The amount of energy contained in empty space turns out to be very difficult to explain without drawing on the multiverse theory. But physicists still have at least one more alternative to study.
The controversial idea that our Universe is just a random bubble in an infinite foaming Universe, logically follows from the most seemingly innocent feature of nature: empty space. Specifically, the multiverse hypothesis grows from an incredibly tiny amount of energy contained in empty space — known as vacuum energy, dark energy, or a cosmological constant. Each cubic meter of empty space contains enough energy to light an electric bulb for only 11 trillion fractions of a second. “It's like a bone in the throat” - this is how the Nobel laureate
Stephen Weinberg once described a problem that should cause at least a trillion trillion trillion trillion trillions of energy in a vacuum because of the
presence of all the fields associated with it matter and interactions. But somehow, all the effects of these fields are almost completely mutually destroyed, and serene peace is obtained. Why is empty space so empty?
Although we do not know the answer to this question — the notorious “
cosmological constant problem ” —the extreme degree of emptiness of our vacuum seems necessary for our existence. In a universe a little more filled with gravitationally repulsive energy, space would expand too quickly for structures such as galaxies or planets to form. Such a finely tuned system suggests that there may be a lot of universes, and each of them may have its own amount of vacuum energy, and we live in the universe with an extremely low index, because it could not have appeared in some other.
Some scholars frown on the tautology of the "
anthropic principle " and dislike the multiverse theory for unverifiable. Even those who are not against this theory would like to have alternative solutions to the problem of the cosmological constant. But while it is almost impossible to solve without the multiverse. “The problem of dark energy is so difficult and inconvenient that people haven’t found one or two solutions for it,” said Raman Sandram, a theoretical physicist at the University of Maryland.
To understand why this is happening, consider what the vacuum energy is. Albert Einstein’s General Theory of Relativity states that matter and energy tell space-time how to bend, and the curvature of space-time tells matter and energy how to move. From the equations automatically follows the ability of space-time to possess its own energy - a constant amount remaining when there is nothing more there, which Einstein called the cosmological constant. For decades, cosmologists have assumed that its value is zero, given the fairly constant rate of expansion of the Universe, and thought why this happened. But in 1998, astronomers discovered that the expansion of space is in fact gradually accelerating, from which the presence of repulsive energy permeates the entire space.
However, the assumed density of this vacuum energy contradicts what quantum field theory says about empty space. A quantum field is considered empty when particles that represent the field excitations do not move through it. But due to the uncertainty principle, the state of the quantum field is never exactly known, so the energy cannot be exactly zero. Imagine that a quantum field consists of small springs located at every point in space. Springs constantly fluctuate, because they are always stretched for some indefinite distance from the most relaxed state. They are always either slightly compressed or slightly stretched, and therefore they always move, which means they have energy. This is called
zero field energy . At interaction fields, zero energy is positive, and at matter fields - negative, and these energies participate in the total energy of the vacuum.
The total vacuum energy should be approximately equal to the sum of the largest contributions. However, the observed rate of cosmic expansion suggests that this value is 60–120 orders of magnitude less than some contributions of the zero energy of the fields, as if all the different positive and negative terms would mutually destroy each other. But to come up with a physical mechanism for this alignment is extremely difficult, for two reasons.
First, the vacuum energy acts only gravitationally, therefore, in order to reduce it, a gravitational mechanism is necessary. But in the first moments of the life of the Universe, when such a mechanism could work, it was so small that all its vacuum energy was negligible compared with the amount of matter and radiation. The gravitational effects of vacuum energy would fade before the gravity of everything else. “This is one of the greatest difficulties in solving the problem of the cosmological constant,” wrote the physicist Raphael Busso in 2007. The gravitational feedback mechanism, which fine-tunes vacuum energy in the conditions of the early Universe, he wrote, “can be roughly compared with an airplane flying in a storm along up to atomic sizes. "
Complicating the situation is that quantum field theory calculations show that the vacuum energy would change as a result of the phase changes of the cooling Universe soon after the Big Bang. As a result, the question arises whether this hypothetical mechanism worked, which equalized the vacuum energy, before or after these changes. And how could the mechanism know how big these effects would be to compensate so accurately?
So far, these obstacles prevent attempts to explain the tiny weight of empty space, without stooping to the multiverse lottery. But recently, some researchers have moved to the study of one alternative: if the Universe did not emerge from nothing, but
rebounded, having undergone a previous compression , then the
contracted Universe in the distant past should have been huge and the vacuum energy should dominate in it. Perhaps it was then that a certain gravitational mechanism could affect the abundance of vacuum energy, and in some way naturally dissipate it over time. This idea inspired the physicists Peter Graham, David Kaplan and Sarzhit Rajendran [Peter Graham, David Kaplan, Surjeet Rajendran] to create a
new model of cosmic rebound , although they have yet to show exactly how vacuum dispersion should work in a contracting universe.
In response to a letter, Bousso called this approach “an extremely worthy attempt”, and “a well-informed and honest struggle with a serious problem.” But he added that major problems remain in the model, and “the technical obstacles that must be overcome in order to fill these holes and make the idea work seem rather serious. This whole design is already reminiscent of
the Goldberg machine , and, at best, will become even more confusing in the future when the holes are filled. ” He and other supporters of the multiverse believe that their answer choices look simpler compared to this.