Content
The Birth of a Universe
Overview
Science is first and foremost an empirical discipline that provides humanity with powerful access to understanding the nature of the physical and living world (Dalai Lama 2005). It provides us with a mode of inquiry—the scientific method—to investigate the possible causes and effects of various phenomena. Although scientific results are limited by the sophistication of available technology and even the parameters of the questions posed, we have learned a tremendous amount about life in our Universe, using the scientific method. One of the great achievements of modern science is that it seems to have brought us closer than ever to an understanding of the conditions and complicated processes underlying the origins of our cosmos (Dalai Lama 2005).
All of the matter in the universe originated from a singularity—a single point that was infinitely dense and infinitely hot. We have no way of knowing where the singularity originated. The laws of physics had not yet been established. In fact, time did not even exist. What we do know is all of space, time, matter, and energy exploded into existence 13.8 billion years ago. In the scientific community, this spectacular birth of our Universe is known as the Big Bang.
Cosmologists have been able to determine that space, time and the four fundamental forces of nature—gravity, weak nuclear force, electromagnetism, and strong nuclear force—came into existence at 10 - 4 3 seconds after the Big Bang (ABB). In ordinary notation, the number looks like this:
.000000000000000000000000000000000000000001 s
That's a decimal point followed by forty-two zeros and a 1—an amazingly quick instant of time (Fleisher 2006). Although the birth of the Universe is an amazing topic, it is beyond the scope of this curriculum unit. However, a brief discussion of some of the evidence supporting the Big Bang is appropriate.
In the 1940s, American physicists Ralph Alpher, George Gamow, and Robert Herman made one of the most important predictions based on the Big Bang theory. The early Universe was extremely dense and compressed. So it must have been tremendously hot. However, it's been expanding and cooling ever since. Consequently, Alpher, Gamow, and Herman realized that even billions of years later, a little bit of the original heat would still be left. They calculated that the remaining energy would have a temperature of about 5 K—only 5 degrees above absolute zero (Fleisher 2006). Absolute zero is the coldest temperature possible (0 K, - 273.15 C or - 459.69 F).
Alpher, Gamow, and Herman also knew the leftover heat, which is referred to as the Cosmic Microwave Background (CMB), would have special properties. It would take the form of black-body radiation. A black-body emits a characteristic spectrum of radiation.
Electromagnetic radiation is the movement of electric and magnetic energy from one place to another. It takes the form of waves that move at the speed of light (3 x 10 5 km/s). In fact, the different forms of electromagnetic radiation can be arranged, according to energy, to form the electromagnetic spectrum. From lowest energy (longest wavelength and lowest frequency) to highest energy (shortest wavelength and highest frequency), the categories of electromagnetic radiation are radio waves (including microwaves), infrared (heat) radiation, visible light, ultraviolet radiation, X-rays, and gamma rays (Fleisher 2006).
The conditions for forming a black-body spectrum are: there must be many collisions between the material that makes up a thermal source and the photons that are radiated by it (Fleisher 2006). Furthermore, a black-body is a perfect absorber and emitter of light. Hence, an interpretation of the black-body spectrum of the CMB is that it formed at a time when the Universe consisted of hot plasma which implies there were many collisions between the photons and free electrons. Since the time of the last scattering surface, the photons have been traveling in straight lines, stretching with the expansion of space. As the Universe has expanded, the wavelengths of the photons have increased, and the black-body spectrum has shifted to longer wavelengths. Consequently, the temperature associated with this black-body spectrum has dropped with the expansion of the Universe (Jones 2004).
No one managed to detect this radiation until 1965. Even then it happened by accident. Arno Penzias and Robert Wilson were doing research on satellite communications at Bell Laboratories in New Jersey. Penzias and Wilson's instrument was a sensitive horn-shaped antenna. It was designed to capture microwaves, which are low-frequency electromagnetic waves. They detected very weak residual radio signals coming from all directions, which they could not explain in terms of known sources. Those waves produced static in radio signals, causing communications problems. Penzias and Wilson discovered these waves, when they could not get rid of the "noise."
Eventually, Penzias contacted Robert Dicke at nearby Princeton University. Dicke had invented the method they were using to look for weak radio signals. Dicke also had a team of researchers preparing to look for weak microwave radiation. But they knew exactly what they were looking for—the fossil heat from the Big Bang. Dicke's team expected the radiation to be spread evenly across the sky. And they calculated it would measure about 3 K.
