Gravitational waves: Do they suggest a bang at the end of the universe?

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How something can come out of nothing is perhaps the biggest mystery related to the Big Bang. The most accepted theory of the universe’s beginning states that it started as an infinitely small, infinitely dense point that expanded outwards and cooled to become the modern cosmos. But what was the cause of this event nearly 14 billion years ago? 

Even this is a question loaded with problems. If the Big Bang created time, as conventional thinking says, then you can’t talk about “before,” or a prior cause, as those are notions that only make sense if time already existed.

Sir Roger Penrose, a long-time collaborator of Stephen Hawking, believes he has a way to banish these problems for good. What’s more, astronomers might have found evidence to confirm he’s right. His theory is called Conformal Cyclic Cosmology (CCC), and it says that the explosive birth of our universe arose during the twilight years of another. In other words, there was a time before the Big Bang.

According to Penrose, there is “a mammoth in the room” that no cosmologist is currently addressing: How the early universe at the Big Bang was, in some ways, very similar to the state our universe is heading in the distant future. In both cases, mass makes a significantly lower contribution to the total energy of the universe than it does today. 

To calculate kinetic energy, or the energy of motion, you half the mass and multiply it by its velocity squared. In the first moments after the Big Bang, when the cosmos was very hot, particles were flying around stupendously fast. That means their speed made the majority contribution to the universe’s total energy, not the particle’s mass.

The same can be said of the universe’s future. In 1998, physicists discovered that the universe was expanding at an ever-increasing rate, shaking the astronomical community to its core. They had expected the cosmos to be slowing down as the power of the Big Bang waned. So, for the expansion to be accelerating again, astronomers believed there must be some invisible entity, known as dark energy, pushing everything apart. Eventually, all matter in the universe will be separated to such a great extent that mass again becomes a trivial factor in the overall energy of the cosmos.

In both cases, the universe will eventually be dominated by light, not matter. And for a photon (a massless particle of light) time and length does not exist. Ride along on a photon and you’d zip across the visible universe in literally no time. That insight was Penrose’s key breakthrough. 

“In both cases, the universe doesn’t know how big it is,” Penrose said. As far as the universe is concerned, its hot, small beginning is physically identical to its cold, huge future. That in itself isn’t controversial, but Penrose goes a step further. “This remote future becomes another Big Bang,” he says. So what happened before the Big Bang? 

According to Penrose another universe ended, and that universe sprung from the death of yet another. Penrose calls each period an aeon. The aeons go further and further back in time with no need for an initial beginning. In some ways, it’s a return to the steady-state model that prevailed before the Big Bang gained significant traction in the mid-20th century.

Penrose admits it’s a wild suggestion, but believes that like all good scientific theories, it might be tested through experiment and observation. These tests stem from the idea that our aeon and the one preceding it were not completely isolated from one another. “Information does get through,” he said. “It gets through in the form of a shock wave in our universe’s initial dark matter.” 

Dark matter, like dark energy, is a shadowy substance, this time needed to account for the way structures such as galaxies and clusters of galaxies formed in the early universe. According to Penrose’s calculations, that shock wave would have had an effect on the cosmic microwave background (CMB), which is the leftover radiation from the Big Bang, released when the universe was under 400,000 years old. “You’d see rings in the CMB that are slightly warmer or cooler than the average temperature,” he said.

The equations of CCC predict that a shock wave arriving from a previous aeon would have dragged matter into our universe. If that caused material to head toward us, we would see light from that region shunted to shorter wavelengths — an effect astronomers call blueshift. Equally, a region carried away from us by a CCC shock wave would be redshifted, meaning its wavelength would be stretched out. 

Blueshifted regions would appear hotter and redshifted areas cooler. It’s these changes Penrose believes we’d see as rings in the cosmic microwave background. Multiple shockwaves might even have produced a series of concentric rings. “I asked whether anyone had looked for these rings in the sky,” Penrose said.

Several years ago, it did seem as if those rings had been found, a veritable smoking gun for CCC. “Except nobody believed us. They said it must have been a fluke or something,” Penrose said. 

“But those signatures have been confirmed by alternative groups,” said Vahe Gurzadyan a physicist at the Yerevan Physics Institute in Armenia and Penrose’s long time collaborator on CCC.

The scientists point to the fact that a team of Polish and Canadian researchers confirmed the presence of the rings to a confidence level of 99.7%. However, there are still many doubters. Gurzadyan remains steadfast. “These structures are real – there is no doubt that our calculations are reliable and correct,” he said. Still, Penrose has been exploring other approaches that might further support the pair’s claims about CCC and a time before the Big Bang.

