Dark matter

[neilmr]

I'm quite confident that the scientists won't be looking for dark matter in 10 years or so, I am much more inclined to believe in "MOND" or modified newtonian dynamics which will answer the questions

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[JulianO]

MOND and TeVeS are good attempts to fit new laws to the universe.

Both of them have problems explaining certain features - in particular the bullet cluster, but then dark matter also has issues (no ones found it yet!).

I think I'd bet on DM ahead of MOND at the moment, but its anyones race in the end.

[K Nixon]

The Universe expanding is not like say my gut expanding as I demolish my breakfast in the office. My gut gets bigger into its local surroundings.

If you run the Universe story backwards, you don't end up with a single point inside a defined area, you end up with all points in one place. That place isn't defined in regular dimensions as it would imply an outside of that point which there is (theoretically) not.

It doesn't matter where you are located in the Universe (as per the post above,) should the Universe spontaneously contract, all points would seem to have all other points contracting in on it.

I wish I could remember who to credit with this (might have been John Gribbin?) but the best description I ever heard was that it is not that things are moving apart but more that the space between them is getting greater. Think of this like a large funnel with water being pumped in from the bottom. things are floating on the surface of the water. As water is forced up from the bottom, the surface they float on becomes bigger so they get more spaced out but they are not moving apart, its just that water is being forced into the gap between them.

If you use the model of space time - it is stretching creating greater dimension between the objects in space, but they themselves are not moving apart. Things like redshift still work with this model, the difference being that the stretch in the emitted wavelengths is not caused by relative separation velocity but by the wavelengths becoming stretched over time as the space time onto which they are written is being stretched.

Basically galaxies just bob around a bit and orbit each other while the universe forces more dimension in between them.

Just to really confuse you (and try to get the concept of weird dimensions across,) there's equally no edge (no expanding face) of the Universe. Theory goes that if you could travel infinitely fast in one direction, you would ultimately end up back where you started

This is one possibility. The universe is shaped a bit like a 'snakes' universe on your mobile phone - sort of wrap around.

If this isn't the case and there is an 'edge' you are still quite correct that it is beyond our ability to reach. Although this somewhat goes against what I said above (these things are all just theories and change every few years depending on the latest measurements) but dimension (space AND time) only exist when matter or energy occupies the space. Following this the very first photons are travelling outwards at the speed of light creating new dimension as they go.

If you 'stood' at the endue of the universe and the stepped forwards you would created space and time as you went as you now occupy that space. All of which is academic as we will never catch up with the photons travelling at the speed of light creating this edge of the universe as we go.

Which version of the Universe's shape we use depends on the density of matter contained within it and it is this which changes. It is also linked to the fate of the universe - whether we expand for ever or go for the big crunch.

Kieron

[K Nixon]

I'm quite confident that the scientists won't be looking for dark matter in 10 years or so, I am much more inclined to believe in "MOND" or modified newtonian dynamics which will answer the questions

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Interesting New scientist articles on this if you can get hold of the last week in Aprils copy

Kieron

[K Nixon]

So the spacetime between galaxies expands but the spacetime within them doesn't because gravity is too strong inside galaxies?

That sort of makes sense, but also doesn't. I think there must be some sort of expansion within galaxies otherwise the size of galaxies would be getting smaller and smaller relative to the size of the expanding universe around them. Conversely, if the universe were to contract back on itself, we would expect the galaxies to start contracting wouldn't we? Otherwise, everything ends up at one point, except galaxies that dont expand or contract??? That doesn't seem logical.

The Hubble constant describes the rate of expansion of the universe. It varies but it is about 70 km per second per megaparsec. That means for every million parsecs, space is expanding away from us at a speed of 70 km per second.

This is quite unnoticeable on local galaxy scales. The milky way is 30-37 000 parsecs (0.037 Megaparsecs) across so the expansion would be negligible when measured against the usual speeds of stars and planets zooming around each other.

It is 0.66megaparsecs to Andromeda so thats why even on a local galactic scale the normal speed at which galaxies bob around is not really affected by the rate of expansio (hence Andromeda is still coming towards us despite the universe' expansion)

So whether expanding or contracting you would not notice a particularly great effect on the size of an individual galaxy.

However I have to presume that you would when we start getting so small that we run out of space, we would start to notice something. I have a feeling that the Hubble constant may be linked to the size of the Universe and that it changes over time? I would not swear to this though, maybe someone knows more about this?

Kieron

[OzDave]

Related to this sub-theme of expansion, someone mentioned a while back about what general relativity has to say about gravity and that started me thinking a bit, which can be dangerous :-)

If I understand (or even remember) well, one of the things that general relativity says is that gravity is actually the curvature of space-time in the vicinity of large objects such as planets or stars. This explains why orbital deflections happen to objects travelling in the space-time near such large objects etc.

