Tweet
THE FABRIC of the COSMOS, Brian Greene, 2004
```(annotated and with added bold highlights by Epsilon=One)
```(annotated and with added bold highlights by Epsilon=One)
Chapter 14 – Up in the Heavens and Down in the Earth
Dark Matter, Dark Energy, and the Future of the Universe
In Chapter 10 we went through the strong theoretical and observational evidence indicating that a mere 5 percent of the universe's heft comes from the constituents found in familiar matter — protons and neutrons (electrons account for less than .05 percent of ordinary matter's mass) — while 25 percent comes from dark matter and 70 percent from dark energy. But there is still significant uncertainty regarding the detailed identity of all this dark stuff. A natural guess is that the dark matter is also composed of protons and neutrons, ones that somehow avoided clumping together to form light-emitting stars. But another theoretical consideration makes this possibility very unlikely.
Through detailed observations, astronomers have a clear knowledge of the average relative abundances of light elements — hydrogen, helium, deuterium, and lithium — that are scattered throughout the cosmos. To a high degree of accuracy, the abundances agree with theoretical calculations of the processes believed to have synthesized these nuclei during the first few minutes of the universe. This agreement is one of the great successes of modern theoretical cosmology. However, these calculations assume that the bulk of the dark matter is not composed of protons and neutrons; if, on cosmological scales, protons and neutrons were a dominant constituent, the cosmic recipe is thrown off and the calculations yield results that are ruled out by observations.
So, if not protons and neutrons, what constitutes the dark matter? As of today, no one knows, but there is no shortage of proposals. The candidates' names run the gamut from axions to zinos, and whoever finds the answer will surely pay a visit to Stockholm. That no one has yet detected a dark matter particle places significant constraints on any proposal. The reason is that dark matter is not only situated out in space; it is distributed throughout the universe and so is also wafting by us here on earth. According to many of the proposals, right now billions of dark matter particles are shooting through your body every second, so viable candidates are only those particles that can pass through bulky matter without leaving a significant trace.
Neutrinos are one possibility. Calculations estimate their relic abundance since they were produced in the big bang, at about 55 million per cubic meter of space, so if any one of the three neutrino species weighed about a hundredth of a millionth (10^-8) as much as a proton, they would supply the dark matter. Although recent experiments have given strong evidence that neutrinos do have mass, according to current data they are too light to supply the dark matter; they fall short of the mark by a factor of more than a hundred.
Another promising proposal involves supersymmetric particles, especially the photino, the zino, and the higgsino (the partners of the photon, the Z, and the Higgs). These are the most standoffish of the supersymmetric particles — they could nonchalantly pass through the entire earth without the slightest effect on their motion — and hence could easily have escaped detection. 9 From calculations of how many of these particles would have been produced in the big bang and survived until today, physicists estimate that they would need to have mass on the order of 100 to 1,000 times that of the proton to supply the dark matter. This is an intriguing number, because various studies of supersymmetric-particle models as well as of superstring theory have arrived at the same mass range for these particles, without any concern for dark matter or cosmology. This would be a puzzling and completely unexplained confluence, unless, of course, the dark matter is indeed composed of supersymmetric particles. Thus, the search for supersymmetric particles at the world's current and pending accelerators may also be viewed as searches for the heavily favored dark matter candidates.
More direct searches for the dark matter particles streaming through the earth have also been under way for some time, although these are extremely challenging experiments. Of the million or so dark matter particles that should be passing through an area the size of a quarter each second, at most one per day would leave any evidence in the specially designed equipment that various experimenters have built to detect them. To date, no confirmed detection of a dark matter particle has been achieved. 10 With the prize still very much up in the air, researchers are pressing ahead with much intensity. It is quite possible that within the next few years, the identity of the dark matter Will be settled.
Definitive confirmation that dark matter exists, and direct determination of its composition, would be a major advance. For the first time in history, we would learn something that is at once thoroughly basic and surprisingly elusive: the makeup of the vast majority of the universe's material content.
All the same, as we saw in Chapter 10, recent data suggest strongly that even with the identification of the dark matter, there would still be a significant plot twist in need of experimental vetting: the supernova observations that give evidence of an outward-pushing cosmological constant accounting for 70 percent of the total energy in the universe. As the most exciting and unexpected discovery of the last decade, the evidence for a cosmological constant — an energy that suffuses space — needs vigorous, airtight confirmation. A number of approaches are planned or already under way.
