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"LCDM Paradigm Is Consistent With All Observations"? - Not So!



 
 
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  #21  
Old August 23rd 14, 08:15 AM posted to sci.astro.research
Phillip Helbig---undress to reply
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Posts: 629
Default "LCDM Paradigm Is Consistent With All Observations"? - Not So!

In article , Nicolaas Vroom
writes:

The fact that 85% of all the matter in universe is Darm Matter
(non-baryonic) is that a precition of the LCDM theory ?


No. LCDM is not a theory in the classical sense. Dark matter is
something which is observed (or, rather, not observed, but the existence
of which is inferred from observations). LCDM is a theory which is
based on the empirical conclusion that dark matter exists.

I would assume that the LCDM predicts that spiral galaxies have
a large halo with Dark Matter. The specific mathematical equations
that describe this halo i.e. the Hernquist profile or the NFW profile
are they also predicted by LCDM ?


The NFW profile grew out of simulations within the context of LCDM, so I
would say yes. LCDM, again, though, is not a theory like Newtonian
mechanics; rather, there is much give and take between theory and
observation.

A slightly different issue is: if the coincidence argument is still
valid? See:
http://en.wikipedia.org/wiki/Anthrop...c_coincidences

The reason why I ask this question is, because the book "Galactic Dynamics"
1994 edition in paragraph about "The cosmological constant" page 637,
contains the following text:
"Thus if Omega0=0.2 we have lambda = 2.5 * 10^-35 h^2
It appears that a model Universe with this value of lambda is consistent
with all available observations (Peebles 1984) as well with inflation.
All of the mass can be in baryons and there is no need for any exotic
particles to comprise most of the mass of the Universe.


True with the very inexact observations in 1984; no longer true today.

However, these
models are subject to the same "coincidence" objection that was made
to models with Omega0 1 in $10.3.6 etc"


There are people who think that this is a problem and people who think
that it is not. The first issue is whether or not there is a
coincidence at all. There is no consensus on this. Even if it is a
coincidence, does it tell us something useful? Something deep? What
about the coincidence of the apparent size of the Sun and Moon (which,
because the Moon is receding from the Earth, only holds around our
time)? Does it tell us something useful? Something deep?
  #22  
Old September 5th 14, 03:44 PM posted to sci.astro.research
Steve Willner
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Posts: 1,172
Default "LCDM Paradigm Is Consistent With All Observations"? - Not So!

In article ,
Phillip Helbig---undress to reply writes:
LCDM is not a theory in the classical sense.


That took me aback. I think what you mean is that the exact
constituents of the Universe aren't predictions of any theory.
That's correct: the various densities and a few other things are free
parameters. If you put specific values for these parameters into GR
and known baryon physics, you can predict lots of observables.

Many people use "LCDM theory" to refer to GR+baryon physics+a
specific set of free parameters. That last doesn't mean specific
values, only the list of parameters. The era of "precision
cosmology" means that each of the parameters is now measured with
multiple observations.

Dark matter is
something which is observed (or, rather, not observed, but the existence
of which is inferred from observations).


In particular, a non-zero density for non-baryonic dark matter is
inconsistent with observations. One set of such observations is the
CMB fluctuations. Another is the cluster velocity dispersions
combined with the nucleosynthesis upper limit on baryon density.

--
Help keep our newsgroup healthy; please don't feed the trolls.
Steve Willner Phone 617-495-7123
Cambridge, MA 02138 USA
  #23  
Old September 7th 14, 12:11 PM posted to sci.astro.research
Phillip Helbig---undress to reply
external usenet poster
 
Posts: 629
Default "LCDM Paradigm Is Consistent With All Observations"? - Not So!

In article , Steve Willner
writes:

In article ,
Phillip Helbig---undress to reply writes:
LCDM is not a theory in the classical sense.


That took me aback. I think what you mean is that the exact
constituents of the Universe aren't predictions of any theory.


Right.

That's correct: the various densities and a few other things are free
parameters. If you put specific values for these parameters into GR
and known baryon physics, you can predict lots of observables.

Many people use "LCDM theory" to refer to GR+baryon physics+a
specific set of free parameters. That last doesn't mean specific
values, only the list of parameters. The era of "precision
cosmology" means that each of the parameters is now measured with
multiple observations.


Right.

Dark matter is
something which is observed (or, rather, not observed, but the existence
of which is inferred from observations).


In particular, a non-zero density for non-baryonic dark matter is
inconsistent with observations. One set of such observations is the
CMB fluctuations. Another is the cluster velocity dispersions
combined with the nucleosynthesis upper limit on baryon density.


Right.

To be more precise, LCDM is not a theory derived from first principles
like, say, Newtonian physics or GR. Rather, it is strongly motivated by
what is observed, in the same way that particle physics is. That
doesn't mean that it is not a theory at all; it is, and can make further
predictions which don't depend on observational input and some of these
have been confirmed.

In particular, LCDM does not "predict non-baryonic matter" because the
idea of non-baryonic matter is an observational input.
 




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