Universe: Brian Cox discusses the time before the Big Bang
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Scientists typically explain gravity in terms of Einstein’s theory of general relativity. In this model, objects in the universe cause the fabric of spacetime to warp around them, creating a depression that affects the movement of other bodies. Or, as the American theoretical physicist John Wheeler glibly put it: “Spacetime grips mass, telling it how to move; mass grips spacetime, telling it how to curve.”
While general relativity offers arguably the simplest explanation for how gravity works, there are alternative interpretations — such as the so-called scalar-tensor theory.
The work of US physicists Robert Dicke and Carl Brans, this theory proposed that alongside spacetime and its contents, the universe has a third component, the “scalar field”.
This field exists across all of spacetime, and serves to change the strength of gravity both locally and over time.
In this way, the Brans–Dicke theory differs from general relativity, in which the strength of gravity is fixed, being that of Newton’s gravitational constant.
What this means is that, under general reality, two planets of the same mass in different parts of the universe would have the same surface gravity — whereas, under the scalar-tensor theory, the gravity could also be stronger, weaker, or even change with time.
As astronomer Dr Paul Sutter of Stony Brook University told Space.com: “Experimentally, general relativity and scalar-tensor theories are equivalent.
“General relativity has passed every single experimental obstacle thrown at it.
“But if you take a scalar-tensor theory and simply assume that your scalar field has a constant value equal to Newton’s constant, then you also get those same results.
“Because general relativity is so much simpler than scalar-tensor theories and there’s no known way to tell them apart, physicists prefer Einstein’s classic theory.”
There lies, however, a small fly in the ointment — dark energy.
Physicists have proposed this mysterious component of our universe to explain how the rate of expansion of the universe appears to be accelerating over time, contrary to theoretical expectations.
To account for this fact in general relativity, researchers add a so-called “cosmological constant” to the equations, one whose value is very small but not zero.
And, as Dr Sutter explains, “adding values to the equations sure does look a lot like scalar-tensor theories.
“So, ever since astronomers discovered dark energy in the late 1990s, physicists have been working to see if there’s a potential way for that long-discarded model of gravity to explain the accelerated expansion more naturally.”
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The accelerated expansion of the universe is not, however, without precedent — cosmologists believe that the universe underwent a period of extremely rapid expansion, referred to as “inflation”, shortly after the Big Bang.
In a new study, physicist Dr Motohiko Yoshimura of Japan’s Okayama University proposed that scalar-tensor theories may provide a direct link between inflation and dark energy.
According to Dr Yoshimura, the scalar field was considerably stronger in the early universe, thereby explaining the cosmos’ early and rapid inflation.
At the end of the inflationary period, he suggests, the scalar field weakened, releasing all its energy in the form of particles.
The scalar field would have remained in the background, Dr Yoshimura argues, until universal expansion diluted the distribution of matter sufficiently until it was able to assert its influence again, thereby causing the “modern” expansion of the cosmos.
Dr Sutter concluded: “While it’s an intriguing story, astronomers still need to test the hypothesis.
“Thankfully, this model produces a lot of potentially observable relics of the early universe.”
For example, he said, in the scenario proposed by Dr Yoshimura gravity would have been so strong in thin places in the early universe that they would have spontaneously formed primordial black holes that would still exist and be detectable in the present day.
Similarly, physicists may be able to detect the presence or influence of the gravitational waves left behind from the early universe when the inflationary period finished.
A preprint of the article, which has not yet been peer-reviewed, can be read on the arXiv repository.
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