As Einstein and Hawking point out, all the above means that time (space-time)is an illusion.

The very existence of time (and space) comes about by the process of observation…electromagnetic observation. The electromagnetic wave/particle duality implies we are on a Holo-Deck. We ARE real, but cosmologically, all this stuff about the expansion of the universe- all motion and change- is frame of reference “bull shit”. We observe “expansion”, spin (implying motion) and particles, but essentially (cosmologically), the universe is none of these things.

conumdroms like “wave-particle duality” and the fact that we measure gravity as propagating at the speed of light when we know full well such a thing is impossible are natures warnings to us….they are the door to the outside of the Holo-Deck…except that in the case of the universe, existence, motion and change are what we observe and measure in a very certain way- there is really NO “outside” or “beyond”. We and everything else we observe exist -permanently- ONLY inside.

If the graviton has any relevance/existence, how would it play out with gravitational effects (much less energy conservation) arising from an expanding universe.

Too much to hold in my mind. But then the idea that there is may be no exchange particle creating gravitational effects has always boggled my mind also.

The fact that open geometry GR does not have a conservation of energy principle casts doubt, I would think as to the veracity of the concept. Einstein’s grand proportion itself implies a cosmological relationship between all (or any part) of what we identify as “matter” and “energy”.

“Space” is created by the way energy densities are observed…their relationships relative to the observer within the manifold.

Tolman test is a fiction. Check out the linked article for example, the measured value of the exponent is between 2.6 and 3.4 but the authors conclude that the difference from 4 is due to galactic evolution and locate some model which gives such an evolution. Seriously, if we have to assume how a particular galaxy evolved billions of years ago then the test is worthless. Tolman original idea was that the measurement should be 4 to begin with so the above value can be seen as a refutation of simple expansion model.

But there are other problems with the test, to do it we have to know gravitational potential difference and relative speed to account for gravitational redshift and Doppler effect and there is no way to know them meaning no way to do the test.

http://www.iop.org/EJ/article/1538-3881/122/3/1084/201175.web.pdf?request-id=f2949343-d90f-46fc-9e42-714622f0f18f

> I am not suggesting that we not look for it, but damning cosmology because of some unknown physics is silly – you may as well damn all of science.

Certainly not all of science, only those disciplines where many experiments needed to verify theories cannot be performed for technical reasons. Most of natural sciences don’t have such problems.

And I am not trying to “damn” cosmology, I am trying to point out that many of it’s predictions are nowhere near the level of reliability that many people expect from natural sciences. For example it is far to early to conclude with certainty that our Universe started this way or that way – for that we have to first understand more then just 5% of its mass-energy content! We also need a proper theory of matter, meaning successfully marrying QM and GR, we need to be able to correctly predict vacuum energy, we need to know where the parameters of SM come from and so on.

There is more to expansion than just the redshift – bit only would photons have to magically lose energy, they would have to space themselves out so the surface brightness drops as (1+z)^4 – the Tolman surface brightness test.

Yes, there *may* some magic that can do this, but currently there is no mechanism for it and not even remotely workable theory (other than random handwaving) for it. Hence, cosmology in terms of GR is it.

I am not suggesting that we not look for it, but damning cosmology because of some unknown physics is silly – you may as well damn all of science.

I am well aware of that but that is not a scientific argument and it’s not convincing in any case.
History provides countless examples of experiments discovering completely novel and unexpected phenomena which were not predicted by currently popular theories.

The possibility that all electromagnetic radiation experiences intrinsic redshift on cosmic scales cannot be excluded simply because we have no idea why it should be so or how to explain it. For all we know it may simply be a fact with no explanation at all, just like existence of electromagnetic radiation has no explanation at all.

Observational astronomers are intimately familiar with redshift, of course, but even they don’t think much about violation of conservation of energy, because when they’re doing flux calculations, they’re either doing them locally (where a static background is a very good approximation) or they’re using the luminosity distance, which by construction hides the non-conservation of energy.

So for dark matter in rotating galaxies, what we are told by experiment is that we do not know all the mechanisms that determine the stress-energy, but we can infer components of the stress-energy from the geodesics (rotations) within the galaxies.

As for the FLWR, it is quite literally the simplest possible stress-energy that conforms to the broad observations we have of the universe, such as angular isotropy. So the fact that it agrees with the data with so few free parameters is stunning.

A final example of a well confirmed metric is the Kerr metric which, as anyone who as used centimeter scale GPS can confirm, is in excellent agreement with observation of the curvature around a rotating body.

One can think of GR more as a really powerful tool, where the specific metrics or stress-energies are the theories being tested. The only way to dispense with GR is to dispense with the assumption that space-time can be represented by a smooth manifold, but testing such a hypothesis would require experiments that are beyond the scale of what can be accomplished in this century. So at the scale of energies and distances we have measured to, space-time is accurately described by a smooth manifold.

Every contender has been ruled out.

Galaxy rotation curves are in stark disagreement with GR. In fact we cannot explain a whooping 95% of mass-energy content of the Universe and yet you claim there is “excellent agreement with observation” and that “other branches of science could only hope to be so lucky”?

>> Cosmology deals with concepts and distances far removed from anything we can experimentally test which puts it on a very shaky ground.

>Is this suggesting that cosmology is not an experimental science?

No, it’s saying that cosmology is on a very shaky ground – it’s based on limited experiments and breathtaking generalizations.

For example we cannot conclude that redshift of distant galaxies is due to expansion of space until we rule out the possibility that it’s an intrinsic property of all electromagnetic radiation. Unfortunately distances involved make such experiments technically impossible so the issue cannot be decided using scientific method.

Nope – as generally the potential they fall into is as deep as the one they climb out of.

Is this suggesting that cosmology is not an experimental science?

So the prediction, and then detection of the wobbles in the CMB was not experimental?

So the prediction that the time scale of distant supernovae would be time dilated was not a prediction?

So the prediction that the temperature of the CMB was hotter in the past was not experimentally verified?

Clearly many of those who criticize cosmology have no real clue about what happens on a day to day basis. We don’t just sit in the pub and make this stuff up! (well, except for the guys who publish in Phys Rev D).

when photons go through a gravitational lens, do they lose energy in addition to what they lose from a Hubble red-shift?

thx.

If we keep track of a certain fixed number of photons, the number stays constant while the energy per photon decreases, so the total energy decreases. A decrease in energy is just as much a “violation of energy conservation” as an increase in energy, but it doesn’t seem to bother people as much.

We’ve “known” about photons’ loss of energy due to the expansion of space-time since Edwin Hubble, but it’s funny how when the concept is expressed in a different format we can miss the connection.

Did it just take a long time to tease out this implication of GR? Or is time-translation invariance a good enough approximation that its treated as a no-harm, no-foul issue?

By comparison the Standard Model has 19 parameters that need to be fit, but many orders of magnitude more data collected.

Perhaps another way to popularize the problem is that stress-energy is a generalization of 4-momentum, which is a generalization of kinetic energy. Because the stress-energy is divergence-less, it has a type of conservation through orthogonally intersecting geodesics.

The assumption that our simple theories which hold on our tiny little planet at this moment in time also hold in the whole expanse of the Universe at all times is well… a bit naive.

Yes, it was worth a try, but now that we know it forces us to accept dark matter, dark energy, naked singularities, accelerating expansion of space, inflation and other such exotic concepts completely inaccessible to experimental verification an intellectually honest person is forced to conclude that the assumption mentioned above is no longer justified and that despite many grandiose claims we still know very little about the Universe.