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Expanding Universe Question


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Sorry for slow response - I posted last night but somehow it disappeared.... maybe due to dark energy. Unfortunately work sleep and thinking seem to be mutually exclusive activities - for me at least.

I'm grateful for the further thoughts, but I don't think any of this addresses to my key question. Let me try and express myself differently.

1. We look at objects in space around us and note that they all seem to moving away from us.

2. Of course this (probably) doesn't mean we are at the centre of our universe - merely that the chunk of observable space we occupy is expanding in every direction.

3. Because distant objects/events move away from us they (eg SN1a's) are red-shifted.

4. SN1a's which are further away have a greater degree of red-shift.

5. SN1a's with red-shift of z=0.8-1.2 appear fainter than expected (Perlmutter et al, 1998) , whereas SN1a's with greater red-shift z=1.7 appear brighter than expected (Riess et al 2001).

6. From this its inferred that in the early stages of the universe, expansion was slowing, whereas now it is accelerating (due to dark energy)

My point is this:

If, (capital letters, underlined and italics) if all matter started from the same locus, with the same acceleration, it would all have traveled the same distance from that locus. Clearly this isn't so. (Either because there wasn't a point origin or because the acceleration wasn't uniform)

If on the other hand matter was formed from a plasma mass of finite size, then we don't know that the SN1a's we are measuring had the same starting point as us - they probably didn't.

Or put another way the reason that SN1a's with z=0.8-1.2 may appear dimmer than expected, ie be farther away than expected, may be precisely because they started from a point some distance away from us - they had a "head-start" if you like.

I'm happy to entertain objections to this - maybe to the point of being proved completely wrong. But hopefully someone can at least see the point I'm trying to make!!

 

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50 minutes ago, Tommohawk said:

Sorry for slow response - I posted last night but somehow it disappeared.... maybe due to dark energy. Unfortunately work sleep and thinking seem to be mutually exclusive activities - for me at least.

I'm grateful for the further thoughts, but I don't think any of this addresses to my key question. Let me try and express myself differently.

1. We look at objects in space around us and note that they all seem to moving away from us.

2. Of course this (probably) doesn't mean we are at the centre of our universe - merely that the chunk of observable space we occupy is expanding in every direction.

3. Because distant objects/events move away from us they (eg SN1a's) are red-shifted.

4. SN1a's which are further away have a greater degree of red-shift.

5. SN1a's with red-shift of z=0.8-1.2 appear fainter than expected (Perlmutter et al, 1998) , whereas SN1a's with greater red-shift z=1.7 appear brighter than expected (Riess et al 2001).

6. From this its inferred that in the early stages of the universe, expansion was slowing, whereas now it is accelerating (due to dark energy)

My point is this:

If, (capital letters, underlined and italics) if all matter started from the same locus, with the same acceleration, it would all have traveled the same distance from that locus. Clearly this isn't so. (Either because there wasn't a point origin or because the acceleration wasn't uniform)

If on the other hand matter was formed from a plasma mass of finite size, then we don't know that the SN1a's we are measuring had the same starting point as us - they probably didn't.

Or put another way the reason that SN1a's with z=0.8-1.2 may appear dimmer than expected, ie be farther away than expected, may be precisely because they started from a point some distance away from us - they had a "head-start" if you like.

I'm happy to entertain objections to this - maybe to the point of being proved completely wrong. But hopefully someone can at least see the point I'm trying to make!!

 

Ok, again, you assume that matter started at certain position with some acceleration.

