Hacker News new | past | comments | ask | show | jobs | submit login
Dark Matter from Scalar Field Fluctuations (aps.org)
46 points by bookofjoe 70 days ago | hide | past | web | favorite | 48 comments

The paper proposes that the entirety of dark matter is a massive free (as in non-interacting and non-self-interacting) field that appeared at the end of inflation, but before big bang nucleosynthesis ("BBN"). In order to make this work with observations of the cosmic microwave background, the field has to have much lower mass earlier during inflation than at inflation's end. The mass-gaining mechanism is not elucidated.

There is only one known scalar field -- the Higgs -- and it is far from a free field.

Any non-gravitational interaction between this proposed field and the rest of the universe (including self-interactions) after the end of inflation is incompatible with this proposal. (One contrast this with other proposals such as WIMPs, which are not free fields, and typically also not scalars) and axions (which are scalars, but are not free); most of these are explicitly non-free because they also try to solve some problems in the Standard Model of Particle Physics via new interactions).

The phase between the end of inflation and before some of the dense matter (mostly the quark-gluon plasma, and ancestors thereof) condensed into nucleons and nuclei is far from the much hotter and much much denser phase in which quantum uncertainties and classical curvature do not obviously play well with one another. Barring very early black holes from direct collapse (perhaps seeded by overdensities in this massive proposed field), touched on in this paper only very briefly as "enhanced structure formation", the post-inflation/pre-BBN universe this paper describes is free from strong curvature requiring any sort of quantum gravity: one is safely in the land of quantum field theory on (classical) curved spacetime, and in practically any small spacetime region after the decay into the proposed massive free scalar, one can even ignore the (global) curvature.

The paper proposes constraints on the mass of the field and on its ancestor field(s) but otherwise does not contemplate the much earlier universe. The central feature of the paper is that it commits totally to minimal-coupling (i.e., the late time field is truly free, other than gravitation). The field contents generate and respond to curvature, and that's it.

The paper also features what appears to be one of the most confusing initial two sentences that science writers have latched onto in many many papers...

Finally, I think it is better to link the abs page than the PDF directly. Speaking only for myself, the abs page has puts significant and useful metadata front-and-centre and other metadata at the same reliable one-click distance from the PDF itself. Not all PDFs make it easy to go to the abs page in a click, and fewer still suggest that the PDF itself may have been superseded by a subsequent revision (or by publication somewhere in final form).

For this paper, the abs link is https://arxiv.org/abs/1905.01214

Can anyone explain the definition of big bang that they're using here? My understanding has always been that inflation happened a very short time after the big bang, arguably as part of the process itself, but pre-big bang is hard to conceive (like going north of north). Is it meant as an antecedent or precondition of the big bang, or as something outside the big bang itself?

The idea that it is meaningless to speak of "before the Big Bang" is not a universal, mathematically-proved truth, but a contingent result of particular theories of the Big Bang that include it being a singularity at a particular time 0.000000..., at which point there is no meaningful "before" or any spatial directions either. Just as physicists discuss how the singularities in black holes may be removed via a theory of quantum gravity, further theories about the Big Bang may or may not remove the singularity and may or may not result in a meaningful "before" to speak about.

In fact there are a number of such theories, it's just that AFAIK none of them have predicted something better than the standard model well enough for the singularity model to be definitively replaced, though I think if you polled physicists and asked them about whether they think there was "really" a singularity there or if it's just an artifact of our current poor theories, the vast bulk would say it's the latter. But in the absence of quantum gravity, General Relativity is the best established tool we have for investigating this sort of thing, and that tool says it was a singularity. We have lots of other tools that say other things, but none of them are established like GR is.

> 0.000000..

This is neither here nor there, but that number would be 0.

It's possible that 'time' has no minimum, but as far as I am aware the solution can be continuously extended to include 0. If not then you might as well say that the universe has existed for all time, the clocks were just running a bit slow at the start.

That is a number with significant digits. 0.00000... emphasizes the fact that it's not just close to zero, but exactly zero. 0 in a physics context can be read as "0 to one significant digit", which can be non-zero. For a real example, see discussions about the total cosmic curvature, which is 0 to some finite number of significant digits, but that doesn't let us quite be sure it's totally zero.

Thank you this is helpful.

Penrose’s very counterintuitive “Conformal Cyclic Cosmology” is such a theory that predicted “Hawking Rings”, which have been observed.

> Can anyone explain the definition of big bang that they're using here?

