Expert: Courtney Seligman - 2/26/2009

Question
QUESTION: Hello,
Does the Light Horizon include cosmic microwave background radiation?  Or is one more distant then the other?

Thank you

ANSWER: The light horizon is identical to the 'surface' defined by the microwave background radiation.

When the Universe was first forming, it was so dense that nothing, including light, could move from one place to another. As it expanded and became less dense, it became less opaque. Eventually, it became sufficiently transparent so that light moving through space was able to just keep going, without much chance of running into anything.

When we look outward, we can see through the nearly empty space created by the expansion of the Universe only as far as the space is empty, and not opaque. Once we've looked so far that we're seeing the Universe as it was when it was opaque, we can't see any farther. That's the light horizon. But that's also where the light of the microwave background is coming from, so the two terms, although describing different concepts, refer to the same distance/place relative to us.

Courtney Seligman

---------- FOLLOW-UP ----------

QUESTION: Thank you Courtney for the indepth and fast response!  Just to make sure I'm understanding you correctly, the light horizon and the cosmic microwave background radiation share the same reliative space in relation to Earths placement, or our vantage point? Everything that I've read in redards to C.M.B.   States that the radiation is more uniformed then it should be, more then anything else in human experence.  How can science be sure of that when we are dealing with 13.7 billion light year brick wall in every direction? Isn't  radiation light also? It to would be bound by the same 186,000 mps, wouldn't it?  Who's to say that an astronomer on the edge of our light horizon doest have a different view of the C.M.B., 13.7 billion light years further down the road?  I have read the theory of inflation and it does give a good explaintion as to the uniformity but with out knowing what's beyond our field of view, does it really answer the question.   I guess what I'm asking is, IF there is more observable universe beyond our light horizon isn't there the possibility C.M.B.could be more non-uniform past our boundry.  Couldn't.Earth be in an area where, because we are confined by the speed of light, we can't see into the non-uniform areas?  Thanks again

Answer
Yes, the light horizon and cosmic microwave background would be in the same place, relative to us.

The uniformity of the cosmic background only refers to its uniformity as we see it, but there is no reason to believe it would look any different (in general terms) anywhere else, unless we are in a 'special' place. However, the history of astronomy is one in which assumptions about our being special are continually overturned, so barring evidence to the contrary, no one is going to seriously consider that possibility.

Skipping forward a few of your points, the astronomer who is now living where we see the cosmic background would admittedly see a different portion of the Universe than we do. He would see the half between us and him, plus a similar half on the other side of him, and would not see the half on the other side of us. It is possible that his view would be different, but we have no way of knowing that, and as discussed in the previous paragraph, no one believes he is likely to see anything different. The details -- the exact stars, galaxies, etc near him -- would be different, but the general nature of things would probably be the same.

Most of your reply seems to be the same question, stated in different ways. The only answer I can give to most points is the one I just presented. As to the parts that aren't the same --

(1) Light is a form of radiation. In the context of your question, the only difference between 'radiation' and light is that light is a form of radiation that our eyes can see, and other forms -- UV, infrared, gamma rays, etc -- are forms of radiation that our eyes can't see.

(2) The cosmic background isn't the most uniform thing that we know of. It's just far more uniform than you might expect if you assume the Universe started off large enough for different parts of it to have different properties. The idea behind the Inflationary theory is that if you start with a point of zero size, there isn't room for anything to be different from anything else, save for quantum fluctuations. Those fluctuations should produce very small fluctuations in the cosmic background which match what we observe in terms of size and brightness variations, so it appears that this 'works'. The idea is also attractive because quantum theory states that quantum fluctuations can be as big as you want, provided they disappear before you have a chance to observe them. The current theory of the origin of the Universe is that in an otherwise empty part of space-time in one Universe, a cosmic glitch produces an infinitely large energy for an infinitely small time. In that instant a whole new space-time, completely separate from the original one, is created out of nothing and instantly expands (per Inflation) to tens or hundreds of billions of light years size, then continues to expand per the 'Big Bang' scenario for the rest of eternity. Whether it makes 'common' sense, it fits all the laws of physics as we currently know them, predicts properties for the cosmic background which match those which we observe, and suggests that our Universe is merely one of an infinity of Universes, none of which have any connection with any other, and each of which is no more unique than any other.

I don't know if this clears things up, or simply muddies the waters, but if you would like a further explanation, just let me know.

Courtney Seligman