Is the Universe Expanding?

[MeteorWayne]

A new home for this part of the Big Bang argument.

A few earlier posts might be moved here later.

 

[xXTheOneRavenXx]

Wow, okay... here we go. Where to start. I can foresee a long drawn out discussion on this topic, however I am sure it will be one for the record books here at SDC. I will start on the side of supporting the Big Bang Theory, while remaining open-minded to other applied data & observations others may post supporting a different concept of the universe. I will remind everyone that while I am very good at understanding literature within the many papers we have cited, I don't not have a background in the larger physics equations that express some of the theories. Please keep this in mind when reading my posts. I will try to stick to what I know.

I guess among the numerous history lessons that will be covered here, and contradictions to these lessons; I will start off with one minor argument that arose between Doppler shift and Redshift, and how things in the universe were observed and appeared to be expanding. Some of this information may be elementary in nature for those more advanced in physics and their knowledge of the cosmos, while it will help teach others about some misconceptions.

A redshift is a shift in the frequency of a photon toward lower energy, or a longer wavelength. The redshift is defined as the change in the wavelength of the light divided by the rest wavelength of light. Here is an example of this equation:

z = (Observed wavelength - Rest wavelength)/(Rest wavelength)

Notice that the positive values of z correspond to increased wavelengths (in others words, redshifts). Different causes will occur depending on the type of redshift observed. Some of those types are:

The Doppler Redshift results from the relative motion of the light emitting object and the observer. If the light source is moving away from the observer then the wavelength of the light is stretched out. (moved toward the Red spectrum). If the light source is moving closer to the observer, then the wavelength of light is condensed. (moved toward the Blue spectrum). These effects called Redshifts and Blueshifts together are called Doppler Shifts. I think this will help explain why there is so much confusion between the two:

The shift in the wavelength is given by a simple formula
(Observed wavelength - Rest wavelength)/(Rest wavelenth) = (v/c)

As long as the velocity (v) is greatly less then the sped of light. A relative doppler formula is required when velocity is compared to the speed of light.

The Cosmological Redshift is a redshift caused by the expansion of space. The wavelength of light is increased as it travels across the expanding universe; between the point of origin and it's point of detection by the same amount that space has expanded during the lights transit.

The Gravitational Redshift is a shift in frequency of a photon to lower energy as it climbs out of a gravitational field.

Firstly, we must remember is the cosmological redshift is NOT the same as Doppler shift. While the Doppler shift reflects the motion between the object and the observer (and therefore becoming redshift or blueshift), the cosmological or better words the Hubble redshift is a direct result of the expansion of space itself.

Your probably asking what the relevance of this is. Well many people such as Vesto Slipher (Lowell Observatory 1917), James Edward Keeler (Lick & Allegheny Observatories 1918), William Wallace Campbell (Lick Observatory) all observed redshifts in distant galaxies. In addition to these observations, Edwin Hubble combined his own measurements of galaxy distances with Vesto Slipher's measurements of the redshifts associated with the galaxies. From here Edwin Hubble and Milton L. Humason discovered a rough proportionality of the objects' distances with their redshifts. Hubble and Humason plotted a trend line from a total of 46 galaxies they studied (In error they estimated a constant of 500 km/sMpc due to errors in their distance calibrations; which was the correction was in 1968 of 75 km/s/Mpc, in Allan Sandage's publication), however it would be years before a consensus on the calculation was achieved). In 1929 they formulated the empirical Redshift Distance Law of galaxies, nowadays termed simply Hubble's law. IF the redshift is interpreted as a measure of recession velocity (as it was first referred to as such by Vesto Slipher), happens to be consistent with the solutions of Einstein’s equations of general relativity. This is considered the first observational basis for strong evidence most often cited in support of the Big Bang (or also referred to as Standard) model.

After Hubble's discovery was published, and Alexander Friedmann found a non-stationary solution to Einstein's field equations in 1922, (printed in his 1924 papers) which was in fact verified by Edwin Hubble's observations in 1929; Einstein then abandoned his work on the cosmological constant (which he initially designed to allow for a static solution to his equations). He later termed this work his "greatest blunder" since the assumption of a static universe had prevented him from predicting the expanding universe. However, the cosmological constant has regained attention in recent years as a hypothesis for dark energy.

I will now leave the thread open to discussion of these foundation points.

 

[dangineer]

So due they attribute redshifts in stars to cosmological redshift because those predictions agree more with observation (in other words, is there a mathematical difference between cosmological redshift and doppler redshift)?

 

[michaelmozina]

So due they attribute redshifts in stars to cosmological redshift because those predictions agree more with observation (in other words, is there a mathematical difference between cosmological redshift and doppler redshift)?

