Why Einstein must be wrong: In search of the theory of gravity

Over a century has seen great achievement with general relativity, Einstein's theory of gravity. It has theoretical flaws, though. This is not shocking as the theory predicts that it will fail at spacetime singularities within black holes, as well as at the Big Bang.

The general theory of relativity has only been tested in weak gravity, in contrast to physical theories that describe the other three fundamental forces in physics: the electromagnetic and the strong and weak nuclear interactions.

By no means are deviations of gravity from general relativity checked or rejected anywhere in the cosmos. Theoretical physicists also assert that deviation is inevitable.

Inconsistencies and quantum physics

Our universe began in a Big Bang, according to a theory first put out by Georges Lemaître and largely embraced by the astronomical community. Within black holes, there are more singularities: There, pressure and energy density become unlimited, and space and time lose all significance. These indicate that there are problems with Einstein's theory and that a more basic one has to be put in its place.

Naively, quantum physics, which operates on extremely tiny scales, ought should be able to resolve spacetime singularities.

Two fundamental principles underpin quantum physics: first, point particles are illogical; second, the Heisenberg uncertainty principle asserts that it is impossible to know with perfect confidence the value of some pairs of values, such as a particle's location and velocity. This is due to the fact that particles behave like waves of matter at microscopic sizes and should not be seen as points.

This is sufficient to realize that such pathologies should not exist in a theory that incorporates both general relativity and quantum physics. All attempts to combine quantum physics with general relativity, however, inevitably include departures from Einstein's theory.

As a result, Einstein's theory of gravity cannot be the final one. In fact, Arthur Eddington—best known for confirming general relativity in the 1919 solar eclipse—began looking for alternatives soon after Einstein introduced the theory in 1915, simply to see whether things might be any different.

Up until now, all of the tests conducted on Einstein's theory have been successful in forecasting a wide range of outcomes, from the precession of Mercury's orbit to the presence of gravitational waves. Thus, where are these general relativity aberrations hidden?

The cosmos is relevant

The Λ-Cold Dark Matter (ΛCDM) model is the mainstream model of cosmology that has been developed over a century of research. In this case, κ represents either the well-known cosmological constant of Einstein or an enigmatic dark energy with comparable characteristics.

Astronomers invented dark energy on the spur of the moment to explain why the expansion of the universe was accelerating. Up until recently, the ΓCDM model fit cosmological evidence quite well, yet it is remarkably inadequate and unsatisfying from a theoretical standpoint.

It has also seen extreme observational tensions during the last five years. The cosmic microwave background can be used to estimate the Hubble constant in the early universe, and supernovae can be used as standard candles in the late universe to calculate the age and spatial scale of the universe.

The findings of these two measurements are inconsistent. More importantly, the nature of dark energy, dark matter, and the field responsible for early universe inflation—a brief period of extraordinarily rapid expansion that gave rise to the first galaxies and galaxy clusters—remains a mystery.

The acceleration of the cosmos seen in 1998 with Type Ia supernovae, whose brilliance is diminished by this acceleration, is the most persuasive observational argument for modified gravity. The general relativity-based ΓCDM model proposes that the cosmos is filled with a highly exotic dark energy that exerts negative pressure.

The issue is that there is no physical basis for this dark energy. Its nature remains entirely unknown despite a multitude of models being put forth. The cosmological constant Ϋ, which is the suggested substitute for dark energy, should be enormous based on dubious quantum-mechanical back-of-the-envelope calculations.

To suit the cosmic findings, however, κ has to be extremely fine-tuned to a minuscule number. If dark energy is real, it is really concerning that we know so little about it.

Einstein's theory alternatives

Is it possible that problems stem instead from an incorrect attempt to incorporate the cosmic findings into general relativity, much like attempting to put an oversize pair of pants on someone? That the enigmatic dark energy just does not exist, and that we are seeing the first departures from general relativity?

The University of Naples scholars who initially put out this theory have seen enormous growth in support, despite the ongoing fervor of the dark energy group.

How do we find out? Experiments on the solar system, current gravitational wave observations, and near-horizon black hole imaging restrict deviations from Einstein gravity.

A vast body of work has been written about alternative theories of gravity to general relativity since Eddington's pioneering studies in 1923. A highly favored category of substitutes is known as scalar-tensor gravity. Since it adds just one scalar field—which corresponds to the most basic spinless particle—to Einstein's geometric account of gravity, it is theoretically extremely straightforward.

However, this program has far-reaching repercussions. One remarkable occurrence is the "chameleon effect," which is the ability of these theories to pass for general relativity in high-density settings (like stars or the solar system) while drastically departing from it in the low-density setting of cosmology.

Because of this, the additional (gravitational) field only manifests at the biggest (cosmological) scales in the first class of systems, when it basically vanishes like a chameleon.

The circumstances of today

The range of alternatives to Einstein gravity has significantly expanded in the modern era. A significantly larger class of theories known as Horndeski theories and its later extensions have been produced even by adding a single massive scalar excitation (a spin-zero particle) to Einstein gravity and keeping the resultant equations "simple" to avoid some known catastrophic instabilities.

For the past ten years, theorists have worked to determine the practical implications of these theories. Gravitational waves have just been detected, and this has made it possible to limit the physical class of Einstein gravity changes that are possible.

There is still more to be done, nevertheless, in the hopes that developments in multi-messenger astronomy in the future will reveal alterations to general relativity when gravity is extraordinarily strong.

Provided by The Conversation