Scientists from the University of Trento and the Trento Institute for Fundamental Physics have discovered a serious problem with current thinking about the expansion of the universe. When the influence of the Laniakea supercluster, which includes the Milky Way, is taken into account, the values of the Hubble constant determined by different methods were found to be even more different. This could be a clue to the existence of new physics, according to physicists’ findings published as preprints in the arXiv repository.
About 20 years ago, astronomers discovered that the universe was expanding faster and faster. Within the framework of a cosmological model that satisfies general relativity and its equations, the so-called Friedman universe, such acceleration requires an exotic source, currently called dark energy. The currently accepted cosmological model, ΛCDM, describes a flat dark energy universe in which the majority of matter is represented by cold dark matter. This model is successful in that it can explain many cosmological observations. However, ΛCDM has many drawbacks. Observations supporting this are based on so-called standard candles, that is, objects of the same brightness, thanks to which it is possible to measure the distance to them (for example, type Ia supernovae in distant galaxies). However, calculating this distance is not always possible without using the cosmological model itself. A possible source of the distortion may be the central cosmological principles of ΛCDM: the assumptions of homogeneity and isotropy (no preferred direction in space).
Although cosmological principles are primarily supported by independent observations, their applicability is limited by the existence of large-scale structures in the universe that attract their surroundings. One such structure is the Laniakea supercluster, which is more than 520 million light-years in diameter and contains more than 100,000 galaxies. The local galaxy group, including the Milky Way, tends toward the center of Laniakea, inevitably distorting cosmological observations.
To assess Laniakea’s contribution to the potential distortions of the ΛCDM, the researchers measured the gravitational effects of the supercluster’s heterogeneity and non-sphericity. They discovered that without considering the expansion of the universe, the structure of the universe could be well described by an ellipsoid that compresses along two axes and expands along the third, longest axis. This makes it possible to correct the brightness of a known standard candle depending on the viewing angle. This led to an overestimation of distances by 2 to 3 percent, which proved to be a disadvantage for cosmologists. Scientists expected that by taking Laniakea’s influence into account, the value of the Hubble constant calculated from standard candles would approach the value calculated based on observations of the cosmic microwave background radiation. This contradiction is considered one of the major problems in cosmology and is known as the Hubble tension. But the new results only heighten this tension, pointing to the possible existence of new astrophysics. But scientists warn that Laniakea’s negative effect on Hubble voltage may have a less exotic explanation. The existence of large voids outside superclusters could affect scientists’ conclusions, and their contribution could potentially eliminate Hubble contamination. This should motivate astronomers to better map the local universe.