Does Space Expand? What Do You Think?

[cristo]

Your assumption of a 'big crunch' would be true if you assumed a 'big bang'

What do you mean by this? Are you meaning to say that if we assume a big bang, then this implies a big crunch in the future? If you are, then this is not true!

 

[Old Smuggler]

The point is though that there is not expansion to 'overcome' since the 'expansion' is merely the result of the metric formed by a homogeneous and isotropic mass distribution. If the mass dosn't obey these principles we shouldn't be surprised that we don't see any 'expansion'.

But the expansion is seen on scales where the mass distribution is far from homogeneous and isotropic - namely in the vicinity of or even within the local group of galaxies. Moreover, the Hubble law is obeyed tightly even at this (cosmologically speaking) local scale, and the local Hubble parameter is of the same order as the global one. This problem of "the quiet local Hubble flow" was first identified by Sandage, and it is still an unsolved problem.

For observational results and references to the original papers, see

T. Ekholm et al., Astron. & Astrophys. 368, L17 (2001) (astro-ph/0103090).

 

[Wallace]

Sure, so there is no harsh cut-off point where the metric goes from FRW suddenly to some other non-expanding form. I would have expected that the expansion would not be dominant on the scale of the local group, so that results is unexpected for me. Thanks for pointing it out, I will have to follow this up.

It remains that case though, that as the local region becomes more and more inhomogenous, the metric must have less and less 'FRWness' in it. The expansion most certainly does not occur on galactic scales or smaller, such as the scale of a solar system for instance.

 

[wolram]

Sure, so there is no harsh cut-off point where the metric goes from FRW suddenly to some other non-expanding form. I would have expected that the expansion would not be dominant on the scale of the local group, so that results is unexpected for me. Thanks for pointing it out, I will have to follow this up.

It remains that case though, that as the local region becomes more and more inhomogenous, the metric must have less and less 'FRWness' in it. The expansion most certainly does not occur on galactic scales or smaller, such as the scale of a solar system for instance.

For sure, it seems that some realistic idea are coming to the fore
.

 

[Wallace]

But the expansion is seen on scales where the mass distribution is far from homogeneous and isotropic - namely in the vicinity of or even within the local group of galaxies. Moreover, the Hubble law is obeyed tightly even at this (cosmologically speaking) local scale, and the local Hubble parameter is of the same order as the global one. This problem of "the quiet local Hubble flow" was first identified by Sandage, and it is still an unsolved problem.

For observational results and references to the original papers, see

T. Ekholm et al., Astron. & Astrophys. 368, L17 (2001) (astro-ph/0103090).

I've followed this up, it looks like this was a unfortunately timed paper, coming out just a month or two before the HST key project to measure H₀. This paper uses a best guess for H₀ of just over $$50 km s^{-1} Mpc^{-1}$$ substantially lower than the $$72 km s^{-1} Mpc^{-1}$$ found from the Hubble key project. This significantly changes the articles conclusions since it really shows that the local group Hubble diagram will not match the slope of the global expansion rate very well at all. I think this is closer to what I would have expected.

 

[hellfire]

You may be interested in the references mentioned in this post.

 

[Old Smuggler]

I've followed this up, it looks like this was a unfortunately timed paper, coming out just a month or two before the HST key project to measure H₀. This paper uses a best guess for H₀ of just over $$50 km s^{-1} Mpc^{-1}$$ substantially lower than the $$72 km s^{-1} Mpc^{-1}$$ found from the Hubble key project. This significantly changes the articles conclusions since it really shows that the local group Hubble diagram will not match the slope of the global expansion rate very well at all. I think this is closer to what I would have expected.

But they didn't measure the Hubble parameter; rather they took a value of 57 km/sMpc as
an input for their model. While it is reasonable to assume that the numbers would change
somewhat if a different input value were chosen, the main results should not be critically
dependent on this. That is, the linearity of the local Hubble law and the small local velocity
dispersion around it should still hold.

As Hellfire pointed out, the mystery of the quiet local Hubble flow could have an explanation in dark energy, suppressing the growth of velocity fluctuations. However, this explanation is not sufficient, since the parameter values necessary to achieve this would
be incompatible with other observational tests. For details, see

M. Axenides and L. Perivolaropoulos, Phys. Rev. D 65 127301 (2002) (astro-ph/0201524).

So what the observations of the velocity field of galaxies in the vicinity of the local group
is telling us, is that there is no obvious link between the expansion and the clumpiness of the Universe. Thus the fact that the Universe is even more inhomogenous on smaller
scales such as galaxies and planetary systems, is in itself not a sufficient argument
to conclude that the expansion cannot exist and be detectable on such scales.

