TThis is a broad and in places unconventional overview of the strengths and shortcomings of our standard models of fundamental physics and of cosmology. The emphasis is on ideas that have accessible experimental consequences. It becomes clear that the frontiers of these subjects share much ground in common.
"Cosmology"
..., one set of exogenous parameters in the standard model of cosmology specifies a few average properties of matter, taken over large spatial volumes. These are the densities of ordinary matter (i.e., of baryons), of dark matter, and of dark energy.
We know quite a lot about ordinary matter, of course, and we can detect it at great distances by several methods. It contributes about 3% of the total density.
Concerning dark (actually, transparent) matter we know much less. It has been “seen” only indirectly, through the influence of its gravity on the motion of visible matter. We observe that dark matter exerts very little pressure, and that it contributes about 30% of the total density.
Finally dark (actually, transparent) energy contributes about 67% of the total density. It has a large negative pressure. From the point of view of fundamental physics this dark energy is quite mysterious and disturbing, as I’ll elaborate shortly below.
Cosmic Rays
Perhaps not quite so sharply posed, but still very promising, is the problem of the origin of the highest energy cosmic rays. It remains controversial whether there so many events observed at energies above those where protons or photons could travel cosmological distances that explaining their existence requires us to invoke new fundamental physics. However this plays out, we clearly have a lot to learn about the compositions of these events, their sources, and the acceleration mechanisms.
Is the Universe a Strange Place?
The observed values of the ratios ...[formulas for the cosmological density ratios] are extremely peculiar from the point of view of fundamental physics, as currently understood. Leading ideas from fundamental theory about the origin of dark matter and the origin of baryon number ascribe them to causes that are at best very remotely connected, and existing physical ideas about the dark energy, which are sketchy at best, don’t connect it to either of the others. Yet the ratios are observed to be close to unity. And the fact that these ratios are close to unity is crucial to cosmic ecology; the world would be a very different place if their values were grossly
different from what they are.
Several physicists, among whom S. Weinberg was one of the earliest and remains among the most serious and persistent, have been led to wonder whether it might be useful, or even necessary, to take a different approach, invoking anthropic reasoning. Many physicists view such reasoning as a compromise or even a betrayal of the goal of understanding the world in rational, scientific terms. Certainly, some adherents of the “Anthropic Principle” have overdone it. No such “Principle” can substitute for deep principles like symmetry and locality, which support a vast wealth of practical and theoretical applications, or the algorithmic
description of Nature in general. But I believe there are specific, limited circumstances in which anthropic reasoning is manifestly appropriate and unavoidable.
The highlighted opinion concurs with my own - for whatever that's worth. The Anthropic Principle seems like one of those things which get invoked when you run out of better ideas. That Weinberg and Wilczek, who have had many very good ideas indeed, are driven to this resort seems like a bad sign to me.
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