Scientists at Trent University and the Trent Institute for Fundamental Physics have discovered a serious problem for current ideas about the expansion of the Universe. It turned out that taking into account the influence of the Laniakea supercluster, which includes the Milky Way, the values of the Hubble constant obtained by various methods diverge even more from each other. According to the findings of physicists, published as a preprint on the arXiv repository, it may hint at the existence of new physics.
About 20 years ago, astronomers discovered that the Universe is expanding at an accelerating rate. In the framework of general relativity and the cosmological model that satisfies its equations, called the Friedmann Universe, such acceleration requires an exotic source, now called dark energy. Currently, the generally accepted cosmological model ΛCDM, which describes a flat Universe with dark energy, in which most of the matter is represented by cold dark matter. This model is successful in that it explains many cosmological observations.
However, ΛCDM has a number of disadvantages. Observations confirming it rely on so-called standard candles – objects with the same luminosity, thanks to which the distance to them can be measured (for example, type Ia supernovae in distant galaxies). However, calculating this distance is not always possible without using the cosmological model itself. A potential source of distortion could be the key cosmological principle for ΛCDM—the assumption of homogeneity and isotropy (the absence of a preferred direction in space).
Although the cosmological principle is largely supported by independent observations, the existence of large-scale structures in the Universe that attract their surroundings limits its applicability. One such structure is the Laniakea supercluster with a diameter of more than 520 million light years, containing more than 100 thousand galaxies. The local group, including the Milky Way, tends to the center of Laniakea, which inevitably distorts cosmological observations.
To assess Laniakea’s contribution to the possible distortion of ΛCDM, the researchers measured the supercluster’s gravitational influence, given its heterogeneity and non-spherical shape. They found that without taking into account the expansion of the Universe, its structure is well described by an ellipsoid, compressing along two axes and expanding along the third, longest axis. This made it possible to make corrections to the luminosity of the known standard candles depending on the viewing angle.
It turned out that this leads to an overestimation of distances by 2-3 percent, but for the worse for cosmologists. Scientists expected that taking into account the influence of Laniakea would bring the value of the Hubble constant calculated from standard candles closer to the value calculated based on observations of the cosmic microwave background radiation. This discrepancy is considered one of the main problems of cosmology and is known as the Hubble tension. However, the new result only increases this tension, indicating the potential existence of new cosmological physics.
However, scientists warn that Laniakea’s negative effect on the Hubble voltage may have a less exotic explanation. Large voids outside the supercluster, if they exist, can influence scientists’ conclusions, and accounting for their contribution can reduce the Hubble stress to zero. This should motivate astronomers to better map the local Universe.
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