
Dark Matter from Scalar Field Fluctuations - bookofjoe
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.123.061302
======
saagarjha
PDF on arXiv:
[https://arxiv.org/pdf/1905.01214.pdf](https://arxiv.org/pdf/1905.01214.pdf)

~~~
raattgift
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](https://arxiv.org/abs/1905.01214)

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tsieling
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?

~~~
fpoling
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.

~~~
networkimprov
> 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.

~~~
fpoling
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...

------
ta1234567890
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.

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

~~~
ta1234567890
Interesting, what's the definition of "physics model"?

Also, can anything in written form _not_ be called playing with words?

~~~
krapp
> 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.

[0][https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics](https://en.wikipedia.org/wiki/Modified_Newtonian_dynamics)

[1][https://xkcd.com/224/](https://xkcd.com/224/)

~~~
ta1234567890
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.

~~~
tsimionescu
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.

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

[https://theconversation.com/keplers-forgotten-ideas-about-
sy...](https://theconversation.com/keplers-forgotten-ideas-about-symmetry-
help-explain-spiral-galaxies-without-the-need-for-dark-matter-new-
research-121017)

------
mytailorisrich
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"...

------
ISL
ArXiv link:
[https://arxiv.org/abs/1905.01214](https://arxiv.org/abs/1905.01214)

------
iamgopal
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 ?

------
ISL
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.

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

~~~
networkimprov
Not sure why this was downvoted...

[https://en.wikipedia.org/wiki/Eternal_inflation](https://en.wikipedia.org/wiki/Eternal_inflation)

