The big bang at the beginning of the universeGravitational wave。stayuniverseAt the first moment of birth, the universe is full of dense matter, so that the gravitational waves generated by collisions between particles are immediately absorbed by other particles.In the inflationary stage of the rapid expansion of the universe, the density of the universe suddenly drops, and the released gravitational waves are no longer absorbed.Since then, those primitive disturbances have spread in the space around us.
The original gravitational wave was proposed by Einstein in the general theory of relativity published in 1916. It is a kind of space-time wave generated at the beginning of the universe, and it is weakened with the evolution of the universe.Scientists say that the original gravitational waves, like the "afterglow" of the Big Bang of Genesis, will help people trace back to an extremely short period of rapid expansion at the beginning of the creation of the universe, the so-called "inflation".
However, since the general theory of relativity was put forward nearly a hundred years ago, other important predictions derived from it, such as the bending of light, the perihelion precession of Mercury, and the gravitational redshift effect, have been confirmed one by one, while gravitational waves have never been directly detected. The problem is that their signals are extremely weak and difficult to measure technically, so some people also call them "century suspense""The greatest futility in the universe".[1]
From classical mechanics to relativity
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Newton, the great scientist, combined the theories of three other great scientists, Copernicus (1473-1543), Kepler (1571-1630) and Galileo (1564-1642), proposed the law of universal gravitation, and solved many problems in his period, including the causes of tides, the movement of the earth ball and the moon, and the orbit of comets.However, although Newton's theory explained what gravity is, in the following 300 years, the reason for gravity is still a mystery.
As scientists developed better astronomical tools, they found that their observations were slightly different from those predicted by Newton's theory.For example, the prediction of the orbit of Mercury by Newton's theory is slightly different from the actual observation.In addition, Newton's theory can not give a satisfactory explanation to the following question, that is, if the sun suddenly disappears, what will happen?
According to Newton's theory, the whole universe will immediately detect the disappearance of the sun.This means that all planets orbiting the sun will fly out of orbit in a tangent direction.Einstein gave a more detailed explanation of this problem. He thought that the planets farther away from the sun would know later that the sun disappeared, so the nearer planets would fly away from the orbit around the sun first.But in the late 19th century, most scientists agreed with the description of the universe at that time.In fact, most of them think that the research of physics has been quite perfect, and all that remains is to understand some details.But the problem is that these so-called "details" are observations or experimental results that cannot be explained by the theory at that time.One of them is that the speed of light in the experiment is always measured at 300000 kilometers per second (186000 miles per hour).Therefore, scientists believe that because the earth is moving around the sun, if we measure the speed of light in different directions, we will get different results.In 1895, Albert Michelson and Edward Morey carried out this experiment, but unexpectedly failed to find any difference in the propagation speed of light in different directions.People at that time could not explain this phenomenon according to their theories at that time, until Albert Einstein gave an accurate explanation in 1905.He believes that unlike the speed of a car, the speed of light is constant and does not change with the movement of the observer.In other words, even if you run very fast, you can't catch up with the light.The first law in Einstein's special theory of relativity is that the speed of light is constant, completely independent of the speed of the observer and the light source.In the following years, Einstein continued to study the details of special relativity.At this time, he began to consider how to integrate Newton's theory of gravity into the new theory.
Relativity Prediction of Gravitational Waves
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Einstein realized that people falling freely on the earth can not feel the effect of gravity just like astronauts in space.Moreover, in the rocket with constant acceleration, people will feel the same gravitational effect as those sitting on the earth.The basic assumption of Einstein's general theory of relativity is that the gravity felt by objects on the earth, the force felt by objects far away from the mass and the force felt by objects with constant acceleration are identical.However, since the two forces are the same, the laws of physics are applicable in both cases.So Einstein found that this needed to modify the definition of gravity.But how to define gravity?
