In the universe, time and space are basically the same thing. Because when you see the depths of the universe, you see the past. The cosmological unit of length we usually refer to is the light year, which looks strange, like a unit of time, but in fact, it is a unit of length, representing the distance traveled by light for one year.

How far is it to walk alone for a year? It's not very far, about 9460 billion kilometers. How big is the Milky Way? With a diameter of over 100000 light years, that is to say, light travels for 100000 years from one end of the Milky Way to the other.

When you see a more distant galaxy, such as the Andromeda galaxy, 2.5 million light years away from Earth, what you see is what it was 2.5 million years ago and may no longer exist today. Another windmill galaxy is even further away, 21 million light years away.

For those who study cosmology and astronomy, 'time flies, I only care about your past', we cannot see the future, we can only understand what happened in the universe through the past.

How to Explore the Evolution History of the Universe

To study the universe, you first need a very good theory, and to create a good theory, you need a smart brain. Einstein established a very great theory: general relativity, but at the same time, you also need to have excellent observation methods to test the theory, among which the multiverse probe uses various different methods to explore the universe.

The cosmic microwave background radiation is one of our celestial tools, and its principle is actually very simple. It is similar to using the wavelength of a microwave oven to detect the distribution of photons in the entire sky. According to the prediction of the Big Bang theory, there is a 3K microwave background radiation in the sky, which carries a lot of cosmological information in the distribution of the sky, because these photons flew from 380000 light-years after the Big Bang to our present day, Carrying a large amount of "scenery" along the way, we say that cosmic microwave background radiation is a sharp tool for exploring the infancy of the universe. The study of cosmic microwave background radiation (CMB) has won the Nobel Prize in Physics twice.

Cosmic Standard Candlelight: Supernova

Not only can we study what happened in the early universe by looking at photons, but we can also observe another type of star - supernovae, which are called cosmic standard candles and are relatively close to us. Why is it called a standard? Because its luminosity varies regularly over time from eruption to death, we can use the luminosity of a supernova to infer its distance from us.

On the other hand, using today's spectral technology, velocity can be calculated. With distance and velocity, scientists can measure many interesting cosmic information, such as cosmological parameters, and so on.

Through supernovae, scientists discovered in 1998 that the universe is not only expanding, but also accelerating. The 2011 Nobel Prize in Physics was awarded for research related to supernovae.

The standard whistle of the universe: Gravitational Waves

Gravitational waves are known as the standard whistle of the universe, and studying the universe can be done not only by looking, but also by listening. When two dense celestial bodies merge and orbit around each other, they form ripples in time and space, just like sound can be heard by everyone, because the pressure of air changes during the propagation of sound. Gravitational waves are ripples in time and space, and essentially vibrations can be heard through "sound". And gravitational waves carry very important cosmological information, so they are called standard whistles. In 2017, research on gravitational waves also won the Nobel Prize.

Universal Standard Scale: Baryon Sound Wave Oscillation

Baryonic sound waves oscillate, known as the cosmic standard ruler. There is candlelight in front to see brightness, a whistle to listen to sound, and a standard ruler to measure distance. The research method is to extract the evolution information of the universe from a large number of galaxy samples using statistical properties. The study of baryonic acoustic oscillations also won the Shaw Award in 2014.

It is not difficult to see from here that every research method is important. Based on these observations, we can understand the energy composition of the universe today: 4% is a familiar ordinary substance. Ordinary matter refers to you, me, him, the sun, black holes, all of which can be called ordinary matter, and you can see and touch them. The remaining 96% is unknown to us. It is divided into two parts, about two-thirds of which is dark energy, and the rest is dark matter.

Since the establishment of the Nobel Prize in Physics in the early 20th century, over a hundred Nobel Prizes in Physics have been awarded to these 4%, and you can speculate that there is still 96% of the "Nobel Prize" level research work waiting for us to discover.

In fact, research on dark matter and dark energy has won several Nobel Prizes, CMB research has won two Nobel Prizes, accelerated expansion research won Nobel Prizes in 2011, gravitational wave research won Nobel Prizes in 2017, and this year's very important research on Peebles has also won Nobel Prizes. He has established a complete framework of cosmic gravitation. Without him, our cosmological research today would be difficult to start, and his work has laid the foundation.

This is the evolution history of the universe that we understand today, which can be basically divided into three parts: the beginning of the universe. At this stage, the expansion of the universe is very rapid, an accelerated process, just like a person growing very quickly in infancy, transitioning from a baby to a giant baby in a very short period of time.

In the middle of this process, the universe is growing and developing, and we all know that there is gravitational force. Under gravitational force, the expansion of space and time should be slowed down, as we can imagine, because it is mutual attraction. Under this effect, the universe slowly expands, and the structure of the universe grows slowly, just like a sapling.

Until 6 billion years ago, the universe inexplicably began to accelerate its expansion and grow in a explosive manner, and we still do not know the reason. Some scientists believe that it is due to the existence of dark energy, but we don't know what dark energy is. We can only slowly approach it and use our sky survey observations today to analyze its nature and explore its essence.

