Will the universe expand forever, or will it eventually turn into a tiny speck?
An article published in June suggests that infinite expansion is not possible, and this has greatly excited the entire physics community.
"People react very emotionally because if it is discovered and proven, it will be impressive," said Timm Vraze, a physicist at the Vienna University of Technology.
Now, Vraze and his colleagues have published a separate study that counterbalances the previous article. This means that the theory of a constantly expanding universe cannot yet be ruled out.
Dark energy and space expansion
Our Universe is permeated with an immense invisible force that seems to oppose gravity. Physicists call this force dark energy. It is believed that it is she who constantly "pushes" our Universe outward, provoking expansion.
But in June, a group of physicists published a paper claiming that dark energy changes over time. This means that the universe will not expand forever, but may eventually collapse to the size that it was before the Big Bang.
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Almost immediately, physicists discovered the theory's flaws: several independent groups subsequently published papers proposing corrections to the hypothesis. The paper, published Oct. 2 in the journal Physical Review D, says that as it stands, the original hypothesis cannot be justified because it cannot explain the existence of the Higgs boson, which we know from the Large Hadron Collider exists. Nevertheless, according to Vraze, who co-authored this article, with some theoretical adjustments, the hypothesis of a collapsing universe may still be viable.
How do we explain everything that has ever existed?
String theory, sometimes referred to as the theory of everything, is a mathematically elegant but experimentally unproven model for combining Einstein's theory of relativity with quantum mechanics. String theory assumes that all the particles that make up the universe are not actually points, but one-dimensional strings that vibrate. The differences in these vibrations allow us to see one particle as a photon and another as an electron.
However, for string theory to be a viable explanation for the universe, it must include dark energy.
According to Vraze, you can imagine dark energy as a ball among mountains and valleys, which represents the amount of potential energy. If the ball is on top of a mountain, it may be stationary, but it may roll down due to the slightest disturbance, so it is unstable. If the ball is in the valley, it is not moving, has low energy, and is in a stable universe, because a strong push would cause it to roll back.
Theorists working on string theory have long assumed that dark energy is constant and unchanging in the universe. In other words, the ball is in the valleys between the mountains, it does not roll off the mountain peaks and does not change over time.
But a hypothesis put forward in June suggests that for string theory to work, the landscape must not have mountains or valleys above sea level. (In this concept, our universe is above sea level, which is a metaphor for the point at which dark energy begins to either expand the universe or contract it.)
Rather, the terrain has a slight slope and the "ball of dark energy" rolls downward. “As it rolls down, there is less and less dark energy,” Vraze said. "The height of the ball corresponds to the amount of dark energy in our universe."
In this theory, dark energy could eventually "slide" below sea level and begin to pull the universe back to its pre-Big Bang size.
But there is one problem.
Such unstable mountain peaks have been shown to exist because the Higgs boson exists, Wraze said. And it has been experimentally proven that Higgs particles can exist on these mountain peaks ("unstable universes") and can be disturbed at the slightest touch.
Difficulties with the stability of universes
Kamrun Wafa, who works on string theory at Harvard and senior author of the June hypothesis, confirmed that, indeed, the original hypothesis has "difficulties with unstable universes." And subsequent articles have reflected this problem.
However, according to Vraze, even if the hypothesis is revised, "we still will not be in a stable Universe - rather, everything else will change." According to the revised version, mountain peaks can exist, but stable valleys cannot (imagine the shape of a horse's saddle). The ball should eventually start rolling and the dark energy should change over time.
But if the hypothesis is completely wrong, then dark energy can be constant, we will sit in a valley between two mountains, and the universe will continue to expand.
He hopes that over the next 10-15 years, satellites that more accurately measure the expansion of the universe will help us understand whether dark energy is constant or changing. Wafa agreed: "This is an exciting period in cosmology and hopefully in the next few years we will see experimental evidence of dark energy changing in our universe."