First Morning—Simple Elements
The first particles came from the collision of gamma ray photons. The collisions also fractured the rays and the photons. Whenever the colliding photons had excess energy beyond that needed to produce particles, photons of lower energy carried the excess away from the interaction region along other rays. This produced the first light. The first morning dawned almost as soon as the first evening began to produce particles.
Heat, another by-product of the collisions, started an expansion that cooled the universe. The cooling and the fracturing of the gamma ray photons of highest energy put an end to particle production a very short time after it began. Now that we know how much energy it takes to make particles, we can understand why particle production stopped.
The End of Particle Production
It may help to compare photons to money. Let’s think of gamma-ray photons as large bills and light or heat photons as small coins. As we spend large bills to buy material possessions, we gradually accumulate change consisting of smaller bills and coins. At the end of a shopping spree we could have a huge bag of small coins. There might still be enough money in the bag to buy an expensive car, but no salesperson would want to sit down and count all the coins!
This is why there is now very little natural creation of particles from photons. Almost all the energetic photons present at the beginning of the universe are by now broken up into weak photons. Weak photons seldom come together in great numbers and therefore do not produce particles.
Doppler Shift, Expansion, and Cooling
A few seconds after the gamma rays began to collide, there were hardly enough of them left to make new particles. There is another reason why particle production stopped. The expansion and the cooling weakened all the photons, even those that had not collided and therefore had not fractured.
If we draw a picture of a wave on a wide rubber band and then stretch it, we will see the distance increase between the wave crests. In other words, the stretching increases the wavelength. But the energy of a photon is inversely proportional to the wavelength, so stretching reduces the energy. A stretched-out wave moves at the speed of light just as any other electromagnetic wave does, but it brings in less energy per unit time than a compact wave. The temperature in a region is proportional to the amount of energy in that region. Reducing the energy lowers the temperature.
This process has another description. Christian Johann Doppler (Austrian physicist, 1803–1853) discovered that stars moving away from an observer appear reddened, and stars moving toward an observer appear bluer than usual. This effect is called the Doppler shift.
People who have heard a car driven by with its horn blowing are familiar with the Doppler shift. The car’s horn plays a musical note, that is, it has a certain pitch. As the car approaches the horn’s pitch is higher than usual. It drops smoothly to a lower-than-usual pitch when the car passes and recedes into the distance. We should not confuse pitch with loudness. It is true that a car’s horn sounds louder as it approaches and softer as it recedes. Loudness and musical pitch in this case go up and down together. But the Doppler Effect is about musical pitch or frequency, not about loudness.
One hears the note the designer intended when the car is stationary. Now let’s consider the sound waves when the car is blowing its horn and approaching the listener. At a certain moment the horn emits the crest of a sound wave. By the time it emits the following wave crest the car is closer. The crests arrive at the listener’s ear closer together than usual. They have a shorter wavelength and therefore a higher frequency. They sound like a musical note of higher pitch. After the car passes the listener, its horn emits each successive wave crest at a greater distance from the observer. This stretches the wavelength and decreases the frequency. The pitch drops to a lower note than the one the horn plays when the car is stationary.
The same thing happens when a star is moving away from an observer. At a certain moment it emits the crest of a wave of light. By the time it emits another wave crest, the star is farther away, since it is moving all the time. The distance between wave crests is greater than it would be if the star were stationary with respect to the observer. Red light has the longest visible wavelengths and visible photons of the lowest energy. Therefore, we say that the light is redder and cooler. If the star happens to be moving toward the observer, the wave crests are closer together. The light is bluer and warmer.
The amount of change in wavelength, divided by the wavelength, is called the Doppler shift. The Doppler shift is positive if the star is moving away from the observer. In the same situation the distance between the star and the observer is expanding, the light is redder, and the light is cooler. All of these are different ways of describing the same thing.
