Earlier this year, a group of astronomers made a very high-profile announcement: They had, for the first time ever, detected the ripple structures in distant space indicative of gravitational waves, remnants of the theorized cosmic inflationary period that occurred after the Big Bang.The group, which used the BICEP2 telescope to make their observations, was met with immediate skepticism. For one thing, the skeptics argued, they had underestimated a crucial complicating factor: dust. As gravitational waves might leave a signature in the cosmic microwave background (CMB), dust might leave a similar mark.
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A report out this week from the European Space Agency found that there would have indeed been enough dust in the patch of sky being observed by the BICEP2 astronomers to account for the would-be gravitational wave observations. Posted to the arVix preprint server, the ESA study found that the signal frequencies involved with interstellar dust polarization and structures in the CMB are just too similar."We show that even in the faintest dust-emitting regions there are no 'clean' windows in the sky where primordial CMB B-mode polarization measurements could be made without subtraction of foreground emission," the ESA astronomers wrote, the foreground emission here being dust.
The dust component of the interstellar medium isn't all that much like dust as we know it
That in itself is an interesting notion: space is dirty. We might look up at the night sky and imagine a pristine cosmos, where blackness indicates emptiness. That's hardly the case, however, and it's often quite the opposite, with that blackness the result of dust clouds shielding us from the visible frequencies of light.Dust is part of what's known as the interstellar medium. The medium is mostly (99 percent) gas, about 75 percent hydrogen, with most of the rest being helium. If these gases become gravitationally unbalanced and collapse, the eventual result is a star.The dust component of the interstellar medium isn't all that much like dust as we know it. For one, grains of cosmic dust are tiny, a fraction of a micron across. Usually composed of carbon or some variety of silicate with a bit of iron or ice, these atoms and molecules might form into clumps, typically surrounding a star-in-progress. They might eventually clump enough to begin forming planets.
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Image: NASA
While interstellar dust can block out entire galaxies from view, as with the Horsehead Nebula above, its density is almost incomprehensibly low. We might find one single atom occupying a cubic-centimeter of otherwise mostly vacuum. Meanwhile, a cubic-centimeter of the air that we breathe will contain somewhere around 30,000,000,000,000,000,000 atoms, according astronomers at the University of New Hampshire.The usual size of interstellar dust particles happens to match well with the wavelength of blue light. This leads to another feature of these space-clouds called "reddening." As light from a distant galaxy travel through cosmic dust, its blue components get scattered, leaving the light that actually makes it to Earth appear more red. If an observer were to observe the same cloud from the side, it would appear to be blue. This is the case with the Egg Nebula, pictured above.
There's an interesting third possibility. A cloud of cosmic dust might act as a mirror, reflecting light from some bright object and directing it towards Earth. This is called a reflection nebula, an example of which was detected by the Hubble Space Telescope in 1999, a structure known just as NGC 1999.Dust is an old problem for astronomers. Edward Barnard is usually given credit for the first direct observation of interstellar dust clouds, describing them as "dark markings" and "curiosities of the sky" in a 1919 paper."I first did not believe in these dark obscuring masses," Barnard wrote. "The increase of evidence, however, from my own photographs convinced me later, especially after investigating some of them visually, that many of these markings were not simply due to an actual want of stars, but were really obscuring bodies nearer to us than distant stars." He went on to catalog a number of them, in part to aid in future study.The source of interstellar dust remains at least somewhat elusive. The going theory is that it comes from stars, a byproduct. "From the stellar winds of evolved stars, new dust is formed and is injected into interstellar space," explained astronomer Amara Graps in a 1996 report. "Young stardust is mixed with old, heavily-processed, diffuse ISM dust, and is subject to passing supernova shocks and ultraviolet radiation.""Dusty clouds form," she said. "The protostar environment is a fertile ground for solids on all size scales from dust grains to planets."So, it's hard to really resent these clouds of cosmic dust, blocking our view of Big Bang history or not. They're what we're all made of: star stuff.
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There's an interesting third possibility. A cloud of cosmic dust might act as a mirror, reflecting light from some bright object and directing it towards Earth. This is called a reflection nebula, an example of which was detected by the Hubble Space Telescope in 1999, a structure known just as NGC 1999.Dust is an old problem for astronomers. Edward Barnard is usually given credit for the first direct observation of interstellar dust clouds, describing them as "dark markings" and "curiosities of the sky" in a 1919 paper."I first did not believe in these dark obscuring masses," Barnard wrote. "The increase of evidence, however, from my own photographs convinced me later, especially after investigating some of them visually, that many of these markings were not simply due to an actual want of stars, but were really obscuring bodies nearer to us than distant stars." He went on to catalog a number of them, in part to aid in future study.The source of interstellar dust remains at least somewhat elusive. The going theory is that it comes from stars, a byproduct. "From the stellar winds of evolved stars, new dust is formed and is injected into interstellar space," explained astronomer Amara Graps in a 1996 report. "Young stardust is mixed with old, heavily-processed, diffuse ISM dust, and is subject to passing supernova shocks and ultraviolet radiation.""Dusty clouds form," she said. "The protostar environment is a fertile ground for solids on all size scales from dust grains to planets."So, it's hard to really resent these clouds of cosmic dust, blocking our view of Big Bang history or not. They're what we're all made of: star stuff.