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# Is Krauss looking at this right?

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(...)You did confuse distance in spacetime with velocity, but that isn't a discussion we should have here.

(...)

Did I?

If I did I am really sorry.

Quoting Spyman:

When distant objects gets carried away by expansion of space they are NOT considered traveling through space.

Which means that the expanding distance in spacetime is NOT a velocity. I agree 100% with that.

But then, why do we all accept velocity addition like this: (from the previous link http://arxiv.org/pdf/astro-ph/0310808v2.pdf) page 3.

Top panel (proper distance): The speed

of photons relative to us (the slope of the light cone) is not constant, but is rather vrec − c. Photons

we receive that were emitted by objects beyond the Hubble sphere were initially receding from us

(outward sloping lightcone at t <

5 Gyr).

(bolded mine)

You cannot add bananas & umbrellas. It is wrong.

And the "ant on a rope" is NOT a good example because it is based on velocity addition.

Edited by michel123456

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Spyman,

Well here are my questions. And yes, I want to learn what we know, but I would like it to make sense, and fit together, from outside in, and inside out.

Ok, the simplest explanation I have of how a changing rate of expansion can allow or prevent us from observing a distant object receding faster than light is the river analogy in post #147, did you have more questions or any logical objections against it?

The photons from my lamp, on for just a few hours, will not forever be in the space between here and Mars, but will they, or will they not, forever be somewhere in the universe, traveling outward from my house, in an ever increasing half shell, in the direction my house was facing during the event of it being on?

Yes, if unhindered all emitted photons will continue to traverse through space forever.

Is the event of a galaxy currently exceeding C recessional speed, not a different consideration, from the information we currently are receiving from that galaxy?

The event when a distant galaxy is emitting photons now and the event imprinted in photons we receive now are entirely separate and different events.

If we will never witness the event, but we currently see the galaxy, then the information we will recieve in the future, from that galaxy, must be events that occurred prior the event. Since a finite amount of events occurred prior the event, and after the events we currently are witnessing, should we not expect to see them all, eventually, given an infinite amount of time?

Yes, we expect to see all events between the event we currently are observing and the last event we will ever witness from that distant object.

Photons seen now will be of shorter wavelengths, than those we wil see (or fail to notice due to lack of huge telescopes and patience), at the end of time, which will be of very long wavelengths. Why is it not mandatory, that these wavelengths be proportionally, though increasingly lentheningly distributed in the spacetime between here and now, and the C exceeding event?

Wavelengths of lightbeams are increased proportionally to expansion of space, at the distant event horizon the redshift is increased to infinity.

Is each successive photon, we witness, in the future, from this galaxy, emitted way prior to the exceeding C event, not required to be just that slight bit more time dilated, that is, it will take a longer and longer time, to witness, a second worth of events, happening at that galaxy's location (prior the exceeding C event).

Yes, signals emitted with one seconds interval local time will get further and further apart when space in between them expands.

Should these wavelengths eventually grow to the length of the Milky Way, at that time, it will take us 400 thousand years to witness one nanosecond of events that occurred at that galaxy, in the years just prior the exceed C event. Given the wave/particle duality of a photon, it raises the question of whether we would be able to collect the photon at the beginning of the 400 thousand years, the end, or at any time, inbetween. That is, what is the nature of a photon, stretched out over 400 thousand ly. If the wave function would collapse, upon reception of the photon, would that require a physical nullification of the electrical and magnetic fields generated by the photon, instantaneously, over a 400thousand ly distance? Or would the nullification itself travel at C, as well?

I don't think the photon itself is considered to be stretched out over 400 thousand ly, my understanding is that it is one single particle somewhere along its wavelength.

The actual detection of the photon is the only detection of the electromagnetic field this photon represents and since a photon only can be detected once there is no need for nullification at locations where it has not been and can not be detected.

