Is the cosmological prinicple wrong? Is Big Bang wrong?

[Garth]

Maybe the 'Cold Spot' isn't quite so real after all?

The mystery of the WMAP cold spot (http://arxiv.org/abs/0712.1118" , my bold):

From that paper:

7. Conclusion
We have re-examined the properties of the Internal Linear combination WMAP CMB map and the co-added WCM map by an analysis of the properties of the signal in the vicinity of the CS. These two maps of the CMB signal display remarkably similar structures on equal latitude rings at |b| > 30◦. We have re-examined the properties of the CS at the galactic latitude b = −57◦ and longitude l = 209◦ and shown that it is associated with the cluster with length D ∼ 3hD(n)i. In addition to the CS, we have also found a few more zones of the CMB signal with almost the same morphology, at b = 57◦, b = −80◦, b = −30◦.
From an analysis of the ILC III map we have shown that the shape of the CS is formed primarily by the CMB signal localized in multipoles between 10 ≤ ℓ ≤ 20 (corresponding to angular scales about 5 − 10◦), in agreement with Cruz et al. (2005, 2007) results. Taking into account that the same modes lead to a modulation of the whole CMB sky, we subtracted these modes from the CMB signal. The demodulated CMB signal looks like a random one without significant over-clustering.
We have investigated the asymmetry of the variance for iso-latitude rings in respect to the Galactic plane. The South hemisphere has excess variance in comparison to the North hemisphere. This is why local defects and large clusters, including the CS and its associated cluster, are mainly concentrated in the Southern hemisphere.
Taking all these investigations together, we believe that the mystery of the WMAP CS directly reflects directly the peculiarities of the low-multipole tail of the CMB signal, rather than a single local (isolated) defect or a manifestation of a globally anisotropic model. This interpretation does not preclude the possibility of an exotic origin of the CS and related phenomena, but it does specify more precisely what properties such explanations must generate. A satisfactory model of the CS must explain the entire range of its behaviour rather than only one aspect.
Our final remark is related to the definition of significance of the CS detection by different methods, based on the assumption that Gaussian statistics apply to the observed CMB sky. Ever since Eriksen et al. (2004), showed that the distribution of the power of the CMB across the sky is very anisotropic at the scales about 10◦, it has been clear that Gaussian statistics are no longer a valid reference for determining the significance of this feature. Our approach to the large-scale angular modulation of the CMB is a possible alternative approach to this issue.

(emphasis mine)

Are they not saying that the Cold Spot (CS) does exist but as a modulation of the Gaussian distribution of power of the CMB? i.e. it is secondary artifact of the CMB imposed on it after it left the Last Scattering Surface?

There is also a void in radio sources in the same location, the two phenomena could well be linked via an inverse late integrated Sachs–Wolfe effect: WMAP's cold spot shows giant void in space.

Therefore, the WMAP cold spot remains a puzzle, no longer as a peculiarity of the very early universe but as an oddity of the time of structure formation.

The question is, " Is there an Age Problem in the Mainstream Model?, IOW was there enough time in the Mainstream $\Lambda$CDM model for such a large void to form?

Garth

 

[George Jones]

Are they not saying that the Cold Spot (CS) does exist but as a modulation of the Gaussian distribution of power of the CMB? i.e. it is secondary artifact of the CMB imposed on it after the Last scattering Surface?

There is also a void in radio sources in the same location, the two phenomena could well be linked via an inverse late integrated Sachs–Wolfe effect:

The penultimate paragraph of

http://arxiv.org/abs/0712.1118"

is

Taking all these investigations together, we believe that the mystery of the WMAP CS directly reflects directly the peculiarities of the low-multipole tail of the CMB signal, rather than a single local (isolated) defect or a manifestation of a globally anisotropic model. This interpretation does not preclude the possibility of an exotic origin of the CS and related phenomena, but it does specify more precisely what properties such explanations must generate. A satisfactory model of the CS must explain the entire range of its behaviour rather than only one aspect.

This seems to say that the an explanation that uses more than just a single void is required.

 

[Garth]

Well yes George, that is more or less what I was suggesting.

There are two distinct observations here, one is a non-Gaussian cold spot in the CMB and the other is the absence of radio signals in the same part of the sky.

There may be other examples elsewhere in the sky.

