Our universe grows a little bigger every day. Empty space is expanding, sweeping galaxies further and further apart. Even starlight traversing this swelling nothingness is stretched like a rubber band.
The astronomical evidence for the accelerating expansion of the universe is overwhelming. But what is pushing the universe apart?
Particle physicists endeavor to answer cosmic-sized questions like this using the most fundamental laws of nature. But this particular query has them in a pickle because it is unlike anything else.
“If we understand gravity correctly, then there is some other substance in the universe that makes up about two-thirds of the total energy density and that behaves totally differently from normal matter,” says theorist Amol Upadhye, a postdoc at the University of Wisconsin, Madison. “So the big mystery is, what is this stuff?”
This stuff is dark energy, but besides its ostensible pushing effect in the cosmos, scientists know little else. However, theorists like Upadhye suspect that if there really is something causing empty space to expand, there is a good chance that it produces a particle. But to mesh with the cosmological observation, a dark energy particle would require a series of perplexing properties. For one, it would need to behave like a chameleon—that is, it would need to alter its properties based on its surroundings.
Cosmic chameleon: You come and go
In the depths of empty space, a chameleon particle might be almost massless, minimizing its gravitational attraction to other particles. But here on Earth (and in any other densely populated regions of space), the chameleon would need to swell to a much larger mass. This would limit its ability to easily interact with ordinary matter and make it nearly invisible to most detectors.
“If matter were music, then ordinary matter would be like the keys on a piano,” Upadhye says. “Each particle has a discrete mass, just like each piano key plays a single note. But chameleon particles would be like the slide on a trombone and able to change their pitches based on the amount of background noise.”
In addition to a sliding mass, the chameleons would need to exert a negative pressure. Classically, pressure is the force particles exert on their container. When the container is made of matter (like the rubber of a balloon), it expands as the internal pressure increases, and relaxes back to normal when the pressure diminishes. But when the container is made of nothing—that is, the container is spacetime itself—the reverse effect happens. For instance, when a birthday balloon fills with air, the surrounding empty space contracts slightly. But as the balloon releases air and the pressure diminishes, space relaxes back to normal.
All known particles contract space as their pressure increases and relax space as their pressure approaches zero. But to actually expand space, a particle would need to exert a negative pressure—an idea which is totally alien in our macroscopic physical world but not impossible on a subatomic scale.
“This was actually Einstein’s idea,” Upadhye says. “If you put in a substance with a negative pressure into the equations of general relativity you get this accelerating expansion of universe.”
A mass-shifting, space-expanding particle would be unlike anything else in physics. But physicists are hopeful that if such a particle exists, it would be abundant both in the depths of space and here in our own solar system.
Several experiments have searched indirectly for chameleon particles by closely monitoring the properties of ordinary matter and looking for any chameleon-like affects. But the CERN Axion Solar Telescope, or CAST experiment, is hoping to catch chameleons directly as they radiate from the sun.
“The sun is our biggest source of particles,” says Konstantin Zioutas, the spokesperson for the CAST experiment. “If chameleons exist, then they could copiously be produced in the sun.”
The CAST experiment is a specialized telescope that looks for rare and exotic particles emanating from the sun and the early universe. Zioutas and his colleagues recently installed a special magnifying glass inside CAST which collects and focuses particles onto a highly sensitive membrane suspended in a resonant electromagnetic cavity. Their hope is that if chameleon particles exist and are produced by the sun, they’ll see the very tiny pressure these particles flux should exert as they are reflected off the membrane when the sun is in view.
So far they haven’t seen anything unexpected, but new upgrades this winter will make their experiment even more sensitive to both solar chameleons and other exotic cosmic-sprung phenomena.
“The dark energy mystery is the biggest challenge in physics, and nothing we currently understand can explain it,” Zioutas says. “We need to look at the exotic of the exotica for possible solutions.”