Not necessarily:

http://arxiv.org/abs/astro-ph/0601171

 

[michaelmozina]

Firstly, we must remember is the cosmological redshift is NOT the same as Doppler shift. While the Doppler shift reflects the motion between the object and the observer (and therefore becoming redshift or blueshift), the cosmological or better words the Hubble redshift is a direct result of the expansion of space itself.

Well, that's what I thought too based on what I've been "told", but maybe not. Since you first asked questions about this topic awhile back I've since run into some papers you may also find rather interesting:

http://arxiv.org/abs/astro-ph/0601171
http://arxiv.org/abs/astro-ph/0610590

 

[xXTheOneRavenXx]

So due they attribute redshifts in stars to cosmological redshift because those predictions agree more with observation (in other words, is there a mathematical difference between cosmological redshift and doppler redshift)?

I guess it is really the terminology used by astronomers that's to blame. I read it all the time where an article reads like it's referring to the Doppler Effect or better worded in the articles as only using the word redshift alone (I think I did it myself a couple times in my last post) when speaking of the recession of distant galaxies. However if you read their work, they have utilized General Relativity. The difference is that the Doppler Effect does NOT factor in the expansion of space, so thus is governed by Einstein's Special Relativity. On the other hand, Cosmological redshift utilizes Einstein's General Relativity which DOES allow for the expansion of space. All Astronomers are doing is shortening the term when writing their paper, and is quite understood among other astronomers on what is meant. However others who read the paper sometimes take the terminology for face value (or they forget) and don't look at the obvious distinction. Or many people like us (when I first came to SDC) don't know the difference.

A decent document to read (I know, I read through it quite extensively) is Misconceptions of The Big Bang. Page 42 explains it in a lot more detail then what I just described. I knew I kept this link around for some reason

http://www.astro.princeton.edu/~aes/AST105/Readings/misconceptionsBigBang.pdf

 

[harrycostas]

G'day from the land of ozzz

Thank you very much for the links and a discussion based on science.

Its good to see.

I'm going away now to read the papers written by
Michał Chodorowski

Maybe in a few years I can understand what the heck is going on.

 

[SpeedFreek]

Okay, but in order to be sure that you are looking at an unbiased view of the whole argument, you should be aware of these two papers, the first of which is the one that Chodorowski was arguing against, and the second of which rebuts his criticisms. So far, Chodorowski has not published anything that refutes the findings of the second paper (he has only published a short note saying that he will refute it, but has not done so as I write this post), so the second paper actually represents where we are at in the argument at present. I originally posted the whole list of related papers, in chronological order, in an earlier thread. Actually, it might pay the readers of this thread to read through that previous thread.

Eppur si espande

A short answer to critics of our article "Eppur si espande"

Then, once you have read both sides of the argument, you should understand something: Neither side is arguing against the expansion of the universe. The question that all these papers are addressing is "Is SPACE expanding"? Or do distant galaxies recede due to some inertial mechanism.

All these papers accept that distant galaxies are receding, and that the universe is expanding.

From Chodorowski:

Since in the real Universe, sufficiently distant galaxies recede with relativistic velocities, these special-relativistic effects must be at least partly responsible for the cosmological redshift and the aforementioned `superluminalities', commonly attributed to the expansion of space.

Let us finish with a question resembling a Buddhism-Zen `koan': in an empty universe, what is expanding?

Do you really understand the argument? It is not an argument as to whether the universe is expanding, it is an argument as to whether "expanding space", i.e. something akin to a cosmological constant, is the cause of the known expansion.

The expansion of the universe is fully accepted by all these authors and none of the links posted by anyone in this thread (including the two links Michael posted) refutes the expansion of the universe.

 

[harrycostas]

G'dya speed freek

You said

Do you really understand the argument? It is not an argument as to whether the universe is expanding, it is an argument as to whether "expanding space", i.e. something akin to a cosmological constant, is the cause of the known expansion.

The expansion of the universe is fully accepted by all these authors and none of the links posted by anyone in this thread (including the two links Michael posted) refutes the expansion of the universe.

I do understand their points and yes they do point out that the thing that does the expanding is space/time which is most in the MATHS and not of reality. When we make observation and look at the actual we do not see expansion we directly see clustering gravity bound objects near and far.

I had this quote in my notes sorry I do not have the name.
But! I do not agree with it. Since there is no evidence but for ad hoc ideas.

The Standard explanation of the Big Bang has it that all matter came from a small point. The matter emerged and was flung (moved) outwards.
No, that is completely wrong. It is a popular myth that survives because it takes too much time and effort to explain what the Big Bang really is. But it is not any kind of explosion into pre-existing space. It has no center and no edge. Instead, the Big Bang is an explosion OF space, not an explosion INTO space. Since the very beginning, all matter and galaxies remain pretty much in place in their local space except for small local motions. The reason that galaxies get farther apart is not because of motion of galaxies through space, but because more empty space is continually being created between them. The whole universe is a 3D analog of an expanding balloon surface with dimes taped to it. All the dimes (representing galaxies) are getting farther apart from all the others even though none of them is moving, and there is no center and no edge to the balloon surface.

