Scientists make progress in studying gravitational waves in the early universe

Since humans first observed gravitational waves in 2015, gravitational wave physics has become one of the hottest research directions at present. As a new window to understand the universe, gravitational waves are gradually showing us a picture of the universe that people have not seen for thousands of years. The physical phenomena in it point out some directions for our future development of physics.

Gravitational waves and new physics Traditional physical experimental research is often greatly restricted by the environment in which we live, such as collider experiments and the observation of astrophysical electromagnetic signals. For now, particle collider is the most effective means of detecting new physics at extremely small scales, and the collision energy scale is an important indicator to measure the detection performance of the collider - the higher the energy scale, the higher the energy scale can help us detect smaller Scale and understand more basic physical laws. However, under the current production conditions, it has become increasingly difficult to increase the energy standard of the collider. Although in the next twenty years, the energy standard of particle colliders is expected to reach around 100TeV, we have not yet discovered definite new physical signals in particle physics experiments with the current highest collision energy standard of 14TeV. In addition, traditional astronomical observations are almost all based on electromagnetic wave signals. Under the technological revolution of the past century, electromagnetic wave astronomy has achieved fruitful results. But to this day, the limitation of the observation depth in the electromagnetic band and the interference of the foreground ("foreground" refers to the celestial body that is close to the observed source but closer to the observer) are still the key to our understanding of the larger and more distant universe. A solid barrier to the history of the universe.

Figure 1: Schematic diagram of the scale and energy scale of the collider

Over the past 100 years, the development of laser interference technology has greatly improved our measurement of extremely small length changes. ability. The leap-forward development of this technology makes it possible for us to detect gravitational waves. At present, major economies around the world have launched or are arranging their own gravitational wave observation projects. Gravitational wave astronomy has become a new fertile ground in astronomy and physics and will bring us a new understanding of the universe and physics.

Compared with electromagnetic waves, the advantages of gravitational wave observation mainly include two aspects: First, gravitational wave signals are generally difficult to be interfered by the foreground, so background signals can be detected; secondly, because Gravitational waves interact very weakly with ordinary matter during the propagation process, so the gravitational wave signals born in the early universe can remain relatively pure until now, becoming a "historical relic" of the universe waiting for scientists to observe. The combination of gravitational wave observations with traditional collider experiments and astronomical observations in the electromagnetic band will greatly expand our understanding of the universe and basic physical laws.

Since the birth of Einstein’s theory of gravity more than 100 years ago, people have made many major breakthroughs in the study of black holes, but to this day we still know very little about the most extreme objects in the universe. . Everyone believes that a complete description of the physics of black holes requires the combination of gravity theory and quantum theory. However, these two theories, which have achieved great success in their respective fields, have encountered various difficulties in combining them. The vicinity of the black hole event horizon serves as a conflict site between gravity theory and quantum theory. It may give us a glimpse of the true content of quantum gravity theory and greatly expand our understanding of basic theories.

In addition, various physical processes in the very early universe will induce random disturbances in space and time, resulting in a random gravitational wave background. If current gravitational wave observations can discover some characteristics of the random gravitational wave background, it will also imply Something unusual happened in the early universe. This last point was the starting point for a recent study, co-led by Professor Yifu Cai from the University of Science and Technology of China and Professor Lin Chunshan from Jagiellonian University in Poland, with postdoc Dr. Bo Wang and doctoral students Yan Shengfeng participated, and the relevant paper has been recently published in the internationally renowned journal Physical Review Letters. This work will be briefly introduced below [1].

Inspiration from the swing

When you play on the swing, you should have discovered something: if you want the swing to swing higher and higher without anyone pushing it, then we It is necessary to swing the body back and forth regularly and use the swing of its own center of gravity to drive the oscillation of the swing. This is a special vibration phenomenon called parametric vibration.

Figure 2: Schematic diagram of swing

Parametric oscillation phenomenon is widely used in various fields of physics. In the field of cosmology, everyone believes that during a period of the evolution of the universe, the parametric oscillation phenomenon is likely to play a decisive role. In the theory of inflation, due to the extreme "dilution" effect of the inflation process, the end of this process leads to death in the entire universe, leaving only the energy left over from the scalar field that drove the inflation or some other light scalar fields. . At this time, parametric vibration is needed to convert the energy of the field driving the inflation into various material components required for the later evolution of the universe, and reheat the entire universe.

