[Mingpao News]During the demonstrations in the mainland last month, students from Tsinghua University in Beijing held up a piece of white paper with a physics equation printed on it. This equation is called Friedmann equation (Friedman equation), the pronunciation is similar to “Free’s man”, a free person. In addition to the special homophony, Friedman’s equation plays a pivotal role in cosmology. It challenged the universe model proposed by Einstein in the 1920s, and subsequent astronomical observations also confirmed that Friedman was correct. To this day, Friedmann’s equation is still the one that best helps us understand the universe.
Introduction: Why do ducklings swim in a row? Do you feel cold when you are empty and lonely? There are hundreds of thousands of strange and interesting questions in the world, and science popularization is never too much. For the new column, we invite the host of the Hong Kong and Taiwan science popularization program “True Good Science” to continue to explore the old news and new knowledge of science. Each issue answers a question and introduces a scientist. I hope readers, even if they don’t hear any great truths, at least feel very friendly and find the fun of science in their daily life.
Friedmann’s Equation Calculates the Rate of Change in the Expansion of the Universe
Astronomy observes and studies individual galaxies, while cosmology takes the entire universe as its research object, hoping to understand the universe from a physical perspective. Dr. Chen Zhihong described Friedman’s equation: “The Friedman’s equation is the most ideal equation that we found out from the general theory of relativity. It helps us describe the past, present and future of the universe.” Friedman’s equation was derived from Einstein’s general theory of relativity Changes come from the Soviet physicist Alexander. Friedman proposed it in 1922. Friedman believed that the universe was expanding, and the equations were able to calculate the rate of change in the expansion of the universe.
This is contrary to Einstein’s claim at the time. Einstein proposed the “static model of the universe.” He believed that the space of the universe would not shrink or expand, although the results of his general relativity calculations showed that the universe was either shrinking or expanding. So Einstein added a “cosmological constant” (cosmological constant) to the general theory of relativity that does not affect the correctness of the equation, and designed its value so that the general theory of relativity can calculate that the universe remains static. It was not until the astronomer Edwin Hubble discovered through astronomical observations that other galaxies are getting farther and farther away from our Milky Way, confirming that the universe is indeed expanding, that Einstein’s claim was overthrown.
Why does the universe expand?
It has become the general consensus in cosmology that the universe will expand, but how does the universe expand? Why is the universe expanding? Let’s start with the Big Bang. Cosmologists still haven’t been able to determine why the Big Bang happened and why it produced such a large amount of energy, but the force at the moment of the explosion is so great that the universe can continue to expand from then on, “seems to be the initial condition for the first expansion of the universe , the big bang was powerful enough at the beginning to drop everything.” Just like you throw a ball upwards, the ball will fly upwards, just like the energy of the big bang caused the universe to expand. But the earth has gravity, the ball will always fall down, and so does the universe. “There is a lot of matter in the universe. There is actually a gravitational force between matter and matter and energy. It tends to bury everything together.” Cosmologists initially assumed that the expansion rate of the universe would become slower and slower, reaching a critical point After that, the universe will start to shrink.
However, a study in 1998 found that our universe is expanding at an accelerated rate. In other words, the universe is not only not shrinking, but may continue to expand endlessly. At that time, two astronomical teams were trying to measure the apparent brightness of Type Ia supernovae, because the maximum brightness and energy emitted by Type Ia supernovae were almost fixed, and astronomers could use their brightness to calculate the distance, just like observing the flame of a candle. When the candle is far away from us Appears darker, closer to us appears brighter. Perlmutter (Saul Perlmutter) led the team at Lawrence Berkeley National Laboratory of the University of California, which began to observe supernova explosions in 1988; Schmidt (Brian Schmidt) from the Australian National University also led the expert team in 1994. Research assisted by Adam Riess. The three published reports almost at the same time in 1998, pointing out that the brightness of Type Ia supernovae is darker than expected, that is, the supernova is located at a farther distance than expected, and the universe is accelerating expansion, which may be related to dark energy. Three astronomers were awarded the Nobel Prize in Physics in 2011 for this discovery.
The endlessly expanding universe returns to loneliness?
What would happen if the universe continued to expand indefinitely? “You can imagine that it is like a drop of ink falling into an infinite ocean.” Although the universe has been expanding, the mass and energy in the universe remain unchanged, and the matter will not increase accordingly. “It will not produce more matter for no reason, nor will it disappear for no reason. At most, it will be transformed, and the easiest thing to be transformed may be transformed into light energy. In simple terms, if it is a proton, it may turn into The second kind of thing, but basically there is only so much mass-energy in the overall universe.” When space continues to expand, but the universe does not produce more mass-energy and form more stars, the universe in 100 billion years may be bigger than it is now Much quieter, much more boring. “The stars in the sky are so sparse and dim, nothing special happens, not too many stars will be born and die, and it will move towards a silence, and even in the end, the energy and matter density in the universe can’t help it to be effective anymore Earth formed to great stars, just some hot nebulae.”
