The age of the universe: easy question - hard answer

[SpaceTiger]

AFAIK, QFT can't be well-defined in curved spacetime, any more than GR can be perturbatively quantized. Is that what you mean by saying GR is inconsistent with QFT? Perhaps QFT is inconsistent with GR?

Yes. The reason we think GR would fail on small scales is that QFT seems to work and they're inconsistent. We have no means of testing gravity on those scales (at the moment), so we're just assuming that GR is the problem. I'm not an expert on quantum gravity, so that's about all I can say.

 

[Chronos]

The paper is an interesting and compelling refutation of MOND at cluster scales. The underlying observations are not a confirmation of the existence of dark matter, however. The existence of dark matter is inferred from the failure of GR to properly predict the cluster lensing and cluster member velocities. Further observations of this type cannot be regarded as independent evidence for the existence of dark matter. We know that GR cannot properly predict galactic rotation curves, cluster binding, or cluster lensing, given the luminous matter observed. Some of us are so certain that GR must be right that we are unwilling to challenge it, and so we have to invent entities to "balance the books".

Let's pretend that non-baryonic dark matter does not exist, and we have to observe the universe with the radiation, particles, matter, of the known classical universe. In that case, GR's gravitation model works extremely well on most scales. GR fails to be predictive on galactic scales and larger, however. If we resist the temptation to enshrine GR as "perfect" (and Einstein's made his views on epistemology very well known), we must admit the possibility that GR's gravitational model is flawed - a better approximation perhaps than the Newtonian model, but still not correct.

Agreed, the existence of dark matter is inferred. So what? The existence of gravity is also inferred by observation. And further observations of this type will be regarded as evidence for the existence of dark matter - at least by the scientific community. Inferring entities from observational evidence is not an 'invention', it is science. The whole point of making additional observations is to test those inferences. While some of us may be so certain GR is right we are unwilling to challenge it, others are so certain it is wrong, they are blind to the evidence it is not wrong. Just what are you saying here - is GR merely an 'epicycle' of Newtonian gravity?

 

[turbo]

Agreed, the existence of dark matter is inferred. So what? The existence of gravity is also inferred by observation. And further observations of this type will be regarded as evidence for the existence of dark matter - at least by the scientific community.

The point is that the existence of dark matter was proposed to bridge the gap between behavior predicted by the standard model and what is actually observed. There is no independent justification for dark matter, aside for the need to resuscitate GR and keep the BB theory alive. None.

Inferring entities from observational evidence is not an 'invention', it is science. The whole point of making additional observations is to test those inferences. While some of us may be so certain GR is right we are unwilling to challenge it, others are so certain it is wrong, they are blind to the evidence it is not wrong. Just what are you saying here - is GR merely an 'epicycle' of Newtonian gravity?

No. GR is a more useful approximation of gravitation, and is a bit more widely applicable than Newtonian gravity. It is not, however, a predictive approximation on the scale of galaxies and clusters, nor is it applicable at quantum scales.

Inventing entities based on the failure of the standard model to properly predict observation is not science, it is fantasy. For instance, I may look up at the sky and see Jupiter moving at rates that are not consistent with what I would expect from observing the stars. If I am diligent, I may be able to deduce that Jupiter is a distant planet orbiting the Sun. If I am dull or credulous (but imaginative), I may infer that Jupiter is being pushed back and forth by angels. Silly idea, but the idea is no less silly than the concept that some mysterious substance can cause galaxies to have flat rotation curves and make clusters appear to have WAY more mass than we can detect visually. Dark matter is no more credible than Jupiter's "angels" and is no more likely to be detected.

 

[Chronos]

You are correct. Dark matter is the best fit model that agrees with both observation and theory. While dark matter has not been directly observed, there are a number of observations that lead to the same conclusion - none of which are required to 'resuscitate' GR or BB. How is inferring dark matter a 'fantasy'? GR does not require dark matter, nor does BB. By your arguments, every unexplained observation somehow yields credibility to 'tired light', 'intrinsic redshift', and 'steady state cosmology'. I think not.

