The history of the universe is a history of expansion
The first Friedman equation.
Compared to today, people's understanding of the universe 100 years ago was very limited. In 1922, the universe was considered static and eternal. At that time, astronomers had not yet observed any clues that there were so many different shapes and sizes of galaxies beyond the Milky Way, nor had they found strange objects such as pulsars and white dwarfs in the universe, nor did they imagine that the universe even had a beginning.
However, it was also in that year that the young Alexander Friedmann (1888-1925) derived the Friedman equations that no one in the field of cosmology knew about after studying Albert Einstein's theory of general relativity and assuming that the universe was isotropic (all directions are the same) and uniform (all places are the same). Most surprisingly, Friedman's solution meant that the universe could expand, contract, collapse, and even have a beginning, rather than being static as Einstein and most other scientists thought at the time.
Of the two equations friedman derived, the first is more important. One side of the equation tells us how the structure of the universe would expand or contract over time. The other side of the equation contains all the matter, radiation, and any other form of energy that makes up the universe, as well as the inherent curvature of space (different curvatures correspond to different shapes of the universe), and even the cosmological constants (denoted by "Λ") that Einstein had introduced in the equation to keep the static universe alive.
Although Friedman's life was short, a century later, the equations he wrote still govern the history of the expansion of the universe and have been extended to a universe containing inflation, dark matter, neutrinos, and dark energy.
The "Guardian" of the Universe
Milgrom proposed a correction to Newtonian dynamics.
When we imagine a galaxy, we think of gorgeous structures in the shapes of spirals, ellipses, and so on. While observing the rotation of the galaxy, astronomers unexpectedly discovered an incredible phenomenon at the edge of the galaxy: the celestial bodies there were moving too fast!
The known law of gravitation tells us that the farther away an object is from the center of a galaxy, the smaller the gravitational pull and the slower its orbital speed will be. But observations tell us that the celestial bodies on the outer side of the galaxy are moving at a rate that does not match theoretical expectations, which means that there is something unknown waiting to be discovered. There are two possibilities: either our understanding of gravity needs to be revised, or the presence of a large amount of invisible matter in the universe creates additional gravitational pull, or the galaxy will fall apart.
While many enthusiastically embrace the possibility of dark matter in the universe, a few theorists have chosen to explore on different paths. In the 1980s, Mordehai Milgrom proposed "modified Newtonian dynamics" (MOND theory), which found that if we changed the acceleration in Newtonian gravity a little bit, we could explain the rotational velocity of galaxies of different sizes and ages.
While theories developed based on MOND can explain the rotation problem of galaxies, these theories have been unable to explain the cosmic microwave background (the most critical observational evidence of the Big Bang), so much so that it has not received enough attention for a long time. It wasn't until last year that a new MOND model proposed by two theoretical physicists finally overcame this major obstacle.
We can't confirm whether dark matter or MOND theory is silently guarding galaxies, but it is certain that if dark matter detection continues to be fruitless, more and more people will start to pay attention to MOND theory.
Advanced civilizations in the universe
The "archaeological form" of Drake's equation.
In the vastness of the universe, are human beings lonely?
In 1961, astrophysicist Frank Drake proposed a famous equation to estimate the number of advanced civilizations that might exist in the Milky Way. With the advancement of space technology, scientists have a better understanding of some variables in the equation. But for variables like the time of the expected existence of advanced civilizations, we can no longer do anything but guess.
In 2016, Adam Frank and Woodruff Sullivan proposed a new equation to solve a slightly different problem: What are the number of advanced civilizations that could have developed into advanced civilizations in the history of the observable universe?
In the new equation, A is defined as "the number of technological species that have formed in the history of the observable universe." On the right side of the equation, Nast represents the number of habitable planets in a given volume in the universe (which could be the Milky Way or the entire universe), and fbt represents the probability of developing an advanced civilization in those planets.
The most bizarre celestial body in the universe
Hawking temperature formula.
Black holes are the most mysterious objects in the universe. One of the simplest forms of black holes is also known as a Schwarzschild black hole, and karl Schwarzschild found the first solution to Einstein's field equations shortly after Einstein proposed general relativity. The solution Schwarzschild found corresponds to a black hole with only mass, no charge, and spin.
Because black holes have such a strong gravitational pull that anything that enters the black hole cannot escape. Stephen Hawking and others once thought that black holes don't emit radiation, but if so, then the second law of thermodynamics, which is considered an iron law, would be violated. In 1974, based on general relativity and quantum field theory, Hawking re-examined the curved space around the black hole and found that the black hole has a temperature, so it also radiates. Hawking radiation was never detected because the temperature of a typical black hole was very small. For example, the Hawking temperature of a solar-mass black hole is about 0.00000006 Kelvin.
In the famous black hole temperature formula, we can see that the formula contains the gravitational constant, Planck's constant, the speed of light, and the Boltzmann constant. That is, the formula links quantum theory, general relativity, and thermodynamics.
Cosmic Tunnel
The new "Einstein" equation.
From this equation alone, you might think that the establishment of this equation should require P=1, but this is not the case. Because this equation actually connects two landmark papers published by Einstein in 1935.
The letters E, P, and R represent three physicists: Einstein, Rosen, and Podolsky. The ER to the left of the equation represents a wormhole (also known as an Einstein-Rosen bridge), which creates a space-time tunnel for two very distant regions of space-time. The right side of the equation represents quantum entanglement proposed by Einstein, Podolsky, and Rosen in another paper: When two particles are entangled, measuring the state of one of the particles immediately tells the state of the other, no matter how far apart they are.
In 2013, Juan Maldacena and Leonard Susskind proposed that the link between wormholes and quantum entanglement wasn't just reflected in the fact that they were proposed in the same year, but that they were essentially identical. Any two particles connected together by entanglement, they are actually connected together through wormholes, and vice versa. This means that perhaps the connection between quantum mechanics and gravity is stronger than we think.
#创作团队:
Written by: Zwicky
Design: Wenwen
#参考来源:
https://arxiv.org/pdf/1302.1498.pdf
https://journals.aps.org/prl/abstract/10.1103/PhysRevLett.127.161302
#图片来源:
Cover image: Pixabay
First image: Pixabay