Is Dark Energy Non-Uniform?

The mass budget of the Universe at the present-time. (Image Credit: Astronomy)Our cosmic ignorance is vastly bigger than our knowledge. A quantitative manifestation of this fact is that 95% of the mass-energy budget of the Universe at the present time is of an unknown nature, whereas only 5% is ordinary matter familiar to us. Out of the 95% in invisible components, 68% is labeled `dark energy’ and 27% is labeled `dark matter’. Whereas dark matter is known to condense into halos around gravitationally-bound objects, like our own Milky-Way galaxy, dark energy is known to drive the accelerated expansion of the Universe and is assumed to be spatially uniform.But is that really the case? Could dark energy be non-uniform? In a new paper (accessible here) that I wrote with the brilliant PhD student, Oem Trivedi, we studied the open fundamental question of whether the Universe might contain voids that are deficient in dark energy. If we happen to be embedded in such a void, the local Hubble expansion rate of the Universe would be different than its global value. Such a discrepancy could explain the difference between the expected and the measured values of the Hubble constant, known as the “Hubble tension” between measurements of the cosmic microwave background (CMB) in the early Universe and local measurements.Quantitative illustration of the Hubble tension. The diagram displays measurements of the present time Hubble constant, H0, based on different cosmological probes with error bars of 3.5 standard deviations. The top part shows estimates based on early-time measurements assuming the standard cosmological model, the middle part contains cosmology-independent direct measurements of H0, while the bottom part shows estimates that contain some form of astrophysical modeling. (Image credit: Wikimedia)Motivated by the recent discovery of dark matter deficient galaxies, our new paper asks the fundamental question: Could sufficiently large regions of the Universe be deficient i