In search of the theory of gravity

Einstein’s theory of gravity –General relativity-It has been very successful for over a century. However, it has theoretical drawbacks. This is not surprising: the theory predicts that space-time singularities inside black holes will fail – and The Big Bang itself.

Unlike physical theories that describe the other three fundamental forces in physics – the electromagnetic force and the strong and weak nuclear interactions – general relativity has only been tested in weak gravity.

Deviations of gravity from general relativity are by no means ruled out or experienced everywhere in the universe. According to theoretical physicists, deflection is bound to occur.

Deviations and quantum mechanics

According to Einstein, our universe arose as a result of the Big Bang. Other singularities hide inside black holes: space and time have no meaning there, while quantities such as energy density and pressure become infinite. These indicate that Einstein’s theory fails there and should be replaced by a more fundamental theory.

It is naive to solve space-time singularities by quantum mechanics, which applies to very small scales.

Quantum physics is based on two simple ideas: Point particles illogical; and Heisenberg’s uncertainty principle, which states that the value of certain pairs of quantities can never be known with absolute precision, for example, a particle’s position and velocity. This is because particles should not be viewed as points, but as waves; On small scales they behave as waves of matter.

This is enough to understand that a theory that includes general relativity and quantum physics must be free of such pathologies. However, all attempts to combine general relativity and quantum physics necessarily lead to deviations from Einstein’s theory.

Therefore, Einstein’s gravity cannot be the final theory of gravity. In fact, it was not long after Einstein’s introduction of general relativity in 1915 that Arthur Eddington, who famously verified the theory in… 1919 solar eclipseI started looking for alternatives just to see how things could be different.

Einstein’s theory has survived all tests so far, and has accurately predicted various outcomes from the theory Prefix of Mercury’s orbit by the presence of gravitational waves. So where do these deviations from general relativity hide?

Cosmology is important

A century of research has given us the standard model of cosmology known as cold dark matter Model (ΛCDM).. Here Λ refers either to Einstein’s famous cosmological constant or to a mysterious dark energy with similar properties.

Dark energy was specifically introduced by astronomers to explain the universe Acceleration of cosmic expansion. Although it fit the cosmological data very well until recently, the ΛCDM model is surprisingly incomplete and unsatisfactory from a theoretical point of view.

In the past five years, I also experienced severe Observational tensions. The Hubble constant, which determines the age and distance scale of the universe, can be measured in the early universe using the cosmic microwave background and in the late universe using supernovae as standard candles.

These two measurements give Incompatible results. More importantly, is the nature of the key components of the ΛCDM model – dark energy, dark matter and the field that drives the early universe Economic inflation (a very short period of extremely rapid expansion that gave rise to galaxy seeds and galaxy clusters) — remains a mystery.

From an observational point of view, the most compelling driver of modified gravity is the acceleration of the universe detected in 1998 with… Type Ia supernovae, whose luminosity diminishes due to this acceleration. The ΛCDM model, based on general relativity, posits the existence of a very strange dark energy with negative pressure permeating the universe.

The problem is that this dark energy has no physical justification. Its nature is not completely known, although A A large number of models It was suggested. A proposed alternative to dark energy is the cosmological constant Λ which, according to quantum mechanics, Back-of-the-envelope calculations (but questionable).It should be huge.

However, Λ must instead be adjusted incredibly small to fit cosmological observations. If dark energy exists, our ignorance of its nature is deeply troubling.

Alternatives to Einstein’s theory

Could the problems arise, instead, from a mistaken attempt to fit cosmological observations into general relativity, like fitting someone into a pair of pants that are too small? That we observe the first deviations from general relativity when mysterious dark energy simply does not exist?

this idea, Suggested for the first time This idea, conducted by researchers at the University of Naples, has gained enormous popularity while the competing dark energy camp remains strong.

How can we tell? Deviations from Einstein’s gravity are Limited by solar system experimentsfinal notes to Gravitational waves And the Close-up images of black holes.

There is now a Great literature On alternative gravitational theories to general relativity, dating back to Eddington’s early investigations in 1923. A very common class of alternatives is the so-called tensor scalar gravitation. It is conceptually very simple because it introduces only one additional component (a scalar field corresponding to the simplest non-spin particle) to Einstein’s geometric description of gravity.

But the consequences of this program are not at all trivial. It is a striking phenomenon.”Chameleon effect“, consisting in the fact that these theories can masquerade as general relativity in high-density environments (such as in stars or in the solar system) while strongly deviating from it in the low-density environment of cosmology.

As a result, the additional (gravitational) field is virtually absent in the first type of system, masquerades as a chameleon, and can only be felt on larger (cosmic) scales.

current situation

Nowadays, the range of alternatives to Einstein’s gravity has expanded dramatically. Even adding a single massive scalar excitation (i.e. a particle with zero spin) to Einstein’s gravity — and keeping the resulting equations “simple” to avoid some known fatal instabilities — gave rise to a much broader class of gravity. Horndesky’s theoriesand subsequent generalizations.

Theorists have spent the past decade drawing physical consequences from these theories. Recent discoveries of Gravitational waves I have provided a way to Physical layer restriction of modifications Gravity allowed Einstein.

However, there is still much work to be done, with hope for future progress Multi-messenger astronomy It leads to the discovery of modifications in general relativity where gravity is very strong.