Could our universe end and does it indeed have to end at all? While the Big Bang Theory has been very well known as an explanation for the creation of the universe – and not only since the sitcom – theories about the possible decline of our universes receive less attention. Higgs vacuum decay proposes a possible answer to the opening question.
Introduction
The universe is very old, enormously big and it is expanding at an accelerating rate. Initially, scientists believed that the earth was flat and centered the universe. Until the late 1920s, scientists did not realize that the Milky Way was just one of many galaxies in this vast universe. Shortly thereafter, astronomers realized that other galaxies exist and do move away from our galaxy in all directions and thus from our planet. Likewise, in the last decades, the idea of acceleration contributed to our understanding of the universe.
Astrophysics initially originated in the early 19th century and has developed very rapidly since then. The last century especially revolutionized our understanding of the universe. Theories about the universe and its beginnings became more popular among astrophysicists. The Big Bang theory, first formulated by Georges Lemaître in 1927, provides a convincing cosmological model that explains how the universe began to exist. In the following decades, theoretical astrophysicists have not only been able to predict the origin of the universe but also some scenarios of its end. Since almost everyone agrees on the idea that the universe has an origin, the question emerges: "How will the universe end or does it indeed have an end?”
The Higgs Field
To understand how vacuum decay could destroy our universe some background information about the Higgs field is necessary.
In the year 1964 the physicist Peter Higgs synthesized some ideas that were floating around at that time and added insights of his own in order to develop a new theory. He proposed the idea that there might be an energy field that permeated the entire universe, an energy field now called “Higgs field”. However, in the press you rather read about the Higgs Boson. To understand their relationship, you can imagine the Higgs Boson as H2O molecules and the Higgs field as the Ocean. When you are immersed in the Ocean, water is everywhere – it is a continuous medium. But at the same time, it consists of molecules. In an analogous sense, the Higgs Boson is the smallest particle of the Higgs field.
Peter Higgs proposed that this energy field interacts with subatomic particles and thus gives these particles their mass. In this sense, very massive particles interact to a greater extent with the field while massless particles would not interact at all. To understand this concept better, we can use a second analogy of fish in water. A barracuda fish interacts only slightly with water and can move through it very easily, while a human can only move very slowly through water compared to a barracuda. Consequently, the barracuda fish equals a low-mass subatomic particle and the human a very massive subatomic particle. The lightest of the familiar subatomic particles is the electron while in the subatomic world the king of mass is the top quark. It weighs as much as an entire atom of gold – about three hundred and fifty thousand times more than the electron. However, it is stressed that the top quark is not more massive because it is bigger, in fact it is not. We believe that both the top quark and the electron are the same size! Actually, they both have zero sizes! The top quark is more massive than the electron simply because it interacts to a greater extent with the Higgs field. If the Higgs field would not exist, neither of these particles would have any mass at all!
The Destruction of the Universe
In the universe, everything tries to reach its ground state, in which it is completely stable and has as little energy as possible. For example, a ball on a hill has a lot of potential energy and thus is unstable. When irritated, it will roll down into the valley and lose its potential energy during the process. Then the ball will be in its ground state and stable.
The key question for our universe is whether the Higgs field has already reached its ground state. In the context of quantum field theory this ground state is called true vacuum. However, it is possible that the Higgs field has not reached its true vacuum yet but is metastable. In such a scenario, this false vacuum may last for a limited (potentially very long) time, but eventually decays to the more stable state, the true vacuum. This transition process is known as false vacuum decay.
Figure 1 describes this process. As one can see, the energy level (E) in the false vacuum (1) is higher than in the true vacuum (3). However, a barrier prevents the field from classically ‘rolling down’ to the true vacuum. Therefore, the transition to true vacuum must be stimulated. There are two possible ways:
- 1. The creation of high energy particles: Here the universe creates such a high level of energy that it can, in theory, push a very small part of itself into the true vacuum.
- 2. Quantum tunneling: Quantum mechanics assumes that a particle can tunnel from one place into another, even in vacuum. So, the universe can easily tunnel through.
No matter what triggers false vacuum decay, if only a small region of the universe reaches a more stable vacuum, this "bubble" (also called "bounce") would expand and clean out the universe.
Nevertheless, Joseph Lykken, a theoretical physicist at the Fermi National Accelerator Laboratory in Batavia, claims that if the transition already happened, it does not necessarily mean that the end of the Earth is near. It may be taking a billion years for that bubble to destroy the entire universe, as the distance between the Earth and the edge of the observable universe is 46 billion light-years. Moreover, if the bubble is far enough, it might never reach us as the universe is expanding at an accelerating rate.