Astronomers say we may live at the center of a cosmic void 2 billion light-years wide that defies the laws of cosmology. A growing list of observations suggests we live in the crosshairs of a giant cosmic void — the largest ever observed. They first suggested such a void in 2013 and evidence for its existence has been stacking up ever since. We may live in a void that shouldn’t exist.
According to a fundamental theory of cosmology named the cosmological principle, matter in the universe should be uniformly distributed on very large scales. This idea is important because it allows scientists to use the same laws of physics for objects near us and those far away, even at the edges of the early universe. Basically, everything follows the same universal rules.
This approach helps us study and understand the universe better and suggests that large empty spaces, like the one we might live in, shouldn’t really exist. However, many observations in the last ten years indicate that matter in the universe may actually gather in areas of high and low density, which means it might not be as evenly distributed as we thought. (Source)
“By now it’s pretty clear that we are in a significant underdensity,” said Indranil Banik, a postdoctoral research fellow at the University of St. Andrews. “There’s a few people that are still opposed to it to a limited extent. For example, some people have correctly argued that such a void shouldn’t exist in the standard model, which is true. That unfortunately doesn’t prove it’s not there,” he added.
Indranil Banik is a cosmologist studying the Hubble tension, which is the observation that the Universe is expanding faster locally than expected based on early Universe observations. He thinks this faster expansion may be caused by a local void or underdensity, creating a “Hubble bubble” where the expansion rate appears higher due to the outflow from the void.
Since late 2020, he has published several papers on this topic. His main evidence suggests that the Universe is older than it would be if the local expansion rate were accurate, aligning with early Universe methods. This implies that the observed redshift gradient with distance is enhanced compared to a uniformly expanding universe, likely due to outward peculiar velocities, indicating we might be in a giant void that’s about 20% less dense than the cosmic average. Watch his work here.
The KBC void defies the laws of cosmology
Banik co-authored a paper published late last year in the peer-reviewed journal Monthly Notices of the Royal Astronomical Society that suggests we may live near the center of this void — named the KBC void — that’s about 2 billion light-years wide. Wide enough to fit 20,000 Milky Way galaxies in a row stretching from one end to the other.
The paper suggests that the Λ cold dark matter (ΛCDM) model of the universe is having serious issues when compared to various observations. The biggest problem is the Hubble tension, which has a confidence level exceeding 5σ (a very high level of statistical significance). Galaxy counts reveal the Keenan–Barger–Cowie (KBC) supervoid, a large area with fewer galaxies out to 300 megaparsecs (Mpc), which doesn’t fit well with the ΛCDM model.
Previous work by Haslbauer et al. showed that a higher local Hubble constant can naturally occur due to gravitational forces pulling material away from the KBC supervoid. Their model predicts that the speeds of galaxies should be much higher than what the ΛCDM model suggests. This aligns with a recent finding by Watkins et al., which discovered that galaxies in the CosmicFlows-4 catalogue are moving significantly faster than expected on scales of Mpc.
The unexpected increase in galaxy speeds creates a 4.8σ tension at certain distances. In this study, we explore what Haslbauer et al.’s model predicts for galaxy speeds on these scales. We find that it matches the observations well, especially if we choose our viewpoint to align with the observed galaxy speeds on a scale of Mpc. This shows that the previous model, which wasn’t based on galaxy speed measurements, could still solve the Hubble tension and explain the KBC void in a consistent way with the observed movement of our Local Group of galaxies. Our results suggest that many cosmological issues could be solved if structures in the universe grow more efficiently than what the ΛCDM model predicts on larger scales.
The KBC void isn’t totally empty. It can’t be, because we live in it. But, if Banik and his colleagues’ calculations are correct, the void would be about 20% emptier than space outside its border.That may not seem like a big deficit, but it’s enough to cause some confusing behavior in our local cosmic neighborhood, according to the recent study.
Nearby stars and galaxies are moving away from us faster than expected. Cosmologists use a value called the Hubble constant to measure how quickly the universe is expanding. This constant should be the same everywhere, but local galaxies and stars seem to be moving away faster than it suggests. This challenges our understanding of how the universe expands.
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Astronomers disagree on the cause of the difference in the Hubble constant, which is called the Hubble tension. Banik and his team think a void in space might be the answer. They suggest that areas with stronger gravity outside the void pull galaxies and stars toward them. This pull could explain why nearby objects show a higher Hubble constant. In the void, things move faster, heading out toward denser regions of space.
Mystery solved? Not yet.
If a void in the cosmos exists, it might change how we understand physical laws. Banik’s theory suggests this void could explain why the Hubble constant is higher in our area of space.
Brian Keating, a cosmologist, thinks the idea is reasonable but raises questions about how far the void’s effects reach. If the void is not typical of the whole universe, it may not fully explain the Hubble tension. Keating also points out that Banik’s results depend on the void model used, which affects predictions about how galaxies move. Therefore, while the void might offer a potential solution, it isn’t definitive yet.
There are other theories, like early dark energy, which could also explain the Hubble tension. However, Banik believes this theory contradicts some facts about the universe, such as the ages of ancient stars.
Banik plans to study supernovae data next to see if the Hubble constant aligns with standard cosmology beyond the void. If his theory is right, there should be no Hubble tension outside the void. For now, the Hubble tension remains unsolved.
