The greatest wall

Sometimes simple words can be the hardest to define accurately.

For example, in physics, what do we mean by the word structure?  The easiest way to conceptualize it is that it's a material object for which whatever force is holding it together outcompetes any other forces that might be involved.  For example, a brick could be considered a structure, because the chemical bonds in the fired clay are stronger than the forces trying to pull it apart.  The sand on a beach, however, doesn't form a single structure, because the forces between the sand grains aren't strong enough to hold them together against the power of the wind and water.

Simple enough, it'd seem, but once you get out into space, it gets a little more difficult.

In astronomy, a structure is something that is bound together by gravity so that on some scale, it acts as a single unit.  The Solar System is a cosmic structure; within it, the gravitational pull of the Sun overwhelms all other forces.  The Milky Way is a cosmic structure by the same definition.  But how big can you get and still call it a single structure?  The question gives astronomers fits, because (to abide by the definition) you have to show that the pieces of the structure are bound together in such a way that the mutual gravitational attraction is higher than the other forces they experience -- and given that a lot of these things are very far away, any such determination is bound to rest on some fairly thin ice.

The largest generally accepted cosmic structure is the Hercules-Corona Borealis Great Wall, a galactic filament that (from our perspective) is in the night sky in the Northern Hemisphere in spring and early summer.  It's ten billion light years in length -- making it a little over a tenth as long as the entire observable universe!

[Image licensed under the Creative Commons Pablo Carlos Budassi, Hercules-CoronaBorealisGreatWall, CC BY-SA 4.0]

In the above image, each one of the tiny dots of light is an entire galaxy containing billions of stars; the brighter blobs are galaxy clusters, each made up of millions of galaxies.

And the whole thing is bound together by gravity.

What's kind of overwhelming about this is that because there are these enormous cosmic structures, there are also gaps between them, called supervoids.  One of the largest is the Boötes Void.  This thing is 330 million light years across, and contains almost no matter at all; any given cubic meter of space inside the void might have a couple of hydrogen atoms, and that's about it.  To put it in perspective; if the Earth was sitting in the center of the Boötes Void, there wouldn't be a single star visible.  It wouldn't have been until the 1960s that we'd have had telescopes powerful enough to detect the nearest stars.

That, my friend, is a whole lot of nothing.

What's coolest about all this is where these structures (and the spaces between them) came from.  On the order of 10^-32 seconds (that's 0.00000000000000000000000000000001 seconds) after the Big Bang, the bizarre phenomenon of cosmic inflation had not only blown the universe up by an amount that beggars belief (estimates are that in that first fraction of a second, it expanded from the size of a proton to about the size of a galaxy), it also smoothed out any lumpy bits (what the cosmologists call anisotropies).  This is why the universe today is pretty smooth and homogeneous -- if you look out into space, you see on average the same number of galaxies no matter which way you look.

But there are some pretty damn big anisotropies, like the Hercules-Corona Borealis Great Wall and the Boötes Void.  So where did those come from?

The current model is that as inflation ended, an interaction between regular matter and dark matter triggered a shock wave through the plasma blob that at that point was the entire universe.  This shock wave -- a ripple, a pressure wave much like a sound wave propagating through the air -- pushed some bits of the regular matter closer together and pulled some bits apart, turning what had been a homogeneous plasma into a web of filaments, sheets... and voids.

These baryon acoustic oscillations, that occurred so soon after the Big Bang it's hard to even wrap my brain around a number that small, are why we now have cosmic structures millions, or billions, of light years across.

So that's our mindblowing science for today.  Gravitationally-linked structures that span one-tenth of the size of the observable universe, and spaces in between containing damn near nothing at all, all because of a ripple that passed through the universe when it was way under one second old.

If that doesn't make you realize that all of our trials and tribulations here on Earth are insignificant, nothing will.

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Here are the answers to the puzzles from yesterday's post.  If you haven't finished thinking about them on your own, scroll no further!

1.  The census taker puzzle

The first clue is that the product of the daughters' ages is equal to 36.  There are eight possible trios of numbers that multiply to 36: (1, 1, 36), (1, 2, 18), (1, 3, 12), (1, 4, 9), (1, 6, 6), (2, 2, 9), (2, 3, 6), and (3, 3, 4).  Clue #2 is that the ages sum to equal the house number across the street, so the next step is to figure out what the house number could be.  Respectively: 38, 21, 16, 14, 13, 13, 11, and 10.

The key here is that when the census taker looks at the house number across the street, he still can't figure it out.  So it can't be (1, 4, 9), for example -- because if it was, as soon as he saw that the house number was 14, he'd know that was the only possible answer.  The fact that even after seeing the house number, he still doesn't know the answer, means it has to be one of the two trios of numbers that sums to the same thing -- 13.  So it either has to be (1, 6, 6) or (2, 2, 9).

Then, clue #3 is that the man's oldest daughter has red hair.  In the first possibility, there is no oldest daughter -- the oldest children are twins.  So his daughters have to be a nine-year-old and a pair of two-year-old twins.

