"Even if there is only one possible unified theory, it is just a set of rules and equations. What is it that breathes fire into the equations and makes a universe for them to describe? The usual approach of science of constructing a mathematical model cannot answer the questions of why there should be a universe for the model to describe. Why does the universe go to all the bother of existing?"
Cosmology is the study of the universe as a whole. The big questions of cosmology include the nature of the universe, how it began, and its ultimate fate. A basic assumption of physical cosmology is that the universe is governed by physical laws. For example, the nature of electromagnetic forces and gravity, and the basic structure of matter, are the same everywhere in the universe and they are the same now as they were in the past and the future. Like other scientists, cosmologists form hypotheses and theories and then test them with observations. However, unlike many other fields of science, the cosmologist cannot test theories by changing the conditions of the experiment. Much of the data was generated billions of years ago, although it is just becoming available to us now.
What exists beyond the universe?
Long ago, the universe was believed to consist of Earth, surrounded by the sun, the moon, the planets, and a sphere of stars. As more information has been obtained about objects in space, we know that the universe is unimaginably larger than that early model—so vast that light traveling at 186,000 miles a second takes billions of years to reach us from the most distant galaxies. Even with a universe that huge, we still ask an obvious question: what exists beyond the universe?
Because the universe is defined to be everything that exists, including matter, energy, space, and time, there cannot be anything outside the universe. Even empty space cannot surround the universe because that space would be part of the universe. On a conceptual basis, it is hard to understand how an expanding universe cannot be expanding into something, but a mathematical description of the universe does not have the same constraints as the human imagination.
Fast Facts
In order to understand the universe at all, we have to make a basic assumption: the laws of the universe apply throughout the entire universe and do not vary from place to place. While it may seem obvious, it is necessary to make this assumption as part of any explanation. For example, the speed of light in a vacuum is a constant value in all of our observations. If this is not true in other parts of the universe, then any data we obtain from them is not useful in describing the universe as a whole.
There are speculations that other universes exist, perhaps even an infinite number of other universes. If there are other universes, it is possible that the natural laws that govern them are completely different from those of our universe. However, we cannot treat these ideas as scientific concepts because we have no way to obtain data from other universes, so there can be no experimental test hypotheses about their existence or the existence of anything else outside the universe that we can observe.
How do we know that the universe is expanding?
The Doppler effect, which is the change in wavelength of a wave based on the motion of its source relative to the observer (see Chapter 4), is a key element of our understanding of the expansion of the universe. What does the Doppler effect have to do with the size of the universe?
Light from stars and galaxies comes from the energy changes of electrons in atoms. Because only certain energy changes are possible, we know what wavelengths of light are emitted by energy sources such as stars. A star that is moving toward our solar system emits light in the same wavelengths as a star that is stationary relative to our solar system. However, the light that we perceive has a shorter wavelength—that is, it is shifted toward the blue end of the spectrum. A light from a star that is moving away from us shows a red shift.
The same effect occurs with galaxies that are in motion relative to our galaxy. The light from some nearby galaxies is blue-shifted, but most of the galaxies in the universe have light that is red-shifted. The interesting observation is that, with the exception of a few nearby galaxies, the farther a galaxy or other object is from the Milky Way, the greater its red shift. That means the most distant objects are moving away fastest. This phenomenon is the same in every direction we look.
Remember that the balloon model is designed to illustrate one aspect of expansion. It is not a model of the universe itself. The Big Bang is often described as an explosion. In our experience, material moves outward from the center of an explosion. This, too, is just a model to explain the beginning of the expansion of the universe. The universe, as a whole, has no center.
If everything is moving apart faster and faster in all directions, then the universe is expanding. The observation does not mean, however, that we are at the center of the expansion, as it may seem. It doesn’t matter what reference point you use, the universe is expanding in all directions from it. A common model to illustrate this concept is a balloon. If you place a number of dots on the surface of a partially inflated balloon and then continue to inflate it, what happens to the distance between dots? The distance between any two dots increases, and the farther the dots are from one another, the greater the increase. Galaxies and groups of galaxies are the dots in our universe.