That was just what Penzias and Wilson had found. Right away Dicke knew Penzias and Wilson had beaten his team to the punch. He told them what they had actually discovered was the CMB. It was the 14-billion-year-old remains of the Big Bang itself. Dicke's team later confirmed that the radiation was in agreement with that of a black-body spectrum. Finding the radiation Alpher, Gamow, and Herman had predicted was convincing evidence that the Big Bang had actually happened (Fleisher, 2006).
The Origin of the Elements
Most cosmologists today believe that a few seconds ABB, the temperature decreased to a point where reactions occurred that began making the nuclei of lighter elements, from which much later all the matter in the cosmos came into being (Dalai Lama 2005). What kinds of atomic nuclei formed in the early Universe? George Gamow proposed that nuclei of all the elements formed within the first few minutes ABB.
The protons and neutrons of every atomic nucleus are held together with what is known as binding (or nuclear) energy. Without binding energy, the positively charged protons in atomic nuclei would repel one another and the nuclei would fly apart. The larger the nucleus, the more protons it has and the more binding energy is needed to hold it together. Nuclear physicists realized that only the lightest elements could have formed in the Big Bang. The Universe expanded and cooled too quickly to bind larger groups of protons and neutrons together to form the heavier elements (Fleisher 2006).
In the earliest moments ABB, the Universe was unimaginably hot and energetic. At the high temperatures that existed in the early Universe, the composition of the Universe, in terms of particles that were present, was determined by the typical energy that was available in particle interactions. This energy is termed the interaction energy, and is related to the temperature by E ~ kT, where k is the Boltzmann constant (k = 1.38 x 10 - 2 3 JK - 1).
Interactions obey a set of conservation rules: the conserved quantities include energy, electric charge, and baryon and lepton numbers. The energy available in an interaction plays a vital role in determining which particles may form (see appendix 2). Provided that all other conservation rules are obeyed, a particle that has a mass m can be formed if the available energy is equal to or exceeds its mass energy, which is given by E = mc 2.
At 10 - 1 2 seconds ABB, the four fundamental forces separated from each other and the material content of the Universe at this time included all types of leptons and quarks and their respective antiparticles. The temperature was 10 1 6 K and the typical energy of a photon or particle was about 10 3 GeV. Thus any interactions that occurred could easily supply 10 3 GeV to create a new particle. This energy exceeds the mass energy of all the quarks and the leptons (Jones 2004)
As the Universe cooled, the fundamental subatomic particles began to form. Electrons came first. Then protons and neutrons formed, just one millionth of one second ABB. Antiparticles also formed. Every kind of subatomic particle has a matching antiparticle. The antiparticle is a particle with the same mass but an opposite electrical charge. For example, the electron and positron are antiparticles. So are the proton and antiproton. At 1 second ABB, most of the particles and antiparticles destroyed each other, creating huge numbers of photons (Fleisher 2006).On average in the Universe, there are 1 million photons and 1 proton in a cubic meter.
At five minutes ABB, some protons and neutrons joined together, forming helium nuclei and small numbers of lithium and beryllium nuclei. Most protons remained separate and later became hydrogen. About 300,000 years ABB, known as the last scattering surface, the Universe cooled enough for atomic nuclei to capture and hold electrons. At this point the Universe became transparent to light. Gravity began pulling atoms together into clumps of matter and the first stars began to form (Fleisher 2006).
From Big Bang to Stardust
Hydrogen is the simplest atom. It is made of one proton and one electron. Helium, the second simplest atom, has two protons, two neutrons, and two electrons. Scientists know how much binding energy is needed to produce each of the various atomic nuclei. They calculated that the Big Bang would have created a Universe in which nearly 75 percent of the matter would be hydrogen and 25 percent would be helium. A minute percentage would be the light elements lithium and beryllium.
Each different chemical element gives off and absorbs certain wavelengths of light. When astronomers view the light from distant stars through a spectroscope, they can tell which elements they are observing. The light from stars has a spectrum which includes many thin black lines, because stars are surrounded by an atmosphere. Each element in this "cloud of gas" absorbs certain wavelengths of light. Since these wavelengths cannot pass through the stars atmosphere, they are not detected by spectroscopes. A series of black lines appear in the spectrum at these wavelengths. These lines tell astronomers which elements make up the gas surrounding the star (Fleisher 2006).
The first stars formed out of the hydrogen and helium mixture, a few traces of the light elements, and rare isotopes like deuterium and helium 3. This gaseous mixture split up into fragments of rather large masses that evolved very fast and quickly produced supernovas that produced a seed of all the heavy elements. The elements ejected by the supernovae quickly intermixed with the abundant hydrogen and helium, and managed to generate a second generation of stars of varying masses, and thus inner temperatures, which "ignited' many different nuclear reactions (S. Sofia, personal communication July 13, 2007). The larger the mass of the star, the higher was its inner temperature, and the larger the atomic mass of the newly synthesized chemical element (Delsemme 1998).
