Cosmologists say this must be the case because of the way the Universe is expanding. But it isn\u2019t just that the Universe is expanding; it\u2019s that the expansion is accelerating, and something must be driving that. We call that unknown force dark energy<\/a>.<\/p>\n But there\u2019s 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<\/a>. What\u2019s more, it also proposes that, maybe, dark energy doesn\u2019t exist at all.<\/p>\n False assumptions about dark energy<\/p>\n \u201cThe 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,\u201d says Hayley Macpherson<\/a>, a General Relativity and cosmology researcher at the University of Chicago.<\/p>\n Or, as cosmologists put it, the assumption is that the Universe is homogeneous and isotropic.<\/p>\n That makes sense when we\u2019re looking at the very early stages of the Universe, when it was small and dense, like a kind of primordial soup.<\/p>\n But as the Universe evolved and expanded, it became more like what we see today \u2013 having some regions that are full of stuff, like stars and galaxies, and other regions that are much more empty.<\/p>\n So, do these assumptions of homogeneity and isotropy still hold? Obviously not in a literal sense \u2013 the Universe clearly doesn\u2019t look like soup \u2013 but are the assumptions good enough for cosmology? That\u2019s the key question affecting calculations.<\/p>\n Some theorists, like Prof David Wiltshire<\/a> at the University of Canterbury, argue that these assumptions are so far off base that they\u2019re leading us to see the Universe in the wrong way.<\/p>\n He started off working as a theoretical physicist, he says, and when he began looking into cosmology he wasn\u2019t particularly looking to criticise the Standard Model<\/a>.<\/p>\n \u201cIt\u2019s just that the more I got into it and started speaking with people who actually do the observations, the more I realised that we\u2019re making huge simplifications,\u201d he says.<\/p>\n \u201cIn looking for some average notion of isotropy and homogeneity, we\u2019re really assuming the answer rather than trying to explain it.\u201d<\/p>\n In his view, if you take Einstein\u2019s work seriously, you need to think of the expansion as something that\u2019s dependent on the \u2018clumpiness\u2019 of the Universe.<\/p>\n 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 \u2013 that is, whether you\u2019re looking at a bunch of galaxies or a void.<\/p>\n And so perhaps we don\u2019t need the concept of dark energy at all. Perhaps the expansion of the Universe only looks like it\u2019s becoming faster, because we\u2019re looking at voids where the expansion appears faster, rather than areas with galaxies in them where the expansion appears slower.<\/p>\n Perhaps it\u2019s all just the result of the counterintuitive effects of gravity on time.<\/p>\n \u201cIt really is saying, right, there\u2019s this effect in Einstein\u2019s theory, which we\u2019ve not thought of before\u2019,\u201d Wiltshire says.<\/p>\n Read more:<\/p>\n A story with missing parts<\/p>\n Mainstream cosmologists wouldn\u2019t deny that the Universe is inhomogeneous, or that there are simplifications built into the Standard Model.<\/p>\n 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.<\/p>\n \u201cWe 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,\u201d says Jessica Muir<\/a>, a dark energy researcher and assistant professor at the University of Cincinnati. \u201cBut we don\u2019t think it\u2019s the whole story.\u201d<\/p>\n There are many oddities in the Standard Model of Cosmology. There\u2019s the Hubble Tension<\/a>, 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.<\/p>\n There\u2019s also something about the distribution of bright radio sources<\/a>, called quasars, that doesn\u2019t seem to fit with the way we think the Universe is expanding. This is known as the quasar dipole anomaly.<\/p>\n But perhaps the biggest issue facing the Standard Cosmological Model \u2013 at least, the thing that most people struggle to wrap their heads around \u2013 is that we know almost nothing about the dark matter and dark energy that make up the majority of the Universe.<\/p>\n \u201cThere are things in our model that don\u2019t make sense,\u201d Muir says. \u201cIt\u2019s not very satisfying that we don\u2019t know what 95 per cent of the Universe is made of.\u201d<\/p>\n The fact that the current model is unsatisfying in some ways doesn\u2019t necessarily mean it\u2019s wrong, though.<\/p>\n 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\u2019re likely unimportant.<\/p>\n \u201cIt would be very exciting if we say, \u2018Oh, we missed this thing, and it turns out we don\u2019t need a mysterious substance that has negative pressure and that makes up 70 per cent of the mass and energy in the Universe\u2019,\u201d Muir says.<\/p>\n But when it comes to questioning the assumptions made about General Relativity, \u201cThose assumptions have already been checked pretty rigorously, to the extent that I don\u2019t feel particularly convinced that the cosmological constant isn\u2019t needed,\u201d Muir says.<\/p>\n Building better models<\/p>\n So, why not put that concept to the test and find out how big the effects of these assumptions about General Relativity really are?<\/p>\n That\u2019s 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.<\/p>\n \u201cThe whole point of my work is to incorporate a more complex description of gravity into cosmological modeling, which we typically don\u2019t do,\u201d she says.