Inflation theory was developed byMassachusetts Institute of Technology（MIT）Alan Guth, a scientist of.The theory points out that the space of the early universe expanded exponentially.This rapid expansion process is called "inflation", which means that the universe expands at a very large growth rate for a period of time.According to Goose's theory, the inflation process occurs inBig BangNext 10-36Seconds~10-32Seconds.After the inflation ended, the universe continued to expand, but at a much lower rate.
Chinese name
inflationary theory
Foreign name
Inflation
Properties
The period of rapid expansion of the early universe
The exponential expansion of space means that the distance between observers in two inertial frames increases at a faster and faster speed.In an observer's static reference system, the polar metric formula of the universe region in inflation is as follows[1]：
This steady exponential expansion of space-time is called de Sitter space.To maintain this state, there must be a cosmological constant, that is, there must be
Proportional vacuum energy.In this case, the equation of state is
。Physical conditions are stable from one moment to the next: the rate of expansion (i.e. Hubble parameter) is almost constant, while the proportional coefficient of the universe is equal to eHtIs proportional.Inflation is sometimes referred to as the accelerated expansion phase, because the distance between two inertial observers increases exponentially (that is, they keep away from each other at an accelerating speed), and
It can be approximately unchanged.
Time conditions
An important condition of inflation is that it must last long enough.The entire visible universe today is inflated from a single Hubble volume.Only if this condition is met, the universe will appear flat, homogeneous and isotropic on the largest scale.It is generally believed that the universe should exceed e in the inflation stagesixty（≈10twenty-six）To meet this condition.
Reheating process
Inflation is a stage of supercooled expansion, during which the temperature of the universe decreased by 100000 times (the actual degree of decline is different between different models, and it is generally believed that the temperature in the earliest models changed from 10twenty-sevenK drops to 10twenty-twoK) During inflation, the temperature remained relatively low.When inflation ends, the temperature returns to the level before inflation. This process is called "reheating" or "heating".This is because the huge potential energy of the inflationary place decays into various particles, making the universe full of standard model particles.This includes electromagnetic radiation, thus launching a radiation dominated period.Because scientists have not yet understood the nature of inflation, they know little about this process, but it is generally believed that it is carried out through the parametric oscillation mechanism.
Elimination effect
An important role of cosmic inflation is to eliminate the non-uniformity, anisotropy and the curvature of smooth space.This makes the universe tend to a very simple state: it is completely dominated by the inflation field, and the inflation field is smallquantum fluctuation Is the only important inhomogeneity.Physicists use inflation theory to calculatequantum fluctuation The subtle temperature difference caused by various regions of the universe has been verified by observation.Inflation can also reduce the number density of large mass exotic particles, such as the existence ofMagnetic monopole。If the universe only had enough temperature to form these particles before the inflation period, the inflation would reduce their density to a very low level, so that they do not exist in today's visible universe.Taken together, these effects can be called the "hairless theorem" of inflationHairless theorembe similar.
The hairless theorem is applicable here because there is no difference between the cosmic horizon and the black hole horizon, and the only difference is the philosophical problem of "what exists at the other end of the horizon".The hairless theorem means that the universe (including visible and invisible universes) expands in an extremely large proportion during the inflation stage.When the universe expands, the energy density will decrease as the volume increases.The density of ordinary "cold" matter is inversely proportional to its volume, the energy density is inversely proportional to the cubic of its volume, and the radiant energy is inversely proportional to the cubic of its volume.In the process of inflation, the energy density of the inflation field is roughly unchanged;However, the number density of inhomogeneity, anisotropy, space curvature and various strange particles will be reduced, as well as the number density of photons, and will be reduced to a negligible level after sufficient inflation.This leads to a nearly vacuum, flat and symmetrical universe after inflation and before reheating[2]。
motivation
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horizon problem
The problem of horizon, that is, according toCosmological principleHow to explain the statistical homogeneity and isotropy of the universe[3]。Taking a box full of gas as an example, these gas particles need to interact for enough time to gradually remove the non-uniformity and asymmetry, so as to reach the thermal equilibrium state with uniformity and isotropy.However, in a universe lacking inflation process, it is impossible for two regions far apart to reach equilibrium, because their expansion speed is far faster than the speed of light, and there is no opportunity to "contact" or contact with each other, so it is difficult to explain that they have the same temperature (that is, they are in thermal equilibrium).This kind of contact requires the transmission of information, and the transmission speed cannot exceed the speed of light, so this contradiction has become a major problem of the so-called Standard Big Bang model.Charles Misner once proposed Mixmaster cosmology in history[4]To explain the statistical uniformity and isotropic inflation theory of the universe, but it was finally proved to be wrong due to too much confusion.