How do we measure the distance to faraway galaxies?
We measure the distance to stars that are relatively close to the sun by using the parallax (see Chapter 16) from observations at opposite ends of Earth’s orbit. This method is useful for objects that are within about 300 light-years of Earth. However, the Milky Way measures about 100,000 light-years across—and most galaxies are millions or billions of light-years away. How do we measure the distance to objects beyond the limit of parallax measurements?
Definition
A light-year is defined as the distance that light travels in a vacuum in one year. Keep in mind that a light-year is a unit of distance, not of time. One light year is equal to about 5.9 billion miles.
To start, let’s look at how we can measure the distance to stars in our galaxy that are farther than 300 light-years from us. Although there are many stars, they fall into a limited range of types, which can be distinguished by the spectra of the light that they emit. All of the stars of a particular type are very similar and emit about the same amount of light. The brightness of the light that we detect, however, decreases as the square of the distance to its source. If we know how bright a star really is (based on similar stars that are close to us) and how bright it appears to be, we can calculate its distance. This method works for finding the distance to the stars in our galaxy and other galaxies nearby. For galaxies that are greater than about 30 million light-years distant, the brightness of individual normal stars is not sufficient for the distance measurement.
The distance to galaxies up to about a billion light years away can be measured by a similar technique, though. Two different measures of brightness are available. First, there are certain stars, called variables, whose brightness increases and decreases in a regular cycle ranging from about 1 day to about 70 days. The brightness of these variables correlates directly to the cyclic period. The period gives a measure of actual brightness. The second measure is available when stars explode at the ends of their lifetimes. These exploding stars, known as novas and supernovas, can be billions of times as bright as a normal star but their brilliance lasts only for a few hours or days. The actual amount of light emitted depends on the type of star and can be identified by its spectrum. A nova or supernova in a galaxy is a benchmark that can be used to determine its distance.
A nova or supernova occurs when the heat of a star is not sufficient to keep the star from collapsing under its own gravitational force. When it does collapse, tremendous energy is generated in its interior, causing the star to explode. In the Milky Way, novas occur several times a year. Supernovas are rare, though, occurring on average about once a century in a galaxy such as ours. The last supernova in the Milky Way occurred in 1680.
For objects that are more distant than about a billion light years, we can measure their distance by the red shift of their light. The lengthening of light waves provides information that can be used both for determining a galaxy’s distance and how fast it is moving.
How old is the universe?
Humans have been around for maybe a few hundred million years, but Earth itself is much older—about 4 billion years or more. Scientists have evidence that even the solar system is a fairly young part of the universe. How old is the universe and how do we know?
We cannot know how the universe looks today by observation alone. When we look at a distant galaxy—for example, a galaxy that is a billion light-years away—we don’t know how it looks now. We know how it looked a billion years ago. Also, the galaxy is not a billion light-years from Earth. Its light has taken a billion years to reach us, but the distance between us and the galaxy is increasing constantly. When the light we see now began its journey, the galaxy was less than a billion light-years away from us. Today, it is much more than a billion light-years away.
Until recently, there were two different ways to estimate the age of the universe—the ages of the oldest stars and the expansion rate. The age of a star is calculated from its mass and the rate that it generates energy. The expansion of the universe can be calculated from the red shift of distant galaxies. To determine the age of the universe, you "play it backwards" to the time when the entire universe was a single point.
Neither of these methods yields an exact result for the age and there is some disagreement between the values. If you retrace the expansion of the universe, you find an age of about nine billion years. However, the estimates for the age of the oldest stars ranges from 11 to 18 billion years. It is obvious that no stars can be older than the universe. There are several possible explanations: the calculation of the expansion of the universe is wrong or our measurements are not accurate enough; the Big Bang theory is incorrect; or there is more mass in the universe than believed. The existence of additional matter is considered a possibility because that would explain a slower expansion of the universe and there is some additional evidence that we are not able to detect everything in the universe (see dark matter and dark energy later in this chapter).