The universe should be divided into stages such as the Big Bang, subsequent inflation, and very rapid expansion. Then there was a period in the universe called the Dark Age, during which there was no glowing object because the first generation of stars did not form, mainly hydrogen.

About 400 million years after the Big Bang, the universe was first illuminated, marking the formation of the first generation of stars. Afterwards, there were more and more diverse structures in the universe, where stars formed galaxies and galaxies formed galaxy clusters.

Recently, around 6 billion years ago, the universe began an accelerated expansion phase, which is also my main research direction.

The stage of accelerated expansion of the universe

Let's start with the acceleration of the universe by listing a few numbers. Let's feel it. Within 10 to the minus 36 power seconds, the universe has expanded by 10 to the 26th power. It's not matter expanding, but spacetime itself expanding.

Due to the expansion of the universe, the temperature also dropped from the first 10 to 32 degrees to 10 million degrees. At this time, there began to be some elementary particle, photons, electrons, etc. in the baby universe.

Cosmic photons are the most important medium. No matter what telescope, optical telescope, radio telescope, etc. you use, they are all photons, just at different frequencies.

The frequency of detecting photons is relatively close to that of microwave ovens, used to observe fluctuations in different temperatures throughout the entire sky.

The blue color in the above image may be lower in temperature, while the orange color may be higher in temperature, which may seem irregular. However, it is precisely this irregular sky map that contains rich cosmological information.

How do we know that the universe is 13.8 billion years old? It is through the figure above. How do 70% of dark energy and 20% of dark matter in the universe know? Also through this picture.

Here comes an accidental discovery: Penzias and Wilson were engineers at Bell Labs at the time, whose job was to reduce the noise of radio antennas. When they aimed the equipment in various directions, they found that there was a noise that could never be removed, and it was very uniform, regardless of which direction it was pointed to.

In the same era, it is necessary to mention a great man, Peebles. Peeble proposed a theory back then, when many people believed that the universe was static, while Peebles believed that the universe was not static, and began a Big Bang. Based on the residual noise of the Big Bang, how much signal was there? He found that it corresponds to an energy scale of approximately 3K, which is minus 270.

When Penzias and Wilson discovered the noise, they immediately associated it with Peebles' work. They quickly ran over to discuss with Princeton scientists, including Peebles, to see if the predicted signal had been discovered by us. Finally, in 1978, Penzias and Wilson won the Nobel Prize.

Photons became free 380000 years after the Big Bang and then flew towards us, making the universe rich and colorful, with dark matter present in the universe. What is dark matter? During lunch, everyone may have eaten a lot of dark matter. It is matter, but it has no interaction with you. However, it is the existence of these dark matter that enables the formation of structures in our universe.

We obtained the formation and evolution of the universe through digital simulation. The figure on the right is a digital simulation, which is very close to the galaxies we observed today, with various forms of galaxies, such as elliptical galaxies, vortex galaxies, and so on. Therefore, the dark matter theory can well meet today's observations.

Why does the universe accelerate expansion?

Next is my research field, which is also the most perplexing aspect we currently know, which is the expansion of the universe or the accelerated expansion of the universe.

When it comes to this question, we must start with a 'bull man', Newton. The story of Newton and the apple may be very familiar to everyone. An apple hit the brain of wisdom, and Newton discovered the law of universal gravitation. He discovered the mutual attraction between any two bodies, but did not tell us why, he only discovered this phenomenon.

At the beginning of the 20th century, Einstein established a very fashionable theory: relativity, which few people could understand back then. When Einstein faced the same phenomenon, he fell into deep contemplation and gave a completely different explanation: why can two bodies attract each other? Einstein believed that the existence of matter can cause distortions in the surrounding spacetime. Matter tells spacetime how to twist, and spacetime in turn tells matter how to move.

After Einstein obtained the theory of relativity, he eagerly applied it to the study of the universe. At that time, he discovered two possible solutions: one is that the universe is about to contract, and the other is that the universe is about to expand.

Einstein was very dissatisfied with both solutions, because no one hoped that we would one day collapse to the starting point or expand to pieces. So Einstein was thinking, how can I keep the universe stable without expanding or contracting?

We don't delve into the equation, it means that the curvature of spacetime is equal to the distribution of matter. The distribution of matter may have an attractive effect. You can imagine a shrinking spacetime that is easier to understand because there is attraction between matter. Einstein believed that the universe cannot expand, so he excluded the solution of expansion. So he thought, 'If the universe is contracting, maybe I have a way. Shrinking is because of gravity, why don't I introduce universal repulsion to prevent it from contracting?'? In 1917, Einstein creatively added a term, cosmological constant, to his equation, which is the earliest dark energy model.

After obtaining this equation, Einstein was very excited: my universe finally calmed down perfectly, and my general theory of relativity could finally be perfectly applied to the universe.

But not long after, in 1929, British scientist Hubble discovered that the universe was not static at all. The universe was expanding, and the farther away it was from us, the faster its expansion rate, which is a proportional relationship. Today's universe is still expanding, proportional to distance.

