Quote (doomchaser @ Mar 29 2013 08:21am)
how do we messure the time of the big bang? my guess is looking at the farthest objects we can see and caculate how long the light would take to reach us...
One way it can be done is by reversing the Hubble constant. Every object in the universe (outside the local group, where gravity is strong) is receding from every other object, at a rate that is correlated with its distance from that object. If you reverse this, every object will be in the same location about 13 billion years ago.
http://en.wikipedia.org/wiki/Hubble's_law
Another way (which yields the same answer) is by temperature. At about 3000k, hydrogen gas is ionized. Ionized hydrogen has a huge cross section for scattering photons (it is opaque, like a shower door). The entire univese appears to have been filled with ionized hydrogen gas in the distant past at essentially the same temperature (making the entire universe opaque). The point where this gas ceased to be ionized (the instant the universe became essentially transparent) is called the surface of last scattering. By taking measurements of this surface you can work out how long ago this transition happened. This surface also puts an upper limit on how far back anything can be measured though the use of photons, photons that existed while the surface was intact can not be used to form images because they were scattered by the surface (like how you can't see though a shower door, it is just a haze).
http://en.wikipedia.org/wiki/Cosmic_microwave_background_radiation
Quote (doomchaser @ Mar 29 2013 08:21am)
so if we can only see 16 billion light years away then they guess it is 16b years old? because thats how long it took the light to reach us? We can't see past the speed of light that got too us so everything farther then 16billion lights years away is out of our sight?
This is essentially right except it isn't quite that simple. Since the further an object is away from the us, the faster is it moving away, there is a distance where objects are moving away faster than the speed of light. New light emitted from these objects can never reach us, but light emitted a long time ago can be seen. It is a difficult concept. Image a straight toy race car track:
A-----------B
You push a toy car at B towards A. But every second someone inserts a new piece of track between every existing piece of track. Will the car make it to the other end? It depends on how long the initial track was (and how fast the car is moving, but we are talking about the speed of light so that is fixed). If the initial track is too long, enough pieces will be inserted during the trip that the car will never make it. But if the initial track is short enough, the car can make it. At a certain length the car will just barley make it. Let's assume this is where we started it. When the car reaches point A we ask how far away is B? Is it the original distance between A and B (a distance shorter than the car's trip), is it the current distance between A and B (a distance longer than the car's trip which is now so large no future cars can be received from B), or is it the distance the car moved (the car's speed multiplied by the time it was moving on the track)? The distance the car moved would be analogous to 13 billion light years.
This post was edited by Azrad on Mar 29 2013 12:05pm