The microwave background experiments play an important role here as well. The size of the splotches iri Figure 14.4 — where, again, each splotch is a region of uniform temperature — reflects the overall shape of the spatial fabric. If space were shaped like a sphere, as in Figure 8.6a, the outward bloating would cause the splotches to be a bit bigger than they are in Figure 14.4b; if space were shaped like a saddle, as in Figure 8.6c, the inward shrinking would cause the splotches to be a bit smaller; and if space were flat, as in Figure 8.6b, the splotch size would be in between. The precision measurements initiated by COBE and since bettered by the WMAP strongly support the proposition that space is flat. Not only does this match the theoretical expectations coming from inflationary models, but it also jibes perfectly with the supernova results. As we've seen, a spatially flat universe requires the total mass/energy density to equal the critical density. With ordinary and dark matter contributing about 30 percent and dark energy contributing about 70 percent, everything hangs together impressively.
A more direct confirmation of the supernova results is the goal of the SuperNova/Acceleration Probe (SNAP). Proposed by scientists at the Lawrence Berkeley Laboratory, SNAP would be a satellite-borne orbiting telescope with the capacity to observe and measure more than twenty times the number of supernovae studied so far. Not only would SNAP be able to confirm the earlier result that 70 percent of the universe is dark energy, but it should also be able to determine the nature of the dark energy more precisely.
You see, although I have described the dark energy as being a version of Einstein's cosmological constant — a constant unchanging energy that pushes space to expand — there is a closely related but alternative possibility. Remember from our discussion of inflationary cosmology (and the jumping frog) that a field whose value is perched above its lowest energy configuration can act like a cosmological constant, driving an accelerated expansion of space, but will typically do so only for a short time. Sooner or later, the field will find its way to the bottom of its potential energy bowl, and the outward push will disappear. In inflationary cosmology, this happens in a tiny fraction of a second. But by introducing a new field and by carefully choosing its potential energy shape, physicists have found ways for the accelerated expansion to be far milder in its outward push but to last far longer — for the field to drive a comparatively slow and steady accelerated phase of spatial expansion that lasts not for a fraction of a second, but for billions of years, as the field slowly rolls to the lowest energy value. This raises the possibility that, right now, we may be experiencing an extremely gentle version of the inflationary burst believed to have happened during the universe's earliest moments.
The difference between a true cosmological constant and the latter possibility, known as quintessence, is of minimal importance today, but has a profound effect on the long-term future of the universe. A cosmological constant is constant — it provides a never-ending accelerated expansion, so the universe will expand ever more quickly and become ever more spread out, diluted, and barren. But quintessence provides accelerated expansion that at some point draws to a close, suggesting a far future less bleak and desolate than that following from accelerated expansion that's eternal. By measuring changes in the acceleration of space over long time spans (through observations of supernovae at various distances and hence at various times in the past), SNAP may be able to distinguish between the two possibilities. By determining whether the dark energy truly is a cosmological constant, SNAP will give insight into the long-term fate of the universe.
Through detailed observations, astronomers have a clear knowledge of the average relative abundances of light elements — hydrogen, helium, deuterium, and lithium — that are scattered throughout the cosmos. To a high degree of accuracy, the abundances agree with theoretical calculations of the processes believed to have synthesized these nuclei during the first few minutes of the universe. This agreement is one of the great successes of modern theoretical cosmology. However, these calculations assume that the bulk of the dark matter is not composed of protons and neutrons; if, on cosmological scales, protons and neutrons were a dominant constituent, the cosmic recipe is thrown off and the calculations yield results that are ruled out by observations.
So, if not protons and neutrons, what constitutes the dark matter? As of today, no one knows, but there is no shortage of proposals. The candidates' names run the gamut from axions to zinos, and whoever finds the answer will surely pay a visit to Stockholm. That no one has yet detected a dark matter particle places significant constraints on any proposal. The reason is that dark matter is not only situated out in space; it is distributed throughout the universe and so is also wafting by us here on earth. According to many of the proposals, right now billions of dark matter particles are shooting through your body every second, so viable candidates are only those particles that can pass through bulky matter without leaving a significant trace.
Neutrinos are one possibility. Calculations estimate their relic abundance since they were produced in the big bang, at about 55 million per cubic meter of space, so if any one of the three neutrino species weighed about a hundredth of a millionth (10^-8) as much as a proton, they would supply the dark matter. Although recent experiments have given strong evidence that neutrinos do have mass, according to current data they are too light to supply the dark matter; they fall short of the mark by a factor of more than a hundred.
