Review of Mainstream Cosmology

[oldman]

the LCDM model... handily describes the observational results. It is a very good model that explains many things.

I'm certainly not disputing this kind of general statement, Chronos, but the devil is always in the details! My original query was about one detail, namely:

my difficulty boils down to: why are large-scale temperature fluctuations not more prominent in the WMAP results?

and in a following post:

I would like to know how one can "have one's cake" (solve the horizon problem) and at the same time "eat it" (accept the deficiency in low-l modes).

.

These quotes should also make it clear that I do agree with Hellfire

... that inflation solves the homogeneity problem (or horizon problem) (and that)... the theory ... provide(s) a mechanism to account for the small inhomogeneities in the CMB that are the seeds of the matter structures...this mechanism could be quantum fluctuations during inflation.

But nether of you have yet mentioned the deficiency in low - l modes, which the WMAP results now seem to show are real. This conflict is what bothers me. Or am I tilting at windmills?

I hadn't seen the 2003 paper by Guth that Hellfire kindly referred me to. I've glanced at it now, and it seems to complement his 1997 book (which I have)excellently. Thanks for the reference, Hellfire.

 

[SpaceTiger]

But nether of you have yet mentioned the deficiency in low - l modes, which the WMAP results now seem to show are real. This conflict is what bothers me. Or am I tilting at windmills?

There is a deficiency in the low-l modes, but it's only a relative deficiency...they still expected much less power on those scales than at the scale of the acoustic peak. The interpretation of this deficiency is still a matter of some debate. Some believe it is a sign of a non-trivial topological structure for the universe, some a signature of modified gravity, and some just a statistical fluke exaggerated by a posteriori statistics. Only time will tell.

 

[Garth]

(Edit: Crossed with ST's post.)

Not many low-l mode anisotropies are to be expected in the first instance, as they are the 'large' ones. Even so there are fewer than predicted by the LCDM model, is this perhaps just a statistical fluke?

It could be, however if the 'Axis of Evil' is removed as being some kind of local contamination then the chance that the remaining increased deficiency at the low-l modes be a statistical fluke becomes vanishing small.

Perhaps the Axis of Evil is indeed real and part of the cosmological structure of the CBM? In which case its alignment with local geometry is an 'a posteri' recognised statistical chance event and not significant. And the low-l mode deficiency may be disregarded as statistically insignificant.

However, one explanation for the deficiency may be that although the universe's spatial geometry appears flat and therefore the universe is infinite or very large compared with our horizons, it is in fact smaller so there was not enough room for the deficiencies to grow in the early, pre-Surface of Last Scattering universe but also spatially conformally flat. This, however, would require a modification of the theory by which the CMB data is analysed, i.e. it would require a modification of GR.

Garth

 

[oldman]

... The interpretation of this deficiency is still a matter of some debate ...Only time will tell.

That satisfies my curiosity. I hope that not too much time will pass before all becomes cut and dried. Thanks.

 

[oldman]

... one explanation for the deficiency may be that although the universe's spatial geometry appears flat and therefore the universe is infinite or very large compared with our horizons, it is in fact smaller so there was not enough room for the deficiencies to grow ...

I'm slightly puzzled by this suggestion, because the uniformity of the universe is postulated to have been grown by inflation to encompass the whole observed universe, thus solving the horizon problem. If there was room for this (call it l = 0?) mode to grow, why not room for low-l modes? But I don't trust my reasoning , and accept that others will probably resolve this anomaly in time.

If it were to "require a modification of the theory by which the CMB data is analysed, i.e. and ... a modification of GR", as you mention, the interpretation of much else might also change --- a welcome astonishment for a somewhat sceptical spectator like myself.

Thank you for your interest.

Garth[/QUOTE]

 

[Chronos]

I feel the non-trivial topology hypothesis has legs. The persistent redshift anomalies certainly suggest this possibility. In a finite universe, this [IMO] is a very plausible explanation - there may be quantum fluctuations embedded in the inflationary phase that produce localized variations in H0. I would be fascinated by any study that examines this possibility. I have toyed with this idea using LEDA and VizieR data and cannot rule it out.