Are the photons, that passed us by, from that galaxy, yesterday, still existant, in our galaxy, somewhere behind us, (figuring we are facing the distant galaxy)?

Yes, if unhindered all emitted photons will continue to traverse through space forever.

These questions raise a logical question in my mind, of what you might mean, by us recieving the "last" photon, from that galaxy, at which time, there will be no more available, or on their way.

If you consider the possibility of a cosmic horizon from which new photons are unable to reach us and that the distance between us and this horizon only contains a finite number of photons. Then the logical conclusion is that in this line of finite photons there must be one last and most distant.

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Daedalus,

Thank you for taking the time to work through the ant on a rope. I actually, basically followed (had taken various levels of calculus in college) although I could not have generated that myself. Very well done, and described.

And I had seen, and pondered over that diagram you offered, many years ago, which I think had given me the subconscious understanding that the ant could reach the end...mathematically speaking.

Iggy,

I do believe I am understanding better now, from the paper you cited, and your comments, what Krauss was talking about, and am willing to answer my own question... that of "is Krauss looking at this right"... as yes, he probably is, but you have to be able to shift perspectives from what we know has to be the case, based on what we observe, to what we observe, that establishes the basis upon which we build the hypothetical models that establish what we know must be the case.

I can accept what Krauss was saying, on some levels. Still have some questions though, as to when he is speaking about a "current" situation from a here and now observer point of view, and when he is commenting on the "state" of the comoving universe, when concieved of, as all having the same cosmological age.

From the here and now observer's point of view, it does not effectively matter how big the universe currently is, or what events are currently occurring on distant galaxies. What matters is only what we observe of it. It is the way the entire universe is currently arriving here, that we observe, and that creates our reality. In essence, I would say, that we see the universe, exactly as we see it, and Krauss is seeing it, exactly as it is. This view however is only translated to "the way it actually is" with very careful analysis and peice by peice, hypothetical model building. We have no effective way, of actually checking such a model, against anything but what we observe, or have record of somebody else observing.

I tend to think it is more valuable to see the universe as we see it, than to think it is something else, because it is not something else.

Spyman,

The river model is not as good an analogy as just using the universe itself. And going by what the universe provides us, as evidence, I would have to say that we have determined, that we can see galaxies now, that are actually moving away from us, at superluminal speeds. Regardless of what might be going on on the expanding river you presented. Although, I will agree that there may indeed be a galaxy somewhere in the universe whose current events, can never reach us, I would point again to the distant galaxies we see now and say that those very distant galaxies we now see, are the same ones whose current events will never get here. Again making the important distinction between seeing a galaxy, and witnessing its current events.

Regards, TAR2

Edited by tar
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No I did not include the acceleration of the expansion of space, which might make it impossible for photons to reach us. However, that is just one interpretation to what accelerated expansion is. If we consider the universe as shaped as a sphere, then the ant on a rope problem still applies to moving across the sphere as it expands. This would result in space expanding at a constant rate, while gravitational bodies are pulling on each other across the sphere (this could apply to a flat universe too). This should impart different velocities onto galaxies which can be interpreted much like the ant moving away from us such that a galaxy would have a velocity $v_a$ with respect to the expansion of space. This will make the galaxy appear to be accelerating away from us while photons that are emitted would only be subjected to the constant expansion of space, and hence could reach us. However, that is just combining certain views on the subject and is not what mainstream science supports, which would mean that such an event horizon as you speak of is possible. I just wanted to use some simple calculus to demonstrate the vast amount of time it would take for the ant to reach the end of the rope : )

I don't think that's helpful, because your demonstration is not at all like what is observed happening in space. I know the science isn't settled, and who knows--perhaps what you describe could be like reality. But I don't think anyone in mainstream science thinks so.

Imagine that space is expanding as you suggest, where our hypothetical galaxy is receding at a constant rate. Photons, or ants, can only complete their journey because the distance that they still have to go is expanding slower than it was before.

