"The discovery of the Nobel laureates is the most important discovery in cosmology in the last decades"

This is what Prof. Avishai Dekal says in an interview with the Hidan website. Dekel says that a fourth scientist, Bob Kirchner, also received part of the prize, and it is not clear what was the consideration of the prize committee, which could have included him as one of the three winners

On the occasion of the winning of the Nobel Prize in Physics by the three researchers: Saul Perlmutter, Brian Schmidt and Adam Rees, says Prof. Avishi Dekel, an astrophysicist at the Rakeh Institute of Physics of the Hebrew University and who serves as the head of the cosmology group at the university: "The prize was awarded for the discovery that the universe is accelerating and expanding with time . This is the most important discovery in cosmology in the last 40 years and among the three most important discoveries in the study of cosmology over the years." At the Hebrew University, we are engaged in the theoretical field of research, while the researchers who won the prize engaged in measurements."
In a conversation with the Hidan website, Prof. Dekel explains: "If you look at history, the first important discovery was the discovery of the expansion of the universe by Hubble in 1929, the second was the discovery of the cosmic background radiation by Penzias Wilson in 1965 for which they received a Nobel Prize, and the third discovery is that The expansion of the universe is increasing at an inflationary rate. This theoretically leads to what we call today dark energy - Einstein's cosmological constant."

"We have known that the universe has been expanding for almost 90 years, and we have always assumed that the force of gravity is only the force of attraction, and from this it follows that the universe will slow down its expansion because all the time the bodies are attracting each other and two bodies that are moving apart are supposed to slow down the rate of their distance."
"The two groups that have now won the Nobel Prize, prepared experiments aimed at measuring the slowing rate of the universe's expansion under the influence of gravitational attraction. Both groups received results more or less at the same time in 1998 that the universe behaves exactly the opposite and its rate of expansion is increasing over time."

"The meaning of this is that in addition to the gravitational force that acts between the bodies on small scales, there is some force that is a force of repulsion that causes the universe to accelerate and increase its speed. This force of repulsion is not similar to the force of attraction because it does not act between bodies. It's not that the bodies repel themselves, the bodies still attract themselves according to the well-known laws of Newton and Einstein, but rather that space itself has an internal characteristic - an internal force of repulsion that operates between all parts of space and causes space to expand at an ever-increasing rate. We call the source of this power dark energy without really understanding what it is. We called it dark energy because if you look at all the energy in the universe, 70% is dark energy and only 30% is energy related to the mass of the materials in the universe."

"Mathematically, the phenomenon of the repulsive force of space is called the cosmological constant. This force has a constant like all other constants in nature. Gravitational force has Newton's constant G, there is a constant for the speed of light, and they have certain numerical values. And to that there is another constant - the cosmological constant that measures the force of repulsion of the universe and therefore the acceleration, so this measurement was actually a measurement of the size of the cosmological constant."
"The cosmological constant is not a new invention. Einstein and people of his generation also thought about this when they formulated the theory of relativity, except that Einstein inserted the constant for the wrong reasons, and later removed it and called it "the mistake of my life". We assumed that the value of the constant is zero or a very small number, and here the discovery revealed that this constant has a certain small value, but not completely negligible. At great distances and in future times it will influence the fate of the universe. We know that the universe will expand forever, not collapse back. It will be sparse - no new galaxies will be created, no new stars will be created, no new life will be created, what is created - is created. But the future will be much less interesting, everyone will move away from everyone else, the ability to communicate with distant regions will decrease and we will be condemned to a cold and boring universe, but on a time scale of tens of billions of years, it will not happen tomorrow."
What is your role as theoretical physicists?
"We the theorists in cosmology, try to understand apart from the mathematical understanding of the cosmological constant - where it comes from. Why did the big bang become a cosmological constant different from zero and we don't understand that. The term "dark energy": we do not know what is hidden behind it, except for the way it affects the universe. Today, the question of what dark energy is, what its source is, is the most important question in cosmology."

Can you elaborate on the discovery itself for which the three won the Nobel Prize?

"As for the discovery itself, how did they measure it? Exploding stars are really used here - a type Ia supernova. The idea was to measure the expansion rate of the universe today (not a new thing) and a few billion years ago (which is a new thing). To measure the rate of expansion a few billion years ago I have to look at very distant objects. Therefore, the light beam that came out of a body that is 4 billion light years away will tell me about the universe 4 billion years ago. These supernovae have two properties that make them very important for cosmological measurements, one - a tremendous luminosity, like an entire galaxy, and this is important because then the star can be seen with great clarity. It is important to have a very strong shining object. And the second - we know what the original luminosity of these supernovae is because we know some in the immediate vicinity and we also know the theoretical aspect. We have a standard candle - an object that has a very large and well-known luminous intensity. If I know the intensity of the illumination at the source and know what I measure with my telescope - I know how to estimate the distance between the source and me because the light flux decreases with the square of the distance. And so these supernovae serve as an excellent measure of the distance to very distant galaxies."
"In order to measure the rate of propagation, you have to measure on the one hand the distance to the object and on the other hand the speed of the object moving away from me - and this is done through the redshift. For each such supernova that sits in a distant galaxy, measure the redshift, the distance, one divided by the other - Hubble's constant is the rate of expansion - the velocity divided by the distance. They did this for a nearby group of galaxies and measured the rate of expansion today, and they had 40 galaxies about 4 billion light years away and so they measured the rate of expansion 4 billion light years ago, compared with each other and found that the rate of expansion is increasing over time. "
"Now we are trying to understand the meaning: what is dark energy and what is its theoretical source, apart from the fact that we know how to quantify it into equations, we do not know how to explain the source of the repulsion, it is an open question and we are trying different methods to learn about it."
You probably know the prize winners?
The people are all my good friends. Adam Rees was a student at Berkeley when he worked with me. Sol Perlmutter, the leader of one of the groups is also a good friend of mine, these are people we work with all the time. There is an interesting story about a guy who didn't get a Nobel Prize.

Who is it about?
"The first group was headed by Perlmutter, while the second group was headed by Bob Kirchner, who will be in Israel in December, and was the instructor of Rees and Schmidt. He was the one who conceived the project but at some point Brian Schmidt who was his student became the head of the project and the formal principal investigator. Rees was an even younger student who stepped in and did much of the work and was the lead author on the first articles. I'm sure the Nobel Prize committee was very confused about what to do with the fact that there are four people. Brian Schmidt was clear that he would accept, because he was the formal leader of the project."

Prof. Dekel was the first to refer to this discovery in a news article published on the Hidaan website in 1998