These mass-produced material components include not only particles that can be described and observed by particle physics models such as photons, electrons, and protons, but also dark matter and dark energy that were produced in the original period. This process is called the preheating of the universe, and then the universe enters the standard thermal history evolution.

The SSR mechanism was first used to study the formation of primordial black holes and predict their abundance. Primordial black holes are a special kind of black holes. They were formed in the very early universe due to the inhomogeneity of local space curvature, which caused the disturbance and collapse of the original material density. Their formation mechanism is different from those formed by the collapse of stars under normal circumstances. black hole. As early as the 1960s and 1970s, Soviet physicist Yakov Zel'dovich and British physicist Stephen Hawking respectively pointed out the formation of black holes in the very early universe. The theoretical possibility [5][6] has been widely discussed in subsequent cosmological research. Due to their formation and their own characteristics, primordial black holes have become an important candidate for cold dark matter, and may also be important candidates for gravitational lensing objects and gravitational wave sources. The primordial black holes predicted by the SSR mechanism are mainly distributed near some special masses, and the distribution density is very high, which is comparable to the energy density of dark matter (that is, most of the dark matter is primordial black holes).

On this basis, Professor Cai Yifu’s team found that since the SSR mechanism greatly amplifies the amplitude of the original scalar perturbation, at the second-order perturbation level, through the nonlinear coupling of scalar and tensor, SSR can also be The radiation-dominated period during and after inflation induces a random gravitational wave background, which may be detected by gravitational wave detectors in the future [7]. In addition, the model implementation and application of SSR is also a content worthy of in-depth study. Currently, there are applications under the inflaton-curvature sub-image [8], the realization of SSR under DBI inflation [9], and in the special dual-field model. There are similar vibration amplification applications [4].

SSR of Gravitational Waves

In more than five years of observation of gravitational waves, the most exciting gravitational wave event for scientists is the observation of gravitational waves from the merger of two neutron stars. (GW170817), and the corresponding multi-band electromagnetic signals were observed simultaneously. The discovery of such a standard whistle event can simultaneously let us know the redshift and distance information of the gravitational wave source, opening a new window for measuring the expansion rate of the universe. More importantly, by comparing the time when the electromagnetic signal and the gravitational wave signal are received, we can also limit the propagation speed of the gravitational wave. Currently, through this event, we believe that the difference between the propagation speed of gravitational waves and the speed of light is within an accuracy of 10-15 orders of magnitude.

However, this speed limit comes from the observational data of the relatively nearby universe (generally the red shift is less than 1), and the current observational evidence has little to do with the propagation speed of gravitational waves in the distant or earlier universe. There are no good restrictions, and during this period, if the gravitational wave propagation speed has larger non-trivial characteristics (that is, it deviates from the speed of light predicted by Einstein's general theory of relativity), it may indicate that there is something beyond the standard theory in the early universe. new physics at work.

Among the modified gravity theories beyond Einstein’s general relativity, there are some theories such as Horndeski theory and 4-dimensional Einstein-Gauss-Bonnet theory, whose scalar degrees of freedom and tensor degrees of freedom are to a certain extent If the effects of these theories are relatively obvious in the early universe, they will have an impact on the propagation speed of gravitational waves in the early universe. One possible situation is that in the very early preheating stage, because the scalar degree of freedom has periodic oscillation behavior at that time, the coupling between the scalar field and the tensor field causes the sound speed of the tensor degree of freedom to have periodicity. Oscillation behavior (that is, the propagation speed of gravitational waves has oscillation behavior), and this oscillation feature will be smoothed out as the universe expands, then the propagation speed of gravitational waves will return to the speed of light in the relatively nearby universe.

Due to the oscillatory behavior of the propagation speed of gravitational waves in the very early stage, gravitational waves will also produce parametric oscillation, which is the SSR of gravitational waves. It causes the gravitational wave amplitude to be exponentially amplified, amplified by 4-5 orders of magnitude in a very short time, and then the oscillation will quickly end and the gravitational wave background will return to normal evolution. This type of SSR belongs to the narrow oscillation type in parametric oscillation. The frequency band where the oscillation occurs is in a very narrow frequency band near the characteristic frequency and at frequencies that are integer multiples of the characteristic frequency, but generally only Characteristic frequencies dominate. At this time, the amplitude of the background gravitational wave will have a peak value near the characteristic frequency. Such a peak feature will remain with the evolution of the universe and will be observed by existing gravitational wave detectors and future gravitational wave detection experiments. The significance of this prediction is that if we can detect this background gravitational wave spectrum feature in the future, then we can infer that the propagation speed of gravitational waves in the very early universe would have significantly deviated from the speed of light, which means that the gravity theory at that time was very likely to No longer described by Einstein's general theory of relativity. This is evidence of new physics.

