If you've ever ready anything about cosmology, you'll have come across an astonishing statistic: all the matter around us, everything we see and touch, every star and gas cloud and planet, makes up just five per cent of the energy in the Universe.
Of everything else, around 25 per cent is made up of dark matter, and the rest – a baffling 70 per cent of everything that exists – is dark energy.
Cosmologists say this must be the case because of the way the Universe is expanding. But it isn’t just that the Universe is expanding; it’s that the expansion is accelerating, and something must be driving that. We call that unknown force dark energy.
But there’s a niche area of cosmology research that disputes this explanation. It argues that things appear to be expanding faster due to a mistake in our understanding of gravity. What’s more, it also proposes that, maybe, dark energy doesn’t exist at all.
False assumptions about dark energy
“The assumptions that we make in General Relativity in cosmology, basically, are that the Universe is very close to being the same everywhere and the same in all directions,” says Hayley Macpherson, a General Relativity and cosmology researcher at the University of Chicago.
Or, as cosmologists put it, the assumption is that the Universe is homogeneous and isotropic.
That makes sense when we’re looking at the very early stages of the Universe, when it was small and dense, like a kind of primordial soup.
But as the Universe evolved and expanded, it became more like what we see today – having some regions that are full of stuff, like stars and galaxies, and other regions that are much more empty.
So, do these assumptions of homogeneity and isotropy still hold? Obviously not in a literal sense – the Universe clearly doesn’t look like soup – but are the assumptions good enough for cosmology? That’s the key question affecting calculations.
Some theorists, like Prof David Wiltshire at the University of Canterbury, argue that these assumptions are so far off base that they’re leading us to see the Universe in the wrong way.
He started off working as a theoretical physicist, he says, and when he began looking into cosmology he wasn’t particularly looking to criticise the Standard Model.
“It’s just that the more I got into it and started speaking with people who actually do the observations, the more I realised that we’re making huge simplifications,” he says.
“In looking for some average notion of isotropy and homogeneity, we’re really assuming the answer rather than trying to explain it.”
In his view, if you take Einstein’s work seriously, you need to think of the expansion as something that’s dependent on the ‘clumpiness’ of the Universe.
Gravity slows down time, so time passes differently in regions of the Universe with more matter than in parts with less matter. Different parts of the Universe will expand at different rates based on how much stuff is in them – that is, whether you’re looking at a bunch of galaxies or a void.
And so perhaps we don’t need the concept of dark energy at all. Perhaps the expansion of the Universe only looks like it’s becoming faster, because we’re looking at voids where the expansion appears faster, rather than areas with galaxies in them where the expansion appears slower.
Perhaps it’s all just the result of the counterintuitive effects of gravity on time.
“It really is saying, right, there’s this effect in Einstein’s theory, which we’ve not thought of before’,” Wiltshire says.
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A story with missing parts
Mainstream cosmologists wouldn’t deny that the Universe is inhomogeneous, or that there are simplifications built into the Standard Model.
They mostly work with a version of General Relativity that makes use of a term Lambda, also known as the cosmological constant, which makes the equations of General Relativity work for an expanding Universe. On the whole, that model has been good enough for decades of productive work.
“We have this sort of vanilla description of the Universe, this minimal lambda CDM cosmological model that, for about 25 years since the discovery of dark energy, has done a remarkably good job at predicting most observations,” says Jessica Muir, a dark energy researcher and assistant professor at the University of Cincinnati. “But we don’t think it’s the whole story.”
There are many oddities in the Standard Model of Cosmology. There’s the Hubble Tension, in which the acceleration of the expansion of the Universe seems to be happening at a different rate depending on what method you use to measure it.
There’s also something about the distribution of bright radio sources, called quasars, that doesn’t seem to fit with the way we think the Universe is expanding. This is known as the quasar dipole anomaly.
But perhaps the biggest issue facing the Standard Cosmological Model – at least, the thing that most people struggle to wrap their heads around – is that we know almost nothing about the dark matter and dark energy that make up the majority of the Universe.
“There are things in our model that don’t make sense,” Muir says. “It’s not very satisfying that we don’t know what 95 per cent of the Universe is made of.”
The fact that the current model is unsatisfying in some ways doesn’t necessarily mean it’s wrong, though.
The difficult equations of General Relativity have certainly been simplified with assumptions to make them usable, but most cosmologists say that any effects these assumptions have are so small that they’re likely unimportant.
“It would be very exciting if we say, ‘Oh, we missed this thing, and it turns out we don’t need a mysterious substance that has negative pressure and that makes up 70 per cent of the mass and energy in the Universe’,” Muir says.
But when it comes to questioning the assumptions made about General Relativity, “Those assumptions have already been checked pretty rigorously, to the extent that I don’t feel particularly convinced that the cosmological constant isn’t needed,” Muir says.
Building better models
So, why not put that concept to the test and find out how big the effects of these assumptions about General Relativity really are?
That’s exactly what Macpherson is doing right now by creating simulations of the Universe that are based on a more complex description of gravity, without so many of the simplifications usually placed on General Relativity.
