Dark matter as matter in parallel universes

[mrspeedybob]

I was watching a documentary the other day and this idea occurred to me. Here are links to the relevant portions on youtube...

http://www.youtube.com/watch?v=4BRhjntvGoE&feature=related
http://www.youtube.com/watch?v=2zgxvGaei6o&feature=related

If gravity is not confined to 3 spatial dimensions and if there are universes parallel to ours then the gravity of matter in 1 or more parallel universes would be felt by ours but not otherwise interact. It would act like dark matter. Matter in our universe would act like dark matter in their universes. The distance between universes would allow the gravity to dissipate somewhat so that large concentrated masses like stars would not be apparent but instead the mass would look fairly homogeneous. There is no reason to believe our universe would not be average in terms of mass density and there would be no reason to believe there were only 2 parallel universes. If there were several parallel universes interacting gravitationally in this way then each would perceive dark matter to make up a large percentage of its mass.

In places where the universes were too far apart galaxies and stars would be less likely to form and there would be no intelligent life to observe the absence of dark matter. In places where the they were too close together the increased gravity would cause a localized big crunch scenario, again, no life to observe the excess dark matter. We observe the density we do because we exist in a place where the density is correct to allow for our existence.

So. My question is has this idea been explored? Has it been dismissed for some reason?

 

[Chalnoth]

If gravity is not confined to 3 spatial dimensions and if there are universes parallel to ours then the gravity of matter in 1 or more parallel universes would be felt by ours but not otherwise interact. It would act like dark matter.

Not quite. The problem is that dark matter also doesn't interact very well with itself. That is a very important distinction, because if dark matter were simply normal matter in a parallel universe, then it would have very different properties. In particular, it would lose energy and fall deeper into gravitational potential wells. It doesn't do this, however.

While it is conceivable that dark matter is matter in a parallel universe, it most certainly isn't normal matter in a parallel universe.

 

[Drakkith]

Not quite. The problem is that dark matter also doesn't interact very well with itself. That is a very important distinction, because if dark matter were simply normal matter in a parallel universe, then it would have very different properties. In particular, it would lose energy and fall deeper into gravitational potential wells. It doesn't do this, however.

While it is conceivable that dark matter is matter in a parallel universe, it most certainly isn't normal matter in a parallel universe.

Wait, dark matter doesn't interact with itself very well? What exactly does that mean? If it still obeys gravity then would there be a noticeable clumping of it like normal matter does?

 

[Ryan_m_b]

Wait, dark matter doesn't interact with itself very well? What exactly does that mean? If it still obeys gravity then would there be a noticeable clumping of it like normal matter does?

IIRC two galaxies in the Bullet Cluster have been observed to be colliding and in the process the dark matter from either galaxy has continued travelling. Oddly the dark matter has passed through the other galaxy and through the other traveling dark matter and out into space.

 

[Chalnoth]

Wait, dark matter doesn't interact with itself very well? What exactly does that mean? If it still obeys gravity then would there be a noticeable clumping of it like normal matter does?

The reason why normal matter tends to clump so efficiently to produce dense objects like stars and planets is because normal matter interacts with both gravity and the electromagnetic force.

To understand this, imagine a single atom. Imagine, if you will, that this atom is in an orbit in some gravitational potential well somewhat like the Earth is in orbit around the Sun. If this atom doesn't interact with anything else except through gravity, it will just continue in this same orbit perpetually, never falling inward. This is the picture you should think of with dark matter: it tends to fall into a potential well, then once there, just continue in the same orbit for a very long time (not quite forever, but still a long time).

But normal matter does tend to interact, and interact strongly, through the electromagnetic force. So when this atom bumps into another atom, it will tend to release some energy. This, on average, tends to bump the atom into a lower orbit. So over time, lots of little electromagnetic interacts tend to push all of the atoms in a particular potential well into lower and lower orbits, getting denser and denser. This is how galaxy clusters, galaxies, stars, planets, asteroids, etc. all form.

Without these interactions, the dark matter just can't lose its energy, so it doesn't move to lower orbits and stays very spread-out, which is precisely what we observe.

 

[Ryan_m_b]

The reason why normal matter tends to clump so efficiently to produce dense objects like stars and planets is because normal matter interacts with both gravity and the electromagnetic force.

