The final results from the Dark Energy Survey (DES) have provided intriguing new clues into mysteries surrounding the Universe’s accelerating expansion. The survey, which scanned 5,000 square degrees of the southern sky over 7 years, found no evidence that cosmic expansion is ‘ripping apart’ galaxies and will lead to an eventual ‘Big Rip’. However, dark energy – the mysterious force driving expansion – may be even more exotic than previously thought.
DES provides largest ever sample of supernovae
A core part of the DES involved precisely measuring over 2,500 ‘standard candle’ supernovae explosions to map cosmic expansion over the last 10 billion years. This is the largest ever sample obtained through a single experiment, nearly doubling all previously recorded supernova observations .
Supernovae provided the original evidence for Universe acceleration in the late 1990s. As lead investigator Chris D’Andrea from the University of Pennsylvania explains, this paved the way for DES:
“Twenty years ago, the discovery of dark energy using supernovae ushered in a new era of cosmology. The Dark Energy Survey was designed to crack the mystery of dark energy using multiple complementary probes.” 
By comparing supernovae distances at different epochs, DES precisely tracked the evolution of cosmic expansion over recent history. As collaboration spokesperson Rich Kron from the University of Chicago states:
“Taken together, these measurements allow us to put the most stringent constraints yet on the behavior of dark energy over the last 10 billion years.”
Table 1: Key discoveries from the Dark Energy Survey
|No evidence Universe is tearing itself apart
|Unlikely we face an eventual ‘Big Rip’
|Dark energy density may have decreased over time
|Points to exotic models beyond cosmological constant
|Most precise measure yet of Universe’s clumpiness
|Better understanding of dark matter properties
Constant cosmic expansion more likely than ‘Big Rip’
The DES analysis challenges theoretical models predicting an exponential expansion of the Universe eventually leading to a ‘Big Rip’. Such models suggest galaxies would get increasingly stretched until the night sky appears totally black. Instead, DES supernova data matches a model where dark energy remains constant or slowly decreases in strength over time. This points to continuous expansion rather than a ‘runaway’ scenario .
As collaborator Nicolás Busca of the University of Paris explains:
“The data suggest that cosmic expansion does not get faster and faster without limit, but rather becomes very close to a constant at large cosmic distances.” 
The implications are that our Universe should continue expanding at a steady rate rather than tearing apart. This grants extra reassurance that Earth and our galaxy are not under threat from such exotic cosmic scenarios.
Dark energy not a simple ‘cosmological constant’
While ruling out a ‘Big Rip’, the survey indicates dark energy acts differently than the simplest theoretical models assume. The canonical picture is of a ‘cosmological constant’ uniformly permeating space and driving acceleration. But supernova measures suggest dark energy density dropped around 5% over the last 7 billion years . This points instead to more complex ‘dynamical dark energy’ models where acceleration strength evolves.
According to Harvard researcher Matias Carrasco Kind:
“If you have dark energy being constant it creates some tension with the cosmic microwave background data. This mild preference for dynamic dark energy alleviates that tension”. 
Reconciling DES findings with other experiments could require updating the canonical Lambda Cold Dark Matter (ΛCDM) model underlying modern cosmology. This may demand new particles like neutrinos or exotic alternative gravity theories. As Carrasco concludes:
“We’re really probing the nature of dark energy now — rather than just its existence.” 
Improved measure of Universe’s clumpiness supports ΛCDM
As well as cosmic expansion history, the DES analysis provides the most precise measurement yet of how ‘clumpy’ matter is distributed through the Universe . Quantified through ‘power spectrum’ fluctuations, this allows testing the ΛCDM paradigm predicting how galaxies cluster over different scales. Lead DES author Francois Lanusse states:
“The Rare experiments tell us how the Universe expands, but not how matter clusters together. We can test this clustering using the Dark Energy Survey galaxies in combination with CMB data.” 
The results precisely match ΛCDM predictions, giving confidence we understand the cosmic web structure. This improves dark matter constraints, though its particle nature remains unknown. The next generation ‘CMB-S4’ experiment will provide complementary CMB data to further probe candidates like axions and sterile neutrinos. 
Ongoing impacts from the influential Dark Energy Survey
Concluding over a decade of work, the trailblazing Dark Energy Survey has ushered cutting-edge datasets to uncover new findings about our Universe’s makeup. With ever larger supernova samples in the pipeline from the Vera Rubin Observatory, there are high hopes coming years could resolve lingering questions around repulsive dark energy. 
Key areas still shrouded in mystery include whether cosmic acceleration is strengthening or weakening, how evenly it permeates space, and ultimately deciphering its origin. Non-standard possibilities like modified gravity theories remain viable if evidence for dynamical dark energy firms up. 
Unifying this picture with particle physics is the holy grail to complete a quantum theory of gravity. Learning cosmic expansion history provides crucial empirical data to bridge general relativity and quantum behaviours. Although tractable experiments to test such ‘theory of everything’ proposals may be centuries away, DES and successor stage important first steps in revealing the evolution of our Universe. 
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