Accelerating expansion of universe is an illusion

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Discussion Overview

The discussion revolves around the theory that the accelerating expansion of the universe may be an illusion caused by the relative motion of our region of space. Participants explore the implications of this theory, its compatibility with current observations, and the nature of dark energy.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation
  • Conceptual clarification

Main Points Raised

  • Some participants mention a theory by Christos Tsagas suggesting that the apparent acceleration of the universe's expansion is an illusion due to our region's motion through space.
  • Others argue that while the dark flow interpretation has merit, substantial observational evidence is needed to validate it as a viable alternative to dark energy.
  • Concerns are raised about the compatibility of Tsagas' theory with current observations of isotropy in the universe, with some expressing skepticism about the plausibility of such coincidences.
  • One participant critiques the Nobel citation for the discovery of the accelerating expansion, suggesting it should focus on the observational techniques rather than the conclusion drawn from a specific cosmological model.
  • There is a discussion about the validity of the distance moduli obtained from type Ia supernovae and how interpretations of this data can vary depending on the cosmological model used.
  • Questions are posed regarding the implications of supernova observations on the historical expansion rates of the universe, with references to different cosmological models and their predictions.

Areas of Agreement / Disagreement

Participants express a range of views, with no clear consensus on the validity of Tsagas' theory or its implications for dark energy. Some agree on the importance of observational data, while others remain skeptical about the interpretations of that data.

Contextual Notes

Participants note that the discussion is influenced by the limitations of current cosmological models and the unresolved nature of dark energy. There are references to specific mathematical and observational challenges that remain unaddressed.

Who May Find This Useful

This discussion may be of interest to those studying cosmology, dark energy, and the interpretation of astronomical observations, as well as individuals curious about the ongoing debates in the field of theoretical physics.

bohm2
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I'm not sure how strong the evidence is for this but I found it interesting:

"Now, a new theory suggests that the accelerating expansion of the universe is merely an illusion, akin to a mirage in the desert. The false impression results from the way our particular region of the cosmos is drifting through the rest of space, said Christos Tsagas, a cosmologist at Aristotle University of Thessaloniki in Greece. Our relative motion makes it look like the universe as a whole is expanding faster and faster, while in actuality, its expansion is slowing down — just as would be expected from what we know about gravity.

If Tsagas' theory is correct, it would rid cosmology of its biggest headache, dark energy, and it might also save the universe from its harrowing fate: the Big Rip. Instead of ripping it to bits, the universe as Tsagas space-time envisions it would just roll to a standstill, then slowly start shrinking."

http://www.astro.auth.gr/~tsagas/Publications/Journals/PRD/PRD14.pdf
http://www.msnbc.msn.com/id/44690771/ns/technology_and_science-science/
 
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The author made good points favoring a dark flow interpretation, but, there remains plenty of observational work to confirm this is a viable alternative to dark energy. The basis for including a cosmological constant [dark energy] in GR is mathematically sound. QM provides a mechanism. The real puzzle is why it is so weak.
 
bohm2 said:
I'm not sure how strong the evidence is for this but I found it interesting:

"Now, a new theory suggests that the accelerating expansion of the universe is merely an illusion, akin to a mirage in the desert. The false impression results from the way our particular region of the cosmos is drifting through the rest of space, said Christos Tsagas, a cosmologist at Aristotle University of Thessaloniki in Greece. Our relative motion makes it look like the universe as a whole is expanding faster and faster, while in actuality, its expansion is slowing down — just as would be expected from what we know about gravity.

If Tsagas' theory is correct, it would rid cosmology of its biggest headache, dark energy, and it might also save the universe from its harrowing fate: the Big Rip. Instead of ripping it to bits, the universe as Tsagas space-time envisions it would just roll to a standstill, then slowly start shrinking."

http://www.astro.auth.gr/~tsagas/Publications/Journals/PRD/PRD14.pdf
http://www.msnbc.msn.com/id/44690771/ns/technology_and_science-science/
I have a really really hard time buying that this could both explain the acceleration and be consistent with our current observations of the isotropy of our universe.
 
Chalnoth said:
I have a really really hard time buying that this could both explain the acceleration and be consistent with our current observations of the isotropy of our universe.