How can we detect a black hole if light can’t escape it?
A black hole is a region in space with a gravitational field so strong that even light is pulled into it if it gets close enough. In fact, that is the source of its name. If light does not leave a black hole, it is invisible. How is it possible to detect a black hole in space?
The escape velocity of Earth is about 7 miles per second, which means that an object must move away at that speed to escape the pull of Earth’s gravity. If the mass of the planet were doubled, the escape velocity would double. If its diameter were halved, the escape velocity would be increased by a factor of 4. That is because the pull of gravity increases with mass and decreases by the square of the distance from the center of the mass. On the surface of Earth, you are about 4,000 miles from its center.
The event horizon is the radius around a black hole at which nothing can escape its gravitational pull. The event horizon is not a solid surface. Matter can pass through it into the black hole. We cannot observe anything beyond the event horizon, so we will never be able to determine what "really happens" inside the black hole. Astronomers calculate that the gravity within the event horizon will pull all of the mass into a single point, called a singularity.
A black hole that has the same mass of the sun has a diameter of less than 2 miles. This diameter corresponds to the event horizon, the sphere around the black hole at which the escape velocity is 186,000 miles per second, the speed of light. Because nothing can travel faster than the speed of light, nothing, including light itself, can move fast enough to escape the black hole if it is located at or inside the event horizon.
Because light cannot get past the event horizon, you cannot see a black hole directly and there can be no actual data about what happens inside the black hole. It is possible, however, to observe a black hole by its effect on other objects.
If there is a large mass in a region, its gravitational effects on other masses will reveal it. Astronomers look for black holes by looking for a large mass in a small volume that does not emit light. They believe that they have found black holes in the center of some galaxies, possibly including the Milky Way. Remember that you can determine the mass of something by the rate at which other objects orbit around it. The speeds of rotation of stars around the center of certain galaxies indicate masses in their cores that range from millions to billions of times the mass of the sun.
The second way to find a black hole is to look for the emission of energy from the material around it. If the black hole is surrounded by gas or near a star or other source of gas, atoms that approach too closely will be pulled toward the black hole. As they move faster and faster, these atoms gain a tremendous amount of energy. When they collide with one another, before reaching the event horizon, the atoms emit radiation as high-energy x-rays or gamma rays. Because this radiation is outside the event horizon, it can travel away from the black hole. These emissions are not a certain indication of a black hole because there are other possible sources for them, but they do give astronomers an idea where to look.
Fast Facts
No one has ever directly observed a black hole. Currently, black holes are a theoretical construction based on our knowledge of physics, but their existence is consistent with a large amount of real data. Even though many astronomers believe that the core of many galaxies consists of a massive black hole, that explanation is based on a number of different observations of the behavior of matter around the core of the galaxy. The data are consistent with the theory of their existence, but scientists are seeking more evidence before stating with certainty that black holes exist. No one will ever directly observe a black hole.
Why are quasars so bright?
Quasars are the most distant objects ever observed. They are also the brightest, generating as much as 100 times the light of the Milky Way from a region that is no larger than our solar system. What are quasars and why are they so bright?
When quasars were first discovered in the 1950s, it was clear that they were much smaller than galaxies but far too bright to be stars. They were called quasars, short for quasi-stellar radio sources. More than 100,000 quasars have been detected. They are so distant that their light has traveled billions of years to reach us, so we are seeing them as they existed long ago. Most quasars appear to have been formed during the first billion years of the universe’s existence.
"The black holes of nature are the most perfect macroscopic objects there are in the universe: the only elements in their construction are our concepts of space and rime."