For example, suppose that the galaxy is at a distance of x and receding at 2c.

Suppose that after n years the photon/ant has made back to its original distance of x from Earth. Assuming (contrary to mainstream science) that the galaxy is still receding at 2c, and that the entire space between the galaxy and us is expanding uniformly, then the space between the photon/ant and us is expanding at a rate less than 2c. So the space from Earth to x used to be expanding at a rate of 2c, and now space across that distance is expanding at a lower rate. What interpretation of expansion of space makes that possible? Your example is like the expansion of a balloon, but not I think like predictions of expansion of space.

My understanding of expansion of space is that all space is expanding uniformly. That means that if the galaxy is 10 LY away, and if that 10 LY is expanding at a rate of c, then after some amount of time when the galaxy is 20 LY away, each of the 2 10-LY sections between it and us will be expanding at a rate of c. This assumes there is no change in the rate of expansion per a given distance over that time. It implies exponential recession of the galaxy.

So, even if the rate of expansion per LY is fixed, the galaxy accelerates its recession. Even if the rate of expansion per LY is decreasing over time, depending on by how much, it's possible for the galaxy to accelerate away from us.

Do I have this wrong?

Edited by md65536
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Spyman,

The river model is not as good an analogy as just using the universe itself. And going by what the universe provides us, as evidence, I would have to say that we have determined, that we can see galaxies now, that are actually moving away from us, at superluminal speeds. Regardless of what might be going on on the expanding river you presented. Although, I will agree that there may indeed be a galaxy somewhere in the universe whose current events, can never reach us, I would point again to the distant galaxies we see now and say that those very distant galaxies we now see, are the same ones whose current events will never get here. Again making the important distinction between seeing a galaxy, and witnessing its current events.

So instead of pointing out any logical objections or asking questions you just choose to reject it before you understand it.

Oh well, maybe the river analogy was a failure, but it does explain how we can see galaxies actually moving away from us faster than light, it shows how some ants on the rope can make it when the expansion is deccelerating and why some ants won't make it if the expansion is accelerating.

The rest of your post to me is expressed so vauge that I am unable to make either head or tails of what exactly you do or don't agree with.

I am giving up on you now since I have no other choice than to agree with zapatos's post #192 (+1 zapatos).

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What interpretation of expansion of space makes that possible?

It's a freely coasting universe. One where gravity (attractive from matter or repulsive from dark energy) doesn't affect expansion.

I did a search and it is correct that such a universe doesn't have an event horizon:

a linearly evolving model is the only power law model that has neither a particle horizon nor a cosmological event horizon.

A Freely Coasting Universe page 2

It's pretty far from mainstream.

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It's a freely coasting universe. One where gravity (attractive from matter or repulsive from dark energy) doesn't affect expansion.

I don't understand the meaning of expansion of space in such a case, because I don't see a difference between the galaxy coasting away from us, and the expansion of space coasting, in that case. If the rate of recession is not in any way a function of the distance between the two, then what makes it an expansion of space, rather than just a simple velocity between the two? It seems like an unnecessary complication to say that space is expanding, if that space has no bearing on the behavior of the recession of objects.

Or in other words, if the universe were freely coasting, then what is the measurable distinction between an object receding due to expansion, and one receding due to velocity?

ANYWAY, I'm digressing.

Observations point to an accelerating expansion, and Krauss's conclusions assume that that is true and will continue, in which case there is a horizon and photons from events behind it will never reach us, even given an infinite passage of time. And, like previously conceded, if everything we know about the universe can turn out to be wrong, then any prediction we make can turn out to be wrong. Not understanding something is not enough justification to argue that it is wrong.

Edited by md65536
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I don't understand the meaning of expansion of space in such a case, because I don't see a difference between the galaxy coasting away from us, and the expansion of space coasting, in that case.

I firmly believe there is categorically no difference.

Expansion just means outward velocity.

