Is the cosmological prinicple wrong? Is Big Bang wrong?

[quantum123]

This is a big hole in the sky. Although you can't see it with the naked eye it actually covers almost 3 degrees of the sky, and to put that into perspective the full Moon covers about half a degree!
Until recently no-one was sure how big or far away this void was but the latest calculations suggest it is 900 million light-years across and 8,000 million light-years away.

So is the cosmological prinicple, which says that the universe is homogeneous and isotropic wrong? Is Big Big theory, which assumes it, wrong as well.?

 

[Chris Hillman]

Don't jump to conclusions!

You are rushing to draw the most drastic possible consequences, but the math by and large supports the general validity of the FRW models as overidealized but good models of the gross behavior of our universe.

As all textbooks stress, the assumption of homogeneity and isotropy used in deriving the FRW models is only an approximation. But you should know that there exist a wide range of exact solutions in gtr which constitute (nonlinear) perturbations of FRW models allowing for a variety of anisotropies or inhomogeneities or both in the distribution of the mass-energy which acts as the source of the gravitational field, and in addition numerical relativists have performed many simulations. Once you know this, you can see that the question should be: how much do inhomogeneities in matter density, possible large and large-scale perturbations, disturb the basic features of the FRW models? Generally speaking, the answer is: not very much. This is why the FRW models --- which are obviously oversimplifications--- nonetheless provide an impressively accurate picture of the gross behavior of our universe on large scales.

 

[Nereid]

This is a big hole in the sky. Although you can't see it with the naked eye it actually covers almost 3 degrees of the sky, and to put that into perspective the full Moon covers about half a degree!
Until recently no-one was sure how big or far away this void was but the latest calculations suggest it is 900 million light-years across and 8,000 million light-years away.

So is the cosmological prinicple, which says that the universe is homogeneous and isotropic wrong? Is Big Big theory, which assumes it, wrong as well.?

(my bold)

One way to test this is to do a great many observations, plot the density fluctuations against scale, and compare what the plot looked like before this 'hole' (it's called a 'void' by astronomers) was discovered with what it looks like now.

http://www.sdss.org/news/releases/20031028.powerspectrum.html" is an example of what the former looks like. I don't have an example of what the latter would look like, but am pretty sure it would be much the same, except that a subset of the three right-most data points would have different (vertical) error bars (probably bigger).

Of course, the plot would be different today in other ways, if only because the WMAP Year 3 results were published after this 2003 plot was published ...

If you're interested, I (or someone else) could dig up some references to the appropriate papers on the power spectrum - just ask; if you're curious, we could walk you through the steps involved in making a plot such as this, at least at a high level (I assume you can see how this plot relates to the 'homogeneous' part of the cosmological principle).

 

[quantum123]

How big must the hole be?

how much do inhomogeneities in matter density, possible large and large-scale perturbations, disturb the basic features of the FRW models? Generally speaking, the answer is: not very much.

How big must the hole be before we can throw away the assumption that the universe is homogeneous and isotropic?

 

[Chris Hillman]

Hi, quantum123,

Looks like you had some trouble quoting something I wrote in my Post #2 above; see https://www.physicsforums.com/misc.php?do=bbcode for was to obtain quotations "by hand" using VB markup.

How big must the hole be before we can throw away the assumption that the universe is homogeneous and isotropic?

Generally speaking, approximations are valid under certain circumstances for certain purposes. An approximation valid for one purpose might not be valid for another. But you shouldn't expect a simple criterion for when a given model is broken "once and for all".

In this case, if you just want to model the gross large scale behavior of our universe, we know from observations that the oversimplified but handy FRW models do quite well. If you are trying to model something like the measured inhomogeneities in the CMB, you obviously can't do that with an FRW model, but a small amplitude linearized metric perturbation of an FRW model might be suitable.

HTH

 

[quantum123]

Ok, let's put it this way.
If you look at the sky 360 degrees all around you and you start to notice one gigantic humongous hole at only one direction, can you still say that the universe is isotropic?
I mean that the universe is homogeneous and isotropic is mentioned in every textbook of cosmology and relativity. This is the material that is being taught to every undergraduate. Is it ok with you?

