Expert: Jayendra Upadhye - 6/9/2008

Question
Hello Jayendra,

I have this question for so long, yet have not found any satisfying answer.

My question is: if the universe is expanding, and its age is now 13.7 billion light years, then is it possible BEFORE REACHING 13.7 BILLION LIGHT YEARS in all directions, an object's recessional velocity equals the speed of light? For example: at 12.4 billion light years away, is the recessional velocity = speed of light? Or is it AT EXACTLY 13.7 BILLION LIGHT YEARS away the recessional velocity = speed of light?

Thanks for your answer.

Answer
Hi Indro,
There is more to this than "meets the eye"!
1) -
You see the early history of the universe is "inflationary".
Meaning the universe doubled and quadrupled in size, and the bang happened not at a single point but "over an area" so to speak.
That implies it expanded faster than light during that phase, "appearing" at more than one location "almost simultanously" in time.
And thus violating the sacrosanct speed/velocity limit of C.
From a "wee bit" later in time, it proceeded to expand at velcity C.

Obviously, two points at either opposing ends of this universe would never see one another!!

We will forever be cutoff from those areas, by virtue of their location in space and time during the inflationary phase.

2) - for a time after the bang, matter / energy or whatever one calls that state, was too hot to exist as separate distinct entities, and so no information is available in the CMB (Cosmic Background Radiation) about it.

The decoupling happened after a considerable time elapsed. That "flash" when electromagnetic radiation decoupled as a separate entity, is the source of the CMB, and also its isoptropy. (And small scale anisotropy, which predicted the large scale structure of the universe, as amplified by graity).

3) -
Then the universe entered   the dark age, when the first stars were in the process of forming, and the skeletal spiral galaxies (globular clusters were already forming in the galactic haloes) emerged. In due course these would collide and form irregular and elliptical galaxies.

The universe was expanding during that dark time and by the time the first stars winked on in the visible light, (which we would some day observe for red shifting), further volume of the universe "went over the visible edge"!

So fitting that 13.7 billion years into all this is a bit tricky.
All i can say is that from that "first winking" of the stars, the time elapsed is 13.7 billion years.

I do not make it a point to remember particulars, after all one can always "look them up". But the foregoing is what happened.

You may trace the recession rate with distance from sites evaluating the hubble's constant, and distance relationships with recession rate, plug in the value and get your answer.

Elsewhere on the site i have listed those sites in other answers.
But i think i should leave some "research" to you so you can share the excitement.

regards
Please do rate the answer if you find it informative.
Jayen