The Beginning of Nucleosynthesis
Physicists speak of many theories of strange particles produced in the high temperature and enormous pressure of the very early universe. The first particles may have been complex and heavy. Many of the strange particles were antiparticles of others. These quickly found particles of the same species and annihilated them, producing two or three gamma rays. Each annihilating pair made photons with a total energy equivalent to the mass of the pair. This contributed to the process of breaking up the original highly energetic photons into many others of lower energy. It also broke up the strange, complex, heavy particles into the more familiar, simpler, low-mass particles. Eventually the components of ordinary atoms predominated: protons, neutrons, and electrons.
At this point nucleosynthesis began. Protons and neutrons collided with each other to form the first nuclei, but they made only the three elements of greatest simplicity and lowest weight: hydrogen, helium, and lithium. The cooling put an end to particle production about the time that nucleosynthesis began. Nucleosynthesis also requires high temperatures, though not as high as those particle production requires. The protons repel each other. They must collide with sufficient speed to overcome their mutual repulsion and get close enough to each other to stick together. The cooling continued so rapidly that nucleosynthesis stopped a few minutes after the universe began.
Light usually accompanies high-energy collisions. Cycles of darkness and light let us trace the two stages of nucleosynthesis in the universe. The first stage made the nuclei of the three simplest elements, and the second stage made the heavier elements. We have gone over the first stage in this chapter, and will treat the second in another, when we come to the second time light filled the universe.
After the first stage of nucleosynthesis stopped, the cooling continued for thousands of years. Eventually the nuclei cooled enough to capture the free electrons and form atoms. At this point the light rays could travel freely. Without free charged particles like electrons and positive nuclei there was much less scattering. The cooling continued, making the light redder and redder.
On Earth the last light of day is reddish because of increasing scattering along longer and longer paths through the atmosphere. Twilight is usually cooler because the Sun’s rays strike the Earth more and more obliquely. Toward the end of the first day there was less scattering but more cooling, for different reasons.
One cycle of darkness and light made the first atoms. The process of formation started with particles. These all came from darkness. The energetic darkness of gamma rays is a cause but also the effect of a previous cause. If we want to get back to the origin of all things, we must understand something about physical forces.
Heat, another by-product of the collisions, started an expansion that cooled the universe. The cooling and the fracturing of the gamma ray photons of highest energy put an end to particle production a very short time after it began. Now that we know how much energy it takes to make particles, we can understand why particle production stopped.
The End of Particle Production
It may help to compare photons to money. Let’s think of gamma-ray photons as large bills and light or heat photons as small coins. As we spend large bills to buy material possessions, we gradually accumulate change consisting of smaller bills and coins. At the end of a shopping spree we could have a huge bag of small coins. There might still be enough money in the bag to buy an expensive car, but no salesperson would want to sit down and count all the coins!
This is why there is now very little natural creation of particles from photons. Almost all the energetic photons present at the beginning of the universe are by now broken up into weak photons. Weak photons seldom come together in great numbers and therefore do not produce particles.
Doppler Shift, Expansion, and Cooling
A few seconds after the gamma rays began to collide, there were hardly enough of them left to make new particles. There is another reason why particle production stopped. The expansion and the cooling weakened all the photons, even those that had not collided and therefore had not fractured.
If we draw a picture of a wave on a wide rubber band and then stretch it, we will see the distance increase between the wave crests. In other words, the stretching increases the wavelength. But the energy of a photon is inversely proportional to the wavelength, so stretching reduces the energy. A stretched-out wave moves at the speed of light just as any other electromagnetic wave does, but it brings in less energy per unit time than a compact wave. The temperature in a region is proportional to the amount of energy in that region. Reducing the energy lowers the temperature.
This process has another description. Christian Johann Doppler (Austrian physicist, 1803–1853) discovered that stars moving away from an observer appear reddened, and stars moving toward an observer appear bluer than usual. This effect is called the Doppler shift.