flatness problem
Another problem is flatness.This problem is sometimes called one of the two Dick coincidences, and the other Dick coincidences isCosmological constantproblem[5]。The macroscopic geometry of a universe can be hyperbolic geometry (open universe), spherical geometry (closed universe) and Euclidean geometry (flat universe) between them, which is determined by the density of matter in the universe.The density of matter in our universe is very close to the critical density required for a flat universe[6]。
Whatever the shape of the universe, the contribution of space curvature to the expansion of the universe is not much greater than that of matter.But with the expansion of the universe, the redshift of curvature is slower than that of matter and radiation.This calculation in the past will cause a fine tuning problem, because the contribution of curvature to the universe must be minimal (for example, it is 16 orders of magnitude lower than the radiation density at the time of early nuclear synthesis).The observational data obtained from the cosmic microwave background verify that the universe is flat, and the error value is within 10%, which makes the flatness problem more obvious: what makes the universe flat[7]。
Magnetic monopole problem
Magnetic monopoleThe problem involvesBig Bang TheoryAndGrand unified theory, sometimes called "exotic relics problem".The grand unified theory proposes that if the temperature of the early universe exceeds the grand unified temperature (about 10K), the electromagnetic force, strong nuclear force and weak nuclear force will be unified into a "grand unified force".Due to the expansion of the universe, the temperature will continue to decrease. When the temperature is lower than the grand unified temperature, spontaneous symmetry breaking will occur, and the physical properties of the electric weak force and the strong nuclear force begin to become different, so a phase transition occurs.This phenomenon is similar to the phase change between water and ice. When the temperature of water is lower than the freezing point, phase change will occur, and water will become ice;Before the phase transition, water molecules have rotational symmetry. After the phase transition, ice crystals become anisotropic, and the symmetry is spontaneously broken.
The phase transition due to the breaking of symmetry usually causes "topological defects".For the formation of ice crystals, because the ice crystals grown at several nucleation positions have different symmetry axes in different directions, two-dimensional topological defects, calledDomain wall。The grand unified theory predicts that the grand unified phase transition will produce a zero dimensional point like topological defect, whose physical properties are just likeMagnetic monopole。The grand unified theory predicts that the grand unified phase transition will not only produce suchMagnetic monopole, because it is extremely stable, thisMagnetic monopoleIt will still exist today, and may even become the main component of the universe.However, today's universe is not full ofMagnetic monopoleScientists have never even found any magnetic monopoles, which sets a very low upper limit for the density of magnetic monopoles in the universe.
The inflation theory shows that the rapid expansion of the universe willMagnetic monopoleThe density in space drops suddenly to an order of magnitude that cannot be detected by current instruments.Therefore, according to this theoryMagnetic monopoleAfter the formation, if there is a period of inflation, this problem can be solved.However, some cosmologists doubt this. As Martin Rees said, "For those who doubt strange physics, a theoretical reason to explain the nonexistence of hypothetical particles may not be so great. Of course, drugs used to prevent nonexistent diseases are 100% effective!"[8]。
history
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Early inflation model
The earliest inflation theory was put forward by Erast Gliner in 1965, but the theory has not received widespread attention[9]。In 1980, Alan Gus independently proposed the inflation mechanism to explain why there is no inflation in the universeMagnetic monopole。At the same time, Starobinsky believes that the quantum correction of gravity can replace the original singularity of the universe with the Desite stage of exponential expansion.In October 1980, Demosthenes Kazanas proposed that exponential expansion can eliminate the event horizon of particles, and even may solve the event horizon problem;Katsuhiko Sato also proposed that exponential expansion can eliminate domain walls (another strange relic) in string theory.In 1981, Martin Einhorn and Katsuhiko Sato published a model similar to Goose, and demonstrated that this model can solve the problem that the grand unified theory is full ofMagnetic monopoleProblems.Their conclusions are similar to those of Gus: this model not only requires fine-tuning of various cosmological constants, but also is likely to lead to a "granular" universe, that is, the macroscopic density difference caused by foam wall collision.
