[Marxism] New theory of gravity dispenses with "dark matter"

Jeff meisner at xs4all.nl
Mon Dec 26 17:18:22 MST 2016


At 15:26 08-11-16 -0500, Fred Murphy via Marxism wrote:
>
>http://phys.org/news/2016-11-theory-gravity-dark.html

As promised, I wanted to call attention to what will surely be a revolution in
physics regarding gravity, if it turns out to be correct. Louis occasionally 
posts Physics articles he finds in the popular press which vary greatly in 
quality and accuracy, but the ones proposing a whole new theory usually are 
on the fringe (but nevertheless taken seriously by physicists until they can 
be disproven) and probably aren't appropriate for the popular press inasmuch 
as they are long-shots. Any new theory of gravity, 100 years after the 
publication of Einstein's very successful General Theory of Relativity, 
would appear to fall in that category. However indeed the theory of Erik 
Verlinde (University of Amsterdam) is being taken very seriously and (as 
indicated in the article posted by Fred Murphy) has recently provided an 
alternate explanation of observed gravitational effects which have otherwise 
been ascribed to "dark matter."

Verlinde's theory explains gravity (an effect normally associated with large 
distances and large masses) as an "emergent property" arising from quantum 
mechanics (normally associated with very small distances and masses) and 
string theory (a rather esoteric but powerful theory underlying quantum 
mechanics itself, controversial among the physics community). That puts the 
details of his work beyond most physicists, and anything I am about to 
describe is therefore superficial as it is all way beyond me. Although this 
explanation of gravity has been worked on for at least 5 years, it has now 
received great recognition after Verlinde showed in a recent paper that the 
theory might be able to resolve the troubling issue of "dark matter," a 
hypothetical form of as-of-yet undetected matter responsible for the 
unusually large gravitational fields indicated by the observed rotation 
rates of galaxies. His claim is that the observations can be explained using 
his theory without requiring a "free parameter," that is an arbitrary 
number, unlike the dark matter explanation which requires one free parameter 
(the ratio of dark matter to ordinary, or "baryonic" matter). On the face of 
it that would mean his theory would be favored on the basis of Occam's Razor 
(accepting the simplest explanation consistent with observation and known 
facts). Also, very recently Dutch astronomers completed an analysis based on 
observations of gravitational lensing around some 33000 galaxies which is 
consistent with the new theory (but which doesn't rule out the dark matter 
explanation).

I should add that Verlinde's theory does not contradict Einstein's 
explanation of gravity as curvature of space-time. However it supplies a 
specific quantum mechanical explanation for that curvature due to the 
presence of mass, and (as I understand it!) the theory goes so far as to 
explain the very existence of space-time as an emergent result of mass 
itself using quantum mechanics.

When the above article was posted I began looking for material on Verlinde's 
theory, and although it had been somewhat covered in the wider press, I was 
disappointed to find that almost all of it was in Dutch. An obvious 
interpretation might be that this was only getting hyped up in the 
Netherlands as a matter of Dutch pride (nationalism!), distorting the view 
from where I stand. However I don't think that's the case (and anyway the 
more important indicator would be in reference to the relatively few 
physicists around the world with the required specialization, and their 
publications). I can now point to some non-technical material in English.

Here are a press release  and popular article regarding the recent 
observational support for the theory of emergent gravity:

http://www.astronomie.nl/#!/actueel/nieuws/_detail/gli/verlindes-new-theory-first-tested-weak-gravitation/

http://earthsky.org/space/1st-test-eric-verlinde-gravity-theory-gravitational-lens

And another non-technical overview of the issues (parts of which I'd 
question, especially calling gravity "an illusion"):
  http://www.forbes.com/sites/briankoberlein/2016/11/08/solution-to-dark-matter-proposes-that-gravity-is-an-illusion

And here is a youtube video of Verlinde speaking in English (if you can 
ignore the promotional crap from the publisher of the video) for a very 
general audience:
  https://www.youtube.com/watch?v=hByJBdQXjXU

Verlinde's recent paper describing his theory's explanation for what 
otherwise is attributed to dark matter can be downloaded from the following 
link. However it is extremely dense mathematical physics which very few 
could follow. But he wrote a conclusion section which he must have 
intentionally written to be very accessible to the rest of us (containing no 
equations!), and which I have pasted below the link.