This is "explosion model" where stuff explodes from a single point and flies of into empty space. But the problem with that is that there is no "empty space outside of our universe". Imagine following scenario:

You are standing opposite another person. Distance between you two is 2 meters. If you were to move you would feel acceleration, so would the other person. None of you feels acceleration but somehow another two meters get inserted between you two. Space itself is being stretched. It looks like you moved further away from each other because you are now at 4 meters apart. But you felt no acceleration, nor did the other person. So you did not "explode" apart from 2m to 4m distance, you did not move at all, but distance increased because space stretched. Now imagine third person, so three of you are now in a line. You are to the left and 4 meters from you is the second person and the third is 4 meters away from second. Same thing happens. Somehow, space is inserted / stretched without any of you moving / feeling acceleration. There is now 6 meters between you and middle person and 6 meters between middle and far right person. You conclude that second person moved another 2m per unit time (so speed is 2) but third person moved from second 2 meters and second moved from you 2 meters so third moved a total of 4 meters from you, you conclude that third's person speed is 4 meters per unit time - so speed is greater because distance is greater.

So you see by having space stretched / inserted between objects - they can appear to move / have speed relative to each other with property that further the object is - faster it is "moving" - but none of these objects feels the "speed" / acceleration of any kind. But what about red shift of light you may ask? it has to do with Doppler effect related to the actual speed thru space. Not really - it turns out that stretching of the space which causes "stretching" of the waves of light is exactly the same as would be Doppler shift if object were moving. So there is no real distinction between space being stretched and things actually moving in red shift. There is however distinction in feeling initial acceleration.

This is why big bang is not "explosion" type event - things did not get accelerated into already existing space around it. Rather space started expanding "between" "stuff" (now here we enter uncharted territory, since we really have no idea what the "stuff" was in those moments, but one thing is very likely, what ever stuff was - it was not in form of normal matter - particles, but rather disturbance in quantum fields permeating newly created space - this is somewhat "poetic" attempt to describe the state of affairs).

Now as for rate of expansion in past, one must note that by looking into distance we also look into the past. So photon emitted from a source both in past and in distance traveled thru space that is expanding at a certain rate locally so photon was a bit stretched in one moment, a little less later on, and a bit more later on - what reaches us is cumulative effect of this small stretches of different rates. Since we observe light from different sources at the same time, it is clear that photons from both sources traveled "last leg" together but one photon from more distant target traveled on its own before - this gives us tool to measure red shift that happened at the time that photon traveled "first leg" of the journey. Buy looking at many, many such sources we can reconstruct the curve of expansion.

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Hi again Vlaiv and thanks for taking the trouble to make a very elegant and detailed response.

I spent some time doing a further reply, but in the course of this I looked at things from a slightly different perspective and got a different view. A bit like one of those optical illusion where one moment you see it as one thing and the next you see it as another.

Basically this has to do with the way the Hubble constant is derived - I need to spend some time forming a structured reply so hopefully you can bear with me!

Cheers

Tom

 

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I think your point about supernovas is flawed. The original stars did not all form at the same time. The nebula from which they formed will therefore have been carried to different distances by the expansion of space. Stars are still forming today and their distance from us is not uniform, so when some go super nova in the future they will be at different distances.

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The fact that they are not formed at the same point in time is in a way exactly my point - unless you can be sure when they were formed you cant use calculations to find there rate of movement from the observer.

That said, I agree with you - I think my point is flawed! However I want to try and form my thoughts and make an ordered reply - no time to think ATM!

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Of course you can determine their rate of apparent movement to observer at the time of explosion (the time that we register their explosion and hence backward in time/space - moment of their explosion, in some given coordinate system). It does not matter when they were created. At a time of explosion they are bright enough to do be photometrically measured with relative ease and have spectral signature that can be easily identified. Also red shift of spectral lines can be determined.

If we only examine one type Ia then we would not get much info - only average red shift - so no expansion curve from that. We need many measurements to determine curve. Also, type Ia is not only standard candle that astronomers use.

When one is doing these type of calculations, one must use sensible set of coordinates. It is not usually very sensible to think only in distance or in time when measuring such things. What does it mean that something is for example 10 billion ly away?

Well if you think the moment the photons used to calculate 10bly distance were emitted distance to from object to "place" where we are now but 10by ago (milky way position 10 by ago) is less spatial distance than it would take light to cross in 10by. Also if we think about distance where objects is "now" - it will be more spatial distance then 10bly. Even "now" is not well defined across such a vast distances. It tends to vary depending on relative motion (due to relativity).

