A big debate about the origins of the universe broke out2017-05-14 01:22:48 611 ℃
The above diagram briefly describes the evolutionary history of the entire universe. The galaxies and complex structures we see today begin with a smaller, hotter, denser, and more homogeneous state. Just after the birth of the universe, itImmediately went through a sceneExponential expansion. The theory describing this process is calledinflationary theory(Inflation theory) has been the mainstream scientific theory for explaining the origin of the universe for decades.
However, in February this year, an article published in the Scientific American magazine broke the harmony situation, . From the Princeton University physicist Anna Ijjas and Paul Steinhardt (one of the proponents of the theory of inflation), and the Harvard University astronomy department director Abraham Loeb said,According to the results of the current astronomical observations show that the theory of inflation is not satisfactory. On the contrary, they think that the universe is from a collapse of the universe and the rebound, the so-called"Big bounce" theory. Of course, putting forward the big bounce theory is not the main cause of the debate, which was introduced by Steinhardt in 2001.
(Note: readers interested in the big bounce theory can read Paul, J., Steinhardt, and Neil Turok's endless universe: the big bang, or the same online course 
However, they wrote in the article:"...... According to our current understanding of inflationary cosmology cannot use the scientific method to evaluate."In other words, they think that the theory of inflation is not a scientific theory, because it did not make any predictions, so cannot be tested.
It is because of this sentence, sparked outrage.
In this article the challenge of cosmic inflation theory proposed by Alan David Kaiser, Andrei Guth combined with Linde and Yasunori Nomura wrote a rebuttal article  immediately. They pointed out a number of problems with the original article, butIn particular, can not accept the articles about the theory of inflation is not the result of inspection of the argument.They wrote: "some of the major international work have clearly indicated that inflation is not only can be verified, and so far has passed all important test."
At the same time, the four authors have assembled the most theoretical physicists and cosmologists of the day to sign and publish their views. These people include Stephen, Hawking, Steven, Weinberg, Frank, Wilczek, Edward, Witten, Martin, Rees, Leonard, Susskind, Lisa, Randall and a total of 33 top global physicists. If these people together, like a modern version of the fifth Solvay conference.
The face of such a strong return did not make three physicists flinch. It was a further analysis, a rebuttal of the rebuttal. They argue that the rebuttal article ignores their main points:Once thought that the difference between the theory of inflation and feasible understanding today's Theory.
A few days ago, a major article,  and physicist Sean Carroll (one of the joint critics), , expressed his views on the debate. Carroll representation,The theory of inflation is likely just a part of the final answer, but there is no doubt that it is science.
The debate will not end soon, and the challenge will make the theory even better. This happens frequently in the history of science. But we want to explore is the core view led to this debate: "what is the theory of inflation is not a scientific theory?"
In February this year, an article entitled "POP goes the universe" article sparked a debate about the origin of the universe. (photo: Scientific, American)
In twentieth Century, one of the greatest scientific achievements is found in the expansion of the universe is: with the passage of time, distant galaxies will gradually away from us, this is because the galaxy and our space is constantly expanding. By the middle of twentieth Century, scientists had come up with a bold idea that if the universe were getting bigger and colder today, its past would be smaller, hotter and denser:big bang. The big bang model made many predictions consistent with the observations at that time, especially when scientists detected the microwave background radiation, defeating the theory of competition in one fell swoop.
In 1990s, the microwave background radiation by the COBE satellite precise measurement of first downs. In 2000s, WMAP carried out more precise measurements. The above figure is measured by the latest Planck satellite. The graph contains a great deal of information about the early universe. (photo: ESA)
But in the framework of the big bang, there are some phenomena that are completely unexplained:
Why is the temperature all over the universe the same? Different areas and different directions in space should not have enough time to exchange information so that the temperature is the same everywhere. But the truth is, the universe has the same background temperature - 2.73 Kelvin.
Why is the universe so flat? Why are the two completely independent quantities equal to the expansion rate and the material / energy density?
If the universe had reached such high energies in the early days, why hadn't we seen some high-energy remnants predicted by theory? Today's universe, for example, should be full of magnetic monopoles, but scientists do not find any magnetic monopoles.
In the left, the blue line represents the composite period of the universe, which is time decoupling of the microwave background radiation. You can see any at the time of the two distant point in the past light cone at t = 0 (Big Bang) do not overlap, which means that there is no causal contact between them, the distance light can arrive in their small circle (the little orange circle). If the two orange circle never overlap, then why are they the temperature would be so consistent? This is known as the horizon problem. (photo: D.Baumann)
The first question above is calledhorizon problem. If the expansion of the universe obeys Einstein's general theory of relativity rule, if two in regional space distance can exceed the speed of light transmission, without each other "contact", which is how to reach thermal balance? The big bang theory can not answer the question at all. We can't say, "it was born this way."". From a scientific point of view, it is totally unsatisfactory.
But there's another possibility. Perhaps, immediately after the birth of the universe, it experienced an exponential expansion. In 1979/1980, Alan Guth proposed a revolutionary theory:Cosmic Inflation. It has changed our understanding of the origin of the universe.
