How can it be understood that the universe is 93 billion light years across and yet only 13.8 billion years old? The age of the universe is about 13.8 billion years. How do we know its size (radius at about 46.5 billion light years)? Is the ratio between the two based on the Hubble constant? The reason for the difference is that, first of all, 13.8 billion light years is essentially the radius of a sphere of the CMB radiation that is being observed by the WMAP and Planck satellites. Therefore the diameter of that sphere would be 27.6 billion light years. So that brings us a little closer to the diameter of the 93 billion light year of the observable universe that is often quoted. Each of the yellow or red bumps seen on the CMB image below, will, by NOW, 13.8 billion years later, have become a supercluster of galaxies. In the meantime the continued expansion of the universe would have resulted in those superclusters of galaxies being 46.5 billion light years from us at this time. So the idea is: if we waited another 46.5 - 13.8 = 32.7 billion years, we should actually be able to see the light emitted right now from those superclusters of galaxies in our telescopes. The light is already on its way towards us but it will take a while to reach us since it will have to come from a sphere with a diameter of 93 billion light years. Unfortunately, it is no longer true that we shall eventually see that light from those super clusters. The problem is that we now know that due to dark energy, the expansion of the universe is actually increasing at an accelerated rate (actually it has been accelerating for at least 3 billion years). Because of the accelerated expansion, those superclusters, which are now 46.5 billion light years from us, will be receding from us at a rate that is greater than the speed of light by the time we wait another 32.7 billion more years. So, the diameter of 93 billion light years is, at most, a theoretical estimate of the current distance of all the matter that we can NOW see, even if the light we see is 13.8 billion years old (as in the case for the CMB images). At 379,000 years after the big bang, there was a bump (overdensity) in the region of space where our super-cluster (the Virgo cluster), and our galaxy (the Milky Way) would eventually develop. You might wonder how far away the bumps were then that would eventually show up on the CMB image show above? Well we can calculate that! The CMB is at a redshift of z=1100. There is a scale factor for the universe that is a function of time1 (a)t. The redshift is related to the scale factor by a(t now)/a(t cmb) =z+1. Which means that the diameter of those bumps that would become our CMB image would be 96 billion light years/1101 which is 87 million light years in diameter. SOURCE: Frank Heile, PhD in Physics from Stanford University
Posted on: Tue, 30 Dec 2014 14:04:17 +0000
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