On March 21, 2013, the European Space Agency held an international press conference to announce new results from a spacecraft called Planck that mapped the cosmic microwave background (CMB) radiation, light emitted more than 13 billion years ago just after the Big Bang revealing some of the greatest mysteries of cosmology.
The new map, ESA scientists told the journalists, confirmed a 35-year-old theory that the universe began with a bang followed by a brief period of hyperaccelerated expansion known as inflation. This expansion smoothed the universe to such an extent that, billions of years later, it remains nearly uniform all over space and in every direction and “flat,” as opposed to curved like a sphere, except for tiny variations in the concentration of matter that account for the stars, galaxies and galaxy clusters around us.
Although cosmic inflation is well known for resolving some important mysteries about the structure and evolution of the universe, other very different theories can also explain these mysteries. In some of these theories, the state of the universe preceding the Big Bang – the so-called primordial universe – was contracting instead of expanding, and the Big Bang was thus a part of a Big Bounce.
A team of scientists has proposed a powerful new test for inflation, the theory that the universe dramatically expanded in size in a fleeting fraction of a second right after the Big Bang. Their goal is to give insight into a long-standing question: what was the universe like before the Big Bang?
To help decide between inflation and these other ideas, the issue of falsifiability – that is, whether a theory can be tested to potentially show it is false – has inevitably arisen. Some researchers, including Avi Loeb of the Harvard-Smithsonian Center for Astrophysics (CfA), have raised concerns about inflation, suggesting that its seemingly endless adaptability makes it all but impossible to properly test.
“Falsifiability should be a hallmark of any scientific theory. The current situation for inflation is that it’s such a flexible idea, it cannot be falsified experimentally,” Loeb said. “No matter what value people measure for some observable attribute, there are always some models of inflation that can explain it.”
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, Pop Goes the Universe, in which they challenged the dominance of the inflationist theory.
In the years since the 2013 ESA news conference, wrote Loeb and colleagues, “more precise data gathered by the Planck satellite and other instruments have made the case only stronger. Yet even now the cosmology community has not taken a cold, honest look at the big bang inflationary theory or paid significant attention to critics who question whether inflation happened. Rather cosmologists appear to accept at face value the proponents’ assertion that we must believe the inflationary theory because it offers the only simple explanation of the observed features of the universe. But, as they explain, the Planck data, added to theoretical problems, have shaken the foundations of this assertion.”
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.
Now, a team of scientists led by the CfA’s Xingang Chen, along with Loeb, and Zhong-Zhi Xianyu of the Physics Department of Harvard University, have applied an idea they call a “primordial standard clock” to the non-inflationary theories, and laid out a method that may be used to falsify inflation experimentally.
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.
“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.”
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.”
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.
Many future galaxy surveys, such as US-lead LSST, European’s Euclid and the newly approved project by NASA, SphereX, are expected to provide high quality data that can be used toward the goal.