16. Self organization of large-scale structures in the universe
(continued) Chapter 2. Long Evolution: Universe Emerging
Large-scale structures and their contents
Looking at the earliest phases of the universe’s birth and evolution, we have considered both the infinitely large and the infinitely small, all together, each a part of the other: the big bang itself; the quantum particles produced by the bang, the few light elements which soon followed; the rapid expansion of everything as a result of the very big bang; and the subsequent condensing or “pulling together” effect which gravity had on clumps of matter even while the universe continued expanding. The stage was set.
Using the forces at hand – the outflinging and expansion caused by the explosive big bang, coupled with the pulling together caused by gravity as soon as it emerged after the bang – structures began forming themselves in the early universe.
Laniakea’s children
Before the first stars or galaxies or indeed any structures at all were born, the universe was a non-uniform sea of atoms and miscellaneous subatomic particles constituting huge, irregular clouds of “gas and dust.” From out of these there evolved clouds-within-clouds that would each become, in time, a galaxy. And the galaxies were forming within clusters of galaxies. And the galaxy clusters were forming within superclusters – i.e., clusters of clusters. Of considerable interest in this context, the July 2016 issue of Scientific American magazine informs us that astronomers have succeeded in defining our positions – yours and mine – within the universe. It goes as follows:
From your street and ZIP code move outward to your state, nation, continent and planet which of course is earth. Earth is the third planet out from our personal star which we call Sol. Sol, along with its four small rocky planets, four more giant gassy planets and assorted planetesimal bodies, comets and plentiful debris, is situated in a neighborhood called the Orion Spur, which is one segment of one of the spiral arms far, far out from the center of our Milky Way galaxy.
The Milky Way galaxy is itself a respected member of a cluster of more than fifty other galaxies known as The Local Group, which spans about seven million light years in our local section of the universe. Our Local Group sits out on the edge of a larger cluster of some thousand galaxies known as the Virgo Cluster, which is about fifty million light years across. In turn, the Virgo Cluster within which we reside sits within The Local Supercluster which contains hundreds of groups (clusters) of galaxies scattered across a hundred million or more light years. But don’t stop there.
With painstaking scientific observation and research, astronomers have lately established that our Local Supercluster is but one lobe of a gargantuan structure they have named Laniakea – the Hawaiian term for “immeasurable heaven.” Laniakea as it happens has in fact been measured to be about 400 light years across and to contain some 100,000 large galaxies. Here too we are far from the center of things, for our Milky Way sits out at the edge of Laniakea. Now that you have improved understanding of where you live, be advised that Laniakea is far from the biggest structure in the universe, and its large population of galaxies is but the merest tiny fraction of all the galaxies out there in our grand universe.
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On self organizing
You may have already heard all this in Sunday School, or perhaps not. In any case, all these large structures began forming – falling inward, coming together – while the universe as a whole continued expanding outward, as indeed it continues doing to this moment. While both things were happening at once, cooling gas and space dust all full of atoms and gradually clumping together inside the newly formed proto-galaxies were simultaneously condensing toward becoming the first stars.
Most amazingly, all these structures organized themselves, all around the same time more or less, out of true chaos, forming themselves in response to gravity’s pull wherever that pull was great enough to overcome the expansionary outflinging from the big bang explosion. It’s almost as if a master planner had designed gravity to guide the process. Without it nothing would have happened – the big bang’s debris would simply have kept spreading out to infinity forever, every thing growing farther apart, no two things ever coming closer to each other.
The role of gravity is of central importance in this story. In the tug of war between gravity pulling inward and universal expansion pushing outward, gravity has always won – at least locally – so far. Some pretty big mindsets say it’s over, that all the big clumps which will ever clump have already clumped, and there will be no more. We-l-l-l, they might be right…
In those early times, as each condensing region and sub-region gradually ran out of further matter to pull in, empty spaces began to appear between local regions, and to grow wider. Since gravity’s attractive force diminishes with increasing distance, further collapse of matter into increasingly dense structures was possible only on ever-decreasing scales as matter got pulled away from those in-between emptied-out spaces. As they emptied, millions of proto-stars-to-be simultaneously came into being, winking on one at a time like Christmas lights, inside the embryonic galaxies where gravity was collapsing everything into clumps, each clump and micro-clump becoming denser and hotter.
In between the clusters of galaxy clusters, all filled with burning stars, the universe is now full of great near-empty spaces. Scientists recently announced the largest such void ever discovered – nearly two billion light years across. Imagine. The writer(s) of Genesis could not have known about this, for they had no sophisticated modern telescopes.
Only 300 million years after the big bang, the small irregularities begun as irregular fluctuations, formed in the primeval slurry of energetic particles, had so segregated things that new galaxies were uncountable and infant stars in their billions were everywhere forming within the new galaxies. All the while temperatures across the universe continued falling – by now averaging a mere few thousand degrees.
