Announcing ... 'Dark Flow'

[Dave2042 (Dave)]

Originally Posted by xelasnave

Dave I think you are correct.
I seem to recall it is movement in space.
Google the great attractor.
That should bring up a diagram of the "flow" of the local group.
Alex

Yes. Great attractor is exactly what I was thinking. Thanks.

[Dave2042 (Dave)]

Originally Posted by alpal

Thanks for that link.
There is something going on that scientists don't understand.
Dark matter & dark energy really mean that we don't know what they are.
Dark flow is just the same -
another unknown.
The universe is stranger than we could ever have imagined.

cheers
Allan

Not sure if I agree. Certainly there is something we don't fully understand here. But it doesn't strike me as particularly unimaginably strange. As I see it, the most likely explanation is simply that the entire universe is much larger than the visible universe, and consequently has larger inhomogeneities than we can see in our little bit.

Interesting, wonderful, but hardly unimaginable.

[alpal]

Originally Posted by Dave2042

Not sure if I agree. Certainly there is something we don't fully understand here. But it doesn't strike me as particularly unimaginably strange. As I see it, the most likely explanation is simply that the entire universe is much larger than the visible universe, and consequently has larger inhomogeneities than we can see in our little bit.

Interesting, wonderful, but hardly unimaginable.

Dark matter, Dark energy & now dark flow -

it's certainly hard to imagine & then you have to wonder what the universe is expanding into?
It seems to be expanding into itself.

If you can imagine & explain that then you need to get a Nobel prize.

[Dave2042 (Dave)]

Originally Posted by alpal

Dark matter, Dark energy & now dark flow -

it's certainly hard to imagine & then you have to wonder what the universe is expanding into?
It seems to be expanding into itself.

If you can imagine & explain that then you need to get a Nobel prize.

Actually, this is reasonably well understood. The universe is not expanding into anything. It's just expanding. And several of the people who contributed to the understanding did indeed get Nobel prizes. So a mediocre intellect such as myself can just read up on as much of it as I want to understand.

To be clear, I'm not suggesting it is perfectly understood by anyone, let alone myself. But it is not a complete mystery.

[xelasnave]

I cant get my head around the Universe expanding but there is no outside so I dont even try but such a situation would not offend the concept of infinite is so far as you can double infinite but it is still infinite.
I do find however when I think of the Universe I am on the outside which does not help.
I look at the structure, filimentary nature of galaxies and their line up, and wonder if it could be a giant nervous system or similar and we are less than a virus...
Thank goodness I do not form beliefs about stuff that last one is very odd.
Alex

[alpal]

Originally Posted by Dave2042

Actually, this is reasonably well understood. The universe is not expanding into anything. It's just expanding. And several of the people who contributed to the understanding did indeed get Nobel prizes. So a mediocre intellect such as myself can just read up on as much of it as I want to understand.

To be clear, I'm not suggesting it is perfectly understood by anyone, let alone myself. But it is not a complete mystery.

I don't think anyone understands it really -

it's outside our normal 3 dimensional world of senses.

Space time - adding that extra dimension of time -
is hard enough to understand - it's bizarre but real -
yet alone a universe expanding into itself.

[markbakovic]

I spent about 4 hours writing an attempt at clarifying some of these topics on Saturday night but TL;DP. It's not that unusual: we deal with more than 3 dimensions all the... time

[alpal]

Originally Posted by markbakovic

I spent about 4 hours writing an attempt at clarifying some of these topics on Saturday night but TL;DP. It's not that unusual: we deal with more than 3 dimensions all the... time

TL;DP ? please write in English.

cheers
Alpal

[markbakovic]

Too long; didn't post. Related to TL;DR "too long; didn't read" which would have been the expected response. I just wanted to say expanding universe isn't hard to imagine if you just... and there didn't seem much point since what I had in mind was simple but seemed to require too much explaining what I meant (cos I don't know how much cosmology is assumed knowledge). The extremely short version is "think of time as a spatial dimension", but that statement on its own is probably unhelpful. Sorry.