The CMB cold spot anomaly may be ameliorated by considerations of peculiarities in the low-multipole tail of the power spectrum, but even then the void exists in the foreground.

The question of whether there is a lack of time problem to form that void still remains.

Garth

 

[George Jones]

The CMB cold spot anomaly may be ameliorated by considerations of peculiarities in the low-multipole tail of the power spectrum, but even then the void exists in the foreground.

The question of whether there a lack of time problem to form that void still remains.

Not everyone thinks that the void exists; see post #33 by SpaceTiger.

I think that the data is at least somewhat suggestive.

Has the Sloan Digital Sky Survey (SDSS) looked at this part of the sky? If so, does this void show up in its data?

Cosmology is very exciting these days!

 

[Garth]

Not everyone thinks that the void exists; see post #33 by SpaceTiger.

I think that the data is at least somewhat suggestive.

Has the Sloan Digital Sky Survey (SDSS) looked at this part of the sky? If so, does this void show up in its data?

Cosmology is very exciting these days!

The fact that they detected a dip of 20-30% in the surface brightness and number counts of NVSS sources smoothed to a few degrees at the location of the WMAP cold spot suggests it is real.


AFAIK SDSS has not yet detected it.

Garth

 

[SpaceTiger]

Not everyone thinks that the void exists; see post #33 by SpaceTiger.

It's mainly the "giant" part that I'm skeptical of. There are many voids in the universe and a large paucity of radio sources would certainly suggest that one exists in that direction. The real question is whether or not it's as large as they say it is. They're trying to use it to explain the WCS, so they need it to be a certain size to produce that temperature decrement from the ISW effect. If, instead, most of the temperature decrement is from the low multipoles of the CMB spectrum, then the void wouldn't need to be as unreasonably large.

 

[Garth]

If, instead, most of the temperature decrement is from the low multipoles of the CMB spectrum, then the void wouldn't need to be as unreasonably large.

The Rudnick, Brown & Williams paper Extragalactic Radio Sources and the WMAP Cold Spot states that in the case of the CS being caused totally by a foreground void:

To create the magnitude and angular size of the WMAP cold spot requires a ~140 Mpc radius completely empty void at z<=1 along this line of sight. This is far outside the current expectations of the concordance cosmology, and adds to the anomalies seen in the CMB.

But as ST says the void could be smaller if the CS is caused partly by peculiarities in the low-multipole tail of the power spectrum, however there are other smaller voids of ~ 40Mpc or less that may still pose a problem; in an older, 2001, paper by Peebles, The Void Phenomenon he concludes:

The CDM model is maturing, most dramatically in its success in relating the power spectrum of the thermal background radiation temperature to observationally acceptable cosmological parameters (eg. Hu et al. 2000). This shows the CDM model likely is a good approximation to how structure started forming on the length scales probed
by the measurements.
The apparent inconsistency between the theory and observations of void is striking enough to be classified as a crisis for the CDM model. It may be resolved within the model, through a demonstration of an acceptable theory of galaxy formation. Or it may drive an adjustment of the model.

(emphasis mine)

Which of those last two options is it to be?

Garth

 

[Nereid]

Not really directly on the topic of voids, giant or not, a very recent ESA PR on INTEGRAL is interesting: http://www.esa.int/SPECIALS/Integral/SEM2K4KV3AF_0.html".

Note the rather neat consistency with the recent Auger project paper ...

 

[Garth]

Yes indeed Nereid, although there may well a connection between those papers and 'voids'.

If we see a cluster in one part of the sky it is not too unreasonable that there will be a void left somewhere else.

The correlation with the Auger Ultra-High Energy Cosmic Rays detections also suggests that the SMBHs at the centre of galaxies may be the UHECRs source.

Such structure in our neighbourhood can be explained, but what about structure at high z, is there a problem?

Garth

 

[SpaceTiger]

Such structure in our neighbourhood can be explained, but what about structure at high z, is there a problem?

Could you be more specific? Which structures are you looking to have explained?

 

[Garth]

Such structure in our neighbourhood can be explained, but what about structure at high z, is there a problem?

Could you be more specific? Which structures are you looking to have explained?

Hi ST! I would appreciate your expertise here.

Mine was an open question, something to look out for as a falsifiable test of the Mainstream model.