2.  The St. Ives riddle

The answer is one.  "As I was going to St. Ives..." -- it doesn't say a thing about where the other people he met were going, if anywhere.

3.  The bear

It's a white bear.  The only place on Earth you could walk a mile south, a mile east, and a mile north and end up back where you started is if your starting place was the North Pole.

4.  A curious sequence

The pattern is that it's the names of the single digit numbers in English, in alphabetical order.  So the next one in the sequence is 3.

5.  Classifying the letters

The letters are classified by their symmetry.  (The capital letters only, of course.)  Group 1 is symmetrical around a vertical line, Group 2 around a horizontal line, Group 3 is around either a horizontal or a vertical line, Group 4 has no line symmetry but is symmetrical through a 180-degree rotation around their central point, and Group 5 are asymmetrical.

6.  The light bulb puzzle

Turn on switch one, and leave it on.  Turn on switch two for ten minutes, then turn it off.  Leave switch three off.  Go up to the tenth floor.  The bulb operated by switch one will be on; the one operated by switch two will be dark, but hot; and the one operated by switch three will be dark and cold.

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The cosmic net

A fascinating new piece of research by the astrophysicists is using information from twelve billion light years away to elucidate what happened 13.8 billion years ago, at the moment of the Big Bang -- they've found the long-hypothesized "cosmic filaments," streaks of (mostly) hydrogen that extend from galaxy to galaxy and cluster to cluster.  In a paper that appeared in Science this week, titled "Gas Filaments of the Cosmic Web Located Around Active Galaxies in a Protocluster," a team led by Michele Fumagalli of Durham University found hard evidence of yet another prediction of the Big Bang Theory: that random variations ("anisotropies") in the first fraction of a second of the universe led to clumps of matter connected by streamers, with huge voids in between.

The authors write:

Cosmological simulations predict that the Universe contains a network of intergalactic gas filaments, within which galaxies form and evolve.  However, the faintness of any emission from these filaments has limited tests of this prediction.  We report the detection of rest-frame ultraviolet Lyman-α radiation from multiple filaments extending more than one megaparsec between galaxies within the SSA22 protocluster at a redshift of 3.1.  Intense star formation and supermassive black-hole activity is occurring within the galaxies embedded in these structures, which are the likely sources of the elevated ionizing radiation powering the observed Lyman-α emission.  Our observations map the gas in filamentary structures of the type thought to fuel the growth of galaxies and black holes in massive protoclusters.

So very early on, the universe was a network of thin (well, thin on a cosmic scale, anyhow) filaments of matter, and where they crossed the matter density was high enough to trigger star, and eventually galaxy, formation.

The large-scale structure of the universe.  Each of those pale blue curves is made up of millions, possibly billions, of galaxies.  [Image is in the Public Domain, courtesy of NASA]

Almost against my will I was reminded of a rather captivating image from Buddhist philosophy called "Indra's Net."  I first ran into this when I was an undergraduate, and I and some friends took a class in which we were required to read Douglas Hofstadter's mindblowing chef d'oeuvre, entitled Gödel, Escher, Bach: An Eternal Golden Braid.  This book, which combines charm, wit, music, art, and a confounding amount of high-level number theory, was a fascinating read, but there are large parts of it that -- although I was a reasonably good math student, and in fact minored in the subject -- went over my head so fast they didn't even ruffle my hair.

But the parts that I found accessible were brilliant, and he drew together a great many disciplines -- one of which was Zen Buddhism.  In that section, he described the great net that stretches across the universe as follows:

The Buddhist allegory of "Indra's Net" tells of an endless net of threads throughout the universe, the horizontal threads running through space, the vertical ones through time.  At every crossing of threads is an individual, and every individual is a crystal bead.  The great light of "Absolute Being" illuminates and penetrates every crystal bead; moreover, every crystal bead reflects not only the light from every other crystal in the net—but also every reflection of every reflection throughout the universe.

It's a cool metaphor for interconnectedness in whatever realm you like to apply it to, be it social interactions, ecological connections, the evolutionary tree of life, whatever.  But I always hesitate to bring this kind of thing up, because it's so tempting to take the metaphor as the reality -- the heart of the problem with books like Frijtof Capra's The Tao of Physics and Gary Zukav's The Dancing Wu-Li Masters, where the authors take a rather hand-waving explanation of quantum physics (about all you can do if you remove the math), draw some comparisons to Taoist and Buddhist philosophy, and forthwith conclude that the success of quantum physics as a model shows that Taoism and/or Buddhism is the real explanation for everything we see.

Confusing the model for the reality is a hazard on a lot of levels, and I had to watch that constantly when I was teaching.  I had a number of analogies I used -- the Krebs Cycle as a merry-go-round where two kids get on and two kids get off on every turn, active transport gateway proteins as revolving doors you have to pay to use, DNA as a universal recipe book.  I tried to keep the comparisons so silly that there was no way anyone would think they were real, but I still remember the student who started an essay, "So, antibodies are trash tags..."