Gus proposed that when the temperature of the early universe dropped, it was in a false vacuum with high energy density, and the effect of the false vacuum was very similar to the cosmological constant.When the very early universe cooled down, it was in a metastable state (supercooled state).To decay out of this state, we must go through the nucleation process of the cosmic bubble caused by the quantum tunneling effect.The vacuum bubble spontaneously forms in the false vacuum background and rapidly starts to expand at the speed of light.Gus realizes the problem of this model: the reheating process is not correct.When the cosmic bubble nucleates, it does not produce any radiation;Radiation only occurs when bubbles collide.But in order to solve the problem of initial conditions, inflation must last long enough, and then the probability of foam collision has been reduced to a very low level.Such a universe would not be full of radiation.
Slow rolling inflation
Andre Linde(Andrei Linde) and Andreas Albrecht andPaul Steinhart(Paul Steinhardt) independently found the solution to the foam collision problem[10]。This model is called "new inflation" or "slow roll inflation", while Gus's model is called "old inflation".In the new inflation model, the mechanism of decay from the false vacuum state is no longer the quantum tunneling effect, but is similar to a scalar field rolling down from the potential energy peak.If the scalar field rolls down the potential energy peak at a speed much slower than the expansion of the universe, inflation will occur.Once the potential energy peak becomes steeper, inflation will end and the reheating process will begin.
The impact of asymmetry on inflation
Finally, people found that inflation did not produce a completely symmetrical universe, and smallquantum fluctuation 。These fluctuations became the seeds of all future cosmic structures.During the analysis of the Starobinsky model, the Soviet Union's Viacheslav Mukhanov and G. V. Chibisov calculated these fluctuations for the first time.The 1982 Nuffield Workshop on the Very Early Universe held in Cambridge University for three weeks also calculated thisquantum fluctuation 。At the seminar, four groups of scientists calculated separately: Stephen Hawking;Starobinsky;So Young Pi;James M. Bardeen, Paul Steinhardt and Migao Turner.
Today the universe is not expanding in an inflationary manner.In this way, there must be a mechanism that can eliminate this great effectCosmological constantSo that the expansion rate changes from the state of acceleration to the state of being slowed down by gravity as it is today.It can be expected that the symmetry between different forces will eventually be broken when the universe expands, just as supercooled water will eventually solidify.In this way, the extra energy of the unbroken symmetry state will be released and the universe will be reheated to just below the critical temperature for symmetry of different forces.Later, the universe continued to expand and cool in the standard Big Bang mode.But now we have found an explanation why the universe just expanded at a critical rate and has the same temperature in different regions.
critique
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Since Alan Gus put forward the theory of inflation in 1980, it has been widely accepted by the scientific community.However, there are still many physicists, mathematicians and philosophers who have expressed their opposition and believe that the theory of inflation lacks practical tests and empirical support.In 1999, philosophers John Earman and Jess ú s Moster í n published a paper criticizing inflationary cosmology, pointing out that "we think there is no sufficient reason to include any inflationary model in the standard core of cosmology for the time being"[11]。
As Roger Penrose put forward in 1986, inflation requires extremely specific initial conditions, so this theory cannot solve the problem of initial conditions required by itself.In other words, the problem of "fine-tuning" of initial conditions can not be solved, and will even become more serious because of inflation.Another criticism of the inflation theory is that the inflation field required by inflation does not correspond to any known field, and the potential energy curve seems to be consistent with almost any observation data, that is, lack of verifiability.The criticism comes from one of the founders of the inflation theoryPaul Steinhart[12]。
