Dark matter and energy explained by negative mass

[KarlM]

Any thoughts on this paper?
https://arxiv.org/abs/1712.07962

 

[mountevans]

It looks like it just came out. I am still reading through it, but I don't see any discussion of what this negative mass dark fluid could be made of.

Also, what would the geodesics around such a fluid look like?

 

[yahastu]

https://phys.org/news/2018-12-universe-theory-percent-cosmos.html

Very interesting paper here. In addition, I would point out that if approximately 95% of the mass in the universe is negative rather than positive, then perhaps its actually 100% -- a perfectly equally balanced universe of positive and negative mass, which seems rather elegant.

 

[Bandersnatch]

I would point out that if approximately 95% of the mass in the universe is negative rather than positive, then perhaps its actually 100% -- a perfectly equally balanced universe of positive and negative mass, which seems rather elegant.

How is 20:0 less imbalanced than 19:1, let alone perfectly balanced?

 

[yahastu]

How is 20:0 less imbalanced than 19:1, let alone perfectly balanced?

Because a generic property of inflation is that the universe began from a small quan-
tum fluctuation. According to Vilenkin, “A small amount of energy was
contained in that [initial] curvature, somewhat like the energy stored in a strung bow. This ostensible violation of energy conservation is allowed by the Heisenberg uncertainty principle for sufficiently small time intervals. The bubble then inflated exponentially and the universe grew by many orders of magnitude in a tiny fraction of a second.”

If the there is the same amount of negative mass as positive mass then the sum total is zero and that makes the idea that the whole thing sprang into existence out of some vacuum potential more plausible to me

 

[Bandersnatch]

But the point is, the paper doesn't claim there to be the same amount of positive and negative mass. It claims that all the energy density currently attributed to dark energy + dark matter is negative mass density. So, it proposes an already unbalanced state with a 19:1 ration of negative:positive mass.
A 100% negative mass density would have 0% positive mass in it.

Also, if I'm reading this paper correctly, this model does away with inflation, going for cyclic cosmology instead.

 

[mitosis]

I'm surprised this hasn't yet generated more discussion. Seems like an elegant theory.

 

[Elroch]

This is the most interesting new hypothesis I have come across in a long while, the idea that a single hypothetical substance (field/particle) could explain the two phenomena that are currently explained by dark energy and dark matter separately. It is this that J.S. Farnes (https://www.researchgate.net/profile/Jamie_Farnes) explains in his paper which is on Arxiv at https://arxiv.org/pdf/1712.07962.pdf

The first thing I thought was "how can a single substance explain both the 'missing mass' that is needed for anomalous galaxy rotation and accelerating expansion of the Universe?" The answer is given rather clearly in the paper by an extremely simple diagram whose most important feature is the fact that with a material which has negative mass, force and acceleration act in opposite directions (Newton would have been happy with this, since it is what F=ma says!). The result is that negative mass, which has positive energy according to Einstein's energy momentum equation (and which makes E=mc^2 a wrong simplification for negative mass!) repels itself, but is attracted to positive mass.Positive mass is repelled by negative mass, but since there is negative mass in all directions, this does not make galaxies fly apart. Farnes assures us that the calculations show it can explain the effect for which dark matter was invented and easily explains the accelerating expansion of the Universe.

Farnes does not claim the hypothesis is definitely true: he identifies experimental tests that can test his hypothesis and discriminate between it and other more established ideas.

 

[Chris W Kullander]

How would negative mass affect spacetime? Also what happens when it comes into contact with "positive" mass?
https://phys-org.cdn.ampproject.org...s/2018-12-universe-theory-percent-cosmos.html

 

[fresh_42]

Several threads about this subject have been merged.
Because of the highly speculative nature of this paper, I want to remind all participants to stay as close as possible within a debate about the publication.

 

[PeterDonis]

Sabine Hossenfelder has posted about this paper:

http://backreaction.blogspot.com/2018/12/no-negative-masses-have-not.html

 

[PeterDonis]

One thing I am having trouble understanding in the paper: The author appears to be arguing that negative mass can have the same effect as a negative cosmological constant. But I don't see how he is arriving at that result. A negative cosmological constant is a negative energy density, but a positive pressure of equal magnitude, so the "source" term $\rho + 3 p$ is positive. A negative mass, as he is modeling it, appears to be a negative energy density with zero pressure, which makes $\rho + 3 p$ negative. So a negative mass should have the opposite effect to a negative cosmological constant.