[alpal]

Originally Posted by markbakovic

Too long; didn't post. Related to TL;DR "too long; didn't read" which would have been the expected response. I just wanted to say expanding universe isn't hard to imagine if you just... and there didn't seem much point since what I had in mind was simple but seemed to require too much explaining what I meant (cos I don't know how much cosmology is assumed knowledge). The extremely short version is "think of time as a spatial dimension", but that statement on its own is probably unhelpful. Sorry.

OK - when you've worked out how to explain a universe expanding into itself then please let us all know.

[markbakovic]

k, be careful what you wish for:

Howdy all, long time lurker, first time poster (sorry for the lack of further introduction), but I might be able to help a bit here.

Originally Posted by xelasnave

I cant get my head around the Universe expanding but there is no outside so I dont even try but such a situation would not offend the concept of infinite is so far as you can double infinite but it is still infinite.

Originally Posted by alpal

Dark matter, Dark energy & now dark flow -

it's certainly hard to imagine & then you have to wonder what the universe is expanding into?
It seems to be expanding into itself.

If you can imagine & explain that then you need to get a Nobel prize.

Think of the surface area of a (perfect) balloon: at different pressures its surface is stretched by different amounts, but never needs to expand into some other area. Don't get too hung up on the balloon as a 3d object with a 2d surface; the point is that the surface forms an unbounded (it has no edge) but finite (anyone with a pi key on their calculator can tell you its area) space, and some underlying property (the air inside the balloon) would cause it to increase, moving points on it further apart, and some other property (the air outside the balloon) would cause it to decrease.

I'm sure most of you have heard this analogy before, but maybe there are some ways in which it seems the analogy simply falls down (it is only a simple analogy after all) and it's easy enough to wave away questions (like "so is the universe expanding because it has one less spatial dimension than the space it resides in?") with an instruction to disregard that part of the analogy, just like I did a few lines ago. But we don't always need to.

The surface is 2D, the volume of the balloon is 3D, and this is what allows the surface to curve back on itsself and have no boundary. Do we need the universe to reside in a container, perhaps with a greater number of spatial dimensions? No. If you imagined yourself as an inhabitant of the balloon surface, maybe the surface is conductive and you're an electron moving freely, but only along the surface lattice, then you could circumnavigate the balloon and end up where you started without finding the edge of your 2D universe. But you'd have traced a circle in the 3 dimensions an observer of the balloon sees, right? Well I don't think its helpful to try and imagine a viewpoint external to the universe, as that's the whole problem we want to avoid (since we keep being told it's not possible). More importantly, we don't have to if we want to relate the 2D analogy to 3D reality: we have another dimension available and that is time. Lets say the universe is not expanding and you set off in one direction to circumnavigate it, like the electron in the conductive balloon. Will you get "back to the same place"? No, because you will have taken so long to go anywhere (relatively speaking) that no part of the universe you travel through will ever bear any resemblance to the place you started out from, even though you may actually wind up meeting the same protons (unless they turn out to have a half life too) several times. Time is the dimension in which the space of our universe is curved.

Does it sound as if I've cheated a bit? I mean, you may be familiar with the lycra-sheet demonstration of "spacetime curvature by massive bodies" where a physics teacher plonks some weights on a stretchy surface and makes marbles "orbit" them, and this is a great way to visualise why, say, massless photons follow curved paths around gravity wells, but extending this (other, but related!) analogy to the "real" universe and saying you can circumnavigate the entire universe but you won't know when you've finished sounds a bit... unfulfilling, right? Well, that's exactly why it's not a bad analogy. The universe, in this respect, is rather mundanely exactly as strange as we imagine (i.e. a little bit). The mass around which the spacetime of the universe is curved is just all the matter in the universe, the paths of all photons form closed orbits of the mass forming this gravity well (a better way to put it is the universe fits inside its own event horizon for the simple reason that the constants and relationships that define an event horizon are the properties of exactly this universe). I don't even need to point out that in a universe expanding as rapidly as ours seems to be the "circumnavigation" part is a total non-issue, as the balloon is being inflated faster than it is possible to travel along its surface (the observable universe is about 93 billion lightyears across but only 13.8bn years old).

