dark matter, dark energy,
and the accelerating universe
Another technique is to study the motions of galaxies within clusters of galaxies . The motions are so slow that we can’t actually see galaxies changing positions within a cluster. However, observations show that different galaxies within a cluster have somewhat different redshifts, which indicates that the galaxies are moving relative to the center of mass of the cluster. The speeds of these motions are related to the gravitational force exerted on each galaxy by the other members of the cluster, which in turn depends on the total mass of the cluster. By measuring these speeds, astronomers can determine the average density of all kinds of matter within the cluster, whether or not the matter emits electromagnetic radiation.
Observations using these and other techniques show that the average density of all matter in the universe is 31.5% of the critical density, but the average density of luminous matter is only 4.9% of the critical density. In other words, most of the matter in the universe is not luminous: It does not emit electromagnetic radiation of any kind. At present, the nature of this dark matter remains an outstanding mystery. Some proposed candidates for dark matter are WIMPs (weakly interacting massive particles, which are hypothetical subatomic particles far more massive than those produced in accelerator experiments) and MACHOs (massive compact halo objects, which include objects such as black holes that might form “halos” around galaxies). Whatever the true nature of dark matter, it is by far the dominant form of matter in the universe. For every kilogram of the conventional matter that has been our subject for most of this book—including electrons, protons, atoms, molecules, blocks on inclined planes, planets, and stars— there are about five and a half kilograms of dark matter.
Since the average density of matter in the universe is less than the critical density, it might seem fair to conclude that the universe will continue to expand indefinitely, and that gravitational attraction between matter in different parts of the universe should slow the expansion down (albeit not enough to stop it).
One way to test this prediction is to examine the redshifts of extremely distant objects. The more distant a galaxy is, the more time it takes that light to reach us from that galaxy, so the further back in time we look when we observe that galaxy. If the expansion of the universe has been slowing down, the expansionmust have been more rapid in the distant past. Thus we would expect very distant galaxies to have greater redshifts than predicted by the Hubble law. Only since the 1990s has it become possible to measure accurately both the distances and the redshifts of extremely distant galaxies. The results have been totally surprising: Very distant galaxies, seen as they were when the universe was a small fraction of its present age), have smaller redshifts than predicted by the Hubble law! The implication is that the expansion of the universe was slower in the past than it is now, so the expansion has been speeding up rather than slowing down.
If gravitational attraction should make the expansion slow down, why is it speeding up instead? Our best explanation is that space is suffused with a kind of energy that has no gravitational effect and emits no electromagnetic radiation,but rather acts as a kind of “antigravity” that produces a universal repulsion.
This invisible, immaterial energy is called dark energy. As the name suggests, the nature of dark energy is poorly understood but is the subject of very active research.
Observations show that the energy density of dark energy (measured in, say, joules per cubic meter) is 68.5% of the critical density times c2. As described above, the average density of matter of all kinds is 31.5% of the critical density. From the Einstein relationship E = mc2. Because the energy density of dark energy is nearly three times greater than that of matter, the expansion of the universe will continue to accelerate. This expansion will never stop, and the universe will never contract. Of this, 68.5% is the mysterious dark energy, 26.6% is the no less mysterious dark matter, and a mere 4.9% is well-understood conventional matter. How little we know about the contents of our universe. When we take account of the density of matter in the universe (which tends to slow the expansion of space) and the density of dark energy (which tends to speed up the expansion), the age of the universe turns out to be 13.8 billion 11.38 * 10102 years.
What is the significance of the result that within observational error, the average energy density of the universe tells us that the universe is infinite and unbounded, but just barely so. If the average energy density were even slightly larger the universe would be finite like the surface of the balloon. As of this writing, the observational error in the average energy density is less than 1%, but we can’t be totally sure that the universe is unbounded. Improving these measurements will be an important task for physicists and astronomers in the years ahead.
and the accelerating universe
Astronomers have made extensive studies of the average density of matter in the universe. One way to do so is to count the number of galaxies in a patch of sky.
Based on the mass of an average star and the number of stars in an average galaxy, this effort gives an estimate of the average density of luminous matter in the universe—that is, matter that emits electromagnetic radiation. (You are made of luminous matter because you emit infrared radiation as a consequence of your temperature; It’s also necessary to take into account other luminous matter within a galaxy, including the tenuous gas and dust between the stars.
