Kevin's Watch

I still don't know how this crap works. Can anyone explain? How can we see a galaxy 15 billion lightyears away? The means we are seeing what it as it was 15 billion years ago.

1) How could there have been galaxies 15 billion years ago? I thought it took a very long time after BB for galaxies to form. If we think the universe is 15 billion years old, the oldest galaxy should be much younger than that.

2) If everything is moving away from everything else, a galaxy that we see 15 BLY away would now have to be waaay farther away. Anything currently 15 BLY from us would have to have been much closer to us 15 billion years ago.

I figure I have some very basic misunderstandings?

Fist and Faith wrote:I still don't know how this crap works. Can anyone explain? How can we see a galaxy 15 billion lightyears away? The means we are seeing what it as it was 15 billion years ago.

1) How could there have been galaxies 15 billion years ago? I thought it took a very long time after BB for galaxies to form. If we think the universe is 15 billion years old, the oldest galaxy should be much younger than that.

2) If everything is moving away from everything else, a galaxy that we see 15 BLY away would now have to be waaay farther away. Anything currently 15 BLY from us would have to have been much closer to us 15 billion years ago.

I figure I have some very basic misunderstandings?

You aren't misunderstanding, mostly...you just think you are because things really are just weird.
We ARE seeing something 15bly away as it was 15b years ago, and not only AS it was, but "where" it was THEN in relation to where we are NOW. Roughly. The real number is just shy of 14 billion...and a little less than that as well, cuz current estimate is somewhere between 200 and 500 million years after Bang the first galaxies formed.

The ones we see "now" ARE, as you think, much farther away now than "where" we see them.

The inflation thing is part of the explanation...as is the "ordinary" expansion once it ended.

I think that's all true..

OK. Let me round off to 14 billion in this discussion? Heh.

14 Billion Years Ago, a galaxy was at a certain spot. That spot is 14BLY away from where we are now. That galaxy was in that spot less than 14BYA, but the light from it went 14BLY because of inflation. (The size of the universe is bigger than c could have made it.)

That galaxy continued moving, and is now (assuming it still exists) 14BLY farther along in that direction.

On the other side of us, we have the same situation with another ancient galaxy.

This means the observable universe is 14BLY in all directions, so a diameter of 28BLY. But we know that, although we cannot see past 14BLY, the actual universe must be at least 28BLY in all directions, diameter of 56BLY.

And it's possible, if not darned likely, that, 14BYA, there were things farther than 14BLY from where we are now, but the light from them has not had time to reach us. So the actual universe could be ANY size.

How much of that is right???

The observable universe is 46.5 billion light years across. This is larger than 13.78 billion light years because of expansion.

When they say a galaxy is "X light years away", that's its current calculated position, not how long the light has been travelling.

We could potentially observe light from objects that are 46.5 billion light years away, which has been travelling for only 13.7 billion years. When such light was emitted the universe would have only been 42 million light years across, from what I've found with some googling.

(Yes, it took light 13.7 billion years and travel of 13.7 billion light years to reach something that was "only" 42 million light years away when it started.)

The limit on what we can see is 378,000 years after the big bang. Prior to that there were no photons being emitted to provide detectable light.

Also spotted on my googling, an estimate that the universe expands by more than 2 light years per year, and the expansion is accelarating.

How do we know light traveled more than 1 light-year in 1 year? That would seem to contradict the very definition of light-year. I can understand that it takes more than one year for light to reach us from a star that was exactly 1 light-year from us at the moment the light was emitted. After all, we and the star are moving away from each other at all times. If we're moving away from each other at less than the speed of light, then the light will, eventually, reach us.

But this idea about the space itself between us and the star expanding is another matter entirely. How do we know that? How can we tell it is not merely the "obvious" case that we and the star are moving away from each other?

I truly try to read these things, but I can't figure them out! They all refer to other pages, sometimes in a circular manner, and I can't usually grasp any of them enough to move on to any others.

Fist and Faith wrote: But this idea about the space itself between us and the star expanding is another matter entirely. How do we know that? How can we tell it is not merely the "obvious" case that we and the star are moving away from each other?

That is the explanation which not only explains what we are able to observe but also gives consistent results when applied experimentally. We call it "true" because it has not been observed to be "false" thus far.

Suppose you and I are on the surface of a beach ball which is being inflated at a rate of 1 m^3/second. Let us also suppose that we know, somehow, that right now we are 100 m away from each other on exactly opposite sides of the ball. V = (4/3)(pi)r^3 so dV/dt = 3(pi)r^2 dr/dt and now we can plug in numbers.
dv/dt = 1, r = 50 (half the distance between us), so
dr/dt = (dV/dt)/[3*pi*r^2] = 1/[3*pi*50^2] = 0.000042441 m/s --> we are moving away from the center of the beach ball at this rate.
Now, suppose you are unable to detect the beach ball itself--from your point of view you are "not moving". It would appear to you that I am moving away from you at a rate of 0.000084883 m/s. In our experiment, let us also suppose that the speed of light is 1 m/s (I am making the numbers easy, clearly). You know that I am 100 m away so you would normally expect a laser to bounce from you to me then back to you to take 200 seconds but because of the expansion your equipment tells you that it took 200.0254677 seconds for the signal to get back to you (found by calculating 200/[1-dr/dt]). There are two explanations--either I am moving away from you (as it appears to be) or that we are moving away from each other. You look at other points which are also on the beach ball and, amazingly, it looks like every other point is moving away from you, too, but at different rates.

If six of us are shot out of cannons from a central point, in six directions along the X, Y, and Z axes, at identical speeds, I will see the other five of you moving away from me at different rates. This does not suggest we are all on a spherical surface, or in a common space, that is, itself, expanding. What *does* suggest this in our reality?

Hmmm. Are you saying that, if I shoot them all out of the same cannon, one after another, all inthe same direction, all with the same force, the one shot first would, soon enough, be going faster than the others?

I gotcha. This is a revelation to me. And one that is extraordinary beyond nearly anything I've ever heard. And we're sure it's not that all things left the Big Bang at the same moment, but some were going faster than others?