Current theories explain that a quasar is a galaxy with a supermassive black hole in its center. Because the quasar is so distant, we do not see the stars and other matter surrounding the black hole, so its diameter appears to be very small. As matter approaches the black hole, it acquires a tremendous amount of energy due to the gravitational pull of a black hole with a mass that may be as large as 100 million times the mass of the sun. Some of this energy is generated as electromagnetic radiation, including light, radio waves, and x-rays. It is this radiation that we detect billions of years later on Earth.
There were more quasars in the early universe because they can only emit light when matter is falling into the black hole. As the gas and dust near the center of the galaxy are consumed by the black hole, it dims substantially. It is possible that most galaxies were quasars at one time and they have now become quiet.
What are dark matter and dark energy?
Recent estimates of the composition of the universe, based on its rate of expansion, observations of the galaxies, have shown that we cannot see everything. If the parts that we can see accounted for the entire universe, it would not act as it does without violating the laws of physics. Astronomers now calculate that we can only detect about 4 percent of the universe. The rest consists of matter and energy that we cannot detect, which they call dark matter and dark energy. What are dark matter and dark energy?
The existence of dark matter was proposed decades ago when scientists determined that the total mass of the stars and the dust between them is too small to generate enough gravitational force to hold the galaxies together. Also, galaxies in clusters move too rapidly for their motion to be explained by the gravity of their known matter. There must be some additional matter around us that holds things together, but which we cannot detect. This matter is called dark because it does not emit any electromagnetic radiation that we can detect.
Even taking into account dark matter, most of the universe is very empty. Consider that the nearest star is 4.3 light-years from the sun. There is very little matter between them and we are inside a galaxy, one of the dense clusters of matter within the universe as a whole. Regions between galaxies and between clusters of galaxies contain very little matter. By contrast, we are in a very cluttered part of the universe.
Some of this dark matter may consist of ordinary matter that we cannot see, such as very dim stars or stars that have not yet ignited. Because the amount of unseen matter is so great, astrophysicists have also proposed that there is dark matter of a different sort. This matter would not consist of protons, neutrons, electrons, or any of the known subatomic particles. They do not interact with the matter we know through the electromagnetic force, although they do interact through the gravitational force. Such matter could be all around us and we would only be able to detect it when it had sufficient concentration to be visible by its effects on gravity.
Dark energy was first proposed in the late 1990s when astronomers discovered that the rate of expansion of the universe is increasing. Because gravity works to slow the expansion, there must be a force opposing gravity to increase the rate. The nature of dark energy is not known, although a number of ideas have been proposed. Some astrophysicists believe that its source may be outside our universe. If data were found to support that theory, it would be the first evidence that our universe is not entirely alone.
What forces shape the galaxies?
A spiral galaxy, such as the Milky Way, is an amazing structure, when you consider that each graceful spiral arm is made of billions of stars. The total number of stars in our galaxy is estimated to be about 100 billion. How did these graceful structures form in the first place?
Just as there are billions of people on Earth, each with their own physical characteristics that distinguish them from one another, each galaxy has its own characteristics. Many are spirals, such as the Milky Way and the Andromeda Galaxy, which is often photographed. Many other galaxies are ellipticals, shaped like a rounded football (American football, not soccer), often very symmetrical around an axis. Most galaxies fall into one of these types, although the variations within a type are very broad.
The spiral galaxies are all flattened disk shapes that are spinning around the center part of the galaxy. Stars wander in and out of the arms as the galaxy rotates. The arms of a spiral are not caused by the streaming of stars as the galaxy rotates. The arms themselves are density waves, just as sound is a density wave in air. The regions between the arms are not voids—they contain gases and other nonluminous material and have nearly as much matter as the arms themselves. We see the arms because the density waves create local concentrations of matter so new stars form in the arms and emit light. The cause of the waves is not clear, but it appears that they are frequently the result of the gravitational effect of nearby galaxies on the spinning cloud of stars and gases.