Einstein thought that gravity was not what Newton thought.He believes that objects attract each other because heavy objects distort space-time, while other objects choose the shortest path in the warped space-time.Einstein discovered mathematically that the structure of space-time is elastic, just like a trampoline.As shown in Figure 1, all objects lie on the trampoline and deform it, which is called space-time distortion.The size of spatiotemporal distortion is related to the mass of the object, and the larger the mass, the larger the deformation.The place where the matter is concentrated is the place where the gravitational field is "dense", and also the place where the space-time curve is the largest. The attraction effect of this space-time curve to produce mass is gravity.Due to the curvature of space-time, the shortest path between two points is no longer a straight line, but a curve along the direction of the gravitational field.This phenomenon has been confirmed by observing light from distant stars.If a huge object is located between the earth and the star, then the light from the star will be affected by the space-time curvature, and its propagation path will be distorted to deviate from a certain angle.This effect also forms an interesting gravitational lens phenomenon, which makes distant stars brighter and sometimes forms double images.If the object is a black hole, the light will be sucked into the gravitational trap and never come out again.
Space time distorted by massive objects
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The discovery of gravitational waves is expected to reveal the mystery of the birth of the universe
Since Einstein predicted the existence of gravitational waves in the universe, people have been detecting gravitational waves without interruption.
Scientists Find Evidence for the Existence of Gravitational Waves in the Early Universe
The first evidence of gravitational waves since 1975Princeton University The pulsar binary - PSR1913+16 (also known as PSRB1913+16) discovered by Russell Hess and Joseph Taylor.This system is composed of two neutron stars spinning in a close eccentric orbit. It is the first pulsar binary discovered and has been observed for more than 30 years.PulsarIt is a stable clock, which enables people to use non relativistic data analysis methods to calculate the basic parameters of the system orbit from the arrival time of the pulse signal (such asellipsetrackSemimajor axisOfProjection、EccentricityEtc.).The parameters related to the relativistic effect can be deduced from the arrival time change caused by general relativity (such asPeristellar pointOfPrecessionAngular rateGravitational redshiftEtc.).From these parameters, we can further calculate the inclination, mass, etc. of the binary star system (the mass of both stars is 1.4 timesSolar massLeft and right).The kinetic energy loss of the system caused by gravitational radiation is shown as the attenuation of the binary orbit, which is further shown as the gradual reduction of the orbital movement period. The period change in the second hour calculated theoretically is -2.40242 ± 0.00002 × 10-12Seconds.The theoretical prediction is in good agreement with the experimental observation results, but the experimental observation error is less than 1%.So far, human beings haveEinstein equationIn the verification of correctness, this experiment is the most accurate.But it only indirectly confirmed the existence of gravitational waves.
The telescope of "BICEP2" in the South Pole has detected gravitational waves
March 18, 2014[2]American physicists announced the first observation of evidence for the existence of primordial gravitational waves in the universe.If this discovery is confirmed, it will be a milestone achievement in the field of physics.
The Harvard Smithsonian Center for Astrophysics and other institutional physicists use the BICEP2 telescope set up in the South Pole to observe the "embers" of the Big Bang - microwave background radiation.The microwave background radiation is formed by the microwave background photons diffused in the cosmic space. The calculation shows that a special polarization mode called mode B will be generated when the original gravitational wave acts on the microwave background photons, and other forms of disturbance can not produce this mode B polarization, so the mode B polarization becomes the "unique mark" of the original gravitational wave.The observed B mode polarization means the existence of gravitational waves.
The South Pole is one of the best places on Earth to observe the microwave background radiation.The researchers found a polarization signal of mode B that was "much stronger than expected" here. After more than three years of analysis, they excluded other possible sources and confirmed that it was caused by the original gravitational wave.Co authors of the studyUniversity of Minnesota"It's like looking for a needle in the haystack, and we found an iron crowbar."
On January 30, 2015, it was still the American team of scientists and the scientists of the European Space Agency (ESA) Planck satellite who officially confirmed that the discovery on March 18, 2014 was a mistake.[3]
LIGO announced on February 11, 2016 that "the existence of gravitational waves has been detected".
