Inconsistency Between the Age and Diameter of the Universe?

-- | May 17, 2015

Question: I have read that the diameter of the universe is 96 billion light years. How can that be if the universe is a mere 14 billion years old? Am I to conclude that the 96 billion figure is some extrapolation based on rapid inflation? — Carlton

Answer: One of my physicist colleagues, Frank Heile from Stanford University, has provided an excellent answer to this question. First of all, the 13.8 billion light years is derived from the radius of a sphere of the Cosmic Microwave Background (CMB) radiation that is being observed by the Wilkinson Microwave Anisotropy Probe (WMAP) and Planck satellites. These satellites have mapped the structures which are precursors to galaxies and clusters of galaxies. Now, the radiation from the CMB measured by the WMAP and Planck satellites has, in the meantime, continued to expand so that the structures measured in the CMB signal would be 46.5 billion light years from us at this time. Second, the 93 billion light year diameter estimate refers to the “observable” universe.

Now, 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 which formed from the CMB structures. 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. This is the explanation for the difference; the “observable” universe is larger than we can see it today.

This is only, at best, a theoretical estimate of the diameter of the universe, though. We now know that due to dark energy the expansion of the universe is accelerating. This acceleration will not allow us to see those superclusters which are now 46.5 billion light years from us. In fact, if we wait the requisite 32.7 billion years those superclusters will be receding from us at a rate that is greater than the speed of light. We just will not be able to observe those galaxies and clusters of galaxies which have formed at the edge of the universe.

Jeff Mangum

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