DID the big bang boil? The birth of our universe could have seethed with hot bubbles and, perhaps, a second period of rapid expansion. Such an episode may have left an imprint on the universe that persists to this day and might mean we’re on the wrong track in our hunt for dark matter.
Just 10-37 seconds or so after its birth, a period of inflation is thought to have caused the universe to balloon in size. This process is thought to have amplified tiny quantum fluctuations in the vacuum, giving rise to the megastructures we see all around us in the universe today.
A second profound transformation is thought to have followed hot on the heels of inflation. Just microseconds old and at trillions of degrees, the universe condensed from a superhot soup of sub-nuclear particles called a quark-gluon plasma (QGP) into particles such as protons and neutrons. But exactly how this happened is far from clear.
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“Just microseconds old, the universe condensed from a superhot soup of sub-nuclear particles”
Decades ago, physicists suspected the change could have been abrupt and violent. In that scenario, the universe expands through inflation, then the QGP cools down to the point that bubbles spontaneously start to form. These bubbles release spectacular amounts of energy once stored in the vacuum, and particles composed of quarks and gluons are formed. The idea was attractive, not least because as these bubbles collided and merged, their interaction could have sown the seeds of intergalactic magnetic structures that persist today but whose origins are a mystery.
Yet computer simulations of conditions in the early universe soon began to hint that the transition from the superhot QGP to the matter we see today wasn’t nearly so dramatic. This culminated in 2006, when Yasumichi Aoki, then at the University of Wuppertal in Germany, and colleagues, reported the results of rigorous simulations that showed that the transition must have been smooth. “That was supposed to be the final word on the subject,” says of North Carolina State University in Raleigh.
Now some physicists are arguing that we ought to reconsider whether the cosmos had a bubbly “boiling” birth after all: new analyses suggest the QGP could have bubbled violently as it cooled and might even have been preceded by an additional phase of rapid expansion of the universe.
Aoki and colleagues had simulated a QGP in which matter and antimatter are found in equal amounts. That closely matches conditions in the early universe, which is thought to have contained a billion-and-one particles of matter for every billion particles of antimatter. Almost all of these particles annihilated each other, leaving the relatively small amount of matter that makes up all the stars and galaxies we see today.
Yet last year, Dominik Schwarz and Maik Stuke at Bielefeld University in Germany calculated that the universe could have bubbled and boiled if the initial number of leptons – particles which are not made of quarks, such as electrons and neutrinos – was sufficiently higher than their antiparticle partners (Journal of Cosmology and Astroparticle Physics, DOI: ).
Now Tillmann Boeckel and Jürgen Schaffner-Bielich at the University of Heidelberg in Germany propose an even more exotic way in which the nascent universe could have boiled. Based on estimates from previous models, they reckon the universe could have bubbled if the quarks outnumbered antiquarks by a ratio of about 2 to 1. Since this asymmetry is significantly greater than generally assumed for the initial state of the universe, the pair suggest that something must have wiped out the evidence. They have dubbed this something a “little inflation”.
The pair posit that in a brief period of secondary inflation, the interactions of quarks and gluons drove the universe to expand exponentially before bubbles formed and the transition was complete. This secondary “little inflation” would have washed out signs of a high matter-antimatter asymmetry, they claim ().
That’s because our knowledge of the relative levels of matter and antimatter in the moments after the big bang comes from the ratio of matter to the number of photons radiated by the cosmic microwave background, the universe’s oldest visible radiation. In Boeckel and Schaffner-Bielich’s scenario, extra photons, matter and antimatter released by the bubbles at the end of a little inflation would have resulted in the ratio we observe.
Based on current observations, the universe could only have expanded by a factor of 1000 or so during a second period of inflation. But even this comparatively small inflation could have big implications for the hunt for dark matter, Boeckel warns. Since matter will have been diluted by the additional expansion, more dark matter would have needed to exist in the early universe than is currently thought.
That means the best dark-matter candidates may have different properties from those that are currently favoured. If dark-matter particles were more abundant in the early universe than we thought, they must have a lower likelihood of annihilating with other dark matter particles.
Problematically, these less-interactive dark-matter candidates, if they exist, may be beyond the range of detection of experiments such as the Large Hadron Collider at CERN, says Boeckel. Conversely, if the LHC does find a dark-matter candidate, then his scenario “is probably ruled out automatically”, he says.
Right now, the two new studies raise more questions than they answer. The physics of QGPs that contain more matter than antimatter is difficult to calculate, so little is known about how they behave. A big uncertainty is how long the universe would expand and cool before bubbles began to form. “Nobody has calculated that in a reliable way,” Schwarz says.
“A big uncertainty is how long the universe would expand and cool before bubbles began to form”
The universe may well have bubbled, but for now the idea is speculative, says Schaefer, who is not associated with either study. To proceed, he says, physicists must first understand what conditions lead to a such violent, bubbly “first-order phase transitions” in QGPs. New laboratory experiments aim to probe the behaviour of QGPs to hunt for this transition (see diagram). Although QGPs can be created by colliding heavy atoms like gold, the high energies involved tend to produce equal and abundant amounts of matter and antimatter.
By lowering the energy of collisions – reducing the noise in the data – a team at the Relativistic Heavy Ion Collider at Brookhaven National Laboratory in Upton, New York, hopes to probe the behaviour of the plasmas in scenarios in which quarks outnumber antiquarks by a significant degree. The Facility for Antiproton and Ion Research in Darmstadt, Germany, will begin collisions as early as 2016, hunting for the QGP’s phase transition.
Spotting it is one thing. Demonstrating that a bubbling transition occurred in the early universe is an altogether tougher proposition. One avenue will be to hunt for slight deviations in the rhythmic timing of spinning stars called pulsars. These searches could turn up evidence of gravitational waves created when the bubbles collided.
It had seemed that the nail was in the coffin of boiling-universe hypotheses in 2006, says Schwarz. “It just shows that the story might not be as simple as we think.”
