If the supernova standard candle is wrong, it could solve the Hubble tension

Last time I wrote about new data that shakes up the standard cosmological model. Before anyone starts dusting off their fringe cosmological models, we should note what this new study doesn’t overturn. It does not say that the Big Bang model is wrong, or that the universe is not expanding, or that Hubble’s redshift-distance relationship should be rejected.

This simply indicates that our constant Hubble model is wrong. But we already knew that thanks to a little thing known as the Hubble tension. These new results could also solve this mystery.

Before we dive into the Hubble voltage, let’s talk about the Hubble constant and the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. In 1929, thanks to the work of Henrietta Leavitt and others, Edwin Hubble managed to show that beyond the local group, the more distant a galaxy is, the greater its redshift.

He discovered 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 added a cosmological constant to general relativity to balance the gravity of galaxies. Like most astronomers of the time, Einstein assumed that the universe was in a stable state. Without this constant, a stable state was not possible.

With the discovery of Hubble, Einstein abandoned 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 the 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.

The Hubble constant H₀ and the cosmological constant Λ are related, but they are not exactly the same. The rate of cosmic expansion depends on several factors: the cosmological constant (dark energy), the quantity of dark matter and ordinary matter in the cosmos, as well as the distribution of this matter.

Simply put, matter tries to bring everything together, while dark energy tries to pull everything apart, and the balance between the two gives the cosmic expansion rate, or Hubble constant.

Naturally, since the early universe was denser than the current universe, one would expect the rate of cosmic expansion to increase somewhat over time. This is why the discovery of accelerated cosmic expansion was so important. This proved the existence of dark energy and the cosmological constant. This is also why the Hubble constant is often called the Hubble parameter these days.

For decades, observational evidence has supported the ΛCDM model. But over the past decade, our measurements of the Hubble parameter have become problematic.

There are several ways to find the Hubble parameter, but the big three are distant supernovae, the cosmic microwave background (CMB), and a pattern of galaxy clustering known as baryonic acoustic oscillation (BAO).

Observations of supernovae give us an expansion rate of around H₀ = 71–75 (km/s)/Mpc, while the scale of CMB fluctuations gives a value of H₀ = 67-68 (km/s)/Mpc. The BAO measurement gives a result of H₀ = 66-69 (km/s)/Mpc. This is what we call the Hubble tension. These results should agree, but they absolutely do not.

You might think this means the supernova measurements are wrong, but things aren’t so clear. All three methods rely on assumptions about patterns and hierarchies of evidence.

At first, astronomers thought that better data would bring the values ​​closer, but things only got worse. Even other methods using things like gravitational lenses or astronomical masers contradict each other. That’s why this new study is so interesting.

The work doesn’t do a comprehensive study of how their results would change various Hubble measurements, but it does look at the big three. When the age of the host galaxies is taken into account, the supernova measurement comes much closer to the other two.

The team even performed a first test of their results using host galaxies of roughly the same age, regardless of their redshift, and the results are slightly better. Accounting for galactic age in supernova data appears to resolve much of the Hubble tension.

The authors emphasize that their results are still somewhat provisional. There are only about 300 distant galaxies that have both an observed supernova and a spectrum from which you can determine the age of the host galaxy. This is a small sample, so while the results are compelling, they are not conclusive.

The good news is that when the Rubin Observatory comes online later this year, we will be able to determine the ages of thousands of distant galaxies. In a few years, we will know if this new model holds up. If this is the case, then we will have to abandon the cosmological constant as the sole source of dark energy.