Table 1. Observed Relic Values

Relic

When Measured

Observed Values

1H (Hydrogen)

1960s

∼76% by mass

2H (Deuterium)

1970s

>1.8 × 10−5 relative to hydrogen

3He (Helium-3)

1970s

<6 × 10−5 relative to hydrogen

4He (Helium-4)

1960s

∼23±1% by mass

7Li (Lithium-7)

1980s

1.5±0.5 × 10−10 relative to hydrogen

Number of neutrino families

1990

Nv = 2.99±0.02

observed in the universe. The mass density of luminous material instars and galaxies is small, Ωvisible < 0.0l, while the hot gas in galaxy clusters, which astronomers can detectby its x-ray emission, contributes perhaps Ωgas ~ 0.03. The sum of these values lies in the range consistent withprimordial nucleosynthesis. At the same time, dynamical models basedon the relative motions of galaxies, and the way spiral galaxiesrotate, argue that galaxies have more mass than is seen in theirdetected stars and gas. These dynamical arguments imply that eachgalaxy has an invisible halo of dark matter that is about 10 timesthe visible mass. Moreover, consideration of large-scale flows seemsto indicate a still larger amount of dark matter on scales much largerthan single galaxies. Perhaps there is even enough to be consistentwith a total matter density of Ω ~ 1, or at least Ω > ΩB. This implies that there must exist some unknown form of matterthat dominates the mass density of the universe—an awkward situationfor cosmologists.

There are other theoretical reasons to expect that Ω = 1, or in other words that the total mass density is exactly equalto the critical value that just closes the universe. Ω is an unstablequantity in an expanding universe. If Ω is below 1 it will rapidlybecome much less than 1 as expansion proceeds. Conversely, if Ω isgreater than 1, it will grow to values much greater than 1. Onlyif Ω = 1 does it stay at 1; all other values diverge to either zeroor infinity. A finite, non-zero value of Ω today, other than Ω = 1, implies that it musthave been extremely close to 1 at the beginning of the universe.Cosmologists have puzzled over this fine-tuning problem for decades,but just in the past decade or so, considerations of the early universehave motivated a sensible resolution to this question—inflation.

Epoch of Inflation and Grand Unified Theories of Matter

After the Big Bang, the temperature of the early universe was sohigh that the four fundamental forces of nature are believed to havebeen merged. In the grand Unification era that followed, the grandunified theory (GUT) predicts that all the forces except gravitywere of equal strength. Modern theories of these forces involve aconcept known as symmetry breaking, in which the lowest-energy state(the vacuum) is not symmetric at the low temperatures of the presentuniverse. As time progressed, the temperature decreased, and thevacuum underwent a phase transition from a symmetric state of higherenergy. The higher energy of the “false vacuum” can in principle act like a non-zerocosmological constant, A ≠ 0, which, according to Einstein's general relativity theory, can drivean extremely rapid,



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