Astronomers may soon have the answer to what is perhaps the greatest mystery of modern science –is dark energy a uniform force across space and time, or has its strength evolved over eons?
The universe is not only expanding – it is accelerating outward, driven by what is commonly referred to as “dark energy.” The term is a poetic analogy to the label for dark matter, the mysterious material that dominates the matter in the Universe and that really is dark because it does not radiate light (it reveals itself via its gravitational influence on galaxies).
Gravity or the Vacuum?
Two explanations are commonly advanced to explain dark energy. The first, as Albert Einstein once speculated, is that gravity itself causes objects to repel one another when they are far enough apart. Einstein introduced a non-zero “cosmological constant” term to his field equations of general relativity in 1917 in order to counterbalance the inward pull of gravity, contriving a static, steady-state universe. However, the cosmological constant was abandoned around 1930 when Edwin Hubble demonstrated that the Universe is expanding. A non-zero cosmological constant was reintroduced in the late 1990s as a source of dark energy to explain the recently discovered outward acceleration of the universe.
The second explanation hypothesizes (based on our current understanding of elementary particle physics) that the vacuum has properties that provide energy to the cosmos for acceleration. Within a random fixed volume of the Universe, roughly 5% is normal baryonic matter such as atoms that compose the stars and planets, 27% is the mysterious attractive dark matter, and 68% comprises the vacuum energy density of the repulsive and even more elusive dark energy.
For several decades cosmologies have successfully used a relativistic equation with dark matter and dark energy to explain increasingly precise observations about the cosmic microwave background, the cosmological distribution of galaxies, and other large-scale cosmic features. But as the observations have improved, some apparent discrepancies have emerged.
Planck Satellite Data vs Baryon Acoustic Oscillation Experiments
One of the most notable is the age of the universe: there is an almost 10% difference between measurements inferred from the Planck satellite data and those from so-called Baryon Acoustic Oscillation experiments. The former relies on far-infrared and submillimeter measurements of the cosmic microwave background and the latter on spatial distribution of visible galaxies.
Harvard-Smithsonian CfA astronomer Daniel Eisenstein was a member of a large consortium of scientists who suggest that most of the difference between these two methods, which sample different components of the cosmic fabric, could be reconciled if the dark energy were not constant in time.
The scientists apply sophisticated statistical techniques to the relevant cosmological datasets and conclude that if the dark energy term varied slightly as the universe expanded (though still subject to other constraints), it could explain the discrepancy.
Dark Energy Spectroscopic Instrument Survey
Direct evidence for such a variation would be a dramatic breakthrough, but so far has not been obtained. One of the team’s major new experiments, the Dark Energy Spectroscopic Instrument (DESI) Survey, could settle the matter, helping astronomers better understand the repulsive force associated with “dark energy” that drives the acceleration of the expansion of the universe across vast cosmic distances.
Uniform Across Space and Time or Evolved?
The survey will reconstruct 11 billion years of cosmic history. It could answer the first and most basic question about dark energy: is it a uniform force across space and time, or has its strength evolved over eons?
The five-year quest to map the universe and unravel the mysteries of “dark energy” began officially May 17, 2021 at Kitt Peak National Observatory near Tucson, Arizona. To complete its quest, DESI will capture and study the light from some 30-million galaxies. Scientists say DESI will help them construct a 3D map of the universe with unprecedented detail, reaching back to objects only a few billion years after the big bang, and should be completed sometime in the mid 2020’s.
Astrophysicist and member of the XENON collaboration, Rafael Lang at Purdue University wrote in an email to The Daily Galaxy: “the groundwork for what may turn out to be yet another revolution in cosmology. These colleagues are mapping out our universe to an amazing precision, and that has far reaching consequences. For example, cosmology always had this issue with the “Distance Ladder”, where we needed to work out our way in baby steps out into the cosmic expanse to draw our map as we went out. Get one early turn wrong, and your map is off, getting you completely lost.
Drawing the Cosmic Map
“Now, for example,” writes Lang. “these dark energy surveys can measure what is called Baryon-Acoustic Oscillations, across the universe. These are like compass points along every turn and corner, an independent confirmation that our map is right, if you like, they are now reading the signs in our cosmic landscape. That really solidifies our pathfinding around the cosmos, our drawing of the cosmic map. And in mapping out the entire universe with these amazing instruments, that better understanding, that better map is what allows them to measure even small oddities. Like, not only if the landscape ever so slightly slops, to stay in the picture, but to map out how that slope changes as you walk around. And that is exactly what we want to know about Dark Energy: Does it change as we travel across the universe? Was it always the same? Is it truly just a constant property of space, or is it something dynamic, something that changes as the universe evolves? We don’t know, and these measurements have the potential to tell us. This is extremely exciting. We have no clue about the Nature of Dark Energy, so measuring it across the universe will tell us a lot about what’s going on.”
“Current observations from both satellite studies of the cosmic microwave background and ground-based survey are consistent with a dark energy that behaves like Einstein’s cosmological constant,” astrophysicist Richard Ellis at University College London and former director of the Caltech’s Palomar Observatory told The Daily Galaxy. “DESI and other ambitious upcoming ground-based surveys such as the Subaru Prime Focus Spectrograph,” he noted, “will provide a sufficient increase in the number of spectroscopic redshifts to constrain possible alternatives and even time variations of dark energy. Regardless of what emerges, these will be crucial results for fundamental physics.”
The Last Word
We’ll be able to precisely constrain not just the average effect of dark energy over time
“While we have successfully pushed the boundaries of what was possible in cosmology using current generation experiments like the Dark Energy Survey, new experiments like the Vera C. Rubin Observatory LSST and Nancy Grace Roman Space Telescope are on the horizon and will fundamentally change the scope of what is possible, Duke University astrophysicist, Michael Troxel, wrote in an email to The Daily Galaxy. “In terms of the impact of dark energy on the evolution of large-scale structure, we will be able see much larger volumes of space and further back in time. For the first time, we’ll be able to precisely constrain not just the average effect of dark energy over time with these kinds of surveys, but also how dark energy has potentially evolved over time, which may finally reveal conclusively if Einstein’s cosmological constant is correct. The Rubin Observatory in particular will observe a huge volume of space regularly every few days, enabling the study of how transient objects in the Universe like supernovae, which we use to help map the expansion history of the Universe, are changing over small timescales. It is hard to predict exactly what this flood of high quality data will enable us to discover, but there is no potential for a boring outcome.”
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