I'm going to leave that there, as the "what's it expanding into?" query is all I really wanted to address, but there are a whole host of ways to relate cosmology to strechy fabric that can be quite informative. Distance is one I would gladly touch on in much less detail: galaxies are getting farther away from us as the universe expands, but it is the space expanding and not them moving through space. A distinction is usually made between ("proper") motion due to the object moving through space and an observed receding motion due to the expansion of the universe (which are not always trivial to disentangle): "redshift" can mean many things in different contexts. But if you try to use texta marks on a balloon to illustrate this: how do you know they're "further apart" when the balloon is inflated? with a ruler? c is the ruler? But if space is expanding and space is really spacetime so time is expanding and c should be useless to measure this, right? But there are other rulers, like energy states and binding energies of hydrogen atoms, which don't depend on how long a meter or a second is. In fact if you think of redshift not as a doppler effect due to relative motion, but as a decrease in energy (not that energy is lost, you're just observing less of it) of the photons you can see how its possible to quantize everything, even expansion rates, distance and, ultimately, time, in terms of energy: the one yardstick that's (afawk) invariant under all known universe topology modifications...

OK, so finally to the reason for my post: "Dark". Specifically flow, but generally energy and matter too. It just means as-yet-not-directly-detected-but-predicted, which is important to remember. It made sense with dark matter (we see all other matter by its emitted/reflected/absorbed "light", even if it's just reradiated as IR etc. but there seems to be this matter, without any light)... Now why you'd expect a flow to emit light... I mean if the galactic centre mass excess ("dark matter", pre hypestorm) had *somehow* come before general relativity gravity waves would probably have been called "Dark Waves(TM)". My point is that half of cosmology is predictions of effects that haven't been observed directly (yet), but then the same is true for most of science. Yet the popsci press often gives a lot of weight to ideas and calculations that may be related to other work which gets no press. The one I'm thinking of here is that the fine structure constant is strongly suspected to vary across the universe (spatially and/or temporally) and it's embedded in G, remember, so we don't need to suppose a differently inhomogeneous universe beyond what's observable to get clusters of things behaving noticeably differently to "the norm". That's not to say that discoveries of same wouldn't be momentous achievements, but just that we don't need to go supposing radically different bizzarro universe just beyond what we can see because of them. That's actually as valid a conclusion to draw as that we see every galaxy moving away from us not because the universe is expanding, but because the earth sits exactly at the epicentre of our universe and we have always been the centre of everything. Our patch of sky is big enough to be a representative sample, especially since there are secondary effects of almost any extreme solution to these sort of unknowns which are the reason they remain unknown (so eg dark flow: if it's real and due to vast mass concentration outside our observable universe we should expect everything in the observable universe to be orbiting it, but orbital motion would result in galaxy redshift being polarised: we'd be orbiting in a plane so the galaxies above and below us would be systematically more redshifted than galaxies ahead or behind us and galaxies closer to/further from the mass concentration would have elongation-dependent redshifts relative to us, none of which is observed (afaik), and in fact these sort of systematic trends would be very obvious in things like CMB, which is such an old, low energy signal that anything affecting galaxy speed in an isotropic way should show up there... and we're back to "dark", again...). Personally I think it's just a rather spectacular supercluster in the early stages of formation, quite interesting from a "population statistics on non-rotating black holes" perspective potentially, etc. etc. but not exactly requiring a rewriting of "what we thought we knew about the universe"...

Also, you can't actually double infinity. Mathematical manipulations that involve such things are just placeholders for "so large it no longer changes the result of this calculation (appreciably)"

Can I pick up my prize in January? I'm busy in December.

</copypaste>

It should be interesting to dig this up in 5 years and see if I still agree with myself...

[xelasnave]

Thank you Markbacovik, may I call you Mark?
I am glad you decided to post and I thank you for taking the time.
I hope we can look to you for guidence in the future.
Most of all may I welcome you to the forum.
Alex

[Dave2042 (Dave)]

Originally Posted by alpal

OK - when you've worked out how to explain a universe expanding into itself then please let us all know.

