The paper has yet to be peer-reviewed, but Will Kinney, an inflationary cosmologist at the University at Buffalo and a visiting professor at Stockholm University, said, “The analysis seems correct to me.” He called the proposal “a very elegant idea.”
The Big Bang may have been one of many.
“If the signal is real and observable, it would be very interesting,” Sean Carroll of the California Institute of Technology said in an email.
Any potential hints about the Big Bang are worth looking for, but the main question, according to experts, is whether the putative oscillatory pattern will be strong enough to detect. It might not be a clear-cut guillotine as advertised.
If it does exist, the signal would appear in density variations across the universe. Imagine taking a giant ice-cream scoop to the sky and counting how many galaxies wind up inside. Do this many times all over the cosmos, and you’ll find that the number of scooped-up galaxies will vary above or below some average. Now increase the size of your scoop. When scooping larger volumes of universe, you might find that the number of captured galaxies now varies more extremely than before. As you use progressively larger scoops, according to Chen, Loeb, and Xianyu’s calculations, the amplitude of matter-density variations should oscillate between more and less extreme as you move up the scales. “What we showed,” Loeb explained, is that from the form of these oscillations “you can tell if the universe was expanding or contracting when the density perturbations were produced”—reflecting an inflationary or bounce cosmology, respectively.
Regardless of which theory of cosmogenesis is correct, cosmologists believe that the density variations observed throughout the cosmos today were almost certainly seeded by random ripples in quantum fields that existed long ago.
Inside the artificial universe that creates itself
Because of quantum uncertainty, any quantum field that filled the primordial universe would have fluctuated with ripples of all different wavelengths. Periodically, waves of a certain wavelength would have constructively interfered, forming peaks—or, equivalently, concentrations of particles. These concentrations later grew into the matter-density variations seen on different scales in the cosmos today.
But what caused the peaks at a particular wavelength to get frozen into the universe when they did? According to the new paper, the timing depended on whether the peaks formed while the universe was exponentially expanding, as in inflation models, or while it was slowly contracting, as in bounce models.
If the universe contracted in the lead-up to a bounce, ripples in the quantum fields would have been squeezed. At some point, the observable universe would have contracted to a size smaller than ripples of a certain wavelength, like a violin whose resonant cavity is too small to produce the sounds of a cello. When the too-large ripples disappeared, whatever peaks, or concentrations of particles, existed at that scale at that moment would have been “frozen” into the universe. As the observable universe shrank further, ripples at progressively smaller and smaller scales would have vanished, freezing in as density variations. Ripples of some sizes might have been constructively interfering at the critical moment, producing peak-density variations on that scale, whereas slightly shorter ripples that disappeared a moment later might have frozen out of phase. These are the oscillations between high- and low-density variations that Chen, Loeb, and Xianyu argue should theoretically show up as you change the size of your galaxy ice-cream scoop.