3 :: “Nucleogenesis and Structure Formation, or We
Are All Just Lumps of Stardust”
Exploring the origins of the stuff we are made of and how it clumped into stars, galaxies and us.
Since life requires heavy elements, life could not form in the early universe. The creation of life had to wait until after several generations of stars had gone up in supernovae, enriching space with heavy materials. In our galaxy, there is a habitable zone where there are enough heavy elements to make metal-rich planets and where the stars are far enough apart to allow stable planetary orbits. Outside of this region, life as we know it on earth cannot exist.
To learn more about the formation of various types of atoms, you are encouraged to explore the following Web sites:
Making the Periodic Table via Stellar Evolution:
Tests of the Big Bang: The Light Elements:
A Hot, Smooth Beginning
No matter what the final shape of space might be, all possible futures can be traced back to a single past. In the beginning, there was — well, we don’t know what there was before the big bang. The cause of the big bang is beyond the reach of our theories and must be left to our imaginings. The moments after the big bang, however, are within the grasp of the human mind. In this lesson, I discuss these moments after the first moment.
Just after the big bang, the universe was a small, hot, dense place. When you think of these first moments, it is important to remember that matter and energy are the same thing. All matter and energy can be broken into three families (see Figure 3-1): quarks, which combine to form protons and neutrons; leptons, such as the electron; and bosons, which carry force. Under normal temperatures and densities, matter takes the form of atoms that have a core of neutrons and protons surrounded by an electron shell (see Figure 3-2).
Figure 3-1: All matter and energy can be broken into three families: quarks, leptons, and bosons. (Graphic by Pamela L. Gay, copyright 2002)
Figure 3-2: An atom is made of an electron shell that surrounds a core of protons and neutrons. The protons and neutrons are made of quarks. (Graphic by Pamela L. Gay, copyright 2002)
Conditions in the early universe were anything but what we think of as normal. For the first .001 second, it was too hot for protons and neutrons to form, and for another 100 seconds after that, collisions between particles were so frequent that protons and neutrons couldn’t fuse to form heavier elements. The majority of the helium in the universe was formed after those first 100 seconds, while the conditions were cool enough to allow atomic nuclei to stay together, but still hot enough to allow nuclear reactions.
Our universe is held together (and sometimes apart) by four basic forces: gravity, the strong nuclear force, the weak nuclear force, and electromagnetism. Gravity is the weakest force, but works over infinite distances, causing books to fall and planets to orbit. Electromagnetic force causes protons and electrons to attract one another, controls how atoms bond together, and causes protons to repel protons. The strong force can overcome this repulsion to hold together atomic nuclei, but only if the protons get close together — separations of 10-15 meters or less. The weak force allows atoms to decay by allowing neutrons to transform into protons and electrons and allows fusion to take place. The weak force works over the smallest distance of all: only 10-17 meters!
Under normal circumstances, atoms do not come within 10-17 meters of one another. This is a good thing — it prevents your desk from undergoing sudden nuclear reactions and your pen from becoming a thermonuclear device. The first step in igniting a nuclear weapon is to use conventional explosives to slam nuclear fuel together so that the density becomes high enough to allow nuclear reactions. In the sun, gravity compresses the center to densities where hydrogen can fuse into helium. In the early universe, the universe was simply so small that the density had to be high. This allowed a few basic nuclear reactions to occur (see Figure 3-3). As long as the universe remained hot and dense, hydrogen could fuse into helium, and some of the helium could go into lithium.
Figure 3-3: Hydrogen nuclei — single protons — can fuse into higher elements if the temperatures and densities are high enough.
When the density of the universe dropped below the level necessary for nuclear fusion, the atomic mixture of the cosmos was 73 percent hydrogen (H), 24 percent helium (He), and ~1 percent lithium (Li). In 15 some-odd billion years since these elements were formed, stars have been the primary source of heavy atoms. The first generation of stars contained nothing but the primordial mixture of H, He, and Li. In the cores of these stars, nuclear fusion formed heavy elements up to iron. This material was mixed back into space through stellar winds that blew off the outer layers of the stars, and more dramatically, through supernovae explosions.
The nuclear reactions in the cores of stars fuse elements together and release energy in the process. This means that there is more energy in two hydrogen atoms than in one helium atom. In order to create elements heavier than iron (26 protons, typically 20 neutrons), energy has to be added to the system. Since stars use nuclear fusion to produce energy, they can’t make elements heavier than iron. These atoms are all made during a supernova, where the explosion of the star that forms the supernova can provide the extra energy necessary to make heavy elements such as gold and platinum.
