Critical Density
The 'critical density' is the average density of matter required for the Universe to just halt its expansion, but only after an infinite time. A Universe with the critical density is said to be flat.
Critical Density
The boundary density between the case where the universe has enough mass/volume to close universe and too little mass/volume to stop the expansion is called the critical density.
CRITICAL DENSITY
Consider a rocket ship launched from the surface of a planet. What is the likely outcome of that motion? There are basically two possibilities, depending on the speed of the ship.
critical density
the density of the universe that provides just enough gravity to bring the expansion to a halt after an infinite time
crust ...
Critical Density
The average density of the universe needed to make its curvature flat.
Critical Point ...
Critical Density - The value that the average density of the Universe must equal or exceed if the universe is closed. If the density of the Universe is less than the critical density, the Universe will continue to expand forever ...
CRITICAL DENSITY (ρc) - Boundary value of mass density between universe models that expand forever (open models) and those that recollapse (closed models).
Critical Density
The density that just stops the expansion of space, after infinite cosmic time has elapsed. In the standard models, the critical density requires that the spatial geometry be flat.
Critical Equatorial Velocity ...
Critical Density
The minimum average density that matter in the universe would need in order for its gravitational pull to slow the universe's expansion to a halt.
Dark Matter ...
Critical Density: The mass density of the universe which just stops the expansion of space, after infinite cosmic time has elapsed.
The critical density depends on the gravitational constant G and the Hubble constant H0 = 100 h km s-1 Mpc-1. The Hubble constant is poorly known, but nearly all modern estimates give 0.5 < h < 1.0.
The flatness problem arises because of the observation that the density of the universe today is very close to the critical density required for spatial flatness..
We define the critical density as the density needed to stop the expansion of the Universe. To complicate matters, the value of the critical density depends on the value of the Hubble constant, Ho, and the shape (curvature) of the Universe.
The critical density of the universe for this value of the Hubble constant is 3 H2/8 pi G, which works out to be 1×10−29 grams/cubic centimeter or about 5×10−6 atoms of hydrogen/cc.
We talk about this in terms of the density of the Universe, and compare densities to the critical density. If the density is greater than the critical density, then eventually gravity will overtake the expansion.
This is true as long as the density is much less than the critical density at which collisional de-excitation happens as often as radiation.
In this case, the universe contains enough mass - it is above the critical density - to actually stop its expansion. Once it stops expanding, it will start to contract.
There is a value called the critical density. If the sum is larger than the critical density, then the universe will eventually stop its expansion and start to collapse. We call it a closed universe.
Omega (the last letter in the Greek alphabet) refers to ratio of the observed density of the Universe (how much mass there is per unit volume) to the critical density of the Universe (the density that would be necessary to stop the expansion of the ...
When the collapse reaches a critical density it stops. At this point, the matter in the star's core is packed so tightly that a block of its material the size of a sugarcube would weigh millions of tons.
The geometry of the Universe is often expressed in terms of the "density parameter", which is defined as the ratio of the actual density of the Universe to the critical density that would be required to cause the expansion to stop.
The total amount of matter in the universe (including baryons and dark matter), as measured by the CMB, accounts for only about 30% of the critical density. This implies the existence of an additional form of energy to account for the remaining 70%.
stars and black holes, gravitational lensing, and the convergence of measurements in observational cosmology to an approximately flat model of the observable Universe, with a matter density parameter of approximately 30% of the critical density ...
is the ratio of the density of the to the critical density needed to close the ,
(1) ...
Work presented in 2002 by Antony Stark and Chris Martin mapping the gas density in a 400 light year region around the galactic center has revealed an accumulating ring with a mass several million times that of the Sun and near the critical density ...
Current theory suggests that the spiral is the result of gravitational pull from neighboring galaxies, which causes gas to compress in certain places. Once the gas reaches critical density, hot stars are created.
flatness problem One of two conceptual problems with the Standard Big Bang model, which is that there is no natural way to explain why the density of the universe is so close to the critical density.
 See also: Density, Universe, Mass, Time, Energy
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