People who have heard a car driven by with its horn blowing are familiar with the Doppler shift. The car’s horn plays a musical note, that is, it has a certain pitch. As the car approaches the horn’s pitch is higher than usual. It drops smoothly to a lower-than-usual pitch when the car passes and recedes into the distance. We should not confuse pitch with loudness. It is true that a car’s horn sounds louder as it approaches and softer as it recedes. Loudness and musical pitch in this case go up and down together. But the Doppler Effect is about musical pitch or frequency, not about loudness.
One hears the note the designer intended when the car is stationary. Now let’s consider the sound waves when the car is blowing its horn and approaching the listener. At a certain moment the horn emits the crest of a sound wave. By the time it emits the following wave crest the car is closer. The crests arrive at the listener’s ear closer together than usual. They have a shorter wavelength and therefore a higher frequency. They sound like a musical note of higher pitch. After the car passes the listener, its horn emits each successive wave crest at a greater distance from the observer. This stretches the wavelength and decreases the frequency. The pitch drops to a lower note than the one the horn plays when the car is stationary.
The same thing happens when a star is moving away from an observer. At a certain moment it emits the crest of a wave of light. By the time it emits another wave crest, the star is farther away, since it is moving all the time. The distance between wave crests is greater than it would be if the star were stationary with respect to the observer. Red light has the longest visible wavelengths and visible photons of the lowest energy. Therefore, we say that the light is redder and cooler. If the star happens to be moving toward the observer, the wave crests are closer together. The light is bluer and warmer.
The amount of change in wavelength, divided by the wavelength, is called the Doppler shift. The Doppler shift is positive if the star is moving away from the observer. In the same situation the distance between the star and the observer is expanding, the light is redder, and the light is cooler. All of these are different ways of describing the same thing.
The Beginning of Nucleosynthesis
Physicists speak of many theories of strange particles produced in the high temperature and enormous pressure of the very early universe. The first particles may have been complex and heavy. Many of the strange particles were antiparticles of others. These quickly found particles of the same species and annihilated them, producing two or three gamma rays. Each annihilating pair made photons with a total energy equivalent to the mass of the pair. This contributed to the process of breaking up the original highly energetic photons into many others of lower energy. It also broke up the strange, complex, heavy particles into the more familiar, simpler, low-mass particles. Eventually the components of ordinary atoms predominated: protons, neutrons, and electrons.
At this point nucleosynthesis began. Protons and neutrons collided with each other to form the first nuclei, but they made only the three elements of greatest simplicity and lowest weight: hydrogen, helium, and lithium. The cooling put an end to particle production about the time that nucleosynthesis began. Nucleosynthesis also requires high temperatures, though not as high as those particle production requires. The protons repel each other. They must collide with sufficient speed to overcome their mutual repulsion and get close enough to each other to stick together. The cooling continued so rapidly that nucleosynthesis stopped a few minutes after the universe began.
Light usually accompanies high-energy collisions. Cycles of darkness and light let us trace the two stages of nucleosynthesis in the universe. The first stage made the nuclei of the three simplest elements, and the second stage made the heavier elements. We have gone over the first stage in this chapter, and will treat the second in another, when we come to the second time light filled the universe.
After the first stage of nucleosynthesis stopped, the cooling continued for thousands of years. Eventually the nuclei cooled enough to capture the free electrons and form atoms. At this point the light rays could travel freely. Without free charged particles like electrons and positive nuclei there was much less scattering. The cooling continued, making the light redder and redder.
On Earth the last light of day is reddish because of increasing scattering along longer and longer paths through the atmosphere. Twilight is usually cooler because the Sun’s rays strike the Earth more and more obliquely. Toward the end of the first day there was less scattering but more cooling, for different reasons.
One cycle of darkness and light made the first atoms. The process of formation started with particles. These all came from darkness. The energetic darkness of gamma rays is a cause but also the effect of a previous cause. If we want to get back to the origin of all things, we must understand something about physical forces.