# Expansion/Inflation and the Separation Velocity

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It seems very counter intuitive that galaxies should be separating from each other at velocities that are proportionate to distance.

But this is what we observe ,it seems.

Does this imply that ,at an early stage all the matter that formed those galaxies "talked to each other"?

Â

What did they "say"?đ

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6 hours ago, geordief said:

It seems very counter intuitive that galaxies should be separating from each other at velocities that are proportionate to distance.

Actually it seems very intuitive.Â  If you take any collection of objects in one place and give them all random velocities (different speeds and directions), and assuming none of these pieces accelerates thereafter, they will all recede from each other at a rate proportional to their current separation.

A grenade only sort of works this way since they're designed to have pieces that go in different directions but fairly uniform speed relative to the center of gravity, thus forming a 2D shell expanding with the above characteristics.Â  The universe doesn't have a center of gravity and thus no meaningful 'place' where the bang happened, so any random object can be assigned the designation of 'here' and everything must recede uniformly from there at a velocity proportional to its displacement. The big bang wasn't an explosion, but rather the expansion of space itself, but the recession effect is the same as the infinite (no outside boundary) explosion at a location in space, and thus this expansion rate is quite intuitive. That rate is simply 1/time-since-bang, or at least it would be if the expansion rate was always constant, but it's currently pretty close.

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It actually seems pretty intuitive to me.
If you place any dots on a 1 dimensionalÂ substrate, like a rubber band, the dots 'move' away from each other with those same properties.
If you put the dots on a 2 D substrate like a rubber sheet, you observe the same kind of 'motion' or effect.
If you put the dots on a 3 D substrate, like raisins in sweet bread then bake it, you observe the same as it expands.

Seems like it work all the time.

7 hours ago, geordief said:

Does this imply that ,at an early stage all the matter that formed those galaxies "talked to each other"?

Of course.
How else would you explain the isotropy and homogeneity of the universe, when parts of it areÂ increasingly distant for light ( information ) to ever reach.
At one time the universe MUST have been small enough to be in causal ( local ) contact.

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Yes,thanks.Â  Â @HalcÂ it was intuitive .(seems there is a thick line between counterintuitivity and lack of imagination

Still,I find it hard to accept this concept of "expansion of space" if (as my mind tells me) space is not a "thing" that can expand.Â Â

Â

Does this "expansion of space" just meanÂ  anÂ increase in distances between objects?

Â

Also ,might the "Big Bang" have occurred over a small regionÂ  and a very short time with thereÂ  actually beingÂ  a concatenation of BBs which blended into one eventÂ  when viewed in retrospect?

Edited by geordief
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30 minutes ago, geordief said:

Does this "expansion of space" just meanÂ  anÂ increase in distances between objects?

Yes, but the objects are (or can be) locally at rest.

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47 minutes ago, geordief said:

Still,I find it hard to accept this concept of "expansion of space" if (as my mind tells me) space is not a "thing" that can expand.Â Â

That part isn't very intuitive I'll admint.

Quote

Does this "expansion of space" just meanÂ  anÂ increase in distances between objects?

No, because if I walk away from you, that's also an increase in distance between us, but it's simply not the same thing.Â  Most matter isn't really moving very quickly 'through' space, but rather the space between objects expands over time. Both involve increase of proper distance between objects over time. The differences are more subtle, but to give an example, if Alice is here and Bob (nearby) is moving away from here at 0.7c, then the proper distance between Alice and Bob is increasing at 210,000 km/sec.Â  Ditto for a galaxy ~10BLY away receding from us at 0.7c.Â  But now there's Charlie (also nearby) receding from Bob at 0.7c.Â  Alice and Charlie are now increasing their proper separation at 282,000 km/sec (per relative velocity addition formula).Â  But recession of galaxies isn't that kind of velocity.Â  The galaxy that is twice as distant as the first one (receding at 0.7c) is thus increasing its proper distance from us at a rate of 1.4c (420,000 km/sec).Â  It isn't moving any faster than we are, but the space between us and them is increasing at that rate.

The proper distance between Alice and the others can be measured with a tape measure that is stationary relative to Alice, and is whatever Bob or Charlie reads on that tape (with Alice at the 0 reading).Â  The proper distance between the galaxies on the other hand is measured by the count of individual meter sticks of equal cosmological age, each one of which is unaccelerated since the big bang.Â  That means all the meter sticks are moving apart from each other and over time it take more and more of them to fill the gaps.

Quote

Also ,might the "Big Bang" have occurred over a small region ...

There's nowhere where the big bang did not occur. It happened everywhere, but places that are far apart now where notÂ so far apart then.

Edited by Halc
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21 minutes ago, swansont said:

Yes, but the objects are (or can be) locally at rest.

So this on going expansion (whether steady or whatever) is like a blank canvas on which the relative motion of bodies Â  is written?

Like a page in a book where the letters get smaller and smaller over time.

( the letters corresponding to regions bound by gravity)

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23 minutes ago, geordief said:

So this on going expansion (whether steady or whatever) is like a blank canvas on which the relative motion of bodies Â  is written?

Like a page in a book where the letters get smaller and smaller over time.

( the letters corresponding to regions bound by gravity)

I like MigLâs descriptions better, since they donât involve things shrinking that donât actually shrink.

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7 hours ago, geordief said:

Does this "expansion of space" just meanÂ  anÂ increase in distances between objects?

Yes, this is a better way to look at it than thinking of space as a thing that expands.
Yet another way to look at it is to say that distance measurements in this type of spacetime are time-dependent - so the outcome of a distance measurement between two otherwise fixed points explicitly depends on when that measurement is taken. There needn't be any reference to motion of any kind.

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