Posted on March 26, 2019
"The question is whether we can test the whole [inflation] scenario, not just specific models, "said Avi Loeb, astrophysicist and cosmologist at Harvard University. "If there is no guillotine that can kill some theories, then what's the point?"
In an article published in September 2018 on the physical preprint site, arxiv.org, Loeb and two Harvard colleagues, Xingang Chen and Zhong-Zhi Xianyu, suggested such a guillotine. The researchers predicted an oscillating pattern in the distribution of matter in the cosmos that, if detected, could distinguish between inflation and alternative scenarios, particularly the assumption that the Big Bang was actually a rebound preceded by a long period of contraction.
"If the signal is real and observable, it would be very interesting," said Sean Carroll of the California Institute of Technology at 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. Scientists are generally divided into two camps: those who defend inflation believe that the universe has experienced a period of exponential expansion in its infancy, while those who adhere to the theory of contraction assert that the universe is going through cyclical periods of contraction and slow expansion.
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"If the universe had contracted in anticipation of a rebound," writes Natalie Wolhover in Quanta, "the ripples in the quantum fields would have been pinched." The observable universe would have possibly contracted to a size smaller than the ripples of a certain wavelength, such as a violin whose resonant cavity is too small to produce the sounds of a cello.When too large ripples have disappeared, the peaks or concentrations of particles present on this scale at that time would have been "frozen" in the universe As the observable universe narrowed further, ripples at smaller and smaller scales would have disappeared, freezing in the form of density variations.
"Ripples of some sizes may have interfered constructively at the critical time, producing peak density variations of this magnitude, while slightly shorter ripples that disappeared a moment later could have frozen. It is the oscillations between high and low density variations that Chen, Loeb and Xianyu theoretically support.
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"These oscillations would also occur if, on the contrary, the universe was experiencing a period of rapid inflation," concludes Wolchover. "In this case, as it grows larger and larger, it would have been able to accommodate quantum ripples of ever greater wavelength. Density variations would have printed on the universe at each scale at the time when ripples of this size could have formed. "
New Harvard CfA discoveries, jointly developed by astronomer lecturer Xiangang Chen, director of the astronomy department Avi Loeb and physics postdoctoral fellow Zhong-Zhi Xianyu, have been accepted for publication in Physical Review Letters as "Editor's Suggestion". one out of six "outstanding" documents.
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The theory of inflation is more popular among academics because it provides a more "elegant" explanation of the creation of the universe, according to Chen. But Chen and his colleagues argue that more rigorous testing is needed to determine the validity of the theory.
"The scientific theory is not a beauty contest," Chen said. "You can not just say, this theory is elegant, so it must be right. You have to deduce the consequences and draw experiments to test it. "
In order to find a characteristic that can separate inflation from other theories, the team began by identifying the property that defines the different theories – the evolution of the size of the primordial universe.
"For example, during inflation, the size of the universe increases exponentially," said Xianyu. "In some alternative theories, the size of the universe shrinks. Some do it very slowly, while others do it very quickly.
"The attributes that people have proposed up to now to measure are generally difficult to distinguish 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 observable attributes can be directly related to this definition property."
The signals generated by the primordial standard clock can serve such purpose. This clock is any type of heavy elementary particle in the primordial universe. Such particles should exist in all theories and their positions should oscillate at a regular frequency, much like the ticking of a clock's pendulum.
The primordial universe was not entirely uniform. Tiny irregularities of density on tiny scales have become the seeds of the large-scale structure observed in the present universe. This is the main source of information on which physicists rely to learn about what happened before the Big Bang. The standard clock ticks generated signals that were printed in the structure of these irregularities. Standard clocks in different theories of the primordial universe predict different signal patterns because the evolutionary histories of the universe are different.
"All the information we have about the primordial universe looks like a movie, but we only have the stack of images," Chen said. "But one way or another, this pile of frames is disrupted and we do not know how to run the film."
"If we imagine all the information that we have learned so far about what happened before the Big Bang comes in the form of rolls of film footage," he said. Standard clock tells us how these images should be read, "explained Chen. "Without any information about the clock, we do not know if the film should be played backwards or forwards, fast or slow, just as we are not sure if the primordial universe has inflated or contracted. and how fast. That's the problem. The standard clock put timestamps on each of these images when the movie was shot before the Big Bang, and tells us how to play the movie. "
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 appear in non-inflationary theories and suggested a search in astrophysical observations. "If a signal structure representing a contracting universe were found, it would falsify the entire inflationary theory," said Xianyu.
The success of this idea lies in experimentation. "These signals will be very subtle to detect," said Chen, "and we may have to look in many different places." Microwave cosmic background radiation is one such example, and the galaxy distribution is another example. We have already started looking for these signals and there are already some interesting candidates, but we need more data. "
The South Antarctic pole telescope detects a signal – "The first direct evidence of cosmic inflation after the Big Bang"
The debate on the falsifiability of inflation began in 2017, when Loeb – with Princeton professor Paul J. Steinhardt and Princeton postdoctoral fellow Anna Ijjas – wrote an article in Scientific American in which they disputed the dominance of inflationary theory.
In Pop Goes the Universe, the authors argue in favor of a bouncing cosmology, as proposed by Steinhardt and others in 2001. They conclude by stating in an extraordinary way that inflationary cosmology "can not be evaluated using of the scientific method ". Some scientists who accept inflation have proposed to "reject one of the [science’s] definition properties: empirical testability ", which" promotes the idea of a non-empirical science.
"One of the inevitable consequences of inflation is the concept of multiverse. Anything that can happen will happen an infinite number of times, "said Loeb. "Is inflation really falsifiable? We think that a scientific theory is a theory that you can falsify. If inflation can support anything, it's a problem. "
The text of 2017 provoked what Loeb called "a strange" answer from Professor Alan H. Guth of the Massachusetts Institute of Technology – a letter co-signed by 32 Guth colleagues, including Stephen Hawking and five Nobel laureates. "People – especially those who invented inflation – got really upset and said that it could not be falsified, it must be true, it should be true, and so it's not necessary to test it, this must be true, "said Loeb.
Guth wrote in an email to Loeb and his team that he had never claimed that inflation "can or should not be tested." paper.
Loeb told Juliet E. Isselbacher of Harvard Crimson that he hoped the data needed to carry out the test would be provided in the next decade.
The image at the top of the page shows the map of all the terrestrial universe of the local universe derived from the 2MASS extended source catalog of more than 1.5 million galaxies. The Milky Way is shown in the center and the other galaxies are color-coded according to 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