**Cosmological constant problem long held scientific minds. However, many theories, to get rid of it, instead of modifying gravity, making phenomena such as tachyons and the spirits that may be worse than the disease. However, according to some researchers, there is a solution.**

Cedric Deffaye, a renowned expert on modern cosmology, working at the National Center for Scientific Research, spoke about the latest developments on the modified theory of gravity. The report was made on Ginzburgovskoy Conference on Physics, which was held at the Physics Institute. Russian Academy of Sciences (FIAN).

The focus of the scientist has given the puzzle of the cosmological constant — one of the major unsolved problems of modern physics.

Let us briefly recall the background. At the beginning of XX century, Albert Einstein formulated the main points and ideas of general relativity (GR), which describes gravity as the effect of deformation of space-time in the presence of mass-energy. Thus the basic theory for the Einstein equation contains the term with the specified parameter — the cosmological constant.

From the beginning, it was introduced there artificially. Initially, this value is implemented for assuming the existence of a static solution — study the possible existence of a static universe. Einstein once could not believe that the universe can expand or contract, but the trouble was that no member of the cosmological constant, it sooner or later "collapses" would be under the influence of gravity. Then, the theoretical work of Alexander Friedmann cosmological model substantiate the expanding universe, and experimental observations of Edwin Hubble revealed the dependence of the red shift of galaxies on their distance, persuaded Einstein to non-stationary universe. To the delight of the scientific community, the cosmological constant, does not follow from the theory, has been eliminated from the equations.

In 1919, Sir Arthur Eddington, based on observations of the total solar eclipse showed that light is bent by gravity from the star, the Sun in exact accordance with the predictions of general relativity. (Negative and Photo FW Dyson, AS Eddington, C. Davidson.) |

"For an explanation in the framework of GR new experimental data showing the accelerated expansion of the universe, requires the return of the hypothesis of the cosmological constant, or other bold assumption — says Cedric Deffaye. — For example, the introduction of a new weakly interacting fields or search for other sources of dark energy, which gives 75% of the contribution to the total energy of the **universe**. A year ago, this discovery Saul Perlmutter, Brian Schmidt and Adam Riess won the Nobel Prize in physics, and the scientific community with fresh forces deepened in the search for new models of gravity. "

Now the **cosmological constant** associated with the vacuum **energy**. And here there are problems. The energy density of this energy scale (based on the cosmological constant, which can be linked to the observation of accelerating expansion of the universe) are very small. They are many orders of magnitude smaller than the energy density of other types of physical interactions and, most importantly, the predictions of quantum **gravity** for this quantity. The reasons for this discrepancy are unclear. In addition, the question of the relationship between the concept of vacuum energy with a mysterious dark energy.

One possible way to explain the abnormal smallness of the cosmological constant using the anthropic principle. According to him, the value of this observed constants such that promotes the emergence and development of intelligent life, the top of which there is a physical observer, who is able to measure this constant. Some tension hypothesis linking the major features of the surrounding world of the observer with the existence of an observer, long debated in the scientific community. David Gross, recall, at the time, said without more ado that the anthropic principle is a means to cover our ignorance of the answers to complex questions. Some scientists do not agree with this principle, and therefore develop alternative models of gravity, general relativity modifying low energy or at longer distances.

A key difficulty of such hypotheses is that they should at the same time different from current theories of gravity for long distances and coincide with them in small, where the existing theories are guaranteed to work. "If we offer a modification of the theory of gravity, which would describe the effects of dark energy on large cosmological scales, it is necessary to make sure that the short-range work continued gravity of Newton or Einstein — notes to this effect leading research worker of the theory of elementary particles FIAN Andrew Barvinsky. — This is a fairly difficult task, which is solved with mixed success in the different models. These models include, in particular, the theory or model of massive gravity with massive gravitons. We study various aspects of this theory — with all its advantages, disadvantages, and internal problems. "

The basic idea of the modified theory of gravity is to eliminate the **cosmological** **constant** and some new fields that simulate such. As already noted, there is no theoretical justification nor a constant, no new fields are not. Instead, the theory changes the very laws of gravity / gravity on large (cosmological) distances and periods of time, which may well lead to the accelerated expansion of the universe. One of these new models — massive gravity, in which the graviton (a vector of gravitational interactions) is a massive particle. Report Cedric Deffaye just based on the fact that to abandon the cosmological constant in the equations of general relativity by adding them to an additional term associated with the mass of the graviton.

But in this model, too, has its difficulties. Thus, in the modified theories of gravity with massive gravitons arise phenomenologically forbidden particles — tachyons (hypothetical particles that move faster than light) and dyhi. Both are bad. Tachyons, if they are able to bring the information to contradict the assertion that the speed of light as the maximum.

