Lighting up the "standard candle" marks a new breakthrough in Type Ia supernova research!

Produced by: Science Popularization China

Author: Zhao Weitao (Yunnan Astronomical Observatory, Chinese Academy of Sciences)

Producer: China Science Expo

Type Ia supernovae, known as "standard candles," play a vital role in studying the expansion rate of the universe and verifying the field of dark energy.

Ultrasoft X-ray source, it is the most likely progenitor of Type Ia supernova. Studying it will be of great help to our related research. However, the origin of its quasi-periodic light curve is still unknown.

It is gratifying that researchers from the Yunnan Observatory of the Chinese Academy of Sciences have recently made a new breakthrough in the research of ultrasoft X-ray sources.

The mysterious 'dark energy' that dominates the universe

In recent years, the study of dark energy has been attracting close attention from scientists around the world. "Dark matter" and "dark energy" may become the core research directions of future science.

Dark energy, this mysterious cosmic energy that is more mysterious and more incomprehensible than dark matter, may be the key factor causing the accelerated expansion of the universe!

(Photo source: Veer Gallery)

What is even more shocking is that scientists from all over the world generally believe that this invisible and intangible dark energy occupies a dominant position in the universe. According to a study published in The Astrophysical Journal at the end of 2020, the universe is composed of dark energy and other various substances, of which dark energy accounts for 69% and various substances account for 31%. In addition, among these substances, dark matter accounts for as much as 80%. Conventional matter such as other stars, galaxies, dust and gas that people are familiar with only accounts for 20%.

In other words, nearly 70% of the universe is still a mysterious area that humans know nothing about.

Since dark energy cannot be directly observed so far, scientists can only infer its existence through indirect means.

So how was dark energy confirmed to exist? To learn more about dark energy, you must know these key words: cosmic expansion, cosmological constant, and Type Ia supernova.

The cosmological constant and Einstein's "biggest mistake"

Is the universe eternal and unchanging? From ancient times to the present, there are various opinions on this topic in the fields of physics, mathematics, astronomy and even philosophy. Even Newton and Einstein had their own unique views on the universe to explain their understanding of the "eternity" of the universe.

(Image source: pixabay)

Einstein once proposed a "finite, unbounded, static universe" model: he believed that at a certain cosmic scale (such as 100 million light years), the universe would not change over time. For this theory, he even introduced the "cosmological constant Λ" into the gravitational field equation in general relativity to support this view. The "cosmological constant Λ" assumes that space itself has an intrinsic energy to offset the gravity of matter on a large scale, thereby ensuring that the universe is static.

But about ten years later, this view was "disproven". American astronomer Edwin Hubble, through observation, and based on the linear relationship between the redshift of galaxy spectra and distance, commonly known as Hubble's law, formally confirmed that the static universe model proposed by Einstein did not conform to the actual situation, and the universe was expanding. There are even rumors that Einstein called the cosmological constant Λ the "biggest mistake" of his life.

In fact, the gravitational field equation before the introduction of the "cosmological constant Λ" was able to predict that an unstable universe might expand or contract.

Einstein's gravitational field equations before and after the introduction of the "cosmological constant Λ"

According to Newton's law of universal gravitation, the expansion of the universe should have gradually slowed down due to the force of the Big Bang and the restraint of gravity until it reached a stable equilibrium. However, in 2011, Nobel Prize winners in physics discovered that the universe is expanding at an accelerating rate.

After a series of observations and calculations, scientists have shown that there should be a mysterious force in the universe that is opposite to the direction of gravity (anti-gravitational force), which has not yet been discovered by humans!

Physics calls this mysterious force, which is opposite to gravity and unknown to humans, "dark energy," and believes that it is this "dark energy" that drives the rapid expansion of the universe and the rapid movement of galaxies away from us. From then on, the causal relationship between dark energy and the expansion of the universe gradually became clear as people paid more attention to it.

In order to crack dark energy, the "cosmological constant" has been revived?

Based on the fact that dark energy has been discovered, Einstein's cosmological constant, which is a countervailing force to gravity, has once again sparked discussion. Since the cosmological constant, or dark energy, does exist, it not only counteracts gravity in the universe, but also affects the expansion rate of the universe, so was Einstein really wrong at the time?

The answer is yes. Although dark energy is the main cause of the expansion of the universe, Einstein introduced the cosmological constant only to prove that the universe is static. At the same time, the cosmological constant also proves that Einstein's static universe model is not correct at all.

Modern science is based on rigorous observation and calculation. After a series of twists and turns, the cosmological constant, which is being resurrected in time, has taken on a new meaning.

In the current standard cosmological model ΛCDM, the equations used to describe the state of the universe are mainly related to three cosmological parameters, namely ΩM cosmological density parameter, ΩΛ cosmological normalization constant and H0 Hubble constant. The two main variables included in the formula - cosmological density parameter and Hubble constant - are both crucial.

As the name suggests, dark energy is the mysterious energy in the universe that we cannot see. Since dark energy cannot be directly observed, how do scientists calculate the value of the cosmological constant? The answer is that scientists observe the redshift and magnitude information of distant Type Ia supernovae, assume different cosmological constant models, and then fit the observations to measure a relatively accurate cosmological constant.

Standard cosmological model

(Image source: wikipedia)

In other words, astronomers estimate the value of the cosmological constant based on observations of distant supernovae and fluctuations in the cosmic microwave background.

In addition, the calculation of the cosmological constant is closely related to the abundance of heavy metal elements in the universe. Current research suggests that most of the heavy metal elements (heavier than iron) in the universe come from supernova explosions. Supernova explosions eject matter into the universe, including heavy metal elements, which are very important for the formation of galaxies, star clusters, stars, and planets.