 

[Garth]

Might the explanation of the low-l mode deficiency and apparent alignment be explained by an exotic topology? CMB Alignment in Multi-Connected Universes Not yet apparently!

We have investigated the question whether the strange alignment observed in low CMB multipoles can be explained by multi-connected space forms. There are several examples of such spaces which can explain the missing anisotropy power at large angular scales as measured by the temperature correlation function C(ϑ), ϑ > 60◦, or the angular power spectrum C_l for l = 2, 3. It is thus natural to ask for the alignment properties in such spaces.

......

But in no case we have found a model, where a significant fraction of the simulations possesses the alignment observed in the CMB sky. It remains to be seen whether there are other space forms which can more easily explain the alignment than the space forms considered in this paper.

Garth

 

[heusdens]

With all the crazy ideas that get thrown around in this forum, I thought it would be good to step back and review the mainstream view on cosmology in 2005. The field is advancing very rapidly, so it's possible that even the most reliable websites will be woefully out of date, both in terms of results and the evidence for them. Let's review, starting from the most secure and ending with the most puzzling/dubious aspects of the standard theories. I'll do this over the course of multiple posts, and feel free to interject and discuss at any point. Note that we are discussing mainstream cosmology, so this is not the place to present your favorite non-standard model for the universe. However, please do feel free to discuss observational evidence (or the lack thereof) for the standard theories.


1) Expansion

The universe is, without a doubt, expanding. The most striking evidence for this is the fact that nearly every object in the sky exhibits a redshift in the spectrum of light that is emitted from it. Furthermore, more distant objects are observed to have larger redshifts, exactly what you would expect for expansion. Alternative theories (such as Zwicky's "tired light hypothesis") were put forth and seriously considered in the first half of the 20th century, but have produced no correct predictions, nor are they consistent with any known physics. They have not been seriously considered by the mainstream for quite some time.

It should be noted that redshift is not the only reason we think the universe is expanding, but it was certainly the first evidence. Since the discovery of Hubble's Law in 1929, many more things have been deduced under the assumption of expansion (most notably, the Big Bang Theory) that also produce testable predictions. The success of these theories can be viewed retroactively as evidence for the expanding universe.


2) The Big Bang Theory

There is a lot of confusion amongst the general public about what the Big Bang Theory is really saying and which aspects of it are taken as gospel truth by the scientific community. In its simplest form, you can think of the argument as follows:

"If space is expanding and the universe has a finite size, then it must have been much smaller in the past".

How much smaller? Well, the standard assumption is that the universe had a creation event and expanded from a singularity to its present size. Such a distant extrapolation can't possibly be verified by the current observations, but we can safely say that the universe expanded from a much smaller size than its current one. There is good observational evidence for an epoch of nucleosynthesis approximately one minute after the creation event (z ~ 10⁸). Physical models of the conditions in this early phase of the universe were able to predict the relative abundances of the light isotopes (including hydrogen, helium, and deuterium) to very high accuracy.

There is even stronger evidence for recombination, an event that occurred when the scales in the universe were a 1000 times smaller than today (~400,000 years after the big bang). Recombination is what gives rise to the cosmic microwave background (CMB) radiation, a nearly blackbody spectrum that can also be modeled very accurately. The models are so accurate, in fact, that they have also allowed us to precisely measure some of the parameters of our universe. More on this later.

In addition, there is increasing evidence for an epoch of inflation, thought to occur 10^-35 seconds after the creation event, during which the universe may have expanded by as much as a factor of 10⁵⁰! If we could observationally confirm such a hypothesis, it would be an overwhelming success for both the Big Bang Theory and the scientific method itself. I'll also discuss the evidence for this in more detail later. There are a lot of nice websites on the Big Bang Theory (see here, for example), so web surf if you want to know more.

