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hab321
2006-Mar-20, 11:05 PM
Hello,
I am new here and found quite a bit of interesting topics and information. I have one question I hope someone can answer for me. I hope to state it logically.
I see that the "after glow" of the big bang has been observed by a telescope. My question is how is that possible, shouldn't the light from this event have gone dark millions (if not billions) of years ago? Or does light from this event go on never ending?
I hope this question makes sense, I was having a hard time trying to word this.
Thanks in advance.
Sincerely Andy B.

antoniseb
2006-Mar-20, 11:14 PM
Saying that we've seen light from the big bang with a telescope understates the complexity of what we've really seen. With microwave telescopes, we've seen photons from a time 380,000 years after the big bang.

Kaptain K
2006-Mar-21, 10:06 PM
Wekcome hab321,
Good question. Basically, it all boils down to definitions. Every field of human endeavor (science, medicine, engineering, politics, economics, sports, etc) out of necessity, developes its own "technical language" (jargon). These words of the jargon have very specific meanings, that are often quite different from the meaning of those same word in everyday use. In everyday use, speed and velocity have the same meaning, but in science, speed is a scalar quantity (100 kph), velocity is a vector quantity (100 kph north-northwest).

To an astronomer, "light" means electromagnetic radiation regardless of wavelength. Gamma rays, X-rays, ultraviolet, visible, irfra-red, and radio are all electromagnetic radiation or "light" even though we can only "see" the tiny slice called "visible".

The "light" of the big bang started out as gamma rays and X-rays at a temperature (another technical term that does not have exactly the same meaning as the lay defintion) of billions of degrees. As the universe expanded, that radiation was "stretched" until, now. we "see" it as microwave (radio) radiation with a temperature of 2.7 degrees Kelvin (degrees Centigrade above absolute zero).

I hope I have helped without confusing you even more.

Ken G
2006-Mar-22, 01:24 AM
I think what the OP is asking is, why hasn't the light escaped away from us? The answer is, the light that was emitted by the gas that went on to become our galaxy, etc., has indeed escaped away from us. The light we see is light that came from regions that are now very far away from us (tens of billions of light years away). So how can light have travelled that far if it all started out so small? There are many ways to answer this, and it depends on the coordinate system you choose to describe the reality. The standard approach is to measure time and space as it would be measured by hypothetical observers that have been sitting in these clouds of gas ever since the beginning, and move with their own local corner of the universe. If you take that measure of space and time, and think of a meter and a second as fixed quantities, then space itself appears to expand. Thus the light that has been travelling to us from what is now so far away came from places that by today's standards would have been very close by, but the light was "swimming upstream" compared to the expanding space, so made very little headway in all that time. Thus the reason the light was able to traverse billions of light years of space was that the space was, in effect, moving under it. It is very much like if we watch someone who was on the bottom step of an upward escalator, trying to walk downward at a speed that was very close to the same as the upward speed of the escalator. They would be making little headway, apparently hanging in space, even as
the step they started on gets carried very far away. By now, the "step" on the escalator that originally emitted the observed CMB we now observe is tens of billions of light years away, but that light has been "hanging" almost in place, starting a distance that now would only reach to the next galaxy over.
Alternatively, you may choose a picture in which clocks have been speeding up ever since the beginning, and rulers have been shrinking. In that picture, the light has only taken billions of years to reach us by today's rapidly-moving clock standards-- by the time standard at the time the light was emitted, it takes more like a million years, and the only reason it has appeared to cover so much more distance is that the rulers have shrunk since then. Basically, what you mean by a year and a light year really alters the way you explain what is happening! Note that the first explanation is the conventional picture, the second one is not normally adopted for cosmology (though it is usually exactly the other way around when people talk about black holes).

Ken G
2006-Mar-22, 01:28 AM
The "light" of the big bang started out as gamma rays and X-rays at a temperature (another technical term that does not have exactly the same meaning as the lay defintion) of billions of degrees. As the universe expanded, that radiation was "stretched" until, now. we "see" it as microwave (radio) radiation with a temperature of 2.7 degrees Kelvin

But the CMB was emitted in its local frame as visible and infrared light, not X-rays or gamma rays-- it comes from long after the Big Bang beginning. Nevertheless, you are right that it has been redshifted a great deal, and depending on which of the above pictures you adopt, the redshift is either due to the increasing time-of-flight effect from the expanding space, or from the fact that clocks have been speeding up since then.

hab321
2006-Mar-23, 09:09 PM
Thank you very much for the answers. I am going to have to re-read them a couple of times to really digest them. It gets quite complex.
Andy B.

five_distinct
2006-Mar-28, 09:42 PM
But the CMB was emitted in its local frame as visible and infrared light, not X-rays or gamma rays-- it comes from long after the Big Bang beginning. Nevertheless, you are right that it has been redshifted a great deal, and depending on which of the above pictures you adopt, the redshift is either due to the increasing time-of-flight effect from the expanding space, or from the fact that clocks have been speeding up since then.

Though the speeding up of clocks may make more sense as the concentration of gravity in any given area decreases... The amount of gravity in the area holding the universe at the time of last scattering would have been absolutely enormous.