Stars are responsible for creating only about two percent of the helium in the universe. If the big bang had not happened and the universe had come about by some other means, we would observe a very different amount of helium. While the primordial nuclear reactions lasted for only a brief time, they provide concrete (or rather helium) evidence of the big bang in the chemical composition of the universe. It was in stars that everything we need for life — carbon, oxygen, iron — was formed. You and I are mostly stardust, but the helium that lifts a child’s balloon was provided almost entirely by primordial nucleogenesis.
Atoms under Pressure
Atoms are mostly empty space. As Timothy Ferris describes it, if an atomic nucleus is the size of a golf ball, the inner edge of the electron shell will be located two miles away. The electro-strong force holds together the nucleus, and the electromagnetic force repels the electrons. As atoms are packed closer and closer together, pressure can overcome the electromagnetic force, causing protons and electrons to smash together to form space-saving neutrons via weak force. Neutron stars are formed through this process. Our understanding of physics begins to break down at higher densities, such as those inside black holes, but we believe that the neutrons break down into a quark-energy soup.
For more information on the forces that hold atoms together and particle physics, you are encouraged to explore:
An Introduction to Particle Physics:
And for those of you wanting a broader exploration of physics as a whole, please go to:
For the first 300,000 years of the universe, the temperatures and densities were too high for atoms to hold onto electrons. Photons — packets of energy that we see as light — constantly collided with electrons and protons. These collisions kept the photons, electrons, and protons in constant contact and allowed them to have the same energy distribution — the same way ice in a cooler transfers its temperature through contact to sodas and potato salad, keeping everything the same temperature. These collisions prevented the formation of atoms by continuously knocking free any electrons that became bound to a proton. Everything was a smooth, hot, expanding plasma of unbound particles.
As the universe expanded, it cooled and collisions became less frequent. About 300,000 years into the universe’s evolution, the collisions finally slowed to the point that protons could hold onto their electrons. The photons that had previously shared the same energy as the protons and electrons become uncoupled. The electrons and protons combined, during an epoch called “recombination,” to form hydrogen. The freed photons maintained the same energy distribution as the primordial plasma, and we can now observe them as the cosmic microwave background , or CMB (see Figure 3-4).
Figure 3-4: This image of the CMB was taken with the Millimeter Anisotropy eXperiment IMaging Array (MAXIMA) by the MAXIMA Collaboration. The variations (white to black) represent variations of 6/10,000 of a degree Kelvin. The smaller brighter blobs are in roughly the same area of the sky as the moon! (Graphic by Pamela L. Gay, copyright 2002)
All materials that are heated — humans, branding irons, stars — radiate energy. If you heat your oven to 350 degrees F, the majority of the atoms will have a thermal temperature near 350 degrees F, but they won’t all be at exactly 350 degrees F. The distribution of the atoms’ temperatures is predicted by quantum mechanics and is called a “blackbody distribution.” (A blackbody is a body, or object, that completely absorbs any heat or light that falls on it.) If an object is perfectly heated (that is, it is an “ideal” blackbody), its energy will exactly match the theoretical energy distribution. Most objects are imperfect blackbodies. Either they have hot and cold spots, or different atoms absorb or emit light at specific energies, causing spikes and dips in the blackbody distribution. The perfection of a blackbody (how closely it approaches the ideal) can be judged by how well the measured energy distribution matches the theoretical distribution.
At the moment of recombination, the temperature of the universe was ~10,000 degrees Kelvin and the photons had a color corresponding to that temperature. Since then, the universe has cooled and expanded, and with it, the photons have cooled, and their wavelengths have expanded, becoming redder. Physicists have calculated that the CMB should now have a temperature of just 2.73 degrees Kelvin. This is exactly what has been observed.
Modern astronomers should be able to observe any irregularities in the temperature distribution of the early universe as irregularities in the CMB’s color distribution. Astronomers have searched for these irregularities and find the CMB to be smooth to one part in 100,000. The small irregularities that prevent it from being a perfectly smooth temperature are very important. Had the universe been completely smooth, we would not exist. No galaxies and no stars would have formed. The universe would simply be a continuous distribution of about one atom per square meter. The small fluctuations that existed led to mass concentrations that produced galaxy clusters and empty voids. With the formation of the first permanent hydrogen atom at the moment of recombination, the age of matter formation could begin.
Light and sound are both waves. This means that they have a series of spaced peaks and valleys. The distance between peaks is the wavelength. In sound, a short wavelength corresponds to a high pitch and a long wavelength to a low pitch. With light, a short wavelength is more blue and longer wavelengths are redder. If the object is moving towards an observer, the motion of the object will shorten the distance between peaks, shifting light to the blue. If the object is moving away, the distance between peaks will lengthen, making light appear redder. For a lengthier (and illustrated) explanation, see:
Introduction to the Doppler Effect:
Lumps Beget Bumps, Which Beget Structure
The smooth, hot early universe wasn’t completely smooth. There were small irregularities, observable as temperature fluctuations in the cosmic microwave background. It was out of these little bumps that all structure would eventually form.
