Origins: Fourteen Billion Years of Cosmic Evolution by Donald Goldsmith, Neil deGrasse Tyson Page A
Authors: Donald Goldsmith, Neil deGrasse Tyson
Ads: Link
received confirmation from detailed observations of the cosmic background radiation, and now that cosmologists have had several years to wrestle with the implications of an accelerating cosmic expansion, two great questions have emerged to bedevil their days and brighten their dreams: What makes the universe accelerate? And why does that acceleration have the particular value that now characterizes the cosmos?
The simple answer to the first question assigns all responsibility for the acceleration to the existence of dark energy, or, equivalently, to a non-zero cosmological constant. The amount of acceleration depends directly on the amount of dark energy per cubic centimeter: More energy implies greater acceleration. Thus, if cosmologists could only explain where the dark energy comes from, and why it exists in the amount that they find today, they could claim to have uncovered a fundamental secret of the universe—the explanation for the cosmic “free lunch,” the energy in empty space that continuously drives the cosmos toward an eternal, ever more rapid expansion and a far future of enormous amounts of space, correspondingly enormous amounts of dark energy, and almost no matter per cubic light-year.
What makes dark energy? From the deep realms of particle physics, cosmologists can produce an answer: The dark energy arises from events that must occur in empty space, if we trust what we have learned from the quantum theory of matter and energy. All of particle physics rests on this theory, which has been verified so often and so exactly in the submicroscopic realm that almost all physicists accept it as correct. An integral part of quantum theory implies that what we call empty space in fact buzzes with “virtual particles,” which wink into and out of existence so rapidly that we can never pin them down directly, but can only observe their effects. The continual appearance and disappearance of these virtual particles, called the “quantum fluctuations of the vacuum” by those who like a good physics phrase, gives energy to empty space. Furthermore, particle physicists can, without much difficulty, calculate the amount of energy that resides in every cubic centimeter of the vacuum. The straightforward application of quantum theory to what we call a vacuum predicts that quantum fluctuations must create dark energy. When we tell the story from this perspective, the great question about dark energy seems to be, Why did cosmologists take so long to recognize that this energy must exist?
Unfortunately, the details of the actual situation turn this question into, How did particle physicists go so far wrong? Calculations of the amount of dark energy that lurks in every cubic centimeter produce a value about 120 powers of ten greater than the value that cosmologists have found from observations of supernovae and the cosmic background radiation. In far-out astronomical situations, calculations that prove correct to within a single factor of 10 are often judged at least temporarily acceptable, but a factor of 10 120 cannot be swept under the rug, even by physics Pollyannas. If real empty space contained dark energy in anything like the amounts proposed by particle physics, the universe would have long since puffed itself into so large a volume that our heads could never have begun to spin, since a tiny fraction of a second would have sufficed to spread matter out to unimaginable rarefaction. Theory and observation agree that empty space ought to contain dark energy, but they disagree by a trillion to the tenth power about the amount of that energy. No earthly analogy, nor even a cosmic one, can illustrate this discrepancy accurately. The distance to the farthest galaxy that we know exceeds the size of a proton by a factor of 10 40 . Even this enormous number is only the cube root of the factor by which theory and observation currently diverge concerning the value of the cosmological constant.
Particle physicists and
The simple answer to the first question assigns all responsibility for the acceleration to the existence of dark energy, or, equivalently, to a non-zero cosmological constant. The amount of acceleration depends directly on the amount of dark energy per cubic centimeter: More energy implies greater acceleration. Thus, if cosmologists could only explain where the dark energy comes from, and why it exists in the amount that they find today, they could claim to have uncovered a fundamental secret of the universe—the explanation for the cosmic “free lunch,” the energy in empty space that continuously drives the cosmos toward an eternal, ever more rapid expansion and a far future of enormous amounts of space, correspondingly enormous amounts of dark energy, and almost no matter per cubic light-year.
What makes dark energy? From the deep realms of particle physics, cosmologists can produce an answer: The dark energy arises from events that must occur in empty space, if we trust what we have learned from the quantum theory of matter and energy. All of particle physics rests on this theory, which has been verified so often and so exactly in the submicroscopic realm that almost all physicists accept it as correct. An integral part of quantum theory implies that what we call empty space in fact buzzes with “virtual particles,” which wink into and out of existence so rapidly that we can never pin them down directly, but can only observe their effects. The continual appearance and disappearance of these virtual particles, called the “quantum fluctuations of the vacuum” by those who like a good physics phrase, gives energy to empty space. Furthermore, particle physicists can, without much difficulty, calculate the amount of energy that resides in every cubic centimeter of the vacuum. The straightforward application of quantum theory to what we call a vacuum predicts that quantum fluctuations must create dark energy. When we tell the story from this perspective, the great question about dark energy seems to be, Why did cosmologists take so long to recognize that this energy must exist?
Unfortunately, the details of the actual situation turn this question into, How did particle physicists go so far wrong? Calculations of the amount of dark energy that lurks in every cubic centimeter produce a value about 120 powers of ten greater than the value that cosmologists have found from observations of supernovae and the cosmic background radiation. In far-out astronomical situations, calculations that prove correct to within a single factor of 10 are often judged at least temporarily acceptable, but a factor of 10 120 cannot be swept under the rug, even by physics Pollyannas. If real empty space contained dark energy in anything like the amounts proposed by particle physics, the universe would have long since puffed itself into so large a volume that our heads could never have begun to spin, since a tiny fraction of a second would have sufficed to spread matter out to unimaginable rarefaction. Theory and observation agree that empty space ought to contain dark energy, but they disagree by a trillion to the tenth power about the amount of that energy. No earthly analogy, nor even a cosmic one, can illustrate this discrepancy accurately. The distance to the farthest galaxy that we know exceeds the size of a proton by a factor of 10 40 . Even this enormous number is only the cube root of the factor by which theory and observation currently diverge concerning the value of the cosmological constant.
Particle physicists and
Similar Books