Cosmologist Joseph Silk has written a beautiful description of the whole process, homing in on the origin of our own galaxy. Paraphrasing his words:
Imagine a region of space in the early universe that will someday become the Milky Way galaxy. It contains all the mass that will be in the Milky Way, but it is thousands of times larger than the future Milky Way-to-be. This vast cloud being fairly “new,” it has not yet condensed out of the surrounding clouds. But its density is not completely uniform with those surrounding clouds – by chance, it happens to be a small fraction of a percent more dense than the surrounding clouds. In the grand scheme of things, it is a “density fluctuation.”
Density fluctuations can grow in strength as the slightest excess of gravity attracts surrounding matter. And so, atom by atom and molecule by molecule, the small overdensity becomes larger. Even infinitesimally small fluctuations can grow exponentially, causing the cloud to break up into clumps, thus allowing gravity to more seriously proceed in its work. Still expanding along with the universal expansion, it nevertheless expands at a fractionally slower rate and, over time, lags farther and farther behind. It eventually becomes dominated by its self- generated gravity, growing cooler, its outward pressure slowly succumbing as it is pulled ever inward to greater density.
It took a long, long time for slightly more dense regions of the universe to overcome universal expansion sufficient to become self-gravitating clouds but, almost from the very beginning, such clouds were destined to form the first galaxies. They would continue forming for the next ten billion years.
Wheels within wheels within wheels – though, less poetically, galaxy clusters tend to be irregular more often than round. While the universe continued expanding, these colossally huge structures continued to separate from each other (not altogether unlike Genesis separations) and to self organize into galaxy superclusters – made of clumped galaxy clusters – made of clumped galaxies each made of lots of individual stars. There is some question as to which end came first, the big or the little – or whether ends and middle proceeded all at the same time. Scientists tend to believe the stars were the earliest to become completely formed, even while so many other structures were building simultaneously. Perhaps so – after all, they were smaller. But nobody really knows.
At the largest end of all, there formed an unimaginably vast something called “the great wall.” Stretching thirty million light years across the night sky, it is the largest known structured matter in the universe, discovered by astronomers only in recent years. One must wonder if perhaps other great walls are even now in the making, all unnoticed.
On Galaxies
Go stargazing on a rural hilltop on a still, clear night; behold the wonder, the majesty, of creation. The seeming infinity of stars and galaxies your unaided eyes will see as minute pin pricks of light actually number only a few thousand. Nor can your eyes distinguish which tiny lights are individual stars in our own Milky Way galaxy and which are other galaxies containing billions of stars. The truth is there are at least a hundred billion galaxies in as much of the universe as we can see, and the true number is probably closer to two hundred billion. One recent estimate places it as high as half a trillion. The number keeps increasing because every time higher-tech telescopes are developed they reveal vastly more galaxies in parts of the universe we thought were empty. Not all great voids are truly void when you take a good look.
For instance, the Hubble Space Telescope, from its vantage point orbiting the earth, has given us direct new images of galaxies formed when the universe was much younger. In a view known as the Hubble Ultra-Deep Field the telescope’s camera was pointed steadily at a region which had previously appeared (aging telescopes had told us) devoid of any matter at all. Hubble’s long open-lens exposure – nearly a million seconds over eleven days – revealed thousands of galaxies in this far distant “void” where none were known to exist. The Hubble’s mighty successor, the James Webb Space Telescope, is scheduled to launch in 2018 and will no doubt bring many more such surprises.
Formation of the first galaxies was well underway by around 700 million years after the big bang. Compared to those commonly seen today these ancient galaxies were mostly smaller, not to mention oddly shaped. Things were crowded in the earliest clusters. Galaxies would form, then repeatedly collide and merge with other galaxies, resulting in new larger galaxies. As their size and density grew they pulled in still more nearby galaxies until many grew to “ordinary large” galactic sizes. The Virgo cluster, around 60 million light years from Earth, contains more than a thousand large galaxies.
Studying galaxies can tell us a great deal about how the universe evolved – if we’re smart enough to interpret the evidence correctly. American astronomer Edwin Hubble specialized in observing galaxies, and he handed us a rich legacy. Among other things, the modern system for sorting and classifying galaxies is built on his work. Looking at those farther away is looking back in time, and Hubble’s work revealed that the ages of galaxies can often be distinguished by their shapes, of which two are most prevalent: older disks tend to be somewhat flat and oval, while younger disks tend to have spiral arms and a central bulge of older stars at the center. The Milky Way we call home is of the latter type. Elliptical galaxies are a third category that formed (usually) when two or more galaxies collided and merged. These mostly contain only older stars because galactic collisions occurred more frequently in the early history of the universe than they do today. A fourth type of galaxies are so diverse and irregular as to defy classification.
Displaying the effects of gravitational attraction, most galaxies appear to form from the inside out, starting with the spheroidal bulge at their centers which, in almost all cases, has a massive black hole centered in the bulge. Certain galaxies, including our own Milky Way, have an elongated central bulge called a bar. Some galaxies contain quasi-stellar objects – “quasars” – mysterious lighthouses which glow with the incredible luminosity of thousands of Milky Ways. We are reminded how little we really know about all this.
