It takes some hubris to name a new project the Dark Energy Spectroscopic Instrument (DESI). After all, dark energy is completely invisible – it gives off no light to be collected and analysed with a spectrograph. In fact, it’s never been seen at all – it has evaded every attempt we’ve made to image or capture it with even our most advanced telescopes and detector experiments.
As far as we can tell, dark energy is something that is indiscernible, perfectly uniform throughout space and has no interaction at all with matter or light. Its only function, through some as-yet-undetermined mechanism, is to make space expand ever faster.
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So how is it, then, that DESI’s just-announced first data release is, as promised, shaking up our understanding of dark energy?
There are only a few observational handles we can get on something as frustratingly elusive as dark energy. Since all dark energy does is stretch space-time, testing different theories of dark energy’s nature involves learning how that stretching has occurred across cosmic time.
One method is charting the expansion history of the Universe; a related method is to look at how quickly matter built up into galaxies and clusters at different points in our cosmic past.
Measuring the expansion rate generally relies on creating extremely precise 3D maps of matter in the cosmos; charting out lots of distant galaxies, quasars (the bright emission from the vicinity of supermassive black holes) or intergalactic gas, and information about the motion of each object.
That’s where the spectroscopy comes in. By analysing the spectrum of the light, we can see how much it’s been stretched as the source is pulled away from us by cosmic expansion. Connecting that measured expansion rate with an exact physical distance can give us invaluable information about the evolution of our cosmos (along with some really cool maps).
DESI’s newly released modelling made a splash by hinting that dark energy might have a more complicated history than we normally assume. If these hints hold up, they could reshape our understanding not only of the Universe’s history, but also of our ultimate cosmic fate.
The concordance model of cosmology encapsulates our current best-guess working model of the Universe and its constituents. In this model, dark energy is a cosmological constant: an inherent property of space-time, uniform and unchanging, that essentially just builds a little stretchiness into every bit of space.
With dark energy as a cosmological constant, the observed density of dark energy would always remain the same over time. Unlike matter that dilutes when the space it’s in gets bigger through cosmic expansion, more space just means more cosmological constant contained in that space.
If dark energy were something dynamical, meaning its density or behaviour were changing over time, that change would show up in detailed measurements of the expansion history.
DESI and other surveys tend to report their dark energy results in terms of the so-called ‘equation of state’ parameter, written as w. If dark energy is a cosmological constant, we expect to see w = –1, exactly, for all time. If w is anything other than –1, or if it seems to be increasing or decreasing, dark energy must be something else.
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DESI’s results provide an intriguing conundrum. When w is assumed to be constant, a value of –1 fits the results just fine. But when the analysis is altered to allow for the possibility that w has been changing, DESI’s new results look different. Combined with other datasets from supernova surveys, they seem to suggest that a varying w, one that was lower in the past and will be higher in the future, matches the data best.
What this would mean for our cosmos is unclear. For a constant w, anything less than –1 is called ‘phantom dark energy’, and its ominous name is earned: it implies a distant future in which dark energy could literally rip apart galaxies, solar systems, stars and even the Universe itself.
Phantom dark energy is disliked by theorists because it seems to break some really important fundamental principles that we think probably hold in the cosmos. While the new results suggest a movement away from the phantom regime, they seem to be implying that those principles could have been broken sometime in the cosmic past, which would give a lot of theorists headaches.
If w really is increasing, that might suggest that dark energy is getting less important over time. That could change our cosmic future in subtle but interesting ways, possibly leading to one in which the expansion of the Universe is no longer accelerating (though probably not allowing the expansion to reverse or stop entirely).
The results from DESI are still just hints, which might disappear in future studies. Still, it’s possible that dark energy has just found a new way to surprise us.
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