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From symmetry

Is this the only universe?

Our universe could be just one small piece of a bubbling multiverse. Image: Sandbox Studio with Ana Kova

Human history has been a journey toward insignificance.

As we've gained more knowledge, we've had our planet downgraded from the center of the universe to a chunk of rock orbiting an average star in a galaxy that is one among billions.

So it only makes sense that many physicists now believe that even our universe might be just a small piece of a greater whole. In fact, there may be infinitely many universes, bubbling into existence and growing exponentially. It's a theory known as the multiverse.

One of the best pieces of evidence for the multiverse was first discovered in 1998, when physicists realized that the universe was expanding at ever increasing speed. They dubbed the force behind this acceleration dark energy. The value of its energy density, also known as the cosmological constant, is bizarrely tiny: 120 orders of magnitude smaller than theory says it should be.

For decades, physicists have sought an explanation for this disparity. The best one they've come up with so far, says Yasunori Nomura, a theoretical physicist at the University of California, Berkeley, is that it's only small in our universe. There may be other universes where the number takes a different value, and it is only here that the rate of expansion is just right to form galaxies and stars and planets where people like us can observe it. "Only if this vacuum energy stayed to a very special value will we exist," Nomura says. "There are no good other theories to understand why we observe this specific value."

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Laura Dattaro

Video of the Day

LHC computing

The LHC is the world's highest-energy particle accelerator, and scientists use it to record an unprecedented amount of data. U.S. CMS Education and Outreach Coordinator Don Lincoln gives us a sense of just how much data is involved and the incredible computer resources that makes it all possible. View the video. Video: Fermilab
Photo of the Day

A leg up

A wood duck at Bullrush Pond gives its right leg a break. Photo: Bridget Scerini, TD
In the News

Synopsis: Entangled mirrors could "reflect" quantum gravity

From Physics, July 28, 2015

Entanglement is a correlation between the quantum states of two objects, usually of microscopic size. But as a new theoretical analysis suggests, current technology could allow entanglement of mirrors weighing as much as 100 grams. The proposed experiment could help test theories that attempt to unify quantum mechanics and gravity.

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Frontier Science Result: CMS

Shedding light on the invisible Higgs

Some of the Higgs boson's decay modes may be invisible to detectors, but that doesn't mean we can't see them.

There are basically two types of detectors used in collider experiments: trackers, which are sensitive to any particles that interact electromagnetically, and calorimeters, which are sensitive to any particles that interact electromagnetically or through the strong force. That's only two of the four forces — there's also the weak force and gravity. Anything that interacts exclusively through the latter two forces would be invisible.

This is not a speculative point. Neutrinos are effectively invisible in collider experiments. Even specialized neutrino detectors can detect only a small fraction of the neutrinos that pass through them. Dark matter is known purely through its gravitational effect on galaxies; no one even knows if it interacts via the weak force as well. Invisible particles could be slipping through detectors at the LHC right now.

But if you can't see them, how can you find them? Fortunately, physicists have developed a few tricks, mostly involving conservation laws. For instance, conservation of charge forces some particles and antiparticles to be produced in pairs, and one may be detected while the other decays invisibly. Conservation of momentum requires particles to be produced symmetrically around the beamline; if the observed distribution is highly asymmetric, that's an indication of an unseen particle.

In a recent study, CMS physicists used the latter technique to determine how often Higgs bosons decay into invisible particles and also a photon. This is interesting because Higgs bosons have been observed only in a few of their predicted decay modes — the rest could be wildly different from expectations. In particular, Higgs bosons could interact with new phenomena like dark matter or supersymmetry, and most of these particles would be invisible. One of the ways supersymmetry might be hiding is by decaying into gravitinos (gravity only), neutralinos (gravity and weak only) and a visible photon.

Through this analysis, the mostly invisible signature has been partially ruled out: At most 7 to 13 percent of Higgs bosons might decay this way, if any at all. Before the measurement, it could have been as much as 57 percent. That's a lot for one bite!

Jim Pivarski

These physicists contributed to this analysis. Also contributing, Dustin Stolp of UC Davis, not pictured.
In the News

What the heck is a pentaquark?

From NOVA's Nature of Reality, July 28, 2015

What do you get when you combine four quarks and an antiquark?

If you think this sounds like the opening of a particle physicist's riddle, you aren't too far off. Hypothetically, this particular quark combo makes a "pentaquark." Despite decades of searching, physicists haven't been able to actually find a pentaquark. Now, though, there's a hint that two pentaquarks have unexpectedly come out of hiding.

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