Almost a century after the discovery that the universe is expanding, astronomers remain divided over one fundamental question: how fast is it expanding? Known as the Hubble constant (H₀), this measurement has sparked a heated debate within the scientific community. Despite remarkable improvements in observational techniques, two independent methods for calculating the expansion rate yield conflicting results—a discrepancy now termed the "H₀ tension."
This growing debate centers on two methods: the "distance ladder" approach and measurements from the cosmic microwave background (CMB). Each method should, in theory, lead to the same value for H₀. However, their results diverge significantly, prompting some to label the issue a "crisis for cosmology" that could require a fundamental shift in our understanding of the universe.
Origins of the Debate
The Hubble constant defines the relationship between the distance to galaxies and how fast they are moving away from us, a phenomenon observed in 1929 by Edwin Hubble. Using redshift—the stretching of light caused by the universe’s expansion—Hubble established that galaxies farther away are receding at faster speeds, leading to the discovery that the universe is expanding.
However, modern attempts to precisely measure H₀ have yielded two distinct values. The SH0ES scientific team, led by Nobel laureate Adam Riess, measures H₀ using the distance ladder method, which observes galaxies’ redshifts and correlates them with distance measurements. Their value for the Hubble constant is 73.2 kilometers per second per megaparsec.
On the other hand, the CMB method, which measures radiation left over from the Big Bang, predicts an expansion rate of 67.4 kilometers per second per megaparsec. This measurement, derived from data collected by the European Space Agency’s Planck satellite, reflects the universe’s conditions when it was just a few hundred thousand years old. The 10% difference between these two values is large enough that it surpasses statistical error and has confounded scientists for years.
Distance Ladder vs. Cosmic Microwave Background
The distance ladder method relies on direct distance measurements of nearby celestial objects, using parallax, a technique that calculates how an object’s position shifts relative to background stars as the Earth orbits the Sun. Astronomers then calibrate measurements of more distant objects, like pulsating Cepheid stars and Type 1a supernovae, creating a "ladder" of increasingly distant objects whose speeds can be measured via redshift.
In contrast, the CMB method uses light emitted from the early universe, long before stars and galaxies formed, to calculate the expansion rate at that time. While the physics of this period are well understood, the method requires scientists to model the universe’s evolution over billions of years to extrapolate H₀.
Both methods have proven remarkably precise within themselves, yet their stark disagreement on the value of H₀ has led to intense scrutiny of each technique. Despite exhaustive analyses, neither team has uncovered errors that could account for the discrepancy.
New Physics or Measurement Error?
One possible explanation for the H₀ tension is that the standard cosmological model—our understanding of the universe’s evolution since the Big Bang—may be incomplete. Some theorists propose that new physics, such as an early phase of rapid expansion shortly after the Big Bang, could explain the mismatch between the two methods. However, any new theory must also align with other well-established observations, such as the detailed patterns observed in the CMB, which makes this solution highly complex.
Other proposed ideas include magnetic fields affecting the formation of atoms in the early universe or the possibility that Earth resides in a region of space that has expanded unusually fast.
Another, more prosaic possibility is that one of the measurements may have overlooked a subtle observational effect. This has led to ongoing investigations of both the distance ladder and CMB data. Yet, so far, no significant errors have been found.
New Methods and Technology
In an effort to break the impasse, astronomers are exploring alternative techniques. One such approach, led by Wendy Freedman, employs a category of stars known as the "tip of the red giant branch" (TRGB) to measure distances. These stars provide a more stable rung in the distance ladder than Cepheids and have yielded a H₀ value of 69.8—somewhere between the SH0ES and CMB estimates.
Additionally, new observations from the James Webb Space Telescope (JWST) have revealed a discrepancy between galaxy distances measured by Cepheids and TRGB stars. If confirmed by future analyses, this could cast doubt on the reliability of the distance ladder approach, further complicating the situation.
The search for answers continues as astronomers plan to leverage new technologies and methods, including gravitational wave measurements from merging black holes, to refine H₀ estimates. Whether these efforts will resolve the Hubble tension—or deepen the mystery—remains to be seen.
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