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 Philosophy of Physics Philosophy of Physics Forum - Philosophical questions about our universe

 Aug 29th 2017, 08:53 PM #11 Junior Member   Join Date: Apr 2017 Posts: 12 Is this sentence correct: The galaxies are receding from us because as space expands they are being carried by the expanding space. If not can you break it down into smaller bits to help me understand what I am missing? A) The galaxies are receding from us. b) space is expanding (Is this why my wife needs to buy me new pants every couple of years?) C) The galaxies are being carried by the expansion of space kiwiheretic likes this.
Aug 29th 2017, 11:35 PM   #12
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ok, I in no way claim to understand all this cosmology speak but I did find this in wikipedia:

 Metric expansion is a key feature of Big Bang cosmology, is modeled mathematically with the Friedmann-Lemaître-Robertson-Walker metric and is a generic property of the universe we inhabit. However, the model is valid only on large scales (roughly the scale of galaxy clusters and above). At smaller scales matter has become bound together under the influence of gravitational attraction and such things do not expand at the metric expansion rate as the universe ages. As such, the only galaxies receding from one another as a result of metric expansion are those separated by cosmologically relevant scales larger than the length scales associated with the gravitational collapse that are possible in the age of the universe given the matter density and average expansion rate.
from https://en.wikipedia.org/wiki/Metric_expansion_of_space

So it seems, from the part I highlighted, that gravity produces a drag and overrules against the effect of a body being carried along in the expansion of space. So it sounds like this "space friction" is a fairly weak force, even weaker than gravity.

Edit: After reading https://www.quora.com/How-does-the-e...new-space-time it seems that we may have been misled. It seems as though there are no rising (expanding) raisin muffins or spots on stretching balloons after all. Seems like its just a relativistic "trick" of inserting new "inertial frame" cards into the deck but the author in that link believes no stretching or creation of new space happens at all !!

Last edited by kiwiheretic; Aug 29th 2017 at 11:57 PM. Reason: Further Info

 Aug 30th 2017, 08:36 AM #13 Senior Member     Join Date: Jun 2016 Location: England Posts: 442 Just to Stir the Pot Another way of looking at it might be that the property of the universe that we interpret as distance is changing with time. __________________ ~\o/~
Aug 30th 2017, 08:56 AM   #14
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 Originally Posted by kiwiheretic Then why is the expansion considered to be isotropic? Surely with all the particles moving with constant velocity it would hollow out in the middle (see attached diagram).
This is a wrong figure to link to. Note that the caption says "expands with constant velocity" - that is NOT how expansion of the universe operates. The universe not expanding at constant velocity, but rather it is expanding at a velocity which is dependent on distance. There's a big difference. A good analogy is to consider the particles of an exploding bomb - particles that start at the outer edge of the bomb move away from the center at a greater speed than particles that start nearer the center. A particle that starts at the very center of the bomb doesn't move at all. The interesting thing is that for every particle the speed with which any other particle recedes from it is dependent on the distance away that the other particle. This is true for every single particle of the bomb - the speed it observes for other particles moving away from it at any point in time is a constant per unit distance - just like Hubble's constant is velocity per unit distance. The attached figure illustrates this better than the one you referenced - there is no hollowed out middle.
Attached Thumbnails

Aug 30th 2017, 11:43 AM   #15
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 Originally Posted by ChipB This is a wrong figure to link to. Note that the caption says "expands with constant velocity" - that is NOT how expansion of the universe operates. The universe not expanding at constant velocity, but rather it is expanding at a velocity which is dependent on distance. There's a big difference.
If that's true then why does the NASA pictures show the expansion of the universe drastically slowing down after the first few minutes of the big bang. (Notice how the expansion becomes approximately linear a short time after the initial explosion and increasingly linear after that.)

Also your picture implies acceleration and greater acceleration radially outwards which in turn implies an increasing force so what force are you suggesting produced your spatially-changing velocity profile? I can't think of any force that would do that (other than some kind of unrealistic constant force that acts independently of mass, charge, distance from source, etc and violates isotropicity (if that's a word) because it still acts in a definite direction).

Edit: Ok, I had a chance to think about it and I think you are referring to a kind of stretching the map (as if a map was imprinted on a rubber sheet and then stretched in all directions). If the stretching is uniform this should mean angles are invariant under the transformation. This implies a mass invariant force acting from the edges of the universe. It's also ignoring everything else like kinetic energy of galaxies which we assume to be nonzero. However this makes no progress in explaining why gravity manages to hold galaxies together, even over vast distances, and overcomes the stretching locally. Neither does it explain why we attribute greater velocities to the outermost galaxies as the stretching ought to be uniform. Could it be we were sold a bad raisin muffin? Or maybe, just maybe, there are no muffins.
Attached Thumbnails

