Does Space Expand? What Do You Think?

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The discussion centers on the interpretation of the universe's expansion, questioning whether space itself is expanding or if it's merely a kinematic effect. Cosmologists often describe redshifts as a result of expanding space, but some, including John Peacock, argue that this interpretation can be misleading. The conversation highlights a thought experiment involving a galaxy held stationary in an expanding universe, illustrating that its behavior contradicts the common understanding of expansion. Participants suggest that focusing on increasing distances rather than expanding space might clarify misconceptions. Ultimately, the debate reflects ongoing discussions about the nature of distance and expansion in general relativity.
  • #91
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.
 
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  • #92
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...
 
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  • #93
"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
 
  • #94
It can't. That equation has nothing to do with photons.
 
  • #95
The full equation for the relativistic energy of a particle is (well not completely general, but more verbose than the famous short version) is:
<br /> E^2=M^2c^4 + p^2c^2<br />
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

<br /> E^2=p^2c^2
<br /> E=pc<br />
 
  • #96
Thank you Wallace, your explanation is concise and clear.
 
  • #97
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
 
  • #98
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!
 
  • #99
mattex said:
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.
 
  • #100
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?
 
  • #101
mattex said:
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 :smile: 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 :smile: 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?
 
  • #102
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.
 
  • #103
marcus said:
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.
 
  • #104
mattex said:
...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. :smile:
 
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  • #105
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.
 
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  • #106
are there three fixed points
 
  • #107
Chronos said:
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.

The wavelength redshift of receding light is understandable. Inertia from the big bang must have propelled spacetime very fast. Looking back, we see light receding very fast into the past, and a lot of redshift. But how does that translate into present-tense "accelerating expansion"?
 
  • #108
In 1982, I purchased a publication advertised in the magazine Scientific American; a book entitled The New Physics of Symmetrical Energy Structures (ISBN 0-910122-67-9). The –not for profit publication, The Theory of Symmetrical Energy Structures (SES) in a Megadimensional Cosmology and was produced by the Alpha Omega Research Foundation, Inc. The theory derives Planck length and all the physical constants, to the accuracy that we know π ! The Theory takes a single nucleon placed in the Earth's center and derives a new value for the gravitational constant G!
In October 1983 the Conference Generales des et Measures (CGPM) met in Paris to change the definition of the meter, where it was suggested they delay their decision until the theory had a proper evaluation.
My question is, why hasn’t there been a scientific revolution of unprecedented magnitude taken place? With this theory the all the forces have been unified, including gravity!
 
  • #109
ummani said:
My question is, why hasn’t there been a scientific revolution of unprecedented magnitude taken place?

Because this book is crackpottery.
 
  • #110
Wallace said:
Sure, that's a pretty clear argument you present and I agree with you. However, there are many, including Peacock, as well as Martin Reese and Steven Wienberg who wrote a New Scientist article about this some years ago who contend that thinking in this way misleads you and it's better to just think kinematically.

The classic test case is this. Imagine you are in an expanding universe and hold a galaxy at rest with respect to you but at a cosmological distance. According to Hubbles law a galaxy at that distance should be receding but you prevent this by using a chain or rockets or something to hold it in place. If you let go of the galaxy, what does it do?


[THINK ABOUT THIS FIRST THEN READ ON]













The answer you may assume is that since space is expanding the galaxy will start moving away from you, joining the Hubble flow eventually. However in a decelerating (but still expanding) universe the particle actually comes towards you! If you think about it it becomes clear why but Peacock argues in the link I posted that it is the idea of expanding space that leads to these misconceptions and hence should be abandoned.