You need a lot of coincidences to be fooled in this fashion. If the universe has conspired against our best science to the extent that we can't figure out what's really going on, we might as well give up trying.
 
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DevilsAvocado said:
https://www.physicsforums.com/showthread.php?t=536522" :smile:


(Hi RUTA, nice to have you back!)

I was away while I was working on this very problem, actually. I bailed on all discussion forums during this time to focus on the problem at hand. I do believe the Nobel citation should not have stated "for the discovery of the accelerating expansion of the Universe," since that conclusion follows from their data only in the context of a particular theory of cosmology. They certainly deserve the prize, but the citation should have said something like, "for observational techniques associated with type Ia supernovae." Then, the Nobel committee doesn't have to worry about changes to our theoretical cosmology that change the interpretation of the data.
 
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RUTA said:
I was away while I was working on this very problem, actually. I bailed on all discussion forums during this time to focus on the problem at hand. I do believe the Nobel citation should not have stated "for the discovery of the accelerating expansion of the Universe," since that conclusion follows from their data only in the context of a particular theory of cosmology. They certainly deserve the prize, but the citation should have said something like, "for observational techniques associated with type Ia supernovae." Then, the Nobel committee doesn't have to worry about changes to our theoretical cosmology that change the interpretation of the data.

Cool, I had some remorse about not recapturing the discussion on simplices manifold/non-separability/RBW... a lot of 'messy things' in world politics got between, and then you were gone...

Anyhow, I’m glad you’re back, and maybe sometime we could continue... I have bunch of 'topics' in pipeline that I’ll try to get posted... but maybe later?

The comment on the Nobel citation is interesting. It’s not the first time those Nobel guys made a "mistake", and of course – not one single human has seen DE, or know what it is, or how it works.

But as you said - they certainly deserve the prize – they proved the accelerating expansion beyond any doubts, even if we don’t know the 'mechanism'.
 
DevilsAvocado said:
Cool, I had some remorse about not recapturing the discussion on simplices manifold/non-separability/RBW... a lot of 'messy things' in world politics got between, and then you were gone...

Anyhow, I’m glad you’re back, and maybe sometime we could continue... I have bunch of 'topics' in pipeline that I’ll try to get posted... but maybe later?

The comment on the Nobel citation is interesting. It’s not the first time those Nobel guys made a "mistake", and of course – not one single human has seen DE, or know what it is, or how it works.

But as you said - they certainly deserve the prize – they proved the accelerating expansion beyond any doubts, even if we don’t know the 'mechanism'.

Thanks, I'm glad to be done with this last calculation. It was nasty :devil:

To clarify on this topic, what everyone agrees these guys did was obtain distance moduli (μ) out to redshift z ~ 1.5 using type Ia supernovae. Whether or not this data provides evidence that the universe is expanding depends on your cosmology model. Using inhomogeneous spacetime models, for example, the data does not show accelerating expansion (arXiv:gr-qc/0605088v2). Likewise, the PRD paper that started this conversation shows the accelerated expansion could be an illusion. However, no one is disputing the validity of the μ versus z data these Nobel recipients obtained, and that process took them years to perfect and employ.
 
Begging patience for an imbecile...

If a supernova far, far away is moving very, very fast does that mean the universe long, long ago was moving faster than today?
 
  • #10
HarryWertM said:
Begging patience for an imbecile...

If a supernova far, far away is moving very, very fast does that mean the universe long, long ago was moving faster than today?

Consider the relationship between the Milky Way and a galaxy that is far away today, for example. In the flat, matter-dominated (pressureless dust) GR cosmology model (Einstein-deSitter, EdS for short) that galaxy was moving away from us at time of emission at a much faster speed than it is today (time of reception). The rate of recession between the two galaxies will continue to diminish, slowing to v = 0 at t = ∞. In the Lambda CDM model (EdS + cosmological constant), the recession rate slowed at first, but then started to speed up again (when cosmological constant outward pressure started to dominate attraction of matter). But, in both models, the galaxies today are still not receding faster than they were at time of emission, although eventually in LCDM they will be.
 
  • #11
RUTA said:
Thanks, I'm glad to be done with this last calculation. It was nasty :devil:

Congrats!

