Far out cosmology: dark matter not found. Why? (Introduction)

by David Turell , Sunday, February 12, 2017, 14:57 (2628 days ago) @ David Turell

Joseph Silk explains why it should exist, but no experiment has found any trace as yet. Perhaps we need new ideas?

http://cosmos.nautil.us/feature/133/will-we-ever-know-what-dark-matter-is?utm_source=Na...

Dark matter is as tangible as stars and planets to most astronomers. We routinely map it out. We conceive of galaxies as lumps of dark matter with dabs of luminous material. We understand the formation of cosmic structure, as well as the evolution of the universe as a whole, in terms of dark matter. Yet a decade of sophisticated searches has failed to detect the material directly. We see the shadow it casts, but are completely unaware of what the dark side of the universe may contain.

It certainly isn’t any ordinary object or particle—that has long since been ruled out. Theoretical prejudice favors a novel type of particle that interacts only weakly with ordinary matter. Vast numbers of these particles should be flowing through our planet all the time, and by rights you’d expect some of them to leave a mark. Physicists have grown crystals and filled cryogenic vats, hauled them deep underground to screen out run-of-the-mill particles, and watched for tiny pulses of heat and flashes of light that would betray the passage of something never before seen. The results so far are not encouraging. In Lead, South Dakota, the LUX experiment operates one mile underground in an abandoned gold mine. It has found nothing. In China, the PandaX experiment in the Jin-Ping underground laboratory operates in a tunnel under 2.4 kilometers of rock. It has found nothing. In a road tunnel near Fréjus in the French Alps, the EDELWEISS experiment, at a depth of 1.7 km, has found nothing. And the list goes on.

The null results are rapidly squeezing the regions of parameter space where dark matter might lurk. Confronted by the drought of data, theoretical physicists have conjectured about more exotic particles, but the vast majority of these candidates would be even harder to detect. One could instead hope to produce dark-matter particles at a particle accelerator, so that we could infer their presence by default: by checking whether energy seemed to go missing in particle collisions. But the Large Hadron Collider has tried precisely this and noticed nothing so far. Some theorists suspect dark matter doesn’t exist and our theory of gravitation—Einstein’s general theory of relativity—has led us astray. General relativity tells us that galaxies would fly apart if not held together by unseen matter, but perhaps the theory is wrong. Yet general relativity has passed all other observational tests, and all rival theories have seemingly fatal flaws.

Eighty-five percent of all matter is unknown. Our greatest fear is that it will always remain so.

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In a sense, we are in the situation every scientist dreams of. Old ideas aren’t working; new ones are needed. These might come from exploring novel types of particles, or we might discover a fully consistent new theory of gravity that dispenses entirely with dark matter.

The nagging worry is that nature has put the new physics in a place where we can’t find it. Although we haven’t completely exhausted the search for WIMPs, there’s only so much more that experiments can do. As they become more sensitive to dark matter, they also become more sensitive to garbage particles, and they cannot always discriminate between the two. At the present rate of improvement, within a decade they will be blinded by neutrinos emitted either by the sun or by cosmic rays colliding with Earth’s atmosphere.

The strongest tool for discovery of dark-matter particles would be a new particle collider. Fast-forwarding some three decades from now, physicists plan to build a collider with seven times the power of the LHC. Studies are underway both in China and in Europe. Crudely scaling up from the LHC, it would cost $25 billion in today’s dollars. Shared among nations and spread over the decades, that might just be feasible. But it is probably the limit. Even if physicists had unlimited resources, nothing would be gained by building anything larger. At that point, any unknown particle would have to be so massive that, were the particle produced in the same way as its lighter counterparts, the big bang would not have produced it in sufficient quantity.

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Despite these immense efforts, we may not find any signals. That would be a gloomy prospect. Maybe there is no dark matter. We keep looking for deviations from general relativity. So far we have found none. On the contrary, the detection of black holes in 2016 by gravitational waves has bolstered Einstein’s theory—and its corollary, the existence of dark matter.

But look on the bright side. There could be immense mysteries and revelations about the dark side of nature that we will never glimpse unless we search. For now, we keep looking for particles. We can do nothing else but press on.

Comment: Frustrating as the Standard Model works so well and Wimps should fit. We are accustomed to rapid science advances, but the easy stuff has been detected. God has not made it easy.