So viewing the universe in this curvy space-time sort of way, I am wondering about the following (maybe this needs a separate thread ?):

Let's say we have a small object, like a satellite or something, that is not travelling in space-time and is near a planet. It is at rest. Now, I'm not actually sure what "at rest" means because everything is relative, but for the purposes of this question, let's assume that it means the satellite isn't orbiting the planet, but is rather just sitting there looking at the planet rotate, a bit like an alien mothership might do :-) Empirical evidence shows us that the satellite would eventually get attracted towards the planet due to the gravitational pull. But if gravity is just the curvature of space-time, and the satellite is just sitting at a fixed point in the curved "grid of space-time" around the planet, what actually accelerates the satellite towards the planet? Since gravity is explained as the curving of space-time, presumably it cannot also be exerting a force? Or can it?

David

[K Nixon]

The model of this is that its a bit like dropping a bowling ball on the bed. A nearby marble would roll downhill towards the bowling ball as your duvet becomes curved.

Of course this explains nothing because the model actually uses gravity to make the marble roll 'downhill'.

The issue is that we are using the duvet to describe 3D space bending in the 5th dimension (space time) modelling it as a 2D surface bending in the 3rd. In the real situation there is no 'downhill'. Our heads cannot imagine the real thing.

All other forces are modelled using exchange particles. Quite literally they fire particles at each other on order to exert a force. Gravitons are the theoretical equivalent for gravity but we have yet to discover them. (we seem to have stopped looking mostly) It was thought at one time that this curvature of space time could be caused by streams of gravitons (nicely described in EventHorizon if you like your scary films).

But we continue with our search for models to adequately describe what gravity is - what it is that actually exerts that force. There are layers and layers of models and none of them quite do the job.

Maybe we don't actually understand what gravity does yet and that's why we can't model it properly.

Kieron

[ismangil]

Ever wonder why night sky is dark? I didn't, until I read Paradox...

It's because the already mentioned expansion, edge of observable universe thing: photons can't reach us...

On mobile (excuse the strange predictive words...)

[Gary Anthony]

Dark Energy, Dark Matter and Supermassive Black Holes – Is There a Connection?

I am fascinated by black holes. When I read that Kretchmann’s Invariant meant that the central singularity of a black hole must be real, I thought this really means that BHs are unique. Because of this fact alone, I thought it verges on insanity to hold that BHs are ordinary and should be regarded like any other massive object in the universe.

Let us suppose that there are such things as BH singularities or as close to being singularities as may be needed to produce the effect of “virtually” infinite density, virtually infinitely tight spacetime radius of curvature and virtually strong gravitational fields. “Infinite” means as great as may ever be necessary to account for whatever effects need accounting. Quantities “tend” toward infinity.

Infinite also means that if one attempts to graph an infinite quantity, one must truncate the plot and draw “asymptotes” that mean that the curve “approaches” infinity. Or perhaps it approaches a surrogate value very close to the value achieved at infinity, and would do so more and more closely if only there was a long enough piece of paper. An asymptotic curve never gets to where it is going.

I thought that if this is true, there must be something wrong with the way the gravitational field of BHs is commonly described. One must draw a gravitational field strength diagram of an ordinary object as a parabola, nearer the center, having a maximum (or a minimum, if a gravitational potential well is intended) passing through the center of the object. With a gravitational singularity, one cannot draw the curve so that it passes through the center of the object. It has no maximum (or minimum). This is not a “parabolic” or inverse square gravitational field strength diagram. This is something entirely different.

I believe that the universe adheres to mathematical laws including the laws of geometry. Never mind about spacetime being curved and non-Euclidean. This is a gravitational field diagram, so spacial curvature is already implied and taken into account. Besides geometry, I believe in the power of symmetry. If there is an asymptote nearest to the vertical axis, there must also be an asymptote nearest the horizontal axis.

Wait!

I know what this is. It is a hyperbola! Black holes must generate a hyperbolic gravitational field.

So, I looked at Newton’s law of gravity,

F = - GMm/r²,

and wondered what would happen if

F = - GMm/rr* where r* is the unit vector of r, for dimensional integrity. I call this an “adjusted” Newton’s law. It is hyperbolic.

Using this, I followed analogous steps as with “raw” Newton’s law and I obtained

v = (GM_bh/r*)^½

which is an amazing result.

This says that the stellar orbital velocity around a spiral galactic center should not depend on r and, in fact, depends only on G and the mass of the galactic SBH. For a given galaxy with its central SBH, v is constant, I call it v_o. This is the anomalous stellar velocity distribution, which is not really a distribution at all since all the stellar velocities are constant beyond the galactic bulge.