 

[Hell_SD]

going back to the analogies...

...has anyone equated space expanding to a cell dividing ?

imagine a single red cell dividing in two. Those 2 red cells then split, but instead split into white cells and every division there after is also white, such that there are only ever 2 red cells. The faster the white cells divide exponentially, pushes the 2 red cells ever further apart and faster...

...it'd make a great cartoon i reckon, cos in the bigger picture you'd see that the 2 red cells are moving apart faster than the rate at which any single white cell is dividing at

now when there weren't many white cells separating the 2 red cells someone living on one red cell shined a torch in the direction of someone on the other red cell, but as the distance between the two cells increased, the light from the torch had to travel further and by a strange coincidence turned red itself...

 

[Wallace]

This doesn't seem like a very helpful analogy.

 

[Hell_SD]

...so its no more or less helpful than the raisin and balloon one. Care to explain why ?

watching cells divide would make for a better cartoon though. Theres just something about how they do that and how space is created which i find intriguing...

...like reconstituting matter from one thing to another in real time

 

[marcus]

...less helpful than the raisin and balloon one. Care to explain why ?

watching cells divide would make for a better cartoon though.

I agree with Wallace about cells analogy not helpful---think it is very likely to be harmful to understanding.
with pennies on the balloon, you go thru the exercise of imagining the pennies all drifting apart and then the all-important final step is IMAGINE THE BALLOON ISN'T THERE.

Picture the pennies all receding from each other, their separation distances increasing by some percentage with each time tick

and then imagine the rubber skin isn't there!

============

likewise if you use the raising dough analogy you should, at the end, imagine that the dough is not there

all there is is increasing distances between raisins
=============
the pennies, or the raisins, are the analogs of GALAXIES, where are the galaxies in your analogy?
=============

I would say your "cells" analogy is mind-damaging and pernicious because it tends to mislead people into thinking that space is a "something" like cells. and it gets mentally messy when you imagine finally that all the cells vanish from existence, then what is left?

 

[Hell_SD]

I would say your "cells" analogy is mind-damaging and pernicious because it tends to mislead people into thinking that space is a "something" like cells. and it gets mentally messy when you imagine finally that all the cells vanish from existence, then what is left?

fair enough but surely it's no more messy than the actuality of distances between objects increasing faster than the speed of light without the objects themselves moving...

...and considering there is supposed to be a universal speed limit based on how fast light travels through space which supposedly can't be exceeded or asking the messy question of where does this extra space come from ?

i just like the organicness of cells dividing so could you please tell me why space isn't like cells ? Whats to stop there being a unit of something which appears and vanishes at superluminal speed leaving space as it's residue ?

 

[marcus]

fair enough but surely it's no more messy than the actuality of distances between objects increasing faster than the speed of light without the objects themselves moving...

...and considering there is supposed to be a universal speed limit based on how fast light travels through space which supposedly can't be exceeded or asking the messy question of where does this extra space come from ?
...

you sound pretty confused HSD. I don't know what to advise you or where you could begin trying to get straightened out. I can't take time to try but maybe someone else can help you.

you are mixing up SR and GR. the speed limit in SR does not apply globally to rates of distance change in GR. you are mixing up recession speed with relative motion within one local framework

there is NOT supposed to be a universal speed limit that applies to recession speeds. if someone told you that they lied, or were mistaken, or maybe you just misinterpreted. (all that kind of talk is with SR only and SR has a very restricted applicability, if you apply it outside its area of validity it gives nonsense)

there is NO problem of "where extra space comes from" because space is not a material substance-----it is just the distances between things a web of geometric relations----you DONT HAVE TO MAKE MORE.

you mistakenly assert that this is messy, I do not see that anything i am telling you is messy.

between things that are not bound together there is no reason to expect distances to stay the same, in fact large scale distances tend to increase at a certain percentage rate----there is nothing messy or funny about this and no reason to expect it to not be the case---and in fact it is how the world is.

largescale distances typically increase by 1 percent about every 140 million years-----so naturally the longer they are to begin with the faster they increase in absolute terms---that's how percents work.
so naturally some long distances are increasing faster than any given speed, you can always pick a distance long enough that it is increasing faster than c.

