Science Forums 2

[coldcreation]

Originally Posted by modest

Well said. I don't see this as an unreasonable position. Non-baryonic dark matter is not an easy pill to swallow.

It's funny, Lawrence Krauss is of the same opinion. After having written that he would be “very surprised if any of these initial models [GUTs or inflation] turned out to be true” (since both were divest of quantum gravity) he promptly then introduces the nonbaryonic dark matter issue to explain the flatness problem, saying the “stuff” must be made of “something else” as opposed to ordinary matter, and that we must have “missed most of it,” then concludes in his most expansive moment; “This is a very large pill to swallow.” (Krauss, 2001, pp. 138-68)

Originally Posted by modest

It seems, though, more and more evidence is coming in that is consistent with non-baryonic and cold dark matter—like dark matter gravitational lensing. Meanwhile, more and more candidates for baryonic dark matter are ruled out as telescopes get better at looking for them.

Eventually—and again, this is just an opinion or a prediction based on an alternative pet theory—all candidates for nonbaryonic cold dark matter should be ruled out. This at least is the hope, as our theoretical underpinnings of how nature works become more robust.

Originally Posted by modest

The indirect evidence is speaking pretty loudly.

True, but direct evidence leaves little to be desired.

Originally Posted by modest

Well, I would prefer GR work at all scales. As far as the solar system goes, I don't think the presence of dark energy or dark matter would really change the motion of the planets.

I would prefer GR work at cosmological scales without artificial contrivance, a voluntary Agent, a divine arm. The outlook is that in absence of a solid basis of experimental corroboration, a projected hypothesis is not on firm ground. In other words, modern cosmology has erected ‘new physics’ to fill the growing breach between theory and empirical evidence, direct or indirect.

Originally Posted by modest

So, General Relativity can be consistent with our observations in the solar system as well as galactic and cosmic observations if dark matter and dark energy do indeed exist...

Yes, and that seems to be the only way around the budgetary problem, without a full-scale revision of cosmology.

What counts (pun non-intended) is that physics must be verifiable, factual when possible and that it be utilized to make accurate interpretations.

Clearly there is a risk here. If physics is over-relaxed, or worse, vacated (extended beyond the empirically testable), the result inevitably leads towards something that is not physics, not natural, and ultimately less desirable, especially when the goal is to explain the physical world.

Tough standards, therefore, should be maintained (and imposed) to reduce the risk of cosmology running amok.

For now, few seem to believe that cosmology should stick to standard particle physics, despite the current low-credibility factor with respect to a mysterious cold dark matter dominating the cosmos.

This is another risk, yet perhaps worth taking: to place the field of cosmology back into the domain of testable physics. This policy might help reinvigorating a constructive debate over such important issues as the SNe Ia survey results.

Originally Posted by modest

But, there can't be two solutions describing the same physical situation that give two different answers. [...] As far as I know, General Relativity (including any exact solutions to GR) give exact answers. I don't think we can just find a new metric, or try to solve GR differently.

Einstein had written, 'It is the theory that decides what we can observe.'

There can be two interpretations (or solutions) describing the same observations that give two different answers. The solution depends on fundamental assumptions made at the outset.

Cosmology is a field where direct observations are made but the interpretation of those observations depend heavily on (in this case) a form a matter (CDM) currently beyond the limits of experimental verification and thus becomes more based on assumptions, a person's philosophy and/or religion. (See here for an interesting discussion: The Problem of Observation and Regional Ontologies).

Some interpretations are treated as if they were observations. These are 'commonsense' interpretations, such as "there's not enough visible mass to cause gravitational lensing, or a flat rotational curve, so there has to be something more: CDM." Here, there is an interpretation—though based on direct observations—presented as if it were the actual observation of CDM, and so the need to support the interpretation with direct evidence or explanation within the bounds of tested particle physics is vacated.

It gives the impression of solid evidence, though with an unstated presumption that a large body of underlying physics must be true, and simultaneously failing to consider opposing interpretations, because of uncritically accepted assumptions inherent within the standard model.

We should challenge the scientific community to offer alternative scenarios based on an underlying physics that has been tested, and challenge the untestable assumptions.

Originally Posted by modest

A great example of this are the first two models of cosmology based on GR—that of de Sitter and Einstein himself. Einstein's metric had matter, was spatially closed, and had a cosmological constant. De Sitter's had no matter or radiation pressure, was spatially open, and had a cosmological constant. These two metrics described two very different situations, but the later development of Friedmann's metric (FLRW) could include both situations. In fact, it was shown that Einstein's and de Sitter's universe were two examples of a family of universes described by FLRW. It is now generally accepted that our universe is turning into a de Sitter universe...

But just because something is generally accepted does not make it true.

It's ironic, too, that the original de Sitter world model was non-expanding and non-contracting (stationary).

Recall Edwin Hubble's words: “In the de Sitter cosmology, displacements of the spectra arise from two sources, an apparent slowing down of atomic vibrations and a general tendency of material particles to scatter. The latter involves an acceleration and hence introduces the element of time.” (Hubble 1929, A Relation Between Distance and Radial Velocity Among Extra-Galactic Nebula).

Though Hubble had slightly misinterpreted the de Sitter effect. The slowing down of clocks with distance is the element of time.

In the same works, Hubble wrote: “The outstanding feature, however, is the possibility that the velocity-distance relation may represent the de Sitter effect, and hence that numerical data may be introduced into discussions of the general curvature of space.”

The de Sitter universe was static, yet the light emitted by objects in it appeared redshifted (the infamous de Sitter effect).

Originally Posted by modest

Such a universe can be described with the de Sitter metric or the FLRW metric. Where they describe the same thing, they are the same. So, I don't think it's a matter of finding a new metric.

A de Sitter universe can either be interpreted as flat and expanding, or curved and stationary.

The observation is the same. The interpretation is different.

Originally Posted by modest

It seems, at this point, we either need to accept dark energy and dark matter as plausible or we need to figure out what has gone wrong with our cosmic solutions to general relativity.

Great point. More work arguably needs to be done on the latter.

Originally Posted by modest

Also, let me be clear—we're talking about the finer points of a big bang model here. The integrity of "the big bang" (i.e. the primordial atom) is not IMHO in jeopardy. The evidence for a big bang persist even if ΛCDM ends up being completely broken. If a person uses math to model a car crash and the model ends up being wrong that doesn't mean the car crash didn't happen. The broken glass and skid marks and whatnot are evidence of the crash with or without the model.

Actually, the primordial atom echoes the full pictorial inventiveness of Biblical creation (a useful parameterization of ignorance).

Truly, the primeval atom has left Himself without witness. Until physics can take us there (to t = 0) the primordial atom is in jeopardy.

The big bang itself, with its inaccessible tenants, has always carried with it a heightened vigilance against guided attacks; freed from the customary checks and balances of natural laws, physics, the ethical codes of science.

If Lambda-Cold Dark Matter ends up being completely broken, say, after more is learned about the applicable fundamental physics, or if next-generation telescopes show it to be untenable, then both the big bang theory and the shrouded primeval detonation (whatever is believed to have happened at t = 0) are in jeopardy.

To use your analogy above, the broken glass and skid-marks may not be evidence of a crash at all, within the framework of a model that would/should/could eventually replace ΛCDM.

The fundamental physical origin of matter (e.g., electrons, protons and neutrons), the physical interpretation of redshift z, the quasi-scale-invariant spectrum of CMBR curvature perturbations, the origin of the CMBR itself, large scale structure observations and SNe Ia data would all be subject to review.




CC