You can imagine Einstein's expression when he learned the news. He said a sentence, which translated into Chinese, "The introduction of cosmological constant was the biggest mistake I made in my life", and none of them was very regretful.

But the story did not end. In 1998, three scientists used supernova research to discover that the universe was not only expanding, but also accelerating. Imagine why Einstein introduced cosmology back then? It is to prevent the universe from expanding. He believes that the universe is contracting, and after adding this constant, the universe can not expand. But the universe is already expanding, does it expand faster after adding universal repulsion? So Einstein came to the correct conclusion entirely from the wrong starting point.

That's why when you look at the cover of Science magazine in 1998, Einstein's expression is very complex, surprising and regretful. Science may be like this.

Accelerated inflation research was listed as the top ten technological advancements in 1998. According to the report of the dark energy Task Force, an authoritative international organization, the existence of dark energy shows that our understanding of elementary particle or the theory of gravity today is either incorrect or incomplete. In short, a revolution in basic physics is needed.

Of course, this is a major opportunity. Today, we live in the era of precise cosmology. We have very high-quality data, from which we can extract information about dark energy. The direction of dark energy is also the highest priority research direction in the U.S. Ten Year Plan, and also the first basic frontier field of our country's 13th Five Year Plan breakthrough.

Large scale galaxy surveys to study dark energy

There are many ways to study dark energy, and my own research is to use large-scale galaxy surveys.

The above image shows some galaxies that we captured from a telescope, with each point representing a galaxy. We use a method similar to a census to conduct statistical research on galaxies, called clustering analysis.

According to the measurement results, it is found that there are many pairs of galaxies on a special scale. We use the astronomical scale called 100 trillion parsec, and there is a local bulge there.

The location of this protrusion is very important because it comes from information from the early universe, which in turn can infer information such as the age of the universe, the expansion rate of the universe, the composition of the universe, and so on. So in cosmology, we call it the cosmic scale, also known as the baryon sound wave oscillation, which is a very important physical tool.

This graph shows another very important effect, called redshift distortion, which is a signal for us.

We use the three-dimensional distribution of black holes in space to measure the expansion history and structure growth history of the universe, which can help us study dark energy. By using galaxy surveys, we can extract three most important pieces of information, corresponding to the three major scientific objectives:

The first one is baryon acoustic wave oscillation, which explores the expansion history of the universe, corresponding to the nature of dark energy.

The second one is redshift distortion, as the growth of the cosmic structure is dominated by gravity, so it can study the properties of gravity.

Third, small-scale clustering can help us measure neutrino mass. Because the absolute mass of neutrino is difficult to study on the earth, you can only detect the relative absolute square difference of the mass of neutrino in different generations, but in cosmology, you can measure the absolute mass of neutrino. Because if there are too many neutrino, neutrino will have a high speed, which will make many small structures unable to form. These studies are also the research direction that will win the Nobel Prize and the Shaw Prize in the future.

Back to dark energy, we measured the expansion rate and structure growth rate of the universe together with the cooperation group, which can measure a very critical cosmological parameter - the equation of state of dark energy. All properties of dark energy are embodied in such an equation or function:

The abscissa is time, and the ordinate is equation of state: the ratio of dark energy pressure to dark energy density. A dotted line is drawn in the figure. This is the equation of state of dark energy predicted by Einstein in 1917, which is strictly equal to minus 1. For many years, people have called Einstein's model the standard dark energy model of the universe. They believe that dark energy is the energy of vacuum. Although there are many theoretical problems, people have generally accepted it.

In 2017, we reconstructed the evolution history of dark energy with the line of sight by using the latest astronomical observations, baryon acoustic wave oscillation, redshift distortion, etc. It is obvious that it is not equal to negative 1, not a constant, but evolves with time.

The blue part is the error we obtained, which is relatively large and significant enough. I believe that with the improvement of our observation ability, we will gradually approach the essence of dark energy.

Dark energy research without answer

If dark energy is really a dynamics, what does that mean? This may be a question worth considering, and I don't have an answer today.

What is dark energy? Is it a cosmological constant? Is it the energy of a vacuum? It cannot be completely ruled out today, but it seems unlikely and may be an unknown form of energy existence.

Is it possible that it is a gravitational effect? Any theory may have a scope of application, such as Newtonian mechanics that may not be applicable at the cosmological scale. General relativity can be applied at larger scales, but does it also need to be modified at larger scales? Does it have a candidate for dark energy? It is also possible. Some people say that dark energy is information. It is all possible.

With the improvement of our observation methods and capabilities, we are currently using the SDSS 2.5 meter telescope, which has detected the spectra of one million galaxies. However, it is far from enough. Starting in the second half of this year, the DESI cooperation project will be based on a larger telescope, which will detect 20 million spectra. The well-known FAST radio telescope, with a diameter of 500 meters, will also provide us with clues in terms of very high accuracy.

Carl Sagan once said that we live in the universe and can be said to be very lonely. We are very ignorant and even somewhat arrogant. But no matter what, it is my honor to be able to share a planet and study the universe together in the vast space and infinite time.