Chapter 22
electroweak force
As an analogy, think about ice, liquid water, and water vapor. These three substances are quite different from one another in appearance and behavior, yet they are just different phases of the single substance H2O.H2O. Experiments have shown that, in a similar way, the electromagnetic and weak forces lose their separate identities under conditions of very high temperature or energy and merge together into a single electroweak force
The intense starlight would heat the dust over time until it too glowed like the Sun or evaporated away. There are only two ways out of this dilemma.
Either the universe has a finite number of stars, in which case we would not see a star in every direction, or it changes over time in some way that prevents us from seeing an infinite number of stars. For several centuries after Kepler first recognized the dilemma, astronomers leaned toward the first option. Kepler himself preferred to believe that the universe had a finite number of stars because he thought it had to be finite in space, with some kind of dark wall surrounding everything. Astronomers in the early 20th century tended to believe that the universe was infinite in space but that we lived inside a finite collection of stars. They thought of the Milky Way as an island floating in a vast black void. However, subsequent observations showed that galaxies fill all of space more or less uniformly. We are therefore left with the second option: The universe changes over time.
two key discoveries caused the steady state hypothesis to lose favor
First, the 1965 discovery of the cosmic microwave background matched a prediction of the Big Bang theory but was not adequately explained by the steady state hypothesis. Second, a steady state universe should look about the same at all times, but observations made with increasingly powerful telescopes during the last half-century show that galaxies at great distances look younger than nearby galaxies. As a result of these predictive failures, the steady state hypothesis is no longer considered to be viable.
The Gut era
The next era is called the GUT era, named for the grand unified theories (GUTs) that predict the merger of the strong, weak, and electromagnetic forces into a single GUT force at temperatures above 1029K1029 K (see Figure 22.3). Although different versions of grand unified theories disagree in many details, they all predict that the GUT era was a time during which two forces—gravity and the GUT force—operated in the universe. It came to an end when the GUT force split into the strong and electroweak forces, which happened when the universe was a mere 10−3810−38 second old; note that this is less than a trillion-trillion-trillionth of a second.
critical density
The precise average density for the entire universe that marks the dividing line between a recollapsing universe and one that will expand forever. If the universe's average density is less than the critical density, then the overall geometry is saddle-shaped. If its average density is greater than the critical density, then the overall geometry is spherical.
Big Bang theory
The scientific theory that predicts what the universe was like early in time -It is based on applying known and tested laws of physics to the idea that everything we see today began as an incredibly hot and dense collection of matter and radiation. The Big Bang theory successfully describes how expansion and cooling of this unimaginably intense mixture of particles and photons could have led to the present universe of stars and galaxies, and it explains several aspects of today's universe with impressive accuracy.
inflation
When we discussed the eras of the universe earlier in the chapter, we noted that the universe is thought to have undergone a sudden and dramatic expansion, called inflation, which may have occurred at the end of the GUT era, when the universe was 10−3810−38 second old. (In some models of inflation, the dramatic expansion can happen later, up until the end of the electroweak era.) This idea first emerged in 1981, when physicist Alan Guth was considering the consequences of the separation of the strong force from the GUT force that marked the end of the GUT era. Some theories of high-energy physics predict that this separation of forces would have released enormous amounts of energy, and Guth realized that this energy might have caused a short period of inflation. He found that, in a mere 10−3610−36 second, inflation could have caused the universe to expand by a factor of 1030.1030. Strange as this idea may sound, it appears to explain several otherwise mysterious features of the present-day universe. Moreover, recent evidence from detailed studies of the cosmic microwave background has provided support for the hypothesis that an early period of inflation really occurred.
what key features of the universe are explained by inflation?
Where did the density enhancements that led to galaxies come from?Recall that successful models of galaxy formation start from the assumption that gravity could collect matter together in regions of the early universe that had slightly enhanced density Section 21.1. We know from our observations of variations in the cosmic microwave background that such regions of enhanced density were present in the universe at an age of 380,000 years, but we have not yet explained how these density variations came to exist. Why is the large-scale universe nearly uniform? Although the slight variations in the cosmic microwave background show that the universe is not perfectly uniform on large scales, the fact that it is smooth to within a few parts in 100,000 is remarkable enough that we would not expect it to have occurred by pure chance. Why is the geometry of the universe flat? Einstein's general theory of relativity tells us that the overall geometry of the universe can be curved, like the surface of a balloon or a saddle Section S3.2. However, observational efforts to measure the large-scale geometry of the universe have not yet detected any curvature. As far as we can tell, the large-scale geometry of the universe is flat, which means it is at the precise balance point between being curved like a balloon and curved like a saddle. This precise balance is another fact that is difficult to attribute to chance.
