Is it Dark Energy? Or is Time Just Running at Varying Speeds? The Timescape Model

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New evidence challenges the existence of dark energy. But does the timescape model really solve the question surrounding the accelerated expansion of the universe?


This graph shows the timeline of the universe, including the accelerated expansion of the universe; Source: Nasa Lambda

We still don’t know what dark energy is. In fact, one theory in particular has gained recent traction that it doesn’t exist at all. And it all relies on one assumption: that the universe is not homogeneous.

On large enough scales, the universe is homogeneous and isotropic—in other words, that the universe essentially looks the same if you look far enough out. This is what the Lambda-CDM model tells us, also known as the standard model of Big Bang cosmology. 

It’s a good theory. Not only because it’s based on general relativity, it also explains why the cosmic microwave background (the first light of the universe) exists, how dark matter holds galaxy clusters together, and that dark energy accelerates the expansion of the universe. 

Yet, like dark matter, scientists have no definitive explanation for what dark energy is. Distance measurements, redshift, and type Ia supernovae (used as standard candles to measure cosmic distances) provide evidence for its existence—specifically that the expansion of the universe accelerates with time.

So, even though scientists have no idea what dark energy is, it must exist… right?


The Cosmic Microwave Background, the farthest we've been able to look back, just 380'000 years after the Big Bang; Source: Wikipedia

For every theory, there will be sceptics. The opposite of the Lambda-CDM model is inhomogeneous cosmology, which takes into account that the universe isn’t homogeneous. That is no speculation, it’s a fact—however, in the prevalent theory, it’s assumed that these inhomogeneities have no effect on large-scale structures like galaxy clusters and voids.

But inhomogeneous cosmology says that they do affect large-scale structures. Regions of higher density, namely galaxy clusters, curve spacetime more strongly than regions of lower density, voids, all according to general relativity. 

Enter the timescape model. This theory, first proposed by David Wiltshire in 2007, claims that dark energy isn’t required to explain the accelerated expansion of the universe. As you can imagine, this was a bold statement to make, given that scientists believe that dark energy makes up more than 70% of all the matter in the universe.

Instead of dark energy, the timescape model points to a far stranger force—time itself.

In regions packed with galaxies, time runs slower due to the curving of spacetime. And in voids, where galaxies only occasionally disturb the emptiness, time runs faster. The result is time not flowing equally in different regions of the universe. And therefore, the universe expands unevenly. 

Now scientists have taken a closer look at the Pantheon+ dataset, the biggest collection of type Ia supernovae. After comparing the Lambda-CDM model and the timescape model to the data, they found that the timescape model fits the collected data better. 

In an article by the University of Canterbury, Christchurch, New Zealand—where the timescape model came into existence—it is noted that in large voids time runs faster, and so much more time would have passed in comparison to us, observers in the Milky Way, making it appear that the universe is expanding at different rates. 

The theory certainly is an interesting one, especially given the evidence that now points to it, it has been argued that there are potential sources of error, such as peculiar velocities that can potentially affect supernovae measurements not being taken into consideration. 

Also, further research is needed to validate the claims. 

So, there’s still a window open for hope. And if the timescape model turns out to be true, then the mystery surrounding dark energy would finally be solved. And if not, scientists can continue to look for this elusive force that affects the literal fabric of the cosmos. 

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