“The whole point of my work is to incorporate a more complex description of gravity into cosmological modeling, which we typically don’t do,” she says.
Now, with more data about the Universe than ever before and increasingly powerful supercomputers to run simulations on, “We’re getting the freedom to be able to remove some of those assumptions and see if we can build a better model.”
Often, when cosmologists see the problems with the Standard Model, their first instinct is to try to introduce something – a new kind of energy, a new constant, a simplification – to fix the model.
But Macpherson’s approach is different: “It’s much rarer in the scope of all cosmologists to say, ‘Hey, we’ve built this model based on simplifying approximations. Maybe all this weird stuff is just a consequence of those specific approximations breaking down’.”
Though the simulation work is still very much in progress, so far the results are leaning toward the effects of the assumptions about gravity being very small and negligible.
Despite this, Macpherson says that, “As with any kind of science, or especially in this kind of numerical modelling, there are a billion caveats to that statement. There’s still a lot of work to be done to be able to say for sure whether the assumptions matter or not.”
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The changing constant
Even those who stand by the Standard Model of Cosmology will happily admit that dark energy is strange.
Dark matter is puzzling enough, with its lack of interaction with light, but we can still detect it through its gravitational effects and think of it as, presumably, analogous to some extent with ordinary matter.
Dark energy is harder to conceptualise. The most prominent theories are that it’s a property of space itself: that there’s something about the existence of empty spaces that pushes outwards. Or, as cosmologists generally put it, that it has negative pressure.
But there are so many questions here.
Why does space have this particular energy density, rather than any other amount? How do we relate this to quantum mechanics, which predicts that an empty vacuum should have an energy density that's orders of magnitude larger than what we actually see?
Though most cosmologists agree that dark energy is a useful term to describe a force that must be there due to the expansion of the Universe, what this force actually is, is far harder to explain.
There’s a lot of reasons to think that there’s deeper physics behind dark energy,” Muir says.
Recent findings from observations carried out by the Dark Energy Spectroscopic Instrument and the Dark Energy Survey, on which Muir works, also support a long-held theory that dark energy could be changing over time.
The idea is not just that as the Universe expands, there’s more empty space and this bigger space therefore means more dark energy. It’s that the density of dark energy itself is changing over time.
In this case, dark energy can’t just be represented by a cosmological constant, Lambda, because it isn’t constant – there are times in the Universe when it’s had a lower value, and there could be times in the future when it has a higher value. That’s hard to fit within the current Standard Model.
“It’s difficult to change the cosmological constant mildly, because there isn’t really a theory behind it,” says Dr Valeria Pettorino, a dark energy theorist at the European Space Agency. “It’s a constant. It’s there. It’s a bit added by hand.”
The idea of assuming dark energy to be a constant has been useful for research thus far. But how well that constant fits at different scales, and in different scenarios, is something that’s very much up for debate right now.
“The problem is, what happens if we are at very large scales?” Pettorino says. “What happens in the future, or in the past? Was it always like that? Will it always be like this?”
This leads into an even more intriguing concept: perhaps dark energy isn’t just one thing. It could be a whole family of forces that work together in complex ways to affect the Universe’s expansion.
“The Universe that we know, the five per cent, it’s full of different particles, different forces,” says Pettorino. “So, imagine this 70 per cent of dark energy. It’s probably a combination of different things. I would be surprised to see this 70 per cent just made of one thing.”
Cracks in the facade
We’re getting closer than ever to understanding dark energy, thanks to both ground-based surveys and the recently launched Euclid space telescope.
Euclid is the most ambitious mission yet for understanding dark matter and dark energy, and it intends to create a map of the large-scale structure of the Universe in great detail.
"In practice, the way we approach this is, we try and test the predictions of this minimal model with greater and greater precision,” says Muir.
“The hope is, if you find cracks in that facade where the simplest model predictions don’t match observations, that could be a clue for where you need to go beyond that simplest description.”
To build up a map with the kind of high-level accuracy required to test these models, the Euclid telescope needs to have extremely precise instruments, like its 600-megapixel camera.
Euclid will be able to assess the expansion rate of the Universe using two methods: observing both supernova explosions (which give out a predictable pattern of light) and leftover patters in the distribution of galaxies from the earliest stages of the Universe, called baryon acoustic oscillations.
By having these two methods, the hope is that measurements of the expansion rate can be more certain. The resulting data can hopefully show which of the many models of dark energy are the best fit.
“The Euclid mission can test the Friedmann equation [the basis of the Standard Model of Cosmology], and it can test my model. So, if I’m wrong, then I can retire in five years,” Wiltshire says with a smile.
Whatever the results from Euclid are, they’re set to revolutionise cosmology.
“We don’t necessarily think lambda is the full story. And so, we want to look for deviations from it that could tell us more,” Muir says.
A more complex understanding of gravity, for example, could change the way that we think about dark energy. But Muir also cautions: “Extraordinary claims require extraordinary evidence.”
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