 

[SpeedFreek]

I do understand their points and yes they do point out that the thing that does the expanding is space/time which is most in the MATHS and not of reality. When we make observation and look at the actual we do not see expansion we directly see clustering gravity bound objects near and far.

When we make the observations, we see that the highest redshift galaxies have large angular diameters, indicating that they were relatively close to us when their light was emitted. Galaxies with smaller redshifts look to have been further away when they emitted their light. Looking out from our local area, angular diameter decreases with redshift as you would expect (the further away an object is, the smaller it looks) but only up to a certain redshift. As redshift increases above z~1.6, angular diameters start increasing again! This is totally consistent with an expanding universe and impossible to explain (so far) in a static universe.

Why do you keep insisting we see "clustering gravity bound objects" at all distances when it is irrelevant to the discussion? Objects have been clustering since structure formation began, but those clusters are all separating, over time, due to the expansion of the universe.

Explain the angular diameter distance - redshift relationship, if the universe is not expanding. Surely you must know of it, after all it is one of the four distance measures used in cosmology (luminosity distance, angular diameter distance, comoving distance, light-travel time). Here is a laymans guide.

Let me furnish you with some relevant links to papers which were published in a reputable journal, like all the papers in this thread so far, but unlike the papers you posted in the other thread.

The Tolman Surface Brightness Test for the Reality of the Expansion. I. Calibration of the Necessary Local Parameters

The Tolman Surface Brightness Test for the Reality of the Expansion. II. The Effect of the Point-Spread Function and Galaxy Ellipticity on the Derived Photometric Parameters

The Tolman Surface Brightness Test for the Reality of the Expansion. III. HST Profile and Surface Brightness Data for Early-Type Galaxies in Three High-Redshift Clusters

The Tolman Surface Brightness Test for the Reality of the Expansion. IV. A Measurement of the Tolman Signal and the Luminosity Evolution of Early-Type Galaxies

We conclude that the Tolman surface brightness test is consistent with the reality of the expansion. We have also used the high-redshift HST data to test the ``tired light'' speculation for a non-expansion model for the redshift. The HST data rule out the ``tired light'' model at a significance level of better than 10 sigma.

Now then, the argument as to whether the Big-Bang was an explosion in space or an expansion of space and matter is not the issue here - all the scientists in the debate accept the second premise. Nobody (except for perhaps Milne), thinks that the observed recession of distant galaxies is purely inertial. The universe seems to act as if space expands, but the question is whether saying that "space actually expands" reflects the reality of the situation, or whether the known expansion can be accounted for without attributing empty space with any intrinsic properties of its own. The distinction here is rather subtle, as it relies on the understanding of the inner workings of GR and the stress-energy tensor, etc.

Nobody in this scientific debate denies the expansion of space-like slices of "space-time", to put it simply.

 

[xXTheOneRavenXx]

I just really had to post this. It brings everything together under one fundamental explanation. Very good for those beginning to understand how all these forces come together:

http://www.youtube.com/watch?v=T6IBoa4FxvY

 

[michaelmozina]

Then, once you have read both sides of the argument, you should understand something: Neither side is arguing against the expansion of the universe. The question that all these papers are addressing is "Is SPACE expanding"? Or do distant galaxies recede due to some inertial mechanism.

FYI, from a "skeptics" perspective, Chodorowski's model and "explanation" of expansion is a whole lot easier to swallow than the notion that "space" (not physically defined) expands. I certainly do not dismiss his statements lightly, nor this model of "expansion". The notion however that "space" expands is much more open to criticism IMO.

 

[SpeedFreek]

FYI, from a "skeptics" perspective, Chodorowski's model and "explanation" of expansion is a whole lot easier to swallow than the notion that "space" (not physically defined) expands. I certainly do not dismiss his statements lightly, nor this model of "expansion". The notion however that "space" expands is much more open to criticism IMO.

This may surprise you, but I tend more towards Chodorowski's view myself! :shock:

We argue that, unlike the expansion of the cosmic substratum, the expansion of space is unobservable.

But what does he mean by the expansion of the cosmic substratum, which he fully accepts is observable?


