“critical density”的概念、定义、翻译、参考文献-科学参考

critical density

Astronomy

The mean density of matter that is required for gravity to halt the expansion of the Universe, equivalent to about 10^–29 g/cm³. A universe with a density below the critical density will expand forever. If the cosmological constant is zero, a universe with a density greater than the critical density will eventually collapse. A universe with exactly the critical density is described by the Einstein–de Sitter model, which lies on the dividing line between these two extremes. The average density of material that can be directly observed in our Universe is less than 1% of the critical value. However, various observations suggest that most of the matter in the Universe is dark matter, which cannot be seen directly but which can be detected through its gravitational effect. Even when this dark matter is added in, the average density of the Universe is still only about 30% of the critical density. It therefore seems likely that the Universe will expand forever, especially because recent observations imply that the cosmological constant is not zero, which will cause the expansion to accelerate. Current observations show that the expansion of the Universe is accelerating, indicating a non-zero cosmological constant; this implies that the Universe will expand forever.

Space Exploration

In cosmology, the minimum average density that the universe must have in order for it to stop expanding at some point in the future.
The precise value depends on Hubble's constant and so is not fixed, but it is approximately between 10^–29 and 2 × 10^–29 g/cm³, equivalent to a few hydrogen atoms per cubic metre. The density parameter (symbol Ω) is the ratio of the actual density to the critical density. If Ω < 1, the universe is open and will expand forever. If Ω > 1, the universe is closed and the expansion will eventually halt, to be followed by a contraction (Big Crunch). Current estimates from visible matter in the universe indicate that Ω is about 0.01, well below critical density, but unseen dark matter may be sufficient to raise Ω to somewhere between 0.1 and 2.

单词	critical density
释义	critical density Astronomy The mean density of matter that is required for gravity to halt the expansion of the Universe, equivalent to about 10^–29 g/cm³. A universe with a density below the critical density will expand forever. If the cosmological constant is zero, a universe with a density greater than the critical density will eventually collapse. A universe with exactly the critical density is described by the Einstein–de Sitter model, which lies on the dividing line between these two extremes. The average density of material that can be directly observed in our Universe is less than 1% of the critical value. However, various observations suggest that most of the matter in the Universe is dark matter, which cannot be seen directly but which can be detected through its gravitational effect. Even when this dark matter is added in, the average density of the Universe is still only about 30% of the critical density. It therefore seems likely that the Universe will expand forever, especially because recent observations imply that the cosmological constant is not zero, which will cause the expansion to accelerate. Current observations show that the expansion of the Universe is accelerating, indicating a non-zero cosmological constant; this implies that the Universe will expand forever. Space Exploration In cosmology, the minimum average density that the universe must have in order for it to stop expanding at some point in the future. The precise value depends on Hubble's constant and so is not fixed, but it is approximately between 10^–29 and 2 × 10^–29 g/cm³, equivalent to a few hydrogen atoms per cubic metre. The density parameter (symbol Ω) is the ratio of the actual density to the critical density. If Ω < 1, the universe is open and will expand forever. If Ω > 1, the universe is closed and the expansion will eventually halt, to be followed by a contraction (Big Crunch). Current estimates from visible matter in the universe indicate that Ω is about 0.01, well below critical density, but unseen dark matter may be sufficient to raise Ω to somewhere between 0.1 and 2.
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