Back toward the first second or so of the big bang there was an initial velocity everything got away from everything else. It has never been explained in terms of a cause, but the momentum everything got from that initial burst is what continues to carry everything along inertially today.

It isn't nearly as mysterious as everyone makes out. It's just that if you fling two things away from each other, they'll tend to keep moving that way.

Gravity affects expansion. It isn't synonymous with it.

If the rate of recession is not in any way a function of the distance between the two, then what makes it an expansion of space, rather than just a simple velocity between the two?

The observation that Hubble made (v=H*d) is that galaxies twice as far from an observer have twice the velocity. That is true even in a freely coasting universe. A galaxy today that has a certain velocity today would have half the velocity of a different galaxy that is (today) twice as far away.

If a specific galaxy always has the same recessional velocity at any point in the future then the rate of expansion is constant and that is freely coasting. It remains that at any point in the future galaxies which are twice as far would still be twice as fast. I would just say that a constant rate of expansion should be considered expansion nonetheless.

It seems like an unnecessary complication to say that space is expanding, if that space has no bearing on the behavior of the recession of objects.

Or in other words, if the universe were freely coasting, then what is the measurable distinction between an object receding due to expansion, and one receding due to velocity?

But, it is always a distinction without a difference even with the more standard models.

Are galaxies redshifted from a recessional velocity or because of expansion? It depends on how you look at it. Two ways of saying the same thing.

If you assume that space is static then galaxies are moving through space. If you assume that space is expanding then galaxies are static relative to their local space. Different coordinate systems offer different descriptions of the same reality.

Are galaxies really moving away from us or is space just expanding?

This depends on how you measure things, or your choice of coordinates. In one view, the spatial positions of galaxies are changing, and this causes the redshift. In another view, the galaxies are at fixed coordinates, but the distance between fixed points increases with time, and this causes the redshift. General relativity explains how to transform from one view to the other, and the observable effects like the redshift are the same in both views. Part 3 of the tutorial shows space-time diagrams for the Universe drawn in both ways.

In the absence of the cosmological constant, an object released at rest with respect to us does not then fly away from us to join the Hubble flow. Instead, it falls toward us, and then joins the Hubble flow on the other side of the sky, as discussed by Davis, Lineweaver & Webb (2003, AJP, 71, 358). In what are arguably the most reasonable coordinates, the cosmic time t and the distance D(t) measured entirely at the cosmic time t, the acceleration is given by g = -GM(r<D)/D2 where M(r<D) is the mass contained within radius D. This gives g = -(4*pi/3)*G*(rho(t)+3P(t)/c2)*D(t). The 3P/c2 term is a general relativistic correction to the otherwise Newtonian dynamics. Galaxies all move under the influence of this acceleration and their initial position and velocity. In other words, F = ma and gravity provides the force. Nothing extra or weird is needed.

ANYWAY, I'm digressing.

Observations point to an accelerating expansion, and Krauss's conclusions assume that that is true and will continue, in which case there is a horizon and photons from events behind it will never reach us, even given an infinite passage of time. And, like previously conceded, if everything we know about the universe can turn out to be wrong, then any prediction we make can turn out to be wrong. Not understanding something is not enough justification to argue that it is wrong.

Agreed 100%

I meant only to say that the ant / rope analogy didn't apply to standard cosmology.

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I firmly believe there is categorically no difference.

I've thought about it some more and figure that the behavior of light will be different, even in the case of a coasting universe. If a light source is receding due to velocity with no expansion of space, a photon approaching Earth will approach at c. If the source is receding due to expansion, the closing rate of a photon will be less than c (or negative if it's beyond an event horizon).

The observation that Hubble made (v=H*d) is that galaxies twice as far from an observer have twice the velocity. That is true even in a freely coasting universe. A galaxy today that has a certain velocity today would have half the velocity of a different galaxy that is (today) twice as far away.