The discovery of gravitational waves is of great significance. From a scientific perspective, gravitational waves can be directly connected with the Big Bang.The gravitational waves predicted in general relativity can also be generated in the Big Bang, which means that the gravitational waves at the beginning of the Big Bang can still be detected 13.7 billion years later.Once the gravitational waves during the Big Bang are discovered, we can uncover various mysteries of the universe, and even understand the beginning and operation mechanism of the universe.[4]
On December 19, 2021, the new theory of "baby universe" helps capture the original gravityWave.[6]
significance
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The discovery of the original gravitational wave is proofBig Bang TheoryThe first conclusive evidence of its establishment.
Gravitational waves have laid a solid foundation for a key theory of standard cosmology, namely the inflation theory.The theory points out that the universe experienced a short period of rapid expansion at the beginning of its birth.
In the inflationary stage, the temperature of the universe, that is, the energy contained in particles, exceeds the maximum value of any laboratory in the world by trillions of times, and even exceeds that of largeParticle accelerator, such as those located near Geneva, SwitzerlandLarge Hadron Collider(LHC).
Since inflation is a quantum phenomenon, and gravitational waves are part of classical physics, this phenomenon has built a bridge between the two fields, and will become the first evidence to prove that gravity also has the quantum essence like other natural forces.
Meaning for relativity
Einstein“General relativity”The last missing piece of the puzzle verified by the experiment was found.
So far, 99 years after the general theory of relativity was proposed, Einstein's last prediction based on the general theory of relativity was also confirmed by experiments.
Significance to astronomy
Gravitational waves may reveal the mystery of black hole formation
The gravitational waves predicted by general relativity come from the universe with strong gravitational fieldastronomyorcosmologyWave source, astrophysical research for nearly half a century has shown that gravitational radiation occurs in a wide range of celestial systems.These wave sources that can be expected includeGalaxyInternalbinary star （White dwarf、neutron starorblack holeDouble stars composed of equidense stars),Extragalactic galaxyInternalSupermassive black holeConsolidation of,PulsarRotation of,SupernovaGravitational collapse,big bangThe background radiation left behind.The observational significance of gravitational waves lies not only in the direct verification of general relativity, but also in its ability to provide a new way to observe the universe, just like observational astronomy fromVisible astronomyExpanding to full band astronomy greatly expands the human vision.The traditional observational astronomy completely relies on the detection of electromagnetic radiation, while the emergence of gravitational wave astronomy marks that the observation means have begun to go beyond the scope of electromagnetic interaction. Gravitational wave observation will reveal more previously unknown information about stars, galaxies and the universe.
And based onelectromagnetic waveThe traditional observational astronomy of observation is different. Gravitational wave astronomy has the following characteristics:
(1) Gravitational waves are directly related to the macro motion of the whole wave source, rather than coming from a single wave like electromagnetic wavesatomorElectronicsTherefore, the information revealed by gravitational radiation is completely different from that observed by electromagnetic radiation.For example, the gravitational wave observed by a binary systempolarizationThe inclination of its binary orbit is revealed, and such macroscopic information about the motion of the wave source cannot be obtained from electromagnetic radiation observation.
(2) If comparingwavelengthIn relation to the size of the wave source, gravitational waves in the universe are not much smaller in wavelength than the size of the wave source, as electromagnetic waves do. This makes gravitational wave astronomy generally unable to photograph and phase the wave source as electromagnetic wave astronomy does, but directly analyze the nature of the wave source from the waveform.
(3) Most gravitational wave sources are difficult or impossible to observe directly through electromagnetic radiation (such as black holes), which is also true in reverse;Considering that it is generally believed that the universe does not emit any electromagnetic wavesdark substanceThe proportion is far greater than that of known substances that emit electromagnetic waves. The interaction between dark matter and the outside world isGravitational interactionGravitational wave astronomy is of great significance for the observation of these dark matter.