This apparent error seems to me to be related to what I find to be a glaring omission throughout the paper: the author only considers the first Friedmann equation and never considers the second (he writes the second down as equation 3 and then never mentions it again). But a proper understanding of the dynamics requires both equations.

 

[PeterDonis]

negative mass, which has positive energy according to Einstein's energy momentum equation

If you mean $E^2 - p^2 = m^2$, that equation can't tell you about the sign of the mass or the energy or the momentum, since all of them are squared.

The paper says that negative mass means negative energy density $\rho$ in the Friedmann equations, which are derived from the Einstein Field Equation. So according to the paper, negative mass has negative energy.

 

[Elroch]

Chris, based on the symmetry and the basic dynamics, the effect of the hypothetical negative mass on space-time has to be the opposite of that of positive mass. . For example, regions near a positive mass have to experience gravitational blue shift compared to regions near a negative mass.

The question is really what is the stress energy tensor for a negative mass density. I believe it has to be -1 times the stress-energy tensor for a positive mass with similar distribution to have the correct properties.

 

[PeterDonis]

what is the stress energy tensor for a negative mass density

Just knowing the energy density can't tell you the stress-energy tensor. You have to make an assumption about it. The assumption the paper appears to be making is that "negative mass" means negative energy density $\rho$, and zero pressure, similar to ordinary matter having positive energy density $\rho$ and zero pressure. Basically, this means that the negative mass the paper postulates, like ordinary matter, is cold.

 

[Elroch]

Yes, Peter. I had spotted my mistaken understanding about the sign of the energy when I couldn't edit it, and you are right that it is merely that the energy-momentum equation is consistent with either sign of energy rather than implying it is positive. A question is whether all positive energies breaks any conservation laws.
My previous post was saying exactly the same as yours: some assumption has to be made about the stress energy tensor. However, that assumption is strongly guided by the basic dynamics you want to produce by definition of what a negative mass is, and assumed symmetries probably provide the rest.

 

[mitosis]

Sabine Hossenfelder has posted about this paper:

http://backreaction.blogspot.com/2018/12/no-negative-masses-have-not.html

Great respect for Sabine Hossenfelder. I found her response to a poster named DreamChaser to be of particular interest, DreamChaser said that they liked the study because it was elegant in offering an explanation for both dark energy and dark matter (I will admit that I also found this appealing) and was therefore simpler solution. This was her reply:
Sabine Hossenfelder said...

DreamChaser,

It does not, please look at the paper. You need to introduce some weird stuff new stuff, then you need to introduce the creation tensor, then you need to assume you have no problem with vacuum stability, then you need to somehow assume that you get around the issue with the spin-2 field while still using GR, then you need to explain how come that a negative cosmological constant is actually in agreement with all the data, and even if you have done that you'd still have to bend over backward to demonstrate that the solution actually does fit the rotation curves which, frankly, I am rather skeptic about because I cannot see how you get the right scaling behavior (Tully-Fisher and all).

B.

 

[mitosis]

Here's Sean Carroll's take on the study: https://twitter.com/seanmcarroll/status/1071066682234261504

 

[Elroch]

Sean Carroll's main point is why I am uncomfortable with negative energy and why I assumed (inconsistently with Franes, it seems) that the energies are all positive and it is only the gravitational interaction that is flipped. I am yet to be completely convinced that this is impossible, but that is not really for this discussion.
[Doing a back of an envelope calculation, I find that the simple dynamics based on gravitational interactions as described in the paper conserve energy and momentum with either sign of energy associated with the rest masses. The weird feature is that the total energy of negative mass goes down as it goes faster, because the (m v²) / 2 term in a Newtonian approximation is negative). This is what makes it possible to conserve energy in the situation where a negative mass chases a positive mass. Another concern of mine is whether with such weird dynamics any system with both types of matter could be stable].

 

[yahastu]

Bandersnatch, yes I see what you mean about the positive/negative mass ratio...I had it wrong

I have been thinking about some of the interesting implications/predictions of this theory:

1) If this is dark matter, this explanation tells us why it is "dark" -- specifically, dark matter would be "dark" because these negative mass particles are mutually repulsive, they would never clump together into anything that would reflect light. Additionally, because most negative mass gets ejected outside of the galaxy into the halo, we wouldn't expect there to be much of it nearby to Earth.

2) I found it interesting in section 3.4 where it is pointed out that the runaway motion that is described for positive-negative mass pairs could explain cosmic rays. This theory should be capable of predicting the frequency or density of cosmic rays as a function of one's location in space, which could potentially be used as an additional testable prediction.