Astronomers estimate the age of stars based on their size. Massive stars burn brightly but use up their fuel in only a few million years. The less massive stars—those with the same mass as the Sun or lower—use fuel more slowly and can burn for billions of years. Astronomers estimate some of the low mass, dim stars in our Galaxy to be over 12 billion years old
The history of a star can be summarized by envisaging the gravitational shrinkage of an interstellar cloud of gas toward its center, interrupted by very long stable periods, every time the temperature, rising under gas compression, leads to the start of a new nuclear reaction. This begins with hydrogen, which the compression heat ignites first. Hydrogen begins its conversion into helium, when it reaches 10 7 K. Deuterium transforms even sooner, but there is so little of it that we can neglect its presence. The nuclear transformation of hydrogen releases a large amount of heat which can make a star radiate for millions or billions of years, depending on its size.
Distant stars look like points of light, even with the most powerful telescopes. Light from these stars has taken a long time to reach us. So we are seeing light those stars produced at a much earlier time in the history of the Universe. When the intrinsic brightness of a star is known, (by means of its spectral properties) astronomers can make rough distance estimates by measuring the apparent brightness of these faint stars. Kilometers or miles are too small for measuring cosmic distances. Instead, astronomers use much larger units: light-years and parsecs. A light-year is a measure of distance as opposed to time. It represents the distance light can travel in a single year (Fleisher 2006).
Cosmologists use this measuring tool because the speed of light in empty space never changes. A light-year is an enormous distance—almost 9 trillion kilometers (about 6 trillion miles). But a light-year is still a tiny distance compared to the size of the Universe. Proxima Centauri, the star nearest the Sun, is about 4.3 light-years away. The most distant object we can see with our most powerful telescopes is more than 10 billion light-years away. Astronomers also measure cosmic distances in even larger units called parsecs and megaparsecs. One parsec is the distance to an object that has a parallax shift of one second (1/360 of 1 degree). One parsec is about 3.26 light-years. A megaparsec is 1 million parsecs, or 3.26 million light-years (Fleisher 2006).
Parllax is the measure of the angle between two different views of an object. Astronomers measure the distance to planets and nearby stars by measuring angles and then doing simple calculations. To calculate the distance to a nearby star, astronomers use two angular measurements of the star taken six months apart, from opposite sides of the Earth's orbit. The diameter of our orbit around the Sun forms the base of a triangle. Knowing that distance and the two angles, astronomers can calculate the distance to the star. Currently, parallax can be used to accurately measure distances up to about 100 light-years (Fleisher 2006).
Since our Galaxy also contains heavy elements, some of its material must have come from the stars. But how did these elements become available to help build our Solar System? This question can be answered by studying the fascinating "lives of the stars." In particular, it is significant to understand stellar demise. For most stars this process is a comparatively gentle one. The outer layers of a star are gradually expelled. This ejected material appears as the nebulae (from nubes, Latin for "cloud"), that surrounds the star and is illuminated by it. However, this gently released material comes from the outer layers of the star, which have not undergone nuclear processing, so they are not efficient in enriching the interstellar medium with heavy elements (S. Sofia, personal communication July 13, 2007). On the other hand, a few stars eject matter much more dramatically at the very end of their lives, in a spectacular detonation called supernova, which blows the star apart (Freedman 2005).
There is a good reason for the overwhelming abundance of hydrogen and helium. A wealth of evidence has led astronomers to conclude that the Universe began some 13.8 billion years ago with an explosive event called the Big Bang. As stated earlier, only the lightest elements—hydrogen and helium, as well as tiny amount of lithium and perhaps beryllium—emerged from the enormously high temperatures following this cosmic event. All the heavier elements were manufactured by stars later, either by thermonuclear fusion reactions deep in their interiors or by magnanimous supernova explosions that mark the end of massive stars. Were it not for these processes that take place only in stars, there would be no heavy elements in the Universe, no planet like Earth, and no humans to contemplate the origin of the elements (Freedman 2005).
Primordial Nucleosynthesis
We have already seen that conditions in the early Universe led to a situation, such that at t ~ 1 s, the temperature was about 10 1 0 K and the baryonic matter in the Universe was in the form of protons and neutrons. At this time the physical conditions became suitable for the onset of nuclear fusion reactions which lead to the formation of nuclides with a higher atomic mass than hydrogen. Such a process is believed to have occurred and is called primordial nucleosynthesis—a term that distinguishes it from the processes of stellar nucleosynthesis that create elements within stars (Jones 2006).
