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Dark Energy

One of theobservational foundations for thebig bang model of cosmology was the observedexpansion of the universe. Measurement of the expansion rate is a critical part of the study, and it has been found that the expansion rate is very nearly "flat". That is, the universe is very close to thecritical density, above which it would slow down and collapse inward toward a future "big crunch". A big conceptual problem has been that we haven't been able to observe more than a fraction of that density in the form of ordinary matter. TheWMAP projection of the ordinary baryonic matter is only 4.4% of critical density, and only 27% even when the projected "dark matter" is included. So we are left having to account for the remaining 73% of the effective density, and the name chosen is "dark energy".

One of the great challenges of astronomy and astrophysics isdistance measurement over the vast distances of the universe. Since the 1990s it has become apparent thattype Ia supernovae offer a unique opportunity for the consistent measurement of distance out to perhaps 1000 Mpc. Measurement at these great distances provided the first data to suggest that the expansion rate of the universe is actually accelerating. That acceleration implies an energy density that acts in opposition to gravity which would cause the expansion to accelerate. This is an energy density which we have not directly detected observationally - hence "dark energy".

If we take the WMAP value for critical density at

and presume that dark energy makes up about 73% of that, then the effective density of the dark energy would amount to just over 4 hydrogen atoms (m = 1.67 x 10^-27 kg) in a cubic meter of space. If we take 5.9 x 10⁹ km as a mean radius of Pluto and calculate the volume of a sphere of that radius, then the dark energy in that sphere would be equivalent to just under 6 x 10¹² kg distributed throughout a space representing the solar system. The density of the asteroidIda has been measured to be about 2.7 g/cm³. So all the dark energy in the solar system would amount to about 2.2 x 10⁹ m³ of the material of Ida, or a sphere of about 800 m radius. Ida has a tiny satellite or moon named Dactyl of dimensions 1.2 x 1.4 x 1.6km, so the mass of that tiniest of satellites is comparable to the dark energy in the entire solar system. Yet extended uniformly throughout the entire universe, this dark energy becomes the dominant influence on the expansion of the universe in this era.

Measurement of the redshift of distanttype Ia supernovae is one of the types of evidence for and accelerated expansion of the universe and hence for dark energy.

The density parameter Ω

Index

Reference
Carroll & Ostlie
Ch 29

Conselice

HyperPhysics***** Astrophysics

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