According to the existing astronomical observations, cosmologists generally believe that the universe is currently expanding at a constant speed, or expanding at an accelerated rate, but the future is still unknown. “Whether the universe is expanding or not, how fast is it expanding, or is it not expanding, or will it even shrink in the future? What is the rate of change of expansion and how to determine it is exactly what Friedman’s equation says.” Friedman’s equation The equation calculates the rate of change of the expansion of the universe by calculating parameters such as the density of all matter in the universe and the curvature of space-time. If we can accurately measure every parameter and figure out every component in the universe, we can calculate the fate of the universe. The problem is, scientists know very little about the universe.
The Unprovable Existence of Dark Matter/Dark Energy
Matter, energy, elements, and all other things known to human beings only account for no more than 5% of the universe. Scientists call “dark matter” and “dark energy” the things that fill the universe and are invisible to humans, and are closely related to the expansion of the universe. “We already know that there are substances that can exert gravitational force, that is, the stars we see, countable, and those substances that can emit light, but there are some that neither emit light nor participate in many physical effects, but only participate in the effect of gravitation , those are dark matter.” Dark matter is a substance that exerts a gravitational force, preventing the expansion of the universe; dark energy is the opposite of gravity, causing things to repel each other and helping the universe expand. But scientists have never had direct evidence to prove the existence of dark matter and dark energy, and can only look for indirect evidence, “For example, galaxies have many spiral arms, many swirl-shaped objects are rotating, and so is our Milky Way, but if there is no explanation for the concept of dark matter At the time, we couldn’t understand why it had this pattern of rotation.”
There is a joke in the field of cosmology. Try to explain why 1+1 equals 3. Ordinary people are at a loss. Cosmologists will say that because 1+1 plus a “dark” value, it will equal 3. The “dark” value is actually a metaphor for dark matter and dark energy. We don’t know how the dark value appears, we can’t measure its value, and we can’t find direct evidence to prove its existence, but in short, the dark value does exist. Chen Zhihong compared, “We are like a poor auditor, we can’t count the numbers. On the one hand, we can see that the universe is expanding, but we are unable to use the Friedman equation to explain why the expansion occurs. Appear”. But the spirit of science lies in constantly correcting mistakes and finding new theories to explain new observations. Einstein tried to challenge Newton’s gravitational force and use general relativity to explain phenomena that Newton could not explain; Friedman challenged Einstein’s cosmology and tried to calculate the universe. Fate, maybe one day someone will actually be able to prove that dark matter exists, or that it doesn’t. “For scientists, if someone finds a way to find a problem with the general theory of relativity in an experiment that has nothing to do with cosmology, everyone will be very happy. Will there be a hope that there is no need to use so much dark matter and dark energy? You only need to modify some things in the original theory and fill in the loopholes, and you can solve other cosmological problems.”
Alexander Friedman (1988-1925)
Friedman was a Soviet mathematician and cosmologist. The most important discovery in his life was the possibility of the expansion of the universe. He proposed the Friedman equation in 1922 and published a paper in 1924, describing the space-time curvature of the universe when When it is positive, zero, and negative, it will be beneficial to appear a closed universe, a flat universe, and an open universe respectively. The closed universe first expands and then contracts, and the latter two continue to expand. Later, the astronomical observation evidence published by the American astronomer Hubble in 1929 confirmed Friedman’s hypothesis. Unfortunately, Friedman died of typhoid fever a year after publishing the paper. He was only 37 years old and missed Hubble’s discovery.
“General Relativity”
Friedman’s equation is derived from the general theory of relativity, so what does the general theory of relativity say? The most important thing about general relativity is to explain the distortion of space-time with mass-energy, so as to explain what is gravity. A substance with mass will bend the space-time around it. Just imagine that space-time is a trampoline. You put a bowling ball on the trampoline. The weight of the bowling ball will make the trampoline sink, just like the mass bends space-time. , when a table tennis ball rolls past the bowling ball, it will roll to the sinking place of the trampoline, and it seems to be attracted by the bowling ball, just like “gravity”.
Wen˙ Zhu Linlin
Art Zhang Yuqi
Editor Cuili Wang
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