Expert: Philip A. Stahl - 5/27/2011

Question
(could be a stupid question) scientist say they see an explosion 13 billion light years from earth, probably from a time the universe was in it's infancy. My question is, knowing that light is the fastest thing in the universe, then how did we get to this point of the universe before the light did. It is quite puzzling.

Answer
Hello,

Okay first, true - in general when one looks out at objects in space, e.g. stars, star clusters, galaxies, etc. one is indeed looking *back in time*. If I see Sirius at 8.7 light years distance, I'm seeing it as it was 8.7 years ago - not as it is tonight. Similarly, if I observe the Andromeda galaxy (M 31) I'm seeing it as it was 2.2 million years ago, not as it is tonight. In each of these cases, there is no problem of "us getting here before the light". Thus, in observing any distant object one must factor in the light aspect and delay owing to its finite speed (300,000 m/s).

Observing cosmological objects, like a quasar, is somewhat different in that according to the Hubble law (that relates distance to an object's redshift), as the redshift increases so does recessional velocity. (I.e. the more distant a cosmological object, the faster its velocity of recession). Now, what they don't tell you - in most books, is that one must make a relativistic correction to the red shift beyond a certain value. If you don't make this correction, which brings the results in synch with special relativity, you get preposterous results.

The issue you're describing is different qualitatively and quantitatively from each of the preceding examples. First, no one "sees an explosion" 13+ billion years ago, say like one observes a quasar 10 billion years ago. While the latter is surely a discrete object with observable properties, the former is an event that occurred before space-time even existed. Hence, no one can "see" the Big Bang. What one does is to identify its relic radiation (e.g. the well known 2.7 Kelvin microwave background radiation - discovered by Penzias and Wilson ca. 1965), or radiant "fingerprint" left behind after so many billions of years, and then trace back in time to its putative origin (using physics, namely thermodynamics).

There is not the scope here to go into how this is done, but if you're interested in finding out, please get hold of the book 'The First Three Minutes' by Steven Weinberg. He does a superb job using more or less lay language in showing how one gets from the cosmic microwave background to the hot, superdense state of the Big Bang. His 'Mathematical Supplement' is also most useful in presenting the actual numbers and he uses nothing more advanced than algebra.