Register to reply

Basic problems

by wolram
Tags: basic
Share this thread:
SpaceTiger
#19
Mar27-06, 12:06 PM
Emeritus
Sci Advisor
PF Gold
SpaceTiger's Avatar
P: 2,977
Quote Quote by Garth
The Pioneer Anomaly is certainly not built on questionable methods. Turyshev et al. The Study of the Pioneer Anomaly:New Data and Objectives for New Investigation
I beg to differ. "Measuring" something from an instrument not designed to make that measurement is always questionable...and I think EL's papers covered the theoretical concerns pretty well.


and whether the reasoning behind the Axis of Evil is questionable or not is a matter of current debate. Land and Magueijo The axis of evil
Did you even read the WMAP paper or the argument I had with you in the associated thread?


1. The model continues to depend wholly on two pieces of undiscovered physics, namely dark energy and cold dark matter.
Agreed, but it's not a problem with matching the data, it's a problem of not having enough of it.


2. The implied dark energy density is so small that it is unstable to quantum correction and its size is fine-tuned to the almost impossible level of one part in ~ 10102.

3. It is difficult to explain the coincidence between the dark energy, dark matter and baryon densities at the present day.
I referred to these, the "fine-tuning" and "cosmic coincidence" problems already.

The rest of your references refer to the "cuspy cores" problem, "small-scale structure" problem, and something that we understand too poorly to really be called a problem. The first two are genuine concerns with [itex]\Lambda CDM[/itex] and have been discussed at length in other threads.

If the first isn't just an issue of numerical resolution (which is looking increasingly likely), then it's an issue of our lack of knowledge about the dark matter, which falls back into what I was saying about needing more measurements of dark matter properties. The second is, again, a possible issue with the simulations and may have nothing to do with fundamental theory. The third is another example of how we're data-starved. We don't have nearly enough high-z data to understand how the feedback process works.

They are fair concerns, however. I wouldn't yet call them "weak points" with the model because it's still not clear to me that they're not problems with the simulations, but it's still worth keeping in mind.

As for MOND, it comes as absolutely no surprise to me that a theory designed to fit rotation curves can do so with fewer parameters than the standard model. It also came as no surprise to me that relativistic MOND was completely inconsistent with the CMB.
Garth
#20
Mar27-06, 12:13 PM
Sci Advisor
PF Gold
Garth's Avatar
P: 3,273
Quote Quote by SpaceTiger
Quote Quote by Garth
and whether the reasoning behind the Axis of Evil is questionable or not is a matter of current debate. Land and Magueijo The axis of evil
Did you even read the WMAP paper or the argument I had with you in the associated thread?
Yes - that was the current debate! And I am not the only one who is willing to go with increasing the chance of a false negative. Not everybody agrees with you ST - that is what makes it interesting.
As for MOND, it comes as absolutely no surprise to me that a theory designed to fit rotation curves can do so with fewer parameters than the standard model. It also came as no surprise to me that relativistic MOND was completely inconsistent with the CMB.
Agreed, but it will be interesting to see how Bekenstein responds.

Garth
wolram
#21
Mar27-06, 12:28 PM
PF Gold
wolram's Avatar
P: 3,682
By Garth,
BTW wolram, marcus was complementing you.

Marcus, sorry if my post seemed gruff, it was not meant to be
SpaceTiger
#22
Mar27-06, 12:35 PM
Emeritus
Sci Advisor
PF Gold
SpaceTiger's Avatar
P: 2,977
Quote Quote by Garth
Yes - that was the current debate! And I am not the only one who is willing to go with increasing the chance of a false negative. Not everybody agrees with you ST - that is what makes it interesting.


What you said:

and whether the reasoning behind the Axis of Evil is questionable or not is a matter of current debate.
You said there was no debate. I'm saying there most certainly is!
Garth
#23
Mar27-06, 12:57 PM
Sci Advisor
PF Gold
Garth's Avatar
P: 3,273
Quote Quote by SpaceTiger

What you said:
Quote Quote by Garth
and whether the reasoning behind the Axis of Evil is questionable or not is a matter of current debate.
You said there was no debate. I'm saying there most certainly is!
ST I think you are reading me wrong, on this we most certainly do agree!
Perhaps if I had emphasised?:
whether the reasoning behind the Axis of Evil is questionable or not is a matter of current debate
BTW in today's Times newspaper Hidden CJD is new threat to thousands Having eaten British beef during the 1980's I live in dread of suddenly finding my mind going......