To me, the most enlightening paper is Expanding Space: the Root of all Evil?
From the abstract:

While it remains the staple of virtually all cosmological teaching, the concept of expanding space in explaining the increasing separation of galaxies has recently come under fire as a dangerous idea whose application leads to the development of confusion and the establishment of misconceptions. In this paper, we develop a notion of expanding space that is completely valid as a framework for the description of the evolution of the universe and whose application allows an intuitive understanding of the influence of universal expansion. We also demonstrate how arguments against the concept in general have failed thus far, as they imbue expanding space with physical properties not consistent with the expectations of general relativity.

From 2.6.4 The Expansion of Space and Redshift

The key is to make it clear that cosmological redshift is not, as is often implied, a gradual process caused by the stretching of the space a photon is travelling through. Rather cosmological redshift is caused by the photon being observed in a different frame to that which it is emitted. In this way it is not as dissimilar to a Doppler shift as is often implied. The difference between frames relates to a changing background metric rather than a differing velocity. Page 367 of Hobson, Efstathiou, & Lasenby (2005) as well as innumerable other texts shows how redshift can be derived very simply by considering the change in the orthonormal basis of observers with different scale factors in their background metrics. This process is discreet, occurring at the point of reception of the photon, rather than being continuous, which would require an integral. If we consider a series of comoving observers, then they effectively see the wave as being stretched with the scale factor.

All I am trying to do is to make sure people who stumble upon discussions like this don't jump to the conclusion that any of these mainstream scientists are arguing that the universe is not expanding such that an observer comoving with that expansion would calculate that galaxies over a certain distance away are apparently receding at superluminal speeds. The "superluminal" expansion of the universe is fully accepted in the mainstream, it is the interpretation of "superluminal" that is being debated. The metric expansion of space, where the metric that defines the distance between two distant comoving coordinates is increasing over time, is fully accepted. The concept that, the further you look, the faster the apparent recession speed, is fully accepted. The notion that the space between cosmological coordinates expands equally is a good way to conceptualise it, as long as we don't attribute "space itself" with the property of expansion.

The superluminal recession speeds do not mean that galaxies are moving through the universe faster than light. Nor do they necessarily mean that all space is expanding, pushing everything apart equally at the cosmological scales and stretching light as it does so. It just acts as if it is to any observer co-moving with the expansion like we are!

So, what is the cosmic substratum, that which expands? Well, in lingustics, a stratum is that which influences, whereas a substratum is that which is influenced. So what is it that influences the cosmic substratum, otherwise known as "The Hubble Flow"?

 

[dryson]

The question is this how can we say that the Universe is expanding? Are we right there on the edge of the Universe watching and taking measurments that would tell us the answer? No we are not. The Universe is not expanding, the Universe is infinite and does not expand, only galaxies within the Universe expand and contract based upon each galaxies galactic core's gravitational pull or push upon the mass or the material contained within the galaxy itself.

 

[MeteorWayne]

The evidence is overwhelming that the Universe is expanding. Please demonstrate the math (since you have, I believe, stated that is all you trust; if I'm mistaken, please correct me) that refutes the redshift of all meausrements of distant galaxies.

It has never been suggested that we are near the edge of the Universe...in fact the whole concept has no meaning based on the Big Bang Theory...

The effects within galaxies are unrelated to the overall expansion of the Universe.

 

[xXTheOneRavenXx]

dryson, in my last post I added a link to a great lecture from Berkeley University on the universe and all the concepts along with it. It's pretty good and may help answer some of your questions about what is currently understood and tested. What it doesn't do is go into the actual formula's used. But it is a great tool to help give people an understanding for these concepts and how they all come together.

http://www.youtube.com/watch?v=T6IBoa4FxvY 1h:21min in length. Just got finished watching it.

 

[SpeedFreek]

The Universe is not expanding, the Universe is infinite and does not expand, only galaxies within the Universe expand and contract based upon each galaxies galactic core's gravitational pull or push upon the mass or the material contained within the galaxy itself.

Then explain the relationships between the luminosity and angular diameters of those galaxies, when compared to the amount that their light is redshifted. Why does their light become more redshifted, the longer it has been travelling?

By the way, the universe can expand even if it is infinite.

 

[harrycostas]

G'day Dryson

You said

The question is this how can we say that the Universe is expanding? Are we right there on the edge of the Universe watching and taking measurments that would tell us the answer? No we are not. The Universe is not expanding, the Universe is infinite and does not expand, only galaxies within the Universe expand and contract based upon each galaxies galactic core's gravitational pull or push upon the mass or the material contained within the galaxy itself.

Simple and to the point, I agree with you.