I did not have any particular structures/voids in mind apart from the ones already discussed in the thread, but interested in any large structures that might be at z ~> 1.

If you watch the The Millennium Simulation, for example, you notice that most of the action takes place after z = 1.

At low z the simulation produces a very realistic universe in the local 'neighbourhood', but does the large scale structure out at z=1 resemble the simulation at those earlier times?

Garth

 

[SpaceTiger]

At low z the simulation produces a very realistic universe in the local 'neighbourhood', but does the large scale structure out at z=1 resemble the simulation at those earlier times?

To my knowledge, there are no data sets that could really answer this question as yet. The trouble is that it's extremely difficult to conduct a wide-angle spectroscopic survey at high redshift because the tracer objects are either too dim (as with ordinary galaxies) or too sparsely distributed (as with quasars) to tell us much about large-scale structure.

At low redshift, my thesis work suggested that LCDM did very well at predicting the length and width distributions of filaments, though it's not yet clear how sensitive a test that is.

 

[Nereid]

COSMOS: 3D weak lensing and the growth of structure (http://xxx.lanl.gov/abs/astro-ph/0701480") is - AFAIK - about the best there is today, wrt your question Garth.

To ST's point, note that the authors are very cautious about the estimated LSS beyond z ~1.2 (and look at how many assumptions/inputs need to be further researched to push much deeper using this technique).

 

[SpaceTiger]

COSMOS: 3D weak lensing and the growth of structure (http://xxx.lanl.gov/abs/astro-ph/0701480") is - AFAIK - about the best there is today, wrt your question Garth.

Cosmic shear is certainly a valid constraint on the cosmological model, but they present small-angle two-point correlation functions (and very noisy ones at that)... it's not really possible to judge the three-dimensional distribution of structure (such as the relationship between voids, filaments, clusters, etc.) from this analysis. Studies of this sort have been done at z~1 before, such as with the DEEP2 redshift survey, but the fields are too small to really compare the higher-order correlations to simulations. The real advantage of the weak lensing surveys is that they don't have to worry about galaxy bias -- the signal comes directly from the overall matter distribution. Unfortunately, the lensing signal is inherently two-dimensional and is difficult to deproject into a three-dimensional map of structure.

 

[Loki Mythos]

Hello all. I am very new to this forum and have been reading this thread with interest. Some of it I understand well and other parts seem a bit over my head. So please forgive me for my ignorance in the subject and please don't brow beat me too hard. But I would truly like to have a few things explained to me about cosmology. Please if anyone can answer a few questions for me, It would help me greatly. Please try to answer in English, meaning layman's terms. Science babble and big words will not impress me as much as something I can easily understand and convey. I'll try to write this in a way that the reader can understand where I'm coming from. Here goes.

I have always had a problem with the big bang. First off, it just smacks of GOD. It sometimes seems to me that cosmology, in general, goes out of its way to prove that the universe was created and that their was a beginning to time. I have trouble with this because I can not conceive of a beginning or end of time, it seems to go against the logic that is somehow hardwired into my brain.(please, no if a tree fell arguments) The same with the size of the universe. I just can't see it as finite. A finite amount of matter outside which, nothing exists.

Chronos, states in an earlier post in another thread “We say the universe is expanding because, in this universe, objects move away from each other over time. The volume occupied by galaxies and such are otherwise meaningless. There is no space outside of space. Space can only be described in terms of the distance between physical objects.”

Yes, I understand that almost all of the galaxies that we can see are red shifted. But I haven't seen anyone argue that an infinite universal sea, sprinkled with matter is simply undulating, just like any large ocean. The idea of Conservation of energy(energy can not be created or destroyed), to me, argues for an infinite undulating universe and against the idea of a big bang.(if anyone knows of such an argument, please direct me to it) It also seems to me that we haven't really been observing for very long. I mean how long has it been since Edwin Hubble and William Huggins? A blink of an eye. Do red shifted galaxy's stay red shifted at the same frequency over time? How much will they change in a millennium? Just the fact that we can see a few blue shifted galaxies and are in fact on a collision course with one seems to indicate that we are not in a isotropic or finite expanding universe. Perhaps only the part that we can see, is expanding, at this time. If we were in an expanding part of the universe, would we be able to observe a distant contracting part moving away from us in an infinite and eternally undulating universe? In view of that statement, it seems to me that we are arrogant to assume that there is no matter beyond what we can see with our limited means of observation.