In summary, the characteristics of dark energy can be inferred from the cosmological constant, which can be calculated from observations of Type Ia supernovae.

Among all the types of supernovae, there is one that is an important tool for astronomers to observe the expansion of the universe. It is the Type Ia supernova that we are about to mention.

Type Ia supernovae: a ray of hope for studying the expansion of the universe

Type Ia supernovae can be said to be the "hot players" in the current international astronomical community. If the relevant academic community has a hot search list of topics, it will definitely be on the list.

Type Ia supernovae have no hydrogen and helium lines in their spectra and have essentially the same peak luminosity, so they are also called “standard candles.” Based on the different luminosities of Type Ia supernovae observed on Earth, we can know their distances from us.

Based on this principle, scientists observed the light curve and redshift value of Type Ia supernovae and found that Type Ia supernovae are getting farther and farther away from us, and their speed is getting faster and faster - this means that the universe is expanding at an accelerated rate. And the unknown force that drives the accelerated expansion of the universe is "dark energy."

The role of Type Ia nova, a "celebrity" in the astronomical world, is also very important - when calculating the Hubble constant H0, and the cosmological parameters ΩM and ΩΛ, people need to know information such as the birth rate of Type Ia supernovae, which is closely related to the progenitor stars of Type Ia supernovae; the evolution of galaxies also requires the nucleosynthesis of Type Ia supernovae, the kinetic energy of ejecta, and radiation as physical inputs; the simulation and understanding of the explosion model of Type Ia supernovae can tell us the initial conditions before the explosion and the environment when the explosion occurred; the identification of the progenitor stars of Type Ia supernovae can provide reasonable constraints on the theory of binary star evolution.

Therefore, if we want Type Ia supernovae to play a greater role in research, we need to understand the progenitor stars of Type Ia supernovae in detail. However, so far, people still do not know the progenitor star model of Type Ia supernovae.

The "previous life" of Type Ia supernovae and ultra-soft X-ray sources

In the past few decades, many theories have been proposed about the progenitor stars of Type Ia supernovae, among which the single degenerate star model and the double degenerate star model are currently the two most popular models.

Schematic diagram of two progenitor star models

Left: single degenerate star model; Right: double degenerate star model.

(Image source: NASA)

The binary degenerate star model refers to two CO white dwarfs rotating around each other, losing angular momentum due to gravitational wave radiation, and eventually merging into a new CO white dwarf. If the total mass after the merger exceeds the Chandrasekhar mass limit, a Type Ia supernova explosion will occur.

The single degenerate star model refers to a CO white dwarf and a companion star, which may be a main sequence star, a red giant or a helium star-1714573025. The white dwarf accretes the companion's matter, burns hydrogen-rich matter on its surface and continuously increases its own mass. When the mass increases to the Chandrasekhar mass limit, a thermonuclear explosion will occur.

Both models have some advantages and disadvantages in theory and observation, so there is still debate about which model is the progenitor of Type Ia supernovae. However, the spectrum and light curve calculated by the single degenerate star model are in good agreement with observations, and it can be said to be the more mainstream model of Type Ia supernova progenitors.

The most likely progenitor star system of the currently recognized single degenerate star model is an ultrasoft X-ray source. However, in the current research on ultrasoft X-ray sources, the origin of the quasi-periodic light curve is still unclear, which has once again hindered the research on the progenitor stars of Type Ia supernovae.

New research progress brings "a ray of hope"

It is worth being proud of that, thanks to the efforts of researchers from the Yunnan Observatory of the Chinese Academy of Sciences, the difficult problem of "quasi-periodic light curves in the study of ultrasoft X-ray sources" has recently achieved a new breakthrough!

Ultrasoft X-ray sources are a special type of close binary system with accretion and thermonuclear burning white dwarfs. Ultrasoft X-ray sources consist of a white dwarf and a massive main sequence companion. The white dwarf accretes matter from the companion and burns stably. The light curve of the ultrasoft X-ray source shows quasi-periodic changes of alternating light and dark.

According to observations, ultrasoft X-ray sources have very bright blackbody radiation luminosity, but their X-ray spectra are very soft, with a peak value of about 20-80eV in typical sources and a blackbody radiation temperature of 105-106 K. The companion star is usually a main sequence star or a subgiant star, with an orbital period ranging from a few hours to a few days. However, the cause of this quasi-periodic variation in the light curve of ultrasoft X-ray sources remains unclear.

To this end, our researchers proposed that ultrasoft X-rays periodically illuminate the companion star, causing the companion star to expand and contract periodically. Therefore, the binary star mass transfer rate increases and decreases periodically, causing the white dwarf photosphere to expand and contract periodically, which reproduces the light curve of the ultrasoft X-ray source very well!

Schematic diagram of the evolution of the companion star model irradiated by ultrasoft X-rays

(Photo credit: Drawn by Zhao Weitao)

The research results not only provide a new way to explain the origin of quasi-periodic light curves in supersoft X-ray sources, but also provide new research ideas for the study of Type Ia supernova progenitors. At present, the relevant results were recently published in Astronomy & Astrophysics (A&A) under the title of A robust model for the origin of optical quasi-periodic variability in supersoft X-ray sources.

Conclusion

Although the study of dark energy has lasted for more than 20 years, scientists still cannot accurately say what it is. The part that humans have detected and understood in the vast universe is still the tip of the iceberg. But with the continued efforts of all researchers, humans will continue to use their wisdom to explore unknown areas, calculate and speculate on the future, and continue to find answers.

I believe that Chinese scientific researchers will continue to make breakthroughs in the field of astronomical exploration with a rigorous and scientific attitude, and the unsolved mysteries will eventually become clear.

Editor: Guo Yaxin