The “Hubble tension,” a term used to describe the conflicting measurements of the universe’s expansion rate, has sparked debate in the field of cosmology. A recent theory suggests that a massive, less dense void in space could be responsible for these measurement discrepancies. This challenges the established understanding of matter distribution in the universe and implies that a revision of Einstein’s theory of gravity may be necessary.
A significant enigma in cosmology is the universe’s expansion rate, traditionally predicted by the Lambda-cold dark matter (ΛCDM) model. This model is based on observations of the cosmic microwave background (CMB), the residual light from the Big Bang.
As the universe expands, galaxies move away from each other, with their speed and distance relationship defined by Hubble’s constant. However, recent measurements using nearby galaxies and supernovas have produced a rate approximately 10% higher than predictions based on the CMB, leading to the aforementioned “Hubble tension.”
A new study proposes an intriguing explanation: our position within a colossal cosmic void with lower-than-average density. This void could influence local measurements by creating matter outflows, as denser regions surrounding the void exert a stronger gravitational pull.
This theory requires our location near the center of a void approximately a billion light years across, with a density about 20% lower than the universal average. This contradicts the standard model, which, based on CMB data, suggests a more uniform distribution of matter. Yet, direct galaxy counts in different regions do hint at a local void.
The study explores this hypothesis further by aligning various cosmological observations with the existence of this large void, originating from an early density fluctuation. This approach utilizes Modified Newtonian Dynamics (MOND) rather than ΛCDM, suggesting that Newton’s law of gravity may not apply under conditions of extremely weak gravitational pull, such as in galaxy outskirts.
In a MOND universe, the cosmic expansion history would mirror that of the standard model, but structures like galaxy clusters would develop more rapidly. This variation could explain why local measurements of the expansion rate differ based on location.
Recent galaxy observations have facilitated a new test of this model, examining the predicted velocities at different locations. These tests have revealed a “bulk flow” of galaxies, with speeds up to four times higher than those predicted by the standard model, further challenging its accuracy.
The study’s findings emerge as conventional solutions to the Hubble tension face scrutiny. Some argue for more precise measurements, while others suggest that the higher expansion rate measured locally is accurate, necessitating minor adjustments to early universe expansion to align with CMB observations. However, such a rapid expansion throughout cosmic history would conflict with the ages of the oldest stars.
The presence of a vast and deep local void, supported by galaxy counts and observed rapid bulk flows, indicates that structure develops faster than predicted by ΛCDM over extensive cosmic distances.
For instance, the El Gordo galaxy cluster, formed early in cosmic history, is too massive and its collision speed too high for compatibility with the standard model. This further suggests a slower-than-expected formation rate in the standard model.
Given that gravity is the predominant force on such large scales, an extension of Einstein’s General Relativity theory might be necessary, though measuring gravity’s behavior on these scales remains a challenge. This situation echoes Einstein’s view that new thinking is needed to address new problems, potentially signaling a need to revise our understanding of gravity.
Authored by Indranil Banik, a Postdoctoral Research Fellow in Astrophysics at the University of St Andrews, this summary is adapted from an article originally published in The Conversation.
Frequently Asked Questions (FAQs) about Hubble tension debate
What is the “Hubble tension” in cosmology?
The “Hubble tension” refers to the discrepancy in measurements of the universe’s expansion rate. Traditional methods based on the cosmic microwave background (CMB) yield a lower rate compared to measurements using nearby galaxies and supernovas. This conflict challenges the standard Lambda-cold dark matter (ΛCDM) model and raises questions about our understanding of the universe’s expansion.
How does the theory of a giant void contribute to the Hubble tension debate?
The theory suggests that a massive, underdense void in space could explain the discrepancies in the universe’s expansion rate measurements. This void, with its lower density, could affect local measurements by creating outflows of matter, challenging conventional models and potentially requiring a revision of Einstein’s gravitational theory.
What is Modified Newtonian Dynamics (MOND) and how does it relate to this theory?
Modified Newtonian Dynamics (MOND) is an alternative theory to the standard ΛCDM model. It proposes that Newton’s law of gravity breaks down under conditions of extremely weak gravitational pull, such as those found in the outer regions of galaxies. This theory was applied in the study to align various cosmological observations with the existence of a large void, suggesting a different expansion history in the MOND universe compared to the standard model.
How do recent galaxy observations support this new theory?
Recent observations have revealed a phenomenon called “bulk flow,” where the average velocity of galaxies in a given sphere is significantly higher than expected in the standard model. This discrepancy supports the theory of a large cosmic void, as it indicates faster-than-expected structure growth, challenging the accuracy of the standard cosmological model.
Why is the existence of the El Gordo galaxy cluster significant in this context?
The El Gordo galaxy cluster, formed early in cosmic history, has a mass and collision speed too high to be compatible with the standard model. This suggests that structures like galaxy clusters form faster than predicted by the standard model, providing additional evidence for the need to revise our understanding of cosmic expansion and possibly gravitational theory.
More about Hubble tension debate
- Understanding the Hubble Tension
- The Mystery of Universe Expansion
- Exploring Einstein’s Gravitational Theory
- Insights into Cosmic Microwave Background (CMB)
- The Role of Modified Newtonian Dynamics (MOND)
- Analyzing the Giant Void Theory
- El Gordo Galaxy Cluster: A Case Study
- Revising Cosmological Models