We "see" about 13.8 by in the past. But our observable universe is 46bly in radius. If you think in normal terms - that does not add up.

Probably easiest way to calculate / think about it is in comoving coordinates. It's a bit like relativity (and in essence related to it) - every observer measures something - and measurements don't agree unless you use set of transforms between reference frames. It is similar here - you calculate in a set of coordinates, and for given question you can use a transform to obtain "sensible" answer to that particular question. Like if galaxy is measured to be 10bly away what was the actual spatial distance at the time photons were emitted - "then" distance. Or what is "now" distance to that object in spatial coordinates, or something like that.

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Tommohawk, try to think of Big Bang not as an explosion of matter and energy into an empty vacuum of space/universe, but as an expansion of space itself, while matter and energy stands (almost) still. There was no empty space waiting for Big bang to happen. There was no time before Big Bang. Actually, naming this event "Big Bang" is really misleading, it should be named something like "Big stretch" instead IMHO.

My brain comprehends it kinda like expanding bubble with time, space and energy. There is no space, no time, no energy or matter outside of this bubble of our universe. There is no point asking "what's outside our universe?" or "what was before big bang?". It's not defined, it's like asking what is the next measurement after "30" on a 30cm ruler. There is no such thing. It's hard to describe, you'll have to create your own mental image.
What baffled me for a long time, is this question: Why did not the universe collapse right into a black hole at the beginning, when everything was superdense, superenergetic and supersmall?

The answer is simple and obvious actually: There was no localised difference in gravity anywhere, where would the black hole form? Gravity was pulling equally in all the spatial directions, as there was equal, homogennous amount of superdense energy spread uniformly everywhere throughout the entire universe. And additionally, the stretching was faster than the speed of light, so there is no way opposite parts of the universe could interact with each other.

And we have not moved much since Big Bang. If there were any 3D coordinates visible in the Universe (something like a coordinate grid), our Earth and pretty much everything in our observable universe would be on the same coordinate point since the Big Bang. Its the point on the coordinate grid itself, that got stretched to 80bly.

We stand on a ruler with that little line marking "1cm" on it. The time is 0. The entire universe is only as big as the thickness of the tiny little line marking "1cm". Now big bang happens. We do not move. During the next 13.8billion years that mark "1cm" is getting stretched enormously to 80billion light years in all spatial dimensions with everything that once resided in one point in space. Now, where is the center of that which was once a point? There is no such thing. Or, everything is center?

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Hi - just to throw in an idea - there are those of us as is reflected in this thread that do not accept the simplistic model of a big bang- John Dobson said it was making Science to fit the observations not coherent logical  theory- I go along with the idea that it was a shift from one state to another not a "singularity" expanding into a vast vacuum - that seems preposterous - we know space itself expands, so before big bang there was nothing to expand into - this is part of inflation I think - an anomalous and sudden expansion of "what was" - we need new concepts to understand these cutting edge views- Tony

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On 2017-2-20 at 21:02, tony210 said:

Hi - just to throw in an idea - there are those of us as is reflected in this thread that do not accept the simplistic model of a big bang- John Dobson said it was making Science to fit the observations not coherent logical  theory- I go along with the idea that it was a shift from one state to another not a "singularity" expanding into a vast vacuum - that seems preposterous - we know space itself expands, so before big bang there was nothing to expand into - this is part of inflation I think - an anomalous and sudden expansion of "what was" - we need new concepts to understand these cutting edge views- Tony

Reverend Georges Lemaitre had a bit of a hard time convincing Einstein that his theory was anything but simplistic. Took Einstein a while but he was eventually swayed by the emerging evidence. Today of course there are a number of other theories some more fanciful than others but the one thing that marks Lemaitre's theory, latterly as modified by the Lambda CDM boundaries, is the ubiquity of the evidence. The model fits what we see remarkably well.  That's not to say that the model is not challenging, of course it is. By its very definition it leaves questions unanswered, notably "what was before".  In truth the model never made any pretence to answer such questions, we simply don't have anyway of applying our understanding of physics there - there is no physics there!  Equally, the "expansion into what" question leaves us unsatisfied. We don't like the answer because it does not fit with our experience in our everyday interaction with the world. But that is the nature of physics, it is not there to reflect our safe understanding of reality.  