Based on the mass of an average star and the number of stars in an average galaxy, this effort gives an estimate of the average density of luminous matter in the universe—that is, matter that emits electromagnetic radiation. (You are made of luminous matter because you emit infrared radiation as a consequence of your temperature; It’s also necessary to take into account other luminous matter within a galaxy, including the tenuous gas and dust between the stars.
Another technique is to study the motions of galaxies within clusters of galaxies . The motions are so slow that we can’t actually see galaxies changing positions within a cluster. However, observations show that different galaxies within a cluster have somewhat different redshifts, which indicates that the galaxies are moving relative to the center of mass of the cluster. The speeds of these motions are related to the gravitational force exerted on each galaxy by the other members of the cluster, which in turn depends on the total mass of the cluster. By measuring these speeds, astronomers can determine the average density of all kinds of matter within the cluster, whether or not the matter emits electromagnetic radiation.
Observations using these and other techniques show that the average density of all matter in the universe is 31.5% of the critical density, but the average density of luminous matter is only 4.9% of the critical density. In other words, most of the matter in the universe is not luminous: It does not emit electromagnetic radiation of any kind. At present, the nature of this dark matter remains an outstanding mystery. Some proposed candidates for dark matter are WIMPs (weakly interacting massive particles, which are hypothetical subatomic particles far more massive than those produced in accelerator experiments) and MACHOs (massive compact halo objects, which include objects such as black holes that might form “halos” around galaxies). Whatever the true nature of dark matter, it is by far the dominant form of matter in the universe. For every kilogram of the conventional matter that has been our subject for most of this book—including electrons, protons, atoms, molecules, blocks on inclined planes, planets, and stars— there are about five and a half kilograms of dark matter.
Since the average density of matter in the universe is less than the critical density, it might seem fair to conclude that the universe will continue to expand indefinitely, and that gravitational attraction between matter in different parts of the universe should slow the expansion down (albeit not enough to stop it).
One way to test this prediction is to examine the redshifts of extremely distant objects. The more distant a galaxy is, the more time it takes that light to reach us from that galaxy, so the further back in time we look when we observe that galaxy. If the expansion of the universe has been slowing down, the expansionmust have been more rapid in the distant past. Thus we would expect very distant galaxies to have greater redshifts than predicted by the Hubble law. Only since the 1990s has it become possible to measure accurately both the distances and the redshifts of extremely distant galaxies. The results have been totally surprising: Very distant galaxies, seen as they were when the universe was a small fraction of its present age), have smaller redshifts than predicted by the Hubble law! The implication is that the expansion of the universe was slower in the past than it is now, so the expansion has been speeding up rather than slowing down.
If gravitational attraction should make the expansion slow down, why is it speeding up instead? Our best explanation is that space is suffused with a kind of energy that has no gravitational effect and emits no electromagnetic radiation,but rather acts as a kind of “antigravity” that produces a universal repulsion.
This invisible, immaterial energy is called dark energy. As the name suggests, the nature of dark energy is poorly understood but is the subject of very active research.
Observations show that the energy density of dark energy (measured in, say, joules per cubic meter) is 68.5% of the critical density times c2. As described above, the average density of matter of all kinds is 31.5% of the critical density. From the Einstein relationship E = mc2. Because the energy density of dark energy is nearly three times greater than that of matter, the expansion of the universe will continue to accelerate. This expansion will never stop, and the universe will never contract. Of this, 68.5% is the mysterious dark energy, 26.6% is the no less mysterious dark matter, and a mere 4.9% is well-understood conventional matter. How little we know about the contents of our universe. When we take account of the density of matter in the universe (which tends to slow the expansion of space) and the density of dark energy (which tends to speed up the expansion), the age of the universe turns out to be 13.8 billion 11.38 * 10102 years.
What is the significance of the result that within observational error, the average energy density of the universe tells us that the universe is infinite and unbounded, but just barely so. If the average energy density were even slightly larger the universe would be finite like the surface of the balloon. As of this writing, the observational error in the average energy density is less than 1%, but we can’t be totally sure that the universe is unbounded. Improving these measurements will be an important task for physicists and astronomers in the years ahead.
ΑΝΑΔΗΜΟΣΙΕΥΣΗ ΑΠΟ ΤΟ UNIVERSITY PHYSICS WITH MODERN PHYSICS, YOUNG, FREEDMAN 11/9/2017
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