Since public information offices in the US never want to mention the redshift of an object, distances are usually given as light travel time distances. This has one simple property: the distance in light years is never greater than the age of the Universe in years, avoiding at least one appearance of speeds greater than the speed of light.
This doesn't really satisfy the simplicity requirement, because a numerate listener will ask: if a distant cluster of galaxies is 9.1 billion light years away in a universe that is 13.7 billion years old, how did the cluster get so far away in only 4.6 billion years? If must have been travelling faster than the speed of light! Apparently it takes several minutes for this question to arise, and by that time the presenters are out the door. So I get an E-mail.
So here I will list the reasons why this press office policy is really dumb:
As a service to astronomers who are frustrated by press releases that give only light travel times and not redshifts, here is a version of cosmology calculator that takes the light travel time and computes the redshift. Of course you need to know the cosmological parameters the PAO used when hiding the useful information, but this calculator defaults to the WMAP 1st year Ho = 71 and OmegaM = 0.27 flat Lambda-CDM cosmology.
Part 1 |
Part 2 |
Part 3 |
FAQ | Age | Distances | Bibliography | Relativity
© 2006-2013 Edward L. Wright. Last modified 02 August 2013