Anyway, thank you for taking the time to read.

 

[russ_watters]

I have always had a problem with the big bang. First off, it just smacks of GOD.

Such implications are not on the mind of scientists when they forumlate theories. It just isn't relevant.

It sometimes seems to me that cosmology, in general, goes out of its way to prove that the universe was created and that their was a beginning to time.

The history shows that the reality is quite the opposite. For most of the existence of actual science about the origin of the universe, it was believed to be static. The Big Bang is the inevitable result of the discovery that it isn't.

I have trouble with this because I can not conceive of a beginning or end of time, it seems to go against the logic that is somehow hardwired into my brain.(please, no if a tree fell arguments)

Our brains are not hardwired with knowledge about the universe, only with the ability to process information. So the proper way to go about learning about what's known in science things is by learning the evidence and what the evidence implies - not by just accepting your own preconceptions.

The same with the size of the universe. I just can't see it as finite. A finite amount of matter outside which, nothing exists.

That last sentence implies there is an "outside". There isn't.

Yes, I understand that almost all of the galaxies that we can see are red shifted. But I haven't seen anyone argue that an infinite universal sea, sprinkled with matter is simply undulating, just like any large ocean. The idea of Conservation of energy(energy can not be created or destroyed), to me, argues for an infinite undulating universe and against the idea of a big bang.(if anyone knows of such an argument, please direct me to it)

Sorry, that doesn't make any sense to me. It doesn't seem to describe what we actually see.

It also seems to me that we haven't really been observing for very long. I mean how long has it been since Edwin Hubble and William Huggins? A blink of an eye. Do red shifted galaxy's stay red shifted at the same frequency over time? How much will they change in a millennium?

It doesn't matter how long we've been observing, what matters is that we've made millions of observations. It wouldn't be possible for all those observed galaxies (and the cmb) to switch to blue shift after a while. You did just say "conservation of energy", right?

Just the fact that we can see a few blue shifted galaxies and are in fact on a collision course with one seems to indicate that we are not in a isotropic or finite expanding universe.

We've had this argument already. The fact that on the small scale, the universe isn't a silk sheet doesn't mean it can't be described as isotropic. The word is just a single-word description. It alone doesn't tell you what is really meant by its use. Ie, is the universe's "grainyness" 1 part in a thousand? A million? A billion?

Perhaps only the part that we can see, is expanding, at this time. If we were in an expanding part of the universe, would we be able to observe a distant contracting part moving away from us in an infinite and eternally undulating universe?

Red-shift data tells us that nowhere within our horizon is there a place where anyone would see a contracting universe.

In view of that statement, it seems to me that we are arrogant to assume that there is no matter beyond what we can see with our limited means of observation.

I don't know if "arrogant" is the right word, but it certainly would be foolish - which is why people don't make that assumption!

 

[Nereid]

Cosmic shear is certainly a valid constraint on the cosmological model, but they present small-angle two-point correlation functions (and very noisy ones at that)... it's not really possible to judge the three-dimensional distribution of structure (such as the relationship between voids, filaments, clusters, etc.) from this analysis.

The authors certainly seem to think estimates of 3D distribution are possible to make, using their technique and data, albeit with many, clearly stated, caveats.

One particularly interesting part of the paper (well, preprint) is where they talk about going from 3 redshift bins to 5 ... and why it didn't work.

Studies of this sort have been done at z~1 before, such as with the DEEP2 redshift survey, but the fields are too small to really compare the higher-order correlations to simulations.

My (admittedly very cursory) search didn't turn up any DEEP2-based papers on using cosmic shear - do you have any refs to hand? Not a biggie, just if you have them immediately to hand.

One potential promise of this approach, if it can be pushed into higher z with (considerably) greater S/N, is that at higher z small (angular) scale surveys probe greater chunks of (physical) space. The paper makes this point too (while also pointing to the formidable obstacles that need to be overcome).

The real advantage of the weak lensing surveys is that they don't have to worry about galaxy bias -- the signal comes directly from the overall matter distribution.

That alone makes it worth spending considerable resources on, IMHO!