3) Because positive masses attract and clump together, I think this implies that any positive mass objects in free space would experience some amount of uniform pressure from all directions from negative mass particles that are colliding with it. It begs the question what these collisions would look like. Since the positive mass particles are clumped closely together, I would think that the EM force would hold them together strongly, and so negative mass particles wouldn't be able to break apart positive mass objects, since gravity is weaker than EM.

4) It would make intergalactic space travel significantly more difficult. In addition to the already known difficulties of needing to escape the planet's gravitational field, and then needing to escape the gravitational attraction of the galaxy you are trying to leave, you would also need to fight your way though the repulsive force of this very large negative mass halo. If you did succeed in crossing the tipping point in getting far away from your home galaxy, once you got outside of the negative mass halo it would then provide a propulsive effect...but entering a new galaxy would again be very difficult.

5) The author explains that this theory corresponds to an Anti-de Sitter space which undergoes a cycle of expansion and contraction. It seems not hard to visualize why this would be so...if we assume all positive mass fits within some finite extent, and negative mass is always created within this radius driving expansion, the negative mass would eventually be ejected outside of the positive mass universe and form a large scale halo of negative mass surrounding the entire universe in the same way that it is proposed to do for galaxies. Eventually, the ratio of negative mass "inside" the universe vs "outside" would change, until it reaches a tipping point where there is so much negative mass outside the universe that it begins to compress and slow down the inflation, and then crunches everything back down.

6) If this theory is correct I think it also tells us what we could expect to happen if someone tried to fly "to the edge of the universe"...basically, it would become increasingly difficult to travel beyond the edge of positive mass, because you'd be fighting against the repulsion of the negative mass halo, so you'd basically just fly until the repulsive mass effect canceled out whatever you were using for thrust.

 

[PeterDonis]

A question is whether all positive energies breaks any conservation laws.

The only conservation law in GR is that the covariant divergence of the stress-energy tensor is zero at every event. That means that locally, stress-energy can't be created or destroyed. Our best current model of the universe, with only positive energies, satisfies this property.

The main open question in this regard is what came before the Big Bang--or before inflation if you think inflation is the best answer to what came before the Big Bang. But the issues with the various proposed solutions have nothing to do with violating conservation laws: none of them do.

 

[Elroch]

RIght: I meant whether energy and momentum are conserved at low energies in local Lorentz frames according to the defining interactions, which would seem desirable. As far as I can see this can be so with all positive energies, but that doesn't mean something else doesn't go wrong.

 

[PeterDonis]

If this is dark matter, this explanation tells us why it is "dark" -- specifically, dark matter would be "dark" because these negative mass particles are mutually repulsive

This has nothing to do with whether or not the negative mass particles can absorb or emit electromagnetic radiation. If they are postulated not to do so, that is an additional assumption that has to be added to the model.

The author explains that this theory corresponds to an Anti-de Sitter space

But that requires that negative mass acts like a negative cosmological constant. As I pointed out in post #12, I don't see how that can work.

If this theory is correct I think it also tells us what we could expect to happen if someone tried to fly "to the edge of the universe".

As far as i can tell, there is no edge of the universe in the proposed model in the paper; the universe is spatially closed without a boundary (spatially a hypersphere).

the edge of positive mass

The proposed model appears to be homogeneous on large distance scales, just as our current mainstream model of the universe is, so there is no "edge of positive mass" any more than there is an "edge of negative mass"; both are evenly distributed, on average, throughout the universe.

 

[PeterDonis]

I meant whether energy and momentum are conserved in local Lorentz frames according to the defining interactions

The paper does not appear to treat this question at all; it simply assumes a negative energy density for the "negative mass" without discussing at all the question of local Lorentz invariance or what fundamental interactions the negative mass particles do or do not participate in. In other words, his model is a large-scale phenomenological model, not a fundamental model of some new fundamental particle or interaction.

 

[PeterDonis]

Another concern of mine is whether with such weird dynamics any system with both types of matter could be stable

I think this is highly unlikely, for the reason to be given below.

I found it interesting in section 3.4 where it is pointed out that the runaway motion that is described for positive-negative mass pairs could explain cosmic rays.

Unfortunately, the presence of these "runaway motion" solutions predicts a lot more than that: as Hossenfelder points out in her article, it predicts that these pairs of positive-negative mass particles should be constantly being created everywhere and emitting huge amounts of energy all over the place. (An often used term for this is "the vacuum is unstable".) Obviously we do not observe this at all.