I agree on general on this outline of BB theory, except for the stuff marked in red. Although it can be theoretized that the theory of GR shows a singularity at the beginning, it is not mainstream scientific or part of the BB theory to claim that the singularity actually "happened", because it would mean that there was a moment in time at which time began (there was no time prior to it). But we don't know that at all, it is NOT a scientific fact!
What we can talk about and back up with observational evidence goes back to the origin of the CMBR, and theoretically we can go back to the Planck time. What happened before the Planck time we can not predict with current scientific theories (GR and QM), unless there is a theory of quantum gravity, and although some scientific theories have emerged here, to my knowledge there is not yet an accepted theory there.
What we can NOT talk about, scientifically, is talking about the actual singularity as a real event (happenening in space and time), since the very theory that comes up with it, is known to break down there, so it ain't scientific to say that there was a singularity at the begin, or to even suggest that there could have been a begin. (*)
So the singulairy is only a fictional point which boils up in models, but is not a scientific fact that this singularity actually exist. It would have contained infinite density for instance.
As a model for the early universe, the theory of inflation has been the most succesfull over the last couple of 20 years, and it has been shown that:

• inflation can start at in fact chaotic conditions (which means, not requiring special conditions for the early universe)
• once started, inflation could go on forever, although for every worldline, the period of inflation is finite. [compare that with a wildfire, every tree just burns for some specific time, but the fire itself can go on forever, at least theoretically; in a theoretically infinite universe, inflation could go on forever].
• there is no need for inflation to have begun at some time, it could have been past eternal.

(*)
And as a remark, from a philosophical point of view, it is necessary to acknowledge that as such the universe can not have a begin, as the universe contains all there is, since if there was, the universe would have needed to start out from nothing. But a nothing is only nothing, not a begin of even an infinitisimal small something. A nothing is merely a pure concept of thought, and not a concept that applies to physical reality. Don't interchange that with a vacuum (the absence of ordinary matter), since a vacuum definitely contains something (energy, fields, spacetime metric, etc.)

 

[hellfire]

SpaceTiger's claim is correct if you take singularity = an event or a location at which general relativity and the classical notion of space-time break down.

 

[Chronos]

A universe from 'nothing' is not as far fetched as it may sound. Under the laws of thermodynamics, it is virtually required. Entropy is illogical in an infinitely old universe that includes causality.

 

[heusdens]

A universe from 'nothing' is not as far fetched as it may sound. Under the laws of thermodynamics, it is virtually required. Entropy is illogical in an infinitely old universe that includes causality.

That is not correct, if 'nothing' causally connects to any something, it is not 'nothing', but already a potential something.

You can't make up arbitrary physics law from nothing.

Qauntum mechanics for instance, is not applicable to 'nothing', because a 'nothing' is not part of any reality and has no physical description, it is just a concept of thought. If it would, then 'nothing' would not be 'nothing' but full of potential something.

And btw. this whole issue is outside the domain of physics, if there ain't a physical description of reality, there is no physician or physical law that can deal with it.

That one can not perceive of reality without contradictions somewhere, does not mean that reality does not exist, but that our way of describing it is incorrect.

The law of Thermodynamics (2nd law) is used over and over again to somehow 'proof' that the world would have needed a begin (a begin from 'nothing' !), but what one does not see is that the law of thermodynamics is not usefull in that context. It is restricted to finite systems only, which are not thermally connected to the rest of the universe.

The infinite universe is not a problem, since in any case, any timeline need to be strictly finite. The bottom line however is that one can not perceive of infinity without a contradiction.

 

[heusdens]

SpaceTiger's claim is correct if you take singularity = an event or a location at which general relativity and the classical notion of space-time break down.

Therefore, it is not an 'event'.

 

[hellfire]

Therefore, it is not an 'event'.

Ok, you may be right with the wording. What I was trying to point out is that the claim of SpaceTiger is correct, as long as you take the definition of singularity that is given within general relativity. This definition can be formulated in a very precise an formal way and there are singularity theorems that prove that the universe must have had such a singularity in its past. This is actually the mainstream view.

 

[Mike2]

The law of Thermodynamics (2nd law) is used over and over again to somehow 'proof' that the world would have needed a begin (a begin from 'nothing' !), but what one does not see is that the law of thermodynamics is not usefull in that context. It is restricted to finite systems only, which are not thermally connected to the rest of the universe.

