Dark energy with a sign change: How the transition from negative to positive density smooths out the Hubble tension ✛ ✎ ✦
Context
The standard cosmological model ΛCDM is remarkably successful, but faces a serious challenge — the Hubble tension. According to Hubble's law, discovered by Edwin Hubble and predicted by Georges Lemaître within the expanding Universe after the Big Bang, the recession velocity of galaxies is proportional to distance, and the proportionality factor — the Hubble constant — determines the rate of expansion of the Universe. However, modern local measurements give a value around 73 km/s/Mpc, while cosmic microwave background from the early Universe points to 67 km/s/Mpc. The discrepancy exceeds 5σ, hinting at incompleteness in our understanding of dark energy. The traditional cosmological constant suffers from theoretical problems, so dynamical models where dark energy evolves are being explored. One intriguing possibility is sign-change models of dark energy density, where it transitions from negative values in the past to positive today.
Methods
To test the ECDM model, the researchers used Monte Carlo Markov chains (MCMC) with a modified CAMB code, based on Friedmann's equations for background dynamics and linearized perturbation theory. The analysis included high-resolution cosmic microwave background data from Planck, ACT, and SPT, photometric light curves of Type Ia supernovae from PantheonPlus (with local calibration by SH0ES), and spectroscopic surveys of galaxies from DESI, revealing redshifts of baryon acoustic oscillations — sort of sound waves of the early Universe. The main technical difficulty: when the density of dark energy crosses zero, its equation of state diverges, making standard perturbation equations unsuitable. The authors avoided this by redefining variables to maintain regularity over the entire range of redshifts. This allowed, for the first time, a consistent treatment of dark energy perturbations.
Results
The analysis revealed two branches: a slow and a fast transition of dark energy. However, supernova and BAO data confidently rule out the slow branch, leaving a fast transition at a characteristic redshift z† ≈ 2.9–3.1. Notably, extremely fast transitions (with logarithmic rate log₁₀η > 0.5) are also inconsistent with observations: they cause a sharp spike in structure growth, conflicting with data on gravitational lensing of the cosmic microwave background and redshift measurements. The resulting Hubble constant in ECDM is ~69.3 km/s/Mpc (combination of CMB+DESI+PantheonPlus), which is noticeably closer to local measurements and reduces the expansion tension to ~2σ. Most statistical criteria (AIC, DIC, WAIC) prefer ECDM over the standard ΛCDM, especially with the SH0ES calibration. The model predicts a slight enhancement of large-scale structure growth and a characteristic signal in the integrated Sachs–Wolfe effect — additional anisotropy in the cosmic microwave background on large angular scales, caused by the evolution of gravitational potentials. The CMB lensing power spectrum is also slightly enhanced compared to ΛCDM.
Implications
The results strengthen the case for sign-change models of dark energy as a viable alternative to ΛCDM. They indicate that dark energy is not a cosmological constant but a dynamical field going through a phase of negative pressure. This could explain why dark energy dominates precisely in the present era, alleviating the coincidence problem. The exclusion of instantaneous transitions suggests a smooth nature of this phenomenon, possibly linked to a phase transition in the dark sector.
Future development
Future research should go beyond phenomenological parameterization and build a theoretical model, for example, based on a scalar field with an appropriate potential. This will allow studying the microphysics of the transition and its impact on nonlinear structure growth. Upcoming missions like Euclid and the Roman Space Telescope, with their data on weak lensing and galaxy clusters, will provide stringent tests. More precise measurements of the cosmic microwave background from future experiments are also important.
Impact
Sign-change models of dark energy directly impact late-Universe cosmology, the interpretation of cosmic microwave background, supernova, and large-scale structure data. They also challenge theoretical physics by requiring an explanation for negative energy density.
Next steps
The first priority is to build a realistic model of dark energy based on quantum field theory, free from instabilities. It is also necessary to conduct a joint analysis with full power spectrum data from future surveys and to account for nonlinear effects.
Key open problems
The work addresses one of the major unsolved problems — the Hubble tension in the expansion rate. It is also connected to the cosmological constant problem, offering a dynamical alternative, and indirectly touches on the nature of dark matter, as the modification of expansion affects constraints on its density.
The knowledge machine's view
This section is addressed to the author of the paper and their colleagues. Neighbouring works were found by meaning-based search across our archive, not by word overlap. We do not review: we show the field around the work.
What the work does
The study presents a model of dark energy with a sign-switching density, transitioning from negative values in the past to positive ones today. The authors performed an MCMC analysis using a modified CAMB, incorporating data from Planck, ACT, SPT, PantheonPlus, and DESI, and circumvented the issue of the equation-of-state divergence at the zero crossing by redefining variables. This allowed, for the first time, a consistent treatment of dark energy perturbations, and the fast-transition model agrees with the data, easing the Hubble tension.
Where its strength is
The strong point is the technically challenging and correct description of dark energy perturbations as the density crosses zero, which opens the possibility of testing the model against the full set of cosmological data without loss of accuracy.
Where it could go next
- It might be interesting to compare predictions for the large-scale structure with models where dark energy is described by a scalar field with a kinetic term, as in the work on kinetic quintessence, to see how much the power spectrum signatures differ.based on: Dancing Shadow: How Kinetic Quintessence Reconciles the Universe's Epochs arXiv:2607.03569
- A related approach models dark energy as a scalar field with a potential that allows the density to cross zero; it would be curious to check whether it reproduces the fast transition and whether it is consistent with the parameter constraints from this work.based on: Dark-Sector Seesaw: How a Scalar Spring Solves Cosmological Puzzles arXiv:2607.27515v1
- There is a paper in the archive that considers sign-switching dark energy in the context of local inhomogeneity; comparing with it could clarify how such a model behaves in the nonlinear regime and whether it can be distinguished from ΛCDM by structures.based on: Bubble in a Sea of Dark Energy arXiv:2607.18376v1
Worth trying
- Test whether the fast-transition model remains consistent when adding weak lensing data (e.g., from KiDS or DES).
- Apply the variable redefinition method to other models with zero crossing, such as those with non-minimal coupling.
- Check how the model predicts structure growth and compare with galaxy cluster data.
Why this may matter
If the sign-switching model is confirmed, it would indicate a dynamic nature of dark energy and require a revision of the theoretical foundations of the cosmological constant, potentially leading to new physics beyond the Standard Model.
Nearby works in our archive
- Symphony of Dark Energy: From Heavy Adagio to Explosive Allegrosimilarity 0.77
- A Dark Deal: How the Universe Reconciles Its Clockssimilarity 0.71
- Dark-Sector Seesaw: How a Scalar Spring Solves Cosmological Puzzlessimilarity 0.69
- Warm Emptiness Instead of Dark Energysimilarity 0.67
- Dancing Shadow: How Kinetic Quintessence Reconciles the Universe's Epochssimilarity 0.67
- Dark Matter Decays, the Universe Smooths Outsimilarity 0.66
- Bubble in a Sea of Dark Energysimilarity 0.66
The archive is crowded around this work: 7 reviews nearby, closest at 0.77. This ground is well walked — and there is plenty to compare against.
Imagine that dark energy is not just a cosmological constant, but a field that in the past had negative density, acting like an extra attraction, and then smoothly switched to repulsion, causing the observed acceleration of the Universe's expansion.
Key numbers
- Hubble constant in ECDM: 69.3 km/s/Mpc
- Transition redshift z†: 3.07
- Transition rate logarithm log10 η: 0.67
- Reduction of the Hubble tension: to ~2σ
- Amplitude of fluctuations S8: 0.83