<\/p>\n Now, with more data about the Universe than ever before and increasingly powerful supercomputers to run simulations on, \u201cWe\u2019re getting the freedom to be able to remove some of those assumptions and see if we can build a better model.\u201d<\/p>\n Often, when cosmologists see the problems with the Standard Model, their first instinct is to try to introduce something \u2013 a new kind of energy, a new constant, a simplification \u2013 to fix the model.<\/p>\n But Macpherson\u2019s approach is different: \u201cIt\u2019s much rarer in the scope of all cosmologists to say, \u2018Hey, we\u2019ve built this model based on simplifying approximations. Maybe all this weird stuff is just a consequence of those specific approximations breaking down\u2019.\u201d<\/p>\n 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.<\/p>\n Despite this, Macpherson says that, \u201cAs with any kind of science, or especially in this kind of numerical modelling, there are a billion caveats to that statement. There\u2019s still a lot of work to be done to be able to say for sure whether the assumptions matter or not.\u201d<\/p>\n Read more:<\/p>\n The changing constant<\/p>\n Even those who stand by the Standard Model of Cosmology will happily admit that dark energy is strange.<\/p>\n 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.<\/p>\n Dark energy is harder to conceptualise. The most prominent theories are that it\u2019s a property of space itself: that there\u2019s something about the existence of empty spaces that pushes outwards. Or, as cosmologists generally put it, that it has negative pressure.<\/p>\n But there are so many questions here.<\/p>\n 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?<\/p>\n 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.<\/p>\n There\u2019s a lot of reasons to think that there\u2019s deeper physics behind dark energy,\u201d Muir says.<\/p>\n Recent findings<\/a> 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.<\/p>\n The idea is not just that as the Universe expands, there\u2019s more empty space and this bigger space therefore means more dark energy. It\u2019s that the density of dark energy itself is changing over time.<\/p>\n In this case, dark energy can\u2019t just be represented by a cosmological constant, Lambda, because it isn\u2019t constant \u2013 there are times in the Universe when it\u2019s had a lower value, and there could be times in the future when it has a higher value. That\u2019s hard to fit within the current Standard Model.<\/p>\n \u201cIt\u2019s difficult to change the cosmological constant mildly, because there isn\u2019t really a theory behind it,\u201d says Dr Valeria Pettorino<\/a>, a dark energy theorist at the European Space Agency. \u201cIt\u2019s a constant. It\u2019s there. It\u2019s a bit added by hand.\u201d<\/p>\n 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\u2019s very much up for debate right now.<\/p>\n \u201cThe problem is, what happens if we are at very large scales?\u201d Pettorino says. \u201cWhat happens in the future, or in the past? Was it always like that? Will it always be like this?\u201d<\/p>\n This leads into an even more intriguing concept: perhaps dark energy isn\u2019t just one thing. It could be a whole family of forces that work together in complex ways to affect the Universe\u2019s expansion.<\/p>\n \u201cThe Universe that we know, the five per cent, it\u2019s full of different particles, different forces,\u201d says Pettorino. \u201cSo, imagine this 70 per cent of dark energy. It\u2019s probably a combination of different things. I would be surprised to see this 70 per cent just made of one thing.\u201d<\/p>\n Cracks in the facade<\/p>\n We\u2019re getting closer than ever to understanding dark energy, thanks to both ground-based surveys and the recently launched Euclid<\/a> space telescope.<\/p>\n![\"Galaxy](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/understanding-nothingness-Abell-1689-cluster.jpg\")
Plotting the warping effects on the light from the galaxies in the Abell 1689 cluster has enabled scientists to infer the distribution of dark matter in the region (in blue) – Image credit: Hubble Space Telescope<\/p>\n![\"Visualisation](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/understanding-nothingness-Visualisation-of-primordial-quark-gluon-plasma.jpg\")
A visualisation of the early Universe, as a quark-gluon soup, during the initial moments after the Big Bang – Image credit: Science Photo Library<\/p>\n![\"The](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/understanding-nothingness-Mayall-Telescope.jpg\")
The Dark Energy Spectroscopic Instrument on the Mayall Telescope in Arizona measures the effect of dark energy on the expansion of the Universe – Image credit: Desi Collaboration\/DOE\/KPNO\/Noirlab\/NSF\/AURA<\/p>\n![\"Photo](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/understanding-nothingness-Euclid-on-launch-pad.jpg\")
The European Space Agency’s Euclid mission ready to launch in July 2023 – Photo credit: SpaceX<\/p>\n
![\"Illustration](\"https:\/\/www.newsbeep.com\/us\/wp-content\/uploads\/2025\/09\/understanding-nothingness-universe-expanding.jpg\")
The Universe has been expanding ever since the Big Bang 13.8 billion years ago (left). Dark energy is assumed to be the reason why the expansion appears to be accelerating – Illustration credit: Science Photo Library<\/p>\n