The requirement of causality is that the next fact or event is materially implied by the previous condition. It is logically valid that a true conclusion can be implied by a falsehood, ie. 0->1 is logical. Thus the universe can come from nothing. Logic was developed from the most complete generalization of physical observations. One thing seemed to lead to another until we generalized the "one thing" as a premise and "another" as the consequences until now we accept that this is the way to think about anything. Now we are in the habit of assigning the property of being true to physical facts which do exist, and false to those supposed facts whcih really don't exist. We assign truth to existence and fasle to non-existence. So the very first fact would represent "True" materially implied by non-existence which is false. The first sample of existence completely distinguishes true from false. And the certainty of its existence is 100%. If anything at all is knowable with absolute certainty, it is that reality exists. What we can say about the universe after that would be less certain. And we can only decompose the absolute certaintly about the property of existence to other properites which are not as certain. So it seems probabilities would enter the calculations about subsystems of reality.

 

[heusdens]

The requirement of causality is that the next fact or event is materially implied by the previous condition. It is logically valid that a true conclusion can be implied by a falsehood, ie. 0->1 is logical. Thus the universe can come from nothing. Logic was developed from the most complete generalization of physical observations. One thing seemed to lead to another until we generalized the "one thing" as a premise and "another" as the consequences until now we accept that this is the way to think about anything. Now we are in the habit of assigning the property of being true to physical facts which do exist, and false to those supposed facts whcih really don't exist. We assign truth to existence and fasle to non-existence. So the very first fact would represent "True" materially implied by non-existence which is false. The first sample of existence completely distinguishes true from false. And the certainty of its existence is 100%. If anything at all is knowable with absolute certainty, it is that reality exists. What we can say about the universe after that would be less certain. And we can only decompose the absolute certaintly about the property of existence to other properites which are not as certain. So it seems probabilities would enter the calculations about subsystems of reality.

I don't think you're on the right track, because to begin with "false" has no meaning without "true". "False" and "true" are just opposites of each other, that belong to each other, and one can not exist without the other.
There is no dark without light. There is no nonbeing without being.

In other words those opposites (like false & true, light & dark, being & nonbeing) have no independend existence, but exist only in their unity of opposites.

Further, and as seen by dialectics, these union of opposites form a higher unity, so the unity of being & nonbeing is just becoming.

See for example Hegel, Science of Logic, Doctrine of Being:
http://www.marxists.org/reference/archive/hegel/works/hl/hlbeing.htm

 

[heusdens]

Ok, you may be right with the wording. What I was trying to point out is that the claim of SpaceTiger is correct, as long as you take the definition of singularity that is given within general relativity. This definition can be formulated in a very precise an formal way and there are singularity theorems that prove that the universe must have had such a singularity in its past. This is actually the mainstream view.

Yeah, but be aware that you might be deluded.
If I measure a tree that is growing, and calculate back to it's past, I might conclude that also that tree once was zero length. [let us suppose the observer here is an intelligent ant, who is able of measuring, but has no notion about how trees reproduce, etc. ]
But that ain't the case and can't be the case. Even if the model correctly predicts that!

The problem is that the model is sometimes seen as reality itself, if the model claimed that at one of it's limiting conditions something weird happens, people tend to think that such a thing was a real event.

Don't confuse the model with reality. Trees don't start their growth from zero length, neither does the universe start out from a singularity.

PS.
Even mainstream ideas can be wrong.
If you would ask any person where the world came from or came into being, considering the fact that there is a majority of people considering themselves religious, the answer would then be that 'God did it'.

 

[logomach]

Yeah, but be aware that you might be deluded.
If I measure a tree that is growing, and calculate back to it's past, I might conclude that also that tree once was zero length. [let us suppose the observer here is an intelligent ant, who is able of measuring, but has no notion about how trees reproduce, etc. ]
But that ain't the case and can't be the case. Even if the model correctly predicts that!

The problem is that the model is sometimes seen as reality itself, if the model claimed that at one of it's limiting conditions something weird happens, people tend to think that such a thing was a real event.

