Solving the second equation for quasar APM 8279, with Z = 3.911, T = 12.10 By The result is given in billions of years (By) The equations are based on the following cosmological parameters: Correlation coefficients are better than 0.999. The relation between redshift and lookback time is quite complex, and is presented here with simple equations derived by regression analysis from actual results. When we observe a remote object, how far back in time do we see? What distance did the light have to travel through an expanding universe in order to arrive at the present time? How long did it take the light to traverse the distance between the object and the observer? How old are the photons presently recorded in the object's spectrum? Lookback time, or (less appropriately) light travel time, answers the following similar questions: Lookback time, T, and distance, D, can be calculated from the object’s redshift, z, and cosmological parameters. Enter the redshift and your assumptions about the Hubble parameter and other cosmological parameter and it will tell you the age of the universe at that redshift as well as the lookback time. Here is a cosmological calculator that can do the job for you. The curves are very different at high redshifts, but converge at small redshifts. The plot below is an example taken from which shows look back time versus redshift for two different cosmologial models (but with the same value of $H_0$). Other information is required.įor low redshifts - let's say smaller than 0.1 - and by that I mean the wavelength increases by 10 percent, you might get away with using Hubble's law to estimate the distance and then get the look back time by dividing by the speed of light.
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