|Mar25-06, 01:33 PM||#1|
What are the (basic) problems with main stream theories of cosmology ?
i do not want crackpotery, just what the people who work in or around
the field think.
I am unqualified to comment and will not, i hope moderators will keep this thead to the point.
|Mar25-06, 03:07 PM||#2|
Ellis was asked to do this recently, by Jeremy Butterfield and John Earman, as a contribution to a book that Butterfield and Earman are editing.
Here is what he contributed
There are several ways to respond to your question Wolram, one way would be to try to think of reasons why the whole field of cosmology is no damn good and totally off track---and maybe fundamentally incapable of usefully adding to human knowledge. I would not respond that way myself.
Another way is more like Ellis which is a very constructive moderate criticism saying what have we possibly got wrong that we might fix, and what are some major things we arent certain about but could study and get more certain.
If you want, I will sift thru Ellis essay and quote a few things he says.
If that is to your purpose.
the whole essay is not critical of cosmology. It is focused on the philosophical questions, but whenever anyone does an analysis of the philosophical issues of a field it is necessarily going to involve identifying areas of shallowness and uncertainty, so if the writer does a thorough job it will contain criticism.
|Mar25-06, 05:01 PM||#3|
The mainstream cosmological model fits very well and is compelling to those who study it.
Astrophysics is the application of physics discovered 'down here' to the universe 'out there'. Cosmology is the extension of that discipline to the largest possible scales.
The major theory of local physics that applies on a cosmological scale is GR. It has been well tested in the solar system but there are a few questions about the cosmological model it produces: It has required the invocation of Inflation, DM and DE whereas none of these have been discovered or verified in a laboratory 'down here'. They all might be positively identified, and their properties found to be concordant, tomorrow, but it hasn't happened yet. Various Higgs Boson/Inflaton/DM particles are thrown up by theoretical particle physics, but yet again they are at the moment only hypothetical particles waiting laboratory confirmation.
The latest WMAP release has confirmed many of its predictions to high precision, yet it has left an open question about the 'Axis of Evil'. Finally in the solar system there is the question of the Pioneer anomaly, and also a few rare studies that claim some kind of orbital, or G, evolution on the time scale of Hubble time.
Just questions that might lead nowhere, or, just possibly, be the window into new physics.
|Mar25-06, 05:35 PM||#4|
Here is a sample of the kind of thing he says in that article, look on page 25:
4.2.3 Determining the RW parameters Given that a RW geometry is a good description of the observable universe on a large scale, the further issue is what are the best-fit parameters that characterize it, selecting the specific universe we observe from the family of all FL models (Sec.2.1). Important observational issues are:
• Determining the Hubble parameter H0, which sets the overall scale of the observed universe region.
• Determining the trio of the density parameter Omega_0, deceleration parameter q_0, and cosmological constant Lambda (or equivalently the density parameter Omega_Lambda), which are the major defining characteristics of a specific Friedman-Lemaitre model. The CBR data, supernova observations, deep number counts, source covariance functions, velocity measurements, and gravitational lensing observations can determine these quantities.
• 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).
• Various parameters are used to characterize the nature of dark matter (Sec.2.3.6) and dark energy (Sec.2.3.5). As their dynamics is unknown, these too have to be determined observationally.
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.
I have found this to be the case, but I expect the mentality of the ordinary professional cosmologist is probably going to change gradually, partly because of gentle criticism like this from Ellis and partly because of better data.
Ellis says that instead of taking the Omega exactly = 1 case for granted, that it is the cosmologist job to FIND OUT the sign of k, that is, whether Omega is less or equal or greater than 1.
A related question is whether the spatial slices come around and close on themselves---which would make the universe FINITE. Ellis, a mainstream cosmologist who is able to take a critical look at his own profession, says that it is a cosmologist job to measure and FIND OUT if the universe is finite (one way is by measuring curvature) and not just take for granted because you favor this or that.
The astronomy building is just a stone's throw from here. I used to spend time in their coffee room some. I will have to drop in on some grad students before long and see what they are saying. I will let you know when i do.
If you read Ellis you can find more criticisms of the state of the profession. they are mild and it is a lot of work to read his long essay. I will sift through some more if you want. or if this is not what you want let me know
|Mar25-06, 11:30 PM||#5|
Yet we have such a poor understanding of much of the theory involved. We seem to have a cosmological constant, but have no idea where it came from (the fine-tuning and coincidence problems). We have cold dark matter, but don't know what it is. We have inflation, but struggle to identify the cause.
How did the field get like this? Simple, we don't have enough measurements. We don't understand the dark energy because we've only measured its properties very crudely. Ditto with inflation. Our missions are getting cut like you wouldn't believe. We've entered the era of precision cosmology with a satellite that cost only 150 million dollars.
