Observing our early universe

Last Edited By Krjb Donovan
Last Updated: Mar 11, 2014 07:56 PM GMT

Question

I just finished reading this article at http://news.yahoo.com/s/space/spaceexplosionisfarthestthingeverseen where it is reported that astronomers are viewing the most distant object at around 13 billion light years distant. It is mentioned this was around the time when the entire universe was only 630 million years old.

I'm having difficulty understanding how it is that we on earth are around 13 billion light years away from the point in time and space when the universe is said to have only been around 630 million years old. I mean, how did earth and our entire galaxy for that matter, get 13 billion lights years AHEAD of our own origin 13 billion years ago so that we can now view the light rays arriving to us now 13 billion years later? I don't think it makes sense to assume we traveled faster than light so that we could beat the very light to our current position which shows our very own beginnings around 13 billion years ago. On the flip side, I also don't think it makes sense to assume the entire universe began at a size of 13 billion light years.

So how did we get 13 billion lights years away from our own origin so that we can view the light rays now finally arriving here or catching up with us from there?

I've recently read an "explanation" to my question which absolutely does NOT account for the simple reality that we are able to view an OBJECT said to be our own early universe during the first 630 million years but yet here we are 13 billion light years away observing an object as it appeared billions of years ago BEFORE WE EVEN EVOLVED FROM IT! SO HOW IS IT THERE AND WE ARE HERE?

Thanks Courtney in advance for an explanation which specifically addresses these issues and doesn't insult anyone's intelligence.

John

Answer

When it is said that we see something 13 billion light years away, what it really means is that the light by which we see the object left it 13 billion years ago, and took that length of time to get here. However, the object was not 13 billion light years away at the time it emitted that light. It was considerably closer, and the main reason it took the light so long to get here is that due to its initial distance, the space between here and there has been increasing at nearly the speed of light, so the 'forward' progress of the light (toward us) has been greatly reduced, thereby greatly increasing the time required to get here.

It requires relativistic calculations to get a correct value for the effect, but as a simple approximation, in the case cited the object is said to have been about 96% of the way from here to the 'edge' of the Observable Universe (13+ billion ly out, compared to 13.7 billion ly to the microwave background). As a result, during the light's travel from there to here the space between here and there would have been expanding at about 96% of light-speed (not very fast in any given region, but the combined expansion of the entire space between here and there), so the light would have made only 4% of light-speed forward progress, even though it would have been moving (always) at light-speed relative to its surroundings.

In this grossly simplified way of looking at things, the 4% forward progress would be 25 times slower than light-speed, so the light would have taken 25 times longer to get here than the actual distance would seem to justify. In other words, the object could have been only 4% of the 13+ billion light-years away (about 630 million light years away), but because of the Universal expansion, the light could have taken 13+ billion years to get here. Note that if it were 630 million light years away 630 million years after the 'beginning' of the Universe, its average motion away from us prior to that time would have only been about the same as the speed of light. So there isn't any real problem with it getting that far away. I might note that Inflationary Theory supposes that the Universe had an initial expansion far greater than the speed of light, which would allow things to get much further early on, than ordinary light-speeds would permit. But as long as you realize that the look-back (or light travel-time) for 'distant' objects is much greater than their actual distance at the time their light left them, you can visualize the situation without relying on Inflation to get them that far away.

To summarize, when the light left the object, it wasn't quite 630 million light years away, but the space between here and there was expanding (due to the 'Big Bang') at so nearly the speed of light, that it took far longer than 630 million years for the light to get here. Hence the light travel time of 13 billion years.

Please forgive any spelling or grammar errors in this note. I received your request while getting ready for a doctor visit, so I had to rush to answer it before leaving. If you need further clarification just let me know, and I'll reply when I return.

Courtney Seligman

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