Could cosmic voids reveal uneven dark energy?

Dark energy is the name given to the component responsible for the Universe’s accelerated expansion. In the standard cosmological model, it is a cosmological constant: its density does not vary from place to place, even inside regions containing very little matter.

Oem Trivedi and Abraham Loeb ask what observations might reveal if that assumption fails. Their theoretical preprint develops a framework for searching for dark energy deficiencies through cosmic voids, galaxy motions and distance measurements. It reports no detection of such regions.

An ordinary void is already expanding differently

A cosmic void is an extended region with less matter than the cosmic average. Its matter deficiency naturally produces faster local expansion than the surrounding Universe. Finding galaxies moving outward from a void therefore would not, by itself, establish anything unusual about dark energy.

The authors instead propose a comparison: reconstruct the expansion expected from a void’s matter distribution, assuming homogeneous dark energy, then measure whether its actual expansion differs.

They describe the central part of an approximately spherical void as a small cosmological patch with its own expansion history. This approach accommodates a deep matter deficiency rather than assuming the whole void is merely a small departure from average density. Matter conservation connects the region’s density evolution to its coherent outward motion.

The researchers then introduce a hypothetical dark energy deficit. At fixed matter profile and effective curvature, their equations predict slower expansion than in the corresponding ordinary void. The region can still expand faster than the cosmic average; it simply expands less rapidly than its matter deficiency alone would predict.

What size of signal might appear?

Their representative example has a central matter density about 27% of the cosmic mean. With homogeneous dark energy, its local expansion rate is approximately 77.6 kilometres per second per megaparsec, compared with an assumed background value of 67.36. A megaparsec is a unit of cosmic distance; the rate expresses how recession speed increases with separation.

Giving this model void 10% less dark energy lowers its expansion rate to about 75.5 in the same units: roughly a 2.7% suppression relative to the ordinary-void prediction.

Across radii of 30–50 megaparsecs, that difference corresponds to radial velocity residuals of about 60–100 kilometres per second. These are illustrative maximal radial signals before projection and averaging, not measured galaxy velocities. Individual line-of-sight signals would generally be smaller.

The proposed search would combine many suitably isolated voids to recover a common radial pattern while reducing random motions. Bulk flows, nonspherical shapes and surrounding structures would need careful treatment.

More than one measurement must agree

Galaxy motions also contain smaller fluctuations around a void’s overall expansion. The authors propose jointly examining the matter distribution, velocity-related momentum statistics and correlations between density and motion.

That combination matters because dark energy fluctuations could alter both galaxy motions and the growth of matter structure. An unexplained velocity excess alone would not uniquely identify dark energy: nonlinear dynamics and observational errors can also produce residuals.

DESI galaxy and peculiar-velocity measurements, Rubin Observatory supernova distances, gravitational lensing and measurements of evolving gravitational potentials could provide complementary tests. However, precision forecasts for an entire survey do not automatically apply to its smaller void-selected sample. The paper’s sensitivity estimates are targets, not demonstrated survey capabilities.

A conditional connection to the Hubble tension

The paper also explores whether a large deficient region could affect the locally inferred Hubble constant. This addresses the discrepancy between local distance-based measurements and early-Universe inferences under the standard model.

In one schematic configuration, a roughly 20% dark energy deficiency extending to a radius of about 600 megaparsecs partially reduces an ordinary matter void’s enhanced expansion, leaving an apparent rate near the local measurement across the relevant distance range. Less dark energy is not what produces the initial enhancement; the matter deficiency does that.

This is not a fit to the observed tension. An observer away from the region’s centre would generally see directional differences in distances and an imprint on the cosmic microwave background. Those constraints could exclude a configuration that otherwise reproduces the average expansion rate.

Finally, a strict cosmological constant cannot sustain the proposed density variations. Alternative dynamical fields would need suitable pressure behaviour, stable evolution and consistency with existing observations. The authors offer preliminary theoretical examples, not a fully evolved, observationally validated model. The preprint’s contribution is a proposed set of linked tests: any dark energy interpretation must survive them together.