In 1979, Alan Guth notes. (photo: SLAClab)
The expansion of other periods with inflation in the history of the universe is not the same. When the universe is full of matter and radiation, the energy density decreases as it expands. As the volume expands, the density decreases, so the expansion rate decreases. But during inflation, the universe is full of energy, the energy is from the inherent space. Thus, as the universe expands, it creates more space and keeps the density constant, thereby preventing the expansion rate from falling. The idea also solved the three puzzles just mentioned:
Today's temperature is consistent because the universe is everywhere in the distant past, between different regions are mutually contact, until inflation will separate them.
The universe is flat because inflation will make it quickly opened, and the flat can not distinguish. It's like the earth we see at the window is flat, but actually it's curved. We cannot observe enough to determine the true curvature of the universe.
Cosmic inflation will make the magnetic monopole away from each other, and when inflation at the end of the universe, it again becomes hot, but the temperature will reach the temperature for making them.
Think the inflation theory, in a very short period of time the universe has experienced an exponential expansion, so that the past two points without any communication actually causal contact. (photo: D.Baumann)
After the inflation model has been further developed by others, including Andrei Linde, Paul Steinhardt and so on. Scientists have also proposed some cosmic models that can reconstruct isotropic (all directions alike) and homogeneous (all the same), consistent with all observations. (it is worth noting that inflation is not a single theory, it is a series of model based on similarity principle. Of course, no one believes that all models are correct
But we also need to remember that a theory to be a successful scientific theory must be able to make new predictions that can be validated. Yes, inflation models have made six important predictions of :
The universe should have perfect flatness.Inflation models predicted the cosmic 100% is "anything material added, while the curvature is 0%. This predicted by a CDM model that we now know that the universe is 5%, 27% is the dark matter, and dark energy is 68%; but the curvature is still 0%.
The energy spectrum of the universe's fluctuation is almost constant.If quantum physics is on, then the universe will experience quantum fluctuations even in the inflationary period. These fluctuations are also exponentially stretched. When inflation is over, it will be converted into these fluctuations of matter and radiation, lead to high density and low density areas, in order to make the stars and galaxies grow. According to the inflation in the last stage is how the different model predicted the ups and downs in either smaller scale or larger scales are a little bit bigger. For a perfect scale invariance, a parameter called n_s is equal to 1, while the observed value is n_s = 0.96.
Since the explosion, there have been fluctuations in the larger scale than the distance light travels.This is a result of inflation. These fluctuations have seen large scale structure in the microwave background radiation and scientists in the universe, further validation of the inflation model.
These quantum fluctuations should be adiabatic.There are several different types of undulations: adiabatic, ISO curvature, or two mixed. Inflation predicted these ups and downs are 100% adiabatic, this will leave a unique signal in the microwave background radiation and large scale structures.
In the distant past, the maximum temperature of the universe was not arbitrarily high, but there was an upper limit.From the microwave background radiation, we can deduce the highest temperature ever reached by the universe. If there is no inflation in the early universe, so when the temperature will reach arbitrarily high. But if inflation occurs, it must occur below the Planck scale in the energy scale (~10^19GeV). From the observations we find that the universe in any one of the highest ever reach the temperature is not higher than ~10^16GeV 0.1%, further validation of the model of inflation.
Finally, we should see the primary gravitational waves with special spectra.The primordial gravitational wave is a space-time fluctuation produced by the birth of the universe. It leaves unique signals in the microwave background radiation, the only one of the six predictions that has not been verified. Different models predict gravitational waves of different sizes, and if the primary gravitational waves can be measured, the argument ends.
Finally, a prediction of cosmic inflation is a primordial gravitational waves (Gravitation wave) the existence of a prophecy is the only non observed verification. (photo: NASA/JPL)
Therefore, the theory of inflation actually achieved great success in the past. Since the end of 1980s, theorists burst out many ideas, the development of a variety of different inflation models. Some of the models are very strange and have unusual behavior. But the most simple inflation models is actually based on apotential energy(as shown in the following curve).
A slow roll inflation model. Phi is a scalar field, namely inflation. In inflation, inflation will be in the field with potential V (PHI) at the bottom of the oscillation. In order to avoid inflation after the end of the universe, empty and inflation must follow the standard model of coupled field. Therefore, the inflaton field energy to convert into ordinary particles. (photo: D.Baumann)
The inflaton field from a point away from the bottom of the start, slowly rolled to the bottom, causing inflation to occur until it stays at the lowest point. Quantum effects play an important role in this field. When inflation eventually stops, inflation will be converted to the energy of matter and radiation, resulting in the beginning of the big bang.
But you can also put forward a multi field model, get away from the flat model or design model. In other words, you can make the model more complex, more complicated, or even go against the six previous predictions. In some models, the multiverse is also predicted.
That's where the debate is now! One party thinks, because we can design with arbitrary behavior model, so the theory of inflation cannot become the standard of scientific theory. The other is that inflation has made these successful predictions, as long as the measurement of the parameters more accurately, which can constraint model is feasible, ultimatelyselected which model can reflect the physical reality.
If there is anyone who proposed an alternative model of inflation, then it must first be copied all the successful inflation model. But there is no alternative model to do now. However, this does not mean that the inflation model is correct, no one said it must be correct. In fact, there are many problems need to solve is the theory of inflation. But it is certain that in the past few decades, there is plenty of evidence to support the theory of inflation, the data also has ruled out some possible model. At present, the theory of inflation is still the mainstream scientific theory describes the origin of the universe.
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