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.
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 spacetime, 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 spacetime, 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 expa...
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.
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 spacetime, 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 spacetime, 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 expa...
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BBC Science Focus (Digital) - 1 Issue, June 2024