 

[turbo]

While dark matter has not been directly observed, there are a number of observations that lead to the same conclusion - none of which are required to 'resuscitate' GR or BB. How is inferring dark matter a 'fantasy'? GR does not require dark matter, nor does BB.

We differ here, my friend...GR absolutely requires dark matter. GR gravitation is not predictive on galactic scales, based on the materials we can sense out there. That is why dark matter was hypothesized in the first place - to close the gap between what GR predicts and what we actually observe. Without dark matter, GR is not predictive on galactic scales - what is troubling is just how much of it is needed to close the gap.

Furthermore, making more and more observations that confirm the failure of the predictive power of GR gravity at galactic and cluster scales does not confirm the existence of dark matter. Such observations refine our understanding of just how the GR model of gravity fails under specific circumstances and they help us quantify the magnitude of the failure (and that is not a worthless endeavor, by any means), but the observations do not confirm the existence of dark matter. That is simply not logical - it is a matter of faith.

 

[Gold Barz]

We differ here, my friend...GR absolutely requires dark matter. GR gravitation is not predictive on galactic scales, based on the materials we can sense out there. That is why dark matter was hypothesized in the first place - to close the gap between what GR predicts and what we actually observe.

Furthermore, making more and more observations that confirm the failure of the predictive power of GR gravity at galactic and cluster scales does not confirm the existence of dark matter. Such observations refine our understanding of just how the GR model of gravity fails under specific circumstances and they help us quantify the magnitude of the failure (and that is not a worthless endeavor, by any means), but the observations do not confirm the existence of dark matter. That is simply not logical - it is a matter of faith.

turbo-1 check your PM's

 

[hellfire]

AFAIK, QFT can't be well-defined in curved spacetime, any more than GR can be perturbatively quantized. Is that what you mean by saying GR is inconsistent with QFT? Perhaps QFT is inconsistent with GR?

I have read that even quantum field theory on classical backgrounds which do not have certain symmetries is not well defined, as there are multiple representations of the canonical conmutation relations which are not related by unitary transformations. Is this correct? Do you know where can I find a proof of that, or a discussion of this topic?

 

[Thor]

Hi all,

I've been struggling with an idea lately and was wondering if anybody could shed some light on it. It seems simple but i don't think the answer is very obvious.

The best guestimate for the age of the universe at the moment is 13.5 billion years old. If we could travel back in time 13 billion years and then measured the age of the universe again what would be the answer? Would it be 0.5 billion years old?

Thank you.

If you were born on a proton which once was part of a nuclear bomb and the bomb had exploded prior to your birth, you would probably think the result of that explosion is all there is of the Universe - and that the Universe 'occurred' on the date and time of the explosion.

You would be wrong.

 

[Scinan Heorte]

I believe it is a mistake to assume that any age of the universe should be applied to all regions. I see the universe as a wholly dynamic environment in which a wide range of ages is possible. Where expansion rates are different from one portion to the next and presumptions concerning its size and age are far too easily made and included in calculations. Current estimates are dependent upon the presumption of a linear and static environment of unchanging conditions which has been as it is now since the begining. I feel this is wrong.
So far as your question goes, when you went back 13 billion years, did you take the universe with you? Last I heard, the hypersurface was an average of 13 billion years out from that point. I wonder would there be anything there but the void of time itself.
So far as an infinite universe is concerned, I do not see an infinite universe. I see a universe of finite energy and finite size expanding outward in a definable hypersphere of limited proportions. I see irregularities to it and non-linearities which make things difficult to understand, and I see difficulties in pinpointing the exact location within the void of time the big bang occurred, but I do not see unimaginable and limitless issues to get lost in.
I do however get seasick in the shift of perspectives involved.

 

[1123581321]

depends on what we find the Hubble constant to be (assuming big bang is correct and acceleration of universe is constant) (which yes, big bang is correct and yes, the acceleration of the universe is constant). If it is 50, then the universe is around 15 billion years old. If it is 80 then the universe is around 8 billion years old (the latter of the two is highly disputed, because we have found stars which we estimate to be older than 8 bill). mostly people think it is around 15 billion years old

Fibonacci