There are some distinct differences between nucleosynthetic processes that could have occurred in the early Universe and those which occur within stars. One difference is that the conditions in the Universe were changing rapidly. The temperature of the Universe dropped markedly in the first few hundred seconds after t = 0. In order for nuclear fusion reactions to have had a significant effect they must have progressed at a rapid rate and this would have required temperatures in excess of 5 x 10 8 K. This is in marked contrast to the conditions in the core of stars where fusion reactions progress at a relatively leisurely rate in lower temperature conditions (Jones 2006). Another difference is that heavy elements are built from lighter ones, and require higher temperatures. Inside stars, this is possible, since the central temperature increases. In the expanding Universe it decreases and that is why you cannot build up those heavier elements (S. Sofia, personal communication July 13, 2007).
As time progressed in the early Universe, one nuclear reaction that did not require high temperatures, the Β - -decay of free neutrons, was proceeding. However, the presence of a large number of free neutrons highlights another difference between the early Universe and stellar cores—that of composition. As we shall see, it is the declining number of free neutrons that plays an important role in determining how many nuclei can be formed before fusion reactions become ineffective at a temperature of about 5 x 10 8 K (Jones 2004).
The first fusion reaction that could occur was that between a proton and a neutron to form a nucleus of deuterium (which was referred to as deuteron). This is the neutron capture reaction:
p + n - -> 2 1H+ Γ
Note that this is a reversible reaction: the deuteron can be broken apart by Γ-rays in a process called photodisintergration. In order to cause photodisintegration of a deuteron, an incident photon must have an energy that exceeds 2.23 MeV. Although at t = 1 s the average interaction energy is less than this, there were so many photons in comparison to the number of baryons, that there was a sufficient number of photons with energies greater than 2.23 MeV (i.e. well above the average value) to rapidly destroy any deuterium that formed. However, as the Universe continued to expand and cool, the average photon energy decreased. This decrease allowed deuterium to survive from about t = 3 s. As soon as there was a significant build up in the abundance of deuterium, other nuclear reactions could then proceed. As the temperature of the Universe dropped below 10 8 K, the nuclear reactions that resulted in the formation of light elements came to a halt. The composition of the Universe at this time was: protons; nuclei of deuterium; helium and lithium; electrons; neutrinos; photons; and dark matter particles (Jones 2004). In fact, the bulk of the baryonic matter was mostly hydrogen (about 75 percent by mass), helium (about 25 percent by mass), and minute traces of lithium.
Stellar Nucleosynthesis
The earliest stars collapsed and exploded as supernovae, creating the heavier elements about 10 9 years ABB (Fleisher 2006). Supernovae may seem remote to our own origins. But on the contrary, only by studying the births of stars, and the explosive way they die, can we tackle such an everyday question as where the atoms we are made of originate. The respective abundances of the elements of the Periodic Table can be measured in the Solar System and inferred, through spectroscopy, in stars and nebulae (Fabian 1988).
Complex chemical elements are an inevitable by-product of the nuclear reactions that provide the power in ordinary stars. A massive star develops a kind of onion-skin structure, where the inner hotter shells are "cooked" further up the Periodic Table. The final explosion ejects most of this processed material. All the carbon, nitrogen, oxygen and iron on the Earth must have been manufactured in stars that exhausted their fuel supply and exploded before the Sun formed. The Solar System would then have condensed from gas contaminated by the debris ejected from earlier generations of stars. The processes of cosmic nucleogenesis can account for the observed proportions of different elements—why oxygen is common but gold and uranium are rare—and how they came to be in our Solar System (Fabian 1988).
Each atom on Earth can be traced back to the stars that died before the Solar System formed. A carbon atom, forged in the core of a massive star and ejected when this exploded as a supernova, may spend hundreds of millions of years wandering in interstellar space. It may then find itself in a dense cloud which contracts into a new generation of stars. It could then be once again in a stellar interior, where it is transmuted into a still heavier element. Alternatively, it may find itself out on the boundary of a new Solar System in a planet, and maybe eventually in a human cell. We are literally made of the ashes of long-dead stars (Fabian 1988).
Hydrogen is the most common element in the Universe by far. But stars and planets like ours have many heavier elements. For example oxygen makes up almost 50 percent of Earth's crust followed by iron (18 percent), silicon (14 percent) and magnesium (8 percent). Where did those elements—and even heavier ones like gold and uranium come from (Fleisher 2006)?