As an aside there was a BBC Horizon programme (presented by Prof Jim Al-Khalili) that compared the current challenges to the Big Bang model, it was very watchable and interesting to see the current direction of research.  I must admit, I wasn't swayed by notions of holographic projections, multiple universes etc so I'm still with Lemaitre. I have a saved copy that I use in class but I'll see if I can find the link on the iPlayer if it is still there.

 

Jim

[youtube]

 

 

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On 15/02/2017 at 13:41, Tommohawk said:

Hi and thanks for those points... but I'm not sure it answers the question.

I'm familiar with the expanding balloon idea - and in a way this makes my point quite graphically. If the singularity was a single instantaneous event, all matter would occupy a single balloon shell - it would expand, and so there would be separation along the surface of the balloon, but all matter would be equidistant from the origin. Imagine standing on the surface of that shell - we could see matter all around us, and below us, because there is no solid surface, but not above us. That clearly isnt the case.

Of course we can say its the space expanding, not the matter itself, but that doesnt make any difference. Looking at this another way, if any matter has travelled further from the origin than any other matter, it must have either had more acceleration, or not started in the same place.

If we try and account for this by saying a singularity isnt actually a point source, but rather the original event was an expansion from all points simultaneously, this could explain the fact that matter extends throughout space - but it would impact on any assumptions made regarding different degrees of redshift when considering changes in the acceleration of the universe.

Get your head around this aswell if you can: If all the matter in the universe was once in 1 very small place, was the entire universe a "black hole"? and if so, how did it become mostly "not a black hole"?

If space and time formed from nothing then why doesnt more stuff spawn randomly around us? (Even women drivers dont actually come out of nowhere:icon_biggrin:)

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9 hours ago, Pluto the Snowman said:

Get your head around this aswell if you can: If all the matter in the universe was once in 1 very small place, was the entire universe a "black hole"? and if so, how did it become mostly "not a black hole"?

If space and time formed from nothing then why doesnt more stuff spawn randomly around us? (Even women drivers dont actually come out of nowhere:icon_biggrin:)

The reality is we don't know about the initial state of the "pre-universe" nor do we have any theories that can address it. It can't have be a conventional black hole as we understand them as they need a substantial sized universe conforming to GR in which to have an event horizon.

Matter, could be being spawned randomly around us but we don't have any evidence for this. This was the assumption made in the steady state theory but the required rate was very very low. As the energy density of dark energy stay constant in the current model of the universe dark energy is being created all the time!

Regards Andrew s

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Hi all and again thanks for all the helpful comments. Its really refreshing having folk pitching in with helpful conceptual comment, rather than just spouting received wisdom and trying to dazzle us all with complex mathematical theory.

One thing I've realised is that before even thinking about the relatively recent observations about the expansion varying over time, I have a bit of an issue just getting to grips with Lemaitres/Hubbles basic concept of an expanding universe. I'm in the process of trying to reconcile this... work in progress.

I've had a couple of sleepless nights, partly because of the brain-ache all this is giving me and partly because we're in the throes of trying to relocate which is proving complex, which really doesnt help.

On the plus side I've just arrived in La Palma for a week break which is lovely, although I had to abandon my plan to bring my mount and scope as the weather seemed a bit iffy. Right now I am the right side of a rather nice bottle of local wine, which may help the sleep issue but probably wont help the rational thinking... or maybe it will!

One thought to be going on with - and TBH this is the least of the issues. Recent thinking has turned to dark energy to explain the increased expansion rate observed by Riess et al. But wouldn't any expansion, even a "linear" (well, 3 or more D, but non-geometric) expansion rate require some energy? Very minor point maybe.

