The scientist explained the acceleration of the Universe by quantum uncertainty

Dark energy, which is believed to be responsible for the accelerated expansion of the universe, may be a manifestation of the quantum properties of space itself. Savvas Kushiappas, a physicist from Brown University, proposed a model combining cosmic acceleration and quantum gravity without introducing new particles or unknown fields. This was reported in the magazine on August 20. Phys.org .

Why dark energy remains a mystery

Quantum mechanics describes processes at the level of atoms and elementary particles, while general relativity explains gravity and the structure of the universe on a large scale. Both theories have been experimentally tested many times, but physicists have not yet succeeded in combining them into a single consistent system.

The problem becomes especially noticeable in conditions where quantum effects and strong gravity must act simultaneously, for example, near the center of a black hole or at the earliest stages of the universe's existence. Modern researchers cannot conduct direct experiments in such conditions.

The nature of dark energy remains equally complex. Observations of supernovae, the distribution of galaxies, and the large-scale structure of the cosmos show that the expansion of the universe is accelerating. In the standard cosmological model ΛCDM, this effect is described by a cosmological constant, but its origin remains unknown.

In addition, theoretical estimates of the vacuum energy and the observed value of the cosmological constant may differ by about 122 orders of magnitude. This contradiction is known as the cosmological constant problem and is considered one of the most serious unsolved problems of modern physics.

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How quantum uncertainty affects the Universe

In a study published on August 13, Kushiappas proposed extending the principle of quantum uncertainty to the universe as a whole. According to his hypothesis, the size of the universe and the rate of its expansion cannot be simultaneously determined with arbitrarily high accuracy.

In cosmology, the size of a homogeneous expanding universe is described using a scale factor. It shows how much the distances between distant objects have changed compared to the selected point in time. The rate of change of the scale factor, in turn, is related to the rate of cosmic expansion.

Kushiappas introduced a modified switching relationship between the scale factor and the rate of its change. Simply put, the scientist suggested that the geometry of the universe may be governed by its own version of the uncertainty principle, which differs from the relations applied to an ordinary particle.

This change leads to an additional geometric correction in the Friedman equation describing the expansion of the universe. The correction does not depend on the amount of matter or radiation, but on the scale factor. Therefore, it refers to the geometry of space itself, and not to its material contents.

This is the main difference between the model and many other explanations of dark energy. It does not require the existence of undetected particles, scalar fields, or any other exotic form of matter. The observed cosmic acceleration in this case may be a macroscopic trace of the quantum properties of space.

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Can the model change the perception of the Big Bang

If the parameter n is sufficiently negative, the model leads to a completely different scenario. If n is less than -2, the quantum geometric correction increases rapidly as the scale factor decreases and prevents the universe from shrinking to a point of infinite density.

Instead of the initial singularity, there is a cosmological rebound. According to this scenario, before the expansion, the universe could go through a phase of contraction, and then, having reached a certain minimum size, it began to increase.

Such a model allows us to mathematically avoid the point at which the density, temperature and curvature of space should become infinite, and the known physical laws stop working. However, we are talking about a theoretical result, not a proof that the real universe actually arose after the rebound.

In addition, the mode capable of describing late dark energy and the mode eliminating the Big Bang singularity correspond to different parameter values. In the minimal version of the model, the same set of them cannot simultaneously provide the observed dark energy, the Planck cosmological rebound and the normal evolution of the early Universe. The author himself directly attributes this circumstance to the limitations of the hypothesis.

Why quantum effects can manifest themselves over long distances

It is usually assumed that quantum gravity should become noticeable only on the Planck scale — at extremely small distances and high energies. Therefore, its effects are more often associated with the first moments after the Big Bang or the inner regions of black holes.

Kushiappas offers a different interpretation. In his model, the characteristic scale of the quantum-gravitational correction can be determined by the cosmological horizon, the maximum area of space from where the signal can, in principle, reach the observer.

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In this case, quantum gravity does not necessarily disappear completely during the transition from the microcosm to cosmic distances. Its properties can manifest themselves directly in how the rate of expansion of the entire universe is changing. The author compares this logic with the Hawking temperature of black holes. It is determined by the size of the horizon of a particular object, and is not always fixed at the Planck level. By analogy, the parameters of the quantum geometry of the universe can also be related to the size of its horizon.

Such an explanation remains an interpretation, not a conclusion from a completed fundamental theory. Neither loop quantum gravity, nor string theory, nor other existing approaches yet allow us to accurately obtain the ratio proposed by Kushiappas.

What questions remain open?

The proposed model does not solve the problem of the cosmological constant definitively. In some cases, it still requires extremely fine-tuning the parameters so that the final value corresponds to the observed acceleration of the universe.

It also does not explain the existing discrepancy between the different ways of measuring the Hubble constant. In the considered scenario, the correction even slightly reduces the estimated value of the expansion rate by about 1.5% for one of the selected sets of parameters. This is directed in the opposite direction from the value obtained from observations of nearby supernovae.

In addition, the stability of the cosmological rebound to perturbations of matter and geometry has not yet been proven. The microscopic origin of the proposed uncertainty ratio has not been established either. A fully gauge-invariant formulation of the model has also yet to be developed.

Thus, the study does not prove that dark energy and quantum gravity have a common origin. It offers a mathematically defined and potentially testable hypothesis in which accelerated expansion can occur as a consequence of the quantum structure of space.