 

[hurk4]

So is the cosmological prinicple, which says that the universe is homogeneous and isotropic wrong? Is Big Big theory, which assumes it, wrong as well.?

Dear quantum,
Very glad that you bring this question and that we now have all these reactions.

Here is one of mine.
"Cosmological Principle".
As we know, Einstein’s bold idea that the universe is homogeneous in the large scale average is what Milne called Einstein’s cosmological principle. (See P.J.E. Peebles “Principles of Physical Cosmology” page 10). Of course we know that at local scale this principle is not valid.
May I ask the question “what does large scale average really mean?” Is it not so that this has to be seen relatively? So, large scale related to us, can be taken as e.g. the observable universe, but if we relate it to the observable universe, as a kind of entity, then its large scale surrounding/environment will be very, very large.
As far as I have read, there are in fact at least 2 cosmological principles: 1) The perfect cosmological principle of Hoyle and Bondy, which says that the universe is homogenous and isotropic at large scale and at each time. The expansion of the observable universe learned us that this was not the case at each time, so the perfect CP was proven wrong.
2) So there was left Einstein’s CP which seems to be a good starting point for the mathematics of FRLW as an (approximate) language to describe our universe.
In the standard cosmological model, I suppose that, the cosmological principle is indeed taken as base for the classical theory of the universe at absolute large scale and not at a relative scale.
I wonder what the model consequences are if one introduces relativity into the cosmological principle, or is this already done so? If homogeneity, at large scales, in absolute sense, is not a basic ingredient what are then the consequences e.g. for assuming eventual other local concentrations of mass and or energies in our universe other than our observable universe and its environment?
How nice it may be to start with ideal mathematics, I am asking what that does help if in reality one has to do with deviations of those ideal conditions as there are mass and or energy concentrations in the observable universe (and far beyond as I might suppose).
So as a consequence what can bring us Bojowald, Ashtekar, Rovelli if their models are inherent too ideal? Or are they not?
Kind regards,
hurk4

 

[sysreset]

Hurk I believe that although you pose additional questions, you have skirted quantum123's question, which I would rephrase as "what percentage of the observable universe can be an absolute void without violating Einstein's Cosmological Principle?" As a starting point, let's generously put the current discovered void as a 0.5 billion light year radius sphere within the observable universe with a radius of 13 billion light years. Thus, the void only occupies a ratio of 1 to 26 cubed of the universe, which is only 0.00569% of the universe. I do not think that that is a sufficiently high percentage to discard Einstein's CP, although I still wonder what percent would.

 

[Chris Hillman]

Recommend a good course on mathematical modeling

This is the material that is being taught to every undergraduate.

Dunno if you are in school yourself, but if you have the chance to take a course on mathematical modeling, I think this would be extremely helpful to you in better understanding how cosmologists/physicists think.

If you look at the sky 360 degrees all around you and you start to notice one gigantic humongous hole at only one direction, can you still say that the universe is isotropic?

Why take a hypothetical? Sometime after the CMB was observed, it was noticed that it exhibits a "dipole anistropy". But this turns out to be consistent with the hypothesis that we are "moving with respect to the CMB" in a certain direction and with a certain velocity, and after subtracting for this effect, the CMB is once again seen to be, to a very good approximation, isotropic and homogeneous. But after much effort, astronomers succeeded in mappling tiny inhomogeneities in the CMB, so it is known that on a large scale our universe is not perfectly homogeneous.

I mean that the universe is homogeneous and isotropic is mentioned in every textbook of cosmology and relativity.

I can't seem to get this across: theoretical physics is all about idealizations, simplifying assumptions, artful approximations to a more complicated reality by extrapolating small variations from an oversimplified model, and so on. IOW, the aim of all theory in science is to build models with which one can make testable predictions and which one can compare with observation and experiment.

It is well established that the FRW models, while clearly idealizations, do provide a surprisingly good model of the gross behavior of our universe on very large scales. Thus, these models are simplifications, but if all you want to model is gross behavior on the largest scales, they are not oversimplifications.