They're using "Big Bang" to mean the hot, dense, rapidly expanding state that is the earliest state of the universe for which we have good evidence. In models with inflation, this state occurs at the end of inflation, when "reheating" transfers all the energy stored in the inflaton field to the Standard Model fields (quarks, leptons, and radiation).

This is actually the standard definition of "Big Bang" used in cosmology, but unfortunately pop science books and articles still use "Big Bang" to mean "the initial singularity". We don't even know if there was an initial singularity (in eternal inflation models, for example, there isn't one).

From the equations and models of modern physics there is no time. The universe is just a curved 4D surface with one of the dimensions having different properties than the three others. This surface has at least one strange point with singularity where meaningful physical quantities go to infinities. Intuitively this seems wrong, so the suspicion is that those points are just artifacts of our models.

As for why we perceive the time with notions of before and after, nobody has an clue. There are philosophical speculations, but nothing that can tested experimentally.

> nobody has a clue

The "arrow of time" is commonly explained as an effect of thermodynamics, i.e. increasing entropy.

I always felt this to be a deeply unsatisfying explanation that implied "nobody has a clue" (a hot, expanding ball of quarks is a "highly ordered state", orly?) but I'm not a physicist.

Arrow of time explained via thermodynamics is a circular explanation. Thermodynamics follows from equation of motions or field equations. In those the time is different from space in that from conditions across 3 space coordinates at particular moment in time one can deduce conditions across the whole time (except black hole and other singularities). But from boundary conditions across two space coordinates and across the whole time one cannot fill the conditions along remaining space coordinate. But those equations just reflects experimental observations. So arrow of time exists because we observe arrow of time...

> From the equations and models of modern physics there is no time. The universe is just a curved 4D surface with one of the dimensions having different properties than the three others.

That dimension is called "time" and it does show up in the equations and models of modern physics.

> why we perceive the time with notions of before and after

Perhaps, it is because of

> having different properties than the three others

As I understand it the current model of the beginning of the universe starts with inflation and "the big bang" is the period that immediately follows. The two are distinct because the inflationary period is described as happening at a rate faster than the speed of light and the big bang is expansion constrained by this limit. The transition happened in a fraction of a second and very little is known about the structure of the universe during the inflationary period.

> The two are distinct because the inflationary period is described as happening at a rate faster than the speed of light and the big bang is expansion constrained by this limit.

This is not correct. "Expanding faster than the speed of light" isn't really a good description of the expansion of the universe at any phase, but if you're going to use it, it can happen during all phases--in fact it's happening now relative to us for parts of the universe beyond the Hubble horizon.

The key difference in the inflation period was that all of the energy was in a single field, the inflaton field, whose properties caused exponential expansion of the universe with a very short time constant, so the universe "inflated" by a huge factor in a very small interval of time. At the end of inflation, all that energy got transferred to the Standard Model fields (quarks, leptons, and radiation), which don't have that property (although now the expansion is dominated by dark energy, which does have the "exponential expansion" property but with a much, much longer time constant so the expansion only accelerates very slowly).

See this article https://www.sciencealert.com/new-study-brings-receipts-to-de... which is an excellent overview of Tenkanen's paper the subject of this thread. It also puts the competing ideas about cosmic inflation into perspective.

> but pre-big bang is hard to conceive It's like trying to imagine what is it like to be before you were alive. You simply can't. Its not really a valid question. There was no time before big bang. So we as humans cannot have a frame of reference to even begin thinking what was before.

That is not to say there wasn't anything (say we as 3D being are unable to experience/imagine 4D).

Another way of putting it is that every person alive today was dead for 13 billion years before being conceived and born, and will (relatively) soon be dead for however long the universe exists.

can you prove that?

(sorry, not talking about religion or life-after-death, just about how you go about proving something that can only happen outside your experience, and you have to rely on secondary evidence to deduce it)

> Can anyone explain

I'll try. The tl;dr is at the end before my own footnote. The author certainly doesn't make it easy, even for people familiar with the standard model of cosmology.

The very first sentence of the paper [ https://arxiv.org/abs/1905.01214 ] reads, "Dark matter (DM) may have its origin in a pre-big-bang epoch. It may have been produced, for example, by decays or annihilations of particles during the Big Bang, i.e. by the so-called ’freeze-in’ [1–3] mechanism, or by e.g. the misalignment mechanism which generated a non-zero DM abundance during cosmic inflation (see e.g. Ref.[4])." which while not strictly speaking self-contradictory is certainly far from clear.

