Dark Energy is a mystery so daunting that it stretches and strains our most robust theories. The Universe is expanding, driven by the unknown force that we’ve named Dark Energy. Dark Energy is also accelerating the rate of expansion. If scientists could figure out why, it would open up a whole new avenue of understanding.
The drive to understand dark energy is so powerful that a special instrument designed just to understand it has been created: DESI, the Dark Energy Spectroscopic Instrument. DESI’s main survey began in 2021 and relies on baryon acoustic oscillations (BAO) to create a map of how matter is distributed in the Universe. The survey covers an enormous volume of the Universe in extreme detail.
DESI is attached to the Nicholas Mayall 4-meter telescope at the Kitt Peak National Observatory. Image Credit: DESI Collaboration/DOE/KPNO/NOIRLab/NSF/AURA/P. Horálek (Institute of Physics in Opava)
Our understanding that the Universe is expanding dates back to the early 20th century when astronomers noticed that the light from some distant objects was shifted toward the red end of the spectrum. This is called redshift, and researchers concluded that it was because the objects were moving away from Earth.
A breakthrough came when Edwin Hubble figured out that the further an object was from Earth, the faster it was moving away. It dawned on us that we’re living in a dynamic Universe rather than a static one.
Cosmologists thought that gravity was slowing the rate of expansion. However, in 1998, astronomers discovered that it was actually accelerating. The Cosmological Constant explains this acceleration and is basically an energy density that seems to be an inherent part of the Universe. The Hubble Constant is intertwined with this, and it’s the rate of expansion of the Universe.
Put simply, the Hubble constant is how fast the universe is currently expanding, while the cosmological constant is a factor that affects that speed. Efforts to measure these constants have yielded different answers. Clearly, we have a mystery on our hands.
“The cosmological constant is essentially just an extra term in the equation that everyone has been using for years,” said Andrew Hearin, a physicist at the U.S. Department of Energy’s (DOE) Argonne National Laboratory, which is a DESI member institution. ”We don’t know why it takes on the particular value that it does, but it’s a very mundane explanation for this unexpected cosmic acceleration.”
DESI’s first year of data tracked the expansion of the Universe over 11 billion years. This initial data largely agreed with our Standard Model of Cosmology, called Lambda CDM. However, there were some small discrepancies suggesting that dark energy was changing over time. If it is changing, it’s a direct challenge for the cosmological constant.
This graphic shows some of the theorized stages in the Universe according to the Standard Model of Cosmology. Image Credit: NASA/ LAMBDA Archive / WMAP Science Team
This might seem disappointing on the surface. One of our most foundational understandings of the Universe is being challenged by powerful observations. However, the reverse is true: it generated excitement.
“If the DESI result holds up, it means that a cosmological constant is not the origin of cosmic acceleration. It’s much more exciting,” said Hearin. ”It would mean that space is pervaded by a dynamically evolving fluid with negative gravity, which has never been observed in any tabletop experiment on Earth.”
To make headway, Hearin and his colleagues turned to supercomputer simulations. They used the Aurora exascale supercomputer at the Argonne Leadership Computing Facility to run large-scale simulations of the Universe. These simulations allow researchers to test DESI’s data.
Hearin and his fellow researchers presented the results of their simulations in a new paper titled “ILLUMINATING THE PHYSICS OF DARK ENERGY WITH THE DISCOVERY SIMULATIONS.” The paper has been submitted to the Open Journal of Astrophysics and is available at arxiv.org. The lead author is Gillian Beltz-Mohrmann, a postdoctoral fellow in the Cosmological Physics and Advanced Computing Group at Argonne National Laboratory
“When this result came out last year, we got really, really excited,” said Katrin Heitmann, a cosmologist and deputy director of Argonne’s High Energy Physics division. “Our team got together to discuss what we could do to help the community look into this from the simulation side. Simulations play a crucial role in disentangling fundamental physics from systematics in the observations or in the data analysis.”
One of the challenges in observing the Universe is determining if what we’re seeing is a true representation of reality or if it contains distortions of our own making, like observational biases or problems in processing observational data. Powerful supercomputer simulations are a way scientists can test their observations in depth.