Almost any shape is possible, but all the possibilities fall into just three general categories
Using analogies to objects that we can see in three dimensions, scientists refer to these three categories of shape as flat(or critical), spherical (or closed), and saddle shaped (or open) -According to general relativity, the overall geometry depends on the average density of matter and energy in the universe, and the geometry can be flat only if the combined density of matter plus energy is precisely equal to a value known as the critical density. If the universe's average density is less than the critical density, then the overall geometry is saddle-shaped. If its average density is greater than the critical density, then the overall geometry is spherical. Inflation can explain why the overall geometry is so close to being flat. In terms of Einstein's theory, the effect of inflation on spacetime curvature is similar to the flattening of a balloon's surface when you blow into the balloon (Figure 22.15). The flattening of space caused by inflation would have been so enormous that any curvature the universe might have had previously would be noticeable only on size scales much larger than that of the observable universe. Inflation therefore makes the overall geometry of the universe appear flat, which means that the overall density of matter plus energy must be very close to the critical density.
about 28% of our sun's mass is helium . about how much of that helium was made in the Big Bang , and how much came from nuclear fusion in stars ? a) most of the sun's helium was made in the Big Bang b) most of the sun's helium came from other stars c) about equal amounts came from the Big Bang and from other stars
a) most of the sun's helium was made in the Big Bang
which fo the four forces that exist today are merged together in what scientist call the GUT force ? a) strong and weak only b) strong , weak and electromagnetism c) strong , weak , electromagnetism and gravity d)electromagnetism and weak only
b)b) strong , weak and electromagnetism
observation demonstrate that the universe is
cooling with time as it expands , implying that it was hotter and denser in the past -at very early times , temperatures were so high that different processes came into play
the universe is filled with a _____ of radiation that appears to be the remnant heat of the big bang
faint glow :this faint glow is light that has traveled freely through space since the universe was about 380,000 years old, which is when the universe first became transparent to light. Before that time, light could not pass freely through the universe, so there is no possibility of seeing light from earlier times.
four forces
gravity , electromagnetism , strong force and weak force -Everything that happens in the universe today is governed by four distinct forces: gravity, electromagnetism, the strong force, and the weak force -These models predict that at the high temperatures that prevailed in the early universe, the four forces would not have been as distinct as they are today.
gravity
is the most familiar of the four forces, providing the "glue" that holds planets, stars, and galaxies together -Gravity therefore becomes the dominant force for such objects, because more mass always means more gravity.
deuterium
the universe was still hot and dense enough for nuclear fusion to take place. Protons and neutrons constantly combined to form deuterium—the rare form of hydrogen that contains a neutron in addition to a proton in the nucleus—and deuterium nuclei fused to form helium (Figure 22.10). However, during the early part of the era of nucleosynthesis, the helium nuclei were almost immediately blasted apart by one of the many gamma rays that filled the universe. -most all the available neutrons should have been incorporated into nuclei of helium-4. Figure 22.11 shows that, based on the 7-to-1 ratio of protons to neutrons, the universe should have had a composition of 75% hydrogen and 25% helium by mass at the end of the era of nucleosynthesis. This match between the predicted and observed helium ratios provides strong support for the Big Bang theory.