"Dyhovoe" (this refers to the "bad spirits», bad ghosts) state means that in theory there are particles with negative energy. The risk that the theory will become unstable. In other words, if some fraction of the particles in the system has a positive energy, and the other share — negative, the energy conservation law does not prohibit an unlimited process of particle production as well as another sign of the energy, which contradicts modern observations. And talk about energy conservation in a situation where, any number can appear and disappear (if the energy comes and goes with a negative sign), will also be difficult.

In 1919, Sir Arthur Eddington, based on observations of the total solar eclipse showed that light is bent by gravity from the star, the Sun in exact accordance with the predictions of general relativity. (Negative and Photo FW Dyson, AS Eddington, C. Davidson.) |

"For an explanation in the framework of GR new experimental data showing the accelerated expansion of the universe, requires the return of the hypothesis of the cosmological constant, or other bold assumption — says Cedric Deffaye. — For example, the introduction of a new weakly interacting fields or search for other sources of dark energy, which gives 75% of the contribution to the total energy of the **universe**. A year ago, this discovery Saul Perlmutter, Brian Schmidt and Adam Riess won the Nobel Prize in physics, and the scientific community with fresh forces deepened in the search for new models of gravity. "

Now the **cosmological constant** associated with the vacuum **energy**. And here there are problems. The energy density of this energy scale (based on the cosmological constant, which can be linked to the observation of accelerating expansion of the universe) are very small. They are many orders of magnitude smaller than the energy density of other types of physical interactions and, most importantly, the predictions of quantum **gravity** for this quantity. The reasons for this discrepancy are unclear. In addition, the question of the relationship between the concept of vacuum energy with a mysterious dark energy.

One possible way to explain the abnormal smallness of the cosmological constant using the anthropic principle. According to him, the value of this observed constants such that promotes the emergence and development of intelligent life, the top of which there is a physical observer, who is able to measure this constant. Some tension hypothesis linking the major features of the surrounding world of the observer with the existence of an observer, long debated in the scientific community. David Gross, recall, at the time, said without more ado that the anthropic principle is a means to cover our ignorance of the answers to complex questions. Some scientists do not agree with this principle, and therefore develop alternative models of gravity, general relativity modifying low energy or at longer distances.

A key difficulty of such hypotheses is that they should at the same time different from current theories of gravity for long distances and coincide with them in small, where the existing theories are guaranteed to work. "If we offer a modification of the theory of gravity, which would describe the effects of dark energy on large cosmological scales, it is necessary to make sure that the short-range work continued gravity of Newton or Einstein — notes to this effect leading research worker of the theory of elementary particles FIAN Andrew Barvinsky. — This is a fairly difficult task, which is solved with mixed success in the different models. These models include, in particular, the theory or model of massive gravity with massive gravitons. We study various aspects of this theory — with all its advantages, disadvantages, and internal problems. "

The basic idea of the modified theory of gravity is to eliminate the **cosmological** **constant** and some new fields that simulate such. As already noted, there is no theoretical justification nor a constant, no new fields are not. Instead, the theory changes the very laws of gravity / gravity on large (cosmological) distances and periods of time, which may well lead to the accelerated expansion of the universe. One of these new models — massive gravity, in which the graviton (a vector of gravitational interactions) is a massive particle. Report Cedric Deffaye just based on the fact that to abandon the cosmological constant in the equations of general relativity by adding them to an additional term associated with the mass of the graviton.

But in this model, too, has its difficulties. Thus, in the modified theories of gravity with massive gravitons arise phenomenologically forbidden particles — tachyons (hypothetical particles that move faster than light) and dyhi. Both are bad. Tachyons, if they are able to bring the information to contradict the assertion that the speed of light as the maximum.

"Dyhovoe" (this refers to the "bad spirits», bad ghosts) state means that in theory there are particles with negative energy. The risk that the theory will become unstable. In other words, if some fraction of the particles in the system has a positive energy, and the other share — negative, the energy conservation law does not prohibit an unlimited process of particle production as well as another sign of the energy, which contradicts modern observations. And talk about energy conservation in a situation where, any number can appear and disappear (if the energy comes and goes with a negative sign), will also be difficult.

It looked like a hypothetical tachyon, if it could be seen. When he passes the point nearest to the viewer, will see arising suddenly object, divided into two retreating in all directions apart. (Fig. Wikimedia Commons.) |

In his works, Cedric Deffaye exploring a mechanism for resolving this problem. To do this, it analyzes various nonlinear massive gravity theory involving a complex dependence of the law of gravity on the spatial scale at which it operates, and ensures that no tachyon and dyhovyh instabilities.

The problem with these theories, as well as almost all modern versions of the general theory of relativity, is that they have to be adjusted to the experimental data with the help of external parameters. At the same time in modern cosmology, such a procedure is a rough adjustment of arms parameters of the **theory** is counter-productive direction, is not fundamentally different from the introduction of the same cosmological constant.

The best theory of modified gravity would be some natural mechanisms that would lead to the famous too small value of the cosmological constant without introducing new third-party options. The search for such mechanisms are actively underway.

Prepared according to the LPI-inform.

Category: Astronomy and Space