This link may interest you

http://arxiv.org/abs/astro-ph/0602344
Scalar potential model of redshift and discrete redshift

Authors: John C. Hodge
(Submitted on 15 Feb 2006)

Abstract: On the galactic scale the universe is inhomogeneous and redshift $z$ is occasionally less than zero. A scalar potential model (SPM) that links the galaxy scale $z$ to the cosmological scale $z$ of the Hubble Law is postulated. Several differences among galaxy types suggest that spiral galaxies are Sources and that early type, lenticular, and irregular galaxies are Sinks of a scalar potential field. The morphology-radius and the intragalactic medium cluster observations support the movement of matter from Source galaxies to Sink galaxies. A cell structure of galaxy groups and clusters is proposed to resolve a paradox concerning the scalar potential like the Olber's paradox concerning light. For the sample galaxies, the ratio of the luminosity of Source galaxies to the luminosity of Sink galaxies approaches $2.7 \pm 0.1$. An equation is derived from sample data, which is anisotropic and inhomogeneous, relating $z$ of and the distance $D$ to galaxies. The calculated $z$ has a correlation coefficient of 0.88 with the measured $z$ for a sample of 32 spiral galaxies with $D$ calculated using Cepheid variable stars. The equation is consistent with $z<0$ observations of close galaxies. At low cosmological distances, the equation reduces to $z \approx \exp(KD) \, -1 \approx KD$, where $K$ is a constant, positive value. The equation predicts $z$ from galaxies over 18 Gpc distant approaches a constant value on the order of 500. The SPM of $z$ provides a physical basis for the $z$ of particle photons. Further, the SPM qualitatively suggests the discrete variations in $z$, which was reported by W. G. Tifft, 1997, Astrophy. J. 485, 465 and confirmed by others, are consistent with the SPM.

 

[michaelmozina]

This may surprise you, but I tend more towards Chodorowski's view myself! :shock:

It doesn't seem shocking, but I'm glad to hear we're on the same page on that issue.

We argue that, unlike the expansion of the cosmic substratum, the expansion of space is unobservable.

But what does he mean by the expansion of the cosmic substratum, which he fully accepts is observable?

I'm assuming he means "spacetime" can spread out as the objects that make up spacetime spread out.

This is the type of "expansion" that I tend to accept and even lean towards, whereas the notion of expanding space seems, well, "hard to believe". Your mileage may vary.

 

[SpeedFreek]

G'day Dryson

You said

The question is this how can we say that the Universe is expanding? Are we right there on the edge of the Universe watching and taking measurments that would tell us the answer? No we are not. The Universe is not expanding, the Universe is infinite and does not expand, only galaxies within the Universe expand and contract based upon each galaxies galactic core's gravitational pull or push upon the mass or the material contained within the galaxy itself.

Simple and to the point, I agree with you.

This link may interest you

http://arxiv.org/abs/astro-ph/0602344
Scalar potential model of redshift and discrete redshift

Authors: John C. Hodge
(Submitted on 15 Feb 2006)

Abstract: On the galactic scale the universe is inhomogeneous and redshift $z$ is occasionally less than zero. A scalar potential model (SPM) that links the galaxy scale $z$ to the cosmological scale $z$ of the Hubble Law is postulated. Several differences among galaxy types suggest that spiral galaxies are Sources and that early type, lenticular, and irregular galaxies are Sinks of a scalar potential field. The morphology-radius and the intragalactic medium cluster observations support the movement of matter from Source galaxies to Sink galaxies. A cell structure of galaxy groups and clusters is proposed to resolve a paradox concerning the scalar potential like the Olber's paradox concerning light. For the sample galaxies, the ratio of the luminosity of Source galaxies to the luminosity of Sink galaxies approaches $2.7 \pm 0.1$. An equation is derived from sample data, which is anisotropic and inhomogeneous, relating $z$ of and the distance $D$ to galaxies. The calculated $z$ has a correlation coefficient of 0.88 with the measured $z$ for a sample of 32 spiral galaxies with $D$ calculated using Cepheid variable stars. The equation is consistent with $z<0$ observations of close galaxies. At low cosmological distances, the equation reduces to $z \approx \exp(KD) \, -1 \approx KD$, where $K$ is a constant, positive value. The equation predicts $z$ from galaxies over 18 Gpc distant approaches a constant value on the order of 500. The SPM of $z$ provides a physical basis for the $z$ of particle photons. Further, the SPM qualitatively suggests the discrete variations in $z$, which was reported by W. G. Tifft, 1997, Astrophy. J. 485, 465 and confirmed by others, are consistent with the SPM.

Would you care to summarise why you think that paper might interest Dryson? What are the conclusions of that paper?

 

[SpeedFreek]

I'm assuming he means "spacetime" can spread out as the objects that make up spacetime spread out.

This is the type of "expansion" that I tend to accept and even lean towards, whereas the notion of expanding space seems, well, "hard to believe". Your mileage may vary.

To me, the key issue is that there is nothing to observationally separate the two - they both predict the same observations. We move from the notion of expanding "space" to the notion of expanding space-like slices of space-time.

