image: oswaldo
By now you've probably heard about the hole in the universe, but it's an interesting enough discovery to bear repeating. Larry Rudnick, a University of Minnesota astronomy professor, is reporting in Astrophysical Journal that his team has fortuitously come across a cosmic void a billion light years wide(!), some thousand times larger than any that should exist based on our current astrophysical models. Rudnick evidently ran across this mother of all vacuums while examining swaths of sky taken by the National Radio Astronomy Observatory radio telescope. One area caught his eye, because a huge percentage of the expected matter seemed to be missing; checking that spot against CMB data he found a corresponding spot of deep cold, right there. Conclusion: a whole lot of nothing, "concentrated" in one region.
Science writers tend to stumble over funny sentences like that one when discussing voids, because voids aren't "things," exactly. They are the absence of things. Voids are just titanic empty spaces, places where nature is currently doing nothing, like a long series of rest notes in the otherwise busy cosmic symphony. They contain no matter, including dark matter: no galaxies, no stars, no gas. And, following on the symphonic metaphor, the particular passage Rudnick has just found seems to have been written by John Cage: the silence goes on, and on, and on.
Interestingly, it's the very emptiness of voids that makes them hard to visualize, without tacitly picturing the matter around them (at least for me). This "pure" quality tends also to bring up a host of philosophical issues that surround the question of what space and time really are, as distinct from what our models of them are. For example:
Does space owe its existence to the presence of matter? Try to imagine a pure spacetime universe, finite or infinite, with no mass in it anywhere. Is such a structure conceivable?
Could the spacetime in such a universe be said to have a curvature of its own, without the presence of mass? (Say, could it be bounded?) What would that mean?
Imagine a universe with only two objects in it: Rock A and Rock B. How large is this universe? Does it have a meaningful volume beyond the line that connects A and B?
Imagine a universe with one object in it, Rock A. Rock A has a firecracker inside it that, when it explodes, blasts A to pieces, all of which drift away from the point of explosion. Is the universe getting larger to accommodate the motion of the pieces? Is space being created as they move?
Are the above questions ones of cosmology, of physics, or merely definitional questions? Are they "merely" word games? Or something else?
A more recent addition: Does the discovery that space has an energy density of its own alter the traditional question of what "empty space" is, or render it invalid?
Such questions are by no means pointless. Einstein used to ponder such pleasantly mind-tingling issues, a habit of thought that led eventually to his highly productive "gedanken" experiments ("Suppose you are standing inside a train and a friend is standing on the platform . . ."). My personal favorite of the brain-teasers Einstein used to ask himself is: suppose you are holding an open box. Inside the box is a certain volume of space. When you move the box around, does the space inside it change or stay the same?
Going back to Rudnick's discovery: cosmic voids have been found before, and are not in themselves surprising. These matter-deserts are thought to come about simply as a matter of gravitational course: slow accretion draws all the massive stuff together, and what's left over is a gradually "expanding" oasis of emptiness between gravity wells. (A passing thought: a cosmic void should still be full of virtual particle pairs. Could such as structure be used to study the properties of these elusive entities, far removed from any real baryons?) What's shocking about this one is the same thing that ties it to Foundational issues: its colossal size.
image: Ralf9
A six billion trillion mile wide hole, if that's really what we're looking at, shows how much we don't yet understand about the evolution of cosmic structure. Not long ago the visible stars were thought to be the whole Universe. Next came the discovery of the galaxy, and other galaxies; then many, many other galaxies; then galactic clusters; then super clusters; then sheets, filaments, and voids; and now, perhaps, super-voids. I suspect -- little more than a hunch, but the success of COBE, WMAP and other projects suggest as much -- that the next era of cosmology will be driven by increasingly detailed observations of super-large-scale cosmic structure.
Until now, we've been looking at relatively tiny pieces of the observable universe. It may be that when we finally see the whole, an overall pattern will finally emerge that no one expected. What will that pattern be? Something beyond our ability to fathom, permanently mysterious? Something that makes sense to human thought, a grand and inspiring order? Or will it be something "obvious," a pattern we should have realized was there all along, had we only thought . . .?