shabeer_quark said:
we are living in this milky way galaxy which got separated from bigbang ,the separation would not have taken place in light speed but how come we are tracking the light that is emitted from bigbang period from our earth
This is a very good question... and finding the answer to it basically marks the point when I at last understood what the big bang really means.
The "Big Bang" refers to a time, not a place. Our universe used to be very hot and dense. Everything was jammed up together, a very thick extremely hot ionized gas. Furthermore, everything was flying apart from everything else... and it still is.
This was not an explosion at some point in space; it was a general state of affairs, through all the universe... at least, all the regions that we have any hope of measuring or observing.
As this hot dense gas disperses, it cools and becomes less dense. The word "cooling" here is fun, because we are talking (at first) about dropping from trillions of degrees, to billions of degrees, to millions of degrees. But after some 380,000 years, it has cooled down to about 3000 degrees. At this point, atoms can start to hold onto electrons, and the material in the universe becomes transparent. Before this, light didn't get very far, because a hot plasma is opaque. But when the matter in the universe is cool enough to be transparent, light just carries on through the expanding gas without stopping.
The key point... we are still talking about a hot expanding gas involving the whole universe... at least, everything we can hope to see or measure.
Since then, the gas has continued to expand, and cool. It pulled together into stars and galaxies... including our own Milky Way. And all this time, light continues to stream through the universe. The oldest light is the light that was emitted 380,000 years after things kicked off (somehow)... and that light is everywhere, moving in all directions... because the gas which emitted it was also everywhere and in every direction.
And now, 13.7 billion years later or so (current best estimate) that light is STILL going. We see it coming to us from every direction in the sky, because everything all around us, as far as we can see, is material that started out, along with us, in that hot dense soup of matter and energy that has since expanded and cooled to form the universe as we see it now.
Cheers -- sylas
PS. A history of events for the "observable universe"... that is, everything we can see or measure, including the oldest light still around, is described nicely at
Brief History of the Universe, courtesy of Ned Wright at UCLA.