Stretching space and kinematical motion relative to space are not the only possibilities that can be conjured up to explain expansion -if space is granular on some scale, and the number of units per volume is increasing - then a tethered galaxy would pick up the Hubble flow when released because of the increasing number of spatial entities (whatever they might be - eg mico vortices and the like) - which multiply in proportion to the volume - consistent with exponential expansion internally driven rather than externally motivated
 
  • #111
Yogi, please explain a little more about your idea. Are you saying that the increase in the number of granular units per volume creates a sort of positive pressure which in effect causes them to repel each other and solid matter, pushing everything apart? If so does that positive pressure bring additional gravity with it, like mass-energy does in GR? And how can positive pressure cause things to move apart if there isn't a pressure gradient somewhere (i.e., an outer 'edge' to the granule-filled universe, surrounded by a region devoid of granules)

Or are the granules just an instantiation of dark energy, characterized by negative pressure, which causes a sort of mutual anti-gravitational repulsion (as well as adding gravity)?

Dark energy of course is an explanation for the recent acceleration of the expansion rate, but it is not an explanation for the "original" expansion which was decelerating due to gravity until dark energy eventually became dominant. (Other than of course attributing inflation to some more powerful form of dark energy).

A model that requires both proliferating granules AND proliferating dark energy seems even more perplexing than the standard model.

I also note that the published analyses of the "tethered galaxy" exercise describe the idea of the untethered galaxy "picking up the Hubble flow" as a fallacy. Rather, unless dark energy dominates, the untethered galaxy moves counter to what the Hubble flow intuitively would cause. (Eventually the galaxy's peculiar velocity decays to the point where it arguably asymptotically "rejoins the Hubble flow", but this may be on the opposite side of the origin.) But you know that, so maybe I'm misinterpreting your comment.
 
  • #112
nutgeb said:
Yogi, please explain a little more about your idea. Are you saying that the increase in the number of granular units per volume creates a sort of positive pressure which in effect causes them to repel each other and solid matter, pushing everything apart? If so does that positive pressure bring additional gravity with it, like mass-energy does in GR? And how can positive pressure cause things to move apart if there isn't a pressure gradient somewhere (i.e., an outer 'edge' to the granule-filled universe, surrounded by a region devoid of granules)

Or are the granules just an instantiation of dark energy, characterized by negative pressure, which causes a sort of mutual anti-gravitational repulsion (as well as adding gravity)?

Dark energy of course is an explanation for the recent acceleration of the expansion rate, but it is not an explanation for the "original" expansion which was decelerating due to gravity until dark energy eventually became dominant. (Other than of course attributing inflation to some more powerful form of dark energy).

A model that requires both proliferating granules AND proliferating dark energy seems even more perplexing than the standard model.

I also note that the published analyses of the "tethered galaxy" exercise describe the idea of the untethered galaxy "picking up the Hubble flow" as a fallacy. Rather, unless dark energy dominates, the untethered galaxy moves counter to what the Hubble flow intuitively would cause. (Eventually the galaxy's peculiar velocity decays to the point where it arguably asymptotically "rejoins the Hubble flow", but this may be on the opposite side of the origin.) But you know that, so maybe I'm misinterpreting your comment.

I really don't have a particular model in mind - maybe what I described is something in the nature of a plenum of quasi-static neutrino like angular momentums - jostling like atoms in a gas, and growing in number wherever stresses permit - there are many ways to envision a granular space - perhaps even with individual dimensions on the order of the Planck scale. My point was that stretching space and velocity wrt space are not exhaustive alternatives. As you might guess, I do have difficulties with the ad hoc standard model - I think its not good to get boxed in as far as explanations go given our inability to relate the tenants of the standard model to a physical form -in fact I don't even think it is altogether wise to blame gravity on curvature - while there appears to be little doubt that curvature exists - it may be consequent rather than causal - gravity may be an inertial reaction and curvature may be the evidence rather than the cause. GR started out with wrong presumptions, a positively curved static universe. To explain the G force - Einstein proposed that inert mass curves static space and time - well maybe it does -The standard model postulates dark matter and dark energy in amounts that agree with what appears to be good data - but the mechanism of expansion is not known - Prior to 1998 almost everyone was convinced the q = 1/2 universe had to be correct - so for me its worth exploring querky alternatives - how boring it would be if we already had all the answers.
 
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  • #113
OK, I see. Yes there are still lots of fundamental questions about the standard model, so it is possible it will be overturned or significantly modified in the future.