Another promising proposal involves supersymmetric particles, especially the photino, the zino, and the higgsino (the partners of the photon, the Z, and the Higgs). These are the most standoffish of the supersymmetric particles — they could nonchalantly pass through the entire earth without the slightest effect on their motion — and hence could easily have escaped detection. 9 From calculations of how many of these particles would have been produced in the big bang and survived until today, physicists estimate that they would need to have mass on the order of 100 to 1,000 times that of the proton to supply the dark matter. This is an intriguing number, because various studies of supersymmetric-particle models as well as of superstring theory have arrived at the same mass range for these particles, without any concern for dark matter or cosmology. This would be a puzzling and completely unexplained confluence, unless, of course, the dark matter is indeed composed of supersymmetric particles. Thus, the search for supersymmetric particles at the world's current and pending accelerators may also be viewed as searches for the heavily favored dark matter candidates.
More direct searches for the dark matter particles streaming through the earth have also been under way for some time, although these are extremely challenging experiments. Of the million or so dark matter particles that should be passing through an area the size of a quarter each second, at most one per day would leave any evidence in the specially designed equipment that various experimenters have built to detect them. To date, no confirmed detection of a dark matter particle has been achieved. 10 With the prize still very much up in the air, researchers are pressing ahead with much intensity. It is quite possible that within the next few years, the identity of the dark matter Will be settled.
Definitive confirmation that dark matter exists, and direct determination of its composition, would be a major advance. For the first time in history, we would learn something that is at once thoroughly basic and surprisingly elusive: the makeup of the vast majority of the universe's material content.
All the same, as we saw in Chapter 10, recent data suggest strongly that even with the identification of the dark matter, there would still be a significant plot twist in need of experimental vetting: the supernova observations that give evidence of an outward-pushing cosmological constant accounting for 70 percent of the total energy in the universe. As the most exciting and unexpected discovery of the last decade, the evidence for a cosmological constant — an energy that suffuses space — needs vigorous, airtight confirmation. A number of approaches are planned or already under way.
The microwave background experiments play an important role here as well. The size of the splotches iri Figure 14.4 — where, again, each splotch is a region of uniform temperature — reflects the overall shape of the spatial fabric. If space were shaped like a sphere, as in Figure 8.6a, the outward bloating would cause the splotches to be a bit bigger than they are in Figure 14.4b; if space were shaped like a saddle, as in Figure 8.6c, the inward shrinking would cause the splotches to be a bit smaller; and if space were flat, as in Figure 8.6b, the splotch size would be in between. The precision measurements initiated by COBE and since bettered by the WMAP strongly support the proposition that space is flat. Not only does this match the theoretical expectations coming from inflationary models, but it also jibes perfectly with the supernova results. As we've seen, a spatially flat universe requires the total mass/energy density to equal the critical density. With ordinary and dark matter contributing about 30 percent and dark energy contributing about 70 percent, everything hangs together impressively.
A more direct confirmation of the supernova results is the goal of the SuperNova/Acceleration Probe (SNAP). Proposed by scientists at the Lawrence Berkeley Laboratory, SNAP would be a satellite-borne orbiting telescope with the capacity to observe and measure more than twenty times the number of supernovae studied so far. Not only would SNAP be able to confirm the earlier result that 70 percent of the universe is dark energy, but it should also be able to determine the nature of the dark energy more precisely.
You see, although I have described the dark energy as being a version of Einstein's cosmological constant — a constant unchanging energy that pushes space to expand — there is a closely related but alternative possibility. Remember from our discussion of inflationary cosmology (and the jumping frog) that a field whose value is perched above its lowest energy configuration can act like a cosmological constant, driving an accelerated expansion of space, but will typically do so only for a short time. Sooner or later, the field will find its way to the bottom of its potential energy bowl, and the outward push will disappear. In inflationary cosmology, this happens in a tiny fraction of a second. But by introducing a new field and by carefully choosing its potential energy shape, physicists have found ways for the accelerated expansion to be far milder in its outward push but to last far longer — for the field to drive a comparatively slow and steady accelerated phase of spatial expansion that lasts not for a fraction of a second, but for billions of years, as the field slowly rolls to the lowest energy value. This raises the possibility that, right now, we may be experiencing an extremely gentle version of the inflationary burst believed to have happened during the universe's earliest moments.
The difference between a true cosmological constant and the latter possibility, known as quintessence, is of minimal importance today, but has a profound effect on the long-term future of the universe. A cosmological constant is constant — it provides a never-ending accelerated expansion, so the universe will expand ever more quickly and become ever more spread out, diluted, and barren. But quintessence provides accelerated expansion that at some point draws to a close, suggesting a far future less bleak and desolate than that following from accelerated expansion that's eternal. By measuring changes in the acceleration of space over long time spans (through observations of supernovae at various distances and hence at various times in the past), SNAP may be able to distinguish between the two possibilities. By determining whether the dark energy truly is a cosmological constant, SNAP will give insight into the long-term fate of the universe.