Any over density — any region where the mass density is higher than average — in the mass distribution of the early universe could create a gravitational tug on surrounding material. The gravitational tug set things into motion, causing mass to collapse. In the universe today, you find galaxies collected into groups and clusters that are gravitationally bound together. These groups and clusters tend to collect into loosely associated super-clusters and networks of galaxy clusters that form structures that look like filaments and walls. Maps of the galaxy distribution are available at http://manaslu.astro.utoronto.ca/~lin/lcrs.html and http://cfa-www.harvard.edu/~huchra/zcat/. The overall structure of the universe resembles a sponge, as illustrated by Figure 3-5.
Figure 3-5: This map shows redshift distribution, which is related to the distance distribution, for 26418 galaxies. As you can see, the galaxies trace out voids and clump into super-clusters to form a sponge-like structure. (Graphic by Pamela L. Gay, copyright 2002)
The size of the largest voids and super-clusters is related to the size of the CMB irregularities, with the largest irregularities producing the largest structures. Any theory that attempts to describe the big bang must reproduce the density distribution — referred to in The Whole Shebang as “the density spectrum” — of irregularities in the CMB. The best current theories describe structure formation as a “top-down” process that caused the mass in the largest structures to collapse together and fragment into smaller structures. This is not a rapid process. Super-clusters were much more spread out in the early universe and continue to collapse even today.
This continued structural evolution is apparent in the flow of galaxy clusters and galaxy groups. The CMB provides a rest frame against which we can measure our own motion. Careful measurements of our motion relative to nearby objects allow us to understand this motion. When we look at the CMB in one direction we observe it to be blue-shifted, and in the other, we see a red shift (see the sidebar, “Shifted Waves”). This tells us that our overall motion, relative to the universe, is toward the bluest spot in the CMB.
On the smallest scales, the earth is orbiting the sun, and the sun is orbiting the center of the Milky Way galaxy. Our galaxy is part of a small group that contains the larger Andromeda spiral galaxy, the Triangulum spiral galaxy, and a few dozen dwarf and irregular galaxies all orbiting around one another. This group, in turn, is on the outskirts of the Virgo Super-Cluster and is falling towards it. The Virgo Super-Cluster also appears to be falling towards a currently unobserved larger structure dubbed “The Great Attractor.” The disk of the Milky Way, apparent as a faint white stripe across the summer sky in North America, blocks the light from objects located behind the disk. The Great Attractor is located in this hard-to-view area. Current theories don’t predict any structures larger than the Great Attractor, and we don’t observe any mass migrations of clusters towards any other super-structures.
Most cosmologists believe the universe is isotropic and homogeneous. This means it looks the same in all directions and material is evenly distributed through space. Initially, observations showing galaxies clumped on large scales were declared as wrong because they didn’t match this ideal. Now, observers acknowledge that although the universe does look roughly the same in all directions, it looks lumpy in the same way in all directions. It is thought that this lumpiness goes away on scales larger than what we can currently observe.
Consider a forest: Leaves are lumped together on branches, which are attached to trees. To an observer climbing out on a limb, the leaf distribution is lumpy, but the forest appears to be evenly foliated when observed from above. It is thought that on a large enough scale, the universe appears evenly filled with mass.
Over time, the voids of space will become emptier, the walls and filaments will become thinner, and the super-clusters will become denser. The universe is constantly evolving. What was once a smooth, hot, dense soup of energy has cooled into a filamentary lattice of dense galaxies bundled together into larger and larger associations. At all scales, from that of the mostly empty atom to the solar system to the voids and super-clusters of large scale structure, the cosmos alters between regions of high and low (or no) density.
In the next lesson, we’ll discuss the details of the universe’s evolution. The standard big-bang model is a good first-order explanation of how our universe was formed, but it does not tell the complete story. For this, we need to delve into the world of quantum mechanics and confront the questions of why the universe is so smooth and where it came from.
Assignment: Making Matter
Read Chapters 4 and 6 of The Whole Shebang: A State-of-the-Universe(s) Report by Timothy Ferris. After you finish the reading, reflect on humankind’s location in space. We are not in the center of anything or located anywhere special. At the same time, if it weren’t for a thousand small coincidences, life could not exist at all. Does this affect your opinion on how special life on Earth is or change your belief that there might be life somewhere else in the universe? (I am not asking you to discuss the probability of UFOs, but rather the probability that there might be life of some kind somewhere in the universe.)