That is, if we assume inflation occurred
we find that the density enhancements, large-scale uniformity, and flat geometry are all natural and expected consequences. Note that inflation is a scientific hypothesis because it is testable with observations we can perform today, and confidence in the hypothesis is growing because it has passed all the tests it has faced so far.
particle creation and annihilation
the universe was so hot during the first few seconds that photons could transform themselves into matter and vice versa in accordance with E = m c^2 -One such reaction is the creation or destruction of an electron-antielectron pair (Figure 22.2). When two photons collide with a total energy greater than twice the mass-energy of an electron (the electron's mass times c2c2), they can create two brand-new particles: a negatively charged electron and its positively charged twin, the antielectron (also known as a positron). -The electron is a particle of matter, and the antielectron is a particle of antimatter. The reaction that creates an electron-antielectron pair also runs in reverse. When an electron and an antielectron meet, they annihilate each other totally, transforming all their mass-energy back into photon energy. In order to conserve both energy and momentum, an annihilation reaction must produce two photons instead of just one. -Similar reactions can produce or destroy any particle- antiparticle pair, such as a proton and an antiproton or a neutron and an antineutron. -physicists have observed the behavior of matter and energy at temperatures as high as those that existed in the universe just one ten-billionth (10−10)(10−10) of a second after the Big Bang, giving us confidence that we actually understand what was happening at that early time. Our understanding of physics under the more extreme conditions that prevailed even earlier is less certain, but we have some ideas about what the universe was like when it was a mere 10−3810−38 second old, and perhaps a glimmer of what it was like at the age of just 10−4310−43 second.
Olber's Paradox
A paradox pointing out that if the universe were infinite in both age and size (with stars found throughout the universe), then the sky would not be dark at night. -Olbers' paradox does not prove that the universe began with a Big Bang. However, we must have some explanation for why the sky is dark at night, and no explanation besides the Big Bang also explains so many other observed properties of the universe so well.
the era of nuclei
After fusion ceased, the universe consisted of a very hot plasma of hydrogen nuclei, helium nuclei, and free electrons. This basic picture held for about the next 380,000 years as the universe continued to expand and cool. The fully ionized nuclei moved independently of electrons (rather than being bound with electrons in neutral atoms) during this period, which we call the era of nuclei. Throughout this era, photons bounced rapidly from one electron to the next, just as they do deep inside the Sun today Section 14.2, never managing to travel far between collisions. Any time a nucleus managed to capture an electron to form a complete atom, one of the photons quickly ionized it. -The era of nuclei came to an end when the expanding universe was about 380,000 years old. At this point, the temperature had fallen to about 3000 K—roughly half the temperature of the Sun's surface today. Hydrogen and helium nuclei finally captured electrons for good, forming stable, neutral atoms for the first time. With electrons now bound into atoms, the universe became transparent, as if a thick fog had suddenly lifted. Photons, formerly trapped among the electrons, began to stream freely across the universe. We still see these photons today as the cosmic microwave background, which we will discuss shortly.
the particle era
As long as the universe was hot enough for the spontaneous creation and annihilation of particles to continue, the total number of particles was roughly in balance with the total number of photons. Once it became too cool for this spontaneous exchange of matter and energy to continue, photons became the dominant form of energy in the universe. We refer to the time between the end of the electroweak era and the moment when spontaneous particle production ceased as the particle era, to emphasize the importance of subatomic particles during this period. -During the early parts of the particle era (and during earlier eras), photons turned into all sorts of exotic particles that we no longer find freely existing in the universe today, including quarks—the building blocks of protons and neutrons Section S4.2. By the end of the particle era, all quarks had combined into protons and neutrons, which shared the universe with other particles such as electrons and neutrinos. -The particle era came to an end when the universe reached an age of 1 millisecond (0.001 second), at which point the temperature had fallen to 1012K1012 K and it was no longer hot enough for protons and antiprotons to be produced spontaneously from photon energy. If the universe had contained equal numbers of protons and antiprotons (or neutrons and antineutrons) at the end of the particle era, all of the pairs would have annihilated each other, creating photons and leaving essentially no matter in the universe. From the obvious fact that the universe contains matter, we conclude that protons must have slightly outnumbered antiprotons at the end of the particle era. -This ratio indicates that for every billion antiprotons in the early universe, there must have been about a billion and one protons. That is, for each 1 billion protons and antiprotons that annihilated each other at the end of the particle era, a single proton was left over.
grand unified theories
At even higher temperatures and energies, the electroweak force may merge with the strong force and ultimately with gravity. Models that predict the merger of the electroweak and strong forces are called grand unified theories, or GUTs for short. The merger of the strong, weak, and electromagnetic forces is therefore often called the GUT force. Many physicists suspect that at even higher energies, the GUT force and gravity merge into a single "super force" that governs the behavior of everything. (Current ideas for linking all four forces go by names that include supersymmetry, superstrings, supergravity, and "theory of everything.") -these changes in the fundamental forces probably occurred before the universe was one ten-billionth of a second old.