The reason that the "expansion of space" has been used to describe the concept for so long is that it aids an understanding of the overall picture, even if it introduces misconceptions. All cosmological distances increase by the same factor, over the same time. As 1 billion light-years expands to become 2 billion light-years, 10 billion becomes 20 billion and 100 billion becomes 200 billion. This cannot be described by simple inertial movement, believe me (or ask Milne!).

So we should not say that "space expands" and indeed I stopped explaining it that way myself over a year ago (except for the posts below!). Instead, I simply say that the universe expands and leave it at that. But it changes nothing observationally, and still leaves us with apparently superluminal recession speeds, if you choose to interpret the expansion of the universe as causing distant galaxies to become more distant.

 

[SpeedFreek]

I think this thread is the most appropriate place to post my "laymans guide" to the expanding universe, as it stands. I have posted versions of these before, but I think it is time to put them all in one place. Apologies to those that have seen these already.

Let's make a model.

Now to model an expanding space we need to assign coordinates within that space. For the moment, forget about any edges to that space, we don't need edges, we just need coordinates in order to measure the expansion of that space. Galaxies come later, so for now just imagine a 3 dimensional grid. At each grid intersection we will assign a coordinate, a point, a dot. Let's say each intersection point is 1 meter apart.

Put yourself on a point somewhere in this space. Whatever axis you look along you see neighbouring points 1, 2, 3, 4, 5 etc meters away, receding off into the distance. Then we introduce some expansion. Let's say the space grows to 10 times its original size in 1 second! That seems fast perhaps, but this is just a model with easy numbers. The key thing to remember is that the grid expands with the space.

So, here we are, still sitting on our point (but it could have been any point) 1 second later. Now lets look along an axis. We see those neighbouring points are now 10, 20, 30, 40, 50 etc meters away. The space increased to 10 times its original size, and so did the distance between each intersection point on that grid.

Our nearest neighbouring point has receded from 1 to 10 meters in 1 second, so it has receded at 9 meters per second. The next point away has receded from 2 to 20 meters in 1 second, so that point receded at 18 meters per second. The fifth point has moved from 5 to 50 meters away in 1 second, so that one has receded at 45 meters per second. The further away you look, the faster a point will seem to have receded!

And the view would be the same, whatever viewpoint you choose in the grid! There is no "centre" of expansion, no origin point within that grid - the whole thing, the whole space has expanded from something where the spaces between things were really small to something where the spaces between things are much larger. The expansion of that space has carried matter and energy along for the ride.

Remember I said the grid of points receded off into the distance.. well a point that was initially 33,000,000 meters away will have moved away to 330,000,000 meters in 1 one second, meaning that it has receded at 300,000,000 meters per second - the speed of light! Any point initially more distant than 33,000,000 meters away from another point will have receded from that point faster than the speed of light. That is the distance were an object recedes at light speed in this "little" model of expansion. If you look at a point that has receded at the speed of light, then from that point, the point you are on has receded at the speed of light. But no object would be moving through space faster than light, no photon would ever overtake another photon, it all just gets carried along by the cosmic flow.

Now I know this is a very simple model, dealing with a simple 10 times expansion in 1 second. This might seem very different from a universe where the rate of expansion was slowing from immense speed and then starting to accelerate, but if you start your grid very small and apply different rates of expansion to that grid, incrementally, over different rates of time, to simulate slowing it down and then speeding it up, when you look at the end result it is essentially the same. (Whenever there is a change in the rate of expansion, it is the rate of expansion for the whole grid that changes).

You might be asking how useful this model actually is. Well you can substitute different distance measures and time-scales if you like but the principle remains. If you sprinkle galaxies throughout the grid and then expand that grid such that the galaxies move with the expansion, you would find that galaxies interact gravitationally with their near neighbours. The further apart galaxies are when they form, the less the gravitational attraction between them. If they are less than a certain distance apart, the galaxies will move towards each other and cluster together, but if there is enough distance they will be moved apart by the expansion of the universe.

Galaxies at the edge of clusters might have some attraction to their neigbouring clusters, but that is countered by the gravity of the closer galaxies in their own cluster. Thus, the edges of the clusters seem to stretch out, "filament like", towards others in a manner reminiscent of the spiders web structures of the SDS Survey.

We end up with clusters of gravitationally-bound galaxies and increasing distance between the centres of those clusters, in a universe where there is no "origin point" or centre of expansion. The whole thing was the origin point and we have no way of knowing how much larger than our observable part of it the whole thing is. We don't even know if it has an edge, and it doesn't actually need one, mathematically. It is not quite as simple as saying "if it has an overall shape, it must have a centre", unfortunately.