I share your uneasiness about the assertion that gravity curves spacetime. It is a wonderful mathematical model and makes for very accessible graphical representations. But as you say the converse doesn't seem to be ruled out either. As has been mentioned before, an arrow flying through a crosswind could be described as traveling through curved spacetime, merely following a geodesic. But is that really a meaningful physical statement in this context? It may just come down to whichever approach makes the metric easier to calculate.

GR seems to justify the application of highly abstract methodologies because of its weirder aspects -- time dilation and spatial curvature. Either something is happening to this entity called 'spacetime' at a very deep level, or else we're missing a cornerstone of how to otherwise describe these observed phenomena.
 
  • #114
nutgeb said:
OK, I see. Yes there are still lots of fundamental questions about the standard model, so it is possible it will be overturned or significantly modified in the future.

I share your uneasiness about the assertion that gravity curves spacetime. It is a wonderful mathematical model and makes for very accessible graphical representations. But as you say the converse doesn't seem to be ruled out either. As has been mentioned before, an arrow flying through a crosswind could be described as traveling through curved spacetime, merely following a geodesic. But is that really a meaningful physical statement in this context? It may just come down to whichever approach makes the metric easier to calculate.

GR seems to justify the application of highly abstract methodologies because of its weirder aspects -- time dilation and spatial curvature. Either something is happening to this entity called 'spacetime' at a very deep level, or else we're missing a cornerstone of how to otherwise describe these observed phenomena.

Exactly - we have eased into the idea that inert matter affects static space - but we can't confirm that conclusion because we are living in a universe where matter is inertial and space is not only not static, rather it is dynamic in some sense - so what we observe might be the curvature or distortion of a momentum flow produced by interaction with the G field. Einstein did a great job with what was available at the time - some of which was wrong, yet the theory survived. I wonder what he would have concluded if he had any idea that the space was accelerating.
 
  • #115
What proof is there that space expands at all
That is just an assumption since you can't see it touch it or sense it in any way.
 
  • #116
Whitewolf4869 said:
What proof is there that space expands at all
That is just an assumption since you can't see it touch it or sense it in any way.

As has been said in this thread several times, the prime evidence is the spectral redshift of distant sources. The idea that this could be caused by some mechanism causing energy loss en route (a.k.a. Tired Light) is ruled out because the redshift is independent of frequency and because supernova light curves exhibit stretching compatible with the redshift (i.e. they are farther away by the time the light fades). Modelling it as a simple Doppler shift doesn't fit the gravitational models, the distance between us and a galaxy at z=2 should be increasing by more than 1 light year per year if they are correct. However, envisaging it as "expansion of space" fits perfectly, including several other tests. We adopt what works as our model and continue to look for discrepancies to improve it.
 
  • #117
So you are saying that expansion will eventually be the end of all because the vacuum of space will overcome gravidly.
 
  • #118
Whitewolf4869 said:
So you are saying that expansion will eventually be the end of all because the vacuum of space will overcome gravidly.

That's the way it looks at present. The effect of gravity slowing expansion depends on the average density of matter in space. As stuff gets farther apart, that density falls as the cube of the expansion. Vacuum energy (or whatever "dark energy" turns out to be) on the other hand seems to have constant density, every cubic metre of vacuum is the same as every other cubic metre. The two were roughly equal about 8 billion years ago and since then the rate of expansion has been increasing. As time goes on, the matter keeps gets thinner and that trend isn't going to reverse. Expansion has already won the contest.
 
  • #119
#1 The theory is all wrong vacuums don't expand
#2 So comparing space to balloons or bread is foolish
#3 Then an equation is created to explain what we see through our telescopes
#4 When that doesn't fit more equations are created to explain the faulty equations
because there isn't enough mater to explain movement and we add dark mater and dark energy and dark flow ignoring space itself the whole time
How do we know that space wasn't already here and mater and time was created by the negative energy of space and the space time distortion associated with mater is nothing more than surface tension.
 
  • #120
Don't be insulted I am just rattling some chains
If Einstein hadn't dropped out of high school and become a free thinker who knows where we would be with this right now
 

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