the era of nucleosynthesis
At this point, the protons and neutrons left over after the annihilation of antimatter began to fuse and make heavier nuclei. However, the temperature of the universe remained so high that gamma rays blasted apart most of those nuclei as fast as they formed. This dance of fusion and demolition marked the era of nucleosynthesis, which ended when the universe was about 5 minutes old. By this time, the density in the expanding universe had dropped so much that fusion no longer occurred, even though the temperature was still about a billion Kelvin (109K)(109 K)—much hotter than the temperature of the Sun's core. -When fusion ceased at the end of the era of nucleosynthesis, the chemical content of the universe had become (by mass) about 75% hydrogen and 25% helium, along with trace amounts of deuterium (hydrogen with a neutron) and lithium (the next heaviest element after hydrogen and helium). Except for the small proportion of matter that stars later forged into heavier elements, the chemical composition of the universe remains the same today.
inflation
However, if the grand unified theories are correct, the freezing out of the strong and electroweak forces may have released an enormous amount of energy, causing a sudden and dramatic expansion of the universe that we call inflation. In a mere 10−3610−36 second, pieces of the universe the size of an atomic nucleus may have grown to the size of our solar system. Inflation sounds bizarre, but as we will discuss later, it explains several important features of today's universe
cosmic background explorer ( COBE)
In the early 1990s, a NASA satellite called the Cosmic Background Explorer (COBE) was launched to test these ideas about the cosmic microwave background. The results were a stunning success for the Big Bang theory, and earned the 2006 Nobel Prize in physics for COBEteam leaders George Smoot and John Mather. As shown in Figure 22.8, the cosmic microwave background does indeed have a perfect thermal radiation spectrum, with a peak corresponding to a temperature of 2.73 K. -COBE and its successor missions, the Wilkinson Microwave Anisotropy Probe (WMAP) and the European Planck satellite, have also mapped the temperature of the cosmic microwave background in all directions (Figure 22.9). The temperature turns out to be extraordinarily uniform throughout the universe—just as the Big Bang theory predicts it should be—with variations from one place to another of only a few parts in 100,000.* Moreover, these slight variations also represent a predictive success of the Big Bang theory. Recall that our theory of galaxy formation depends on the assumption that the early universe was not quite perfectly uniform; some regions of the universe must have started out slightly denser than other regions, so that they could serve as seeds for galaxy formation Section 21.1. The small variations in the temperature of the cosmic microwave background indicate that the density of the early universe really did differ slightly from place to place.
steady state universe hypothesis
One of the cleverest alternatives, developed in the late 1940s, was called the steady state universe hypothesis. This hypothesis accepted the fact that the universe is expanding but rejected the idea of a Big Bang, instead postulating that the universe is infinitely old and always looks about the same on large scales. This claim might not have seemed extraordinary before Edwin Hubble discovered that the universe is expanding. However, the claim was made two decades after that discovery and raised the following obvious question: If the universe has been expanding forever, shouldn't every galaxy be infinitely far away from every other galaxy? Proponents of the steady state hypothesis answered by claiming that new galaxies continually form in the gaps that open up as the universe expands, thereby keeping the average distance between galaxies the same at all times
However, like any scientific model, the Big Bang model had to make testable predictions about other observable features of the universe, and it has gained wide scientific acceptance because of two major predictions that have been verified:
The Big Bang theory predicts that the radiation that began to stream across the universe at the end of the era of nuclei should still be present today. Sure enough, we find that the universe is filled with what we call the cosmic microwave background. Its characteristics precisely match what the theory predicts. The Big Bang theory predicts that some of the original hydrogen in the universe should have fused into helium during the era of nucleosynthesis. Observations of the actual helium content of the universe closely match the predicted amount of helium.
how do the abundances of elements support the Big Bang theory ?