Colors too help decipher a galaxy’s age. Certain galaxies whose light is on the red end of the spectrum consist of surviving low mass stars that are long lived because they have become “cooler,” relatively speaking. These older red stars typically are located predominantly in the galaxy’s central bulge. “Blue galaxies,” by contrast, contain younger stars that are often massive, super hot – and burn out quicker, hence cannot be as old as the reds. Modern telescopes show us a mixture – red galaxies where star forming has grown old and mostly ended, and beautiful blue spiral disks with many new stars still forming.
Named by the early Greeks, our Milky Way came into being about 8.8 billion years ago – only five billion years or so after the big bang. Still fairly young. A bit larger than average among galaxies in the modern sky, the Milky Way contains at least a hundred billion stars – reasonable estimates put the number more likely between 200 and 400 billion. One of those billions is our sun. We are at home in the Milky Way.
Like many other younger galaxies, our spiral-shaped Milky Way is turning so that the spiral arms appear to be “trailing,” like a starfish riding on a speeding potter’s wheel. Its overall diameter is more than a hundred thousand light years – the almighty distance light can travel at its fixed speed of 186,000 miles per second. A galactic “halo” of gas and stray stars reaches vastly farther out. From our location near the outer edge of one of the spiral arms, the galactic core is around 27,000 light years distant. After you get there you’ll find the core about 20,000 light years in diameter, and with the predicted bulged shape looking – as one wag put it – rather like two fried eggs back to back.
This central region of our Milky Way contains vast swirling gas clouds, clusters of large powerful stars, and a gigantic black hole about two and a half million times more dense than our sun. A black hole, remember, cannot be directly seen for the very good reason that light cannot escape its immense gravity and so there is nothing to see – it looks like a black circle surrounded by a background of stars. You have to deduce its existence. Astronomers have deduced this monster because the stars near it, which can be seen, are observed to be orbiting the galactic center at thousands of miles per second – a speed which could only be produced by a black hole’s enormous mass and superdensity.
Such dramatic understandings – obtained by some observations augmented with reasonable inferences and deductions – help illustrate why so many scientists hold a firm mindset – some would say closed – that their findings are more reliable than an ancient tribal tale which – whether literal or metaphor – locates a place called “heaven” between two large layers of water, one being the earth’s oceans, the other somewhere in distant reaches of the sky.
Many are the believers in science. Yet many also are believers that literal truth is found in every word of ancient holy works such as the Bible and Koran. Among intelligent people, each thinking the other quite wrong, how is such disagreement possible? Let us continue our study of the bases out of which mindsets spring.
Evolution of stars, the element factories
Suppose there are “only” 200 billion galaxies (rather than that 500 billion estimate above) in the universe. That’s still quite a number, considering that a single galaxy may contain hundreds of billions of stars. Some galaxies have quite many more than the 200 to 400 billion stars in our Milky Way, so simple arithmetic thus tells us there are many quintillions of stars in the wide universe. The true figure may range into the sextillions or septillions. Let’s say a whole lot. How all these stars form is both interesting and very important for our own human story.
Being at the smaller end of large-scale structure formation, each and every star now in existence came into being by the same gravity-driven process of self-organizing evolution that brought the galaxies and other large-scale structures into existence. At first, only large intra-galactic clumps collapsed – big clumps forming within bigger clumps. The irregular clumping process that had begun soon after the big bang continued inside millions of more localized gaseous clouds of matter and energy that formed within every protogalaxy.
In each of these large local clouds of gas and space dust, gravity continued pulling vast clouds of atoms inward until huge amounts of material, mainly hydrogen, accumulated at a center. The centers, irregular at first, took on a round shape. As the centers grew in size and became ever more dense, heating up as inward pressure grew, they further condensed into mighty spheres that would soon turn into stars. The process was fairly straightforward. When the atoms of these spherical clouds were squeezed down sufficiently tight to form a star-sized ball, the increasingly intense heat (around 18 million degrees Fahrenheit) caused the ball to self ignite and start burning as an atomic fusion furnace. You could do this yourself by squeezing a golf ball in your hand if – a rather big if – you could squeeze the ball hard enough. Thus a star is born.
The first stars were massive – many a hundred times and more over the mass of our present-day sun. These early suns started the process that would eventually produce all our elements – gold for rings, iron for trains, oxygen for breathing, the everyday stuff we take for granted. But, at first, almost no elements heavier than hydrogen and helium yet existed.
Using hydrogen atoms as fuel, each star-furnace first fused hydrogen into helium, while releasing heat and light into the surrounding space. When after a few eons the hydrogen was about burned up, the star started burning the helium, producing other new elements. The process then repeated, each repetition producing a newer element to be burned in its turn, each new generation of elements came out slightly heavier than the one before. Coincidentally, radiation spreading outward from the furiously burning hot core kept knocking electrons back out of the recently-formed atoms, so that eventually each new star was surrounded by a huge bubble of hot gas thousands of light years across. You can see these in certain telescopes.
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…to be continued in one week…
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