 

[SpeedFreek]

The observable universe:

Imagine the beginning of time. If light were around, it would take time to reach you, but this is the beginning of time so
no light has had time to move yet. Right at the beginning, your observable universe has no size at all! As time
moves forward, any light that exists will move at the speed of light. Suddenly you might see a small distance all around
you, as light starts coming in from different directions.

After a year, you would be able to see 1 light-year in all directions. After 100 years therefore, your observable universe would be a sphere, 100 light-years in radius. After 13.7 billion years, your observable universe would be 13.7 billion light-years in radius, as you receive light that has been travelling for 13.7 billion years.

If only it were that simple! The problem comes when considering that the universe is expanding. At the start of things our observable part of the universe was very small and the universe was expanding incredibly fast, much faster than light. Also, light could not move freely until around 370,000 years after the Big-Bang. Before that, photons were frequently interacting with other particles and atoms didn't exist as everything was very hot and mixed up!

But at 370,000 years in, the universe had been expanding and the temperature had cooled enough for atoms to form in a flash of light (the universe finally became transparent and photons first moved freely throughout it). These photons filled the universe at that time, and we still receive these photons today. They are now stretched into microwaves (by the expansion of the universe) and are known as the Cosmic Microwave Background Radiation (CMBR). All the CMBR was pretty much emitted at once, nearly 13.7 billion years ago.

As all this was happening, the universe was expanding. When we worked out how much we thought those CMBR photons had been "stretched" by the expansion, it told us how much bigger the universe is today, than it was when those photons were emitted. We estimate that, when the CMBR was emitted, our observable universe was around 42 million light-years in radius, around 1100 times smaller than it is today.

Hang on though! Didn't I earlier imply that, 370,000 years after the BB, our observable universe would be 370,000 light-years in radius? Well, that radius, based on the time that light takes to travel, is not actually a useful measure of distance at all! In an expanding universe like ours, it is a measure of time elapsed only. When astronomers say the universe is 13.7 billion light-years in radius they are not giving you a distance through space, they are giving you a distance through time, known as the light-travel time. The "actual" distance across an expanding universe, known as the comoving distance, is a different thing entirely (although at distances closer to today, they are essentially the same).

The CMBR photons we receive today have been travelling for nearly 13.7 billion years, but they were emitted at a proper distance of only 42 million light-years away, all that time ago. The reason they have taken so long to reach us is that the universe is expanding, putting more distance in between those CMBR photons and their eventual "targets". Near the beginning of time, if a coordinate point in space was only a few centimetres from another, and those points moved apart with the expansion, then only 370,000 years later those points in space were 42 million light-years apart - that's how fast the universe was expanding, early on!

Then those CMBR photons were emitted and the space they were travelling through was receding from this point in space so fast that, to us, it was as if the photons themselves were receding from us too! The gradual deceleration of the expansion allowed those photons to eventually start making actual progress towards us, from our point of view. By the time they found themselves in regions of space where an object was receding from us slower than light, they were 5.7 billion light years away from this point in space, and the universe was around 4.5 billion years old! (This is when those photons crossed into our Hubble Sphere as it was at that time).

13.7 billion years after they were originally emitted, 9.1 billion years after they found themselves in space that was receding from us only sub-luminally, we receive those CMBR photons that were only emitted 42 million light-years away. And the real mind-bender is that we think that the original emission point is now over 46 BILLION light-years away. The edge of our observable universe, the most distant point from which we have received CMBR photons, is 46 billion light years away and continues to recede from us. That "edge", known as the surface of last scattering, was receding from this point in space at over 58 times the speed of light when those CMBR photons were emitted, it is still receding at around 3 times the speed of light today and we assume there are galaxies there now, but all we see is the radiation emitted from there, long ago.

The other mind-bender is that the whole universe is probably larger than our observable universe. After a fraction of a second, when our observable universe only had a radius of 10cm, there may well have been the same thing happening 20cm away. When the CMBR was emitted, and our observable universe was only 42 million light-years in radius, there might have been CMBR emitted 80 million light-years away, or much further away than that. Today, when we think our observable universe has a radius of 46 billion light-years and we assume, as galaxies formed in these parts that there would be galaxies throughout, there could be galaxies whose own observable part of the whole universe is totally separate from ours, galaxies that are 100s of billions of light-years away, outside of our observable part of the universe but still a part of our universe nonetheless.

 

[SpeedFreek]

Consider the distance where an object (or a comoving coordinate) is apparently receding at the speed of light. As the rate of expansion (or more accurately, the change in the scale factor of the background metric) was very fast in the early universe, this means that the distance at which a co-moving coordinate was apparently receding at c was close to this point in space. Imagine, if you will, that right after the Big-Bang (or inflation) distances down at the Planck length were increasing at the speed of light, but that the rate of expansion instantly decelerated from that value.