True, but the redshift of a coasting galaxy should remain constant, so it should never be increasingly redshifted toward 0.

The example I used earlier, of a galaxy receding at 2c but remaining at that recession rate, doesn't make any sense!, because it would have had to increase its rate of recession up to that rate, and then start coasting, and an explanation of how that could possibly be would then be required.

Agreed 100%

I meant only to say that the ant / rope analogy didn't apply to standard cosmology.

I agree with you too, I was just curious about if it's even possible to reason about some of the alternatives others have posted (including some that contradict Krauss's predictions). I think the ant analogy would be fine as long as the rope is given the same properties as space, and the ant the properties of a photon.

After some thought, I realize I don't quite know enough of the details related to this thread.

Let's assume that space *is* expanding as has been observed, and that there is a cosmic horizon beyond which any events that occur will never be seen.

Is the horizon at the same distance all around? (I think yes because space is homogeneous and expanding uniformly???)

Is the distance to the horizon increasing over time? Or decreasing, or staying the same?

For a given distant galaxy to be forever visible, the horizon would have to be receding fast enough that the galaxy never crosses it.

If the horizon remains at a fixed distance over time, that would mean that space of that length is expanding at a fixed rate. This is the simplest case in my mind but I don't think it matches observations.

If the horizon is approaching, then the rate of expansion per unit of length is increasing.

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What interpretation of expansion of space makes that possible?

It's a freely coasting universe. One where gravity (attractive from matter or repulsive from dark energy) doesn't affect expansion.

I did a search and it is correct that such a universe doesn't have an event horizon:

a linearly evolving model is the only power law model that has neither a particle horizon nor a cosmological event horizon.

A Freely Coasting Universe page 2

It's pretty far from mainstream.

Iggy, that is definitely an interesting paper. I researched the authors who wrote the paper and found that Daksh Lohiya was a common factor in a series of papers written about a linear coasting cosmology. A list of publications by Daksh Lohiya and his colleagues can be found here:

http://en.scientific...rg/daksh_lohiya

In Daksh Lohiya's paper, "Power law cosmology - a viable alternative", published in 2008, the authors summarize the following:

In this paper, we study observational constraints on the power law cosmology, $a(t) \propto t^{\alpha}$. This model of the universe has very interesting features which makes it unique when compared to the other models of the universe. Firstly, for $\alpha \ge 1$ it does not encounter the horizon, flatness and age problem [2, 3, 4]. Secondly, such an evolution is a characteristic feature of models that dynamically solve the cosmological constant problem. Statistically this model may be preferred over other models as we have to fit only one parameter, $\alpha$.

...

Since the joint test of SNLS and H(z) data presented in this work favours an open power law cosmology, for the sake of completeness we find bounds on $\alpha$ in open model separately using the SNLS data and the H(z) data. We once again marginalize over $h$ to find $\chi_{v}^2$ and the best fit values using each test. We find that:

1. Constraints from SNLS data: For set A prior, we get the best fit value $\alpha = 1.421_{-0.07}^{+0.08}$ with $\chi_{v}^2 = 1.07$. With set B, we get the same constraints on the parameter $\alpha$ as obtained from set A but with $\chi_{v}^2 = 1.09$. We find that the constraints on $\alpha$ do not depend upon the choice of the prior. We, therefore, conclude that the SNLS data favours $\alpha > 1$ (best fit value being $\alpha = 1.42_{-0.07}^{+0.08}$). This observational data rules out linear coasting cosmology ($\alpha = 1$) even at [the] $2\sigma$ level.

2. Constraints from H(z) data: We find that the H(z) data provides tight constraints on the model parameter $\alpha$. With set A prior we obtain best fit value $\alpha = 1.02_{-0.06}^{+0.09}$ with $\chi_{v}^2 = 0.834$. With set B prior we get the best fit value as $\alpha = 1.07_{-0.09}^{+0.11}$ with $\chi_{v}^2 = 1.06$. We observe that for this test the constraints on $\alpha$ weakly depend upon the choice of priors. The H(z) data, however, strongly favours linear coasting cosmology with the best fit value.

