In that grand cosmic theater, the Milky Way plays an animating role that is something of a defiance of traditional cosmological assumptions. It is in this rarefied location, this thing called the KBC void, that huge chasm in space stretching 2 billion light years across, the largest known void in this observable universe. The publication was supposed to do nothing less than revolutionize our understanding of the cosmos and the underlying predilections that determine it.
The concept of the KBC void was founded in the year 2013, and evidence of this concept has been coming up since then. According to the cosmological principle, matter in the universe must be homogeneous on large scales. This is the backbone of modern cosmology, as scientists can use elementary physical laws everywhere in the universe. The possible existence of the KBC void, however, may suggest that the thought of the homogeneity of matter is not true after all.
The University of St. Andrews postdoctoral research fellow Indranil Banik is at the vanguard of this research and has been quoted as saying, “By now it’s pretty clear that we are in a significant underdensity.” This void is big enough to hold 20,000 Milky Way galaxies in a row from one end to the other. Although not wholly empty, the KBC void is about 20 percent emptier than the space outside of its borders. This difference carries some profound implications for our universe.
One of the strangest implications of the KBC void is what it says about nearby stars and galaxies. They are all zooming away at us at speeds much faster than expected; this has been called the Hubble tension. The Hubble constant is a number that reveals how fast the universe is expanding, and it should be the same everywhere in the universe. The catch is that this observed expansion rate in our locality region is somewhat high. This points toward the fact that the universe could be expanding at a faster-than-expected rate. For Banik and his colleagues, it could be that the gravitational tug of high-density regions beyond the void has become strong enough to draw galaxies and stars toward them in a way that causes this anomaly.
The plausibility of such a local void hypothesis, which could explain this Hubble tension from very large outflows, is particularly given the observational evidence referenced in the paper according to Brian Keating, cosmologist and physics professor at UC San Diego. He also said that many questions remain open—for example, how large an area does the void encompass?
Alternative, more exotic theories of the same class include early dark energy, forming a hypothesized radiation that influenced the expansion rate of the universe in its earlier stages. To do exactly that, though, this one would necessitate massive modifications to our understanding— say, the age of the oldest stars.
The implications of the KBC void reach beyond just the Hubble tension. It’s these kinds of questions that motivate one of the powerful tests of different cosmological models and gravitational theories through the spatial arrangement of galaxies, be it on scales ranging from 100 kpcs to a Gpc. In this way, the large KBC void indicates a problem, as the standard model of cosmology, dubbed the Λ CDM model, prepares to generate much smaller root-mean-square density fluctuations compared to those observed. In a word, ΛCDM could fail to describe the large-scale structure of the universe.
On the other hand, it may be best understood via Milgromian dynamics, a model whereby gravity experiences an enhancement at long ranges. Supposing fast-moving collisionless matter, such as sterile neutrinos, allows this model to explain the observed structure of the Universe. The enhanced growth of structure in Milgromian gravitation generates much larger and deeper voids compared with Einsteinian/Newtonian gravity, leading to the formation of KBC-like voids.
The mystery of the KBC void and its implications for our understanding of the universe remains, quite literally, an open question as he continues his research. The Hubble tension acts more as a pointer toward the complications and wonders of our cosmos, which are lying in wait for future discoveries that can unravel them.