Here are six big questions about our universe (dark matter and black holes) that current physics can’t answer:
- What is dark energy, the mysterious energy that appears to be accelerating the expansion of the universe?
- What is dark matter, the invisible substance we can only detect by its gravitational effect on stars and galaxies?
- What caused inflation, the blindingly fast expansion of the universe immediately after the Big Bang?
- For that matter, what caused the Big Bang?
- Are there many possible Big Bangs or universes?
- Is there a telltale characteristic associated with the death of a universe?
Despite the efforts of some of the world’s brightest brains, the Standard Model of particle physics – our current best theory of how the universe works at a fundamental level – has no solution to these stumpers.
A compelling new theory claims to solve all six in a single sweep. The answer, according to a paper published in European Physical Journal C by Herb Fried from Brown University and Yves Gabellini from INLN-Université de Nice, may be a kind of particle called a tachyon.
Tachyons are hypothetical particles that travel faster than light. According to Einstein’s special theory of relativity – and according to experiment so far – in our ‘real’ world, particles can never travel faster than light. Which is just as well: if they did, our ideas about cause and effect would be thrown out the window, because it would be possible to see an effect manifest before its cause.
Although it is elegantly simple in conception, Fried and Gabellini’s model is controversial because it requires the existence of these tachyons: specifically electrically charged, fermionic tachyons and anti-tachyons, fluctuating as virtual particles in the quantum vacuum (QV). (The idea of virtual particles per se is nothing new: in the Standard Model, forces like electromagnetism are regarded as fields of virtual particles constantly ducking in and out of existence. Taken together, all these virtual particles make up the quantum vacuum.)
But special relativity, though it bars faster-than-light travel for ordinary matter and photons, does not entirely preclude the existence of tachyons. As Fried explains, “In the presence of a huge-energy event, such as a supernova explosion or the Big Bang itself, perhaps these virtual tachyons can be torn out of the QV and sent flying into the real vacuum (RV) of our everyday world, as real particles that have yet to be measured.”
If these tachyons do cross the speed-of-light boundary, the researchers believe that their high masses and small distances of interaction would introduce into our world an immeasurably small amount of ‘a-causality’.
Fried and Gabellini arrived at their tachyon-based model while trying to find an explanation for the dark energy throughout space that appears to fuel the accelerating expansion of the universe. They first proposed that dark energy is produced by fluctuations of virtual pairs of electrons and positrons.
However, this model ran into mathematical difficulties with unexpected imaginary numbers. In special relativity, however, the rest mass of a tachyon is an imaginary number, unlike the rest mass of ordinary particles. While the equations and imaginary numbers in the new model involve far more than simple masses, the idea is suggestive: Gabellini realized that by including fluctuating pairs of tachyons and anti-tachyons he and Fried could cancel and remove the unwanted imaginary numbers from their calculations. What is more, a huge bonus followed from this creative response to mathematical necessity: Gabellini and Fried realized that by adding their tachyons to the model, they could explain inflation too.
“This assumption [of fluctuating tachyon-anti-tachyon pairs] cannot be negated by any experimental test,” says Fried – and the model fits beautifully with existing experimental data on dark energy and inflation energy.
Of course, both Fried and Gabellini recognize that many physicists are wary of theories based on such radical assumptions.
But, taken as a whole, their model suggests the possibility of a unifying mechanism that gives rise not only to inflation and dark energy, but also to dark matter. Calculations suggest that these high-energy tachyons would re-absorb almost all of the photons they emit and hence be invisible.
And there is more: as Fried explains, “If a very high-energy tachyon flung into the real vacuum (RV) were then to meet and annihilate with an anti-tachyon of the same species, this tiny quantum ‘explosion’ of energy could be the seed of another Big Bang, giving rise to a new universe. That ‘seed’ would be an energy density, at that spot of annihilation, which is so great that a ‘tear’ occurs in the surface separating the Quantum Vacuum from the RV, and the huge energies stored in the QV are able to blast their way into the RV, producing the Big Bang of a new universe. And over the course of multiple eons, this situation could happen multiple times.”
This model – like any model of such non-replicable phenomena as the creation of the universe – may be simply characterized as a tantalizing set of speculations. Nevertheless, it not only fits with data on inflation and dark energy, but also offers a possible solution to yet another observed mystery.
Within the last few years, astronomers have realized that the black hole at the centre of our Milky Way galaxy is ‘supermassive’, containing the mass of a million or more suns. And the same sort of supermassive black hole (SMBH) may be seen at the centres of many other galaxies in our current universe.
Exactly how such objects form is still an open question. The energy stored in the QV is normally large enough to counteract the gravitational tendency of galaxies to collapse in on themselves. In the theory of Fried and Gabellini, however, when a new universe forms, a huge amount of the QV energy from the old universe escapes through the ‘tear’ made by the tachyon-anti-tachyon annihilation (the new Big Bang). Eventually, even faraway parts of the old universe will be affected, as the old universe’s QV energy leaks into the new universe like air escaping through a hole in a balloon. The decrease in this QV-energy buffer against gravity in the old universe suggests that as the old universe dies, many of its galaxies will form SMBHs in the new universe, each containing the mass of the old galaxy’s former suns and planets. Some of these new SMBHs may form the centres of new galaxies in the new universe.
“This may not be a very pleasant picture,” says Fried, speaking of the possible fate of our own universe. “But it is at least scientifically consistent.”
And in the weird, untestable world of Big Bangs and multiple universes, consistency may be the best we can hope for.
Robyn Arianrhod is a senior adjunct research fellow at the School of Mathematical Sciences at Monash University. Her research fields are general relativity and the history of mathematical science.
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