Unfortunately, the lensing signal is inherently two-dimensional and is difficult to deproject into a three-dimensional map of structure.

Indeed.

Do you think the authors were unreasonably optimistic in reporting how well* they were able to do such de-projection (up to z ~ 1.2)?

*I think they are pretty frank about how cautious one must be in interpreting the outputs from their analyses ...

 

[SpaceTiger]

The authors certainly seem to think estimates of 3D distribution are possible to make, using their technique and data, albeit with many, clearly stated, caveats.

They infer the three-dimensional two-point correlation function, which is not quite the same as constructing the three-dimensional distribution of matter. Presumably, one could construct a very crude 3-dimensional map (with huge bins along the line of sight), but it wouldn't be of any use for, for example, identifying filaments. It's a 3D distribution in the same way that a three-color diagram is spectroscopy. :)

My (admittedly very cursory) search didn't turn up any DEEP2-based papers on using cosmic shear - do you have any refs to hand?

DEEP2 wasn't a lensing survey, but a redshift survey done in a very small field (in contrast to SDSS and 2dF, which covered a large fraction of the sky). With it, they estimated the correlation function and power spectrum of the galaxy distribution at z ~ 1. This can then be related to the matter distribution by way of the galaxy bias parameter. The advantage of redshift surveys over lensing surveys is that they can much more accurately resolve an object's position along the line of sight (though there are, of course, still redshift distortions).

One potential promise of this approach, if it can be pushed into higher z with (considerably) greater S/N, is that at higher z small (angular) scale surveys probe greater chunks of (physical) space. The paper makes this point too (while also pointing to the formidable obstacles that need to be overcome).That alone makes it worth spending considerable resources on, IMHO!Indeed.

Absolutely, cosmic shear is very promising for constraining cosmology (most notably dark energy)! There are many weak lensing surveys in the works right now.

Do you think the authors were unreasonably optimistic in reporting how well* they were able to do such de-projection (up to z ~ 1.2)?

Nothing immediately struck me as problematic in their analysis, but I didn't read it very carefully. I would be more suspicious if they were making more dramatic claims.

 

[Loki Mythos]

Thanks for the fast reply Russ Watters. I have a lot more questions if you wouldn't mind.

Such implications are not on the mind of scientists when they forumlate theories. It just isn't relevant.

Sorry but your singing to the choir. It seems to me that a lot of scientists need to separate their theories from their theology. And the big bang still smacks of creationism.

That last sentence implies there is an "outside". There isn't.

If there is no outside that would indicate by default that the inside must be infinite right?

It doesn't matter how long we've been observing, what matters is that we've made millions of observations.

A billion observations are worthless if they are taken in such a small time frame on something that changes scale of billions of years, wouldn't you think? And you didn't answer my question. Has the frequency of red shifted galaxy's changed during the time that man has observed them? Are they speeding up? Why?

It wouldn't be possible for all those observed galaxies (and the cmb) to switch to blue shift after a while.

so a galaxy can't just change course? why are we heading right at another galaxy if all galaxy's came from the big bang?

You did just say "conservation of energy", right?

I was referring to the fact that energy can not be destroyed.

Red-shift data tells us that nowhere within our horizon is there a place where anyone would see a contracting universe.

I'm trying to say that part could be contracting as another part is expanding, ok, think BIGGER, like infinitely bigger. In a infinite universe couldn't one part be contracting wile another is expanding? And would you be able to see one part from another?
What happened before the big bang?
Has the frequency of red shifted galaxy's changed during the time that man has observed them? Are they speeding up? Why?
was there time before the big bang?

 

[russ_watters]

Sorry but your singing to the choir. It seems to me that a lot of scientists need to separate their theories from their theology. And the big bang still smacks of creationism.

The phrase is "preaching to the choir" and I'm not sure you know what it means. It means you agree - and you clearly don't!

If there is no outside that would indicate by default that the inside must be infinite right?

No, it wouldn't.

A billion observations are worthless if they are taken in such a small time frame on something that changes scale of billions of years, wouldn't you think?

Since they are observations of space, they do cover billions of years.

And you didn't answer my question. Has the frequency of red shifted galaxy's changed during the time that man has observed them?

We couldn't possibly measure a change on such a short timescale.

so a galaxy can't just change course?