To understand this, imagine a single atom. Imagine, if you will, that this atom is in an orbit in some gravitational potential well somewhat like the Earth is in orbit around the Sun. If this atom doesn't interact with anything else except through gravity, it will just continue in this same orbit perpetually, never falling inward. This is the picture you should think of with dark matter: it tends to fall into a potential well, then once there, just continue in the same orbit for a very long time (not quite forever, but still a long time).

But normal matter does tend to interact, and interact strongly, through the electromagnetic force. So when this atom bumps into another atom, it will tend to release some energy. This, on average, tends to bump the atom into a lower orbit. So over time, lots of little electromagnetic interacts tend to push all of the atoms in a particular potential well into lower and lower orbits, getting denser and denser. This is how galaxy clusters, galaxies, stars, planets, asteroids, etc. all form.

Without these interactions, the dark matter just can't lose its energy, so it doesn't move to lower orbits and stays very spread-out, which is precisely what we observe.

Nice explanation I'm not an expert in the field at all, are there any ideas about how dark matter sticks to itself? Or is it all gravity?

 

[Chalnoth]

IIRC two galaxies in the Bullet Cluster have been observed to be colliding and in the process the dark matter from either galaxy has continued travelling. Oddly the dark matter has passed through the other galaxy and through the other traveling dark matter and out into space.

Well, I wouldn't call that odd. Dark matter had to behave that way to explain our other observations of it. But yes, the key take-away point of the Bullet Cluster observation is that the dark matter was quite undisturbed by the collision of the two clusters of galaxies (as were the galaxies, because clusters have a lot of empty space, and most of the galaxies, if not all, just missed one another).

 

[Chalnoth]

Nice explanation I'm not an expert in the field at all, are there any ideas about how dark matter sticks to itself? Or is it all gravity?

Thanks :)

The expectation is that it's all gravity, with possibly some very limited, short-range interactions, such as the weak nuclear force.

Note that it's exceedingly difficult to get dark matter that has no interactions whatsoever besides gravity, because it has to be produced through some interaction or other. But weak nuclear interactions are a good candidate.

 

[mrspeedybob]

So far all the arguments I see raised against this idea actually support it...

Not quite. The problem is that dark matter also doesn't interact very well with itself. That is a very important distinction, because if dark matter were simply normal matter in a parallel universe, then it would have very different properties. In particular, it would lose energy and fall deeper into gravitational potential wells. It doesn't do this, however.

While it is conceivable that dark matter is matter in a parallel universe, it most certainly isn't normal matter in a parallel universe.

If what we call dark matter is normal matter that exists in 100 different universes then it should not be expected to interact very strongly with itself. For 2 particles to collide they would have to be in the same universe and with 100 different universes there is only a 1% chance of that. I don't know how weakly dark matter interacts with itself but you can pick a number of universes to match whatever level of interaction there is.

Dark matter would not appear to fall into dense gravity wells because It would be blurred out both by physical distance between universes and by the fact that stars and galaxies are not going to exist in exactly the same place in each universe. Gravity should keep the galaxies and galaxy clusters close but stars will be completely random within them so with a large number of universes in play it will look fairly homogeneous.

IIRC two galaxies in the Bullet Cluster have been observed to be colliding and in the process the dark matter from either galaxy has continued travelling. Oddly the dark matter has passed through the other galaxy and through the other traveling dark matter and out into space.

This is also exactly what my idea would predict. If there are again 100 other universes in play we may not expect each galaxy in ours to be shadowed by galaxies in all 100 other universes. If each of our galaxies is shadowed by galaxies in 50 of the 100 other universes then galactic collisions would only occur in 25% of them. Most of the galaxies would continue on their collective way though there should be some smearing back towards the collision caused by gravitation. The level of smearing would be an indication of what fraction of the universes gravitationally interacting with ours contain galaxies which are shadowing ours.

 

[Chalnoth]

If what we call dark matter is normal matter that exists in 100 different universes then it should not be expected to interact very strongly with itself. For 2 particles to collide they would have to be in the same universe and with 100 different universes there is only a 1% chance of that. I don't know how weakly dark matter interacts with itself but you can pick a number of universes to match whatever level of interaction there is.

Dark matter would not appear to fall into dense gravity wells because It would be blurred out both by physical distance between universes and by the fact that stars and galaxies are not going to exist in exactly the same place in each universe. Gravity should keep the galaxies and galaxy clusters close but stars will be completely random within them so with a large number of universes in play it will look fairly homogeneous.