 

[quantum123]

Can you define what do you mean by large scale? There has to be some clear thinking into this. What size? How many light years is considered large scale? We are scientists here and we want things to be quantified.
Again what do you mean by tiny inhomogeneities? What is tiny? How many lightyears is considered tiny?
Without some kind of quantities, and some math, we cannot start talking about mathematical modelling.

 

[Chris Hillman]

Mentor! Mentor!

Can you define what do you mean by large scale? There has to be some clear thinking into this.

Suppose someone asked: "In the definition of the derivative from difference quotients,
\frac{f(x+\varepsilon)-f(x)}{\varepsilon}
how small should we take \varepsilon?" The answer is: it depends. That is, you need to say something about the nature of f and how much error is acceptable to you; then one can say how small we should take \varepsilon! But without that information, one can't say "the answer is that we need to assume that \varepsilon<1/10".

If that doesn't work for you, perhaps some mentor can step in because I feel this thread is in danger of devolving into pointless repetition.

 

[Nereid]

Ok, let's put it this way.
If you look at the sky 360 degrees all around you and you start to notice one gigantic humongous hole at only one direction, can you still say that the universe is isotropic?
I mean that the universe is homogeneous and isotropic is mentioned in every textbook of cosmology and relativity. This is the material that is being taught to every undergraduate. Is it ok with you?

Can you define what do you mean by large scale? There has to be some clear thinking into this. What size? How many light years is considered large scale? We are scientists here and we want things to be quantified.
Again what do you mean by tiny inhomogeneities? What is tiny? How many lightyears is considered tiny?
Without some kind of quantities, and some math, we cannot start talking about mathematical modelling.

Earlier I answered your questions by referring to observations, which - I'm sure you'll agree - are the ultimate arbiter in science.

For these new questions, different kinds of answers - in addition to those already given by Chris Hillman - may be of interest.

Take simulations. Have you heard of http://www.mpa-garching.mpg.de/galform/millennium/" ? This - and other simulations - provides one kind of answer: analyses of the (simulated) voids gives you a handle on just how well (or badly) an arbitrary 'gigantic humongous hole' fits within the theory underlying the simulation. And, as Chris Hillman has already indicated, any decision on 'goodness of fit' requires prior decisions on how to measure such goodness, what threshholds to set, and so on.

Another approach: suppose our vantage point were not the surface of our dear Earth; suppose we - intelligent, scientific enquirers - evolved on the surface of Venus, or in the Ganymede ocean; or on a planet orbiting a star in the Arches cluster, or one wandering between galaxies in the Virgo cluster; or somewhere in an ordinary galaxy at the heart of the Shapley supercluser, or a rogue planet at the edge of (or near the centre of) the Bootes void; or ... Could such scientific enquirers have developed a cosmology which included a principle of homogeneity and isotropy? If they were to try to construct a plot like the SDSS one I linked to in my previous post, how different would it be?

You could take an even more extreme 'what if': suppose you were a cosmologist at the time of the radiation-matter decoupling, or 100 billion (co-moving) years into the future, or ... how would your questions be answered then? Or would you, the über-Nobel Prize winning cosmologist, have ruled that homogeneity and isotropy as cosmological principles were completely unsupported by the best observational evidence?

Finally, what do you think about this: the observational basis of contemporary concordance cosmological models is considerably broader than just estimates of P(k)? Or, putting this another way, what role do you think a single observation (or millions of observations of a single object) plays - or should play - in cosmology (as a science)?

 

[quantum123]

If you were to walk along a road one day and fall into a hole, would you complain to the authorities, or say that : hmmm, the road is still homogeneous and isotropic?

 

[russ_watters]

If you were to walk along a road one day and fall into a hole, would you complain to the authorities, or say that : hmmm, the road is still homogeneous and isotropic?

That analogy doesn't work, since we observe roads from close up while the universe's homogeneity is large-scale.

Look, the Big Bang theory is not going to be discarded because one out of a million observations doesn't quite fit one specific, minute piece of it. That just isn't how science works. The fact that there is a large void (assuming it is larger than expected, which I don't actually know) doesn't change the validity of the basic pillars of the theory ( http://www.damtp.cam.ac.uk/user/gr/public/bb_pillars.html ).