“Since we can’t create a mini-universe to conduct experiments, we can test theories by using really big computers like Aurora to simulate the growth of structure in the universe over time,” said Gillian Beltz-Mohrmann, a postdoctoral research fellow at Argonne.
The researchers conducted two separate simulations called the Discovery simulations. Both had the same initial conditions, but in one, dark energy was constant, and in the other, it changed over time. Simulations can’t prove outright that we’re right or wrong, but they’re an important next step.
The Discovery simulations simulate two large-scale regions of the Universe with different parameters. The standard model of cosmology is on the left, and the dynamical dark energy model is on the right. There are subtle differences between the two, which aren’t apparent on a large scale but become clearer on a small scale. Image Credit: ALCF Visualization and Data Analytics Team and the HACC Collaboration.
“The Discovery simulations are a pair of boxes with identical initial conditions; the only difference between the two simulations is cosmology,” the authors write in their paper. “The first box uses a Lambda CDM cosmology, while the second box contains a dark energy equation of state w, which evolves in time.” The values of cosmological parameters are based on DESI’s year one results.
“The idea is that you create a model universe under one set of assumptions, and then you compare your model universe to the real universe. If the agreement is very good, it gives you some confidence that your assumptions are correct,” Hearin said. ”But if you have some gross discrepancy, then it tells you that your assumptions don’t align with the real universe and don’t represent the truth.”
This is only possible because of massive increases in computing power. What would once take weeks and weeks to simulate now takes only days with the Aurora supercomputer. “These simulations serve as a demonstration of a unique new capability to run high-resolution simulations of cosmological volume in ~2 days, allowing for close-to-real-time investigations of new cosmological results,” the researchers explain in their paper.
“Using Aurora’s immense processing power to rapidly run large-scale simulations at sufficiently high resolution, we can respond much faster to new insights from cosmological observations,” said Argonne computational scientist Adrian Pope. ”These simulations would have taken weeks of compute time on our earlier supercomputers, but each simulation took just two days on Aurora.”
“This pair of simulations really illustrates our ability to take a result that’s hot off the presses from a collaboration like DESI, immediately run a simulation based on those results and then see what it looks like,” Beltz-Mohrmann said.
There are only small discrepancies between the two simulations, but they’re there, and they’re important.
These panels visually compare a small region in the simulations. On the left is the standard model of cosmology, and on the right is the dynamical dark energy model based on DESI’s first year of data. The differences are subtle but still clearly visible at the substructure level. Image Credit: ALCF Visualization and Data Analytics Team and the HACC Collaboration.
“If looking at these two simulations gives us an idea of the type of measurement we should make to help us narrow in on a cosmological model, then we can go back to the real DESI data and make that same measurement and see what it tells us,” Beltz-Mohrmann said. Observations and simulations are in a feedback loop together.
The simulations showed that our understanding of the dark matter halo mass function might be in error. The halo mass function is part of understanding how dark matter is distributed in the Universe. “At all redshifts and for all masses, the halo mass function is suppressed in the ΛCDM simulation compared to the w0waCDM simulation,” the authors write. The w0waCDM simulation is based on DESI’s data showing fluctuating dark energy.
The simulations also showed a difference in the rate at which dark matter haloes accrete mass. At low redshifts, w0waCDM haloes accrete mass slightly faster than haloes in the ΛCDM simulations. At greater redshifts, the difference shrinks.
The simulations also produced different star-formation rates (SFRs). “Among low mass halos, the largest differences in star formation rates between the two cosmologies appear at high redshift (z > 1), where the star formation rate of w0waCDM galaxies is ~ 2% lower than the star formation rate of ΛCDM galaxies,” the authors write in their paper. Among intermediate and high-mass halos, the difference grows.
The results support the dynamic dark energy model, where dark energy changes over time. However, the authors caution that these results are not conclusive yet.
“It should be noted that because the Discovery simulations contain differences in all of their cosmological parameters, we are not isolating the effect of evolving dark energy but rather examining the differences in these two simulations based on their overall cosmologies,” the authors explain in their paper.
More: