Imagine the entire universe is a balloon, constantly expanding. Now, imagine we’re trying to measure just how fast that balloon is inflating. Sounds simple, right? Wrong! If our cosmic yardstick – the supernova ‘standard candle’ – is off, it could completely rewrite our understanding of the universe’s expansion and potentially solve a major cosmic puzzle known as the Hubble tension.
Last time, we discussed some surprising data suggesting the standard cosmological model might need a serious rethink. But before you jump to conclusions about the Big Bang being wrong, let’s be clear: this new study doesn’t throw out the Big Bang, the expanding universe, or Hubble’s redshift-distance relation. What it does challenge is our model for the Hubble constant, a discrepancy we already knew existed thanks to the Hubble tension. These new findings might just be the key to resolving it.
So, what’s this Hubble tension all about? To understand it, we need to talk about the Hubble constant and the Friedmann–Lemaître–Robertson–Walker (FLRW) metric. Back in 1929, Edwin Hubble, building on the work of Henrietta Leavitt and others, discovered that galaxies farther away from us have a greater redshift – they’re moving away faster. This relationship between distance and redshift is linear, leading Hubble to propose a cosmological constant, now known as the Hubble constant. Think of it as the universe’s expansion rate at the present time.
Now, here’s a bit of historical context: In 1917, Einstein, trying to make his theory of general relativity fit with the prevailing belief of a static universe, added a cosmological constant. He needed something to counterbalance gravity and prevent the universe from collapsing. When Hubble discovered the universe was expanding, Einstein famously called it his “biggest blunder.” However, Alexander Friedmann and Georges Lemaître independently showed that Einstein’s equations, even with the cosmological constant, could describe an expanding universe that began with a Big Bang.
And this is the part most people miss… In 1935, Howard Robertson and Arthur Walker proved that the FLRW metric is the only solution to general relativity that describes a uniform, expanding universe. This FLRW metric is the foundation of the standard cosmological model, often called the ΛCDM model because it uses Λ (Lambda) to represent the cosmological constant.
The Hubble constant (H0) and the cosmological constant (Λ) are related, but they’re not the same thing. The rate of cosmic expansion depends on several factors: dark energy (represented by the cosmological constant), the amount of dark matter and regular matter, and how that matter is distributed. Imagine a tug-of-war. Matter tries to pull everything together through gravity, while dark energy pushes everything apart. The balance between these two forces determines the expansion rate, or the Hubble constant.
Because the early universe was denser, we’d expect the expansion rate to have changed over time. That’s why the discovery of accelerating cosmic expansion was such a big deal – it provided strong evidence for dark energy and the cosmological constant. This also explains why the Hubble constant is often called the Hubble parameter – it can change over cosmic time.
For decades, the ΛCDM model was supported by observation after observation. But in the last decade or so, our measurements of the Hubble parameter have become, shall we say, problematic… This is where the Hubble tension really comes into play.
We have several ways to measure the Hubble parameter, but the three main methods are: 1) observing distant supernovae, 2) studying the cosmic microwave background (CMB), the afterglow of the Big Bang, and 3) analyzing Baryon Acoustic Oscillations (BAO), a pattern in the distribution of galaxies. Supernova observations give us an expansion rate of about H0 = 71–75 (km/s)/Mpc, while the CMB gives a value of H0 = 67–68 (km/s)/Mpc, and BAO comes in at H0 = 66–69 (km/s)/Mpc. These results should agree, but they don’t! This discrepancy is the Hubble tension.
Now you might think, “Okay, the supernova measurements must be wrong!” But here’s where it gets controversial… all three methods rely on assumptions about models and evidence hierarchies. It’s not as simple as one measurement being inherently better than the others. Early on, astronomers hoped better data would resolve the differences, but the situation has only gotten worse. Even other methods, like gravitational lensing and astronomical masers, give conflicting results. This is what makes this new study so interesting.
This new research doesn’t provide a complete overhaul of all Hubble measurements, but it does focus on the big three. The key finding? When the ages of the galaxies hosting the supernovae are taken into account, the supernova measurements shift much closer to the CMB and BAO values. The team even performed a preliminary test using host galaxies of similar ages, regardless of their redshift, and the results improved further. Accounting for galactic age in supernova data seems to significantly alleviate the Hubble tension!
But the authors themselves emphasize that their results are still preliminary. They only have data for about 300 distant galaxies that have both an observed supernova and a spectrum from which they can determine the host galaxy’s age. That’s a relatively small sample size. While the results are promising, they’re not conclusive.
However, there’s good news on the horizon! When the Vera C. Rubin Observatory comes online, we’ll be able to determine the ages of thousands of distant galaxies. Within a few years, we’ll know whether this new model holds up. If it does, we might have to rethink our understanding of dark energy and potentially abandon the cosmological constant as its sole source. This could mean dark energy is something more dynamic, something that changes over time. Think of it like this: instead of a constant push on the expanding universe, it might be more like a fluctuating wave.
What do you think? Is the Hubble tension a sign that our standard cosmological model needs a fundamental revision, or is it just a matter of refining our measurement techniques? And if the cosmological constant isn’t the whole story of dark energy, what else could it be? Share your thoughts in the comments below!