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Dark Energy not essential after all? (1)

ErkDemon (1202789) | more than 6 years ago | (#21852670)

Well, one of the problems with applying Einstein's general theory to cosmological problems is that the textbook version really doesn't work very well in that context. Einstein's implementation of a general theory was devised around the assumption that it'd have to reduce to special relativity over small regions, and as a result, it tends to come with some additional baggage that doesn't really work in the context of modern cosmology.

Einstein originally constructed his general theory around the assumption that the universe was "quasi-Euclidean" over large scales - effectively flat and infinite in extent, and of constant (infinite) size. Modern GR reckons that it's more natural to describe the universe as a spatially-closed bubble of expanding spacetime. So where Einstein's GR was engineered around the idea of "static", constant, stable behaviour, modern cosmology describes a more dynamic and unstable evolving structure that is downright chaotic in places (but the universe had to be slightly crazy in order to be able to produce tree-frogs and rap music and us, right?).

The original design brief of Einstein's GR shows itself in two "problem" areas.

The first problem is its description of the behaviour of geometrical horizons. In Einstein's GR, assumptions inherited from special relativity lead us to conclude that a gravitational horizon is a perfectly-sealed one-way surface (giving us GR's "black hole" behaviour). But this description doesn't fit cosmological horizons, which must leak information. A cosmological horizon seems to behave statistically in a way indistinguishable form the predictions of quantum mechanics for gravitational horizons, but modelling a cosmological horizon in this way suggests that spacetime may be operating according to rules that are more like those of an acoustic metric than those of special relativity.
Fixing the "cosmological horizon" problem seems to require a rewrite (or at least a serious rethink) of the rules that we often use to apply general relativity to large-scale problems. Einstein wasn't confronted with this issue when developing his implementation of a general theory, because in the sort of steady-state cosmology that he was originally using, these cosmological horizons didn't exist. Better-known variations on the "horizon problem" are recognised as demonstrating a fundamental mismatch between current GR and quantum theory.

The second problem (to get back on topic) is variable expansion rates. Basic gravitational principles suggest that the rate of timeflow in a region should increase as the local mass-density reduces. Throw in some entropic arguments, and we might say that if the expansion of spacetime itself can be taken as a measure of entropic timeflow, then lower-density regions ought to expand faster, making them even more rarefied.
In this sort of model, relative expansion becomes an unstable and unregulated positive-feedback process: the universe expands as a lobelike instability, and less-dense regions within it can in turn "lobe out" to form regions that expand faster than their immediate neighbours. You end up with a shape (in projection) that looks something like a raspberry, with the universe's radius (and age) being greater in the lobes than in the folds between. What you then expect to see in the pattern of mass-distribution within a parent lobe is a series of expanding voids, with matter tending to collect in the folds between lobes, as a series of wall-like structures.
And that seems to be pretty much what we see.

The reason why this behaviour isn't immediately obvious in GR is that again, Einstein was trying to design a tidy, stable universe in which the natural tendency of spacetime to collapse under its own large-scale gravitational influence was exactly countered by a compensating "Cosmological Constant" invented for the purpose. In Einstein's original model, everything was supposed to be in perfect equilibrium, giving us an effective gravitational "floor" ... a baseline background mass-density that permeated everywhere and represented a field minimum. In Einstein's "constant" universe, there were gravitational peaks and troughs, but the overall quasi-Euclidean geometry meant that the inverse square law still worked over arbitrarily-large scales, and you couldn't really dip below that constant background field-strength. Although GR's geometry was supposed to be self-contained and free-standing, the people applying it tended to use old ideas and conventions, and tended to think of the background field as something that could be intensified (by piling up more local matter), but not diluted.

But if we step away from the old configurations and embrace instability and evolution as the essence of entropic timeflow, this old picture goes out of the window. Instead of saying that the apparent voids and walls that we've now noticed represent the result of some new form of field or energy, we can describe them as being regions that have aged and expanded faster than our own (surrounding) reference-regions, due to their having originally had a fractionally lower initial mass-density, which has since been exaggerated by the resulting difference in expansion rates.

So, no, we don't need to invent new and exotic things to explain the voids, if we're prepared to look at general relativity again from scratch, without some of the original comforting simplifying assumptions.
But that can be a scary thing to do, and people often prefer to force an "old" model to make "new" predictions by retrofitting "new" effects to it, rather than by going back and reevaluating it from first principles.

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