Almost a century of astronomical observations have firmly established that galaxies are receding from each other. More recent observations which were awarded with a Nobel prize have shown that the speed at which the galaxies are receding from each other – per unit distance – is increasing over time.
If we run the film backward we conclude that there must have been a state of matter with huge density 13.8 billion years ago (Big Bang). According to this NASA article “The Big Bang did not occur at a single point in space as an explosion. It is better thought of as the simultaneous appearance of space everywhere in the universe.”
The phrase “simultaneous appearance of space everywhere in the universe” refers to the so-called “cosmic inflation” that happened at the very first instances of the Big Bang. There is a lot of confusion regarding the terms “Big Bang” and “cosmic inflation”. To me, both terms refer to the sudden appearance of external space. After its sudden appearance, the external space kept expanding at a slower rate. That’s the rate that we have been measuring.
In an earlier post I pointed out that the “expansion of space” is more like the “stretching of space”. The fabric of space is stretching and giving the appearance that galaxies are receding from each other. This is also known as the “metric expansion”.
During the metric expansion the galaxy sizes are not changing because the gravitational force produced by the mass of the galaxy (including dark matter) holds the galaxy together. This point is sometimes overlooked.
Whatever it is that causes the stretching of the fabric of space is called dark energy. Is dark energy a property of space? Is dark energy a field? We don’t know the answer but there are many theories.
The most popular theory of dark energy assumes that dark energy is a property of space. More specifically, the assumption is that the energy density of space remains constant as the fabric of space stretches. This view is summarized pictorially in the image below.
The “Expansion rate vs. Time” graphs on the right side of the image above do not provide any numbers. The “energy density of space” is very small positive number (close to zero) therefore the total dark energy is relatively small in the beginning. As the space gets bigger, total dark energy increases and becomes larger than the sum total of matter/radiation energy.
Wait a minute! Energy is not conserved in this picture. The total energy of the universe is increasing. Isn’t energy supposed to be conserved?
At the cosmic scale energy is not conserved. If you don’t believe me I can refer you to professional physicists , , ,,.
Technical reminder: if space (curvature or metric) is changing energy is not conserved.
- The farther away the galaxy the faster it is receding from us.
- Expansion rate depends on the energy density of the universe.
- In matter dominated universe the energy density therefore the expansion rate decreases over time
- In radiation dominated universe the energy density therefore the expansion rate decreases over time as well.
- We are currently in the dark energy dominated universe where the energy density of the universe stays nearly constant. This implies that the recession speed (per unit distance) of galaxies will increase over time. This phase is known as the “accelerating universe”. I suppose one has to study the Friedmann equations to gain better insights on this.
Is dark energy a field?
Maybe dark energy is a field. This article in the Symmetry magazine talks about the search for such a possibility in the collider experiments. I couldn’t resist taking this quote from the same article: “The landscape of dark energy theories is enormous.”
I have already mentioned this speculation in my older articles where I talked about the action/reaction relationship between the external space and the internal space (quantum vacuum).
In my view “dark energy” is a measure of the activity level in the internal space. Vacuum (internal space) activity develops as a reaction to the expansion of the external space. Vacuum fluctuations start as soon as the external space emerges. Particle fields and their quanta are created out of the vacuum fluctuations. The elementary particles that emerge from vacuum fluctuations will tend to be singularities because their formation is governed by the singularity seeking ‘binding action’. Elementary particles would be singularities without the ‘freedom seeking’ quantum effects. The interplay between ‘binding action’ and ‘freedom seeking’ never stops. I hope to say more on this subject in the book I am preparing (only God knows if or when I will finish this book).
Not exactly a re-phrasing
The action/reaction relationship between external and internal space as expressed in the previous section is not exactly a re-phrasing of the “dark energy is a property of space” view.
I need to clarify what I mean by ‘activity level in the internal space‘. I am not talking about the total energy. My term ‘activity level in the internal space’ is closer to the term “energy density of vacuum” (or “energy density of space”) used in standard cosmology.
If there is an action/reaction relationship between the external space and the internal space then we expect the activity level of the internal space to increase as the external space expands faster and faster. This means that the energy density of space will increase over time (not constant as described in the image above). This is a feedback loop that causes amplification of both the expansion and the level of quantum fluctuations in empty space. This is a fundamental instability.
I would not bother you with this post if I did not see Ethan Siegel’s article discussing the tentative astronomical observations indicating that the energy density of space is increasing over time as space expands. My speculation is consistent with those observations. Sorry, I don’t have a mathematical theory but I believe that the concept of action/reaction relationship between external and internal space can be useful.
My prediction: No Big Rip
In Ethan Siegel’s article it is explained that if the energy density of space increases over time then it will lead to the scenario known as the “Big Rip “where “bound structures, like galaxies, solar systems, planets, and even atoms themselves will be torn apart after enough time passes“. This view ignores the possibility that the activity level in internal space is increasing over time.
The ‘action/reaction relationship between external and internal space’ predicts an instability but this instability does not mean “Big Rip”. After the activity level of internal space reaches a certain level, processes totally unknown to us may be initiated. My best guess is that the fundamental instability eventually results in baby universes.
image credit: Jeremy Teaford/Vanderbilt University
 Sean Carroll: Why Does Dark Energy Make the Universe Accelerate?
 Sean Carroll: Energy is Not Conserved
 Sabine Hossenfelder: Where does dark energy come from and what is it made of?
 Ethan Siegel: Where Does ‘Energy’ for Dark Energy Come From?
 John Baez: Is Energy Conserved in General Relativity?