Thursday, January 5, 2012

The Case For the Cosmological Constant

One way of describing the universe is to describe it as having a uniform distribution of dark energy. Another is to simply conclude that the correct statement of the equations of general relativity include a cosmological constant. They aren't equivalent, although current evidence provides no means to distinguish the two theories. Marcus, at the Physics Forums, sums up the arguments for a cosmological constant (for which I have a great deal of sympathy) rather than a "substance" that is dark energy having physical reality (formatting revised to better fit this blog's style conventions):

[Loop quantum gravity physicists Bianchi and Rovelli's] "constant prejudices" paper which is the topic of this thread opens by quoting the first sentence of an article in Physics World co-authored by cosmologist Ofer Lahav (prof Astro. at University College, London). This is the kind of hype B&R are targeting (quoting Calder and Lahav in Physics World 23 (June 2010), 32–37):

Arguably the greatest mystery of humanity today is the prospect that 75% of the universe is made up of a substance known as "dark energy" about which we have almost no knowledge at all.

Earlier I quoted an excerpt from the version that Bianchi and Rovelli published in Nature journal "News and Views" section, the 15 July issue. Anyone who has read the piece in Nature carefully will realize that the operative word is "substance". They argue that it is misleading to talk about Λ (a small constant curvature) as a "substance". Quoting B&R's piece in Nature:

But it is a conceptual mistake to confuse Λ with QFT’s vacuum energy. Λ cannot be reduced to the ill-understood effect of QFT’s vacuum energy — or that of any other mysterious substance. Λ is a sort of ‘zero-point curvature’; it is a repulsive force caused by the intrinsic dynamics of space-time.

Efforts are under way to understand how this "zero point curvature" arises from the underlying quantum dynamics of space-time.

As quantum relativists the authors are naturally interested in how the zero point curvature relates to QG degrees of freedom: "the intrinsic [quantum] dynamics of space-time". There have been several articles about this. For a recent examples see page 41 of the 2010 paper by Meusburger and Fairbairn--also the paper by Han (a member of the Marseille group who has co-authored with B&R.) Continuing the B&R excerpt:

Tests on the ΛCDM model must continue and alternative ideas must be explored. But it is our opinion — and that of many relativists — that saying dark energy is a ‘great mystery’, for a force explained by current theory, is misleading. It is especially wrong to talk about a ‘substance’. It is like attributing the force that pushes us out of a turning merry-go-round to a ‘mysterious substance’...

For the full Nature article see:
The Bianchi, Rovelli, Kolb piece has a link to B&R's Arxiv article
"Why all these prejudices against a constant?"

If you accept that the cosmological constant is not a substance and is not mysterious and may very well be as fully understood as it will ever be, in terms of an accurate mathematical expression of it, and in terms of mechanism, then only dark matter and not "dark matter" is mysterious.

As I've repeatedly noted in this space, new data on the amount of ordinary matter in ellipical galaxies based on astronomy observations (which have not yet been widely assimilated) also indicates that the amount of dark matter is closer to 50% of all matter in the universe, not 80%. And, there are credible arguments that some significant part of that dark matter is due to general relativistic effects that were ignored in making the estimates, that are present in spiral galaxies and indeed, in an galactic structure with angular momentum.

After accounting for all those factors, and probably some slight remaining undercount of ordinary matter in the universe due to an inadequate ability to detect dim matter in certain kinds of distant galaxies and galactic clusters, this still leaves significant dark matter. My own best guess is that this is probably nothing more than fancy than plain old left handed neutrinos, possibly in a condensate, with little kinetic energy, and that standard leptogenesis scenarios have greatly underestimated how many neutrinos can plausibly be generated by standard weak force processes.

If that is true, than the lure of beyond the standard model physics motivated by the need for a dark matter candidate to account for 80% of the matter in the universe is not well founded.


Marcus said...

Hi O-W,
just visiting for the first time.
Glad you liked that PF post, or at least found it useful.
The link to a Dutch copy of the piece in Nature does not work anymore, I regret to say.

Nice blog you have here! :-D
Best wishes

Andrew Oh-Willeke said...


PF is one of my go-to sites for newly published physics pre-prints and LQG developments, and I pick up ideas from there when they seem suitable for a post.