I don't think this idea makes sense. If the dark matter were normal matter in "another universe" it would have to be in a different brane parallel to our own. It doesn't make sense to have more than one or two such branes parallel to our own and yet close enough to interact gravitationally.

 

[Ryan_m_b]

So far all the arguments I see raised against this idea actually support it...



If what we call dark matter is normal matter that exists in 100 different universes then it should not be expected to interact very strongly with itself. For 2 particles to collide they would have to be in the same universe and with 100 different universes there is only a 1% chance of that. I don't know how weakly dark matter interacts with itself but you can pick a number of universes to match whatever level of interaction there is.

Dark matter would not appear to fall into dense gravity wells because It would be blurred out both by physical distance between universes and by the fact that stars and galaxies are not going to exist in exactly the same place in each universe. Gravity should keep the galaxies and galaxy clusters close but stars will be completely random within them so with a large number of universes in play it will look fairly homogeneous.



This is also exactly what my idea would predict. If there are again 100 other universes in play we may not expect each galaxy in ours to be shadowed by galaxies in all 100 other universes. If each of our galaxies is shadowed by galaxies in 50 of the 100 other universes then galactic collisions would only occur in 25% of them. Most of the galaxies would continue on their collective way though there should be some smearing back towards the collision caused by gravitation. The level of smearing would be an indication of what fraction of the universes gravitationally interacting with ours contain galaxies which are shadowing ours.

This makes about as much sense as "If my magic idea is true then X=Y!"

 

[twofish-quant]

The problem with assuming parallel universes is that at some point, you haven't explained anything. If you say it's mysterous parallel universes, that's not different from saying mysterious unknown factors.

I don't know how weakly dark matter interacts with itself but you can pick a number of universes to match whatever level of interaction there is.

If you can pick and choose your numbers to make things work, that's not a good thing. If you can say, things will work out exactly right if we assume 134324 universes that would be impressive, but I don't think you've gotten to that point.

Dark matter would not appear to fall into dense gravity wells because It would be blurred out both by physical distance between universes and by the fact that stars and galaxies are not going to exist in exactly the same place in each universe. Gravity should keep the galaxies and galaxy clusters close but stars will be completely random within them so with a large number of universes in play it will look fairly homogeneous.

But you've made the problem worse. It's not a bad thing to say "I don't know." If you say "I think it's parallel universes" then you have to have the parallel universes act in a certain way, and you have to explain why the parallel universes act in exactly that way, that increases the numbers of "I don't know"'s.

This is also exactly what my idea would predict. If there are again 100 other universes in play we may not expect each galaxy in ours to be shadowed by galaxies in all 100 other universes.

Why? Also, why 100. Why not 10 or 1 million?

If each of our galaxies is shadowed by galaxies in 50 of the 100 other universes then galactic collisions would only occur in 25% of them. Most of the galaxies would continue on their collective way though there should be some smearing back towards the collision caused by gravitation. The level of smearing would be an indication of what fraction of the universes gravitationally interacting with ours contain galaxies which are shadowing ours.

But why parallel universes rather than "something unknown that interacts gravitationally." I'm pretty sure that you could come up with some rules involving parallel universes that "work" but if you have to randomly make up rules that may things fit that doesn't get you very far.

Also, the problem with the model is that to make this interesting, you have to predict something that we *haven't* seen. What theorists do is to think, if this is parallel universes, then you must see X, and if you don't see X, then it's not parallel universes.

So what could you observe that would kill the idea of dark matter being parallel universes? If it turns out that you can make *any* data fit with the idea, then it's not a very good one.

 

[twofish-quant]

One problem with parallel universes is that it's hard to figure out how to show that the idea is wrong. With either dark matter or non-standard gravity, it will take you about a page to show "this assumption will give you this galaxy rotation curve". It's not obvious how to do this with parallel universes.

Also one thing that you have to think about is how parallel universe matter is different from "shadow matter."

 

[mrspeedybob]

Why? Also, why 100. Why not 10 or 1 million?.

I haven't gotten that far but I don't think the answer would be that simple. Different universes would be separated by different amounts of distance which may also very from place to place. The point of the thread was to see if anyone had been down this train of thought and found compelling reasons to dismiss it. If no one has then maybe it's worth investing some time in.

Also, the problem with the model is that to make this interesting, you have to predict something that we *haven't* seen. What theorists do is to think, if this is parallel universes, then you must see X, and if you don't see X, then it's not parallel universes.

