Resolving the Hubble Tension: The Case for a 'Hubble Bubble'
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The Expanding Universe Dilemma
For many years, astronomers have grappled with a puzzling issue: the conflicting measurements of the Universe's expansion rate. Recent studies imply that the Milky Way may reside within a low-density bubble, potentially eliminating the necessity for new physics.
Since the early 1900s, scientists have recognized that the Universe is expanding. By the late 20th century, it was established that this expansion is accelerating. However, many questions persist, particularly regarding 'dark energy'—an undefined force responsible for this acceleration. This leaves scientists facing even more fundamental challenges.
Researchers have developed two distinct methods to gauge the rate of Universal expansion. Unfortunately, these methodologies yield conflicting results. At first glance, this might seem like a straightforward issue to resolve; one of the methods must be incorrect.
This assumption, however, is misleading.
As scientists have refined their techniques, the precision of their results has improved, yet the disagreement remains stark. This ongoing conflict is termed the 'Hubble tension.' How can both measurements be accurate yet fail to converge on a consistent value?
Section 1.1: Local vs. Global Measurements
The discrepancy might stem from the fact that one approach focuses on local space while the other examines the Universe on a grand scale. The local method measures redshifts—the changes in light frequency as cosmic objects recede from us—using light signatures from Type Ia supernovae and distant galaxies.
Conversely, the global method relies on the Cosmic Microwave Background (CMB)—the relic radiation from an event shortly after the Big Bang—as a measurement tool.
What if the Universe in our vicinity differs from the larger Universe, contributing to this discrepancy?
This concept is explored further by Lucas Lombriser, a professor at the University of Geneva’s Department of Theoretical Physics. In a paper published in Physics Letters B, Lombriser proposes that the Milky Way and its neighboring galaxies exist within a 'Hubble bubble'—a region of low density.
“The Hubble tension is one of the most pressing issues in modern cosmology,” Lombriser explains. “Measurements from the two methods have been in conflict for nearly a decade, but the uncertainties were previously large enough that they didn’t necessarily indicate a problem. Recently, however, these uncertainties have diminished while the discrepancies remain.”
The idea that local and global densities can vary might seem contrary to the cosmological principle—asserting uniformity throughout the Universe. Yet, Lombriser argues that this principle is only applicable on a large scale and does not necessarily apply locally.
Section 1.2: Understanding Local Inhomogeneities
“We understand that the Universe nearby is highly irregular,” Lombriser states. “The densities of particles on Earth, in the atmosphere, and in the space between Earth and the Moon or Sun vary significantly.”
These variations extend from within the Milky Way to distant regions outside it. When astronomers observe the CMB, they note an almost perfectly uniform temperature of 2.7 K across the Universe. “However, tiny fluctuations—1 part in 100,000—do exist,” Lombriser adds. “These fluctuations are remnants from the early Universe, when it was only about 400,000 years old, and have developed over time into larger structures, leading to clusters of matter, stars, galaxies, and planets.”
These inhomogeneities expand due to gravitational forces, causing smaller clumps of matter to merge into larger ones. This indicates that while the Universe appears 'clumpy' at short distances, it remains relatively homogeneous over vast distances.
Chapter 2: The Hubble Bubble Concept
Lombriser theorizes that the Milky Way and its neighboring galaxies are situated within a low-density bubble spanning approximately 250 million light-years in diameter. This size is crucial for reconciling the differing expansion rate measurements.
“The notion of a Hubble Bubble isn’t new, and we expect variations in local density around the cosmic average,” Lombriser clarifies. “Previous studies suggested a bubble diameter of up to 4 billion light-years to account for all the supernovae in the dataset. However, it is unlikely that such a large bubble would significantly deviate from the overall average density of the Universe.”
This expansive bubble size was proposed to encompass the supernovae used for local expansion rate measurements. Lombriser's findings indicate that a smaller low-density bubble suffices.
According to Lombriser, "The bubble doesn’t need to be vast to explain the differences in measured expansions. Supernovae provide relative distances, but an absolute distance is necessary to convert this into a measurement of the Hubble constant."
If the Milky Way and Messier 106—used to establish the absolute distances—are indeed in the same low-density bubble, it could clarify the inconsistencies in local and global expansion rate measurements.
“If both galaxies are within the same bubble, we may miscalculate the distance used to estimate the average expansion of the Universe,” Lombriser points out. “This distance needs to be recalibrated for an average cosmological density before it can be utilized to infer the overall expansion rate.”
Despite this, the proposed bubble is still ten times larger in diameter than what is necessary to encapsulate these two galaxies, which allows for the inclusion of Cepheid stars—key variable stars used in distance measurements.
Lombriser indicates that for the Hubble Bubble to account for the disparities in measurements, its density must be 50% lower than that of the surrounding Universe. “Standard cosmology suggests that such under-densities are not uncommon for a conservatively sized bubble of 250 million light-years in diameter,” he states.
“Living in a bubble of this size with the required under-density is still plausible, with chances estimated between 1 in 20 and 1 in 5.”
Bursting the Bubble? Validating the 'Hubble Bubble'
Lombriser's theory could potentially be tested using gravitational waves as 'standard sirens' or by counting the number of nearby galaxies and comparing them with the number densities of galaxy clusters outside the bubble.
However, the host galaxy of the only gravitational wave source currently observed electromagnetically—event GW170817—falls within the proposed 250 million light-year diameter of the bubble, complicating its utility as a reference.
“GW170817 was detected through both gravitational waves and light,” Lombriser elaborates. “This dual observation allows us to determine both the distance to the event and its redshift, making it a 'Standard Siren' for measuring the expansion rate of the Universe. Since the emitter galaxy NGC 4993 lies within our local bubble, we would expect its expansion rate to align with local measurements rather than global ones. Conversely, measurements taken outside this bubble should match the global expansion rate.”
“The impact of gravitational wave measurements and their potential for enhancing precision in this area is something I plan to investigate further.”
Lombriser’s research holds significant implications, solving what he describes as “one of the most exciting problems in modern cosmology” without necessitating changes to current cosmological models or the introduction of new physics.
“While new physics would certainly be an intriguing solution to the Hubble tension,” Lombriser concludes, “if conventional physics can explain this tension, it offers a simpler explanation and underscores the effectiveness of established theories, albeit in a less thrilling way.”
Special thanks to Lucas Lombriser
Original research: Lombriser. L, ‘Consistency of the local Hubble constant with the cosmic microwave background,’ Physics Letters B, (2020).
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