This article was published in Scientific American’s former blog network and reflects the views of the author, not necessarily those of Scientific American
...living in a place that makes doing cosmology hard.
Let's backtrack a little. Unless you've been living under a particularly thick and insulating rock you'll know that in recent months the world of experimental cosmology (what would have previously been called observational cosmology, or just plain old astronomy) has been on tenterhooks waiting to see if the BICEP2 measurement of polarized cosmic microwave background radiation is truly a signature of an inflationary universe, or - to put it bluntly - dust.
The tale has been told well in many places, but the key criticism of the BICEP2 data is that the mucky old galaxy - the Milky Way - could be entirely responsible for the signal that's being seen. It's a galactic foreground, part of a veil between us and the cosmos. Now, with the publication of results from the Planck satellite, it seems increasingly likely that this is the case (although the lid is by no means sealed).
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Microscopic dust - grains of silicates, carbon, aluminum oxide, and other compounds, perhaps coated with water or organics - is everywhere in our galaxy, just as it is in most galaxies. It's the product of stars themselves - condensing and crystallizing material cast into the void. We can get an idea of the sheer extent of this filth by taking a look at our cosmic neighbor the Andromeda galaxy - here's a mid-infrared map made with NASA's Spitzer observatory showing where the 'warm' dust is (temperatures greater than some 20 Kelvin).
It's a lot of dust, and it's all over the place. The Milky Way is in a similar condition and, if you're inside a galaxy like this, your view of the universe is going to be complicated. And it's not just dust that gets in the way. There's also interstellar gas, and there are stars.
Of course, like Andromeda, our galaxy has a disk-like shape. While its diameter is around 100,000 to 120,000 light years, the thickness, or depth, of where stars are distributed can narrow to just a couple of thousand light years. This is why we can look up and see the plane of the disk, the milky splash across the darkened sky that gave our galaxy its name.
All of this obscuration of what lies beyond has long impacted cosmological studies. Maps made of the positions of distant galaxies have often simply cut out the parts of the sky where its hard to find these objects, or left them blank. Here's an example of one of the best maps we have of the location of relatively nearby galaxies (each dot is a galaxy).
The 2MASS survey of tens of thousands of 'nearby' galaxies, plotted in Galactic coordinates, where the Milky Way lies across the horizontal.
That dark horizontal stripe? That's the plane of the Milky Way, hiding who-knows-what from us.
It's a big problem if you're trying to, for example, figure out the gravitational pull on our local group of galaxies, or indeed whether or not we're part of much larger cosmic structures - like a supercluster of galaxies.
The problem isn't just that our view of the cosmos is obscured, galactic stuff also gets confused with extragalactic stuff. Before astronomers like Slipher and Hubble managed to measure the recessional velocity of distant galaxies in the early 1900s it wasn't clear that these objects were anything more than other 'nebulosities' - blobs of light that wouldn't resolve into individual stars, like the true nebulae of molecular gas and dust that dot the Milky Way's spiral arms. Similarly, there is a reason why quasi-stellar radio sources, or quasars, are named this way. Visible light images of these objects made in the 1960s looked like what you'd see peering at a star in our galaxy - a point of light. Until their true (vast) cosmological distance was measured, astronomers generally assumed these peculiar things were in our galaxy, not for a moment realizing that they were actually the distant homes of supermassive black holes busy tearing up matter.
And then there's the cosmic microwave background. Here's an older map from ESA's Planck satellite, showing what experiments have to contend with.
The blue-white swirls and clouds represent microwave emission and obscuration from the Milky Way's gas and dust, seen from our location (the plane of the galaxy running horizontally across the middle here). The mottled orange-mauve stuff? Some of that is primordial, the imprint of structure in our universe when it was just a few hundred thousand years old - a bit of gold at the end of the cosmologists rainbow.
It's easy to see how tough this is. We're sitting deep inside a lot of murk that can absorb or emit radiation at the same wavelengths we'd like to be peering out into the distant universe. This is one reason why an immense amount of astronomical effort goes into characterizing our own back yard, so we can better peer through the gaps in the fence.
But if we step back for a broader perspective there is something interesting about this situation. Cosmologists have pointed out that in many respects we exist at a particularly good time for studying the universe. Five billion or more years ago we'd have been hard pressed to detect evidence for an accelerating cosmic expansion. A hundred billion years in the future and that same expansion will have isolated our galaxy from the electromagnetic signal of most of the rest of the universe - making it a lonely island where it may be impossible to correctly deduce the underlying properties of the surrounding cosmos.
Yet where we are is not ideal for cosmology. The galactic swirl of wonderfully enriched elements, and plentiful stars and planets that has enabled life to emerge here, also complicates - and sometimes confounds - attempts to understand universal truths.
What, if anything, do we learn from this? For one thing, it's a good reminder that whenever someone suggests that there is something 'special' or optimal about our place in the universe they're probably not an experimental cosmologist. It's also a reminder that the cosmos has yet to lay its secrets at our feet, and that many surprises must surely await.
