One of the controversial arguments about the universe is that our universe is like a random bubble in an endless from empty space. It goes the same if we talk about the multiverse. The multiverse hypothesis is about the tiny amount of energy infused in empty space, namely the vacuum energy, dark energy, or the cosmological constant. So, why is empty space so empty?
Likely, the extreme vacuity of our vacuum appears necessary for our life. Space could expand too quickly for structures, such as a planet or people to form. This also suggests that probably, there is a huge number of universes with different doses of vacuum energy.
Additionally, some scientists dislike the multiverse and bristle at the tautology of “anthropic reasoning” simply because these things are untestable. Raman Sundrum, a theoretical physicist at the University f Maryland said that dark energy is so difficult and thorny that people have a single or two solutions.
If we want to know more about dark energy, we need to know first about vacuum energy. The general theory of relativity from Albert Einstein says that matter and energy tell the space-time how to curve. Also, space-time curvature tells matter and energy how to move.
Cosmologists spent many decades and argued the value was zero since the universe’s rate of expansion was steady, and they wondered why. But, it was in 198 the astronomers discovered that the expansion of cosmos is gradually accelerating showing that there is repulsive energy permeating space. Astronomers dubbed the dark energy and it is almost equivalent to cosmological constant presented by Einstein. So, this existence causes the cosmos to expand more quickly and form the new space.
On the other hands, the inferred density of vacuum energy cannot go with the theory of the quantum field and the language of particle physics. When there are no particle excitations rippling through a quantum field, it will be empty. But, the state of the quantum field is never certain due to the uncertainty principle in quantum physics. For this reason, energy is never being zero.
We can say that a quantum field has little springs at each point in space. The springs are wiggling and they are always a bit too stretched. Since they are always in motion, they possess energy, and we call it zero-point energy of the field. Force fields consist of positive zero-point energies and matter fields have negative ones. Therefore, these energies add to and subtract from the total energy of the vacuum. When it comes to the total vacuum energy, it should equal the largest of the contributing factors. Just like when you receive $10,000 and spend it $100, or find $3 in your pocket, you will still have about $10,000. The observed rate of cosmic expansion indicates the value is between 60 and 120 orders of magnitude. This is smaller than some of the zero-point energy contributions to it. Well, it is quite difficult to come up with a physical mechanism for this equalization due to two reasons.
First, the only impact of vacuum energy is gravitational. Dialing it down requires a gravitational mechanism. But, the universe’ first few moments, when such mechanisms might have operated, the universe was so physically small. Its total vacuum energy was negligible compared to the matter and radiation. The gravity of everything else would have completely dwarfed the gravitational effect of vacuum energy. Raphael Bousso, the physicist wrote in 2007 that this is one of the biggest difficulties in solving the problem related to the cosmological constant.
Adding more difficulty, quantum field theory calculations show that the vacuum energy would have shifted in value due to the phase changes in the cooling universe after the Big Bang. This thing raises a big question whether the hypothetical mechanism that equalized the vacuum energy kicked in before or after the shifts. Besides, how can the mechanism know how big their effect will be?
Up to now, these obstacles could thwart the attempt to explain the tiny weight of empty space. On the other hands, recently researchers have been researching one possible way; If the universe did not bang into existence, but bounce with the earlier contraction phase, then the contracting universe in the distant past would have been dominated by vacuum energy. Get motivated with this idea, Peter Graham, David Kaplan, and Surjeet Rajendran started to learn the new cosmic bounce model even though they have yet to show how the vacuum dilution in the universe has worked.