The curse of the fear of the false positive - ("Our meat is unsafe" - when it was safe) - you are left with the consequences of the false negative. ("Our meat is safe - when it was not)

Garth
SpaceTiger
#24
Mar27-06, 01:07 PM
Emeritus
Sci Advisor
PF Gold
SpaceTiger's Avatar
P: 2,977
Quote Quote by Garth
Perhaps if I had emphasised?:
Oy. I need to renew my eyeglass prescription.
wolram
#25
Mar28-06, 11:57 AM
PF Gold
wolram's Avatar
P: 3,682
By Marcus.


So, for instance, he is critical of his fellow cosmologists, at least of the run-of-the-mill university cosmologist, because they often simply ASSUME that k = 0 exactly. That is, they favor the EXACTLY FLAT case so much that they oftentimes just take it for granted, according to Ellis.

Thanks for your help so far Marcus. At the moment i can not see an alternative to making some assumptions, if cosmologists have no data
to work with, what else can they do.
I would like a laymans review of the search for matter in QG theories, from
what i have read string theories are all most out of the picture for now, i
may have the wrong take on that, is there anything new ?

Thankyou, Garth, Space Tiger, Can you shed any light on the (particle)
supersymetry, higs, side of cosmology?
SpaceTiger
#26
Mar28-06, 12:15 PM
Emeritus
Sci Advisor
PF Gold
SpaceTiger's Avatar
P: 2,977
Quote Quote by marcus
Determining the sign of the curvature k, showing whether the universe has closed spatial sections and also whether it is possible for it to recollapse in the future or not. Analyses of the observations should always attempt to determine this sign, and not assume that k = 0 (as is often done).
Just to clarify this point a bit, the assumption of a flat universe (in CMB fits) is made to simplify the analysis. The data are fit with Markov Chain Monte Carlo simulations and the more free parameters there are, the longer it takes to cover the space. You'll see that in the third release, the WMAP team fixes the curvature to flatness for many of their simulations for exactly this reason. They do, however, run a few separate fits that allow the curvature to vary and this is the section you've been referring to in other posts. In other analyses, you'll see the assumption made simply because the flatness has already been measured (in the CMB) to more precision than they could reach.
SpaceTiger
#27
Mar28-06, 12:22 PM
Emeritus
Sci Advisor
PF Gold
SpaceTiger's Avatar
P: 2,977
Quote Quote by wolram
Thankyou, Garth, Space Tiger, Can you shed any light on the (particle)
supersymetry, higs, side of cosmology?
The LHC (Large Hadron Collider) is expected to find the Higgs. It also has some chance of detecting a signature (energy deficit) indicative of the dark matter particle. According to Michael Peskin, this won't be enough, and we'll need the ILC (International Linear Collider) to measure the dark matter particle properties. These are all maybes and will depend on which (if any) of the theories of supersymmetry is correct.

However, I don't think a detection or non-detection of the Higgs boson will have much of an impact on cosmology. Inflation is more of a phenomonological model at this point and doesn't rely on the existence of a particular particle species.
Garth
#28
Mar28-06, 12:28 PM
Sci Advisor
PF Gold
Garth's Avatar
P: 3,273
(Edit crossed with ST's post)
Big Bang studies has been a forced marriage between two incompatible partners: GR and QM, nevertheless the marriage has been, or has the potential of being, very fruitful.

On the one hand GR cosmology has required Inflation, DM and DE, on the other fundamental particle physics has required higher and higher energy accelerators to test the Standard Theory, and you cannot get any higher energy than the BB itself!

The Standard Theory has thrown up countless hypothetical particles that need experimental verification, if they cannot be found in a large accelerator, such as the LHC being built at present, then an alternative is to try to find them in the BB.

Cosmological constraints have filtered out possible candidates, although there are many still to go.

