Why is the cosmic microwave background radiation coming toward us instead of going away?

Question

I mean, I suppose some of it is going away, but how is there any coming toward us.

I understand that the matter that became our galaxy was contained inside the matter that emitted the CMBR, therefore the CMBR would reach "us" at some time, but why didn't it all pass by us billions of years ago? Correct me if I'm wrong, but wasn't the universe much smaller at the time the CMBR was emitted, say less than 1 billion light years across. So the most distant emitter of CMBR would be 1 billion light years from "us" or less.

I get special relativity to some extent, so I know I need to think of an inertial reference frame. Let's pick the frame in which the matter that eventually became our galaxy was at rest at the time the CMBR was emitted, and call its origin the position of that matter. The most distant CMBR emitter was less than 1 billion light years away, and more than 1 billion years has passed in that reference frame, so the CMBR should have passed the origin. Our galaxy might have accelerated, but my understanding is we should have "decelerated" (i.e. accelerated toward the center of the universe, therefore toward the most distant origin of CMBR). Currently the expansion of the universe is accelerating, but I read that it was not always so. So the light should have passed our galaxy too.

I'm guessing the answer has something to do with general relativity, which I don't understand much. Does the mass of the entire universe affect distance or time in some way to resolve this discrepancy? Is it possible to explain how general relativity affects the way the CMBR reaches us, without teaching me to understand the whole thing? Or if not just tell me it has to do with general relativity and leave it at that.

Anonymous · Accepted Answer

So here is the answer to the CMBR light question:
http://www.astro.ucla.edu/~wright/photons_outrun.html

All the light *we* will ever receive from the CMBR's source, is trapped inside our Rindler horizon, and it will play out over the rest of time.  Same for any other "we".

"The most distant CMBR emitter was less than 1 billion light years away, and more than 1 billion years has passed in that reference frame, so the CMBR should have passed the origin."

There *is* no origin.  The "origin" was the Big Bang event, and everything is moving uniformly away in time.  Do not think that the concepts of SR can help you in cosmology.

"Our galaxy might have accelerated, but my understanding is we should have "decelerated" (i.e. accelerated toward the center of the universe, therefore toward the most distant origin of CMBR). Currently the expansion of the universe is accelerating, but I read that it was not always so. So the light should have passed our galaxy too."

Acceleration or not, just changes the diameter of our Rindler horizon.

You can read more about the topic at this next link.  Take your time...
http://www.astro.ucla.edu/~wright/cosmo_01.htm

[EDIT: "But WHY does light move at the speed of light relative to "nearby galaxies"?"

Light moves at a constant speed c for all inertial observers.  This is a requirement of Maxwell`s equations.  Physics is the same for every observer, which is where Relativity comes in.

"Though it makes a nice little circle, relative to the reference frame of the animation the light moves faster to the right than to the left. (Notice it starts near the center of the animation, and by the end it goes past the right edge but does not reach the left edge.) And relative to the reference frame of the left galaxy, it moves even slower to the left. I think that animation is probably authoritative, but how do I reconcile it with the concept of a constant speed of light?"

Sorry, but I see it expanding exactly symmetrically...
]

? · Answer

The microwave history will not be from the time of the massive bang. The microwave historical past is from a time quickly after the enormous bang. There was once a time when the universe was so dense and hot that electromagnetic waves might now not penetrate very a long way before encountering whatever that will block the progress of the wave/photon. When the Universe had multiplied sufficiently and sufficient numbers of particle anti-particle pairs had annihilated themselves the electromagnetic waves were regularly equivalent to a targeted temperature. The waves had been everywhere moving into all recommendations. The Universe has persevered to expand over the several Billion years since that point. The waves which are arriving now have had their wave length made longer as they traversed Billions of light years of the increasing universe from someplace that used to be a ways from the place we presently are.

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Why is the cosmic microwave background radiation coming toward us instead of going away?

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