I also noticed that, for a series of galaxies having different SBHs with different masses,

v_o² = GM_bh

This is the M-sigma relation wherein a simple monotonically increasing relation exists between central SBH mass and the orbital velocities of outer stars, where no correlation is supposed to exist when there is an inverse square gravitational force. This explanation of the M-sigma effect might be complicated by the fact that galaxies contain tens of thousands of large stellar black holes too. These would add significantly to M_bh in ways that would be hard to measure or predict.

But critics of the HSBH G-field postulate say that according to GR, gravitational force falls off as 1/r^(n-1) where n = the number of spacial dimensions. So, a hyperbolic gravitational field can exist only in a 2-D region of spacetime.

I said “No problem.” All real black holes spin very fast, are non-spherical, greatly perturbed and cannot be expected to obey Birkhoff’s theorem. This says that perfectly spherical, stationary, unperturbed, black hole gravitational fields must be “asymptotically flat”. This means that, among other things, the field diagram must have no asymptotes. But, central supermassive black holes must spin even faster than most stellar black holes because so many stars constantly in-fall concertedly toward the BH singularity, increasing its angular momentum. Eventually, this produces “infinite” spin rate at the singularity.

Yes, the Kerr metric sort of stops or pauses (or at least the interpretations do) at the event horizon where a measurable finite non-zero angular momentum must momentarily exist for this material. If it falls into the singularity, once there it should form a mere “ring” singularity. But, I deduce a possible new solution.

After all, what about all the comments that have been made to the effect that we do not really know what happens when spacetime curvature becomes so tight? Everybody says of black hole singularities, as r → 0, GR → ???

Might Kerr have got it slightly skewed? Perhaps SBH EMS (energy/matter/spacetime) collapses not to a ring singularity, but to a virtually infinite spin rate SBH (below the event horizon) with a very wide 2-D disk singularity. This singularity is unique enough to be associated with the amazing properties of a real black hole. It befits the legend that we make of it.

It becomes a very special 2-D EMS singularity that is a very broad, thin flat disk. It is like a big whirling German pancake that spills beyond the griddle onto the floor and out the door. It is a 2-D spacetime entity that can support a hyperbolic gravitational field. Maybe this is actually what the Kerr metric actually implies, but rigid consensus mongers and obsessive seekers after conventional wisdom consistently misinterpret his analysis of GR.

How broad is “broad”?

I began to think large. Perhaps the mass or matter in the spacetime disk singularity stays under the event horizon (which must also be distorted), but the gravitational spacetime coordinate component, which is immune to the event horizon, spreads out to include the periphery of the galaxy and beyond, even to the environs of other galaxies in a cluster or supercluster. It casts its hyperbolic 1/r gravitational field influence outward to infinity or to whatever passes for infinity in our universe. Maybe this is what it would mean to be a 2-D infinite spin rate disk singularity.

The hyperbolic 1/r black hole gravitational field has a potential energy profile that is generally higher than the equivalent inverse square gravitational field. The difference might constitute Dark Matter because M = E/c². The 2-D 1/r gravitational disk influence would display thickness in the galactic stellar assembly because the stars are the observers that would measure the disk’s location. They are limited by the Heisenberg uncertainty principle, so they see the 2-D flat plane as a volume having thickness. This flat plane is not totally flat, however. Being hyperbolic in nature, it would have an overall shape like a hyperboloid of one sheet. Looking like an hourglass or a saddle shape, It would not really be flat beyond the galactic periphery. It surface could therefore align easily with other hyperbolic field surfaces generated by other SBHs in other galaxies. This alignment tendency may account for the large scale structure effect observed in sky surveys.

I reasoned that the enhanced 1/r gravitational field effect accounts for not only the M-sigma relation and the anomalous velocity dispersion, but the weak gravitational lensing, the Sunyaev-Zel’Dovich, the Sachs-Wolfe and yes, even the large scale structure effects. These are precisely the phenomena that are cited in support of Exotic Dark Matter. But DM need not be exotic. It may result simply from how we compute the masses of cosmological objects containing large black holes.

Cosmologists at Cambridge, Cal Tech and the Perimeter Institute will not like this idea. Dozens of graduate students depend on Dark Matter research funding. I wouldn't dream of upsetting anybody's special interests

But, maybe Dark Matter could still be real. A little extra thinking let me see that if extended to include the whole universe from the time of the BB, the hyperbolic field becomes Dark Energy, as well as Dark Matter, by virtue of its generally higher gravitational potential energy compared to the equivalent inverse square field that must pervade the universe, because M = E/c².