 

[Nereid]

If you were to walk along a road one day and fall into a hole, would you complain to the authorities, or say that : hmmm, the road is still homogeneous and isotropic?

russ_watters has already noted that this kind of approach has, it seems, little relevance to how the science of cosmology is actually undertaken, and as we use 'within the framework of modern cosmology, as a science' as our scope here, your post seems way off-topic.

But perhaps that's simply because you wrote too tersely? Perhaps you might like to try to re-cast your question into a form more atune with this section's scope?

 

[SpaceTiger]

If you were to walk along a road one day and fall into a hole, would you complain to the authorities, or say that : hmmm, the road is still homogeneous and isotropic?

Depends on how big the dip is relative to your model of the road. The cosmological principle is never exactly true on any scale and so the FRW model will not be a perfect description of the universe. The observation of an unusually large void might suggest that the FRW model is not quite as good an approximation as we might have thought on scales comparable to the void's size, but it certainly doesn't invalidate the Big Bang.

The real issue is whether or not inflation could explain such a large void, since it claims to predict the initial spectrum of fluctuations. The vast majority of observations done so far support gaussian random phase initial conditions, as predicted by inflation. If it could be shown that the distribution of matter is inconsistent with these initial conditions, then it might bring inflation into question, but it's hard for me to see how it could challenge the Big Bang framework.

 

[sysreset]

Can you define what do you mean by large scale? ... How many light years is considered large scale? ...What do you mean by tiny inhomogeneities? ... How many lightyears is considered tiny?

Quantum I think you raised a valid question. The principles of Isotropy and Homogeneity are invoked on a regular basis, independent of any mention of a specific numerical scale. Maybe it would be useful to start at some specific numerical extremes and work our way toward the muddled middle.

(For the sake of discussion let's adhere our own frame of reference at this point in space-time, rather than reconstructing what we think the universe looks like from another perspective such as at the edge of the void or 100 billion years into a hypothetical future.)

Assume an observable universe from our current perspective of 13 billion light year radius, giving a volume of 9.2 trillion cubic light years.

If the void had been discovered to be a sphere with a radius of 9 billion light years, it would occupy 1/3rd of the volume of the entire observable universe. I would venture to say that that would violate isotropy and homogeneity assumptions.

(Even then, I am out on a limb. I am ignoring the portion of the universe outside our observable horizon.)

If the void were instead 5 billion light years in radius, it would occupy only 5% of the universe, so I would venture to claim that isotropy and homogeneity would be preserved.

The actual void only occupies 5/1000 of 1%. That's one tiny pothole.

The link in Nereid's post to the Millennium Simulation shows beautiful images of the fine, lattice-like structure matter distributed over billions of light years. Those images for me embody validity of the cosmological principle.

 

[russ_watters]

Depends on how big the dip is relative to your model of the road. The cosmological principle is never exactly true on any scale and so the FRW model will not be a perfect description of the universe. The observation of an unusually large void might suggest that the FRW model is not quite as good an approximation as we might have thought on scales comparable to the void's size, but it certainly doesn't invalidate the Big Bang.

The real issue is whether or not inflation could explain such a large void, since it claims to predict the initial spectrum of fluctuations. The vast majority of observations done so far support gaussian random phase initial conditions, as predicted by inflation. If it could be shown that the distribution of matter is inconsistent with these initial conditions, then it might bring inflation into question, but it's hard for me to see how it could challenge the Big Bang framework.

I've never simulated anything quite so complex, but it is worth noting that for some systems, small changes in the initial conditions can produce large differences in the resulting simulations. I suspect these models work the same way so if, in fact, the void is larger than the simulations would predict, the actual error in the model's starting parameters could still be quite small.

And as sysreset noted, even if that pothole turns out to be a lot larger than predicted, it is still one tiny pothole.

 

[SpaceTiger]

I've never simulated anything quite so complex, but it is worth noting that for some systems, small changes in the initial conditions can produce large differences in the resulting simulations. I suspect these models work the same way so if, in fact, the void is larger than the simulations would predict, the actual error in the model's starting parameters could still be quite small.

The location and amplitude of large-scale voids should be easily traceable to the post-inflation perturbation spectrum. The growth of structure is still approximately linear (meaning it's still governed by linear differential equations) on scales comparable to the claimed size of this void, so you wouldn't expect any extreme sensitivity to initial conditions. You would, however, see such sensitivity in the positions of galaxies near clusters (for example).

