There are hints that the universe may be behaving unexpectedly, and astrophysicists are racing to explain why. Their ideas to account for the surprising result include allowing dark matter and dark energy to interact, and arguing for the existence of strange “dark radiation” that is similar in nature to regular light but invisible.
In April, researchers using the Dark Energy Spectroscopic Instrument (DESI) in Arizona released the biggest 3D map of the universe ever created, and it hinted that we may have been wrong about dark energy – the still-mysterious force causing the accelerating expansion of the universe. The data contained tentative indications that dark energy may be changing over time, meaning the rate of expansion of the universe isn’t accelerating as smoothly as we thought. This runs contrary to the standard model of cosmology.
Dark energy is thought to make up nearly 70 per cent of the cosmos, so if its behaviour really is changing as time passes there could be huge implications for our understanding of the universe.
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Since the DESI data came out, researchers around the world have been working on ways to explain the apparent changes in the universe’s expansion rate. Dark energy is not directly observable, so there are several options that could fit. “There are many ways to potentially solve this – too many,” says at the Federal University of Rio Grande do Sul in Brazil. “Each one has its advantages.”
One solution suggested by Sabogal GarcĂa and his colleagues involves letting dark energy interact with dark matter, a mysterious form of matter we can’t see and that is far more plentiful than standard matter. This interaction is forbidden by the standard model, but if energy could in fact flow from the universe’s dark matter to its dark energy – essentially converting the former to the latter – the researchers’ simulations showed that this would match the DESI measurements. However, the mechanism for sure an energy transfer is not yet clear.
One upside of this solution is that it could also help resolve a long-standing cosmic dispute called the Hubble tension. This tension arises from a lack of agreement in the two main ways we measure the expansion rate of the cosmos. Measurements made by analysing nearby galaxies (known as “local” measurements) give an expansion rate, or Hubble constant, that is slightly higher than measurements made by analysing the light left over from the big bang, called the cosmic microwave background (CMB).
However, the CMB measurements require using a theoretical model of the cosmos to extrapolate the observations forward in time from the big bang. Sabogal GarcĂa and his colleagues’ proposal that dark matter and dark energy interact suggests we might need to change that theoretical model. This would bring the CMB measurement of the Hubble constant closer to alignment with the local measurements made using nearby galaxies. Recent observations have also suggested that the local value may be slightly lower than cosmologists thought, so Garcia’s approach could be a step towards resolving the tension.
Interacting dark energy isn’t the only way to do that, though. at Brown University in Rhode Island and his colleagues have come up with another way to explain the strange DESI data while similarly relaxing the Hubble tension: dark radiation.
Regular radiation is made up of massless particles called photons, and dark radiation would be just the same, except the dark photons that it is made from would not be visible or interact much with regular matter. “The data do not rule out dark radiation like previous datasets seemed to – and we even see a slight preference for it,” says Allali. “If you have more radiation, the expansion is faster in the early universe.” This could create the changes that the DESI researchers chalked up to possible shifts in dark energy.
While still just theoretical, dark radiation may also be a simpler resolution than arguing for changes in the behaviour of dark energy. “If these modifications of dark energy were confirmed, they could imply a breakdown in the laws of physics,” says at the University of Barcelona in Spain, part of Allali’s team. “What we did is much more conservative. It’s just adding an extra ingredient that’s similar to things we already observe – nothing extremely exotic.”
For now, both of these models seem to fit the DESI data, and there will almost certainly be many more that do so too. “It is compelling to test one’s favourite exotic model, and the current results allow for a plethora of models which could fit the data well,” says at the University of Edinburgh in the UK. “However, other than the [current standard model of cosmology], no other dark energy model really stands [up] from a fundamental physics point of view.”
The standard model matches most observations of the universe astonishingly well, so some researchers say it may not be time to start modifying it just yet. “Ultimately, we need even more data from DESI and other cosmological surveys on the horizon to pass a conclusive judgement on the standard cosmological model,” says at the University of Portsmouth in the UK. Thankfully, that data should be available in a matter of months or a few years.
Reference:
arXiv ,
Article amended on 14 May 2024
We clarified that the local value of Hubble tension measurements may be lower than previously thought
