From Peter Pan to Battlestar Galactica, one of the most famous concepts in all ** is the idea of cyclical repetition: all of this has happened before and will happen again. But does this apply to the universe itself? The universe as we know it began with the blazing great**, which itself was established and caused by a previous state called the expansion of the universe, which expanded rapidly and inexorably in an unknown time. When the inflation ends, the energy of the entire space – previously the field energy or the energy inherent in the space itself – is converted into various quanta, and the thermal boom begins.
Yet, billions of years later, we still have one form of energy inherent in space itself: dark energy. Will this one day trigger a similar situation, leading to a new kind of big **? That's Sara Wright's question, she asked:
If I understand correctly, a big ** occurs when the inflationary field energy is suddenly converted into all the particles and radiation that exist today. But there is also a false vacuum decay situation, where the zero energy of space may reach a lower state than it is now. Is there any relationship between these two events? If so, is it possible that the big ** that produced us is still far beyond the cosmic horizon of our observable universe?
It's a very big question and a fascinating possibility, and honestly, no one knows if it describes the future of our universe. It's a big idea, and why it's worth pondering.
In the top panel, our modern universe has the same properties everywhere (including temperature) because they originate in regions with the same properties. In the middle panel, the space that could have had any arbitrary curvature is inflated to the point where we cannot observe any curvature today, solving the flatness problem. In the bottom plate, the original high-energy relics were inflated and taken away, providing a solution to the problem of high-energy relics. This is how inflation solves the three major problems that the big ** itself cannot explain. **e. siegel/beyond the galaxy
For the uninitiated, the expansion of the universe came into being a puzzle to explain the properties that the universe must have at the beginning of the hot big **, but the big ** itself does not provide any explanation. The original idea of inflation was that there was a phase before the hot **, in which:
The energy of the universe does not exist in the form of matter and radiation, but in the form of an energy inherent in space itself, which causes the universe to expand not to cool and slow down as it expands, but to expand at an exponential, relentless rate, so that its size doubles with every tiny fraction of a second passes, then doubles again in the next fraction of a second, and so on, until all that remains is flat, empty space, full of empty spaces with the same properties, such as energy density, In addition to the quantum fluctuations superimposed on this uniform background.
This explains the conditions known to exist at the beginning of the thermal maximum, including temperature and density that are almost completely uniform in all directions and positions, an observable universe that is almost perfectly spatially flat, and a universe with no remaining high-energy relics, such as magnetic monopoles. It also provides a series of initial fluctuations that are slightly larger than the small cosmic scale on a large cosmic scale, all of which are adiabatic rather than isocurvature, where the fluctuations exist on a scale larger than the cosmic horizon, which are Gaussian in nature and obey the distribution of bell curves.
If you want to study signals in the observable universe and find clear evidence of extra-horizon fluctuations, you need to look at the extra-horizon scale on the TE cross-correlation spectrum of the CMB. With the final (2018) Planck data in hand, the evidence overwhelmingly supports their existence, validating the extraordinary ** for inflation** and in the face of inflation, this volatility should not exist without it**. ESA and Planck cooperate; e.Siegel's note.
But for the puzzle we are considering, the relevant part of inflation—if we know the beginning of the universe, defined by the transition from inflation to hot greatness, has to do with the potential fate of our universe through vacuum decay—requires us to study how inflation ended. In physics, we call it a phase transition: something that previously existed in a way that seemed stable or quasi-stable, and then transformed into another way in a short period of time, and this new way is now stable or quasi-stable and does not go back to what it was before.
In general, the way we look at phase transitions is to imagine that you have a ball living on a hill that – a stand-in for what physicists know as "potential energy" – can be any general shape that describes the behavior of your system. The simplest type of hill (or potential) we can draw would be a one-dimensional line, which is continuous (i.e., without any gaps) and differentiable (i.e., smooth), but can take any form or shape that meets these criteria. Initially, when it was first proposed, people who were thinking about inflation thought it had the potential for multiple local minima, but only one true minimum, and they imagined inflation as a state stuck in one of these "false" minimums.
In many physical instances, you will find yourself trapped in a local, false minimum, unable to reach the lowest energy state, the so-called true minimum. Whether you've been kicked to cross an obstacle, which usually happens, or whether you take the pure quantum mechanical path of quantum tunneling, getting from one state to another is always possible as long as you don't violate the fundamental conservation laws. Cranberry Wikimedia Commons.
It was the first proposal about inflation, known today among cosmologists as the "Old Inflation". Alan Guth, who was first popularly regarded as the founder of inflation, soon realized that the idea of the old inflation faced a major problem: it could not reproduce the hot big as required. During expansion, you can think of the area of the expanding space as a three-dimensional volume, where the inside of that volume is the expanded area. The end of inflation can be thought of as a specific "bubble" that applies to that volume, in the example above, the bubble:
There is an interior (the inside of the bubble), there is a boundary (the wall of the bubble), and where the inflation ends by quantum tunneling.
