by The inflationary epoch that caused our universe to expand rapidly in its early days may be connected to the modern age of dark energy, thanks to a phantom component of the cosmos that changes the strength of gravity as the universe evolves, proposes a physicist in a new role.
The traditional approach to understanding. gravity involves Einstein’s famous general relativity theory. For such a powerful idea, which can explain everything from the orbit of Moon to the evolution of the entire universe, it’s a fairly simple concept. In general relativity, there is only space-time and the contents within it. The contents of the cosmos cause space-time to bend and warp, and the curvature and warping of space-time dictate how the contents must move.
For example, the presence of a planet distorts the space-time around it, causing other objects to follow it in orbits, or distortions caused by a star can deflect the path of passing light.
Related: Was Einstein wrong? The case against the space-time theory
Although general relativity is the simplest approach to gravity, it is not the only one. An alternative, known as scalar-tensor theories, dates back to the early 1960s and is the work of physicists Robert Dicke and Carl Brans, for which it is sometimes called the Brans-Dicke theory.
In scalar tensor theories, in addition to space-time and its content, there is a third ingredient, known as the scalar field. The scalar field sucks up all of space-time, and its only job is to change the force of gravity from one place to another or from time to time. In vanilla general relativity, the force of gravity is fixed; it’s just newton‘s gravitational constant, forever and ever. No matter where or when you are in the universe, a given amount of mass and energy will always distort space-time in exactly the same way.
But in scalar-tensor theories, that can change. A planet on one side of the universe could have a weaker or stronger impact on the surrounding space-time, depending on the local value of the scalar field. The strength of gravity can also change over time, if the scalar field itself evolves.
tuning the cosmos
Experimentally, the general relativity and scalar tensor theories are equivalent. General relativity has overcome all the experimental obstacles that have been presented to 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. But because general relativity is much simpler than the scalar tensor theories and there is no known way to tell them apart, physicists prefer EinsteinThe classical theory of .
Except there’s one little problem: dark energy. According to the observations, the expansion of the universe is accelerating, but at a very smooth pace. The only way to explain this in general relativity is to include a cosmological constant, an extra value in the equations that has an incredibly small value, but not quite zero. That feature of the cosmological constant worries most physicists because it seems incredibly unnatural. If dark energy had almost any other value, the expansion of the cosmos would have ripped the cosmos apart long ago, rendering it incapable of supporting life (including anyone who might observe it), and yet it’s not perfectly zero either.
“Adding extra values to the equations” certainly sounds 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 is a potential way for that long-discarded model of gravity to explain the accelerated expansion more naturally.
Interestingly, the current era is not the only one in which the expansion of the universe has accelerated. Cosmologists believe that very early in the big Bangthe universe underwent a period of extremely rapid expansion known as inflation. You may be wondering if there is a connection between the early period of inflation and the modern period of dark energy, and you’re not the only one.
gravitational threads
Now Motohiko Yoshimura, a physicist at the Research Institute for Interdisciplinary Sciences at Okayama University in Japan, has proposed that scalar tensor theories provide a direct link between inflation and dark energy.
In this model, described in an article published in the preprint database arXiv, the scalar field part of the scalar tensor theory (the “tensor” refers to spacetime itself) is much stronger in the early universe, triggering the epoch of inflation. At the end of inflation, the scalar field weakens and releases its energy in the form of all the particles in the standard model (such as quarks and electrons).
Crucially, the scalar field never goes away. It maintains some background presence as the universe continues to evolve, forming stars Y galaxies in the meantime. Then, after cosmic expansion dilutes all matter to a low enough level, the scalar field kicks in again, but at a much weaker level, giving rise to the current age of dark energy.
But while it’s an intriguing story, astronomers still need to test the hypothesis. Fortunately, this model yields many potentially observable relics of the early universe. For example, in this scenario, gravity can be so strong in places that black holes they form spontaneously and survive to this day. find evidence of these primordial black holes It would help reinforce the idea.
Another approach is to look for gravitational waves from the early universe that are left behind when inflation takes place. Astronomers can search for these gravitational waves directly, trying to detect them in the faint background hum of the universe, or through their influence on this so-called cosmic microwave background.
Physicists know that dark energy and inflation represent the current limits of our knowledge, and only radical suggestions like this one, and the accompanying experiments, will help us push past that limit.
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