So what could you observe that would kill the idea of dark matter being parallel universes? If it turns out that you can make *any* data fit with the idea, then it's not a very good one.

Matter in multiple parallel universes should stick together on large scales, but only a portion of its gravity would reach us. If a large mass of dark matter were to appear more massive as measured by its momentum then as measured by its gravity that would be a good indicator that the matter exists in other universes. If this disconnect between momentum and gravity is not observed then it is far more likely invisible matter in our own universe.

 

[Ryan_m_b]

I think you're missing the point that dark matter doesn't act like normal matter. So your matter from other universes not only would have to leak gravity to ours but also be fundamentally different.

Your parallel universe idea basically requires the existence of dark matter anyway yet with the added complexity of being in another universe.

 

[mrspeedybob]

I think you're missing the point that dark matter doesn't act like normal matter. So your matter from other universes not only would have to leak gravity to ours but also be fundamentally different.

Your parallel universe idea basically requires the existence of dark matter anyway yet with the added complexity of being in another universe.

So far what I know about dark matter...
Dark matter interacts very weakly, if at all, with electromagnetism.
Dark matter interacts very weakly, if at all, with itself or with normal matter.
Dark matter is approximately 25% of the observed mass/energy content of the universe, dark energy - 70%, normal matter - 5%
Dark matter is observed by gravitational lensing of light and by its effects on galactic rotation.
Dark matter halos surround galaxies.
The dark matter in the universe usually exists in roughly, but not exactly the same places that luminous matter exists.

What properties of dark matter do I not know about about? What properties of dark matter are incompatible with normal matter spread over multiple parallel universes?

If the objection is more to the idea of many parallel universes then that is a different (though related) discussion.

I'm sorry if it seems like I am, but I'm not stuck on this idea. I just want to understand if/why it is incompatible with previous observations before I go wasting a bunch of time on it. If it is incompatible with previous observations then please use it to shine a light on the gap in my understanding so I can fill that gap in.

 

[Chalnoth]

So far what I know about dark matter...
Dark matter interacts very weakly, if at all, with electromagnetism.
Dark matter interacts very weakly, if at all, with itself or with normal matter.
Dark matter is approximately 25% of the observed mass/energy content of the universe, dark energy - 70%, normal matter - 5%
Dark matter is observed by gravitational lensing of light and by its effects on galactic rotation.
Dark matter halos surround galaxies.
The dark matter in the universe usually exists in roughly, but not exactly the same places that luminous matter exists.

What properties of dark matter do I not know about about? What properties of dark matter are incompatible with normal matter spread over multiple parallel universes?

If the objection is more to the idea of many parallel universes then that is a different (though related) discussion.

I'm sorry if it seems like I am, but I'm not stuck on this idea. I just want to understand if/why it is incompatible with previous observations before I go wasting a bunch of time on it. If it is incompatible with previous observations then please use it to shine a light on the gap in my understanding so I can fill that gap in.

The most important observation for dark matter, really, is the cosmic microwave background. Basically, before the CMB was emitted, the universe was a plasma. Normal matter in a plasma interacts very strongly with light, and when it falls into a gravitational potential well, this interaction causes the normal matter to feel pressure and bounce. Dark matter, on the other hand, doesn't interact with light, and so experiences no such bounce.

Observing the relationship between how much matter falls into a gravitational potential well and how much bounces gives us a very accurate measurement of the ratio between normal matter and dark matter.

Your "normal matter in an alternate universe" would bounce just like normal matter here does, so it would show a dramatically different signature in the CMB.

 

[twofish-quant]

What properties of dark matter do I not know about about? What properties of dark matter are incompatible with normal matter spread over multiple parallel universes?

Dark matter causes the following galaxy power spectrum

http://magnum.anu.edu.au/~TDFgg/Public/Survey/Overview/sld023.htm

Dark matter causes a weird peak in galaxy sizes

http://cmb.as.arizona.edu/~eisenste/acousticpeak/acoustic_physics.html

One problem with popular accounts of science is that they tend to use words, and with words you can't describe things quite as accurately as with numbers. If I tell you that something is "big" it could be an elephant, but if I tell you that it is 200.5 meters high, then it's not an elephant.

If you can come up with a set of equations that can generate those graphs starting with parallel universes, that would be interesting. The problem is that if I tell you that I'm wrong, and there are actually two peaks instead of one, and you can still match the data, then that says that your model is too vague.

