reply to post by Cyberbian
Originally posted by Cyberbian
Our matter is here at this precise distance from the big bang over this precise time period, yet we can supposedly view light just arriving here from
relatively shortly after the big bang.
You present what is, at first sight, rather a nice paradox. If you thought it up yourself, well done. You're thinking like a physicist. A little
further thought, however, makes clear why this idea is wrong.
Our matter is not 'at this precise distance from the Big Bang'. Our matter is at the precise
location of the Big Bang. And so, interestingly
enough, is all the other matter and energy in the universe.
The Big Bang was not an explosion. Matter and energy did not radiate outward from a point. Space itself expanded (and continues to do so). It expands
equally in all directions. This is the first thing you have to remember. All points in the universe are the location of the Big Bang.
Now let's see what happened to that pesky Big Bang light.
According to currently accepted theory, the outer boundary of the expanding universe (you could call this the wavefront of the Big Bang) is actually
moving faster than light. This motion has two components: the first is the velocity of photons radiating outwards at - obviously - the speed of light,
c; the second is the inflation of space itself. Inflation is not occurring outwards from a central point (the Big Bang was not an
explosion). It takes place equally in all directions. Some of this expansion is also outwards from the Big Bang, so we can add that velocity component
to the velocity of the radiant photons to get a value higher than
c and constantly increasing because the rate of inflation is
increasing. Relativity is not violated because space is not matter or energy; the photons are still moving at the speed of light relative to any point
in the space they occupy, but that space is expanding, so the total velocity of the BB wavefront is travelling faster than light. (I am, by the way,
assuming a flat spacetime geometry.)
This gorgeous diagram (any excuse to post it will do for me) may help you visualize things. The outer curve of the figure is the rate of expansion of
space (if you click on the picture, you'll be able to see all of it).
Light from the BB wavefront doesn't reach us; its source is moving away from us so fast, radiation emitted in our direction is redshifted down to
invisibility. We cannot see to the end of the universe. For all we know, it could be infinite in extent, although there are reasons for doubting
this.
So how far, exactly,
can we see? Light always travels at the speed of light. The universe is some 13.7 billion years old. So our visible
universe is a sphere of radius 13.7 billion light-years (in practical terms rather less due to the limitations of our instruments, but we're getting
very close).
And what we see at the limit of this 13.7-billion-light-year boundary is light from the Big Bang. We do not see it as superhot and superbright because
the inflation of space has stretched and stretched its wavelength; what were once gamma rays are now radio waves of the lowest detectable frequency.
This is known as cosmological redshift, and it is calculated differently from the better known Doppler redshift.
Now remember, the location of the Big Bang is everywhere. Every point in the universe was involved in it. But light emitted during the BB from points
inside the 13.7LY sphere has already reached us, because it had less than 13.7LY to travel.
13.7LY from us, however, the BB (to our eyes) is
still happening. We're still seeing the light from it. Only it isn't the Big Bang any more,
more a kind of wispy, barely-glowing radio haze.
There now, that wasn't so hard, was it?
[edit on 23/8/09 by Astyanax]