Last edited by kiwiheretic; Aug 30th 2017 at 01:52 PM. Reason: readability

 Aug 30th 2017, 03:32 PM #16 Physics Team     Join Date: Jun 2010 Location: Morristown, NJ USA Posts: 2,280 The picture I attaced shows constant velocity of all particles starting immediately after the explosion of the bomb. There is no acceleration shown or implied, and no force needed to keep it going. Do you agree that this is how an ideal bomb would work? The bomb analogy is only helpful in seeing ng how the relative velocity of two particles can be a function of the distance between them. But it's not very good at illustrating how the universe expands as a whole, because a bomb exploding like in the figure has a distinct center and a distict edge, but the universe does not. In our universe ALL points are at the "center" (the universe is essentiall homogenous in all directions, from all points of view). As for gravity holding galaxies together - yes, that's true, but only over relatively "short" distances, such as in our local galactic group. You have to look at many galaxies over huge distances to see the Hubble Constant at work. But it would seem that over time Hubble's Constant should decrease, as the motion of galaxies away from each other should slow under mutual gravitational attraction. The fact that it appears that the expansion is actually increasing, rather than decreasing, is a mystery. There must be some sort of repulsive force at work. No one knows what that is - it's referred to as "dark energy" because its source has never been determined, yet. Regarding the early expansion phase - that's a theory that helps explain the structure and distribution of galaxies. No one really knows what force may have caused this inflation. It lasted for only a very short time - less than 10^-34 second - but had a profound influence on the structure of the universe we see today.
Aug 30th 2017, 04:40 PM   #17
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 Originally Posted by ChipB I wouldn't describe it that way. The distance between galaxies is indeed increasing, at an average rate of about 72 kilometers per second per megaparsec (a rate that depends on distance, known as Hubble's Constant). Thus from our perspective sitting here in the Milky Way we see that distant galaxies are receding from us at a rate that averages out to Hubble's Constant.
That doesn't mean that our galaxy[/u] is moving. The increase in distance is a result of an increase in the space between the galaxies. Think of it like this: imagine a rubber sheet with a Cartesian grid laid out on it with a galaxy located at the intersection of each grid line. Now imagine that the rubber sheet is being stretched out. The galaxies would remain at the intersection points of the grid but they'd measure the distance to be increasing, not because they're moving but because there's more space being created. And the rate of increase is neither constant or decreasing. In fact its increasing at an accelerating rate.

Aug 30th 2017, 04:43 PM   #18
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 Originally Posted by ChipB The picture I attaced shows constant velocity of all particles starting immediately after the explosion of the bomb. There is no acceleration shown or implied, and no force needed to keep it going. Do you agree that this is how an ideal bomb would work?
You're neglecting cosmological inflation. There as a period of extremely rapid expansion right after the big bang (by which I don't mean t =0). The cause being referred to as the presence of dark matter as well as a positive cosmological constant.

Aug 30th 2017, 05:11 PM   #19
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 Originally Posted by ChipB The picture I attached shows constant velocity of all particles starting immediately after the explosion of the bomb. There is no acceleration shown or implied, and no force needed to keep it going. Do you agree that this is how an ideal bomb would work?
No, I don't think it does. Once a bomb has exploded all the ejected shrapnel should obey Newton's first law. One idealisation I made was that all the pieces of shrapnel were of identical size which looks similar to your scenario anyway. If all particles are being ejected with the same velocity then they cover the same distance in the same unit of time. However what you have is a linear scaling which means that particles further away from the origin travel further in the same unit of time just because they were further away from the origin. I don't know of any ideal bomb which works this way.

To be clear, what you have is something like:
• a particle 1cm from the bomb moves 2cm in a unit time
• a particle 5cm from the bomb moves 10cm in a unit time
• a particle 10m from the bomb moves 20m in a unit time

etc.

If the particles were travelling at say 10m/s you would have
• a particle 1cm from the bomb moves 10.01m in a second
• a particle 5cm from the bomb moves 10.05m in second
• a particle 10m from the bomb moves 20m in a second

This is not a linear scaling but a translation.

So I can only assume you are presuming the further away particles to start with higher initial velocities but I can't see what kind of motivation you might have for making such an assumption as it seems very arbitrary.

Also the rubber sheet analogy, which might produce something like this, isn't really like an idealised bomb either (the forces come from tension forces at the edges and stretching the manifold) so I am at a loss to know how you derived these results.

Edit:
I just did some calculations on ChipB's expansion model.

Basically his model comes down to v=kx where v is the velocity at position x offset from the big bang and k is some suitable constant. This equation ensures that all particle distance ratios remain constant as they expand which was indicated in ChipB's particle expansion diagram.

Constant ratio expansion requires $\displaystyle v=\frac{dx}{dt} = kx$

integrating as an ODE...

$\displaystyle \int_{x_0}^x \frac{dx}{kx} = \int_{t_0}^t dt$

yields

$\displaystyle \frac{1}{k}(ln(x-x_0)) = t-t_0$

results in an expansion of $\displaystyle x = e^{k(t-t_0)}+x_0$ therefore the ChipB's linear expansion model is actually an exponential expansion model in practice !! If you differentiate that expression twice you will get a non vanishing acceleration.
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Burn those raisin muffins. Burn 'em all I say.

Last edited by kiwiheretic; Aug 30th 2017 at 11:33 PM.

 Aug 30th 2017, 05:49 PM #20 Junior Member   Join Date: Apr 2017 Posts: 12 aside from redshift what evidence is there that space is expanding? kiwiheretic likes this.

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