So, to answer your question, the problem with mainstream cosmology is that it's poorly constrained. [itex]\Lambda CDM[/itex], for all its success, is only a crude parameterization of the real universe and it will be forced to remain that way until we start getting more data to test our theories. Cranks look elsewhere, we don't need new theories, we need new data.
|Mar26-06, 03:22 AM||#6|
Wolram, I am trying to sympathize with the alternative views, but, it's difficult. 20 years ago, after a few beers, I would have challenged you to an ***-kicking contest in the parking lot! I was a rebel, back then. I wanted to 'reinvent' the world in my own image. Unfortunately, I did not die a good death. In fact, I did not die at all. My friends were shocked and dismayed by this unlikely turn of events.
|Mar26-06, 10:15 AM||#7|
[several different answers]
I now think that I did not understand the original question, and gave an inappropriate response. I will try again.
I now think W. is asking a very simple question. WHAT IS WRONG WITH the standard consensus "Lambda CDM" model?
Every human construct has its weak points---the little imperfections/incompletenesses where it is most easily challenged.
And we should all have these at our fingertips----anyone who knows a theory and works with it should be able to tell you its weaknesses or where it is least good. Even an attentive WATCHER should be aware of the achilles heels of the going theory.
I think Wolram is asking us to tell these. He is not asking us to DEFEND the consensus model. He is not asking us to render a balanced overall judgement and weigh successes against failures and say how-surprisingly-good-all-things-considered type of things.
Since maybe I didnt understand the question the first time, I will try again to answer.
Remember guys, we are NOT ATTACKING the standard LambdaCDM model, we are identifying its most vulnerable least satisfactory points.
When I say something, you do not immediately have to DEFEND the beloved model against me. I am not saying it is wrong I am just trying to point to a "least satisfactory" aspect.
what I hope is that if I can come up with one proposal for a "least satisfactory" part of the model then someone else can suggest some other----and in the end Wolram will have a list of weaknesses of Lambda CDM.
my candidate for the lamest thing about LCDM is the ROTATION CURVE FIT
the Bekenstein TeVeS Mond model fits the rotation curves much better than dark matter does.
I will get a reference link for that. We should probably give Wolram references for the achilles-heels that we offer him.
Please note that I am not ADVOCATING Bekenstein modified gravity. I just use it to illustrate that LCDM could do better in the rotation curves department
Oh, another lame thing about LCDM might be that nobody has a clue what DM is or where Lambda the "cosmological constant" comes from. And yet those are like 75% and 20% of the whole shebang. In other words it could be said there is a kind of preposterous look and feel to the conventional model. But since that is kind of vague criticism, I will make my nomination for lame thing be the comparatively poor fit of rotation curves.
Now I see that Garth mentioned some of these things already, and other stuff too like CMB "Axis". Why should something as grand as the Background line up with local things like galaxy and solar system? And he mentioned the Pioneer anomaly. Relevant response point for Garth :-) Maybe we should have a poll! what do you think is the weakest aspect of the consensus LCDM model?
|Mar26-06, 11:54 AM||#8|
by Marcus, Remember guys, we are NOT ATTACKING the standard LambdaCDM model, we are identifying its most vulnerable least satisfactory points.
When I say something, you do not immediately have to DEFEND the beloved model against me. I am not saying it is wrong I am just trying to point to a "least satisfactory" aspect.
This is what i mean Marcus, i would hate this thread to become a free for all
debate on why people think the SM is wrong, i only want to know what you
guys think are the (problem areas of the SM) with respect alternate theories
and side issues do not belong here.
|Mar26-06, 01:04 PM||#9|
There is of course the small point of the problem of developing a quantum gravity theory.
Apart from indicating that their integration will probably require both QM and GR to be revised in the future, the whole Big Bang would obviously have been a quantum gravity event, so this would seem to be quite a weak point!
|Mar26-06, 02:01 PM||#10|
|Mar26-06, 02:35 PM||#11|
I am a new member with no experience in physics. I do have a question.
before the big bang, the singularity was like a black hole, I think. If light cannot escape a black hole and nothing can exceed the speed of light, how could there be enough energy for the big bang? Can a black hole explode?
|Mar26-06, 04:46 PM||#12|
I see you are a retired welder, so you probably have mature patience and are used to the idea that scientific research rarely supplies certainty.
I think you ask very good questions. In certain sense, which I wont try to explain right now, it is thought that a black hole can indeed explode. In more than one way. Also in a sense the quantum picture of the pit of a black hole DOES resemble the picture at least one school of quantum cosmology has of the big bang. But those are big leaps to take without preparation and clarification.