QM requires the Higgs Boson to impart inertial mass to particles and GR requires the energy of its scalar field, or of another hypothetical inflaton particle, to impart a massive exponential expansion in the first 10-35sec or so of the universe's history. It was predicted to be detectable in present particle accelerators but so far without success, perhaps the LHC will deliver.

DM requires a particle with all the right properties to explain the large scale features of the universe including the rotation rates of spiral galaxies. One likely candidate at the moment is the LSP or lightest supersymmetric particle.

We will only know what we are talking about when these particles have actually been discovered, their properties measured and been found concordant with the cosmological constraints.

Until then GR and the Standard Model must remain, well, provisional.

Garth
wolram
#29
Mar28-06, 12:29 PM
PF Gold
wolram's Avatar
P: 3,682
ST, this may be a stupid question, but what evidence do we have that the
CMB is only observed as it is from our view, could observation from another galaxy give different results ?
Garth
#30
Mar28-06, 12:38 PM
Sci Advisor
PF Gold
Garth's Avatar
P: 3,273
Quote Quote by wolram
ST, this may be a stupid question, but what evidence do we have that the
CMB is only observed as it is from our view, could observation from another galaxy give different results ?
ST will certainly answer this, but if I may butt in, the answer to your question depends on what the CMB actually is. It is most certainly (IMHO) the radation emitted by the surface of last scattering (SLS) when the universe emerging from the Big Bang cooled enough for atomic hydrogen to form and the universe become transparent.

As such it would look more or less the same from any galaxy at this present epoch. Its temperature will depend on the epoch of observation.

However the largest anisotropy of the CMB, 100X larger than the rest, is the dipole caused by our peculiar motion relative to that SLS. As each galaxy has its own peculiar motion, and planets their own trajectories within those galaxies it will be this dipole that will differ from planet to planet, star to star and galaxy to galaxy.

Garth
SpaceTiger
#31
Mar28-06, 12:39 PM
Emeritus
Sci Advisor
PF Gold
SpaceTiger's Avatar
P: 2,977
Quote Quote by wolram
ST, this may be a stupid question, but what evidence do we have that the
CMB is only observed as it is from our view, could observation from another galaxy give different results ?
If inflation is correct, then the CMB should look different from other locations, but have the same statistical properties. Sometimes, we can remotely infer the properties of the CMB by looking at gas that is coupled to it. Here's one such example:

In situ measure of the cosmic microwave background temperature at a distance of 7.5 kiloparsecs

It's not a very powerful technique, however, and can only give us very crude measurements of the temperature. Measuring the anisotropies remotely is probably a long way off, if possible at all.
wolram
#32
Mar29-06, 07:01 AM
PF Gold
wolram's Avatar
P: 3,682
A question i have with held for lack of research, but, how can it be shown that the CMB is a relic of the BB, every one seems to assume it is, but i guess, there is not a unique signature for this radiation.
Garth
#33
Mar29-06, 10:51 AM
Sci Advisor
PF Gold
Garth's Avatar
P: 3,273
There have been sugestions from the Steady State 'school' in the 1970s that the CMB might be either the sum total of background galaxies - Olber's paradox red-shifted into the micro-wave region.

Or, in Fred Hoyle's mass field theory fundamental particle masses decrease as you out in space until you reach the membrane where m = 0, which we interpret as the BB. The CMB was then claimed to be the smoothed out radiation from galaxies beyond that membrane, (presumeably with negaitve masses.)

These suggestions were long shots, which did not explain the CMB isotropy and would not have survived the discovery of the anisotropies at the 10-5 level.

Garth
wolram
#34
Mar29-06, 01:02 PM
PF Gold
wolram's Avatar
P: 3,682
Thankyou , Garth, i admit i am learning a lot about the fundamental reasons
for the SM, but i am cursed with the ability to see other views, i think it is
like a retail outlook, one sees an opening and tries to fill it, but this is wrong
for cosmology, one should not try to fill in the gaps untill the market ressearch
has been done.


Register to reply

Related Discussions
Basic Circuit Problems - KVL and KCL Engineering, Comp Sci, & Technology Homework 6
2 more basic problems Introductory Physics Homework 22
Basic Limit problems Calculus & Beyond Homework 18
Basic Basic problems Introductory Physics Homework 1
Basic Newtonian Problems #2 Classical Physics 1