- Jeff



https://arxiv.org/pdf/1611.02269.pdf


8 Discussion and Outlook

8.1 Particle dark matter versus emergent gravity

The observational evidence for the presence of dark matter appears to be overwhelming.
The first known indications came from the observed velocity profiles in (clusters of)
galaxies. Other strong evidence comes from strong and weak gravitational lensing
data, which show signs of what appears to be additional clumpy matter in clusters and
around (groups of) galaxies. Dark matter also plays a crucial role in the explanation
of the spectrum of fluctuation in the cosmic microwave background and the theory of
structure formation.

Since up to now there appeared to be no evidence that general relativity or New-
tonian gravity could be wrong at the scales in question, the most generally accepted
point of view is that these observations indicate that our universe contains an enor-
mous amount of a yet unknown form of dark matter particle. However, the discrepancy
between the observed gravitational force and the one caused by the visible baryonic
matter is so enormous that it is hard to claim that these observations provide evidence
for the validity of general relativity or Newtonian gravity in these situations. Purely
based on the observations it is more appropriate to say that these familiar gravitational
theories can only be saved by assuming the presence of dark matter. Therefore, with-
out further knowledge, the evidence in favour of dark matter is just as much evidence
for the possible breakdown of the currently known laws of gravity.

The real reason why most physicists believe in the existence of particle dark matter
is not the observations, but because there was no theoretical evidence nor a conceptual
argument for the breakdown of these laws at the scales where the new phenomena
are being observed. It has been the aim of this paper to provide a theoretical and
conceptual basis for the claim that this situation changes when one regards gravity as
an emergent phenomena. We have shown that the emergent laws of gravity, when one
takes into account the volume law contribution to the entropy, start to deviate from
the familiar gravitational laws precisely in those situations where the observations tell
us they do. We have only made use of the natural constants of nature, and provided
reasonably straightforward arguments and calculations to derive the scales and the
behavior of the observed phenomena. Especially the natural appearance of the accel-
eration scale a0 should in our view be seen as a particularly convincing aspect of our
approach.

In our view this undercuts the common assumption that the laws of gravity should
stay as they are, and hence it removes the rationale of the dark matter hypothesis. Once
there is a conceptual reason for a new phase of the gravitational force, which is governed
by different laws, and this is combined with a confirmation of its quantitative behavior,
the weight of the evidence tips in the other direction. Admittedly, the observed scaling
relations have played a role in developing the theoretical description, and motivated our
hypothesis that the entropy of de Sitter space is distributed over de bulk of spacetime.

But the theoretical arguments that support this hypothesis together with the successful
derivation of the observed scaling relations are in our view sufficient proof of hypothesis.
Our main conclusion therefore is:

     The observed phenomena that are currently attributed 
     to dark matter are the conesquence of the emergent 
     nature of gravity and are caused by an elastic 
     response due to the volume law contribution to 
     the entanglement entropy in our universe.

In order to explain the observed phenomena we did not postulate the existence of
a dark matter particle, nor did we modify the gravitational laws in an ad hoc way.
Instead we have to tried to understand their origin and their mutual relation by taking
seriously the theoretical indications coming from string theory and black hole physics
that spacetime and gravity are emergent. We believe this approach and the results we
obtained tell us that the phenomena associated with dark matter are an unavoidable
and logical consequence of the emergent nature of space time itself. The net effect
should be that in our conventional framework one has to add a dark component to
the stress energy tensor, which behaves very much like the cold dark matter needed
to explain structure formation, but which in its true origin is an intrinsic property
of spacetime rather than being caused by some unknown particle. Indeed, we have
argued that the observed dark matter phenomena are a remnant, a memory effect, of
the emergence of spacetime together with the ordinary matter in it.