It really depends what you mean by explain.

I think there are three broad things you might mean.

1. We have a set of equations that describe the process pretty well. Essentially the idea is that the metric, which tells you how far apart things appear to be, is changing, so that things seem to be getting further apart without moving. The metric is internal to the manifold of space-time and requires no 'background space' to be meaningful. This is all 'just' maths (sometimes referred to as the 'shut up and calculate' view of physics), but I think it counts as a real form of understanding/explanation. The description is good, and we can calculate and observe all kinds of interesting consequences.

2. We can interpret the equations as meaning that the universe is expanding, though not into anything, and that the universe is 'curved' or 'warped'. This is fairly removed from our day-to-day experience, but it seems a fairly reasonable conclusion from where I stand. You can see light curving - this is the warpedness. Blowing up a balloon while standing on the surface is a reasonable illustration of the expansion with fewer dimensions. As to what the universe is 'in', I don't see the need for anything 'outside', particularly since no one has ever glimpsed it. If you can't visualise 4-D spacetime, or a non-embedded manifold, then fair enough - they are difficult abstract concepts. But generations of maths and physics students have successfully absorbed the concepts to a reasonable degree, self included. I find this personally to be a good 'explanation' that I 'understand'.

3. Finally you can ask what is 'really' going on. Here you are outside physics, and indeed science. The generally accepted reality is that you can do maths and experiments and just leave it at that (point 1 above), or you can come up with a useful way of visualising or interpreting the maths and experiements (point 2 above), and that's it. There is no way I've ever heard of to meaningfully understand physics, other than to do maths and experiments and try to sum it up in a useful interpretation. If you insist that 'really' is what you need, then I'm not sure anyone can help, and I'd suggest your bar may be set a bit too high. Or maybe you are doing philosophy, which is not a field I really know a lot about.

[markbakovic]

I think taken to an extreme philosophy comes to the conclusion that in the end the only reality is what we perceive, and there are fairly low limits on even the degree to which we can truly agree on a collective perception of reality. So point 3 boils down to points 1 and 2 anyway: consensus and distribution of their results amounts to the "global" perception and hence is as real as anything can get for us. This of course can be taken to imply that reality changes over time, but the philosophical argument would run "prove that it doesn't then..." which is exactly what 1 and 2 try to do: rewrite past perceptions in light of new information.

To me that's not troublesome at all; small children have a hard time coping with a world without absolutes (when is it ok to lie? never! but then why did mummy say daddy doesn't look like a potato now that his hair's falling out? mummy is a Bad Person because she LIED!) but we become comfortable with an uncertain world eventually. Asking the questions is illuminating, needing the answers to be absolute is just a great way to be disappointed. So I guess a "complete explanation" should be a recasting of the question to include as much as possible that is currently known and thus better point to what is currently unknown. That would mean a mix of both point 1. (the part with assumed knowledge) and point 2. (ideally with as little assumed knowledge as possible) in repeated iteration. The result of doing this as many times as possible either approaches point 3. given infinite time, infinite grad students on typewriters and infinite episodes of Cosmos/youtube videos with a physicist named Brian in them (if you take "reality" as external), or is, by definition, point 3. if the definition is as I've wished for above.

To me that way preserves more of the "sense of wonder" and is thus motivating to keep asking questions. But otoh I can see how the whole "you can never be sure anyway" realisation can make the whole thing seem futile if you dwell on it, though I like to think that as a species a lesser reliance on absolute truth as the carrot to motivate critical thinking is the more helpful direction.

(and thanks for the welcome Alex: of course! It just seemed silly to do the brackets thing given my not-so-pseudo-nym)

[xelasnave]

So what is going on with the attractor?
Alex

[alpal]

Originally Posted by markbakovic

k, be careful what you wish for:

Howdy all, long time lurker, first time poster (sorry for the lack of further introduction), but I might be able to help a bit here.





