Can we get an inkling of what existed before our universe began? Some theories suggest that, before the Big Bang, whatever existed was contracting, rather than expanding, as our universe is today. Perhaps what was contracting was an earlier universe, for example. If so, what we perceive as a Big Bang was actually a part of a Big Bounce. But a popular theory of our universe, called the inflation theory, doesn’t call for the idea of a previously contracting universe.
So what if inflation theory could be proven false? If so, the door would open to other theories, some of which do suggest a state of contraction before our universe began. If inflation theory could be proven false, we’d have some potential to probe – via these other theories – the universe before the Big Bang.
Now a team of scientists at the Harvard-Smithsonian Center for Astrophysics (CfA) has laid out a method that might be used to falsify inflation experimentally. The study will appear in the physics journal Physical Review Letters as an Editors’ Suggestion.
Let’s start from the beginning here … literally. Inflation is the theory that speaks of a time immediately after the Big Bang. It describes a universe that dramatically expanded in size for a fleeting fraction of a second. Inflation theory solves some important mysteries about the structure and evolution of our universe. But, according to the CfA scientists, other very different theories – including those that do allow for a previously contracting universe and a Big Bounce – can also explain these mysteries. These scientists said in a statement:
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 CfA – who is a part of the new study – had previously raised concerns about inflation, on the grounds that it was difficult, if not impossible, to falsify. Loeb said:
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. No matter what value people measure for some observable attribute, there are always some models of inflation that can explain it.
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. Xianyu said:
For example, during inflation, the size of the universe grows exponentially. 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.
So, we wanted to find what the observable attributes are that can be directly linked to that defining property.
According to these scientists, the signals generated by the primordial standard clock can serve such a purpose. They explained:
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. 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. Co-author Xianyu said:
If a pattern of signals representing a contracting universe were found, it would falsify the entire inflationary theory.
Chen added that the success of this idea lies with experimentation. He said:
These signals will be very subtle to detect, 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 …
As always, they said, more observational data is needed to bear out these theoretical ideas.
Bottom line: We don’t know what happened before the Big Bang, but some cosmological theories suggest a contraction prior to it. Perhaps an earlier universe was contracting. Unfortunately, the most popular cosmological theory of today – inflation theory – doesn’t call for this idea. Now scientists at CfA have devised a way that inflation theory might be falsified. If it were falsified, the door would be open to some of the other theories that hint at a pre-Big-Bang contraction.
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