And as sysreset noted, even if that pothole turns out to be a lot larger than predicted, it is still one tiny pothole.

He's right in that the supposed void wouldn't be evidence against the Big Bang. It would, however, be difficult to explain it with gaussian random phase initial conditions and what we currently think is the amplitude of the power spectrum. I think it's far more likely that their estimate of the void's size is just wrong and that the "extreme" CMB cold spot can be explained by a combination of effects at the surface of last scattering and at intermediate redshifts. This explanation would be ad hoc if it weren't for the a posteriori nature of the analysis on the cold spot.

 

[quantum123]

Fine, the pothole is 0.001% of the observable universe.
So you think it is small.
But some people think that it is big enough to be a sign that another alternate universe exists.
So is the pothole big or small?

 

[Nereid]

Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae - a recent preprint (http://arxiv.org/abs/0712.0370" ) might be of interest to you quantum123 (and other readers) - it presents a 'minimal void' model, in which we are located inside a void (region underdense by ~40%) of several hundred million light-years in radius, and asks how well such a model fits the relevant cosmological observations.

The answer the authors give is, 'rather well'! (Of course, it's just a preprint at this stage)

Of particular interest may be the range of observations they consider: not only the high-z Ia SNe (high redshift type Ia supernovae) and the Year 3 WMAP observations of the CMB, but also BBN observations (primordial abundance of light nuclides), BAO observations (baryon acoustic oscillations - a peak observed in the galaxy-galaxy correlation function of LRGs), observations from large-scale structure and weak lensing, and ISW correlations (integrated Sachs-Wolfe effect)!

This also points to something Space Tiger mentioned: relating the observed CMB 'extreme' cold-spot to only one cosmological parameter is not so clean at this stage - Alexander et al. do also look at a couple of additional 'tweaks' (and mention that at least one class of 'alternative universe' models is a poor fit) ...

Oh, and the 'pothole' is very, very small quantum123 ... it just happens to be, apparently, the biggest pothole we've found so far ...

 

[Chris Hillman]

it is worth noting that for some systems, small changes in the initial conditions can produce large differences in the resulting simulations. I suspect these models work the same way so if, in fact, the void is larger than the simulations would predict, the actual error in the model's starting parameters could still be quite small.

Just wanted to point out that the question of stability in cosmological models (including but not limited to ones which undergraduate students of gtr are likely to encounter) has been extensively studied using the methods of dynamical systems; see for example Dynamical Systems in Cosmology, J. Wainwright and G. F. R. Ellis (editors), Cambridge University Press, 1997. Notice this typically involves reducing a complicated nonlinear PDE (the EFE) to a system of nonlinear ODEs.

(For those who have no idea what "stability" means in this context, suppose we consider an exact solution which is a perturbation of an FRW model and imagine that we have a slicing which approximates a homogeneous isotropic Riemannian three-manifold at some "time". Then, roughly speaking, we can ask whether initially perturbations grow larger as "time" progress? Or as we run time backwards?)

Thanks to all the mentors who showed up to help!

 

[Nereid]

One more thing on the difference between a big void (if that's what the extreme CMB cold spot turns out to be) and a 'pothole': contrary to the impression you may get reading some of the popular press articles, 'voids' are not empty ... they are simply regions in which the average density is (significantly, somewhat, ...) lower than the universal average.

For example, the Boötes void - the largest (?) before the CMB extreme cold spot was discovered - is certainly (very) under-dense, but it is also certainly not empty! For example, http://adsabs.harvard.edu/abs/1996AJ...111.2141S", a paper published in 1996, reported the discovery of many new galaxies in the small part of the void the authors studied; I think the number of known Boötes void galaxies is over 50. Also, given that some of these galaxies have AGNs, that some are undergoing mergers or other interactions (or have done so), and that many stars have gone supernova there, the medium between the galaxies in the void will certainly contain gas, dust, 'rogue' stars, planetary nebulae, ... and maybe even some dwarf galaxies.

