- dark matter -- the stuff that (by its gravitational influence) seems to make up 26% of the mass/energy of the universe, and yet has resisted every effort at detection or inquiry into what other properties it might have.
- dark energy -- a mysterious "something" that is said to be responsible for the apparent runaway expansion of the universe, and which (like dark matter) has defied detection or explanation in any other way. This makes up 69% of the universe's mass/energy -- meaning the ordinary matter we're made of comprises only 5% of the apparent content of the universe.
- the conflict between the general theory of relativity (i.e. the theory of gravitation) and quantum physics. In the realm of the very small (or at high energies), the theory of relativity falls apart -- it's irreconcilable with the nondeterministic model of quantum mechanics. Despite over a century of the best minds in theoretical physics trying to find a quantum theory of gravity, the two most fundamental underpinnings of our understanding of the universe just don't play well together.
A while back I was discussing this with the fiddler in my band, who also happened to be a Cornell physics lecturer. Her comment was that the mess physics is currently in suggests we're missing something major -- the same way that the apparent constancy of the speed of light in a vacuum, regardless of reference frame, created an intractable nightmare for physicists at the end of the nineteenth century. It took Einstein coming up with the Theories of Relativity to show that the problem wasn't a problem at all, but a fundamental reality about how space and time work, to resolve it all.
"We're still waiting for this century's Einstein," Kathy said.
[Image licensed under the Creative Commons ESA/Hubble, Collage of six cluster collisions with dark matter maps, CC BY 4.0]
There's no shortage of physicists working on stepping into those shoes -- and just last week, two papers came out suggesting possible solutions for the first two problems.
One claims to solve all three simultaneously.
Both of them start with a similar take on dark matter and dark energy as Einstein did about the luminiferous aether, the mysterious substance that nineteenth-century physicists thought was the medium through which light propagated; they simply don't exist.
The first one, from Rajendra Gupta of the University of Ottawa, proposes that the need for both dark matter and dark energy in the model comes from a misconception about how the laws of physics change on a cosmological time scale. The prevailing wisdom has been "they don't;" the laws now are the same as the laws thirteen billion years ago, not long after the Big Bang. Gupta suggests that making two modifications to the model -- assuming that the strength of the four fundamental forces of nature (gravity, electromagnetism, and the weak and strong nuclear forces) have decreased over time, and that light loses energy as it travels over long distances, explain all the astrophysical observations we've made, and obviates the need for dark matter and dark energy.
The second, by Jonathan Oppenheim and Andrea Russo of University College London, suggests a different solution that (if correct) not only gets rid of dark matter and dark energy, but in one fell swoop resolves the conflict between relativity and quantum physics. They propose that the problem is the deterministic nature of gravity; if a quantum-like uncertainty is introduced into gravitational models, the whole shebang works without the need for some mysterious dark matter and dark energy that no one has ever been able to find experimentally.
The mathematics of the model -- which, I must admit up front, are beyond me -- introduce new terms to explain the behavior of gravity at low accelerations, which are (not coincidentally) the regime where the effects of dark matter become apparent. It's a striking approach; physicist Sabine Hossenfelder, who is generally reluctant to hop on the latest Grand Unified Theory bandwagon (and whose pessimism has been, unfortunately, justified in the past) writes in an essay on the new theory, "Reading Oppenheim’s new papers—published in the journals Nature Communications and Physical Review X—about what he dubs 'Post-Quantum Gravity,' I have been impressed by how far he has pushed the approach. He has developed a full-blown framework that combines quantum physics with classical physics, and he tells me that he has another paper in preparation which shows that he can solve the problem of infinites that plague the Big Bang and black holes."
Despite this, Hossenfelder is still dubious about Post-Quantum Gravity. "I don’t want to withhold from you that I think Oppenheim’s theory is wrong, because it remains incompatible with Einstein’s cherished principle of locality, which says that causes should only travel from one place to its nearest neighbours and not jump over distances," she writes. "I suspect that this is going to cause problems sooner or later, for example with energy conservation. Still, I might be wrong... If Oppenheim’s right, it would mean Einstein was both right and wrong: right in that gravity remained a classical, non-quantum theory, and wrong in that God did play dice indeed. And I guess for the good Lord, we would have to be both sorry and not sorry."
So we'll just have to wait and see. If either of these theories is right, we're talking Nobel Prize material. If the second one is right, it'd be the physics discovery of the century. Like Sabine Hossenfelder, I'm not holding my breath; attempts to solve definitively the three problems I started this post with are, thus far, batting zero. And I'm hardly qualified to make a judgment about what the chances are for these two. But like many interested laypeople, I'll be fascinated to see which way it goes -- and to see if we might, in the words of my bandmate/physicist friend, be "looking at the twenty-first century's Einstein."
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