I'm sorry if it seems like I am, but I'm not stuck on this idea. I just want to understand if/why it is incompatible with previous observations before I go wasting a bunch of time on it. If it is incompatible with previous observations then please use it to shine a light on the gap in my understanding so I can fill that gap in.

What you need to do is to show how parallel universes can fit those curves and do it in a way that is better than other explanations. There's a lot that we don't know, but there is also a lot that we do know.

 

[Chalnoth]

Dark matter causes a weird peak in galaxy sizes

http://cmb.as.arizona.edu/~eisenste/acousticpeak/acoustic_physics.html

A bit of clarification here: this isn't about galaxy sizes, but instead about the typical separation between galaxies (the so-called two point function). Pay attention to the horizontal axis: this feature in our universe is vastly, vastly larger than any single galaxy. So instead it describes where galaxies are likely to form relative to one another.

Secondly, the dark matter sets the smaller-scale behavior, while the extra bump at longer distances is due to the normal matter. That said, this picture, when taken together, is incredibly powerful evidence for dark matter.

 

[Tanelorn]

Is there a reason why DM cannot be found in our galactic neighbourhood? If not why have we not found even 1 particle of it? Assuming it is made out of particles. Hard to believe that something can be so rare here as not to be present at all.

 

[Chalnoth]

Is there a reason why DM cannot be found in our galactic neighbourhood? If not why have we not found even 1 particle of it? Assuming it is made out of particles. Hard to believe that something can be so rare here as not to be present at all.

The dark matter is very diffuse, such that its gravitation is just not measurable within our own solar system. In order to have the cosmological properties it does, dark matter also needs to interact very weakly with normal matter, and thus hard to detect. Those who study dark matter are (for the most part) not surprised at all that we haven't yet observed it in our current detectors.

 

[Tanelorn]

Thanks Chalnoth, DM is diffuse, yet DM is meant to be 20% of the mass energy content of the universe, many times more than baryonic matter. Sometimes seems like believing in ghosts :)

 

[Chalnoth]

Thanks Chalnoth, DM is diffuse, yet DM is meant to be 20% of the mass energy content of the universe, many times more than baryonic matter. Sometimes seems like believing in ghosts :)

Well, um, space is really, really big. The quantity of dark matter really starts to add up once you get to larger scales. The inability to detect the dark matter in our own solar system is exactly what we expect when we take our observations of dark matter on galactic (or larger) scales and apply them to our own solar system.

 

[Tanelorn]

Chalnoth, so is it correct then that unlike normal matter, DM remains diffuse, never clumping at all into small bodies, with some very low number of particles per cubic meter and it dominates because it is spread everywhere very thinly, including, for example, galaxy voids and super voids? Do we expect to find any particles of DM within our solar system at all?

 

[Chalnoth]

Chalnoth, so is it correct then that unlike normal matter, DM remains diffuse, never clumping at all into small bodies, with some very low number of particles per cubic meter and it dominates because it is spread everywhere very thinly, including, for example, galaxy voids and super voids? Do we expect to find any particles of DM within our solar system at all?

Right, because dark matter experiences precious little friction. Because it interacts so weakly, once a dark matter particle gets into an orbit around a gravitational potential well, it stays in that same orbit pretty much forever. By contrast, normal matter will lose energy through various interactions with other bits of normal matter and fall into progressively lower orbits, forming things like galaxies and stars.

 

[Misericorde]

Just to be clear, if something like DM were in a "parallel" universe, we wouldn't see ANY evidence of it because it would be completely inaccessible. By definition, it's part of our universe or it isn't DM, and while it could be all around us right now, its interactions are so incredibly weak it makes neutrinos look burly and interactive. Would that be correct?

 

[n1person]

@Tanlorn: Also, many theories of dark matter predict it is concentrated in the Galactic Halo, which is not where we are.

Also one should note that there is little evidence to suggest that dark matter has been detected via gravitational lensing. MACHOs (Massively Compact Halo Objects) have not been observed via gravitational lensing in any reasonable quantity. One paper I read (http://adsabs.harvard.edu/abs/2007A&A...469..387T) recently looked at ~3 million stars over a 6.5 year period looking for any evidence of microlensing, only finding one event, when any model showing considerably MACHO content predicted at least 30-50 events with that sample size. There was a paper a few years back which did find a number of supposed events (http://adsabs.harvard.edu/abs/2000ApJ...542..281A) , but various papers since showed that the vast majority of them were supernova or other non-lensing phenomena.

