The Big Bang Signal –“That Could Kill the Theory”

The Universe with Milky Way


“The question is whether one can test the entire [inflation] scenario, not just specific models,” said Avi Loeb, an astrophysicist and cosmologist at Harvard University. “If there is no guillotine that can kill off some theories, then what’s the point?”

In a paper that appeared in September of 2018 on the physics preprint site,,  Loeb and two Harvard colleagues, Xingang Chen and Zhong-Zhi Xianyu, suggested such a guillotine. The researchers predicted an oscillatory pattern in the distribution of matter throughout the cosmos that, if detected, could distinguish between inflation and alternative scenarios — particularly the hypothesis that the Big Bang was actually a bounce preceded by a long period of contraction.

“If the signal is real and observable, it would be very interesting,” Sean Carroll of the California Institute of Technology said in an email to Natalie Wolchover at Quanta.

Now, a new Harvard paper provides a possible test to determine what happened before the Big Bang. The question of what preceded the event has long puzzled physicists and astronomers alike. Scientists are generally divided into two camps: those who advocate for inflation believe the universe underwent a period of exponential expansion at its inception, while those who subscribe to the theory of contraction assert that the universe goes through cyclic periods of contraction and slow expansion.

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“If the universe contracted in the lead-up to a bounce,” writes Natalie Wolhover in Quanta, “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.

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“These oscillations would also arise if instead the universe experienced a period of rapid inflation, concludes Wolchover. “In that case, as it grew bigger and bigger, it would have been able to fit quantum ripples with ever larger wavelengths. Density variations would have been imprinted on the universe at each scale at the moment that ripples of that size were able to form.”

The new Harvard CfA findings, co-developed by Astronomy Lecturer Xiangang Chen, Astronomy Department Chair Avi Loeb, and Physics postdoctoral fellow Zhong-Zhi Xianyu, have been accepted for publication in Physical Review Letters as an “Editors’ Suggestion” — a distinction awarded to one in six “outstanding” papers.

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Inflation theory is more popular among academics because it serves as a more “elegant” explanation of the universe’s inception, according to Chen. But Chen and his colleagues argue that more rigorous testing is needed to determine the validity of the theory.

“Scientific theory is not a beauty contest,” Chen said. “You cannot just say, this theory is elegant, so it must be right. You have to derive the consequences and derive experiments to test it.”

In an effort to find some characteristic that can separate inflation from other theories, the team began by identifying the defining property of the various theories – the evolution of the size of the primordial universe.

“For example, during inflation, the size of the universe grows exponentially,” Xianyu said. “In some alternative theories, the size of the universe contracts. Some do it very slowly, while others do it very fast.

“The attributes people have proposed so far to measure usually have trouble distinguishing between the different theories because they are not directly related to the evolution of the size of the primordial universe,” he continued. “So, we wanted to find what the observable attributes are that can be directly linked to that defining property.”

The signals generated by the primordial standard clock can serve such a purpose. That clock is any type of heavy elementary particle in the primordial universe. Such particles should exist in any theory and their positions should oscillate at some regular frequency, much like the ticking of a clock’s pendulum.

The primordial universe was not entirely uniform. There were tiny irregularities in density on minuscule scales that became the seeds of the large-scale structure observed in today’s universe. This is the primary source of information physicists rely on to learn about what happened before the Big Bang. The ticks of the standard clock generated signals that were imprinted into the structure of those irregularities. Standard clocks in different theories of the primordial universe predict different patterns of signals, because the evolutionary histories of the universe are different.

“All the information we have about the primordial universe is like a movie, but we only have the stack of frames,” Chen said. “But somehow that stack of frames gets messed up, and we don’t know how to run the movie.”

“If we imagine all of the information we learned so far about what happened before the Big Bang is in a roll of film frames, then the standard clock tells us how these frames should be played,” Chen explained. “Without any clock information, we don’t know if the film should be played forward or backward, fast or slow, just like we are not sure if the primordial universe was inflating or contracting, and how fast it did so. This is where the problem lies. The standard clock put time stamps on each of these frames when the film was shot before the Big Bang, and tells us how to play the film.”

Chen explained that astronomers could theoretically determine the direction of the film, or the universe, by following the ticking of the watch.

The team calculated how these standard clock signals should look in non-inflationary theories, and suggested how they should be searched for in astrophysical observations. “If a pattern of signals representing a contracting universe were found, it would falsify the entire inflationary theory,” Xianyu said.

The success of this idea lies with experimentation. “These signals will be very subtle to detect,” Chen said, “and so we may have to search in many different places. The cosmic microwave background radiation is one such place, and the distribution of galaxies is another. We have already started to search for these signals and there are some interesting candidates already, but we need more data.”

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The debate about the falsifiability of inflation started in 2017, when Loeb — along with Princeton professor Paul J. Steinhardt and then-Princeton postdoctoral fellow Anna Ijjas — wrote an article in Scientific American in which they challenged the dominance of the inflationist theory.

In Pop Goes the Universe, the authors make the case for a bouncing cosmology, as was proposed by Steinhardt and others in 2001. They close by making the extraordinary claim that inflationary cosmology “cannot be evaluated using the scientific method” and go on to assert that some scientists who accept inflation have proposed “discarding one of [science’s] defining properties: empirical testability,” thereby “promoting the idea of some kind of nonempirical science.

“One of the inevitable consequences of inflation is the notion of the multiverse. Anything that can happen will happen an infinite number of times,” Loeb said. “So is inflation really falsifiable? We think that a scientific theory is one that you can falsify. If inflation can accommodate anything, it’s a problem.”

The 2017 piece provoked what Loeb characterized as a “odd” response from Massachusetts Institute of Technology Professor Alan H. Guth — a letter co-signed by 32 of Guth’s colleagues, including Stephen Hawking and five Nobel Prize Laureates. “People — especially people that invented inflation — got really upset, and said that it cannot be falsified, it must be true, it should be true, and therefore there is no need to test it because it must be true,” Loeb said.

Guth wrote in an email to Loeb and team that he has never argued that inflation “cannot or should not be tested.” Loeb said Guth’s letter prompted them to search for a way to test the theory of inflation, leading them to publish their most recent paper.

Loeb told the Harvard Crimson’s Juliet E. Isselbacher that he hopes the data needed to complete the test will come within the next decade.

The image at the top of the page shows the all-sky map of the local universe derived from the 2MASS Extended Source Catalog of more than 1.5 million galaxies. The Milky Way is shown at the center, and other galaxies are color-coded by their distances, obtained from several different galaxy surveys. IPAC/Caltech, by Thomas Jarrett

The Daily Galaxy via Harvard CfA, Quanta, Harvard Crimson and Scientific American


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