RUTA said:
To clarify on this topic, what everyone agrees these guys did was obtain distance moduli (μ) out to redshift z ~ 1.5 using type Ia supernovae. Whether or not this data provides evidence that the universe is expanding depends on your cosmology model. Using inhomogeneous spacetime models, for example, the data does not show accelerating expansion (arXiv:gr-qc/0605088v2). Likewise, the PRD paper that started this conversation shows the accelerated expansion could be an illusion.

Oops... I took it for granted this was some 'cranky solution'...

A lot of questions arise:
  • How could a contracting universe look like an expanding universe? We will not get the same 'setup', playing 'the movie' backwards, right?

  • How could we observe the (extreme) red-shift in the CMB?

  • Why do we observe red-shift at all?

  • When did the universe swap 'direction'? BB did happen, right?

  • Are all data from Type II Supernovae wrong?

500px-HST_SN_1987A_20th_anniversary.jpg
 
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  • #12
HarryWertM said:
Begging patience for an imbecile...

If a supernova far, far away is moving very, very fast does that mean the universe long, long ago was moving faster than today?

According to the Lambda-CDM model the answer is: At early inflation, yes, later, no.

700px-CMB_Timeline300_no_WMAP.jpg
 
  • #13
Oops, I'm sorry, I've given the wrong impression. The universe is expanding, no one disputes that. It's just a question of whether or not the expansion is speeding up or slowing down. In LCDM it was slowing down until the cosmological constant started dominating the matter density at which point it started speeding up. This demarcation occurs in the data at about z = 0.752 (arXiv:1105.3470).
 
  • #14
RUTA said:
Oops, I'm sorry, I've given the wrong impression. ...

Phew, big relief!

Thanks

(:smile:)
 
  • #15
How could a contracting universe look like an expanding universe? We will not get the same 'setup', playing 'the movie' backwards, right?

If you think of space and time as opposite directions, you would know that looking out in space is looking back in time to the same point.
 
  • #16
petm1 said:
If you think of space and time as opposite directions, you would know that looking out in space is looking back in time to the same point.

Thanks, I understand, but I was thinking more on the second law of thermodynamics, the entropy of the universe, and the arrow of time.

AFAICT, you will not end up with an "exact copy" of the early universe, if you tried to reverse the current expansion for 13.75 billion years...
 
  • #17
DevilsAvocado said:
Thanks, I understand, but I was thinking more on the second law of thermodynamics, the entropy of the universe, and the arrow of time.

AFAICT, you will not end up with an "exact copy" of the early universe, if you tried to reverse the current expansion for 13.75 billion years...
Why not? If you have the current configuration of our universe exactly, then in running the clock backward it will necessarily reach an identical state.
 
  • #18
DevilsAvocado said:
How could a contracting universe look like an expanding universe?

As an aside, in answer to this question, it is possible in theory to see increased redshift with increased distance in a collapsing universe. The reason is that the redshift only tells you the universe scale factor is bigger at reception than at emission. Thus, z is independent of what happens dynamically between emission and reception. So, if a photon is emitted during an expanding phase and received shortly after the collapsing phase begins, as in the closed model, then you will see increased redshift with increased distance even though the universe is now collapsing. I explained this in a paper some years ago (“Kinematics between Comoving, Photon Exchangers in the Closed Matter-dominated Universe,” W.M. Stuckey, American Journal of Physics 60, No. 6, 554 - 560 (1992)).
 
  • #19
RUTA said:
As an aside, in answer to this question, it is possible in theory to see increased redshift with increased distance in a collapsing universe. The reason is that the redshift only tells you the universe scale factor is bigger at reception than at emission. Thus, z is independent of what happens dynamically between emission and reception. So, if a photon is emitted during an expanding phase and received shortly after the collapsing phase begins, as in the closed model, then you will see increased redshift with increased distance even though the universe is now collapsing. I explained this in a paper some years ago (“Kinematics between Comoving, Photon Exchangers in the Closed Matter-dominated Universe,” W.M. Stuckey, American Journal of Physics 60, No. 6, 554 - 560 (1992)).
Well, that's not really a contracting universe looking like an expanding one, though. The redshift combined with a distance measure still captures accurately the expansion history. The recent start of the collapse would also be captured in the relationship between redshift and distance (though if the collapse was recent enough it could only be extrapolated).
 
  • #20
Chalnoth said:
Why not? If you have the current configuration of our universe exactly, then in running the clock backward it will necessarily reach an identical state.

