Why Do We See Old Galaxies in Every Direction?

[stuart100]

TL;DR

Further away in every direction we can see galaxies as they were near start of expansion already there then.

Look, I had to digest a lot from answers to first question. Still, I'm missing some main idea. As far as we look in any direction we can see stuff out there from long ago. A grenade explodes in some void. The material travels outward as a sphere of leading faster bits. Slower material follows, and being slower, falls back. Likewise the slowest particles make up a trailing sphere of debris falling further behind.Ahead and behind_nothing. This is an expanding debris-field though not because of anything but relative velocity. Our local universe blew apart, fine. And space expands, fine. The galaxies in opposite ends of the sky were closer before though the space between has grown, but nearly 14 billion years ago they are so far from each other. Wouldn't most of the distance between come from the expansion of space given they started-out in one place.

 

[Ibix]

A grenade explodes in some void.

This isn't a good model of the universe. As far as we know it's infinite in spatial extent and the Big Bang happened everywhere. It's not everything coming from a point - if the universe is infinite it was always infinite.

Our local universe blew apart, fine.

Not sure what you mean by "local universe". And I don't think "blew apart" is really a helpful description. It's probably better to characterise it as everything moving away from everything else. That emphasises the lack of a special point (anywhere you are, everything is always moving away from you) and leaves open how that state of affairs came to be. The latter is important because the details of the early universe are still a topic of a lot of discussion.

Wouldn't most of the distance between come from the expansion of space given they started-out in one place.

Yes and no. "Space is expanding" is, strictly speaking, an optional description of what's going on. Others are available, but this one is extremely convenient for many reasons and very widely used. If we use it then yes, the vast majority of distance between distant galaxies is due to space expanding. In fact, in this description, observers who see the cosmic microwave background as isotropic only move apart due to expansion. Most galaxies are moving slightly compared to such observers, which is why nearby galaxies such as Andromeda can be moving towards us (neither of us sees the CMB as perfectly isotropic - one side of the sky is slightly redshifted and the other blueshifted) but distant galaxies are all moving away (the increasing distance due to the expansion of space dominates any other motion at large enough distances).

 

[sophiecentaur]

A grenade explodes in some void.

You must surely have seen (many times?) the old model of dots on a balloon as it is inflated, showing how the spacing of the any two dots increases proportionally with their initial spacing. That model doesn't involve any 'location' for an 'explosion'. I would have thought that extrapolating from that two / three dimensional demonstration to a similar effect for Hubble expansion of a 3D arrangement of galaxies. There doesn't have to be either a start point or an outer edge. But of course you need to chuck away some of your intuitive ideas about what's happening in the Universe.

 

[DrStupid]

That model doesn't involve any 'location' for an 'explosion'.

Involving a 'location' for the 'explosion' doesn't make much of a difference. Let's say we have an explosion at $r_0$ and all particles fly away from that center with their individual constant 'velocity'. That results in the 'velocity' distribution

$v\left( r \right) = k \cdot \left( {r - r_0 } \right)$

The relative 'velocities' as seen from an arbitrary point $x$ are

$v'\left( r \right) = v\left( r \right) - v\left( x \right) = k \cdot \left( {r - x} \right)$

which is the same distribution but with $x$ in the center. That means that every observer (who cannot see the outer edge) can assume himself to be in the 'center' of the 'explosion'. We don't even need constant 'velocities' as long as the 'accelerations' have an analogue distribution.

The problem with this model is not the center but the outer edge (and of course the classical kinematics).

 

[sophiecentaur]

nvolving a 'location' for the 'explosion' doesn't make much of a difference.

If you have two different observers, in different parts of the Universe it would.

 

[DrStupid]

If you have two different observers, in different parts of the Universe it would.

Could you please explain that?

 

[Drakkith]

A grenade explodes in some void. The material travels outward as a sphere of leading faster bits. Slower material follows, and being slower, falls back. Likewise the slowest particles make up a trailing sphere of debris falling further behind.Ahead and behind_nothing. This is an expanding debris-field though not because of anything but relative velocity. Our local universe blew apart, fine. And space expands, fine. The galaxies in opposite ends of the sky were closer before though the space between has grown, but nearly 14 billion years ago they are so far from each other. Wouldn't most of the distance between come from the expansion of space given they started-out in one place.

Let's go back to your grenade analogy. Right before the explosion you have one grenade surrounded by empty space. This is not an accurate description of how we think the early universe was. Instead of a single grenade, we need grenades everywhere. All of space, everywhere, is filled with grenades. There is very little, if any, empty space anywhere in the universe.

And that's still not very accurate, as the simultaneous explosion of all these grenades wouldn't expand, because space is already (or very nearly) full.

What you're missing is that 'space itself' is expanding, meaning that the distance between all objects not bound together by strong enough forces increases over time, without acceleration or the application of a force.

There was no explosion like you and I are used to. That implies the moving away of parts from a central point. No such central point exists.

 

[DrStupid]

What you're missing is that 'space itself' is expanding, meaning that the distance between all objects not bound together by strong enough forces increases over time, without acceleration or the application of a force.

I don't think he is missing that:

Wouldn't most of the distance between come from the expansion of space given they started-out in one place.

 

[sophiecentaur]

I don't think he is missing that:

He's getting bits of it but using the grenade idea doesn't cope with the density variation (does it?).

 

[DrStupid]

He's getting bits of it

At least he got enough of it to wonder if the grenade idea is obsolete. All he needs is the confirmation that most of the distance indeed comes from the expansion of space.

but using the grenade idea doesn't cope with the density variation (does it?).

I again need to ask for an explanation.

 

[Ibix]

The problem with one grenade is that if the matter distribution is initially finite in extent then it is always finite in extent. I'm not sure that it leads to isotropic curvature either, and it fails to account for the CMB.

The problem with grenades everywhere is that that's a matter-dominated FLRW universe, isn't it? Do they even need to explode?

 

[DrStupid]

The problem with one grenade is that if the matter distribution is initially finite in extent then it is always finite in extent.

Yep, that's what I mean with the outer edge.

I'm not sure that it leads to isotropic curvature either

It's just an analogy. You can simply define that the resulting mass distribution is isotropic (or assume that the grenade has been designed to guarantee it).

and it fails to account for the CMB.

Does it? Even in this model every observer sees in the past - the more the greater the distance. If the outer edge of the distribution expands near the speed of light, it should also be possible to see the red-shifted heat emission of the matter shortly after the initial blast.

However, the relativistic effects resulting from the required velocities might result in problems. That's what I mean with the classical kinematics that needs to be assumed for this model to work.

 

[sysprog]

I like the term "old galaxies" being posited, as if there were such a thing as 'young galaxies' -- well, maybe there is such a thing, comparatively, but to us, all galaxies are very old, and almost all of the information we have by which to consider their origin and their temporal-spatial characteristics is also from very long ago and from very far away.