It can't hang a left at the next stoplight, that's for sure! What you are suggesting is quite a radical change and wouldn't be possible.

why are we heading right at another galaxy if all galaxy's came from the big bang?

We are part of a gravitationally-bound cluster. That's not the same as what you were suggesting before, distant galaxies suddenly reversing direction.

I'm trying to say that part could be contracting as another part is expanding. ok, think BIGGER, like infinitely bigger. In a infinite universe couldn't one part be contracting wile another is expanding? And would you be able to see one part from another?

Conjecture about things that aren't observed and wouldn't be possible given what we already know is pointless. We don't live in the universe you are describing.

What happened before the big bang?
was there time before the big bang?

There's no such thing as "before the big bang". That's when time began.

 

[Loki Mythos]

are you going to teach me anything? plese back up what your saying

 

[Loki Mythos]

There was nothing before the big bang. LOL. That answer sucks. I was hoping that someone here would take the time to give more of an explanation then brow beating one liners. Oh well. Guss I'll try somewhere else.

 

[Garth]

Loki, (BTW Welcome!) this is a physics forum, so we deal with what we can observe, test and falsify. There are other forums on PF where we discuss philosophical issues, but not this one.

Cosmology is the furthest extension of astrophysics. Astrophysics is the understanding of the universe 'out there' ("astro-") by applying the physical laws ("-physics") we have developed in the laboratory 'down here'.

What we observe and interpret 'up there' using the physical and mathematical theory developed 'down here' is that on the largest scales the universe has expanded from a 'Big Bang'.

Whether the first moment of the BB was a genuine singularity, where the laws of physics break down, a genuine beginning of time, or 'simply' a regime far outside our present understanding which marks a transition from one pre-BB state to the universe we now observe, is a matter of debate, and perhaps 'faith' (as we cannot observe that pre-BB state).

We all need a little humility in the face of the unknown, but what is known is magnificent and the subject of this Forum!

Garth

 

[marcus]

Loki raised some questions about which respected scientists disagree. I will try to focus on responding to those parts of his post. Also his Undulating Universe picture (some regions expanding some regions contracting) is interesting and it may be worth explaining why it runs contrary to a fundamental postulate---this thread is partly for discussing the status of basic assumptions like uniformity and what their justification is, since we can't claim they are true.

Loki himself may not be around any more but I will respond to certain interesting issues anyway

... I have trouble with this because I can not conceive of a beginning or end of time, it seems to go against the logic that is somehow hardwired into my brain.(please, no if a tree fell arguments) The same with the size of the universe. I just can't see it as finite. A finite amount of matter outside which, nothing exists.
...
...
...I understand that almost all of the galaxies that we can see are red shifted. But I haven't seen anyone argue that an infinite universal sea, sprinkled with matter is simply undulating, just like any large ocean. The idea of Conservation of energy(energy can not be created or destroyed), to me, argues for an infinite undulating universe and against the idea of a big bang.(if anyone knows of such an argument, please direct me to it)
...

This year Ashtekar, who works with a model of the universe where there is no edge to time----no beginning or end of time---was elected president of the international professional body that puts on the conference on Gen Rel and Cosmology every three years. they just had the GR18 conference this summer. Bojowald who also works on that kind of model was awarded the Xanthopoulos prize at the conference.

In January of this year Bojowald was one of the organizers of a workshop at Santa Barbara on removing spacetime singularities. Mainly on how to fix General Relativity so that it won't break down at the beginning of expansion. So there were all these prominent people gathered from all over the world (string and non-string alike) trying to see how there might NOT be a beginning of time, and how it might extend back before where the classic model breaks down.

So a lot of prominent respected people apparently share Loki's attitude that it isn't intuitive for time to begin at where GR crashes and fails to compute. Different ways to fix the model so it doesn't crash are a hot research area. Loki is in respectable scientific company on that one, wherever he comes to it from (which I don't know.) In any case there is room for legitimate disagreement.
========================

This UNDULATING picture which Loki presents is actually pretty cute. And I don't see how you could actually rule it out on empirical grounds. But there are basic AXIOMS or postulates in cosmology and we can justify using them on Occam grounds or Pragmatism grounds even tho we can't show observational evidence. And that is interesting. It is interesting that part of what science is based on is certain philosophical justifications.