Figure 3: Schematic diagram of the sonic vibration mechanism of gravitational waves

In addition, in this study, the researchers also found that because gravitational waves are severely amplified under linear theory, It is also possible to induce relatively obvious high-order nonlinear effects. If vibration amplification and nonlinear effects are observed at the same time, it will greatly increase the possibility of the existence of this mechanism. These nonlinear effects may also explain the suspected background gravitational wave signal currently observed by the NANOGrav experiment, and this research is still ongoing.

In terms of particle physics, this work is also of great significance: the energy scale where gravitational wave vibration amplification occurs is above the TeV energy scale, which is basically higher than the existing particle collisions. Machine test energy standard. In other words, if this phenomenon is discovered, it may also indicate the early existence of some new physics beyond the standard model of particle physics. For example, by modifying the coupling and some scattering of the scalar field and the Higgs field in the gravity theory, the scalar field affects the graviton. behavior, thereby changing the propagation speed of gravitational waves. These predictions are waiting to be corroborated by improvements in observation levels in the future.

References: [1] Y.-F. Cai, C. Lin, B. Wang, S.-F. Yan, "Sound speed resonance of the stochastic gravitational wave background", Phys. Rev . Lett. 126 (2021) 071303 . [2] Y.-F. Cai, X. Tong, D.-G. Wang, S.-F. Yan, “Primordial Black Holes from Sound Speed ??Resonance during Inflation”, Phys . Rev. Lett. 121, no.8, 081306 (2018). [3] B. Carr, F. Kuhnel, “Primordial Black Holes as Dark Matter: Recent Developments”, Ann. Rev. Nucl. Part. Sci. 70 , 355-394 (2020). [4] Z. Zhou, J. Jiang, Y.-F. Cai, M. Sasaki, S. Pi, “Primordial black holes and gravitational waves from resonant amplification during inflation”, Phys. Rev. D 102, no.10, 103527 (2020). [5] Ya. B. Zel'dovich, I. D. Novikov, Sov. Astron. 10 (1967), 602. [6] S. Hawking, “Gravitationally collapsed objects of very low mass", Mon. Not. Roy. Astron. Soc. 152, 75 (1971). [7] Y.-F. Cai, C. Chen, X. Tong, D.-G. Wang, S. -F. Yan, “When Primordial Black Holes from Sound Speed ??Resonance Meet a Stochastic Background of Gravitational Waves”, Phys. Rev. D 100, no.4, 043518 (2019). [8] C. Chen, Y.-F . Cai, “Primordial black holes from sound speed resonance in the inflaton-curvaton mixed scenario”, JCAP 10, 068 (2019). [9] C. Chen, X.-H. Ma, Y.-F. Cai, “ Dirac-Born-Infeld realization of sound speed resonance mechanism for primordial black holes”, Phys. Rev. D 102, no.6, 063526 (2020).

Mozi Salon is named after the Chinese sage “Mozi” "A large-scale public welfare science popularization forum named after the Shanghai Research Institute of the University of Science and Technology of China, hosted by the University of Science and Technology of China Shanghai Research Institute, the University of Science and Technology of China New Alumni Foundation, the University of Science and Technology of China Education Foundation, Pudong New Area Science and Technology Association, China Science and Technology Association and Pudong New Area Science and Technology Co-organized by the Economic Committee and others.

Mozi is a famous thinker and scientist in ancient my country. His thoughts and achievements are the embodiment of the early emergence of science in my country. The establishment of the "Mozi Salon" aims to inherit and carry forward the scientific tradition and build a society that advocates science. social atmosphere, improve citizens’ scientific literacy, and advocate and carry forward the scientific spirit. The target audience of science popularization is the general public who love science, have a spirit of exploration and curiosity. We hope that the public with middle school equivalent or above can understand and appreciate the most cutting-edge scientific progress and scientific ideas in the world.

About "Mozi Salon"