Think of the surface area of a (perfect) balloon: at different pressures its surface is stretched by different amounts, but never needs to expand into some other area. Don't get too hung up on the balloon as a 3d object with a 2d surface; the point is that the surface forms an unbounded (it has no edge) but finite (anyone with a pi key on their calculator can tell you its area) space, and some underlying property (the air inside the balloon) would cause it to increase, moving points on it further apart, and some other property (the air outside the balloon) would cause it to decrease.

I'm sure most of you have heard this analogy before, but maybe there are some ways in which it seems the analogy simply falls down (it is only a simple analogy after all) and it's easy enough to wave away questions (like "so is the universe expanding because it has one less spatial dimension than the space it resides in?") with an instruction to disregard that part of the analogy, just like I did a few lines ago. But we don't always need to.

The surface is 2D, the volume of the balloon is 3D, and this is what allows the surface to curve back on itsself and have no boundary. Do we need the universe to reside in a container, perhaps with a greater number of spatial dimensions? No. If you imagined yourself as an inhabitant of the balloon surface, maybe the surface is conductive and you're an electron moving freely, but only along the surface lattice, then you could circumnavigate the balloon and end up where you started without finding the edge of your 2D universe. But you'd have traced a circle in the 3 dimensions an observer of the balloon sees, right? Well I don't think its helpful to try and imagine a viewpoint external to the universe, as that's the whole problem we want to avoid (since we keep being told it's not possible). More importantly, we don't have to if we want to relate the 2D analogy to 3D reality: we have another dimension available and that is time. Lets say the universe is not expanding and you set off in one direction to circumnavigate it, like the electron in the conductive balloon. Will you get "back to the same place"? No, because you will have taken so long to go anywhere (relatively speaking) that no part of the universe you travel through will ever bear any resemblance to the place you started out from, even though you may actually wind up meeting the same protons (unless they turn out to have a half life too) several times. Time is the dimension in which the space of our universe is curved.

Does it sound as if I've cheated a bit? I mean, you may be familiar with the lycra-sheet demonstration of "spacetime curvature by massive bodies" where a physics teacher plonks some weights on a stretchy surface and makes marbles "orbit" them, and this is a great way to visualise why, say, massless photons follow curved paths around gravity wells, but extending this (other, but related!) analogy to the "real" universe and saying you can circumnavigate the entire universe but you won't know when you've finished sounds a bit... unfulfilling, right? Well, that's exactly why it's not a bad analogy. The universe, in this respect, is rather mundanely exactly as strange as we imagine (i.e. a little bit). The mass around which the spacetime of the universe is curved is just all the matter in the universe, the paths of all photons form closed orbits of the mass forming this gravity well (a better way to put it is the universe fits inside its own event horizon for the simple reason that the constants and relationships that define an event horizon are the properties of exactly this universe). I don't even need to point out that in a universe expanding as rapidly as ours seems to be the "circumnavigation" part is a total non-issue, as the balloon is being inflated faster than it is possible to travel along its surface (the observable universe is about 93 billion lightyears across but only 13.8bn years old).

I'm going to leave that there, as the "what's it expanding into?" query is all I really wanted to address, but there are a whole host of ways to relate cosmology to strechy fabric that can be quite informative. Distance is one I would gladly touch on in much less detail: galaxies are getting farther away from us as the universe expands, but it is the space expanding and not them moving through space. A distinction is usually made between ("proper") motion due to the object moving through space and an observed receding motion due to the expansion of the universe (which are not always trivial to disentangle): "redshift" can mean many things in different contexts. But if you try to use texta marks on a balloon to illustrate this: how do you know they're "further apart" when the balloon is inflated? with a ruler? c is the ruler? But if space is expanding and space is really spacetime so time is expanding and c should be useless to measure this, right? But there are other rulers, like energy states and binding energies of hydrogen atoms, which don't depend on how long a meter or a second is. In fact if you think of redshift not as a doppler effect due to relative motion, but as a decrease in energy (not that energy is lost, you're just observing less of it) of the photons you can see how its possible to quantize everything, even expansion rates, distance and, ultimately, time, in terms of energy: the one yardstick that's (afawk) invariant under all known universe topology modifications...