 

[Elroch]

Actually, I think that problem is due to having negative energy particles.

I am thinking of the first order approximation, like Newton's gravitation as an approximation to GR.. I believe this is useful for the dynamics of a galaxy consisting of components all moving at small fractions of the speed of light, and may be sufficient to describe the halo of negative matter around a positive matter galaxy,

 

[yahastu]

Sean Carroll's main point is why I am uncomfortable with negative energy and why I assumed (inconsistently with Franes, it seems) that the energies are all positive and it is only the gravitational interaction that is flipped. I am yet to be completely convinced that this is impossible, but that is not really for this discussion.

Sean argues that it fails Occam's razor because he says "dark matter and dark energy are a simpler theory that explain the same effect"...but is that really so?

Dark matter is not something that was predicted in advance by any theory. Rather, it was observed that galaxy rotation curves do not agree with the predictions of GR. Rather than rejecting or revising GR due to its inability to match observations, it was argued that the theory is infallible, and therefore there must be some type of "dark matter" that exists and can explain the observed rotation curves without modifying GR...but there is still no consensus on what this dark matter would actually consist of, or how exactly it would be distributed to account for the observed failure of GR to properly predict galaxy rotation curves. Farnes points this out in section 3 when he says, "This provides a resolution to the cuspy-halo problem (eg, de Blok 2010) and to my knowledge makes negative masses the only dark matter candidate that can provide a non-contrived solution."

Farnes negative mass proposal is a specific example of a theory to resolve the galaxy rotation problem, and it happens to do so by defining a type of "dark matter" distributed almost exactly as predicted...so if it is is correct, it would be a concrete proposal for "dark matter" that can finally replace the vague and undefined concept of "dark matter" which is currently lacking a concrete theory.

 

[yahastu]

Unfortunately, the presence of these "runaway motion" solutions predicts a lot more than that: as Hossenfelder points out in her article, it predicts that these pairs of positive-negative mass particles should be constantly being created everywhere and emitting huge amounts of energy all over the place. (An often used term for this is "the vacuum is unstable".) Obviously we do not observe this at all.

Farnes only proposed a creation term of negative mass particles, not positive ones...so I don't think you'd have positive-negative mass pairs spontaneously appearing in the vacuum. I think that they would only come about when a negative mass particle happens to encounter a free positive mass particle of equal mass which happens to be found in isolation from any other positive masses...a pretty rare scenario.

In any case, when a positive mass encounters a negative mass, it wouldn't lead to the creation of new energy...rather it would just lead to their existing mass energy being converted into pure energy after they accelerated to the speed of light and turned into photons. Conservation of energy would be preserved due to this effect.

It does however seem to raise the question of how these negative masses, which are constantly being created, have energy, and persist indefinitely do not violate conservation of energy

 

[Elroch]

Regarding vacuum instability with negative energy particles, to preserve conservation laws you would appear to need to spontaneously generate four species: positive energy particles, their antiparticles, negative energy particles and their antiparticles, to satisfy both energy conservation and other conservation laws. Admittedly photons would do for two of these, and negative energy particles could also feasibly be their own antiparticles.
It seems plausible to someone with my limited knowledge that this would be an unlikely interaction (is this notion even meaningful given the Lorentz invariance of the vacuum?). If it was unlikely but not impossible, it might be a handy way of generating the negative mass, along with some photons. (Is this how cosmic rays got mentioned?)

 

[yahastu]

This has nothing to do with whether or not the negative mass particles can absorb or emit electromagnetic radiation. If they are postulated not to do so, that is an additional assumption that has to be added to the model.

If negative masses exist and are mutually repulsive than this would make them behave like an extremely thin gas, and would fail to absorb or emit EM radiation for the same reason that we can see through air...they are spatially separated so much that they simply don't create any significant interaction. Their density would be less than any conventional gas because the mutual repulsion is driven by gravity rather than collisions.

But that requires that negative mass acts like a negative cosmological constant. As I pointed out in post #12, I don't see how that can work.

If negative masses exist in free space, and are created so as to maintain equal pressure, then their effect would be to push all positive masses apart from each other. This would cause all positive masses to expand through spacetime (rather than causing spacetime itself to expand), but I think perhaps the point is that when you take into account the creation term, we no longer need to invoke the concept of spacetime itself expanding...