Open Universe
Written by John Carl Villanueva
If the density of the Universe is below what is known as the critical density, it will expand forever, ending in either a Big Freeze or a Big Rip. Furthermore, with this density, its geometry of space is supposed to be saddle shaped or open. Since the most widely accepted and recent data indicates an increasing rate of expansion, many scientists may be inclined to adopt the open universe model.
However, as you will learn in the text below, some recently gathered data lead to contradictory implications.
Just a point of clarification. Although the accelerated expansion rate is largely attributed to the presence of an unknown and never before seen entity, aptly called dark energy, the Universe will still expand forever if its geometry is open.
In terms of the Friedmann equations, an open universe is obtained when the density parameter, represented by the capital Greek letter Omega (Ω), is less than unity or Ω < 1. For comparison Ω = 1 denotes a flat universe, while Ω > 1 denotes a closed one. Ω is defined as the ratio of the observed density to the critical density. The currently accepted value for the critical density is about 10-30 grams per cubic centimeter.
The Friedmann equations were introduced by Alexander Friedmann in order to relate all factors that contribute to the expansion of space based on a homogeneous and isotropic model of the Universe and derived out of Einstein's General Relativity.
Much of the recent measurements of basic parameters related to the Big Bang model, such as density and composition, are being gathered through the WMAP (Wilkinson Microwave Anisotropy Probe). These parameters are based on accurate measurements of the cosmic microwave background (CMB) fluctuations.
Now, since results from WMAP's measurements favor a flat universe model despite indicators that the Universe is expanding at an accelerated rate (which supposedly favors an open model), the temptation to believe in the existence of a dark energy is difficult to resist.
Allow me to elaborate further.
Remember that if the universe is found to be flat, as was found by WMAP through measurements of the CMB, this was supposed to mean that its overall density should be equal to the critical density. However, WMAP has discovered some data contradicting this assumption. Also by measuring the CMB, WMAP has found that the total amount of mass/energy can only account for 30% of the critical density.
To make matters more confusing, a flat universe is expected to expand forever but at a decreasing rate. As stated earlier, this is not what observations have shown. The Universe is expanding at an increasing rate, remember?
To reconcile the two observations, scientists have adopted the notion that there exists a dark energy that makes this increasing rate of expansion possible.
We've got a few articles here in Universe Today that are related to the concepts of an open 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:
Filed under: Astronomy
Tags: Big Bang, close universe.WMAP, Cosmology, open universe