We summarize: An open power law cosmology with $\alpha > 1$ is in excellent agreement with the present day observations. This makes it an attractive alternative. In fact, the possibility of an open linear coasting model as a viable model cannot be ruled out (as suggested by the H(z) data). Concordance of the power-law cosmology with CMB anisotropy measurement is a major area to be explored. There are large numbers of surveys that are ongoing or have been proposed. With the flood of new data (and the possibility that the observational techniques will be improved), the task ahead is to find models of the universe that can explain these observations. It will be interesting to investigate how the future observations will change the constraints on $\alpha$.

Although your references support a cosmological / particle horizon, as of 2008, they were not able to rule out an open linear coasting model as supported by the H(z) data. However, one does have to note that power law cosmology is still outside the currently favored ΛCDM model.

If you assume that space is static then galaxies are moving through space. If you assume that space is expanding then galaxies are static relative to their local space. Different coordinate systems offer different descriptions of the same reality.

What about the case where space is expanding at a constant rate but the galaxies are in motion relative to their local space? Wouldn't that also produce an apparent acceleration in the expansion of space?

Edited by Daedalus
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I've thought about it some more and figure that the behavior of light will be different, even in the case of a coasting universe. If a light source is receding due to velocity with no expansion of space, a photon approaching Earth will approach at c. If the source is receding due to expansion, the closing rate of a photon will be less than c (or negative if it's beyond an event horizon).

Ned wright explains how both descriptions of light amount to the same thing here under "many distances".

The condensed version is that the coordinates of special relativity (static space) are not the same as FLRW coordinates (expanding space). Distance and time mean something different under the two different assumptions. Velocity (which is distance divided by time) is naturally going to be different as well. The coordinate system changes, not the reality of the situation being described. You get the same predictions either way.

True, but the redshift of a coasting galaxy should remain constant, so it should never be increasingly redshifted toward 0.

Right.

The example I used earlier, of a galaxy receding at 2c but remaining at that recession rate, doesn't make any sense!, because it would have had to increase its rate of recession up to that rate, and then start coasting, and an explanation of how that could possibly be would then be required.

The same explanation would then have to be required of the standard model. Either way the thing that caused the initial expansion isn't know.

Let's assume that space *is* expanding as has been observed, and that there is a cosmic horizon beyond which any events that occur will never be seen.

Is the horizon at the same distance all around? (I think yes because space is homogeneous and expanding uniformly???)

Is the distance to the horizon increasing over time? Or decreasing, or staying the same?

For a given distant galaxy to be forever visible, the horizon would have to be receding fast enough that the galaxy never crosses it.

If the horizon remains at a fixed distance over time, that would mean that space of that length is expanding at a fixed rate. This is the simplest case in my mind but I don't think it matches observations.

If the horizon is approaching, then the rate of expansion per unit of length is increasing.

Yes to the same distance all around.

The bolded one is where we're headed. We aren't there yet. The horizon is still receding, it's just that space is receding faster where it is because of acceleration. In the future the hubble parameter should end up being about constant over time which gives a horizon of a constant proper distance.

Of course, it depends on what you mean by "horizon" and "distance". The comoving distance to the event horizon shrinks indefinitely. A galaxy that has a comoving distance of 10 billion lightyears today will always have that same distance. That's how the coordinates of standard cosmology work. The coordinate distance to a galaxy that is expanding away from us doesn't increase over time.

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For a given distant galaxy to be forever visible, the horizon would have to be receding fast enough that the galaxy never crosses it.If the horizon remains at a fixed distance over time, that would mean that space of that length is expanding at a fixed rate. This is the simplest case in my mind but I don't think it matches observations.

If the horizon is approaching, then the rate of expansion per unit of length is increasing.