As a matter of fact, Alan Guth says that the universe was once a quantum object. So, I say that it still is. Thus, the 2-D hyperbolic field may have been the highly excited “inflaton field” which decayed into the 3-D + time universe that we see today. This quantum transition would have been time dependent. It may still be going on. The higher potential energy of the hyperbolic field may currently be being realized as accelerating expansion of spacetime and the apparent kinematic “repulsion” between cosmological objects. The superposition of yesterday fades into the reality of today.

I did not yell “Eureka!” But, Dark Matter and Dark Energy are essentially the same. It all hangs together quite nicely.

[Ronnie67]

Black holes are singularities. This means that they must exist as point masses, according to general relativity. The gravitational field around a point mass must have a hyperbolic potential profile, not a parabolic one as assumed by Newton’s Law of Gravity. This is because the event horizon of a black-hole is not a true surface like that of a planet or a surface zone like that of a star. The only characteristic of a black-hole that makes any difference here is the point-mass at the Heisenberg uncertainty constrained center. Mordechai Milgrom's work (see below) proves that there is more to a black-hole than just its event horizon.

A hyperbola is characterized by asymptotes that define the extreme behavior of the curve far from the origin. Hyperbolic curves merely approach, but never reach the asymptotes. Proper parabolas, on the other hand, approach and reach their extreme values eventually. For instance, a proper parabola will actually pass through the origin, but the equivalent hyperbola never will.

The hyperbolic gravitational field profile (a 2D graph) around a black hole has significant consequences. These effects include a much slower fall-off of the gravitational potential as one proceeds to larger distances, r, from the origin. The mass of the supermassive black hole at the center of most spiral galaxies is a few million solar masses at most. But, the mass of the entire galaxy is several hundred billion solar masses. So, it would seem that the mass of the central black hole is insignificant relative to the galaxy as a whole. But, since the gravitational fields of the black hole and of the galaxy must be perfectly aligned and co-axial, the fields must superpose, reinforce and merge as one. So, the effective mass of the central black hole must be in the hundreds of billions of solar masses. The consequence of this is that the gravitational field experienced by any given star or group of stars near the periphery of a spiral galaxy must be declining as 1/r*, by the definition of a hyperbola, not as 1/r², according to Newton’s Law. The unit vector, r, allows proper dimensional analysis. It is a constant, so the equation, say, F = GmM/ r*, is hyperbolic in nature. The symbol, r*, is the variable r (distance from the graphical origin or from the center of a galaxy) multiplied by the dimensioned value of the unit vector associated with the vector equivalent of r, (r₁). This ensures that the "hyperbolic" Newtonian equation F = GmM/r* passes dimensional analysis. But, there are other fundamental justifications for its use. Normally we use the "parabolic" form F = GmM/r². But not when super-massive black holes are involved.

This explains the MOND effect, the residual gravitational potential constant that was observed by Mordehai Milgrom in 1983 for stars near the peripheries of spiral galaxies. MOND stands for Modified Newtonian dynamics, a proposed revision of Newton’s Law. I call it the “MOND effect” to distinguish it from MOND itself because when the hyperbolic (proportional to 1/r*) gravitational field is considered, no modification of Newton’s Law is required. Instead, a footnote must be appended to acknowledge that a hyperbolic 1/r* field can exist around black holes.

Another consequence is that there is no need to hypothesized “Dark Matter”. The rotational distribution effects around galaxies and the behavior of galactic clusters and super-clusters is explained by the hyperbolic black hole gravitational field effect. The behavior of colliding galaxies having hyperbolic gravitational fields that are in the process of merging is also explained.

Since the hypothesis of the existence of a huge point mass called an “inflaton” is used to explain numerous characteristics of the cosmic microwave background radiation (CMB) and the expansion of the universe, it makes sense to consider that the universe was once immersed in a hyperbolic 1/r* gravitational field. The inflaton itself can be said to have existed before the Big Bang (BB). So, its hyperbolic gravitational field must have existed then too.

The question arises: What happened to the hyperbolic field after the BB? At the instant of the BB, the hyperbolic field must have begun to collapse or transition from a hyperbolic field to a parabolic Newtonian filed. This transition should still be occurring, the process requiring the entire lifetime of the universe to complete. The hyperbolic field is inherently more intense than the equivalent parabolic field, so the transition to the weaker field must be releasing potential energy. This energy will show up as kinetic energy in the form of the accelerating expansion rate of the universe.

So, the source of “Dark Energy” is gravitational, not due to a new type of field called "quintessence". It would give a positive cosmological constant, Lambda, in the Friedmann equations under the FLRW metric. These are relativistic treatments, but the hyperbolic 1/r* gravitational field equations are purely Newtonian. Nevertheless, no “quintessence” field is needed, just as no “Dark Matter” is needed to explain the MOND effect.