Heavy elements are made in the cores of massive stars. Stars turn hydrogen into helium by the process of nuclear fusion. In fusion the nuclei of two or more small atoms join together to form one larger nucleus. The process releases large amounts of energy. When all of the hydrogen from a stellar core has been fused into helium, the star begins to fuse helium into larger nuclei, creating all the elements up to iron. Finally, the star is out of usable fuel. It collapses and explodes as a supernova. During the supernova explosion, a large number of free neutrons are produced, which can enter atomic nuclei without the repulsive barrier of a Coulomb force (due to their neutral charge). This creates nuclei of the elements heavier than iron. The exploding supernova then spreads these elements through the Galaxy (Fleisher 2006).
The Abundance of Elements on Earth
By studying the abundances of the elements, we are led to a remarkable insight. Some 4.56 x 10 9 years ago, a collection of hydrogen, helium, and heavy elements came together to form the Sun—a third generation star—and all of the objects that orbit around it. All of those heavy elements, including the carbon atoms in your body and the oxygen atoms that you breathe, were created and cast off by stars that lived and died long before our Solar System formed, during the first 8 to 9 billion years of the Universe's existence (Freedman 2005) It is mind boggling to think that we are literally made of star dust.
Stars create different heavy elements in different amounts. For example, carbon (as well as oxygen, silicon, and iron) is readily produced in the interiors of massive stars, whereas gold (as well as silver, platinum, and uranium) is created only under special circumstances. Consequently, gold is rare in our solar system and in the Universe as a whole, while carbon is relatively abundant (although still much less abundant than hydrogen or helium). A convenient way to express the relative abundances of the various elements is to say how many atoms of a particular element are found for every trillion (10 1 2) hydrogen atoms. For example, for every 10 1 2 hydrogen atoms in space, there are about 100 billion (10 1 1) helium atoms. From spectral analysis of stars and chemical analysis of Earth rocks, Moon rocks, and meteorites, scientists have determined the relative abundances of the elements in our part of the Milky Way Galaxy today (Jones 2004).
Hydrogen, the most abundant element, makes up nearly three-quarters of the combined mass of the Sun and planets. Helium is the second most abundant element. Together, hydrogen and helium account for about 98% of the mass of all the material in the Solar System. All of the other chemical elements are relatively rare; combined, they make up the remaining 2%. The dominance of hydrogen and helium is not merely a characteristic of our local part of the Universe. By analyzing the spectra of stars and galaxies, astronomers have found essentially the same pattern of chemical abundances out to the farthest distances attainable by the most powerful telescopes. Hence, the vast majority of the atoms in the Universe are hydrogen and helium atoms. The elements that make up the bulk of the Earth—mostly iron, oxygen, and silicon—are relatively rare in the Universe as a whole, as are the elements of which living organisms are made—carbon, oxygen, nitrogen, and phosphorous, among others (Freedman 2005).
The composition of the Earth's atmosphere is unique within the solar system. The Earth is situated between Venus and Mars, both have atmospheres consisting primarily of CO 2; the outer planets (Jupiter, Saturn, Uranus, Neptune) are dominated by hydrogen and helium and by reduced compounds, such as CH 4. By contrast, CO 2 and CH 4 are only minor (although very important) constituents of the Earth's atmosphere. Nitrogen represents - 78% of the molecules in air, and life-sustaining oxygen accounts for - 21%. The presence of so much oxygen is surprising, since it might appear to produce a combustible mixture with many of the other gases in air (e.g. sulfur to form sulfates, nitrogen to form nitrates, hydrogen to form water).
The Earth's atmosphere is certainly not in chemical equilibrium, since the concentrations of N 2, O 2, CH 4, and NH 3 are much higher than they would be for perfect equilibrium. Four of the most abundant elements in the Earth's atmosphere (nitrogen, oxygen, hydrogen, and carbon) are also among the top five most abundant elements in the biosphere. This suggests that biological processes have played a dominant role in the evolution of the Earth's atmosphere and that they are probably responsible for its present non-equilibrium state.
In comparison to the Sun (or the cosmos) the atmosphere of the Earth is deficient in the light volatile elements (e.g. H) and the noble gases or inert gases (e.g. He, Ne, Ar, Kr, Xe). This suggests that either these elements escaped as the Earth was forming or the Earth formed in such a way as to systematically exclude these gases (e.g. by the agglomeration of solid materials similar to that in meteorites). In either case, the Earth's atmosphere was probably generated by the degassing of volatile compounds contained within the original solid materials that formed the Earth (Hobbs 2000).
The science behind the origin of the elements is impeccable. Cosmologists can describe the conception of matter to 10 - 4 3 seconds after the birth of the Universe. Arising from an innate curiosity to understand the physical and living world, scientists have collected substantial data to support the idea that our deepest roots, stem from the nothingness before time and space. Hence, it is imperative to remember that there is infinitely more to learn about this fascinating topic.
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