Unless you immediately come around and stop talking nonsense i will have to give up on you, and I do not expect you to. So maybe someone else will be willing to try to explain. I hope so.

 

[Hell_SD]

distances between objects increasing faster than the speed of light without the objects themselves moving

c'mon marcus that is a hell of a concept to try and wrap your head around...

the fact that the objects aren't moving apart at superluminal speed but that space is growing between them at faster than lightspeed while maintaining that nothing can go faster than c

...sounds messy so surely I have a right to be confused ?

 

[Nick666]

Well there are theories that say space is nothing. So, it might be that space can move with speeds > c.

 

[BoomBoom]

Dividing cells is an absolutely horrible analogy...no offense. It seems to me that the main point of the discussion was to try and describe increasing distances without implying that space itself was driving it apart. Describing space as white cells in this analogy defeats that purpose and implies that space is like matter.

I've been struggling to remember what this gadget is called (perhaps someone can help me out) but if anyone has ever seen these toys that are a lattice type structure with moveable joints that when fully contracted is like a ball. When you pull it apart, it expands in size uniformly to many times it's original size. If you can pictue this and also picture each joint as a galaxy, I've always thought that would be a decent visualization of an expanding universe...:uhh:

 

[Wallace]

distances between objects increasing faster than the speed of light without the objects themselves moving

c'mon marcus that is a hell of a concept to try and wrap your head around...

the fact that the objects aren't moving apart at superluminal speed but that space is growing between them at faster than lightspeed while maintaining that nothing can go faster than c

...sounds messy so surely I have a right to be confused ?

Superluminal recession is something that easily causes confusion. The issue is that velocity is the rate of change of distance and with but in GR neither distance or time are trivial concepts. The distance that we use in FRW cosmology is most commonly the proper distance, and if we look at the rate of change of this with 'cosmic time' (the time as measured by co-moving observers) we note that this velocity exceeds c for sufficient distance from the origin.

All this tells us is how one of many different possible definitions of distance change. If you accept the FRW metric then you have to live with that fact. Other metrics that use difference co-ordinates but make the same physical predictions do not contain any apparent superluminal recession.

You need to realize that your everyday notion of distance doesn't work on cosmological scales, and hence your everyday intuition is useless.

 

[wolram]

If we want to measure distances in our own galaxy, we can use a rigid ruler,
and we can get quite an accurate measurement from some point (earth) +,- bodies motion (orbits) within the galaxy.
If we want to measure between galaxies, we have to throw away the rigid ruler and use a flexible one, the rate of flex (stretch) depends on distance from our vantage point, the rate of stretch increases with distance.
The stretch factor is not considered as a (speed) just an increase in the total volume we, the universe exists in.
This sounds total nonsense, but may be i have the above wrong?

 

[wolram]

Another paper.

arXiv:0707.1350 (cross-list from gr-qc) [ps, pdf, other]
Title: Cosmological expansion and local physics
Authors: Valerio Faraoni, Audrey Jacques (Bishop's University)
Comments: 17 pages, LaTeX, to appear in Phys. Rev. D
Subjects: General Relativity and Quantum Cosmology (gr-qc); Astrophysics (astro-ph)
The interplay between cosmological expansion and local attraction in a gravitationally bound system is revisited in various regimes. First, weakly gravitating Newtonian systems are considered, followed by various exact solutions describing a relativistic central object embedded in a Friedmann universe. It is shown that the ``all or nothing'' behaviour recently discovered (i.e., weakly coupled systems are comoving while strongly coupled ones resist the cosmic expansion) is limited to the de Sitter background. New exact solutions are presented which describe black holes perfectly comoving with a generic Friedmann universe. The possibility of violating cosmic censorship for a black hole approaching the Big Rip is also discussed.

 

[pervect]

The issues arising with measuring the distance to distant galaxies are not really related to the problem of constructing a rigid ruler (though of course a rigid ruler is an idealization - the most rigid ruler that's actually possible would be counting the wavelengths on a lightbeam, which is only approximately rigid. This is essentially a limiting case when the velocity of sound in the ruler is equal to 'c'.)

