Cosmic Dust
And yet, to me, what is this quintessence of dust?” Hamlet, Act 2
You may have heard fanciful tales of physicists talking in hushed tones about strange-sounding things like the multiverse, wormholes, and dark energy. These esoteric wonders, while delightful to dream about, remain inscrutable, far from definitively proven. Indeed, they may not exist and, worse yet, we may never, ever have any evidence of their existence.
While physicists wax philosophical on the latest cosmic sensations, the impact of the most essential substance in the Universe — dust — remains largely unexplored.
Dust is a humble substance. It literally litters the cosmos. But what do astronomers mean when they talk about dust, and where does dust come from? The ancient Incas were the first astronomers to describe the appearance of dust. They saw dust as the stuff of life (Figure 1). These Incan astronomers were not too far off. We now know that the dark dusty rivers concealing the Milky Way’s stellar luminaries perhaps even harbor life itself, in regions such as the so-called pillars of creation.
Figure 1. The dusty constellations of the Incas as seen from the Atacama Desert of Northern Chile. © Shaffer Grubb
Our knowledge about cosmic dust grains comes from their interaction with starlight. New information is also now arriving from cosmologists like me studying the Universe’s oldest light: the cosmic microwave background radiation, the fossil heat left over from the formation of hydrogen and helium a few minutes after the Big Bang. In looking back to the Big Bang, cosmologists unavoidably must look through a lot of dust.
Humble, but essential
According to the current best models of the formation of our solar system, interstellar dust allows for the formation of planets, themselves the product of so-called protoplanetary disks. In these disks, tiny grains of rock and ice congeal over time into kilometer-wide “planetesimals.” With enough mass, a runaway accretion process can produce embryonic versions of planets in under a million years. These embryos are the progenitors of rocky planets like the Earth, forming in the hundreds of millions of years following the embryonic phase.
When the Voyager 1 space probe turned its cameras back toward the inner solar system on Valentine’s Day, 1990, it saw the Earth, bathed in starlight from our Sun (Figure 2). In a beautiful paean, Carl Sagan captured the significance of our planet as a mere “mote of dust suspended on a sunbeam.” 1 What we’ve learned about other planets since Sagan’s quip has only magnified our insignificance. The Kepler space telescope has shown astronomers that there are at least as many planets in our galaxy as stars. Stars like TRAPPIST-1 even harbor multiple planets in the so-called habitable zone, where liquid water, considered by many scientists to be a prerequisite for life to arise, can exist on the surface of a suitable rocky planet like the Earth.
Figure 2. Pale Blue Dot, by Voyager 1 NASA. Public domain.
Through a telescope, dimly
Astronomers were slow to appreciate dust’s importance. For much of astronomy’s history, dust seemed irrelevant. Dust has long been our galaxy’s “dirty windshield,” obscuring the view of the road ahead, and history’s most renowned astronomers — from Galileo to Herschel to Shapley — have been deceived by dust.
Galileo was the first astronomer to be fooled by dust. He was tricked into thinking the dust in the Pleiades was the result of innumerable unresolved stars. Soon after Galileo, the English astronomer William Herschel was deceived into thinking the Sun was at the center of the galaxy. Again, it was dust that duped him into thinking so. A few years later, the American astronomer Harlow Shapley ignored dust, which led him to vastly overestimate the Milky Way Galaxy’s size, implying our galaxy was so big it had to be the entire Universe. In in the mid-1900s, astronomers slowly began to realize that dust was behind all these misapprehended apparitions.
Only recently has dust’s importance been recognized. In dust’s latest act, it has impacted the quest to ascertain whether our Universe is unique, or whether it is one of an infinite number of universes within the so-called multiverse. The multiverse controversy is a natural continuation of a series of debate–first initiated by Copernicus and Galileo–revolving around a central question: Are we humans central, in any way, to the existence or arrangement of the Universe? Dust often makes it appear that we are. In these debates, only one viewpoint could be correct: We were either at the center of our solar system or galaxy, or we weren’t. Ours was either the only solar system or galaxy, or it wasn’t. Yet the one constant in all these debates was dust.
In 2014, using observations of the Universe’s oldest light — the cosmic microwave background radiation (CMB) — the BICEP2 team announced the discovery of what was believed to be the spark that ignited the Big Bang: the so-called inflationary epoch. Inflation was created to explain certain missing ingredients in the standard model of the cosmos’s formation, the Big Bang. And inflation is intimately related to the multiverse: many astronomers believe you can’t have the former without having the latter.
But, less than a year after BICEP2’s historic announcement in early 2015, this signal, called “B-mode polarization” was shown to be caused by the same source that had bedeviled astronomers for centuries: tiny grains of dust, produced from early generations of stars that exploded as supernovae in our galaxy. These grains of dust aligned with the Milky Way galaxy’s magnetic field, mimicking the exact inflationary pattern the BICEP2 was seeking. But dust is not all bad: some of the detritus in this star dust includes iron which eventually like that in the hemoglobin flowing through our veins. Cosmic dust not only made our planet, it makes us, as well!
Throughout the history of modern astronomy, dust has been the Janus-faced spoiler and savior, creator and concealer, founder and flummoxer of astronomical aspirations. To understand dust is to understand the life cycle of the cosmos and the objects that inhabit it, including us, and requires a deep dive into the astrophysics of dust as well as a dose of cosmic humility, for as Mahatma Gandhi said: “The seeker after truth should be humbler than the dust.”