The term "Big Bang" only appears in that first sentence, and the abstract.

If one does a case-independent search for "bang" in the paper's reference [1], there are no matches at all.

There is hope, however.

Reference [2] of the paper carefully uses only "big bang nucleosynthesis" and its abbreviation BBN.

BBN occurs after the universe has cooled via expansion ("adiabatic cooling") so that some of the hot, dense matter filling the universe earlier than BBN can "freeze" into atomic nuclei. That probably happened in steps: first quarks and gluons (and perhaps other particles feeling nuclear forces) could freeze into individual protons and neutrons, then those could join into atomic nuclei. It was still too hot for electrons to bind for long with these nuclei, so they were completely ionized.

Reference [3] uses "big bang nucleosynthesis" and "BBN" too, but also introduces "hot big bang cosmology". With respect to that new term, it defers to a further reference, which describes the typical picture of an arrangement of matter fields that undergo a phase transition wherein the result is BBN preceded by a plausible technical description of pre-BBN matter.

Reference [4] discusses a particularly speculative particle, the axion, and its role in the lead-up to BBN, compared with "the usual hot big bang"; it also leaves many of the details of "hot big bang cosmology" to other papers (e.g. at footnote 45).

I think it is fair to say that the widely circulated paraphrasings of the paper's first sentence are at best begging the question of whether the big bang is that of the standard model, or one of the variations or extensions in the first four of the paper's references. I also think it is fair to say that the author should have anticipated these paraphrasings, and that most readers would have even more trouble distinguishing exactly what is meant by "pre-big-bang" than working physical cosmologists.

For "professionals", the sentences immediately following equation (1) explain the picture: a field with very little mass gains mass during cosmic inflation, with the result that after inflation stops the field contents have the characteristics of a form of cold dark matter that interacts only gravitationally (it is a "free field", which is more amenable to modelling than an "interacting field" or a "self-interacting field" or a field that is both[a]). The paper considers constraints imposed by other observations, how generic a solution remains after considering those constraints, and that the entire idea would be obliterated by evidence favouring any sort of non-gravitational dark matter interaction (including non-gravitational interactions between DM and itself, or different types of DM).

Given this, one would tend to read "pre-big-bang" as used by the author as a region between the end of inflation and the beginning of big-bang nucleosynthesis. The epoch wherein one runs into conflicts between General Relativity and Quantum Field Theory is well before the end of the inflationary epoch, so one should feel free to completely ignore any sort of explanation which invokes things like the beginning of time, or even the differences in the nature of time in these two sufficiently-fundamental-for-these-purposes theories.

- --

[a] Strictly speaking the field is "minimally coupled to gravity"; it is non-interacting in the sense that there is no associated (non-gravitational) force-carrier, whereas interacting fields generally involve things like gauge bosons. Here because the end of inflation is so far from the part of the early universe that's hot and dense enough that quantum uncertainties and classical curvature cause problems, we can safely use textbook quantum field theory on curved spacetime -- the new physics is in the "decay" from a very light field to a massive field through the inflationary period, as well as the presence of a free field at all (no known fields are "free"). The mass-gaining mechanism is not described, but in the paragraph after the one containing eqn (21), the author claims that a wide range of possible mechanisms is allowed without conflict with other observations, and without conflicting with the central claim that dark matter experiences no non-gravitational interactions (including no self-interactions) after inflation.

Here's an alternative theory about dark matter. Warning: this is not a proven physics model.

Everything in the universe can be considered to be either light/electricity or magnetic (basically an electromagnetic wave). One cannot exist without the other.

Using the above as a base, you can think of vacuum as "noise", basically equal ("minimal") amounts of oscillating magnetic waves that connect and stretch across the whole universe, just like the background radiation. And you could also think of it as a sort of "medium" through which information travels.

Then you can think of objects/particles as our perception of "standing waves". So under this model, the planets can be described as clumps of light/electricity, sorounded by a "magnetic vacuum".

The interesting thing is you can then consider dark matter to be everything magnetic that is not light. However, it all in the end depends on the observer (us), because any light also has a magnetic component and viceversa, so "dark" just means "not in the human-visible light spectrum". But in reality not a single bit of space is "dark", that's just our (very limited) perspective/perception.

So if you ask me what dark matter is, I would say it's just the same stuff as everything else, it's already all around us, but we just can't see it with our human eyes.