OK, so finally to the reason for my post: "Dark". Specifically flow, but generally energy and matter too. It just means as-yet-not-directly-detected-but-predicted, which is important to remember. It made sense with dark matter (we see all other matter by its emitted/reflected/absorbed "light", even if it's just reradiated as IR etc. but there seems to be this matter, without any light)... Now why you'd expect a flow to emit light... I mean if the galactic centre mass excess ("dark matter", pre hypestorm) had *somehow* come before general relativity gravity waves would probably have been called "Dark Waves(TM)". My point is that half of cosmology is predictions of effects that haven't been observed directly (yet), but then the same is true for most of science. Yet the popsci press often gives a lot of weight to ideas and calculations that may be related to other work which gets no press. The one I'm thinking of here is that the fine structure constant is strongly suspected to vary across the universe (spatially and/or temporally) and it's embedded in G, remember, so we don't need to suppose a differently inhomogeneous universe beyond what's observable to get clusters of things behaving noticeably differently to "the norm". That's not to say that discoveries of same wouldn't be momentous achievements, but just that we don't need to go supposing radically different bizzarro universe just beyond what we can see because of them. That's actually as valid a conclusion to draw as that we see every galaxy moving away from us not because the universe is expanding, but because the earth sits exactly at the epicentre of our universe and we have always been the centre of everything. Our patch of sky is big enough to be a representative sample, especially since there are secondary effects of almost any extreme solution to these sort of unknowns which are the reason they remain unknown (so eg dark flow: if it's real and due to vast mass concentration outside our observable universe we should expect everything in the observable universe to be orbiting it, but orbital motion would result in galaxy redshift being polarised: we'd be orbiting in a plane so the galaxies above and below us would be systematically more redshifted than galaxies ahead or behind us and galaxies closer to/further from the mass concentration would have elongation-dependent redshifts relative to us, none of which is observed (afaik), and in fact these sort of systematic trends would be very obvious in things like CMB, which is such an old, low energy signal that anything affecting galaxy speed in an isotropic way should show up there... and we're back to "dark", again...). Personally I think it's just a rather spectacular supercluster in the early stages of formation, quite interesting from a "population statistics on non-rotating black holes" perspective potentially, etc. etc. but not exactly requiring a rewriting of "what we thought we knew about the universe"...

Also, you can't actually double infinity. Mathematical manipulations that involve such things are just placeholders for "so large it no longer changes the result of this calculation (appreciably)"

Can I pick up my prize in January? I'm busy in December.

</copypaste>

It should be interesting to dig this up in 5 years and see if I still agree with myself...

Thanks Mark,
a nice attempt & it shows our current knowledge & theories.

It's all quite strange.
Consider that when we look out from our galaxy - no special place -
there are galaxies receding faster than light which we will never be able to see.
Anyone in those galaxies would see the same as us - other galaxies
receding at close to the speed of light & obviously theorise others they couldn't see
moving faster than light.

So - the universe is expanding faster than light!

cheers
Allan

[billdan (Bill)]

Well we have Dark matter, Dark Energy, Dark Flow, now we have also Dark Radiation.

http://www.abc.net.au/news/2016-06-0...iverse/7472074

All very confusing.

Bill

[alpal]

Originally Posted by billdan

Well we have Dark matter, Dark Energy, Dark Flow, now we have also Dark Radiation.

http://www.abc.net.au/news/2016-06-0...iverse/7472074

All very confusing.

Bill

Nice post - thanks.

[avandonk]

I reckon Andrei Linde has a few good ideas. Not all cosmologists agree with him.

This is one of his lectures


https://www.youtube.com/watch?v=V9MV...Pprj-&index=53



It is just one of many on cosmology lectures in this set looking at all the knowns and unknowns.

I have been attempting to get up to speed on all of this. I will most probably die not being too much the wiser.

This is real Physics folks not philosophy.

This is also worth a long look.


https://www.youtube.com/watch?v=2hIa...gRpWSQ3oXCPprj-







Bert

[alpal]

There are plenty of ideas:
here's a recent short paper:

An Explanation for Galaxy Rotation
Rates without Requiring Dark Matter

http://vixra.org/pdf/1606.0218v2.pdf