YSBSFA: how did our matter get here ahead of the light?

[marcus]

Matter hardly moves. It doesn't "get here ahead" of anything, it was always more or less where it is.

Light, including the oldest, comes towards us from all over the universe. So naturally there is some arriving now, at this moment.

Look at the 2D toy model:
just google "wright balloon model"--it is so easy to understand!

As time goes on, the ancient light reaching us will come from matter that is farther and farther away. It will have taken longer to get here, and its wavelengths will be stretched out.
But some will always be arriving.

And it will always be light that was emitted at the same moment that today's oldest light was. The moment 380,000 years after the start of expansion, when the hot gas filling the universe became sufficiently transparent (due to expansion and cooling) to let the light set out on its journey.

Matter being approximately stationary (e.g. galaxies moving only very slightly relative to ancient ight) does not prevent DISTANCES from expanding. That's what spacetime curvature is about. Spatial geometry is dynamic. It doesn't mean that anybody, any galaxy, is GOING anywhere.

About matter moving...what started out as hot gas with only very disorganized motion has picked up a relatively small amount of more organized motion by falling together. Galaxies and stars within galaxies have comparatively small speeds (a few hundred clicks of individual motion) acquired because random patches in the original gas had slightly more or less than average density and stuff began to collect and fall towards overdense regions. It's not an issue. Although complex structures have developed over cosmological time, matter is still on average roughly stationary and evenly spread out.

So you should be shot for asking "how did our matter get here ahead of the ancient light?"

 

[bcrowell]

I think the underlying problem is that they imagine the big bang as an explosion that took place within a preexisting background of empty space. This is probably the basis of 90% of the confusion we see, such as people wanting to know where the big bang happened, talking about the leading edge of the expanding cloud of matter, etc.

 

[marcus]

I think the underlying problem is that they imagine the big bang as an explosion that took place within a preexisting background of empty space. This is probably the basis of 90% of the confusion we see, such as people wanting to know where the big bang happened, talking about the leading edge of the expanding cloud of matter, etc.

Amen! I personally intend to encourage (to whatever extent I can) our newcomers to read the FAQ section before the post their "help I'm puzzled" or "silly doubt" or "layman's question". The stuff is not all that hard, and the FAQ section is pretty clear. Ought to be used more.

Here is a recent example:
https://www.physicsforums.com/showthread.php?t=510528

In this case it comes off of something by Kaku. These questions often seem to come off Discovery Channel or some other Science TV or a popularization (Kaku Hawking etc.) We always seem to be cleaning up after Kaku or the like. The pop-sci tripe is misleading and people get fundamental misconceptions which interfere with their understanding. It's nobody's fault, has to do with the structure of commercial media---you have to pitch so as to boost ratings. So standards go by the board. Anyway maybe I will try a few more of these YSBSFA posts (unless you would prefer a less obstreperous approach)

 

[Cosmo Novice]

I like this explanation; that we are pretty much where we started, which would be same of all matter locally. Essentially the matter density of the region of spacetime that eventually became our galaxy was slightly elevated, and then other slightly elevated localised areas (originally in causal contact but some now separated from us by inflation initially then expansion) became the other matter dense regions we can now observe within our OU/not observe out of our OU as galaxies older and older as we look further back - spatially and temporally.

The light that has been traveling the longest is the last to get here so we essentially "look back", although essentially there just pertubations in the matter density of the localized space time region, at different stages :)

I think the misconception of a "leading edge of space" especially comes into play when people think of a finite Universe.

Anyway there are some very good FAQs on here which people should definitely read before posting to give them a better grasp of what the scientific communities accepted model of thinking is. Happy posting everyone.