Bjørn Ekeberg
| Philosopher of science, author of Metaphysical Experiments
The idea that the universe started with a Big Bang is a key tenet of the standard model of cosmology. But that model is a lot less scientific than it’s taken to be. To begin with, we can never have direct evidence of the Big Bang itself, and so if we are to accept it, it must be as a metaphysical, not a scientific hypothesis. Furthermore, the standard model of cosmology has had to adapt to a number of observational discrepancies, postulating entities like dark matter and dark energy for which there is no direct evidence. To add to the above, another central assumption, the cosmological principle, stating that the laws of the universe are the same everywhere, is also under scrutiny. The universe might turn out to be a lot stranger than we think, or could possibly imagine, argues Bjørn Ekeberg.
We usually talk about the Standard Model of particle physics, but you also talk of a standard model of cosmology, and how it’s also on shaky foundations. What do you see as the key tenets of cosmology’s standard model?
The standard model of cosmology (sometimes called the Concordance or Lambda-CDM model) resembles a game of Jenga, with building blocks stacked on top of each other. At the bottom is an evolutionary model of General Relativity in which space expands with time, based on the hypothesis that the universe originated in a 'Big Bang' - along with a few simplifying assumptions. Two key interpretations were hailed as evidence for this theory: the redshift of galaxies was taken to mean that space is expanding, and the so-called cosmic microwave background was interpreted as residue from the Big Bang. With these core building blocks, this theoretical framework became the first established 'scientific cosmology' in the 1960s. The core tenets of standard cosmology are that the universe has a finite origin in time - and more fundamentally, that the mathematical laws deduced from our own galaxy apply universally, that we can know the entire universe with certainty.
It's a very bold project painting a grand picture that many observations contradict. And so new blocks were added on top to account for discrepancies between theory and observations. Dark matter was invented as an explanation for why our observations of galaxies didn't fit our mathematical model. Inflation was invented as an explanation for how the universe possibly could have gone from nothing to the kind of structure we observe today. Dark Energy was invented to account for ostensible acceleration of space, and so on. Today there are challenges to some of these upper blocks but very few scientists want to question the blocks below. In my understanding, this reluctance is not based on these blocks being proven beyond doubt but because if they were to be in doubt, the entire Jenga tower of standard cosmology would fall apart.
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Dark matter was invented in response to the problem that observations of galaxies didn't match theoretical predictions. But after 50 years of research there is still no direct evidence of it.
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What’s your main reservation when it comes to the existence of dark matter and the belief of many physicists that it makes up 85% of our universe?
Dark matter was invented in response to the problem that observations of galaxies didn't match theoretical predictions. At a stroke, it solved the discrepancy and offered a very simple physical explanation to grasp. But after 50 years of research there is still no direct evidence of it. While it's not my role to judge the scientific merit of the theory, my reservation is that this belief that dark matter or something like it "must exist" is really a belief in the universalized theoretical model we use to understand the cosmos in the first place. It looks suspiciously like the epicycles that astronomers in ancient times postulated to make sense of planetary movements within the geocentric model.
If we were to abandon the idea of dark matter as having no evidential basis, would other parts of our cosmological model fall apart, and if so, do we have an alternative model that could replace it?
I use the Jenga metaphor for this reason - to suggest dark matter is not a core tenet of the standard model but rather an added block on top. It is potentially replaceable without threatening the blocks underneath. Dark matter now faces a serious challenger in the so-called MOND theory (modified Newtonian dynamics), which may well eclipse it by doing a better job of matching theoretical predictions with observations. But this would not constitute a radical shift. Basically, it would replace a simple physical explanation with a complicated mathematical description, which has taken decades to develop. In a way, MOND looks more like an evolution than a revolution. That said, its key idea, that gravity does not work the same way everywhere in the universe, does have interesting implications for the model as a whole. It's another simplifying assumption of the original framework that is called into question.
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You can't build a cosmological model without metaphysics; to think cosmology is pure science is delusional.
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One of the striking claims that you’ve made is that the Big Bang is a metaphysical hypothesis, rather than a claim based on astronomical observation. What do you mean by ‘metaphysical hypothesis’ here, that it’s not possible to test empirically?
From the outset, the 'Big Bang' was always a hypothetical premise - if t=0, then... it allowed for calculation of scenarios. When this in turn could yield models that conformed to observations, it was seen to validate the original premise. This was repeated and became an integral assumption built into the framework. The 'Big Bang' also carried a powerful analogy with nuclear physics, the image of a cosmic explosion. It seemed to make sense to a certain generation and it was a very compelling narrative for many reasons - the ultimate origin story. We have since been scouring the skies for signs that we infer as proof of this cosmic origin. But this is retroactive reasoning - there is no direct evidence of the Big Bang because it lies beyond the horizon of the observable. So as a grounding assumption that can never itself be verified, it's a metaphysical hypothesis in this precise sense. Of course, you can imagine an alternate premise, that the universe is infinite in time, and this is metaphysical too. My point is you can't build a cosmological model without metaphysics; to think cosmology is pure science is delusional.