 

[Chalnoth]

@Tanlorn: Also, many theories of dark matter predict it is concentrated in the Galactic Halo, which is not where we are.

Um, the galactic halo is where we are. We're near the center of it. It's just that the galactic halo extends far beyond the stars, so that there just isn't that much mass here.

Also one should note that there is little evidence to suggest that dark matter has been detected via gravitational lensing. MACHOs (Massively Compact Halo Objects) have not been observed via gravitational lensing in any reasonable quantity. One paper I read (http://adsabs.harvard.edu/abs/2007A&A...469..387T) recently looked at ~3 million stars over a 6.5 year period looking for any evidence of microlensing, only finding one event, when any model showing considerably MACHO content predicted at least 30-50 events with that sample size. There was a paper a few years back which did find a number of supposed events (http://adsabs.harvard.edu/abs/2000ApJ...542..281A) , but various papers since showed that the vast majority of them were supernova or other non-lensing phenomena.

MACHOs are completely ruled out by CMB observations anyway.

 

[Lost in Space]

Perhaps dark matter is the result of a two brane interaction by gravitons?

 

[Misericorde]

Perhaps dark matter is the result of a two brane interaction by gravitons?

Then why isn't it uniformly distributed along with "normal" matter?

 

[twofish-quant]

Just to be clear, if something like DM were in a "parallel" universe, we wouldn't see ANY evidence of it because it would be completely inaccessible.

That depends on the rule for parallel universes. If you can invent a rule for parallel universes that gives the graphs that I showed, and also doesn't have any other bad effects, people would be interested.

One problem is that if I assume that dark matter is jupiter-sized planets of ice, I can make some predictions. If you start talking about parallel universes, it's not clear how to start making predictions.

By definition, it's part of our universe or it isn't DM, and while it could be all around us right now, its interactions are so incredibly weak it makes neutrinos look burly and interactive.

I think right now the limits on interaction puts them at the same level as neutrinos. Also as we push down the limits, there are some candidates that get crossed off the list of possible things.

 

[Misericorde]

Interesting, I didn't know that there were formulations of parallel universes that allowed for any observable effects in ours; that's pretty cool! If I understand the second part of your response correctly, then you mean that we're basically deploying more sensitive techniques and equipment, and as we do so and find nothing, some previous options are no longer viable. That makes perfect sense, thanks for the information.

 

[DavidMcC]

Not quite. The problem is that dark matter also doesn't interact very well with itself. That is a very important distinction, because if dark matter were simply normal matter in a parallel universe, then it would have very different properties. In particular, it would lose energy and fall deeper into gravitational potential wells. It doesn't do this, however.

While it is conceivable that dark matter is matter in a parallel universe, it most certainly isn't normal matter in a parallel universe.

In my version of cosmology, at least, dark matter isn't in just one other universe, Chalnoth.
If there is a 4D hyperspace continuum supporting the space we exist in and a lot of others, then matter in many other universes can surely gravitationally interact with this one when they happen to be close by in our three dimensions, even if not in the fourth.
BTW, long time no "see" from RD.net, Chalnoth.

 

[Chalnoth]

In my version of cosmology, at least, dark matter isn't in just one other universe, Chalnoth.
If there is a 4D hyperspace continuum supporting the space we exist in and a lot of others, then matter in many other universes can surely gravitationally interact with this one when they happen to be close by in our three dimensions, even if not in the fourth.

Yes, but as I've been saying, it can't be normal matter, because it doesn't dissipate energy.

BTW, long time no "see" from RD.net, Chalnoth.

I've been on a few other forums, but I'm pretty sure I've never been on RD.net.

 

[DavidMcC]

Chalnoth, I understand that dark matter shows no sign of gravitational interaction even with itself, but, as I said in my previous post, in my cosmology, that can be because bits of it aren't really as close as they they seem in the LQG hyperspace.

BTW, it was richarddawkins.net/forum to be more precise. I can't believe there's another "Chalnoth" interested in cosmology!

 

[Chalnoth]

Chalnoth, I understand that dark matter shows no sign of gravitational interaction even with itself, but, as I said in my previous post, in my cosmology, that can be because bits of it aren't really as close as they they seem in the LQG hyperspace.

Presumably you meant non-gravitational interaction. But then, if the dark matter wasn't that close to itself, it wouldn't be close to us either, and thus we wouldn't feel its effects very much.

