Volume renderings of the density field in a region of the simulation at 55,000 years of evolution. The left panel shows a polar view, and the right panel shows an equatorial view. The fingers feeding the equatorial disk are clearly visible. Images by Krumholz et al
That is, if the density is less than what is known as the critical density, then the expansion will prevail. Otherwise, if the density is greater, then gravity will emerge victorious. The critical density, or the value that separates the two possibilities, is believed to be about 10-30 grams per cubic centimeter.
Different measurements made on the Universe such as red shifts of the outermost galaxies indicate that not only is the Universe expanding, but its rate of expansion, particularly on the outer rims, is undergoing acceleration. That is, the galaxies that are farthest from us are moving away much faster than those that are nearer.
Density was first thrust into a major role in this grand event when Alexander Friedmann introduced his Friedmann equations, a set of equations that strive to relate all factors that contribute to the expansion of space based on a homogeneous and isotropic model of the Universe. The equations are based on Einstein’s General Relativity.
Measurements of the Universe’s density, as with all other objects, is easily obtained once its mass and volume is known. Since it would be impossible to measure these physical quantities directly, scientists improvise by sampling a region larger than the scale on which the Universe becomes homogeneous.
To obtain the volume of the selected region, typically taken as a sphere, the radius has to be known. This is calculated based on the maximum redshift at the edge of the said region. The procedure for obtaining the mass is much more sophisticated, making use of an angular size and again the redshift measurements among others.
Based on data gathered by powerful equipment like the WMAP (Wilkinson Microwave Anisotropy Probe), the total density of matter is believed to be equal to the said critical density. If this is true, then the consequence of which is a spatially flat universe. Much of this is also believed to be composed of what they have called as dark energy.
Dark energy, if the scientist’s speculations are correct, provides the driving force that can keep the Universe expanding forever.
We’ve got a few articles here in Universe Today that are related to the density of the Universe. Here are two of them:
NASA also has some more:
Tired eyes? Let your ears help you learn for a change. Here are some episodes from Astronomy Cast that just might suit your taste:
Source: NASA WMAP
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