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.

  • #31
So, HUP predicts a pencil balanced on its point can only remain in balance for a few seconds, yet, has deterministic predictive powers for the future [or past] of the universe over the course of billions of years? I doubt that.
 
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  • #32
Chronos said:
So, HUP predicts a pencil balanced on its point can only remain in balance for a few seconds, yet, has deterministic predictive powers for the future [or past] of the universe over the course of billions of years? I doubt that.
The Heisenberg Uncertainty Principle only talks about the accuracy of specific sorts of measurements. One way of thinking about it is that the wavefunction can (in principle) be determined precisely, but the wavefunction can only give precise information about either position or momentum, not both. This is, fundamentally, because once you have fully-defined the wavefunction in terms of position (for example), the wavefunction is completely determined. You can simply calculate the wavefunction as a function of momentum from the wavefunction as a function of position. This relationship, the fact that the position-space wavefunction and the momentum-space wavefunction are really the same exact thing represented differently, is what gives you the Heisenberg Uncertainty Principle.

But the principle itself says nothing whatsoever about what happens when you just deal with the dynamics in a purely quantum-mechanical sense, just talking about how the wavefunction evolves in time. In that case, it's all purely deterministic.
 
  • #33
Chalnoth said:
But the principle itself says nothing whatsoever about what happens when you just deal with the dynamics in a purely quantum-mechanical sense, just talking about how the wavefunction evolves in time. In that case, it's all purely deterministic.

Yes, it is, but just mathematically.

You make a wrong physical assumption: "just need to know the full wavefunction of the universe at anyone point in time". This is a physical impossibility, I'm sorry. So, for us the Universe will be forever non-deterministically.
 
  • #34
Skolon said:
Yes, it is, but just mathematically.

You make a wrong physical assumption: "just need to know the full wavefunction of the universe at anyone point in time". This is a physical impossibility, I'm sorry. So, for us the Universe will be forever non-deterministically.
Well, yes. But that is as much true classically as it is quantum-mechanically. This is a practical limitation, not a fundamental one.

Edit: Actually, let me amend that slightly. It is a fundamental limitation in the sense that the information needed to contain the full state of the universe would require a computer more complex than the entire universe. So in that sense it is a fundamental limitation. But what I'm trying to say here is that quantum mechanics doesn't change anything, at least not as far as this is concerned.
 
  • #35
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.

I agree, because even after it's proven wrong, using type 1A supernova will still be useful, and we'll just need to consider an extra variable of our local dark flow or whatever else is causing our relative acceleration. However, the average person wouldn't understand what the big deal with type 1A supernova is, so I can understand the naming the Nobel committee used.

I also can see why they picked to honor it now because of the threat of evidence mounting against universal acceleration of expansion. Good science is good science, even if they are eventually shown to be "wrong" because they missed some detail. Newton was eventually shown to be "wrong", for instance, however Newtonian physics still accurately describe specific macro-phenomena like how two billiard balls will interact at the macro level when we don't need to be so precise to consider the minute relativistic and quantum level effects. Einstein may be shown to be "wrong", especially if we can't refute the CERN neutrino results (if this doesn't turn out to be some experimental error). But then it will probably be some edge case / exception, and not the general rule. So "right" or "wrong" isn't the best way to look at it, but instead experimental and mathematical results add conditions to previously accepted theories. Most theories are eventually found to be incomplete, but if they stood the test of time for a while then they are at least correct in some limited scope. So in that light, the men being honored still did a good job.

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...

Doesn't the "no cloning" theorem prevent that? Or is it allowed as long as the "exact copy" doesn't exist simultaneously as the original?

Edit: No, in both Copenhagen and revised Everett interpretations, ways around "no cloning" exist for this discussion as long as no exact copy is made but instead separate measurements are made on the same quantum wave function. Instead, quantum decoherence is the issue. QC is reversible but wave function collapse isn't. See later.

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...

Or?

That would be interesting and I've thought about this quite a bit. I think our view of causality is biased and people who were experiencing everything backwards would just be used to it.

Edit:

Well, yes. But that is as much true classically as it is quantum-mechanically. This is a practical limitation, not a fundamental one.

Edit: Actually, let me amend that slightly. It is a fundamental limitation in the sense that the information needed to contain the full state of the universe would require a computer more complex than the entire universe. So in that sense it is a fundamental limitation. But what I'm trying to say here is that quantum mechanics doesn't change anything, at least not as far as this is concerned.

I agree more with your Edit better than your first statement but complexity of the computer is irrelevant when no computer is powerful enough to solve a problem. You seemed to have taken a "hidden variable" interpretation and believe there is actually a deterministic state versus a probabilistic one and that we can fully "know" the state if we crunch hard enough and have a complex enough computer. I can't read for sure if that's your viewpoint, or if you were just using terms that have different meanings in Computer Science. It's a common misconception that since you can describe the entire universe as a wave function, that you can deterministically describe its state. I'm a Computer Scientist and have also studied quantum computing and the mathematics behind it (which sadly isn't required study yet at most universities). It is useful when having these discussions to refer to theories of both classical and quantum computation and just view the universe as a giant quantum computer with some gates that perform a measurement to break superposition and create classical states (through decoherence). First of all, we do not know yet whether a classical computer can be as powerful as a quantum computer, and the general consensus is "no". Nevertheless, even if a classical computer could do everything a quantum computer can do, and therefore an ideal turing machine (the most powerful classical computer possible) could model the universe's classical states and superpositional states, then you still have another fundamental blocker: You can crunch all you'd like, even with an ideal turing machine -or- a quantum computer for that matter, and you'll never be able to deterministically "calculate" the "the universe" in a way you could predict 100% what the next state was going to be. This has been shown mathematically using a combination of the undecidable halting problem and Godel's incompleteness proof to show a T.O.E. can never completely describe the universe in a predictive way. The best you can do is an approximation, a probabilistic result!
This is a lot more than a practical limitation, it is a fundamental blocker. http://adsabs.harvard.edu/abs/2008PhyD..237.1257W"