It sounds like you read the word "dark matter" and didn't do any research beyond that. It's not just that we can't see dark matter, it's that it interacts very weakly with the electromagnetic field. It doesn't even react with itself very much. At the risk of suggesting other leaps based on the name, a better layman's term would be "ghost matter".

No, please refrain from calling this a 'physics' model. It's just playing with words.

Interesting, what's the definition of "physics model"?

Also, can anything in written form not be called playing with words?

> Interesting, what's the definition of "physics model"?

A model which is based upon physics, which itself is (according to Wikipedia,) "the natural science that studies matter, its motion and behavior through space and time," and which, being a natural science, ascribes attributes and behavior to the material world and its processes through experimentation and mathematical inference.

>Also, can anything in written form not be called playing with words?

What "playing with words" means in the context of your former comment is that what you presented was a fantasy which ignored any of the observed and known principles (read: actual definitions) of the terms being used, and the science which led to them.

Alternative theories for dark matter are all well and good (MOND[0] is popular) but your alternative only makes sense if one neither knows, nor cares, about actual physics. We already know that planets are not clumps of electricity and light surrounded by a magnetic vacuum which is the medium in which information travels. That's not physics, it's word-salad, it doesn't even make sense.

That said, I still upvoted your comment because ridiculous as it is, fringe theories for dark matter aren't uncommon and they can and should serve as a basis for discussion, not just be quashed. It's understandable that people are uncomfortable or unsatisfied by dark matter and dark energy - particularly since the "dark" in those terms refers to the nature of the phenomena being unknown, and they seem counter-intuitive and humans (and perhaps CS/engineering types in particular) want the universe to not just be intuitive, but elegant and simple.

Unfortunately, the more we study it, the less sense it makes. To reference XKCD[1], the universe isn't built in Lisp, it's hacked together in Perl.

But that doesn't mean we should just throw out what we know and start again on first principles until we have a model that makes sense to us first, and describes reality second.



That was a very fair and insightful comment. I really appreciate you taking the time to respond intelligently and respectfully.

Now, regarding physical models, you are assuming there is a universal truth or reality that is the same for everyone. And that is just a belief. I don't think you can force anyone to believe something. You are also free to believe in whatever you want to believe.

>Now, regarding physical models, you are assuming there is a universal truth or reality that is the same for everyone. And that is just a belief.

There appear to be universal constants which can be experimentally verified and whose attributes and behaviors remain consistent. In other words, the universe appears to have properties which can be known. There is no evidence that the fundamental nature or properties of reality change based on the beliefs of the individual observer. Therefore, it is reasonable to assume that universal physical reality does exist, and that data gathered through repeated observation and measurement more accurately describes that reality than speculation without such evidence.

>I don't think you can force anyone to believe something.

Wasn't trying to. Just pointing out that the universe isn't arbitrary, and not all conjectures are equally valid. You can believe the moon is made of green cheese if you like, just don't expect to be taken seriously, because there is a vast amount of evidence that it isn't.

The hypothesis that there is no universal truth is self-defeating. It isn't necessarily wrong, but any deduction you would make from it meaningless, and if you truly choose to believe you should fall into solipsism.

You yourself must believe in some universal truths, otherwise you would not attempt to communicate with other beings.

So, in order to have any kind of discussion, you must start from a point where you believe that at least a large part of everyday experience (including other beings, their minds, physical objects, their interactions, our observation thereof and many others) exist in a meaningful sense, outside your own cognition.

Now, any extrapolation from these base assumptions, is what we should think of as physics. For example, if I assume my eyes exist and my perception of the world is meaningful, then I must also conclude that the moon I see through a telescope exists to the same extent, and it's motion as I observe it exists, and I can search for explanations of that motion etc.

If I were to not assume that my eyes perceive something which truly exists, I would have no reason to stand in front of a computer screen, hitting keys on my keyboard and watching the letters appear on the screen - it's possible in a very absolutist way, but it's simply not a productive way of looking at the world.

you say:

> regarding physical models, you are assuming there is a universal truth or reality that is the same for everyone

which is basically an axiom one must adopt to engage in science. E.g., here's the formulation found in the Wikipedia article "Philosophy of Science"[1]:

> that there is an objective reality shared by all rational observers

So if one wishes to engage in an inquiry that doesn't hold this axiom, it's of course a perfectly fine thing to do, but it's best for all concerned not to call it science or scientific. Many issues and lines of inquiry that are important to people aren't amenable to scientific inquiry, but it does nothing but harm to dissimulate.

