Origins: Fourteen Billion Years of Cosmic Evolution by Donald Goldsmith, Neil deGrasse Tyson
Authors: Donald Goldsmith, Neil deGrasse Tyson
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mass equivalent, E/c 2 , then dark energy furnishes 73 percent of all the mass.)
Cosmologists have long known that if the universe has a non-zero cosmological constant, the relative influence of matter and dark energy must change significantly as time passes. On the other hand, a flat universe remains flat forever, from its origin in the big bang to the infinite future that awaits us. In a flat universe, the sum of Ω M and Ω Λ always equals 1, so if one of these changes, the other must also vary in compensation.
During the cosmic epochs that followed soon after the big bang, the dark energy produced hardly any effect on the universe. So little space existed then, in comparison to the eras that would follow, that Ω Λ had a value just a bit above zero, while Ω M was only a tiny bit less than 1. In those bygone ages, the universe behaved in much the same way as a cosmos without a cosmological constant. As time passed, however, Ω M steadily decreased and Ω Λ just as steadily increased, keeping their sum constant at 1. Eventually, hundreds of billions of years from now, Ω M will fall almost all the way to zero and Ω Λ will rise nearly to unity. Thus, the history of flat space with a non-zero cosmological constant involves a transition from its early years, when the dark energy barely mattered, through the “present” period, when Ω M and Ω Λ have roughly equal values, and on into an infinitely long future, when matter will spread so diffusely through space that Ω M must pursue an infinitely long slide toward zero, even as the sum of the two Ω s remains equal to 1.
Observational deduction of how much mass exists in galaxy clusters now gives Ω M a value of about 0.25, while the observations of the CBR and distant supernovae imply a value close to 0.27. Within the limits of experimental accuracy, these two values coincide. If the universe in which we live does have a non-zero cosmological constant, and if that constant is responsible (along with the matter) for producing the flat universe that the inflationary model predicts, then the cosmological constant must have a value that makes Ω Λ equal to a bit more than 0.7, two and a half times the value of Ω M . In other words, Ω Λ must now do most of the work in making ( Ω M + Ω Λ ) equal to 1. This means that we have already passed through the cosmic era when matter and the cosmological constant contributed the same amount (with each of them equal to 0.5) toward maintaining the flatness of space.
Within less than a decade, the double-barreled blast from the Type Ia supernovae and the cosmic background radiation has changed the status of dark energy from a far-out idea that Einstein once toyed with to a cosmic fact of life. Unless a host of observations eventually prove to be misinterpreted, inaccurate, or just plain wrong, we must accept the result that the universe will never contract or recycle itself. Instead, the future seems bleak: a hundred billion years from now, when most stars will have burnt themselves out, all but the closest galaxies will have vanished across our horizon of visibility.
By then, the Milky Way will have coalesced with its nearest neighbors, creating one giant galaxy in the literal middle of nowhere. Our night sky will contain orbiting stars, (dead and alive) and nothing else, leaving future astrophysicists a cruel universe. With no galaxies to track the cosmic expansion, they will erroneously conclude, as did Einstein, that we live in a static universe. The cosmological constant and its dark energy will have evolved the universe to a point where they cannot be measured or even dreamt of.
Enjoy cosmology while you can.
CHAPTER 6
One Universe or Many?
T he discovery that we live in an accelerating universe, with an ever-increasing rate of expansion, rocked the world of cosmology early in 1998, with the first announcement of the supernova observations that point to this acceleration. Now that the accelerating universe has
Cosmologists have long known that if the universe has a non-zero cosmological constant, the relative influence of matter and dark energy must change significantly as time passes. On the other hand, a flat universe remains flat forever, from its origin in the big bang to the infinite future that awaits us. In a flat universe, the sum of Ω M and Ω Λ always equals 1, so if one of these changes, the other must also vary in compensation.
During the cosmic epochs that followed soon after the big bang, the dark energy produced hardly any effect on the universe. So little space existed then, in comparison to the eras that would follow, that Ω Λ had a value just a bit above zero, while Ω M was only a tiny bit less than 1. In those bygone ages, the universe behaved in much the same way as a cosmos without a cosmological constant. As time passed, however, Ω M steadily decreased and Ω Λ just as steadily increased, keeping their sum constant at 1. Eventually, hundreds of billions of years from now, Ω M will fall almost all the way to zero and Ω Λ will rise nearly to unity. Thus, the history of flat space with a non-zero cosmological constant involves a transition from its early years, when the dark energy barely mattered, through the “present” period, when Ω M and Ω Λ have roughly equal values, and on into an infinitely long future, when matter will spread so diffusely through space that Ω M must pursue an infinitely long slide toward zero, even as the sum of the two Ω s remains equal to 1.
Observational deduction of how much mass exists in galaxy clusters now gives Ω M a value of about 0.25, while the observations of the CBR and distant supernovae imply a value close to 0.27. Within the limits of experimental accuracy, these two values coincide. If the universe in which we live does have a non-zero cosmological constant, and if that constant is responsible (along with the matter) for producing the flat universe that the inflationary model predicts, then the cosmological constant must have a value that makes Ω Λ equal to a bit more than 0.7, two and a half times the value of Ω M . In other words, Ω Λ must now do most of the work in making ( Ω M + Ω Λ ) equal to 1. This means that we have already passed through the cosmic era when matter and the cosmological constant contributed the same amount (with each of them equal to 0.5) toward maintaining the flatness of space.
Within less than a decade, the double-barreled blast from the Type Ia supernovae and the cosmic background radiation has changed the status of dark energy from a far-out idea that Einstein once toyed with to a cosmic fact of life. Unless a host of observations eventually prove to be misinterpreted, inaccurate, or just plain wrong, we must accept the result that the universe will never contract or recycle itself. Instead, the future seems bleak: a hundred billion years from now, when most stars will have burnt themselves out, all but the closest galaxies will have vanished across our horizon of visibility.
By then, the Milky Way will have coalesced with its nearest neighbors, creating one giant galaxy in the literal middle of nowhere. Our night sky will contain orbiting stars, (dead and alive) and nothing else, leaving future astrophysicists a cruel universe. With no galaxies to track the cosmic expansion, they will erroneously conclude, as did Einstein, that we live in a static universe. The cosmological constant and its dark energy will have evolved the universe to a point where they cannot be measured or even dreamt of.
Enjoy cosmology while you can.
CHAPTER 6
One Universe or Many?
T he discovery that we live in an accelerating universe, with an ever-increasing rate of expansion, rocked the world of cosmology early in 1998, with the first announcement of the supernova observations that point to this acceleration. Now that the accelerating universe has
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