The principle we are talking about here is Uniformity and the justification is wanting to keep things simple enough to make progress.

If we allow ourselves to contemplate big differences out beyond what we can see then it makes the whole job of analyzing the data too complicated. Too many extraneous possibilities: suppose this, suppose that. what if this other thing! Dragons and Sea Serpents.

The simplest thing is to assume that its UNIFORM more or less like what we can see, on average. And you work with that as long as it fits the data. And if fits amazingly well! And you keep on assuming uniformity and making progress fitting more and more data, until maybe someday you run into a wall.
And maybe someday you find something that you absolutely cannot explain without assuming that something over beyond the horizon is drastically different. BUT THIS HASN'T HAPPENED YET. So we postulate uniformity.

It is not something we KNOW for a fact is true, it is, instead, a basic GROUND RULE OF DISCOURSE which is to say that you can't play the game unless you accept the Cosmological Principle. If you don't accept that basic postulate then either you don't graduate, or you get banned from forum, or you fail the course, or you get called a crackpot.
There is a good practical reason for this: it is a waste of time and a distraction to try to do cosmology without a few minimal assumptions like that---that you just postulate.

Well there are also the multiverse and eternal inflation people, but that almost proves the point about distraction and waste of time. Regions out there with different physical laws etc etc. An awful morass of fantasy. And unnecessary.
What I am presenting is conventional cosmology viewpoint, in which the assumption of uniformity is basic and General Relativity applies to the whole thing---at least at large scale.
==========================

There is this other issue that Loki raised which is Epistomological----how do we know? And I guess the point to make is that General Relativity is our best theory of gravity and it is ALSO a theory of the changing geometry of space. We didn't want space to be expanding but GR describes gravity very accurately---the bending of light, the fine details of orbit behavior, the differences between atomic clocks at different altitudes, GPS signal corrections etc etc.

It is all one theory, so if you buy GR because it does a precision job on the solar system, then you have to buy expansion. And then the amazing thing is that expansion turned out to be observed! A really impressive surprise.

So GR, plus the uniformity assumption (that matter is distributed throughout space roughly how it is in the part we can see) has a lot of creds. have to go. may get back to this.
==================
The same with the size of the universe. I just can't see it as finite. A finite amount of matter outside which, nothing exists.
I object to the word "same"---I think there's a common confusion here. picturing space as finite is NOT analogous to imagining that time has a beginning. Picturing space as finite does not require any kind of singularity or boundary or edge or arbitrary termination. It just has finite volume the way the surface of a sphere has finite area. So it doesn't have the same problems as presuming that time doesn't continue back before the big bang. NOT the same as your rejecting idea of time having a beginning.
Many respectable cosmologists reject that idea about time along with you but that's different from what we are saying about space.

It hasnt been decided yet whether space is more likely to have finite volume or infinite volume. But the idea that space might have finite volume is not something to reject on philosophical grounds. It might have a finite, ever-increasing volume. Or it might have infinite volume, and still be expanding in the sense of distances increasing by a certain percentage each year. In neither case are there any boundaries or edges to worry about or find philosophically objectionable.

 

[Loki Mythos]

Thank you Marcus, your very nice, and you have gave me enough info to keep me busy for a few days at least. Thank you for pointing me in the right direction. Just in time too, as I was starting to feel the Inquisition had grabbed me up and was going to burn me at the steak for heresy.
Doesn't GR break down at the quantum scale? Could it be possible that it breaks down on some gigantic cosmic scale also? I seem to have heard this somewhere. Thanks again.

 

[marcus]

as I was starting to feel the Inquisition had grabbed me up and was going to burn me at the steak for heresy.
Doesn't GR break down at the quantum scale? Could it be possible that it breaks down on some gigantic cosmic scale also? I seem to have heard this somewhere. Thanks again.

heh heh maybe you SHOULD be burnt at the stake for heresy, but not yet.
GR breaks down at very small scale yes, if you mean break down at very high energy density which means very high curvature. the breakdowns are called "singularities"
the technical meaning of the word singularity comes from mathematics and means the failure of a theory or model, or the failure of a mathematical function to compute meaningful values------like where the function blows up, that is a singularity.

