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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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 a close-up of a block of ramen 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 seven hundred 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, and the skies every night would be a uniform pitch black.

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 (extremely deep) 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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The cosmic spiderweb

I've posted here before about dark matter, the mysterious substance which -- if current models are even close to correct -- makes up 27% of the matter/energy density of the universe.  Even more mysterious is dark energy, which is thought to be responsible for the acceleration of the universe's expansion; that's another 68% percent.

Yes, the implication is what it sounds like.  Ordinary matter -- like you, me, the Earth, all the planets and stars and galaxies -- accounts for a measly 5% of the matter/energy in the universe.

Most frustrating of all is how little we know for sure about dark matter and dark energy.  To date, every experiment to detect a particle of either one has failed.  In fact, we don't even know if either one is made of particles.  (If not, what it might be made of is an open question.)  Some scientists have compared it to the luminiferous aether, which according to nineteenth century physicists was the substance through which light waves allegedly propagated.  Every wave they knew about traveled in some sort of medium; water waves, sound waves in the air, vibrations in a fiddle string.  That light might travel in a vacuum -- that it might not need a medium -- was incomprehensible.

In the words of one of my college physics professors: "If light waves don't need a medium, then what, exactly, is waving?"

It took Einstein to come up with the answer to this, and in the process proved that the luminiferous aether didn't exist.  The result revolutionized physics.  I've heard physicists say that dark matter and dark energy are this century's aether -- artifacts of measurement created by a fundamental piece of our model being misunderstood, missing, or flat-out wrong.

Be that as it may, if dark matter is an error, it's a pretty persistent one.  We haven't been able to detect it other than by its gravitational signature, but that signature is a bold flourish.  The discovery of it, by astronomer Vera Rubin and others, came about because measurements of the spin rate of galaxies indicated there had to be some extra mass holding them together; at the measured rotation rates, they should fly apart.  That extra mass turned out to be huge.  The best estimates were that there had to be over five times as much of this invisible matter as there was ordinary matter -- and that estimate held for every galaxy studied, so it wasn't a local phenomenon.

A paper last week in The Astrophysical Journal adds a new layer to dark matter not being local.  Astrophysicists Sungwook Hong, Donghui Jeong, Ho Seong Hwang, and Juhan Kim, of Pennsylvania State University, have created the most detailed map yet of the dark matter in the universe, using the known motion of seventeen thousand galaxies.  Strangest of all is that whatever this weird, invisible -- but extremely common -- substance is, it's not distributed uniformly.

Not only are there lumps of it within galaxies, holding them together, there are long filaments of dark matter between them -- threading the galaxies together like dewdrops clinging to an enormous spiderweb.

Another feature that makes the spiderweb analogy even more apt is that there are huge voids, nearly devoid of... just about everything, including dark matter.  One of them, the Boötes Void, is 330 million light years across.

[Image licensed under the Creative Commons El C at English Wikipedia., Boovoid, CC BY-SA 2.5]

To put that number in some perspective, if you took the Sun and the rest of the Solar System and put it in the middle of the Boötes Void, the night sky would be completely dark.  No stars at all.  In fact, it wouldn't have been until the 1960s that we would have had telescopes powerful enough to see the nearest galaxies; until that time, we would have thought the Sun was the only star in the entire universe.

So whatever dark matter is, we're gradually closing in on it.  We know how it affects ordinary matter gravitationally, and now we have a map of how it's distributed in the universe.  Maybe soon we'll have an idea of what it actually is.

Of course, then we still have dark energy to tackle, and there's over twice as much of that stuff as there is dark matter.  So, as is usual in science, we're not going to run out of mysteries to investigate any time soon.

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Astronomer Michio Kaku has a new book out, and he's tackled a doozy of a topic.

One of the thorniest problems in physics over the last hundred years, one which has stymied some of the greatest minds humanity has ever produced, is the quest for finding a Grand Unified Theory.  There are four fundamental forces in nature that we know about; the strong and weak nuclear forces, electromagnetism, and gravity.  The first three can now be modeled by a single set of equations -- called the electroweak theory -- but gravity has staunchly resisted incorporation.

The problem is, the other three forces can be explained by quantum effects, while gravity seems to have little to no effect on the realm of the very small -- and likewise, quantum effects have virtually no impact on the large scales where gravity rules.  Trying to combine the two results in self-contradictions and impossibilities, and even models that seem to eliminate some of the problems -- such as the highly-publicized string theory -- face their own sent of deep issues, such as generating so many possible solutions that an experimental test is practically impossible.

Kaku's new book, The God Equation: The Quest for a Theory of Everything describes the history and current status of this seemingly intractable problem, and does so with his characteristic flair and humor.  If you're interesting in finding out about the cutting edge of physic lies, in terms that an intelligent layperson can understand, you'll really enjoy Kaku's book -- and come away with a deeper appreciation for how weird the universe actually is.

[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!]