Exotic star-like objects made up almost entirely of subatomic particles called pions could have formed in the earliest fractions of a second after the big bang. If proven to exist, these cosmic oddballs would be one of the strangest additions to the stellar catalogue.
Pions are lightweight particles formed from just two quarks, as opposed to more familiar matter like protons and neutrons, which each contain three quarks. The universe was suffused with a primordial pion soup one-ten-thousandth of a second after the big bang, and this soup was studded with other particles such as electrons, muons and neutrinos.
Ordinarily, pions decay within a few hundred millionths of a second. But at Goethe University Frankfurt in Germany and his colleagues hypothesise that a massive blob of positively charged pions could have glommed together during this early era, creating a star-like ball of an unusual state of matter called a Bose-Einstein condensate.
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Bound together like this, the pions would all interact with one another and start behaving as a single particle. The Bose-Einstein condensate鈥檚 oscillations would be something like those of a violin string, says of Washington University in St Louis, Missouri.
鈥淭he available ways in which it can twang do not allow it to twang in the right way that it can start decaying,鈥� he says. So, this pion 鈥渟tar鈥� could remain stable.
The team鈥檚 calculations suggest that such objects would be roughly 250 times the mass of the sun and about half the size of Jupiter. This is significantly larger and more massive than other compact, super-dense objects such as white dwarfs and neutron stars.
Whether astronomers could detect such peculiarities from the distant early cosmos is unclear. It is possible that a facility like the Laser Interferometer Gravitational-Wave Observatory (LIGO) in the US, which is designed to search for massive objects, might be able to spot the gravitational waves produced by two enormous pion stars spinning around one another before they smashed together, much like the signal it captured in October of two neutron stars merging.
Conventional telescopes could potentially spot pion stars too. The outermost pions would be only weakly bound to the rest of the Bose-Einstein condensate and constantly evaporating away, the team suggest. If so, most pion blobs would last about a million years before vaporising into nothing. But a few could have survived until now and might appear as warm, featureless, massive 鈥渟tars鈥� in our telescopes.
Pockets of pions
But for some astrophysicists, our chances of detecting pion stars are too remote to take the idea seriously. 鈥淭hey say that it is conceivable鈥� pion stars could form, says at the University of Tennessee, Knoxville. 鈥淭hat鈥檚 the kind of thing I say when I want to say, 鈥楾his is a crazy idea, but I have no idea if it really works or not.鈥欌��
On Earth, we can form Bose-Einstein condensates only by cooling material down to ultra-low temperatures. For pockets of pions to form a condensate in the hot early universe, the team say it would require a somewhat non-standard model of how the big bang played out, with more positively or negatively charged pions than typically assumed.
鈥淚 would have to say it鈥檚 a very speculative hypothesis,鈥� says Alford. 鈥淚t鈥檚 interesting because no one has suggested it before, so it鈥檚 worth exploring.鈥�
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