In particular, we have made clear why the apparent dark matter behaves exactly in
the right way to explain the phenomenological success of modified Newtonian dynamics,
as well as its failures, without the introduction of any freely adjustable parameters. We
have found that in many, but not all, aspects the apparent dark matter behaves similar
to as one would expect from particle dark matter. In particular, the excess gravity and
the gravitational potential wells that play a role in these scenarios also appear in our
description.

Perhaps superficially our approach is similar in spirit to some earlier works [62, 63,
64, 65, 66] on the relationship between dark matter and the thermodynamics of space-
time. But the details of our derivations and especially the conceptual argumentation
differs significantly from these papers. Our theoretical framework incorporates and
has been motivated by the recent developments on emergent gravity from quantum
information, and is in our view a logical extension of this promising research direction.

8.2 Emergent gravity and apparent dark matter in cosmological scenarios

In this paper we have focussed on the explanation of the observed gravitational phe-
nomena attributed to dark matter. By this we mean the excess in the gravitational
force or the missing mass that is observed in spiral or elliptical galaxies and in galaxy
clusters. Of course, dark matter plays a central role in many other aspects of the cur-
rent cosmological paradigm, in particular in structure formation and the explanation
of the acoustic peaks in the cosmic microwave background. In none of these scenarios
is it required that dark matter is a particle: all that is needed is that its cosmologi-
cal evolution and dynamics is consistent with a pressureless fluid. In our description
we eventually end up with an estimate of the apparent dark matter density that in
many respects behaves as required for structure formation and perhaps even for the
explanation of the CMB spectrum. Namely, effectively the apparent dark matter that
comes out of our emergent gravity description also leads to a gravitational potential
that attracts the baryonic matter as cold dark matter would do.

However, the arguments and calculations that we presented in this paper are not yet
sufficient to answer the questions regarding the cosmological evolution of our equations.
In particular, we made use of the value of the present-day Hubble parameter H0 in
our equations, which immediately raises the question whether one should use another
value for the Hubble parameter at other cosmological times. In our calculations the
parameter H0 was assumed to be constant, since we made the approximation that our
universe is entirely dominated by dark energy and that ordinary matter only leads to
a small perturbation. This suggests that H0 or rather a0 should actually be defined
in terms of the dark energy density, or the value of the cosmological constant. This
would imply that a0 is indeed constant, even though it takes a slightly different value.

A related issue is that in our analysis we assumed that dark energy is the dominant
contribution to the energy density of our universe. According to our standard cosmo-
logical scenarios this is no longer true in the early times of our universe, in particular
at the time of decoupling. This poses again the question whether a theory in which
(apparent) dark matter is explained via emergent gravity would be able to reproduce
the successful description of the CMB spectrum, the large scale structure and galaxy
formation. These questions need to be understood before we can make any claim that
our description of dark matter phenomena is as successful as the deltaCDM 
paradigm in describing the early universe and cosmology at large scales.

By changing the way we view gravity, namely as an emergent phenomenon in which
the Einstein equations need be derived from the thermodynamics of quantum entan-
glement, one also has to change the way we view the evolution of the universe. In
particular, one should be able to derive the cosmological evolution equations from
emergent gravity. For this one needs to first properly understand the role of quantum
entanglement and the evolution of the total entropy of our universe. So it is still an
open question if and how the standard cosmological picture is incorporated in a theory
of emergent gravity. How does one interpret the expansion of the universe from this
perspective? Or does inflation still play a role in an emergent cosmological scenario?
All these questions are beyond the scope of the present paper. So we will not make
an attempt to answer all or even a part of these questions. This also means that before
these questions are investigated it is too early to make a judgement on whether our
emergent gravity description of dark matter will also be able to replace the current
particle dark matter paradigm in early cosmological scenarios.

 





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