Don't confuse the model with reality. Trees don't start their growth from zero length, neither does the universe start out from a singularity.

PS.
Even mainstream ideas can be wrong.
If you would ask any person where the world came from or came into being, considering the fact that there is a majority of people considering themselves religious, the answer would then be that 'God did it'.

Just to chime in as a completely unrelated observer, I agree with your reasoning, Heusdens, and I found reading your perspective quite interesting. But the title of this thread is "Review of Mainstream Cosmology".

"Even mainstream ideas can be wrong" is not really a position that should be described and championed so much in this thread. Maybe you should start a new thread, perhaps entitled "Critique of Mainstream Cosmology"?

I, at least, would enjoy reading it. But I think you will agree that the topic is separate from the one that this thread was intended for.

 

[heusdens]

Just to chime in as a completely unrelated observer, I agree with your reasoning, Heusdens, and I found reading your perspective quite interesting. But the title of this thread is "Review of Mainstream Cosmology".

"Even mainstream ideas can be wrong" is not really a position that should be described and championed so much in this thread. Maybe you should start a new thread, perhaps entitled "Critique of Mainstream Cosmology"?

I, at least, would enjoy reading it. But I think you will agree that the topic is separate from the one that this thread was intended for.

Although my post relates to the physical theory and what it tells is about the world (and just to note, I am not an opponent of the BB theory as a scientific theory, I am just commenting on some commonly held beliefs and intepretations of the BB theory, which are not even part of the theory itself (there is nothing in the BB theory that says or states anything about the singularity - even if it digs up in the underlying theory - that it would have to be regarded as a 'real event'), but are misnoted common beliefs or interpretations of the BB theory, which seem to be pretty mainstream - it is not a comment that has to deal with the physics/cosmology itself, but the philosophical implications it might or might not have.

The comment is not just in regard to any specific physical theory, but in general on all physical theories. So, philosophy of science is in fact the forum it belongs to.

 

[Allday]

many responses

hi everyone, i just read thru this whole thing so this post might drag on a bit but i wanted to address some of the points brought up.

1. There is another piece of evidence for dark energy that is not mentioned very often. This paper (http://arxiv.org/abs/astro-ph/0307335) describes a measurement of the correlation between the integrated Sachs-Wolfe effect and the location of LRGs from Sloan. Basically photons falling into potential wells that are shrinking due to accelerated expansion come out with extra energy because the hill they have to climb out of isn't as steep as the one they went down. Scranton et al. measure a positive correlation between hot spots on the CMB and luminous red galaxies in Sloan.

2. As has been mentioned, the study of pop III stars is at a very early stage and little is known about their properties. Here is a recent paper detailing a simulation (http://arxiv.org/abs/astro-ph/0610174) Of course this is not as good as observation, but it is a good start and I believe that simulations will be necessary to help analyze the observational data when it comes in.

3. A promising way to get at some of the properties of pop III stars is to make maps of the nuetral hydrogen in the early universe and look for the dark spots (ionized regions). These observations have to be done with large radio telescopes because the 21cm line from that era has been redshifted into the radio. LOFAR and PAST are two such telescopes that have the potential to make these kind of measurements in the next couple years.

Here are some recent papers on reionization that I've found interesting ...

self regulated reionization
http://arxiv.org/abs/astro-ph/0607517

lecture notes on observations of the high Z univese
http://arxiv.org/abs/astro-ph/0701024

short description of LOFAR
http://arxiv.org/abs/astro-ph/0610652

Also I've read a couple people say something about the Universe decelerating in the present era? that's news to me.

 

[heusdens]

What is expansion of space? Is it really physical?

The most important idea behind the Big Bang theory is that, acc. to the general relativity theory, the metrics of space are expanding, which becomes observable in the form of an appearant redshift-distance relation (Hubble relation).

But what is in fact this concept of "expansion of space". Is it some physical event, with an underlying physical process explaining it, or what?

