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Old August 26th 18, 03:19 AM posted to sci.astro.research
Phillip Helbig (undress to reply)[_2_]
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Posts: 241
Default Missing matter found in the cosmic web

In article , Bruce Scott
writes:

On 2018-07-12, Steve Willner wrote:

Dark energy, the remaining 68%, is something different still. There
is little evidence for what it is, but all the evidence I'm aware of
is consistent with its being a cosmological constant. I personally
have no problem with that. The cosmological constant has to have
_some_ value, and there's no reason that value must be zero.


Nice summary in general, just want to comment on this... you can take
the position it should be zero unless you have a reason for it.


Actually, it is usually the reverse in science. If nature has a degree
of freedom, she uses it. If it is NOT observed, then there is a reason
for it---a conservation law, for example. In particle physics, "if it
can happen, it will" is the standard approach. If something doesn't
happen, then it has to have a reason. The burden of proof is then on
the person claiming that it is zero if this is just a theoretical, as
opposed to observational, claim.

Fitting
the data is well enough (but I've seen that go wrong many times in
plasma physics where the underlying asumption of the thing and its cause
both being totally wrong and the community taking 20 years to wake up
to it).


Not really an issue here, as we are talking 1920s cosmology. If
anything, the surprise is that despite the high quality and quantity of
current data, all of it can be fit with ideas which have been around for
decades.

I take the position that zero is a reasonable a priori assumption,


No; see above.

but that if it has a value there should be a reason for it (ie, why is
it not very large).


Large compared to what? Some people complain it is way to small,
compared to the expectation from quantum field theory. Others are
surprised that the energy density in the cosmological constant is
comparable to that in ordinary matter (with, for some, the additional
puzzle that this is not always the case, but is now).

It may be like the photon mass, so small as not to
rock the boat with a theory in which it is zero and which is successful
for anything else which is known (at least below whatever it is... 5 MeV
or so for the nonlinearity in the electron scattering cross section).


I'm sure that upper limits on the photon mass are much smaller than 5
MeV.

Do we have solid evidence that it is _different from zero_


Yes. This is essentially what the Nobel Prize in physics for 2011 was
awarded for.

and if so
what does the curvature of the universe have to be?


The curvature of the universe depends on the sum of the cosmological
constant and the density parameter. Observations indicate that the
universe is close to being flat and perfect flatness is not yet ruled
out.

I guess if we say
68 percent of the curvature is due to the quoted dark energy fitting
then this should be something. I think if we know enough it may be a
property of space-time rather than a species of field/particle... but
I guess this is the same thing as "cosmological constant".


Right. This is essentially the question whether the cosmological
constant is "geometric" and belongs on the left side of the Einstein
equation, or is a source term with a certain equation of state and
belongs on the right side. This goes back to a discussion between
Einstein and Schr=F6dinger:

E. Schr=F6dinger, _Physikalische Zeitschrift_, 19, 20, 1918.
A. Einstein, _Physikalische Zeitschrift_, 19, 165, 1918.

(does this follow from universe accelaration as per the supernovae
observations from 20 years ago? but that's negative curvature isn't it)


Yes, the supernova observations are an important reason for believing in
a positive cosmological constant. But even without them, the data point
to it. Not negative curvature, though; both matter and a (positive)
cosmological constant make the curvature more positive. You might be
thinking of negative pressure. Contrary to what one might think,
positive pressure acts like normal matter: causes deceleration.
Negative pressure thus causes acceleration. Since matter thins out as
the universe expands and the cosmological constant doesn't (which is why
it is called the cosmological CONSTANT), early on matter dominates, then
with time (already in our past) the cosmological constant dominates.
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