However, all that is irrelevant since unitary quantum calculations are reversible, and therefore every part of the universe that's in quantum superposition is reversible. Of course, the classical states are reversible. What isn't reversible, unfortunately, are the wave function collapses "due to measurement" (really due to quantum decoherence). Therefore, at best, the universe could have a certain probability of reversing into the same state it was before. However, to say that it is 100% likely to reverse into the same state is not something we can acertain unless the universe had a completely known state which is not possible.
 
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  • #36
netdragon said:
I also can see why they picked to honor it now because of the threat of evidence mounting against universal acceleration of expansion.
There is no evidence mounting against universal acceleration. A cosmological-constant driven accelerated expansion remains the best fit to all current data.

netdragon said:
I agree more with your Edit better than your first statement but complexity of the computer is irrelevant when no computer is powerful enough to solve a problem. You seemed to have taken a "hidden variable" interpretation and believe there is actually a deterministic state versus a probabilistic one and that we can fully "know" the state if we crunch hard enough and have a complex enough computer.
I'm only assuming unitary quantum mechanics. And there's no such thing as wave function collapse, by the way.
 
  • #37
Chalnoth said:
And there's no such thing as wave function collapse, by the way.
Isn't that a matter of interpretation?
 
  • #38
bapowell said:
Isn't that a matter of interpretation?
Only if you think making up dynamics counts as "interpretation".
 
  • #39
Great. Thanks for the informative answer!
 
  • #40
RUTA said:
As time went on you would see all the redshifts diminish, passing from redshifts to blueshifts.

Thanks for the answer RUTA.

(I’m getting more and more convinced that spacetime is nothing but a sheet of rubber... :smile:)
 
  • #41
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.

Chalnoth said:
Well, we'd be running back in time too, so we wouldn't see anything at all out of the ordinary :)

DevilsAvocado said:
Agreed! :smile:

): yranidro eht fo tuo lla ta gnihtyna ees t'ndluow ew os ,oot emit ni kcab gninnur eb d'ew ,lleW


Could I change my mind...? :rolleyes:

Thinking a little bit more about this, it (maybe) doesn’t work, because:
  • AFAICT it’s more or less impossible to refute BB and the expansion of the universe, due to the red-shift in the CMB.

  • This means that at some point in history, intelligent observers would have to go thru the horrific experience of living their lives backwards, when the arrow of time change direction. I will not get into the disgusting details in the everyday process of getting food thru the body... in the wrong direction... Please note that we are talking about observers that only 'yesterday' did this in normal manners. I don’t think a civilization could cope and survive this kind of madness.

  • It also means that at some point in history, our laws of nature have to change, e.g. the second law of thermodynamics. You could hardly call it an "exact copy" of the early universe, if you have different/reversed laws of nature...

  • Personally, I don’t think that the human brain could function in a world with a reversed arrow of time. We need to have memories of yesterday and plans and expectations for tomorrow. My guess is that this is 'hardwired' in the brain, and we stop being humans without this functionality. This video about Clive Wearing shows a man with just 30 sec memory:


    https://www.youtube.com/watch?v=wDNDRDJy-vo

  • And then of course you have the problem with a supercomputer, bigger than the universe, calculating "the universal wavefunction"... :smile:
 
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  • #42
netdragon said:
... Doesn't the "no cloning" theorem prevent that? Or is it allowed as long as the "exact copy" doesn't exist simultaneously as the original?

Edit: No, in both Copenhagen and revised Everett interpretations, ways around "no cloning" exist for this discussion as long as no exact copy is made but instead separate measurements are made on the same quantum wave function. Instead, quantum decoherence is the issue. QC is reversible but wave function collapse isn't. See later.

We are not talking about "copying" the universe, "just" a very hypothetical change of expansion to contraction. The "main problem" would then be; the arrow of time, the second law of thermodynamics, and the entropy of the universe, i.e. if you’re looking for an "exact copy", meaning exactly the same state and entropy that was at BB.
 
  • #43
Chalnoth said:
Only if you think making up dynamics counts as "interpretation".
Actually, after staring at this sentence for the past 2 hours, I have absolutely no idea what it means. But, seriously, only attempt a clarification if you intend to actually explain something, rather than giving another vague and sarcastic response that is manifestly unhelpful.
 
  • #44
DevilsAvocado said:
We are not talking about "copying" the universe, "just" a very hypothetical change of expansion to contraction. The "main problem" would then be; the arrow of time, the second law of thermodynamics, and the entropy of the universe, i.e. if you’re looking for an "exact copy", meaning exactly the same state and entropy that was at BB.
Actually, I think the main problem would be that so far, by all indications, dark energy is truly a cosmological constant. As our experimental accuracy increases further, it only looks more and more constant.
 
  • #45
Chalnoth said:
Actually, I think the main problem would be that so far, by all indications, dark energy is truly a cosmological constant. As our experimental accuracy increases further, it only looks more and more constant.

Absolutely agree. (and this put another very solid point on "my list" ;)