What is the force driving the galaxies?

The universe contains billions of galaxies and in each of them billions of suns. From each galaxy, all the others are seen to 'run away' at a speed (increasing with distance) of about 25 km per second per million light years. The escape began with the Big Bang about 12 billion years ago. In 1998, it became clear that there is still a force that accelerates and increases the rate

Prof. Yuval Na'am, courtesy of the Rosh Gadol system

The Year of the Miracle and Cosmology

In physics it is not customary to talk about miracles, yet the physicists call 1905 "the year of the miracle"; Albert Einstein then published five works that changed the face of modern physics, among them special relativity. From then until 1915, Einstein invested himself in building the theory of general relativity, which replaced Newton's theory of gravity and shaped the science of cosmology.
Newton's theory of gravitation explained the attraction relationships that exist between bodies in the universe as conditioned by their mass and the distance between them. The theory of general relativity brought into play the idea of ​​curved space as affecting the mutual movement of bodies and their speed.
Basically, Newton's theory is an excellent approximation to its Einsteinian counterpart, as long as the speeds are not close to the speed of light and as long as the gravitational field is "weak". A gravitational field is considered "strong" in this context, if its strength is close to what is required for the formation of a black hole.
Our sun, for example, whose radius is 500,000 km, has a "weak" gravitational field. If it had collapsed to a radius of 2 kilometers, it would have turned into a black hole.
Einstein's theory of relativity has accumulated verifications since it was presented, it was accepted by the scientific community and in light of its light, astrophysicists began deciphering the structure of the universe and the trend of its expansion.

What is the cosmological constant?
When Einstein approached the problem of the universe, he assumed that it was possible to use an approximation (model) in which the stars are a kind of 'dust grains' and then calculate the forces within this dust*.
It was clear that in Newton's dynamics this is an unstable system where everyone attracts everyone else and such a universe will collapse while crushing each other.
Einstein started from the premise that the universe is generally stable and therefore tested in equations the possibility that there is a factor in it that is able to balance the gravitational forces and stabilize the system. Indeed, in the new theory of gravity there is an element that can represent repulsion and it is called the 'cosmological constant'. But, here things start to get complicated, since the constant size of the 'cosmological constant' makes it difficult for it to fulfill the role of stabilizer. This is because of the incessant movement of the 'grains' (the galaxies, as you remember), which constantly changes the distances between them, and thus changes the forces of attraction that the 'immutable' constant is supposed to balance.

The expanding space
Are the only two choices facing the universe either to collapse or to expand forever? In 1922, a Russian atmospheric researcher named Alexander Friedman, who was engaged in cosmology as a hobby, showed that, according to the equations of general relativity, several states of instability, and not just collapse, are possible following the Big Bang. If we consider for example the initial expansion following the big bang, then it slows down after a time due to the mutual attraction of the gravitational fields. Or, the gravity of the fields is too weak to stop the expansion and it continues and results in the formation of isolated clumps (galaxies) that move away and cause the universe to 'inflate'.
A few years later, Hubble indeed discovered the redshift** in the spectrum of the light of the galaxies, indicating a 'frightened' escape of the galaxies from each other. They move away at an increasing speed, which accumulates with the distance - another 25 km per second for every million light-years distance***.

What is in the universe and where does it spread?
Since the fifties, scientists have been busy with another issue in the theory of relativity. This is a gap between the amounts of matter and radiation, which according to the calculations should constitute an average energy density (the 'critical' density) in the universe, and the detected amount (the visible matter) which is less than 10% of that. Therefore, it is estimated that there are still vast amounts of matter in forms that we have not discovered because they do not reflect normal light. This material is collectively referred to as "dark matter" and it constitutes another 20% of the amount of matter in the universe. There is still 70% missing from the critical density and it is believed that there are additional forms of energy that we have not noticed exist and they play a role in the behavior of the universe.

When galaxies move away from each other, a void (although not absolute) is created between them. But when the speed of their escape changes in context and depending on time, it is a sign that an intervening force is at work here now and not only at the moment of the bang that happened 12 billion years ago. Is this force induced by what scientists call "vacuum energy", as attraction is induced by the masses?

Recently it is assumed that the missing percentages (70%) are found in one of two forms. One is the same "vacuum energy" that we have not yet learned to calculate and the other is related to a new term called 'quintessence' which describes an entity (which is not material in the usual sense) that radiates a repulsion field. Both forms have been included under the name "black energy" (as opposed to "black matter").

At the beginning of the 21st century, we are facing the conundrum of a force that is gravitational in nature, but its action is repulsion and it runs the galaxies. So this is the cosmological mystery ahead of the year of physics that will begin in 2005 - the 100th anniversary of the 'year of the miracle'.

*In 1926, the American astronomer Edwin Hubble discovered that the universe is much larger than they had estimated and that the nebulae that were thought to be gas clouds in our galaxy are actually galaxies in themselves.
** This is what is called the 'Doppler effect' - the lengthening of the wavelengths when the light source moves away from us, and their shortening when the source gets closer.
***A light year - the distance a light beam travels in one year (the speed of light = 300,000 km per second, which is about ten thousand billion km)

Thanks again to the editor of "Rosh Gadol" Zohar Guri for the article, and of course also to Prof. Yuval Naman
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