Keep in mind that the defining characteristic of a thermal large ** is that space needs to be uniformly filled with matter and radiation (within the range of quantum fluctuations): the same in every position and the same in all directions. But to do that, the energy trapped in that bubble-like area must end up inside the bubble, not the bubble wall.
It's the biggest problem with this quantum tunneling expansion model – the "old inflation" – that makes it so suspicious: this type of phase transition puts energy into the bubble wall, not inside. Due to this problem, this model was eventually abandoned.
When the expansion of the universe occurs, there is a lot of energy inherent in space because it is located at the top of this mountain. When the ball rolls into the valley, the energy is converted into particles. This not only provides a mechanism to not only set up a hot large, but also to solve the problems associated with it and make new ones. e. siegel/beyond the galaxy
What has it been replaced? Through an inflationary model with different types of potential and different types of transitions: cleverly named "New Inflation". Everyone should know that in physics, there are two main categories of phase transitions in physics, very cleverly named "first-order" and "second-order" phase transitions.
First-order phase transitions are similar to quantum tunneling, in which there is a false minimum that abruptly transitions to a true minimum. In this case, the "ball" spontaneously switches from being in a false minimum to a real minimum, and the change from one state to another is instantaneous.
However, the second-order phase transition, like the picture above, has a ball starting at the top of the plateau and gradually rolling into the valley below, where it oscillates downward at the bottom around the minimum it encounters.
In the case of inflation, the first-order transition will result in all energy being within the bubble wall in any one area where inflation ends. However, the second-order transition causes all energy to be converted into various quanta (e.g., particles and antiparticles of the Standard Model) within that bubble region. This is important because, in the context of inflation, the regions where inflation ends – that is, different "bubbles" within the inflationary space – never overlap or connect with each other.
During the expansion of the universe, the space contained in the expanded region grows exponentially, doubling in all three dimensions with every fraction of a second that passes. Where inflation ends, a hot ** ensues. But due to quantum effects, each region where the large ** occurs will be surrounded by more expanding, exponentially expanding space, thus ensuring that no two regions where the hot ** occurs collides, intersects, or overlaps. **k**li impu
This is because during inflation, the rate of expansion never goes down. However, as soon as inflation ends anywhere, the rate of expansion is sudden**. The reason is simple: because the energy density of the universe determines the rate of expansion. When your universe is filled with field energy or the energy inherent in empty space, the energy density remains the same. However, once your universe is filled with quanta – such as matter, antimatter, and radiation – the energy density decreases because the number of quanta remains the same while the volume of space expands.
Thus, in an expanding universe, the expansion rate remains high indefinitely as long as expansion occurs. As long as there are areas of expansion, it expands relentlessly, exponentially separating all areas from each other.
However, the rate of expansion begins immediately whenever inflation ends, so any inflationary end "bubbles" will always expand more slowly than the inflationary zones around them. This means:
No two bubbles that end inflation will touch, no two bubbles will overlap or collide, therefore, you can't have multiple bubbles hitting each other, you can't exchange energy between the various bubble walls, and therefore, inflation cannot end with a first-order phase transition (i.e., by quantum tunneling), but it must end with a second-order phase transition: by rolling down a hill and into a valley.
The analogy of a ball sliding on a high surface is when the expansion persists, while the structure collapses and releases energy to represent the conversion of energy into particles, which occurs at the end of the expansion. This transition—from inflationary energy to matter and radiation—represents an abrupt change in the expansion and nature of the universe, as well as a huge increase in entropy at the end of inflation. **e. siegel/beyond the galaxy
Within each individual bubble at the end of the expansion, the "ball" that rolls down from the mountain into the valley will oscillate at the bottom, converting the field energy (or energy inherent in space) into particles: a process known as cosmic reheating. The fact that there is an upper bound on the temperature reached during the thermal maximum—a temperature well below the Planck temperature—is further evidence in favor of inflation and against extrapolating the macro** to arbitrary temperatures, densities, and energies.
This is the concept of inflation: we used to lock all the energy of the universe in the form of a field, or in other words, in the form of energy inherent in space itself. This energy causes the universe to expand inexorably and inexponentially, doubling in volume with every second that passes. As long as this "ball" remains on a relatively flat plateau, the expansion continues and ends when the ball rolls into the valley: this is an example of a second-order phase transition. This shift led to an end to inflation, leading to a hot ** as part of the consequences.