According to most current theories, the great majority of galaxies, including our own, formed during the first billion years or so after the Big Bang. During this period, huge clouds of gas pulled into clumps due to the effect of gravity. Frequently, a force caused the mass of material to begin spinning, making disk-shaped clouds. Within these clouds, smaller clumps pulled together to form stars. As gravity pulls the matter of the spinning cloud closer to its axis, the spin increases. Over time, the stars burn out and die and new stars form, but gravity keeps the mass of the galaxy together.
Looking at a picture from the Hubble telescope showing hundreds of galaxies of different sizes and shapes, it is easy to think of it as a snapshot in time. Galaxies are constantly changing, though, and the light from two galaxies in a single photo carries information from an incredible range of times. While we see nearby galaxies as they appeared millions of years ago, the light we see from the most distant galaxies shows us what they looked like billions of years ago. Many of these galaxies no longer exist.
The force of gravity also causes galaxies to collide. As they come together, the stars in each galaxy exert a force on the passing stars of the other galaxy. The stars and gas of the two individuals merge into a larger galaxy. The result of the merger is a large elliptical galaxy.
Galaxies move through space as a mass of stars and other matter but, in general, they do not exist in isolation. Most galaxies appear to be part of a group of up to 50 galaxies that are bound to one another by gravity. They move together and exert forces on one another. These groups frequently form part of a cluster, which can consist of as many as a thousand galaxies.
The total number of galaxies in the universe is not known because we cannot see them all. Some are too faint to detect with our current technology and others are hidden behind closer galaxies or clouds of dust. Using photographs from the Hubble space telescope, astronomers have estimated that there are between 250 billion and 500 billion galaxies. These estimates could increase when more powerful telescopes are trained on the distant parts of the universe.
How will the universe end?
As the study of the universe, cosmology is concerned not only with the universe as it is today, or as it existed in the past. Another key question is: In the long run, where are we going? How will the universe end?
This is a hard question to answer. We have a lot of data about the universe today (at least the little section of it that we can observe around us). We also have a record of the past in the light that is constantly reaching us from distant galaxies. It seems like we should be able to just look at the past, project through the present, and into the future. But, as with most things in science, it is not that simple.
For one thing, we still are not certain of the mass of the universe. This is a key bit of knowledge. The universe is expanding and gravity fights that expansion. If we add up all the matter in the universe that we can’t see, it is not enough to pull the distant parts back together, so the universe would just keep expanding. This is where dark matter comes in. We see evidence that the gravitational force throughout the universe is greater than can be explained by what we see, so there must be matter that we don’t see. Is there enough dark matter to pull the universe back together, eventually causing it to die a fiery death as an unimaginably big black hole forms?
To answer this question, astrophysicists began to measure the expansion of the universe, to see if it is slowing. That’s where things became complicated again. They found that the universal expansion is not slowing, but accelerating. That discovery led to the concept of dark energy, acting as an "anti-gravity."
We do know that in billions of years, the sun and other stars will use up all their fuel and go dim. Beyond that, it is not clear. The matter in galactic clusters may, over many trillions of years, pull together into black holes, leaving an empty universe speckled with invisible holes. Another possibility is that matter will spread thinner and thinner through expansion of the universe, gradually losing energy and leaving an unimaginably cold, almost empty space of immense proportions. Still another possibility is a repeat of whatever event led to the Big Bang in the first place, starting everything all over.
We may never know whether any of these descriptions is correct. Scientists continue to collect evidence to describe the past and present of the universe. From this evidence, they expect to be able to make better predictions about its future. The concepts of dark energy, dark matter, and even the expansion of the universe are relatively new. They are all based on advances in our ability to collect data about our most distant past. As better tools and techniques are developed, the concepts will be modified based on new data. Theories about the fate of the universe will also build on the new data.
"The only reason for time is so that everything doesn’t happen at once." -Albert Einstein (1879-1955)