I think another way to look at it is that for a distant galaxy to be forever visible, it must be within our supercluster. If it is not, it will eventually cross the horizon. Unless of course the Hubble Constant continues to change over time and eventually splits apart our supercluster, cluster, galaxy, etc.

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I think another way to look at it is that for a distant galaxy to be forever visible, it must be within our supercluster. If it is not, it will eventually cross the horizon.

I agree, but what you and MD said are not mutually exclusive unless we have acceleration of expansion. I believe MD was pondering otherwise.

Unless of course the Hubble Constant continues to change over time and eventually splits apart our supercluster, cluster, galaxy, etc.

Indeed. What I said last post about the proper distance to the horizon eventually being constant would be true only if the cosmological constant's equation of state is -1. In that case, everything that is gravitationally bound today will be bound indefinitely. If it is less than that then we all better believe every little thing is going to be ripped to pieces.

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Iggy,

Well that paper,

A “Freely Coasting” Universe

Savita Gehlaut, A. Mukherjee, S. Mahajan & D. Lohiya∗

Department of Physics and Astrophysics, University of Delhi, Delhi–110

007, India

included many concepts and terms, that I have "heard" before, but don't fully understand the derivations, and implications of.

Interesting, that 10 years ago, we seem to have thought the universe was 15 billion years old. (if I read it right). Wouldn't that change the figures and figuring some?

And boy, I just can not get my head around seeing the effects of pressure waves (sound) in the CMB. So long ago, and so far away, and so small, yet subtending measurable angles in our emmense sky. (If the mars rover would subtend an angle equal to hydrogen atom, at arms length, how could a swath of sky measureable in seconds of arc, be representative of something on the scale of photons and baryons pushing each other around.) These areas of sky are "behind"" and farther away than the most distant galaxy we have found. They must represent HUGE chunks of current space. What ever kind of dynamics are figurable from the anisotropies witnessed and measured, must be something the universe does on GRAND scale, hardly comparable to the speed of sound. I would tend to think that small things, far away represent hugher dynamics, than what we are familiar with, not smaller and closer dynamics. I guess its a grain size thing. And I don't think I have the mental equipment to scale EVERYTHING up and down consistently, in the appropriate manner.

I suppose, under the circumstances, I will just have to leave cosmology to the geniuses that have the equipment required to make the shifts in scale, in both time and space, required to match the model up, with the reality. I just can't carry all the varibles through.

I will have to retire with Spyman, and float about on the boats on the expanding river. Maybe just do some fishing, and drink icetea, and bask in the sun.

Take some peanut butter cups.

Thank you for trying, Iggy. I hope that somebody contributing or reading, learned something, or had a new insight along the way.

Let's just say that Krauss is looking at it right.

Regards, TAR2

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I'm actually wondering what Iggy's reply is to the post (#210) I recently made.

"Although your references support a cosmological / particle horizon, as of 2008, they were not able to rule out an open linear coasting model as supported by the H(z) data. However, one does have to note that power law cosmology is still outside the currently favored ΛCDM model."

and

"What about the case where space is expanding at a constant rate but the galaxies are in motion relative to their local space? Wouldn't that also produce an apparent acceleration in the expansion of space?"

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Although your references support a cosmological / particle horizon, as of 2008, they were not able to rule out an open linear coasting model as supported by the H(z) data.

In your quotes from the paper they say "This observational data rules out linear coasting cosmology", but then they conclude that it can't be ruled out? Are the observational data invalidated?

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In your quotes from the paper they say "This observational data rules out linear coasting cosmology", but then they conclude that it can't be ruled out? Are the observational data invalidated?