Space itself, when we see it from a "classical" point of view, is not a substance or something. The metrics of space in the classical sense are purely defined by measuring distances between bodies in space. Measuring "space itself" is something clearly out of the question. At least in our classical conception of space. In the classical conception of space, the expansion of space could not even be stated, we could only state that bodies distantiate themself in space, which is of course the same as stating that those bodies recede away from each other, and have therefore a nonzero velocity relative to one another.
In the classical sense a change of the metrics of space would not be a physical phenomena. In the same way as changing our rulers (units of measure) would not change anything physically. All physical phenomena stay the same when we would switch our units of measurement. (the only thing that would change were our textbooks of physics, which would have to be rewritten in the new measurement units). This is even true when we would have a "flexible" measurement unit (one that changes as a function of some other physical entity, for instance time; if we were to state that the unit of length were to double each year, then of course all physical interactions become much more difficult to calculate, yet the physical world itself does not change because of this weird choice of measurement unit).

Now quantum mechanics and relativity come into play, in which our classical concept of space ain't correct any more. We can adress physical properties to space. Electro magnetic theories also adress properties of space.
Also we know that space can't be exactly empty, acc. to the Heisenberg Uncertainty Principle.

So, my question then is, if we apply this to the concept of expansion of space, and reason that only the changing metric does not change anything physically (which is the case in classical point of view), what physical changes take place in space that would cause space to expand.
It can't just be a change in the metric or scale factor, as they are not themselves something physical (i suppose), or are they?

In a theoretical situation of a universe as large as ours, and we have two very distantiated objects (like some billion light year), how can we ever state that the appearant increase in distance (which we somehow measure) is due to expansion of space, instead of a relative velocity (objects receding from one another)?

Sometimes this is misnoted as that in one case, the two very far apart objects are not moving (relative to surrounding space), and therefor the appearant recission speed is due to expanding of space, and in the other case, the objects do recede from each other (they are moving relative to surrounding space).

Yet, this whole explenation is somehow inconvincing, since it makes use of the concept of "moving relative to surrounding space". Acc. to relativity itself , this is not a valid concept, since we can only state something about objects moving relative to each other (and not relative to space itself!).

Am I misconceiving something, or how is it that all of a sudden we must make statements to explain what expansion of space really is (and how it differs from normal relative motion), which uses concepts which by the very same theory that would cause this effect in the first place, is simply not a valid concept.

So either the motion of an object relative to space itself is a valid concept, or not. If it is not a valid concept, then how do we explain the difference between simple motion of objects relative to each other (very far remote objects) and the expansion of space?

If it is a valid concept, then how can we calculate our relative motion to surrounding space. Perhaps relative to the CMBR?

 

[SpaceTiger]

the standard assumption is that the universe had a creation event and expanded from a singularity to its present size. Such a distant extrapolation can't possibly be verified by the current observations, but we can safely say that the universe expanded from a much smaller size than its current one.

Seems fine to me. Standard models of the universe use a simple GR model with a beginning and an end, the beginning marked by a singularity. Nobody views this as scientific fact, just an approximation that covers the essential features that are currently verifiable by experiment. Eternal inflation is possible, as is a quantum gravity removal of the singularity, but I didn't feel it was appropriate to get into these details in a basic BB summary. I think you're just picking at semantics.

 

[Allday]

Heusdens,

let me address some of your statements.

"Space itself, when we see it from a "classical" point of view, is not a substance or something. The metrics of space in the classical sense are purely defined by measuring distances between bodies in space. Measuring "space itself" is something clearly out of the question."

The fact that space is expanding was deduced from measurements of the distances to galaxies and supernovae, and the fact that their recession velocity increased the further away they were.

If you want to explain this fact by postulating that the galaxies are simply moving through a static space away from us and that space is not expanding, then we are at the center of this "explosion" and occuppy a very special place in the universe. Also in this scenario the galxies that are distant enough will be moving faster than the speed of light. Expanding space solves both of these problems.

special and general relativity introduced the concept of a dynamic space-time that can curve and stretch. In some sense this is the assumption of GR and as its predictions are verified to a higher and higher degree you should think of space in this sense as opposed to the static sense.

If you are looking for the answer to the question "what exactly is spacetime and where does it come from?" then you will have to look beyond GR.