True, but it would be a terrible bad "illusion", because of the change of the direction of time; watching the omelet jumping out of the pan to 'regenerate' into 4 complete eggs... we would just know that there’s something 'fishy' going on... :smile:

Or?
 
  • #21
Chalnoth said:
Well, that's not really a contracting universe looking like an expanding one, though. The redshift combined with a distance measure still captures accurately the expansion history. The recent start of the collapse would also be captured in the relationship between redshift and distance (though if the collapse was recent enough it could only be extrapolated).

Except that at the shortest distances we don't see expansion regardless, we see collapse (gravity overcomes cosmological expansion within Local Group), so very close to the onset of collapse you would see only increasing redshift with increasing distance once you ignore the smallest z (which are negative). But, you're right, you should be able to fit the overall curve to see where you're at in the evolution.
 
  • #22
RUTA said:
As an aside, in answer to this question, it is possible in theory to see increased redshift with increased distance in a collapsing universe. The reason is that the redshift only tells you the universe scale factor is bigger at reception than at emission. Thus, z is independent of what happens dynamically between emission and reception. So, if a photon is emitted during an expanding phase and received shortly after the collapsing phase begins, as in the closed model, then you will see increased redshift with increased distance even though the universe is now collapsing. I explained this in a paper some years ago (“Kinematics between Comoving, Photon Exchangers in the Closed Matter-dominated Universe,” W.M. Stuckey, American Journal of Physics 60, No. 6, 554 - 560 (1992)).

I buy this, no problem. But something ought to happen to the CMB when we are collapsing (a strange mix of red-shift and "blue-shift"?), unless we assume the collapse just started...?

Or is this just a dumb guess... (<-- probably yes)
 
  • #23
DevilsAvocado said:
True, but it would be a terrible bad "illusion", because of the change of the direction of time; watching the omelet jumping out of the pan to 'regenerate' into 4 complete eggs... we would just know that there’s something 'fishy' going on... :smile:

Or?
Well, we'd be running back in time too, so we wouldn't see anything at all out of the ordinary :)
 
  • #24
RUTA said:
Except that at the shortest distances we don't see expansion regardless, we see collapse (gravity overcomes cosmological expansion within Local Group), so very close to the onset of collapse you would see only increasing redshift with increasing distance once you ignore the smallest z (which are negative). But, you're right, you should be able to fit the overall curve to see where you're at in the evolution.
Yeah, that's why I said that you'd have to extrapolate if the transition to collapse was near enough. The peculiar velocities just make the measurement too noisy in the local area.
 
  • #25
Chalnoth said:
Well, we'd be running back in time too, so we wouldn't see anything at all out of the ordinary :)

Agreed! :smile:

): yranidro eht fo tuo lla ta gnihtyna ees t'ndluow ew os ,oot emit ni kcab gninnur eb d'ew ,lleW
 
  • #26
DevilsAvocado said:
I buy this, no problem. But something ought to happen to the CMB when we are collapsing (a strange mix of red-shift and "blue-shift"?), unless we assume the collapse just started...?

Or is this just a dumb guess... (<-- probably yes)

As time went on you would see all the redshifts diminish, passing from redshifts to blueshifts. The point at which blueshifts turn to redshifts would (over eons) creep out to what is now the largest z, i.e., the CMB.
 
  • #27
I doubt you could predict the state of the universe by running it backwards due to the HUP. Similarly, the current state of the universe is not predictable by running the numbers forward from any given set of initial conditions. QM is probabilistic, not deterministic.
 
  • #28
Chronos said:
I doubt you could predict the state of the universe by running it backwards due to the HUP. Similarly, the current state of the universe is not predictable by running the numbers forward from any given set of initial conditions. QM is probabilistic, not deterministic.
If you want to get into the quantum details, you just need to know the full wavefunction of the universe at anyone point in time, and you can perfectly-predict that wavefunction at any other point in time. There is no non-determinism to be found there: the wavefunction simply evolves.
 
  • #29
Agreed, the wave function evolves - just not predictably.
 
  • #30
Chronos said:
Agreed, the wave function evolves - just not predictably.
No, exactly predictably. That's what quantum mechanics says. There is an appearance of probabilistic collapse only because are also quantum-mechanical.