCDM version performs better for several background datasets and offers an interesting alternative framework.
However, it does not solve the Hubble tension when early-Universe calibration is included.
The next major challenge is to test thermogravity using the CMB, galaxy formation, weak lensing and the growth of cosmic structure.
If those future tests support the theory, scientists may have to rethink how gravity, thermodynamics and cosmic expansion are connected.
For now, the biggest question remains fascinating: Could the Universe be accelerating not because of dark energy, but because gravity itself allows a small amount of energy to be created over cosmic time?
Reference: Dong Ha Lee, João Magueijo, Carsten van de Bruck, Eleonora Di Valentino, "Testing cosmic acceleration from thermogravity without vacuum energy", Arxiv, 2026. https://arxiv.org/abs/2609.29206
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