Origins: Fourteen Billion Years of Cosmic Evolution by Donald Goldsmith, Neil deGrasse Tyson Page B
Authors: Donald Goldsmith, Neil deGrasse Tyson
Ads: Link
cosmologists have long known that quantum theory predicts an unacceptably large value for the dark energy, but in the days when the cosmological constant was thought to be zero, they hoped to discover some explanation that would, in effect, cancel positive with negative terms in the theory and thereby finesse the problem out of existence. A similar cancellation once solved the problem of how much energy virtual particles contribute to the particles that we do observe. Now that the cosmological constant turns out to be non-zero, the hopes of finding such a cancellation seem dimmer. If the cancellation does exist, it must somehow remove almost all of the mammoth theoretical value we have today. For now, lacking any good explanation for the size of the cosmological constant, cosmologists must continue to collaborate with particle physicists as they seek to reconcile theories of how the cosmos generates dark energy with the value observed for the amount of dark energy per cubic centimeter.
Some of the finest minds engaged in cosmology and particle physics have directed much of their energy toward explaining this observational value, with no success at all. This provokes fire, and sometimes ire, among theorists, in part because they know that a Nobel Prize—not to mention the immense joy of discovery—awaits those who can explain what nature has done to make space as we find it. But another issue stokes intense controversy as it cries out for explanation: Why does the amount of dark energy, as measured by its mass equivalent, roughly equal the amount of energy provided by all the matter in the universe?
We can recast this question in terms of the two Ωs that we use to measure the density of matter and the density equivalent of dark energy: Why do Ω M and Ω Λ roughly equal one another, rather than one being enormously larger than the other? During the first billion years after the big bang, Ω M was almost precisely equal to 1, while Ω Λ was essentially zero. In those years, Ω M was first millions, then thousands, and afterward hundreds of times greater than Ω Λ . Today, with Ω M = 0.27 and Ω Λ = 0.73, the two values are roughly equal, though Ω Λ is already notably larger than Ω M . In the far future, more than 50 billion years from now, Ω Λ will be first hundreds, then thousands, after that millions, and still later billions of times greater than Ω M . Only during the cosmic era from about 3 billion to 50 billion years after the big bang do the two quantities match one other even approximately.
To the easygoing mind, the interval between 3 billion and 50 billion years embraces quite a long period of time. So what’s the problem? From an astronomical viewpoint, this stretch of time amounts to nearly nothing. Astronomers often take a logarithmic approach to time, dividing it into intervals that increase by factors of 10. First the cosmos had some age; then it grew ten times older; then ten times older than that; and so on toward infinite time, which requires an infinite number of ten-times jumps. Suppose that we start counting time at the earliest moment after the big bang that has any significance in quantum theory, 10 -43 second after the big bang. Since each year contains about 30 million (3 x 10 7 ) seconds, we need about 60 factors of 10 to pass from 10 -43 second to 3 billion years after the big bang. In contrast, we require only a bit more than a single factor of 10 to stroll from 3 billion to 50 billion years, the only period when Ω M and Ω Λ are roughly equal. After that, an infinite number of ten-times factors opens the way to the infinite future. From this logarithmic perspective, only a vanishingly small probability exists that we should find ourselves living in a cosmic situation for which Ω M and Ω Λ have even vaguely similar values. Michael Turner, a leading American cosmologist, has termed this conundrum—the question of why we find ourselves alive at a time when Ω M and Ω Λ are
Some of the finest minds engaged in cosmology and particle physics have directed much of their energy toward explaining this observational value, with no success at all. This provokes fire, and sometimes ire, among theorists, in part because they know that a Nobel Prize—not to mention the immense joy of discovery—awaits those who can explain what nature has done to make space as we find it. But another issue stokes intense controversy as it cries out for explanation: Why does the amount of dark energy, as measured by its mass equivalent, roughly equal the amount of energy provided by all the matter in the universe?
We can recast this question in terms of the two Ωs that we use to measure the density of matter and the density equivalent of dark energy: Why do Ω M and Ω Λ roughly equal one another, rather than one being enormously larger than the other? During the first billion years after the big bang, Ω M was almost precisely equal to 1, while Ω Λ was essentially zero. In those years, Ω M was first millions, then thousands, and afterward hundreds of times greater than Ω Λ . Today, with Ω M = 0.27 and Ω Λ = 0.73, the two values are roughly equal, though Ω Λ is already notably larger than Ω M . In the far future, more than 50 billion years from now, Ω Λ will be first hundreds, then thousands, after that millions, and still later billions of times greater than Ω M . Only during the cosmic era from about 3 billion to 50 billion years after the big bang do the two quantities match one other even approximately.
To the easygoing mind, the interval between 3 billion and 50 billion years embraces quite a long period of time. So what’s the problem? From an astronomical viewpoint, this stretch of time amounts to nearly nothing. Astronomers often take a logarithmic approach to time, dividing it into intervals that increase by factors of 10. First the cosmos had some age; then it grew ten times older; then ten times older than that; and so on toward infinite time, which requires an infinite number of ten-times jumps. Suppose that we start counting time at the earliest moment after the big bang that has any significance in quantum theory, 10 -43 second after the big bang. Since each year contains about 30 million (3 x 10 7 ) seconds, we need about 60 factors of 10 to pass from 10 -43 second to 3 billion years after the big bang. In contrast, we require only a bit more than a single factor of 10 to stroll from 3 billion to 50 billion years, the only period when Ω M and Ω Λ are roughly equal. After that, an infinite number of ten-times factors opens the way to the infinite future. From this logarithmic perspective, only a vanishingly small probability exists that we should find ourselves living in a cosmic situation for which Ω M and Ω Λ have even vaguely similar values. Michael Turner, a leading American cosmologist, has termed this conundrum—the question of why we find ourselves alive at a time when Ω M and Ω Λ are
Similar Books