The Milky Way's helium fraction is about 28%, and no galaxy has a helium fraction lower than 25%. Although helium is produced by hydrogen fusion in stars, calculations show that this production can account for only a small proportion of the total observed helium. We therefore conclude that the majority of the helium in the universe must already have been present in the protogalactic clouds that preceded the formation of galaxies. -The Big Bang theory makes a specific prediction about the helium abundance. As we discussed earlier, the theory explains the existence of helium as a consequence of fusion that occurred during the era of nucleosynthesis, when the universe itself was hot enough to fuse hydrogen into helium. Combining the current microwave background temperature of 2.73 K with the number of protons we observe in the universe tells us precisely how hot the universe must have been in the distant past, allowing scientists to calculate exactly how much helium should have been made. The result—25% helium—is another impressive success of the Big Bang theory.
origin of the cosmic microwave background
The cosmic microwave background consists of microwave photons that have traveled through space since the end of the era of nuclei, when most of the electrons in the universe joined with nuclei to make neutral atoms, which interact less strongly with photons. With very few free electrons left to block them, most of the photons from that time have traveled unobstructed through the universe ever since (Figure 22.7). When we observe the cosmic microwave background, we essentially are seeing back to the end of the era of nuclei, when the universe was only 380,000 years old. -The Big Bang theory predicts that the cosmic microwave background should have an essentially perfect thermal radiation spectrum Section 5.4, because it came from the heat of the universe itself. Moreover, the theory predicts the approximate wavelength at which this thermal radiation spectrum should peak. As we discussed earlier, the theory tells us that the radiation of the cosmic microwave background broke free when the universe had cooled to a temperature of about 3000 K, similar to the surface temperature of a red giant star. The spectrum of the cosmic microwave background therefore should have originally peaked at a wavelength of about 1000 nanometers, just like the thermal radiation from a red star. Because the universe has since expanded by a factor of about 1000, the wavelengths of these photons should by now have stretched to about 1000 times their original lengths Section 20.3. We therefore expect the peak wavelength of the cosmic microwave background now to be about a millimeter, squarely in the microwave portion of the spectrum and corresponding to a temperature of a few degrees above absolute zero.
how do observations of the cosmic microwave background support the Big Bang theory ?
The discovery of the cosmic microwave background was announced in 1965. Arno Penzias and Robert Wilson, two physicists working at Bell Laboratories in New Jersey, were calibrating a sensitive microwave antenna designed for satellite communications (Figure 22.6). (Microwaves fall within the radio portion of the electromagnetic spectrum; see Figure 5.7.) Much to their chagrin, they kept finding unexpected "noise" in every measurement they made. The noise was the same no matter where they pointed the antenna, indicating that it came from all directions in the sky and ruling out any possibility that it came from any particular astronomical object or any place on Earth.
electromagnetic force
The electromagnetic force, which depends on the electrical charge of a particle instead of its mass, is far stronger than gravity. It is therefore the dominant force between particles in atoms and molecules, responsible for all chemical and biological reactions. However, the existence of both positive and negative electrical charges causes the electromagnetic force to lose out to gravity on large scales, even though both forces decline with distance by an inverse square law. Most large astronomical objects (such as planets and stars) are electrically neutral overall, making the electromagnetic force unimportant on that scale
The eras of atoms and galaxies
The end of the era of nuclei marked the beginning of the era of atoms, when the universe consisted of a mixture of neutral atoms and plasma (ions and electrons), along with a large number ofphotons. Because the density of matter in the universe differed slightly from place to place, gravity slowly drew atoms and plasma into the higher-density regions, which assembled into protogalactic clouds Section 21.1. Stars then formed in these clouds, and the clouds subsequently merged to form galaxies. -The first full-fledged galaxies had formed by the time the universe was about 1 billion years old, beginning what we call the era of galaxies, which continues to this day. Generation after generation of star formation in galaxies steadily builds elements heavier than helium and incorporates them into new star systems. Some of these star systems develop planets, and on at least one of these planets life burst into being a few billion years ago
The plank era
The first era after the Big Bang is called the Planck era, named for physicist Max Planck. It represents times before the universe was 10−4310−43 second old. According to the laws of quantum mechanics, there should have been substantial energy fluctuations from point to point during this very early time. Because energy and mass are equivalent, Einstein's theory of general relativity tells us that these energy fluctuations must have generated a rapidly changing gravitational field that would have randomly warped space and time. These fluctuations are predicted to have been so large that our current understanding of physics is inadequate to describe what might have been happening. The main problem is that we do not yet have a theory that links quantum mechanics (our successful theory of the very small) and general relativity (our successful theory of the very big). Perhaps someday we will be able to merge these theories of the very small and the very big into a single "theory of everything" (see Special Topic).Until that happens, science cannot describe the universe during the Planck era.
how did hey come to have the same temperature and density ?