 

[Misericorde]

It doesn't exactly bowl me over than a name from Star Trek would be something you find cropping up in more than one forum related to science in general.

I keep hearing in this thread about dark matter as a product of universes other than our own, but even if possible (and twofish-quant has made that sensible), doesn't Occam's Razor cut far too close for that? I'd want to see native matter eliminated through process and trial before looking to other realities.

 

[DavidMcC]

It doesn't exactly bowl me over than a name from Star Trek would be something you find cropping up in more than one forum related to science in general.

I keep hearing in this thread about dark matter as a product of universes other than our own, but even if possible (and twofish-quant has made that sensible), doesn't Occam's Razor cut far too close for that? I'd want to see native matter eliminated through process and trial before looking to other realities.

I don't see how "Occam's razor" can be defeat the right kind of multiverse - on the contrary. The problem of conventional, one big bang cosmology are very severe, in that nothing makes sense. Dark matter is but one little aspect of it. Slight parity violation in particle physics, the appearance of fine tuning of the universe for life (the values of the fine structure constant and strong interaction), the variability of the cosmological "constant", inflation, are all better explained by the right kind of multiverse (ie a "fecund universe" type once proposed by Smolin, but I don't want to go through that yet again, there are now so many threads on this , it gets tiresome). I suspect two things : (1) Smolin was too soon with this hypothesis, so that there wasn't enough evidence for it at the time, and (2) there were many rubbish versions of the multiverse, which do, indeed, just complicate things.
EDIT: Also, a single big bang is highly improbable if big bangs are a natural process. Therefore, we only DETECT one with light - the one we're in.

 

[Chalnoth]

I don't see how "Occam's razor" can be defeat the right kind of multiverse - on the contrary.

Yes, Occam's Razor actually prefers a multiverse, even if we don't look at any data. This simply comes down to the fact that within mathematics, it is much easier to get a class of objects than it is to get a single member of that class. So it is a unique universe, not a multiverse, that needs evidence to support it. And the fact is that all of the evidence that has any relevance to this question at all so far points more to a multiverse than it does to a unique universe.

 

[Misericorde]

Let me rephrase: the notion of an infinite number of universes is not subject to Occam's Razor if you believe in eternal inflation, or a multiverse of many isolated pockets. It does cut finely (I think) in the case of QM interpretations such as MWI where it's just another variable to remove the "artifact" of collapse.

 

[Chalnoth]

It does cut finely (I think) in the case of QM interpretations such as MWI where it's just another variable to remove the "artifact" of collapse.

In the MWI, there are actually fewer assumptions than in other interpretations. The MWI just assumes the wavefunction dynamics of quantum mechanics, full stop. Other interpretations assume some sort of collapse or other mechanism that looks like collapse. MWI just looks at the wavefunction dynamics and notes that the appearance of collapse is already there, no extra bells and whistles required.

This is just one example (among many) of why a multiverse is preferred over a unique universe.

 

[twofish-quant]

In the interest of relating everything to observations, let me point out that a lot of the interest in parallel universes was inspired by recent observations of extra solar planets. It turns out that when we look at extra solar planets, that hot jupiters are very common, but we don't see them in our own solar system, which makes people wonder why our solar system seems to be specially tuned for life, and of course the answer is obvious. If our solar system had a hot jupiter, we wouldn't be here, and it turns out that the orbits of Jupiter and Saturn are specially tuned so that they are well-behaved...

The other problem is that it turns out that in standard particle physics theories none of the fundamental constants are uniquely determined. The value of the fine structure constant at "zero energy" happens to be because the universe settled at this energy state after symmetry breaking happened.

So this got people thinking about how this applies to the whole universe. I know one scientist that wrote a critical paper on the possibility of multi-universes and he mentions this explicitly as one of the things that got him thinking about it.

 

[DavidMcC]

Twofish-quant, I thought all the other observational reasons for suspecting a multiverse are actually rather more convincing than mere exoplanet tendencies, which have nothing much to do with the issue, as far as I'm concerned.
However, I agree about your second para.

 

[Misericorde]

MWI has fewer than other interpretations except the only one that has any evidence to support it: shut up and calculate. I don't understand why people think they're going to form a valid ontology based on a theory which is not a "final" theory. Occam's Razor cuts closer to the bone than an attempt to rationalize why we don't recognize quantum behaviour in everyday life (dead/alive cats).