 

[Garth]

And a cosmologist living on a planet in the middle of the Bootes void, who compared the Hubble flow of nearby galaxies with those further away, would conclude the universe was accelerating...

Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae

It is now a known fact that if we happen to be living in the middle of a large underdense region, then we will observe an ``apparent acceleration'', even when any form of dark energy is absent.

Garth

 

[Nereid]

And a cosmologist living on a planet in the middle of the Bootes void, who compared the Hubble flow of nearby galaxies with those further away, would conclude the universe was accelerating...

Local Void vs Dark Energy: Confrontation with WMAP and Type Ia Supernovae

Garth

Hmm ... I seem to recall that someone else cited that preprint, earlier in this thread, ... I wonder who?

 

[Garth]

Yes, I wonder!

It just shows you that great minds think alike and fools seldom differ!

The point of me citing that link was, of course, that the hypothesis of us being in a relative void need not be too much of a violation of the Copernican Principle.

Garth

 

[quantum123]

Would a doorway to another universe mean that the cosmological principle is incorrect?

 

[Garth]

The point of me citing that link was, of course, that the hypothesis of us being in a relative void need not be too much of a violation of the Copernican Principle.

Another revision of Professor Sarkar's paper, which has now been accepted for publication in a special issue of General Relativity and Gravitation, Is the evidence for dark energy secure?.

This paper makes the point that:

The precision CMB data can be equally well fitted without dark energy if the spectrum of primordial density fluctuations is not quite scale-free and if the Hubble constant is lower globally than its locally measured value. The LSS data can also be satisfactorily fitted if there is a small component of hot dark matter, as would be provided by neutrinos of mass 0.5 eV. Although such an Einstein-de Sitter model cannot explain the SNe Ia Hubble diagram or the position of the `baryon acoustic oscillation' peak in the autocorrelation function of galaxies, it may be possible to do so e.g. in an inhomogeneous Lemaitre-Tolman-Bondi cosmology where we are located in a void which is expanding faster than the average.

It remarks:

Such alternatives may seem contrived but this must be weighed against our lack of any fundamental understanding of the inferred tiny energy scale of the dark energy. It may well be an artifact of an oversimplified cosmological model, rather than having physical reality.

So might it be the case that the Copernican Principle does not apply to our position in space and instead we are in a 'special' place, i.e. in a void?

If we see other voids, such as those in Bootes and the WMAP Cold Spot, then this hypothesis may not be so "contrived" after all.

And yes quantum123 the Cosmological Principle would still apply to the universe on the largest scales, even if the void 'pothole' is a "doorway to another universe", but if Sarkar's hypothesis is correct then it would not apply to the 'immediate' region around our Local Group.

Garth

 

[pervect]

If you were to look at Mt Everest, would you say that the figure of the Earth is not an ellipsoid because the mountain is so tall? Or would you say that the Earth is essentially an ellipsoid, because the mountain is so small?

 

[fasterthanjoao]

Would a doorway to another universe mean that the cosmological principle is incorrect?

I think, as has been shown by a good people already, you are misunderstanding and mixing priorities. Paraphrasing the same question and point doesn't help! It would be useful to know to what level you have studied physics: the issue of homogeneity and isotropy is only assumed as a starting point. Physics at any level needs to take a starting set of assumptions, and gradually perturb your model until the results match observation.

The FRW model (as you will find when you actually start studying cosmology) is not only a very nice starting point but does yeild some useful results, even if the finer points aren't accurate. To say that the model is worthless because absolute homogeneity and isotropy are not observed is naive, I'm sure you will know if you have studied chemistry at any level the value of the Bohr model of the atom - to a certain level results can be useful in making predictions, even though the fundamental essence of the model is 'wrong' (more correct to say that it has been replaced in mainstream physics by a quantum model).

 

[quantum123]

You have made a very good point indeed, fasterthanjoao!
In the light of what we know, that the hydrogen atom is just a solution of the Schrodinger wave equation in a spherically symmetric potential well, and that the energy levels are but eigenstates in Hilbert space, we no longer think of it as a Earth and moon orbits anymore. The framework has totally shifted from Newtonian mechanics to Quantum Mechanics. You can perturb all you want in Newtonian mechanics, it will not lead to you to nowhere.
Isn't there a possibility that with all these alternate universes at the doorstep, and so many anomalies being observed, a shift in framework is already taking place?