 

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On 20/02/2017 at 22:44, saac said:

As an aside there was a BBC Horizon programme (presented by Prof Jim Al-Khalili) that compared the current challenges to the Big Bang model, it was very watchable and interesting to see the current direction of research.  I must admit, I wasn't swayed by notions of holographic projections, multiple universes etc so I'm still with Lemaitre. I have a saved copy that I use in class but I'll see if I can find the link on the iPlayer if it is still there.

PS Would love to see this if you can find the link.

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  • 2 weeks later...

OK I'm back - got tied up with stuff in general. I hope some folk are still willing to follow this thread for a while.

Seems to me we can safely say that if matter started from the same point at the same time and expanded somehow, it would all be equidistant from the origin. This clearly isn't the case, so either it didnt all start at the same point, or it didn't start at the same time, or both. Hopefully that's not too contentious.

I think I'm right in saying that La Maitre and Hubble started with an unsatisfactory static universe model, inherited from Einstein, but observing that there was a relationship between the distance of other matter, galaxies etc, and their speed relative to Earth, they concluded we have an expanding universe. Hence the Hubble constant. This in itself doesn't account for the objection raised in my initial comment, ie it doesn't account for the fact that matter isnt all equidistant from an initial point, but La Maitre and others resolved this by proposing a period of initial rapid expansion, so that matter didnt all originate from the same locus.

TBH this still seems a bit of a fudge, because it doesn't account for the differential within the initial inflationary event. Why did the initial matter/plasma mass have areas with greater velocity? What caused the separation? Maybe it could be explained by the idea that the initial event wasn't instantaneous, but occurred over a finite period - small, but finite. It seem this may accord with some models.

In any event, it seems generally accepted that the expansion is caused by the expansion of space itself - known as metric expansion.

Leaving this aside for a moment, more recent observations suggest that more distant supernovae have a greater velocity than would be expected using the Hubble constant - from which it is claimed that the acceleration is increasing. This could be explained by the existence of dark energy. 

However, given that so little is known about the early stages of the universe,  doesn't the dark energy model seem a little premature? For example, how do know that the greater relative velocity of more distant SNs isnt simply the result of them having a great initial velocity? Or that they started sooner? Or that gravitational constraints weren't so great in the early expansion as supposed? And can we really be sure that red-shift, used to assess relative velocity, isn't in itself subject to other effects or distortions given the huge distances involved? Does space expansion, or metric expansion, in itself cause separation of waves and hence greater red-shift? It's hard to see how it wouldn't.

Its not that I have any objection to dark energy - I just think there are too many assumptions being made. Of course, it may simply be that I'm not smart enough to follow the science - but even the experts cant reconcile dark energy with existing models, and so many recent theories have been subsequently rejected I guess I'm just surprised that so many folk have jumped on the dark energy band-wagon. Another fudge factor maybe?

 

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17 hours ago, Tommohawk said:

However, given that so little is known about the early stages of the universe,  doesn't the dark energy model seem a little premature? For example, how do know that the greater relative velocity of more distant SNs isnt simply the result of them having a great initial velocity? Or that they started sooner? Or that gravitational constraints weren't so great in the early expansion as supposed? And can we really be sure that red-shift, used to assess relative velocity, isn't in itself subject to other effects or distortions given the huge distances involved? Does space expansion, or metric expansion, in itself cause separation of waves and hence greater red-shift? It's hard to see how it wouldn't.

The current LCDM theory is a self consistent description of what we know about the universe based on a wide range of observations and consistent with GR. These observations range from the CMB via studies of SNs, galaxies and much more. 

Reading what you say I think you have the wrong conceptual model of the initial and ongoing expansion. i hope this helps.

Firstly you can't view it from the outside as there is no outside. 

While we don't know about the instant of creation we do know (based on the theories we do have) is what happened just very shortly afterward.

The whole universe was of very small dimension and was essentially a fluid of very energetic particles. (imagine being in a drop of water or better still hot plasma but with no outside.)