I'd add, if we're going to be willing to boil everything down to imponderables such as "before" the BB, or an infinite multiverse, eternal inflation, and so on... well... why not just say that this universe is all that there is, period. What's outside of the universe? Well, it's possibly as salient as asking for a before the BB; it's just not something human beings can contemplate; eternity, nothingness, a lack of time.

We wisely ignore those issues as being beyond the scope of physics, or at least not falsifiable or verifiable, right? Well, my view is that using the data we have now to even wonder about the 'why' of our existence and the ability of a universe or multiverse to support wondering organisms is predicated on failure. We have imperfect theories, but the benefit of them is the technology and progress they allow; I suppose if a final theory were developed then moving on to ontology would make sense, but right now it's hubris.

 

[DavidMcC]

Presumably you meant non-gravitational interaction. But then, if the dark matter wasn't that close to itself, it wouldn't be close to us either, and thus we wouldn't feel its effects very much.

It's a tricky point, Chalnoth. Maybe gravity simply has a short range in hyperspace, so we only feel the gravity from those bits that are very close in it, and by the same token, those bits are isolated from each other.

 

[Chalnoth]

It turns out that when we look at extra solar planets, that hot jupiters are very common, but we don't see them in our own solar system, which makes people wonder why our solar system seems to be specially tuned for life, and of course the answer is obvious.

Well, the difficulty here is that this is almost certainly a selection effect. Hot Jupiters have high masses and short orbital periods, and so are the easiest to observe. The fact that we see a lot of them seems to indicate more that planets in general are likely to be very common. In fact, given that within our own solar system, every one of the more massive planets has a plethora of moons argues rather strongly that nearly every star out there will similarly have a number of planets (though obviously the more violent star systems, such as, say, tight-orbiting binaries, may end up destroying any planets that form).

If our solar system had a hot jupiter, we wouldn't be here, and it turns out that the orbits of Jupiter and Saturn are specially tuned so that they are well-behaved...

Well, Jupiter and Saturn couldn't exist if their orbits weren't well-behaved, because our solar system is some 4.5 billion years old. Anything that wasn't well-behaved that was that massive would have collided with something over the billions of years our solar system has been around.

 

[Chalnoth]

It's a tricky point, Chalnoth. Maybe gravity simply has a short range in hyperspace, so we only feel the gravity from those bits that are very close in it, and by the same token, those bits are isolated from each other.

That doesn't help.

And by the way, in order to explain our own observations of gravity, gravity in a higher-dimensional space would have to be stronger than it is in our 3+1 dimensions, not weaker.

 

[Chalnoth]

MWI has fewer than other interpretations except the only one that has any evidence to support it: shut up and calculate.

You mean the Copenhagen interpretation? Sorry, but that one has been falsified by observations of quantum decoherence.

 

[Misericorde]

You mean the Copenhagen interpretation? Sorry, but that one has been falsified by observations of quantum decoherence.

I mean exactly what I said: "shut up and calculate", not worry about collapse unless decoherence is the focus on your study. The solution to a falsified theory is not to invent countless interpretations of a theory, a majority of which are not falsifiable even if they are elegant. The solution is to calculate and work toward a greater understanding of what is right and wrong in the current formulation, observing and recording, and creating a NEW THEORY.

Clearly GR and QM have holes in them, but they also predict nature with astonishing fidelity. To assume that doesn't mean we have an infinite series of half-steps to a final theory however, again, seems like hubris. Shut up and calculate, and fine tune until we get so close it makes no difference, or we're all dust. It's good to identify the ontological problems uncovered, and some areas such as DCQE experiments demand explanation. I do not believe that interpretation of existing theories is of value in finding that however, but rather only new theories which are MORE predictive and explanatory. The rest is very admirable, but still very clearly intellectual masturbation.

 

[Chalnoth]

I mean exactly what I said: "shut up and calculate", not worry about collapse unless decoherence is the focus on your study. The solution to a falsified theory is not to invent countless interpretations of a theory, a majority of which are not falsifiable even if they are elegant. The solution is to calculate and work toward a greater understanding of what is right and wrong in the current formulation, observing and recording, and creating a NEW THEORY.

Sounds like a hand-wavy excuse to stop thinking about it to me.

My entire point was that absent evidence to discern which way to go, we should always consider the explanation with the fewest assumptions to be the most likely. That's MWI. And it turns out that this has been confirmed by observations of quantum decoherence. To claim that there is still some "real" collapse besides simple decoherence is an unjustified and unjustifiable claim and should be considered nonsensical.