Keep in mind that the only type of models that have a chance of working at the (bigbang or blackhole) singularities are the fuzzy wavy pictures---the quantum models. The clearcut classical ones break down---they simply do not compute at a certain point. But that is OK because various quantum approaches are being worked out and in at least one case may be testable against observations.
The fuzzy models remove the singularities and replace them with a "quantum regime" of very high (but not infinite) density pressure temperature. Your questions are pushing into the area of quantum cosmology.
One thing you could do is look at this scientific (not popular) article
The guy, Martin Bojowald, is a recognized expert in quantum cosmology of one particular school (loop quantum cosmology).
This is his most recent SHORT overview paper for interested non-specialists (physicists like him, but not in the same specialty) It is 16 pages with relatively few equations. He also has a encyclopedia-like Living Reviews article of 100-some pages and many equations that is even more recent (2006) and a chapter in recent book by World Scentific press. But that is all too hard to read. What I told you is the easy one, of his recent writings.
Another thing you could do is start a thread with your questions. You are always welcome to do this at PF, except when your questions are noticeably similar to what someone else has already asked when they started their thread. I would say yours are different enough but someone else may remember a thread we already have about this.
|Mar26-06, 07:05 PM||#13|
MOND habitats within the solar system
Jacob Bekenstein, Joao Magueijo
"MOdified Newtonian Dynamics (MOND) is an interesting alternative to dark matter in extragalactic systems. We here examine the possibility that mild or even strong MOND behavior may become evident well inside the solar system, in particular near saddle points of the total gravitational potential. Whereas in Newtonian theory tidal stresses are finite at saddle points, they are expected to diverge in MOND, and to remain distinctly large inside a sizeable oblate ellipsoid around the saddle point. We work out the MOND effects using the nonrelativistic limit of the TeVeS theory, both in the perturbative nearly Newtonian regime and in the deep MOND regime. While strong MOND behavior would be a spectacular "backyard'' vindication of the theory, pinpointing the MOND-bubbles in the setting of the realistic solar system may be difficult. Space missions, such as the LISA Pathfinder, equipped with sensitive accelerometers, may be able to explore the larger perturbative region."
This should have references to previous papers that show how Mond can fit galaxy rotation curves better.
Bekenstein was who discovered about Black Hole temperature and entropy and gets credit along with Stephen Hawking. The "BH-entropy formula" should probably be called the "Bekenstein-Hawking entropy formula". He is a famous reputable guy, but he is creative. So he has this TeVeS ("TEnsor VEctor Scalar") modification of gravity that fits rotation curves noticeably better.
In my mind the fact that another model can do better stamps a question mark on the idea of Dark Matter. It doesnt disprove it. DM is still sort of OK but it ruffles darmatter's complacency a little.
|Mar27-06, 04:22 AM||#14|
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
1. The model continues to depend wholly on two pieces of undiscovered physics, namely dark energy and cold dark matter.
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.
4. Any model with a positive [itex]\Lambda[/itex] also creates fundamental difficulties for superstring theories of quantum gravity.
A number of astrophysical observations have proved difficult to 'shoehorn' into the [itex]\Lambda CDM[/itex] model:-
1. The mass profiles of low surface brightness galaxies appear to be less sharply peaked than predicted by CDM models. (Moore et al. Cold collapse and the core catastrophe)
2. The large numbers of sub-haloes predicted in galaxy haloes may make spiral disks subject to tidal disruption on timescales of less than a Gigayear. (Moore et al. Dark Matter Substructure within Galactic Halos)
3. The observed galaxy luminosity function is much flatter than the mass distribution predicted by CDM; attempts to suppress star-formation by invoking significant feedback in low-mass haloes appear to create further problems at higher masses. (Benson et al. What Shapes the Luminosity Function of Galaxies?)
Just a few thoughts to chew over.
|Mar27-06, 08:47 AM||#15|
I find myself in general agreement.
(Having seen some, but not all, of the papers you cite. And numerous others supporting your main points.)
Cant vouch for your last 3 minor points of technical astrophysics, not having read the supporting papers you give.
this seems like a thorough carefully thought answer, Garth. I wonder if Wolram will be content with it
He has considerable ability to be dissatisfied with answers.
A bottomless pit of doubt, like any good critic.
|Mar27-06, 09:26 AM||#16|
|Mar27-06, 10:47 AM||#17|
I can and do sympathise with ST, the lack of data is annoying to say the
It seems to me,(a not very clever person) that a quantum theory of gravity that includes matter, is the first rung on the ladder.
Getting rid of or explaining DE,DM, is the second.
GPB, may have some good news, or bad for GR, i just hope the data it
gives is not inconclusive, but what comes after GPB?
Theories,that can be tested are all well and good, but if the means is not
there to test them they will have to stay on shelf gathering dust.
Forgive me if i am wrong, but trying to fit observations to theories can
only be secondary to meassurments.
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