But the comparison between the cosmic filaments crossing and generating galaxies at each intersection, and the magical Net of Indra spanning the cosmos with a reflecting jewel everywhere the threads cross, was just too pretty not to mention.  I hope it won't get in the way of your appreciation of the actual research, though, which is even more beautiful.  It's astonishing that sitting here, on this little spinning ball in the outer reaches of a quite ordinary galaxy, we've been able to learn about the structure of the universe from the very largest scales to the very smallest.  So whatever else you can say about human accomplishments, you have to admit that one is pretty impressive.

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This week's Skeptophilia book recommendation is by the team of Mark Carwardine and the brilliant author of The Hitchhiker's Guide to the Galaxy, the late Douglas Adams.  Called Last Chance to See, it's about a round-the-world trip the two took to see the last populations of some of the world's most severely endangered animals, including the Rodrigues Fruit Bat, the Mountain Gorilla, the Aye-Aye, and the Komodo Dragon.  It's fascinating, entertaining, and sad, as Adams and Carwardine take an unflinching look at the devastation being wrought on the world's ecosystems by humans.

But it should be required reading for anyone interested in ecology, the environment, and the animal kingdom. Lucid, often funny, always eye-opening, Last Chance to See will give you a lens into the plight of some of the world's rarest species -- before they're gone forever.

[Note: if you purchase this book using the image/link below, part of the proceeds goes to support Skeptophilia!]

A whole lot of nothing

Anybody have any ideas about what this is?

I've shown a bunch of people, and I've gotten answers from an electron micrograph of a sponge to the electric circuit diagram of the Borg Cube.  But the truth is almost as astonishing:

It's a map of the fine structure of the entire known universe.

Most everyone knows that the stars are clustered into galaxies, and that there are huge spaces in between one star and the next, but far bigger ones between one galaxy and the next.  Even the original Star Trek got that right, despite their playing fast and loose with physics every episode.  (Notwithstanding Scotty's continual insistence that you canna change the laws thereof.)  There was an episode called "By Any Other Name" in which some evil aliens hijack the Enterprise so it will bring them back to their home in the Andromeda Galaxy, a trip that will take three hundred years at Warp Factor 10.  (And it's mentioned that even that is way faster than a Federation starship could ordinarily go.)

So the intergalactic spaces are so huge that they're a bit beyond our imagining.  But if you really want to have your mind blown, consider that the filaments of the above diagram are not streamers of stars but streamers of galaxies.  Billions of them.  On the scale shown above, the Milky Way and the Andromeda Galaxy are so close as to be right on top of each other.

What is kind of fascinating about this diagram -- which, by the way, is courtesy of NASA/JPL -- is not the filaments, but the spaces in between them.  These "voids" are ridiculously huge.  The best-studied is the Boötes Void, which is centered 700 million light years away from us.  It is so big that if the Earth were at the center of it, we wouldn't have had telescopes powerful enough to see the next nearest stars until the 1960s.

That, my friends, is a whole lot of nothing.

What I find most mind-bending about the whole thing, and in fact what sent me down this particular rabbit hole this morning, is that the location of the filaments is thought to reflect quantum fluctuations in the matter immediately after the Big Bang, when the whole universe was only a fraction of a centimeter across.  As inflation took over and the universe expanded, those tiny "anisotropies" -- unevenness in the composition of space -- were magnified until you have filaments which are densely filled and gaps where there is almost nothing at all, and the universe resembles a Swiss cheese made of stars.

Of course, I'm using "densely filled" in a comparative sense.  The cold vacuum of space between the Sun and Proxima Centauri, the nearest star, really doesn't have much in it.  Dust, comets, possibly a rogue planet or two.  But even this is jam-packed by comparison to the Boötes Void and the others like it, wherein it is thought there are light-years of space without so much as a single hydrogen atom.

All of which makes me feel awfully small.  Our determination to act as if what happens down here is of cosmic import is shaken substantially by looking up into the night sky.  It's smashed to smithereens by considering the scale of the largest structures in the universe, which are threads of billions of stars making up a latticework -- and between which there is nothing but eternal silence and such profound darkness that it contains not even a single star close enough to see.

I don't know about you, but that makes me want to climb back under the covers and hug my teddy bear for a while.

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This week's recommendation is a classic.

When I was a junior in college, I took a class called Seminar, which had a new focus/topic each semester.  That semester's course was a survey of the Book Gödel, Escher, Bach: An Eternal Golden Braid by Douglas Hofstadter.  Hofstadter does a masterful job of tying together three disparate realms -- number theory, the art of M. C. Escher, and the contrapuntal music of J. S. Bach.

It makes for a fascinating journey.  I'll warn you that the sections in the last third of the book that are about number theory and the work of mathematician Kurt Gödel get to be some rough going, and despite my pretty solid background in math, I found them a struggle to understand in places.  But the difficulties are well worth it.  Pick up a copy of what my classmates and I came to refer to lovingly as GEB, and fasten your seatbelt for a hell of a ride.

[If you purchase the book from Amazon using the image/link below, part of the proceeds goes to supporting Skeptophilia!]