The inflation hypothesis answers this question by saying that even though the two regions cannot have had any contact since the time of inflation, they were in contact prior to that time. Before the onset of inflation, when the universe was 10−3810−38 second old, the two regions were less than 10−3810−38 light-second away from each other. Radiation traveling at the speed of light would therefore have had time to bounce between the two regions, and this exchange of energy equalized their temperatures and densities. Inflation then pushed these equalized regions to much greater distances, far out of contact with each other. Like criminals getting their stories straight before being locked in separate jail cells, the two regions (and all other parts of the observable universe) came to the same temperature and density before inflation spread them far apart.
The electroweak era
The splitting of the GUT force marked the beginning of an era during which three distinct forces operated: gravity, the strong force, and the electroweak force. We call this time the electroweak era, indicating that the electromagnetic and weak forces were still merged together. Intense radiation continued to fill all of space, as it had since the Planck era, spontaneously producing matter and antimatter particles that almost immediately annihilated each other and turned back into photons. -The universe continued to expand and cool throughout the electroweak era, dropping to a temperature of 10151015 when it reached an age of 10−1010−10 second. This temperature is still 100 million times hotter than the temperature in the core of the Sun today, but it was low enough for the electromagnetic and weak forces to freeze out from the electroweak force. After this instant (10−1010−10 second), all four forces were forever distinct in the universe. -The end of the electroweak era marks an important transition not only in the physical universe, but also in human understanding of the universe. The theory that unified the weak and electromagnetic forces, which was developed in the 1970s, predicted the emergence of new types of particles (called the W and Z bosons, or weak bosons) at temperatures above the 1015K1015 K temperature that pervaded the universe when it was 10−1010−10 second old. In 1983, particle-accelerator experiments reached energies equivalent to such high temperatures for the first time. The new particles showed up just as predicted, produced from the extremely high energy in accord with E=mc2.E=mc2. We therefore have direct experimental evidence concerning the conditions in the universe at the end of the electroweak era. We do not have any direct experimental evidence of conditions before that time.
strong and weak force
The strong and weak forces operate only over extremely short distances, making them important within atomic nuclei but not on larger scales. The strong force binds protons and neutrons together in atomic nuclei Section 14.2. The weak force plays a crucial role in nuclear reactions such as fission and fusion, and it is the only force besides gravity that affects weakly interacting particles such as neutrinos.
the strongest tests of inflation to date come from studying what ?
patterns of temperature differences in the cosmic microwave background, and in particular the maps of the cosmic microwave background made by the WMAP and Planck satellites (see Figure 22.9). Remember that these maps show tiny temperature differences corresponding to density variations in the universe at the end of the era of nuclei, when the universe was about 380,000 years old. However, models of inflation predict that these density enhancements were created much earlier, when inflation caused tiny quantum ripples to expand into seeds of structure. Moreover, detailed models of inflation predict how the density enhancements should have been distributed in the early universe, and this allows scientists to calculate the angular separation that we should observe among the temperature variations in the cosmic microwave background. For example, if the universe is precisely flat, as inflation predicts it should be, then the largest temperature differences in the cosmic microwave background should be separated by an average of about 1°.
the Big Bang model makes specific predictions that we have observationally verified, including
predictions about the characteristics of the cosmic microwave background and the composition of the universe.
Neutrons are slightly more massive than protons, and therefore
reactions that convert protons to neutrons require energy to proceed (in accordance with E=mc2E=mc2). As the temperature fell below 1011K,1011 K,the energy required for neutron production was no longer readily available, so the rate of these reactions slowed. In contrast, reactions that convert neutrons into protons release energy and therefore are unhindered by cooler temperatures. By the time the temperature of the universe fell to 1010K,1010 K, protons had begun to outnumber neutrons because the conversion reactions ran only in one direction. Neutrons changed into protons, but the protons didn't change back.
