The universe's expansion, governed by the Hubble-Lemaitre constant, has sparked a cosmic conundrum known as the “Hubble tension.” This enigma arises from conflicting values obtained through different measurement methods. Enter a proposed resolution from researchers at the Universities of Bonn and St. Andrews: an alternative gravity theory. Their study, now published in the Monthly Notices of the Royal Astronomical Society (MNRAS), suggests that embracing this alternative theory can effortlessly reconcile the measured value disparities, making the Hubble tension vanish.

The cosmic dance of galaxies moving apart due to the universe's expansion is intricately tied to the Hubble-Lemaitre constant. Edwin Hubble, a pioneering US astronomer, was among the first to discern the proportional relationship between the speed of galactic separation and the distance between them. To calculate this speed accurately, the crucial factor is knowing the distance between two galaxies. Yet, this determination hinges on a constant—the Hubble-Lemaitre constant, a cosmological linchpin. Its value, vital for these calculations, is derived from observations of far-flung corners of the universe, yielding a velocity of nearly 244,000 kilometers per hour per megaparsec distance (where one megaparsec spans just over three million light-years).

## 244.000 kilometers per hour per megaparsec—or 264,000?

Prof. Dr. Pavel Kroupa, affiliated with the Helmholtz Institute of Radiation and Nuclear Physics at the University of Bonn, sheds light on an alternative approach to gauge the Hubble-Lemaitre constant. By examining category 1a supernovae—specific types of exploding stars—in close proximity to Earth, precise distance measurements become feasible. Notably, these supernovae exhibit color shifts as they move away, akin to the Doppler effect experienced with an ambulance siren.

Deriving the speed of 1a supernovae from their color shifts and correlating it with their distance yields a distinct Hubble-Lemaitre constant value, approximately under 264,000 kilometers per hour per megaparsec distance. The intriguing twist emerges as this suggests a faster expansion of the universe in our vicinity—up to around three billion light years—compared to its overall rate, presenting a cosmic anomaly.

However, recent observations propose a potential explanation. It suggests that Earth resides in a region of space characterized by a relatively sparse matter distribution, resembling an air bubble in a cake. The surrounding higher matter density exerts gravitational forces, pulling galaxies within the bubble toward its edges. Dr. Indranil Banik from St. Andrews University explains that this phenomenon could elucidate why galaxies in this space are receding from us at a swifter pace than anticipated—a deviation attributable to a local “under-density.”

Adding to the intrigue, another research group measured the average speed of galaxies situated 600 million light years away. The findings unveiled that these galaxies are departing at a rate four times faster than the standard cosmological model predicts, as highlighted by Sergij Mazurenko from Kroupa's research group, who contributed to the current study. This anomaly further deepens the exploration into the dynamics of our cosmic neighborhood and challenges established cosmological paradigms.

## Bubble in the dough of the universe

The standard model grapples with the existence of under-densities or “bubbles” in the cosmos, a phenomenon not anticipated in a model where matter should uniformly fill space. This challenges conventional wisdom, as evenly distributed matter would struggle to account for the forces propelling galaxies to their heightened velocities.

Prof. Dr. Pavel Kroupa remarks on the foundation of the standard model, rooted in Albert Einstein's gravitational theory. However, he highlights the potential variance in gravitational forces from Einstein's expectations. In response, the collaborative efforts of the University of Bonn and St. Andrews deploy a computer simulation utilizing a modified theory of gravity—specifically, “modified Newtonian dynamics” (MOND). This theory, proposed by Israeli physicist Prof. Dr. Mordehai Milgrom four decades ago, remains unconventional but gains credence in the current study.

Kroupa notes that MOND, in their calculations, successfully anticipates the existence of these cosmic bubbles. Entertaining the notion that gravity aligns with Milgrom's assumptions could resolve the Hubble tension. In this scenario, a singular constant governs the universe's expansion, with observed deviations attributed to irregularities in matter distribution. This departure from Einstein's envisioned gravitational dynamics presents a compelling avenue for unraveling cosmic mysteries and reconciling the observed anomalies.

Source: University of Bonn