Is the Universe Really Expanding?

[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.