The drop grew in size ( as measured by the particles inside it - metrical expansion) and the density of particles dropped ( i.e. they were all on average further apart).

This continued through the inflationary phase (rapid expansion of the metric with particles getting further and further apart on average causing the temperature to drop). This evens out any density fluctuations due to the random motions giving us the very smooth CMB we have to day.

At some pointt the particles / fields we know today condensed out as the temperature drops (protons, helium nuclei, electrons, photons...) during several phase transitions

The expansion continues but now gravity takes a bigger hand as the temperature drops and structure starts to form - stars and galaxies.

Now dark energy is taking a bigger hand as the universe continues to expand.

Regards Andrew

 

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Hi Andrew - thanks for that very precise account, and sorry for the very slow response.

I still feel I have a coupe of valid questions which remain unanswered, and I'm going to have one last ditch effort at posing them - I'd be grateful for any thoughts.

1. If we could "run back the clock" we would presumably see the separation between two given objects/particles reduce. This would be the reverse of the expansion you refer to. If we keep reversing time we would get to the point in time when the particles condensed out from the plasma, or in your words:

On 06/03/2017 at 15:09, andrew s said:

At some pointt the particles / fields we know today condensed out as the temperature drops (protons, helium nuclei, electrons, photons...) during several phase transitions

Now we know that at this time the plasma didn't occupy a single point - because as you said:

On 06/03/2017 at 15:09, andrew s said:

The drop grew in size ( as measured by the particles inside it - metrical expansion) and the density of particles dropped ( i.e. they were all on average further apart).

So the particle formation you refer to didn't all happen at one point/locus. There was separation/distance between the particles at the point in time when they condensed out.

To restate this running forward in time, at the end of the period of rapid inflation there was a zone (the expanded drop to which you refer, but not a point source) of plasma. It has a finite or possibly infinite size, but it definitely isn't all in the same locus. So at the time when particles - which will go on to slowly form matter - condense out from the plasma, they will also not all occupy the same point - they will separated by some distance.

So the question is, do we know the extent of this separation? Because this would impact, one would think, on the relative position of these particles over time.

One last thought - folk sometimes ask "how big is the universe - where is the boundary?". The usual answer is, we don't know, but we do know the size of the observable universe. Similarly, folk ask "how big was the universe in its early phase following the rapid initial inflation?" As I understand it, the answer again is we don't know, but we could calculate the size of the boundary which would go on, following expansion, to represent the boundary of the observable universe - estimates seem to vary: grapefriut or beach ball size seem popular estimates.

BUT my point is that if when we calculate the expansion rate based on the position of known galaxies or SNs etc NOW, we need to remember that the location of the originating particles from which these are composed wasn't common. That's all!

I wanted to make a second point but TBH this has exhausted me, so I'll post that later.

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Hi Tom, sorry about the last post but I had a residual reply in reply box and could not get rid of it.

I think we are getting to the limits of my imagination if not yours.

You seem to be holding onto a model of particles which are points. I may have encouraged this by talking about the distance between then getting bigger etc. 

In reality they are quantum particles and especially in the early phase of the universe will have been strongly de-localised.

In your reversed universe as the "particles", which are excitations in the relevant quantum field, get confined into a smaller and smaller space. This causes their momentum to increase and the pressure and temperature rise significantly. In parallel the energy density of the particles goes up and up but you can't locate them at any precise point. In this phase it is more like an ideal fluid (i.e. no particles just a continuous field). 

Only when the temperature and pressure drop sufficiently in the normal expanding universe does it become possible to" localise" to a degree  the excitations of the field into point like particles. Even then it is only an approximation. For the photon (the excitation of the EM field) it can't be localised at all at any epoch! (Technically there is no position operator for a photon - you can never even in principle know where it is.)

The underlying problem is I think that we tend to see the world as discrete individual objects and idealise the atomic and sub-atomic world as points. However, the best theories we have on the underlying physics is modern QM and GR both of which are based on continuous fields with energy densities and excitaions (waves). 

Regards Andrew

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