It is for this reason that we no longer think of the great ** as a single beginning in time and space, but only the beginning of the universe as we know it. Before it there was a phase, the expansion of the universe, and before it and set it all up.
From the smallest imaginable region of space (Planck scale), the expansion of the universe causes space to expand exponentially: every second that passes, it relentlessly doubles and doubles again. While this empties the universe and flattens it, it also contains quantum fluctuations superimposed on it: these fluctuations will in turn provide the seeds for the cosmic structure in our own universe. Although the universe is stretched very large, it will still have some spatial curvature, as caused by inflationary dynamics. **ben gibson/big think
Now, we fast forward to today: the end of inflation and the beginning of the hot big ** are in the last billion years of the universe. Today, our universe is no longer even dominated by matter or radiation, but by a mysterious form of energy that causes the expansion of the universe to accelerate over time. We call this form of energy dark energy and infer its properties by measuring the expansion of the universe throughout its cosmic history. Perhaps surprisingly, its characteristics look somewhat familiar.
It appears to be a form of energy that was undetectable in the early days of cosmic history, when matter and/or radiation were densely concentrated.
About 8 billion years ago, its effects began to appear in the data, causing the universe to expand faster than it could have been without dark energy.
About 6 billion years ago, it became the dominant effect in the expanding universe, marking the transition between distant objects slowing down (as before) as they move away from us and them accelerating (as they did subsequently) and retreating faster and faster.
About 4.5 billion years ago, just as the Earth was formed, it became the dominant form of energy in the universe: more than dark matter and normal matter combined, as well as radiation and neutrinos combined.
Notably, it behaves as if the dark energy density remains the same over time: just as the energy density behaves during inflation.
While matter (normal and dark) and radiation become less dense as the universe expands due to its increased volume, dark energy, as well as field energy during expansion, is a form of energy inherent in space itself. As new spaces are created in the expanding universe, the dark energy density remains the same. **e. siegel/beyond the galaxy
So, where does dark energy come from and what is its true nature?
In every theory we have described the fundamental forces and interactions in the universe, the possible origin of dark energy has emerged. One is our theory of gravity in the context of general relativity. In the context of general relativity, there is one form of energy that behaves like observed dark energy: Einstein's cosmological constant. It's a form of energy inherent in space itself, and it represents what's left if you remove all matter, antimatter, radiation, and any other quanta from the universe. There is still energy in the open space, and the cosmology that we measure as positive quantifies it. It may be more than 100 orders of magnitude (with a coefficient of 10100) less than the energy density present during the expansion, but it is still there.
Another possible ** comes from the quantum vacuum: the energy of the empty space due to the zero-point energy of the quantum field theory that describes our universe. The lowest energy state of a quantum system is not necessarily zero, but it can usually exhibit a positive non-zero value, just like a hydrogen atom. It is in this context that the scenario of vacuum decay has been proposed, theorizing that the universe may not be in a true minimum state, but as evidenced by the positive energy density of dark energy, it may be in a false minimum, with the possibility of transitioning to a lower state in the future.
Scalar field in a false vacuum. Note that the energy e is higher than the energy in the true vacuum or the ground state, but there is an obstacle that prevents the magnetic field from classically scrolling down to the true vacuum. Also note how the lowest energy (true vacuum) state is allowed to have finite, positive, non-zero values. It is well known that the zero energy of many quantum systems is greater than zero, and no one knows whether the dark energy observed today is due to a true or false vacuum state. **stannered/wikimedia commons
But for now, we must remember that there are two types of phase transitions that we talked about: first-order phase transitions, which correspond to quantum tunneling from false minima to true minimum, and second-order phase transitions, which correspond to a slow roll from a mesa to the valley below. Cosmic inflation, in order to reproduce the universe as we observe it, must be a second-order phase transition. But vacuum decay scenarios all rely on quantum tunneling, which strongly suggests that the first-order phase transition will be the culprit.
In other words, although dark energy and the storm may be related and similar in many ways, the end of the storm and the beginning of the heat and the scenario of the dark energy vacuum decaying into a low-energy state are fundamentally important for them! There may still be some relationship between the inflationary states of long ago and today's dark energy, and it is still possible that, for inflation and any future vacuum decay scenarios, the shape of the potential that leads to this transformation will change over time. However, any discussion of our future vacuum decay scenario remains purely speculative, as there is no evidence that such decay ever occurred or will occur in the future.
When it comes to the great cosmic unknowns, we must be open to all possibilities that have not yet been ruled out. However, in the absence of any evidence to validate a fantasy scenario like vacuum decay, we have to admit that there is no evidence that something like this could happen, and if it did, we would have to learn something very unexpected from it to be able to link it to inflation. The idea cannot be ruled out, but at this point it falls squarely into the realm of pure speculation.