If the expansion rate had remained constant, then so would the distance at which a comoving coordinate was receding at c would have remained constant. But the rate of expansion decelerated over the first six billion years or so and therefore the distance at which a co-moving coordinate was apparently receding at c became larger.

After inflation the observable universe was the size of a grapefruit, but 370,000 years later it had a radius of around 42 million light years. The edge of the observable universe had, at that point, receded from what would become this point in space at many multiples of the speed of light in order to move 42 million light years in only 370,000 years, so at that point the distance where a comoving coordinate was receding at c would have been well within that radius.

And yet, we receive photons today that were emitted from the edge of the observable universe all those years ago. We receive photons that were emitted from the "surface of last scattering", which was receding from this point in space at least 50 times of the speed of light at the time those photons were emitted. They were only 42 million light years away when they were emitted, but they took 13.7 billion years to reach us.

The rate of expansion continued to slow, and after something over 100 million years, the earliest galaxies formed. The observable universe was something around 2 billion light years in radius at that time. We have seen dim blobs that might be these galaxies, but the oldest, dimmest, most distant galaxy we have reliable measurements for emitted its light around 800 million years after the Big-Bang, it has a redshift just under z=7 and is estimated to have been 3.5 billion light years away when it emitted its light.

Now lets look at a galaxy at redshift z=3. This (much brighter) light was emitted when the universe was 2.2 billion years old, 11.5 billion years ago when that galaxy is estimated to have been 5.3 billion light years away.

Now we move closer still to redshift z=1.4 and here is where we find the galaxies that are apparently receding at the speed of light – that is, they were receding at the speed of light when they emitted the light we are now seeing. The light we are seeing was emitted when the universe was around 4.6 billion years old, just over 9 billion years ago. These galaxies are estimated to have been 5.7 billion years away when they emitted the light we see, and what is more, they are the most distant objects we have seen in the universe! Let me say that again. Objects that are apparently receding at the speed of light are the most distant objects we have actually seen. Let me explain what I mean by this...

We use measurements of a galaxy's angular diameter (how big the object actually looks in the sky) to help determine how far away they were when they emitted the light we are now seeing. This makes sense, as you always see any object at the distance it was when the light left it, regardless of whatever it does or however it moves afterwards. Of course, we also have to determine what the galaxy's actual or absolute size was to do this, and this is a whole other subject unto itself, involving various models for galaxy formation.

Using the models that best fit our observations, we find that the most distant galaxies by angular size are the ones that are apparently receding at c, and yet we see light from more distant (in time) galaxies that are dimmer and more redshifted and yet those galaxies have increasing angular diameter the further we look in that direction.

Lets look at the figures (The first line is the CMBR or surface of last scattering).

Redshift____Distance then____Time since emission
z=1089_____42 million ly_____13.7 billion years ago
z=7________3.5 billion ly_____12.8 billion years ago
z=3________5.3 billion ly_____11.5 billion years ago
z=1.4______5.7 billion ly_______9 billion years ago
z=1________5.4 billion ly______7.7 billion years ago

So you can see that if our criteria is the object that was furthest away when it emitted the light we are now seeing, then the most distant object we have seen, seen as it was when it was that distant, was a galaxy at redshift z=1.4 at 5.7 billion ly. But we have also seen objects that are a lot older, were a lot closer when they emitted the photons and are now estimated to be a lot more distant as we receive those photons, than the objects that were apparently receding at the speed of light!

Now if I haven't lost you or bored you to death so far, hopefully you will be getting an inkling into how this all works and what an apparent recession speed of c actually represents.

The key thing to remember is that light never overtakes light. If you look at those figures above and also remember that we received all those photons at pretty much the same time you will find that:

Photons were emitted 3.5 billion light years away, 12.8 billion years ago. 1.3 billion years later, photons were emitted 5.3 billion light years away and if light never overtakes light then those older photons must have “been moved away by the rate of expansion” to that distance. 2.5 billion years later still, photons were emitted 5.7 billion years away and so our older photons must have moved away that far by then. And all those photons reached us at the same time.

So the light from that redshift=7 galaxy was receding from us (as it made its way towards us) from emission at 12.8 billion years ago until it passed the point where objects are apparently receding at lightspeed from us, 9 billion years ago. All light we receive that was emitted before that time was effectively moving away from us whilst it made its way towards us until it passed that point 5.7 billion light years away that was receding at c, 9 billion years ago, and then took another 9 billion years to reach us after that through a universe where the rate of expansion was levelling out and starting to accelerate again.

 

[harrycostas]

G'day

Speedfreek, first define your BBT than try to fit your data.

There are so many theories of the BBT.

"Which way did they go?"

Your definition is not as per the standard model.