Ask Ethan: What Could Solve The Cosmic Controversy Over The Expanding Universe?
“As you pointed out in several of your columns, the cosmic [distance] ladder and the study of CMBR gives incompatible values for the Hubble constant. What are the best explanations cosmologists have come with to reconcile them?”
If you had two independent ways to measure a property of the Universe, you’d really hope they agreed with one another. Well, the situation we have with the expanding Universe is extremely puzzling: we have about 10 ways to do it, and the answers all fall into two independent and mutually incompatible categories. Either you make the measurement of an early, relic signal that’s observable today, and you get a value of 67 km/s/Mpc, with an uncertainty of about 1%, or you measure a distant object whose emitted light comes directly to our eyes through the expanding Universe, and you get a value of 73 km/s/Mpc, with an uncertainty of about 2%. It’s looking increasingly unlikely that any one group is wrong, in which case, we absolutely require some new, exotic physics to explain it.
While many ideas abound, there are five of them that are eminently testable in the next decade or so. Here’s how we could solve the expanding Universe controversy in the best way possible: with more and better science!
Cosmology’s Biggest Conundrum Is A Clue, Not A Controversy
“This is not some fringe idea, where a few contrarian scientists are overemphasizing a small difference in the data. If both groups are correct — and no one can find a flaw in what either one has done — it might be the first clue we have in taking our next great leap in understanding the Universe. Nobel Laureate Adam Riess, perhaps the most prominent figure presently researching the cosmic distance ladder, was kind enough to record a podcast with me, discussing exactly what all of this might mean for the future of cosmology.
It’s possible that somewhere along the way, we have made a mistake somewhere. It’s possible that when we identify it, everything will fall into place just as it should, and there won’t be a controversy or a conundrum any longer. But it’s also possible that the mistake lies in our assumptions about the simplicity of the Universe, and that this discrepancy will pave the way to a deeper understanding of our fundamental cosmic truths.”
In science, if you want to know some property of the Universe, you need to devise a measurement or set of measurements you can make to reveal the quantitative answer. When it comes to the expanding Universe, we have many different methods of measuring light that fall into two independent classes: using the imprint of an early relic and using the cosmic distance ladder. These two techniques each give solid results that are mutually inconsistent: the distance ladder teams find results that are higher than the early relic teams by about 9%. Since the errors are only about 1-2% on each measurements, this has been dubbed cosmology’s biggest controversy.
But perhaps it’s not about “who is right,” but rather about “what is the Universe doing?” Perhaps it’s a clue, not a controversy. Come learn about the cutting-edge science behind this fascinating and unexpected result.
Why Supersymmetry May Be The Greatest Failed Prediction In Particle Physics History
“There are two very different camps of scientists when it comes to the issue of SUSY. On the one hand, we have a large group of people, both theorists and experimentalists, are are following the evidence closely, seeking alternative explanations for these various puzzles, and responsibly constraining the viable scenarios to progressively tighter bounds. To rule out a theory that has dominated a sub-field of physics for nearly two generations would be a tremendous advancement for science.
But on the other hand, there’s a large and powerful group of (mostly) theorists who will go to their graves as true believers in not only SUSY, but electroweak-scale SUSY, regardless of what the evidence says. Yet with every new proton the LHC collides, we see the same answer again and again: no SUSY. No matter how often we fool ourselves, nor how many scientists get fooled, nature is the ultimate arbiter of reality. The experiments do not lie. As of today, there is no experimental evidence in favor of SUSY.”
Supersymmetry, or SUSY for short, is an incredibly compelling idea. You should know why, and I’m happy to tell you in this article. But it also doesn’t appear to describe our Universe, and not only should you know that, but so should many of the theorists still clamoring for more research into it.
Supersymmetry is, for my money, the greatest failed prediction in the history of particle physics. The experimental data says it’s time to move on.