[1] https://en.wikipedia.org/wiki/Philosophy_of_science#Naturali...

Physics already acknowledges that there is no universal truth. Maybe not with those words.

All physical models depend on a "frame of reference". The most common one being the inertial frame of reference.

They do that because it all depends on how and from where you look at something, which is just another way of saying that the models depend on an observer.

Alternatively you could interpret that as there actually being some sort of universal truth, but then the experience of it is different for every single observer.

And then again, you are the one picking an interpretation or belief over another.

The terminology as understood in physics is not the same terminology that you're using.

The term "frame of reference" refers to how you have to set up the equations and how you define values, for example physical coordinates of a location. It doesn't change the actual predictions of the evolution of a physical system (and if it did, that's generally a good sign that the model is missing something!).

For example, you can model gravity in Newtonian mechanics on the surface of Earth as a force that pulls all objects down at a constant 9.8m/s^2 acceleration. Or you can step off the planet, and model the entire Earth as a closed gravitational system, and measure the gravitational attraction between the mass of the Earth and the things on or near its surface using F=G * m_e * m / r_e^2.

When you get to relativity, you discover that the problem is that spacetime is defined in such a way that there is no well-defined global "ruler" or "clock" that applies independently to all observers, so that 1 meter or 1 second for me on Earth is not the same 1 meter or 1 second on a spaceship travelling at 0.2c. What is independent to all observers is the speed of light in a vacuum, and from these two facts, you can in fact derive how to map the definitions of how your rulers and clocks would be distorted if you moved to a different reference frame.

The universal truth, in that case, is described by relativity. Different reference frames don't imply that the laws of physics differ between frames - in fact it's a fundamental axiom of special relativity that the laws of physics and speed of light are invariant (remain the same in all reference frames[0].)


> Now, regarding physical models, you are assuming there is a universal truth or reality that is the same for everyone. And that is just a belief.

Can we prove that the laws of physics are the same for everyone? No. That's an assumption. It seems to be a reasonable one, but it's an assumption.

But we got to our current understanding of physical models from other ideas. [Edit: That is, other ideas of what the models should be.] How did we get here? By people believing other things, and them finding out experimentally that things didn't work that way, even for the people who believed that they did.

Force is a strong word. But if your model can be verified by independent experiments and make useful predictions, it is more likely that it will be considered a valid model by other people.

Freeman Dyson's essay "Why is Maxwells Theory so hard to understand" is worth checking out. Words are usually not the best medium to talk about abstract stuff.

There is a difference between “playing with words” and “just playing with words”. If something is said to be “just”/only “playing with words”, this suggests that there is “no meat to it”, that it doesn’t actually describe something coherent and actionable.

Things can, of course, include both real and actionable content, and also wordplay.

A model has to be precise enough that you can actually draw conclusions from it. Precise: As in, precise enough to be formulated in mathematics. Conslusions: Actual, externally-visible conclusions, not just "conclusions" about how the terms you yourself have defined relate.

True, which is why we use maths.

“Kepler’s forgotten ideas about symmetry help explain spiral galaxies without the need for dark matter – new research”


To me dark matter (and, worse, dark energy) is a hint that our current models miss something crucial that we haven't figured out yet.

Essentially we're saying "our model is fine as long as 80% of the mass is made up of some unknown matter"...

I'm sufficiently good at understanding everyday engineering including related mathematics. I have no clue about this things apart from concepts that's read here and there. In half a century most of people with advanced knowledge will not be here, how do we are planning to pass on the understanding ? I think Khan academy for advance physics needed, or that would be too primitive to go deep enough ?

I'm not certain what the author intends by stating that inflation is pre-Big-Bang. It is my impression that there is no bigger bang than superluminal inflation.

Cosmic inflation is the inflation of space-time. The Big Bang refers to the transition period generally at and slightly after the universe's mass/energy interactions went from occurring in a singularity to occurring in a significant volume of space-time.

edit: in other words - superluminal inflation is big, but that's not a bang

I think the author should have written "pre-big-bang-nucleosynthesis" ("pre-BBN"), since the paper is about the appearance of a particular type of zero-spin particle at the end of inflation but before BBN, and forming the whole of the non-neutrino dark matter sector.

Some more details at my two comments above:



See the eternal "inflaton" field... or is that what you meant by "superluminal"?

Not sure why this was downvoted...


Guidelines | FAQ | Support | API | Security | Lists | Bookmarklet | Legal | Apply to YC | Contact