Rather, the issue with measuring distance it is the problem of "what curve" to measure the length of. The usual notion of distance proceeds as follows - one takes space-time, and separates it out into space and time. One then measures the distance in some hypersurface of constant time.

Unfortunately, the split of space-time into space and time is in general arbitrary and depends on the choice of coordinates.

Note that the usual notion of distance ("proper distance") defined in this manner (measuring the distance along a curve of constant cosmological time) does not actually measure the distance along a straight line (or the equivalent of a straight line in a curved space-time, a space-like geodesic).

This is because a curve of constant cosmological time connecting two points in a FRW universe is not a "straight line", i.e. it is not a geodesic.

 

[smallphi]

The so called 'physical' distance in cosmology doesn't have the status of invariance (independence of coordinate system) like the line element ds^2 because the 'physical' distance is a coordinate quantity.

It can be measured if you have a gazillion of comoving observers in straight line from you to the point where you want to measure. Those observers have clocks that all show the 'cosmic' time which is a coordinate time in FRW metric. You simply tell them at given fixed cosmic time to record the distance to the closest comoving observer. Assuming that distance is small it will be the line element ds. Then you tell them to report the distances to you and you add them up getting the coordinate distance to the object at that time. Basically this is integration of ds over a specified curve which as pervect pointed out is NOT a geodesic.

 

[chrisina]

I recommend the following article :

Misconceptions about the Big Bang
Baffled by the expansion of the universe? You're not alone. Even astronomers frequently get it wrong
By Charles H. Lineweaver and Tamara M. Davis

http://www.sciam.com/article.cfm?articleID=0009F0CA-C523-1213-852383414B7F0147


As far as explaining a complex issue in a precise, didactic and relevant manner, I haven't found a better paper so far...

 

[dilletante]

"photons are massless but they do have energy"

I am sure this is a stupid question, but if M=E/c^2 and E is nonzero, how can M be zero?

dilletante

 

[russ_watters]

It can't. That equation has nothing to do with photons.

 

[Wallace]

The full equation for the relativistic energy of a particle is (well not completely general, but more verbose than the famous short version) is:
$$E^2=M^2c^4 + p^2c^2$$
where p is the momentum of the particle

This reduces to $$E=Mc^2$$ when the momentum is small (i.e. particle isn't moving or is not moving very fast). Photons do have a momentum, but not rest mass, so for them

$$E^2=p^2c^2$$
$$E=pc$$

 

[dilletante]

Thank you Wallace, your explanation is concise and clear.

 

[dilletante]

It could be that "space expands" is an unfortunate popularization choice of words because space is not a substance that can expand.

This sounds very categorical but is there really unanimous agreement with this? We assume space is not a "substance" but yet it contains energy, can create virtual particles, and there seems to be agreement that you can curve it to explain gravity -- why then can you not "stretch" it?

dilletante

 

[mattex]

This thread is extremely interesting...but at the end of the day, it proves one thing - that we (layperson AND scientific professional) don't really understand anything!? It all amounts to fumbling with words. What we need is a model we can picture in our heads, something we can visualize. If the universe is beyond human understanding/visualization, why do we bother?

Space is "nothingness" but it is stretching and is infused with "virtual matter" popping in & out of existence...? Huh? The universe came out of "nothingness"? Huh?

Nothing makes sense - or should I say, "nothing" doesn't make sense. I find the universe we are in disturbing - frightening!

We are "made of star dust" and are intimately/subatomically connected to this universe, and yet we seem like aliens/strangers/outsiders to it all...

Plus, there seems to be so much disinformation out there, so much noise clogging up the signal... Stark contradictions, ambiguities, unspoken assumptions... It's hard/impossible to find answers...

From a layman's point of view (me), I am completely unsatisfied with our state of knowledge today.

Damn, I'm in a miserable mood today! :yuck:

 

[marcus]

This thread is extremely interesting...but at the end of the day, it proves one thing - that we (layperson AND scientific professional) don't really understand anything!? It all amounts to fumbling with words. What we need is a model we can picture in our heads, something we can visualize. If the universe is beyond human understanding/visualization, why do we bother?