You have a good question there, Pervect. It is true then that the Earth is an ellipsoid. But recently some people say that this Mt Everest is a sign there is another Earth somewhere. Then the whole thing look more like a pair of balls, not a ellipsoid anymore. lol

 

[fasterthanjoao]

You can perturb all you want in Newtonian mechanics, it will not lead to you to nowhere.
Isn't there a possibility that with all these alternate universes at the doorstep, and so many anomalies being observed, a shift in framework is already taking place?

Absolutely! It would be foolish (and I suspect this forum would have difficulty existing) if researchers and students subscribed only to a 'standard' model, I know I have certainly had some reservations about points in things I've been taught in physics and astronomy. Every time (i might be over-generalizing, but you get the point ) there has been a huge advance in physics, it has involved a rejection of what was considered standard at the time, so it is definitely important to have people exploring other possibilities, even 'just in case'.

The issue of FRW is a strange one at the moment: you should consider that Quantum Mechanics is now 100 years old, whilst cosmology may not have even been considered a science for half that amount of time (not that I wish to look down upon the great progress that has certainly been made in that time). We are still learning in [every area of] physics, so there is still some missing pieces of the puzzle; good thing for everyone in the field now then!

 

[SpaceTiger]

It shouldn't require a professional astronomer to remind people that the vast majority of fantastic claims made in the literature turn out to be wrong. Giant voids, gates to other universes, dark galaxies, quark stars... these things usually disappear upon closer inspection. Astronomy is a living, just like any other job, so it is sometimes worth taking chances on fantastic claims if your other research is not so well funded.

 

[andrewj]

You are rushing to draw the most drastic possible consequences, but the math by and large supports the general validity of the FRW models as overidealized but good models of the gross behavior of our universe.

As all textbooks stress, the assumption of homogeneity and isotropy used in deriving the FRW models is only an approximation. But you should know that there exist a wide range of exact solutions in gtr which constitute (nonlinear) perturbations of FRW models allowing for a variety of anisotropies or inhomogeneities or both in the distribution of the mass-energy which acts as the source of the gravitational field, and in addition numerical relativists have performed many simulations. Once you know this, you can see that the question should be: how much do inhomogeneities in matter density, possible large and large-scale perturbations, disturb the basic features of the FRW models? Generally speaking, the answer is: not very much. This is why the FRW models --- which are obviously oversimplifications--- nonetheless provide an impressively accurate picture of the gross behavior of our universe on large scales.

theories are an easy answer so did you really answeri

 

[Nereid]

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):

The first and third year data releases from the WMAP provide evidence of an anomalous Cold Spot (CS) at galactic latitude b=-57deg and longitude l=209deg. We have examined the properties of the CS in some detail in order to assess its cosmological significance. We have performed a cluster analysis of the local extrema in the CMB signal to show that the CS is actually associated with a large group of extrema rather than just one. In the light of this we have re-examined the properties of the WMAP ILC and co-added ``cleaned'' WCM maps, which have previously been used for the analysis of the properties of the signal in the vicinity of the CS. These two maps have remarkably similar properties on equal latitude rings for |b|>30deg, as well as in the vicinity of the CS. We have also checked the idea that the CMB signal has a non-Gaussian tail, localized in the low multipole components of the signal. For each ring we apply a linear filter with characteristic scale R, dividing the CMB signal in two parts: the filtered part, with characteristic scale above that of the filter R, and the difference between the initial and filtered signal. Using the filter scale as a variable, we can maximize the skewness and kurtosis of the smoothed signal and minimize these statistics for the difference between initial and filtered signal. We have discovered that the shape of the CS is formed primarily by the components of the CMB signal represented by multipoles between 10<=L<=20, with a corresponding angular scale about 5-10 degs. This signal leads to modulation of the whole CMB sky, clearly seen at |b|>30deg in both the ILC and WCM maps, rather than a single localized feature. After subtraction of this modulation, the remaining part of the CMB signal appears to be consistent with statistical homogeneity and Gaussianity.

 

[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 \LambdaCDM 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.