Fifty years ago, our understanding of space included only some of the properties we now find most intriguing from the standpoint not only of physics but also of potential propulsion. Dark energy was not suspected then, while Fritz Zwicky’s inference of dark matter (1933) wouldn’t really inspire a wave of investigations until the 1970’s. The presence of the quantum fluctuations that would later be dubbed Zero Point Energy had only recently been examined. For that matter, the cosmic microwave background was still almost a decade from discovery.
Knowing that such properties exist out there in the cosmos offers the potential of future technologies that might be able to make use of them. But clearly, we are a long way from understanding whether or if such phenomena could eventually be harnessed. Just how far becomes apparent every time we get new dark energy news, as we recently did from the University of Toronto, where astronomers studying supernovae in nearby galaxies found when comparing them to far more distant ones that the older supernovae were brighter.
That adds yet another problem to the dark energy picture, because the assumption of uniform brightness in these exploding stars has been helpful in understanding the universe’s accelerated expansion. Correcting for varying brightness could be a thorny problem, says Andrew Howell, lead author of the study:
“You can think of supernovae as light bulbs. We found that the early universe supernovae had a higher wattage, but as long as we can figure out the wattage, we should be able to correct for that. Learning more about dark energy is going to take very precise corrections though and we aren’t sure how well we can do that yet.”
We’d also like to know whether dark energy varies over cosmic time. After all, the term in play is ‘cosmological constant,’ but just how constant is it? Trying to figure out how dark energy behaved in the early universe may require studying radio emissions from neutral hydrogen, redshifted by the expansion of the universe. Stuart B. Wyithe (University of Melbourne) and Avi Loeb (Harvard-Smithsonian Center for Astrophysics) want to study such emissions, believing that although most hydrogen from that era was ionized, surviving neutral clumps may still be detectable.
Does neutral hydrogen show the same kind of distribution patterns as galaxies, caused by early fluctuations in energy density and pressure? Studying its distribution should provide clues as to dark energy’s role in the first few billion years. Wyithe and Loeb believe instruments now being built such as the Murchison Wide-field Array could detect the faint signals from the neutral hydrogen that could tell the tale.
Given the revolution in our understanding of the universe’s expansion, we can assume that the investigation of dark energy will continue to be a primary thrust of modern physics. Which reminds me that a focus of the Breakthrough Propulsion Physics program at NASA was to introduce a targeted perspective — applications to spaceflight — within which recent physics advances could be studied, in hopes of generating new lines of inquiry. The hope: Even if propulsion breakthroughs do not emerge, their study may add to the pool of scientific knowledge.
That’s a worthy goal, and we will need all the perspectives we can find to approach problems as thorny as dark energy. The Toronto paper is Howell et al., “Predicted and Observed Evolution in the Mean Properties of Type Ia Supernovae with Redshift,” Astrophysical Journal 667, pp. L37-L40 (abstract). You can find Loeb and Wyithe’s paper “Fluctuations in 21cm Emission After Reionization” here. The mind boggles at how vast the future bibliography of dark energy studies will become.