Could a quantum horizon timescale shape cosmic expansion?

A clock suggested by horizons

In their preprint, Fast Scrambling Cosmology, Sahoo and colleagues ask whether a timescale discussed in quantum gravity could influence the universe’s expansion. Their proposal is a theoretical model, not a detection of quantum-gravity effects.

The starting point is “scrambling”: the spreading of information through a system until it cannot be recovered from a sufficiently small part. Black holes are conjectured to scramble information exceptionally quickly. Applying related reasoning to a cosmological horizon gives a characteristic timescale involving the expansion rate and a logarithm of the enormous separation between cosmic and Planck scales.

A similar timescale appears in the trans-Planckian censorship conjecture, a proposed restriction on how expansion stretches initially tiny fluctuations. The authors use this connection as motivation, rather than deriving their model from an established microscopic theory.

Changing the expansion equation

The researchers add one term to the Raychaudhuri equation, which describes how the Hubble expansion rate changes with time. A dimensionless parameter, Gamma, sets the correction’s strength and direction. Positive Gamma makes the expansion rate decrease in an otherwise empty universe; negative Gamma makes it increase.

Either choice replaces an exactly constant vacuum expansion rate with slow evolution. The authors solve this vacuum case exactly. To include matter and radiation, they also use an approximation in which a related parameter, sigma, stays constant. This is useful because the logarithm controlling sigma changes slowly, particularly between the release of the cosmic microwave background and today.

The resulting effective dark energy has two parts. One is a slowly evolving vacuum contribution, which becomes a cosmological constant when the correction vanishes. The other depends on the accumulated history of matter and radiation. Some contributions track those components as the universe expands; changes in a species’ pressure relative to its density can also leave a lasting residual.

Neutrinos leave a mathematical memory

Neutrinos provide the authors’ example of this history dependence. As cosmic expansion reduces their momenta, massive neutrinos evolve from radiation-like to matter-like behavior. The researchers calculate their contribution using a distribution for particles that decoupled while relativistic, neglecting small distortions from non-instantaneous decoupling.

After separating tracking and initial-condition terms, they identify a residual contribution that scales approximately with the fourth power of neutrino mass. It eventually settles into the slowly evolving vacuum contribution.

Importantly, this is not a sudden release of dark energy when neutrinos become non-relativistic. In the authors’ example, most of the residual accumulates much earlier, while the neutrinos remain relativistic.

For small positive sigma, around one millionth to one hundred-thousandth, the residual can approach the observed dark-energy density if the remaining initial-condition contribution is negligible. But that result depends on the chosen starting epoch and initial conditions. It also requires Gamma much smaller than the order-one magnitude motivating the model. Near the negative parameter values suggested by their observational proxies, the neutrino residual is instead negative and much too large in magnitude, requiring compensation by the initial-condition contribution.

Comparisons are not direct constraints

The authors estimate plausible parameter values by translating existing results from primordial light-element abundances and a different modified-gravity model. One light-element estimate gives Gamma = −3.7 ± 6.1 at 68% confidence, consistent with no correction. A tighter translation from the other gravity model gives approximately −2.8 ± 0.9.

These are explicitly proxy estimates, not measurements obtained by fitting fast-scrambling cosmology itself. The other gravity model can behave differently when small density fluctuations grow, limiting the translation’s reliability.

The team also compares its dark-energy evolution with reconstructions using DESI’s second data release combined with other observations. A negative-sigma example resembles the reconstructed behavior at redshifts above roughly one. It does not reproduce a lower-redshift bump in the comparison model. For a fixed sign of sigma, the proposed dark-energy density changes monotonically and, where positive, cannot cross the equation-of-state boundary associated with a cosmological constant.

Physical consistency remains open

Matching average expansion is not enough. The authors construct a scalar “clock” field that reproduces their approximate background equation, but its simplest form has an instability that makes short-wavelength disturbances grow.

An additional term regulates that problem during dark-energy domination on the negative-sigma branch. The authors caution that this does not establish viability across cosmic history; matter, transitions between eras and the theory’s range of validity still need examination. The extension also does not fix the problematic scalar dynamics on the positive-sigma branch used in the neutrino-abundance example.

Nor does the model itself satisfy the integrated trans-Planckian censorship bound that helped motivate it. Its contribution is a concrete expansion-history proposal with calculable consequences—not a completed quantum-gravity explanation of dark energy. A consistent treatment of cosmic structure and a dedicated observational fit remain necessary.