(4) The interaction between gravitational waves and matter is very weak, and it will not occur as easily as electromagnetic waves in the propagation pathattenuationorscattering, which means that they can reveal some information deep in the corner of the universe. For example, the gravitational radiation formed at the birth of the universe is still spreading in the universe almost without attenuation, which provides the only possibility for direct observation of the Big Bang.Therefore, the detection and research of gravitational waves is crucial to our understanding of the universe.Moreover, the study of gravitational waves is likely to bring us into the era of "gravitational wave astronomy".
Xinhua News
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American physicists announced on the 17th (Washington time) that the first evidence of the existence of primordial gravitational waves in the universe was observed[5]。
If this discovery is confirmed, it will be a milestone achievement in the field of physics.
First, this discovery fills the last missing piece of the puzzle of experimental verification of general relativity.
Einstein's general theory of relativity, published in 1916, predicted the existence of a kind of space-time wave - the original gravitational wave - generated at the beginning of the universe.In the past hundred years, other predictions of general relativity, such as the bending of light, the perihelion precession of Mercury, and the gravitational redshift effect, have been confirmed. The only reason why the original gravitational wave is always out of the sight of astronomers is that the signal is extremely weak and it is difficult to measure technically.Ma Yinzhe, Ph.D. of Cambridge University and "CITA National Researcher" of the University of British Columbia, Canada, believes that the discovery of the original gravitational wave is another strong evidence supporting general relativity. All experimental phenomena predicted by relativity have been verified, and the experiment and theory are in good agreement.
Secondly, this discovery opened a new window to observe the universe.
In the development of astronomy for hundreds of years, the main means for people to observe the universe is to observe light, which means that almost all astronomical experiments are collecting photons.According to the standard Big Bang theory, about 400000 years after the Big Bang, photons, electrons and other particles were mixed together, and the universe was in a dark fog, and light could not penetrate.Gravitational waves are different. They were born at the beginning of the Big Bang and spread at the speed of light.Engaged in gravitational wave research for many yearsArizona State University Theoretical physicistLawrence Klaus believes that gravitational waves are measured, which means that people can trace back to the very early period of only 10 minus 35 seconds after the Big Bang through gravitational waves. At the same time, gravitational waves can also be used as another means of observing the universe.The door of gravitational wave astronomy, a new discipline, was opened.
Third, this discovery is helpful to truly understand the physical process of the original moment of the Big Bang.
According to the inflation theory gradually developed in the 1980s, 14 billion years ago, less than 10 negative 35 seconds after the Big Bang, the universe expanded rapidly at an exponential speed, which is the so-called "inflation process".The original gravitational wave faithfully recorded the physical process during the boom period.Ma Yinzhe told reporters that there were hundreds of theoretical models about the original moment of the Big Bang, but "it was not clear until today which one was right or wrong. But if the results (of American scientists) were true, many theoretical models would be excluded".
Fourth, this discovery means that the measurement of cosmic microwave background radiation will enter the next important milestone.
The cosmic microwave background radiation is the "ember" of the Big Bang, which is a weak electromagnetic wave signal diffused throughout the universe.In the past decades, people have measured the microwave background radiation, in fact, they mainly measured the information of the temperature field, but they have not measured the unique imprint of gravitational waves - B mode polarization.Many teams around the worldDetecting gravitational wavesThe new findings will undoubtedly greatly boost their morale and encourage relevant countries to further increase the investment in scientific research funds and human resources.
Ma Yinzhe said, "If this work is confirmed, it deserves to beNobel PrizeLevel of work.And after that, the Nobel Prize for gravitational waves may reappear.The proposer of the theory of "inflation" of the universe may also win the prize. "Klaus also told Xinhua News Agency that the new research still needs further verification, but if confirmed, it "can be among the most important cosmological discoveries in the past 25 years" and may win the Nobel Prize.
