Before the Big Bang theory — an explanatory model for how the universe began — there was the belief that the universe had been in the same state since the beginning of time. Thanks to further research, we now know that the universe is expanding and that — according to a 2026 study from Astronomy & Astrophysics — it’s happening much faster than physicists previously thought. 

Evolving theories

Without a set ‘beginning’ to the universe, one would assume that it has existed indefinitely. One such concept is the Steady State theory, which proposes that creating cosmic matter — which consists of all physical substances in the universe — is an ongoing process. In essence, new matter is formed to replace matter that is lost, such that the universe maintains a constant density.

However, there are quite a few problems with this model. Notably, it suggests that the universe has no beginning or end and will not evolve. This theory was falsified, in part due to the discovery of quasars — luminous galactic centres at very distant ends of the universe — billions of light years away, meaning that the light they emit is billions of years old. The fact that quasars are only found in the early universe, where the first galaxies and stars created reside, provides strong evidence that the universe has changed over time.

After the Steady State theory was disproven, scientists adopted the Big Bang theory, which posits that the universe has a definite beginning and continues to evolve. But how did scientists come to this conclusion? 

Raisin bread and Hubble’s flying galaxies

By observing the light spectra emitted by distant galaxies, astronomer Edwin Hubble discovered in 1929 that galaxies farther away from the Milky Way appear to move faster. Hubble’s observation provided evidence for the Big Bang theory and the idea of an expanding universe. 

To further conceptualize this idea, imagine baking a loaf of raisin bread. As it bakes, the loaf expands; the raisins stay in their place while the bread, the space between them, moves. From the perspective of each raisin within the bread, all of the surrounding raisins appear to move farther away. 

Now, replace the raisins with individual galaxies, and the bread with spacetime. On Earth, we observe other galaxies moving away from us, when in reality, all galaxies are moving relative to each other. As galaxies move farther, they appear to move away from us faster.  

Hubble’s observation revealed that the rate at which our universe is expanding is accelerating. As more time passes, the universe expands even faster, at a rate of change known as Hubble’s constant (H0).

The Hubble tension problem

We can predict H0 by measuring temperature fluctuations in the cosmic microwave background — the light located at the edge of the observable universe from 380,000 years after the Big Bang. Alternatively, we can calculate it by measuring the distance between us and a stellar object, such as a star. However, these methods predict differing values of H0. 

This dilemma, known as the Hubble tension, has remained unsolved for over a decade. In an email to The Varsity, U of T Associate Professor Michael Reid explains that measuring H0 is a meticulous process, no matter the method, and our understanding of the universe shapes the resulting value.

How fast is the universe expanding? 

A 2026 study published in Astronomy & Astrophysics set out to rectify the calculation of H0 using a novel approach. Rather than using a single distance method, the authors combined numerous distance techniques to make a large network. 

Ultimately, the researchers’ calculations yielded an H0 value that was significantly higher than what was predicted. This suggests that the universe is expanding faster than other models predict.

Scientists’ confusion stems from the significant discrepancy between values. As of now, there is no explanation as to why these methods differ in their prediction. Assuming that the latest value of H0 is correct, the results of the study pose a new problem: what is causing this rapid expansion? 

Just another physics mystery?

The unresolved tension could mean that our current understanding of the universe is flawed, or that both measurements yield incorrect values. However, according to Reid, it is less likely to be a problem with the methodology. Numerous studies over the last decade have sought to identify problems with the measurements, yet the Hubble tension has persisted.

For the time being, the exact rate at which the universe is expanding remains unknown. While the Hubble tension may complicate our existing understanding of spacetime expansion, discoveries like this allow scientists to revise existing knowledge.  

As Reid explains, there is something missing in our fundamental knowledge of the nature of our universe. It is possible that the theory behind the rate of expansion is wrong in the first place, which may be why our predictions don’t align. Until we explore further and uncover more truths of our universe, the Hubble tension remains a physics mystery.