What's going on here is two different data sets. One rules out linear coasting cosmology, and the other strongly supports it. My questions, "What about the case where space is expanding at a constant rate but the galaxies are in motion relative to their local space? Wouldn't that also produce an apparent acceleration in the expansion of space?", would resolve such discrepancies. If space is expanding at a constant rate, then it would produce the linear coasting cosmology $\alpha = 1$. Without including gravity we can show that we would still observe an apparent acceleration in the expansion of space. However, we can assume that there is more matter outside a radius surrounding our super cluster, than within it. Double that radius and it's possible that there could be even more matter outside this new radius, and so forth. If these things are gravitationally interacting, then the net force acting up inner rings would increase such acceleration allowing for an observed $\alpha > 1$ while maintaining the constraints imposed by the H(z) data.

Edited by Daedalus
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I'm actually wondering what Iggy's reply is to the post (#210) I recently made.

"Although your references support a cosmological / particle horizon, as of 2008, they were not able to rule out an open linear coasting model as supported by the H(z) data. However, one does have to note that power law cosmology is still outside the currently favored ΛCDM model."

and

"What about the case where space is expanding at a constant rate but the galaxies are in motion relative to their local space? Wouldn't that also produce an apparent acceleration in the expansion of space?"

I don't understand what you're asking with the first thing. To the second, if every galaxy at a certain distance has a peculiar velocity directly away from us then we must live in a very special place in the universe.

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I don't understand what you're asking with the first thing. To the second, if every galaxy at a certain distance has a peculiar velocity directly away from us then we must live in a very special place in the universe.

If you are driving with your car on the highway, and if every car around you , traveling in the same direction, in front of you & back, is accelerating at the same rate, i.e. they all pass through the same point at the same speed, then you will observe all cars receding from you.

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If you are driving with your car on the highway, and if every car around you , traveling in the same direction, in front of you & back, is accelerating at the same rate, i.e. they all pass through the same point at the same speed, then you will observe all cars receding from you.

and so will every other car. In that case it isn't a peculiar velocity. It isn't something extra added on to the scale factor. It isn't something above and beyond normal expansion... which is the thing Daedalus proposed.

In any case, I do agree with that much the same as I recently agreed on your behalf in the case of constant expansion on the same topic.

Edited by Iggy
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I saw that my post #201 was by-passed.

If when distant objects gets carried away by expansion of space they are NOT considered traveling through space,

Then recession "speed" is not a standard "speed" and that is the reason why this "speed" can overpass C.

So

I understand that this "speed" is a phenomenon of other nature than regular speed through space.

Or do I understand nothing?

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• 2 weeks later...

I saw that my post #201 was by-passed.

If when distant objects gets carried away by expansion of space they are NOT considered traveling through space,

Then recession "speed" is not a standard "speed" and that is the reason why this "speed" can overpass C.

So

I understand that this "speed" is a phenomenon of other nature than regular speed through space.

Or do I understand nothing?

Yes, this expansion speed is not the ordinary speed of an object moving in space but the speed of expansion of space itself. Being essentially a metric speed it is not constrained by the c limit as an ordinary speed is.

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Yes, this expansion speed is not the ordinary speed of an object moving in space but the speed of expansion of space itself. Being essentially a metric speed it is not constrained by the c limit as an ordinary speed is.

Right.

And since these are not the same kind of "speeds", can we add them ?

Can we add bananas and umbrellas?

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Right.

And since these are not the same kind of "speeds", can we add them ?

You should be able to use velocity composition and Doppler calculations to get something meaningful, if you're careful. A simple straightforward addition is probably not meaningful.

Can we add bananas and umbrellas?

Off topic, but yes.

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You should be able to use velocity composition and Doppler calculations to get something meaningful, if you're careful. A simple straightforward addition is probably not meaningful.

That doesn't sound rigorous.

Adding a speed that is subjected to velocity composition (never faster than C) and another "speed" that is not subjected to velocity composition (expanding space) can only give wrong results IMHO.

If you make the attempt to add bananas & umbrellas, the result will certainly not be bananas, nor umbrellas: the result will be "something else" (objects?). Units don't match.

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