Parallel universes are no longer just a feature of a good sci-fi story. There are now some scientific theories that support the idea of parallel universes beyond our own. However, the multiverse theory remains one of the most controversial theories in science.
Our universe is unimaginably big. Hundreds of billions, if not trillions, of galaxies, spin through space, each containing billions or trillions of stars. Some researchers studying models of the universe speculate that the universe's diameter could be 7 billion light-years across. Others think it could be infinite.
Around 13.7 billion years ago, everything we know of was an infinitesimal singularity. Then, according to the Big Bang theory, it burst into action, inflating faster than the speed of light in all directions for a tiny fraction of a second. Before 10^-32 seconds had passed, the universe had exploded outward to 10^26 times its original size in a process called cosmic inflation. And that's all before the actual expansion of matter that we usually think of as the Big Bang itself, which was a consequence of all this inflation: As the inflation slowed, a flood of matter and radiation appeared, creating the classic Big Bang fireball, and began to form the atoms, molecules, stars and galaxies that populate the vastness of space that surrounds us.
That mysterious process of inflation and the Big Bang have convinced some researchers that multiple universes are possible, or even very likely. According to theoretical physicist Alexander Vilenkin of Tufts University in Massachusetts, inflation didn't end everywhere at the same time. While it ended for everything that we can detect from Earth 13.8 billion years ago, cosmic inflation in fact continues in other places. This is called the theory of eternal inflation. And as inflation ends in a particular place, a new bubble universe forms, Vilenkin wrote for Scientific American in 2011.
Those bubble universes can't contact each other because they continue to expand indefinitely. If we were to set off for the edge of our bubble, where it might butt up against the next bubble universe over, we'd never reach it because the edge is zipping away from us faster than the speed of light, and faster than we could ever travel.
But even if we could reach the next bubble, according to eternal inflation (combined with string theory), our familiar universe with its physical constants and habitable conditions could be totally different from the hypothetical bubble universe next to our own.
These neutrinos also come in a variety of energies, with the most energetic ones (unsurprisingly) being the rarest and, to many physicists, the most interesting. Neutrinos are mostly invisible to normal matter — it would take about a light-year's worth of lead to have a 50/50 shot of stopping one — so they can realistically come from any direction.
However, most of the high-energy neutrinos that we see aren't produced from far away, but are produced when other cosmic particles (also of extremely high energies) strike the upper atmosphere, producing cascades of particles that also result in neutrinos. Some of these neutrinos will pass through the Earth almost completely, only interacting with the final layers of Earth's crust (or ice), where they can produce a signal that our detectors are sensitive to.
The rare events that ANITA saw were consistent with a neutrino coming up through the Earth and producing radio waves, but at energies that should be so high that passing through the Earth uninhibited should not be possible.
How many events like this did they see? Three.
Did they have to come through the Earth? No. The first two could have been normal air-shower tau neutrinos (one of the three types of neutrino allowed), while the third was probably just part of the experimental background.
In fact, there's an extraordinary piece of evidence that disfavors them coming through the Earth: the IceCube neutrino detector exists, and if high-energy tau neutrinos are regularly passing through the Earth (and the Antarctic ice), IceCube would have definitively seen a signal. And, quite unambiguously, they have not.
Scientifically, this means that:
ANITA saw radio signals that it could not explain,
their leading hypothesis was that high-energy tau neutrinos are traveling upwards through the Earth,
and that hypothesis was refuted by IceCube observations,
teaching us there is no astrophysical point source out there that is creating the particles that ANITA is indirectly seeing.
So where, in all of this, do the parallel Universes come in?
Because there were only three explanations for what ANITA saw: either there was an astrophysical source for these particles, there's a flaw in their detector or their interpretation of the detector data, or something very exotic, remarkable, and beyond the Standard Model (known as CPT violation) is happening. Some very good science ruled out the first option (back in January), which means it's almost certainly the second option. The third? Well, if our Universe cannot violate CPT, maybe this comes from a parallel Universe where CPT is reversed: an explanation that's as unlikely as it is poorly reasoned.
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