Space is "nothingness" but it is stretching and is infused with "virtual matter" popping in & out of existence...? Huh? The universe came out of "nothingness"? Huh?From a layman's point of view (me), I am completely unsatisfied with our state of knowledge today.

a good bit of the problem comes from using the English language instead of math.
we all expect "understanding" to correspond to English sentences and dictionary definitions

IndoEuropean root language goes back to what, before 5000 BC probably.
You expect distance between two stationary locations, village A and village B, to stay constant.

in the GR model of dynamic geometry it CANNOT stay constant, unless A and B are bolted onto some material framework. If it is galaxy A and galaxy B , not bound together, it MUST change. It is of the nature of distance to change and the GR main equation describes how.

But this is not part of the 10,000 year old linguistic tradition to which English belongs. So we think it is confusing.

So be of good cheer Mattex. It is just some obsolete language interfering with your contentment.

 

[mattex]

No, it's not increasing distance between 2 galaxies that is the problem. It is increasing distances between ALL galaxies! How can this be? Do we really know?

It's damn frustrating! Nothing adds up! Does the universe curve round on itself? Or is it flat & infinite? What is going on? Why aren't people disturbed?

 

[marcus]

No, it's not increasing distance between 2 galaxies that is the problem. It is increasing distances between ALL galaxies! How can this be? Do we really know?

I can't tell you the REAL TRUTH :rofl: I can just tell you how it looks. The universe looks amazingly uniform, and same largescale scattering of galaxies in whatever direction.

do some simple picture-math. make a uniform distribution of dots on a piece of paper and suppose that you are on one dot and the distances to all the other dots are increasing by 1 percent each day

then you will find that the distances between any two dots must be increasing 1 percent per day.

it is like similar triangles in 9th grade trig. If the distance from A to B increases 1 percent and the distance A to C increases 1 percent then if the triangle stays same shape the distance from B to C must also increase by same proportion.

It's damn frustrating! Nothing adds up! Does the universe curve round on itself? Or is it flat & infinite? What is going on? Why aren't people disturbed?

I am not sure you are serious It doesn't matter to what we were talking about whether space is finite or infinite. Locally the expansion looks the same. We can tell that if it is finite then it is very very large.
All the evidence points towards expansion continuing indefinitely. What's to worry?
Why be disturbed?

Are you sure you are not exaggerating?

 

[wolram]

I think a lot of the confusion about expansion would stop if we had some idea of the total energy the universe has within it, and what these energies do, pull, push.

 

[mattex]

What's to worry? Why be disturbed?

Are you sure you are not exaggerating?

Well, I'm one of those poor sods who suffers this universe - something in my genes, perhaps. I've never really understood why others don't have this emotional reaction also? I've always put it down to self-denial, or something...

So no, I'm not really exaggerating when I say this universe terrifies me at times. What is ultimately going on? It's cruel if you ask me.

 

[marcus]

...something in my genes, perhaps. I've never really understood why others don't have this emotional reaction also? ...this universe terrifies me at times...

I am delighted that you have an emotional reaction to the universe and are open about it.
We can't talk too much about our emotional relations to nature and the universe, here, or the thread will be moved to some more literary or philosophical forum. So be restrained.

However I think it is human nature to relate emotionally to the universe and I do that myself, but with a different attitude. I deeply love it. I totally admire its lawfullness. I am also very glad that in the view of modern cosmology the universe is not expected to eventually collapse (I thought it was very sad when they were predicting a big crunch.)

According to modern understanding of gravity, the ONLY WAY IT CAN AVOID COLLAPSE is to continue expanding. Fortunately this is what it seems inclined to do!
Moreover it is expanding in a rather gentle regular way----the horror stories which sensationalist fringe scientists make up about catastrophic expansion or "big rip" are not accepted by mainstream. Honestly we could hardly have it better.

And we are only beginning to find out the real laws of physics. All we have now are rough approximations (though even they are elegant) and must always be trying to improve them, to get closer to the real laws.

that is my attitude. You have yours, which is one of dread. Any attitude, if it is genuine, is valid, I believe. There is no "correct" one.

Be well, and try to ask only scientific questions in the scientific forums.

 

[Chronos]

All our knowledge of the universe is indirect. The long and the short of it is we think it's expanding because of redshift - a fairly well established scientific principle called the doppler effect. Wipe that concept from the blackboard of science and most modern models of the universe are dead on arrival. The problem is, it does not easily erase. All other independent indicators of distance strongly agree with the redshift interpretation. Halton Arp has railed against the redshift interpretation for many years, but his arguments have not been well received. That does not falsify his claim, but places it squarely within the fringe camp.