Related: Video: Historic gravitational waves discovery explained by experts

The transition between aeons would do something more fundamental that just create a shock wave in our dark matter and rings in the cosmic microwave background. “A new material, the dominant material in the universe, is created at the crossover,” Penrose said. He regards that new material as the initial form of dark matter itself. 

“But in order that it doesn’t build up from aeon to aeon, it has to decay,” he said. He calls these initial dark matter particles erebons after Erebos, the Greek god of darkness.

On average it would take 100 billion years for an erebon to decay, but there are some that will have decayed in the 14-billion-year history of our universe. Crucially, as they decay, Penrose says erebons dump all their energy into gravitational waves.

Gravitational waves are distortions in the fabric of space-time, predicted by Einstein over a century ago as part of his theory of general relativity. For most of the past century, we didn’t know if gravitational waves even existed. But that changed on Sept. 14, 2015, when physicists using the Laser Interferometer Gravitational-Wave Observatory (LIGO) announced the detection of gravitational waves arriving at the Earth, from two black holes which had smashed together at nearly one-half the speed of light. Several other detections have followed, including more black hole mergers, along with the collision of two neutron stars — the collapsed cores of massive stars (which are still too small to form black holes) that have gone supernova.

In the summer of 2017, the astronomical community was buzzing with rumors that these detections might not have been what they seemed after all. A team from The Niels Bohr Institute in Copenhagen published a paper suggesting that the signals were not gravitational wave events, but ghosts in the data instead. By the time a gravitational wave makes it to the Earth its signal is very weak, making it difficult for physicists to pick out these disturbances above the background noise of more mundane terrestrial events that might also wiggle LIGO’s sensitive mirrors. If the same signal is picked up by both detectors, that is a massive clue that it has come from space. The noise, however, should not be correlated in the same way. A truck trundling by in Washington State should not affect a detector over 1,800 miles (3,000 kilometers) away in Louisiana.

The Copenhagen team performed their own independent analysis of the LIGO data and found that the noise was indeed correlated, too. LIGO physicists may have been fooled into thinking they were picking up gravitational waves when they weren’t. Perhaps there was something up with the detectors that meant they produced gravitational wave signals where none existed. 

The Copenhagen paper was met by a swift rebuke from Ian Harry, a physicist at the Max Planck Institute for Gravitational Physics in Germany and a member of the LIGO team. He argued that the Copenhagen team hadn’t performed their analysis correctly and there was no correlated noise.

When Roger Penrose heard about this debate he thought more about what could be causing any correlated noise. “Maybe they are seeing erebon decay,” he said. He soon published his own paper setting out more details behind his claim. 

The arrival of gravitational waves from erebon decay would be correlated between the two detectors, as the waves meet one before reaching the other. Yet as they’d have nothing to do with black holes or neutron stars, they might be dismissed as noise. Indeed, Penrose argues that what the Copenhagen team found isn’t correlated terrestrial background noise, but correlated noise from background erebon decay out there in the universe.

So how likely is that to be the case, and CCC to be the right approach to the troublesome questions surrounding the Big Bang? 

“It’s classic Roger Penrose,” said Andrew Pontzen, a cosmologist at University College London. “It’s a very thought-provoking idea that brings together a lot of very clever strands into a really nice vision for the way the universe could behave over extremely long timescales,” he said. “It’s a beautiful theory and it deserves a lot of attention.” 

However, Pontzen points out that the original data analysis on the CMB rings — Penrose’s first proposed test of CCC — was “quite badly flawed” and “reached conclusions that couldn’t really be supported.” Similarly, he supports the conclusions of the LIGO collaboration, which found that the correlated noise between its detectors isn’t real, and so can’t be caused by erebon decay. “Data analysis is tremendously subtle. There are these pitfalls that are waiting to be fallen into,” he said.

That doesn’t mean CCC is wrong, but it does appear that compelling evidence for its veracity has yet to be found in either the CMB or the LIGO gravitational-wave detectors. Even if the correlated noise trumpeted by the Copenhagen team is fictitious, future gravitational-wave detectors might pick up correlated noise from erebon decay.

“I’m hopeful that it might be possible to see these effects from distant galaxies,” Penrose said. “If so, it would give you a remarkable way of telling the dark-matter distribution across the universe. It might also allow us to talk about a time before the Big Bang.”

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