In an ordinary low energy low curvature regime, GR doesn't actually break down AFAIK, it is just that people are suspicious that it doesn't give a good description at very small scale. There are good reasons to mistrust it.

But at least it doesn't blow up. Practically speaking, the main places it blows up are the big bang and in black holes.

It could also be wrong at very large scale. Some people are working on modifications that take over at large scale---they want to see if they can fit data better with modified GR.

I guess the situation as I see it is paradoxical. On the one hand GR is overwhelmingly prevalent in cosmology. virtually everybody's research is based on it. On the other hand there are people constantly poking and prodding GR to find soft spots, trying to tweak it to make it do better, dreaming up ways to test it and maybe find a flaw. maybe that is normal. maybe a dominant theory always has some scientists attracted to trying to discover its faults, while the rest just go on using it.

 

[Garth]

heh heh maybe you SHOULD be burnt at the stake for heresy, but not yet.

But note Loki that “It is a heretic that makes the fire, not she which burns in it”!(William Shakespeare)

Garth

 

[Garth]

To my knowledge, there are no data sets that could really answer this question as yet. The trouble is that it's extremely difficult to conduct a wide-angle spectroscopic survey at high redshift because the tracer objects are either too dim (as with ordinary galaxies) or too sparsely distributed (as with quasars) to tell us much about large-scale structure.

At low redshift, my thesis work suggested that LCDM did very well at predicting the length and width distributions of filaments, though it's not yet clear how sensitive a test that is.

Thank you ST for your answer, I appreciate the observational difficulties.

It is great that we can see so far out and probe the early universe albeit in small areas of sky.

I understand the $\Lambda$CDM model fits observations at low z, my idea of a test for the $\Lambda$CDM model, would be to see if it does the same at high z as well.

Garth

 

[SpaceTiger]

I understand the $\Lambda$CDM model fits observations at low z, my idea of a test for the $\Lambda$CDM model, would be to see if it does the same at high z as well.

It would be nice to make the constraint more precise, even at low z. Believe it or not, at low z, we're actually more limited by our methodology than by our data.

Although Nereid's link (and the many other similar studies) don't directly address your question, they are suggestive of the answer. If the morphology of structure at low z matches the simulations, the two-point statistics match at intermediate redshifts, and we know from the CMB that it all started from a gaussian random field, then it's very hard to imagine that we'll see surprises in the morphological properties of large-scale structure at intermediate redshifts. Nevertheless, I agree that the study should be done and am in fact in the process of producing some results in that direction. Stay tuned!

 

[Garth]

Nevertheless, I agree that the study should be done and am in fact in the process of producing some results in that direction. Stay tuned!

Will do!

Garth

 

[Loki Mythos]

I have been looking at some of Van Flandern's work. The idea of how gravity waves could effect red shift. The more I think about it, the more the Big Bang Theory looks like a polished turd.

I would hope that fans of the BB here could answer some of Flandern's questions.

A short list of the leading problems faced by the big bang in its struggle for viability as a theory:
1.Static universe models fit the data better than expanding universe models.
2.The microwave "background" makes more sense as the limiting temperature of space heated by starlight than as the remnant of a fireball.
3.Element abundance predictions using the big bang require too many adjustable parameters to make them work.
4.The universe has too much large scale structure (interspersed "walls" and voids) to form in a time as short as 10-20 billion years.
5.The average luminosity of quasars must decrease with time in just the right way so that their mean apparent brightness is the same at all redshifts, which is exceedingly unlikely.
6.The ages of globular clusters appear older than the universe.
7.The local streaming motions of galaxies are too high for a finite universe that is supposed to be everywhere uniform.
8.Invisible dark matter of an unknown but non-baryonic nature must be the dominant ingredient of the entire universe.
9.The most distant galaxies in the Hubble Deep Field show insufficient evidence of evolution, with some of them apparently having higher redshifts (z = 6-7) than the faintest quasars.
10.If the open universe we see today is extrapolated back near the beginning, the ratio of the actual density of matter in the universe to the critical density must differ from unity by just a part in 1059. Any larger deviation would result in a universe already collapsed on itself or already dissipated.
From: Meta Research Bulletin, v. 6, #4, December 15, 1997. The full list and details appeared in "The top 30 problems with the Big Bang", Meta Research Bulletin, v. 11, #1, March 15, 2002.