This Is How Dark Energy’s Main Competitor Failed
20 years ago, our understanding of the Universe underwent a revolution. For generations, we had known the Universe was expanding, but we didn’t know its fate. Whether it would recollapse (with gravity defeating the expansion), expand forever (with the expansion defeating gravity), or live right on the border between the two cases (with expansion and gravity perfectly balanced) was one of cosmology’s greatest open questions.
Then, in 1998, two independent teams — the high-z supernova search team and the supernova cosmology project — both released their results that showed that ultra-distant supernovae were far too faint to be consistent with any of these. The Universe wasn’t just expanding, the expansion was accelerating. Expansion defeats gravity, and a new form of energy was required to explain the observations: dark energy.
But many scientists were skeptical. After all, if things were fainter than expected, maybe the Universe wasn’t accelerating. Maybe it was just dust? For years, that notion was the main competing idea to dark energy. Here’s how it died.
The way the Universe expands is inextricably linked to the matter and energy present within it. A Universe dominated by matter will expand differently than one dominated by radiation; the composition of your Universe and how it changes over time determines how it expands. Because of this, a primary goal of cosmology, for a long time, was to measure two major features: the expansion rate and how it changes over time.
But we can’t measure the expanding Universe directly. We can only measure objects within the Universe. So we don’t measure the Universe’s expansion; we measure how bright or how big objects appear to be. If we know some things about them — their intrinsic brightness, their apparent brightness, and their redshift — we can infer their distance from us, and use that to calculate the expansion history of the Universe.
Unless, of course, there’s a confounding, polluting factor in there. If you knew you had a 60-watt light bulb and you observed it to have a particular brightness, you’d be able to calculate how far away it is. The brightness-distance relation is very simple: observed brightness falls off as the inverse of the distance squared (b ~ 1/r2).
But if it’s foggy out, you’re going to have a problem. The light will appear fainter than the simple brightness-distance relation predicts, in proportion to the density of the fog. If you just measured that distant light and applied the brightness-distance relation, you’d conclude its distance was greater than it actually is. Your results would be biased, because you didn’t account for the fact that something is blocking a portion of the light.
So if you apply this logic to these fainter-than-expected supernova, you might wonder if there was some kind of cosmic “fog” blocking this distant light. We don’t have fog in the Universe, but we do have light-blocking dust. And if you put enough dust at great enough distances, you could potentially explain why supernovae appear fainter without dark energy. It’s the first thing you’d consider; additional dust is far less of a revolution than a new type of energy permeating the Universe.
So that became a proposition: there was some additional dust in the distant Universe, and the reason the supernovae appeared fainter wasn’t because they were farther away due to an extra expansion of space, but because dust was blocking the light.
Dust grains, however, come in particular sizes, and the size of the dust grains determines which wavelengths of light are preferentially blocked, with most dust better at blocking blue than red light. This is why there are many dark nebulae in the Universe that block the visible light, but if you look with an infrared telescope, you can see the stars behind that nebula.
Measurements of different wavelengths of light, however, didn’t show a preferential light-blocking phenomenon. They instead showed that both red and blue light were reduced by equal amounts. You might think that rules out dust as an explanation, but that’s not necessarily so. What if the dust in the distant Universe was of a new type, that blocked all the wavelengths of light equally?