Last time I wrote about new data that turns the standard cosmological model on its head. Before anyone starts dusting off their fringe cosmological models, let’s note what this new study doesn’t upend. It does not say that the Big Bang model is wrong, nor does it say that the universe is not expanding, or that Hubble’s redshift-distance relationship must be discarded. It really just says that our Hubble constant model is wrong. But we already knew that thanks to a little thing known as Hubble tension. These new results could also solve this mystery.
Before we get into the Hubble voltage, let’s talk about the Hubble constant and the Friedmann-Lemaître-Robertson-Walker metric (FLRW). Thanks to the work of Henrietta Leavitt and others, Edwin Hubble was able to show in 1929 that – beyond the local group – the further away a galaxy is, the greater its redshift. He found that the relationship between distance and redshift was linear, leading him to propose a cosmological constant now known as the Hubble constant.
In 1917, Einstein had added a cosmological constant to the general theory of relativity to balance the gravity of galaxies. Like most astronomers of his time, Einstein assumed that the universe was in a stable state. Without the constant, a stable state would not be possible. With Hubble’s discovery, Einstein rejected the idea, but Alexander Friedmann and Georges Lemaître independently discovered that solutions to Einstein’s equations with a cosmological constant could describe an expanding universe beginning with a Big Bang. In 1935, Howard Robertson and Arthur Walker proved that the FLRW metric is the only solution to GR that describes a uniformly expanding universe. This is the metric used in the Standard Model. Since the FLRW metric uses Λ as a symbol for the cosmological constant, it is the ΛCDM model.
Graphic showing how the fate of the universe depends on dark energy, dark matter and matter. Photo credit: NASA and A. Feild (STScI)
The Hubble constant H0 and the cosmological constant Λ are related, but are not exactly the same. The rate of cosmic expansion depends on several things: the cosmological constant (dark energy), the amount of dark matter and regular matter in the cosmos, and the distribution of that matter. To put it simply, matter tries to bring everything together while dark energy tries to push everything apart, and the balance between the two gives the rate of cosmic expansion, the Hubble Constant. Since the early universe was denser than today’s universe, one would naturally expect the rate of cosmic expansion to increase somewhat over time. This is why the discovery of accelerated cosmic expansion was such a big deal. It proved the existence of dark energy and the cosmological constant. For this reason, the Hubble constant is now often referred to as the Hubble parameter.
*Over time, our measured Hubble values began to diverge. Photo credit: Wendy Freedman*
For decades, observational results supported the ΛCDM model. But over the last decade or so, our measurements of the Hubble parameter became problematic. There are several ways to determine the Hubble parameter, but the big three are distant supernovae, the cosmic microwave background (CMB), and a pattern in galaxy clusters known as the Baryon Accoustic Oscillation (BAO). The supernova observations give an expansion rate of about H0 = 71 – 75 (km/s)/Mpc, while the magnitude of fluctuations in the CMB give a value of H0 = 67 – 68 (km/s)/Mpc. The BAO measurement gives a result of H0 = 66 – 69 (km/s)/Mpc. We call this the Hubble voltage. These results should agree, but that is absolutely not the case.
Now you might think that this means that the supernova measurements are wrong, but things are not that clear. All three of these methods rely on assumptions about models and evidence hierarchies. Early on, astronomers assumed that better data would bring the values together, but they only got worse. Even other methods that use things like gravitational lensing or astronomical masers contradict each other. That’s why this new study is so interesting.
*Old methods disagreed, but this new result brings things together. Photo credit: Son et al.*
The paper doesn’t give a complete overview of how its results would change various Hubble measurements, but it does address the big three. When the age of the host galaxies is taken into account, the supernova measure shifts significantly closer to the other two. The team even conducted an initial test of their results using host galaxies of roughly the same age, regardless of their redshift, and the results were slightly better. Accounting for galactic age in supernova data appears to resolve much of the Hubble tension.
The authors note that their results are still somewhat preliminary. There are only about 300 distant galaxies that have both an observed supernova and a spectrum from which the age of the parent galaxy can be determined. Because this is a small sample size, the results are compelling but not conclusive. The good news is that when the Rubin Observatory comes online later this year, we will be able to determine the age of thousands of distant galaxies. In a few years we will know whether this new model holds up. If so, we must reject the cosmological constant as the only source of dark energy.
So what then? What alternative is there if ΛCDM is wrong? I’ll talk about it next time.
Reference: Son, Junhyuk et al. “Strong progenitor age bias in supernova cosmology – II. Agreement with DESI BAO and signs of a non-accelerating universe.” *Monthly Communications of the Royal Astronomical Society* 544.1 (2025): 975-987.
Reference: Hubble, Edwin. “A distance-radial velocity relationship between extragalactic nebulae.” *Proceedings of the National Academy of Sciences* 15.3 (1929): 168-173.
Reference: Robertson, Howard Percy. “Kinematics and World Structure.” *Astrophysical Journal*, vol. 82, p. 284 82 (1935): 284.
Reference: Peebles, P. James E. and Bharat Ratra. “The cosmological constant and dark energy.” *Reviews of Modern Physics* 75.2 (2003): 559.
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