Shulou(Shulou.com)12/24 Report--

A recent paper puts forward a new hypothesis that the origin of the universe may be not just one big bang, but two big bangs, one is the familiar big bang, the other is a dark big bang.

Background We know that matter and radiation in the universe are mainly produced by the Big Bang, and this process can be verified by the evidence of prokaryotic synthesis. Prokaryotic synthesis refers to the process in which photons and baryons form a hot plasma when the temperature is higher than one trillion electron volts in the early universe, and then when the temperature decreases, the baryons combine to form light elements such as hydrogen, helium and lithium. This process can well explain the abundance of light elements in the universe we have observed.

However, there is another kind of matter in the universe that we don't know much about, that is, dark matter. Dark matter refers to matter that does not interact with electromagnetic radiation, or does not emit light, which can only be detected by gravitational effects. Dark matter accounts for about 85% of the total matter in the universe, and it has an important influence on the formation and evolution of the structure of the universe. But what dark matter is, how it is produced, and whether it has the possibility of interacting with matter we are familiar with are all unsolved mysteries in physics at present.

A paper on the Dark Big Bang proposes a possible answer: dark matter (and possibly dark radiation) is produced in a dark big bang caused by a phase transition in the dark matter part. Phase transition refers to the process in which matter changes from one state to another, such as water from liquid to solid or gaseous. In this process, the energy and entropy of matter will change, sometimes accompanied by some violent phenomena, such as foam formation and collision.

In this paper, the author assumes that the part of dark matter is in a high-energy pseudo-vacuum state in the early universe, and then a first-order phase transition occurs at some point, that is, from a pseudo-vacuum state to a vacuum state. In this process, the vacuum energy in the dark matter part is released to form a hot dark plasma, which is the starting point of the Dark Big Bang.

The time of the dark big bang can be before, during, or after pronuclear synthesis, as long as it does not affect the history and observations of the universe as we know it. The authors calculated the temperature range of the dark big bang and found that it must occur when the temperature in the visible part of matter is higher than a few thousand electron volts, which is equivalent to when the universe is less than a month old. At this temperature, the visible matter sector has formed neutral atoms rather than plasma, which means that its interaction with the dark matter part will be very weak, thus ensuring the concealment of the dark big bang.

The new model predicts that the Big Bang produced dark matter, but what is the abundance of dark matter? This paper presents several possible mechanisms to explain the abundance and properties of dark matter. One is the autophagy of dark matter, that is, there is a strong interaction between dark matter, so that they can be converted into other dark particles, thus reducing the numerical density of dark matter. The other is the cancellation of dark matter, that is, dark matter and anti-dark matter can annihilate each other into dark radiation, thus reducing the energy density of dark matter.

Both of these mechanisms can lead to the freezing of dark matter, that is, at some point, the interaction rate of dark matter is lower than the expansion rate of the universe, so that the abundance of dark matter is fixed. Another possibility is that dark matter is overweight, that is, its mass is much larger than that of visible matter, which can be produced directly from foam collisions in the Dark Big Bang without going through any freezing process. The authors call this dark matter "Dark Girl" and its mass can reach 10 ^ 16 kilograms, which is equivalent to the mass of an asteroid.

The Dark Big Bang produced not only dark matter, but also gravitational waves. Gravitational waves are ripples of time and space, which are caused by the accelerated motion of mass or energy. Phase transitions and foam collisions in the Dark Big Bang are strong energy sources. They can excite very strong gravitational waves, which will continue to the present universe and become the background of cosmic gravitational waves. The author calculates the frequency and intensity of these gravitational waves and finds that they can be detected in pulsar timing array experiments. Pulsar timing array is a method to detect gravitational waves by using the stable pulse signals of pulsars. It can detect gravitational waves with frequencies between 10 ^-9 Hz and 10 ^-6 Hz.

The gravitational waves produced by the dark big bang are within this range, and have a very special feature, that is, their frequencies change with time, because the temperature of the dark matter part is different from that of the visible matter sector. their expansion rates are also different, which leads to the frequency shift of gravitational waves. If we can observe such gravitational waves in pulsar timing array experiments, we can confirm the existence of the Dark Big Bang and measure the temperature and energy density of the dark matter sector.

The Big Bang can also explain some other dark matter properties, such as the self-interaction of dark matter and the temperature of dark matter. The self-interaction of dark matter means that there are other forces between dark matter in addition to gravity, which can affect the distribution and motion of dark matter, thus explaining some observed anomalies of small-scale structures.

The temperature of dark matter refers to the average kinetic energy of dark matter, which determines the motion state of dark matter, whether it is cold, warm or hot. The temperature of dark matter is related to the temperature of the dark big bang. If the temperature of the dark big bang is very high, then dark matter will be hot and its speed will be close to the speed of light. Such dark matter will be difficult to form a structure and difficult to detect.

If the temperature of the Dark Big Bang is very low, then dark matter will be cold and its speed will be very slow, so that dark matter can easily form a structure and be easily detected. If the temperature of the dark big bang is between the two, then dark matter will be warm and its velocity will be in the medium range, and such dark matter will have some special properties, such as it can smooth out the density disturbance on a small scale. in order to explain some of the inconsistencies in observations. According to the parameters of the dark big bang, the author gives the temperature range of dark matter and finds that it can cover all the possibilities from cold dark matter to hot dark matter.

This article comes from the official account of Wechat: Vientiane experience (ID:UR4351), author: Eugene Wang

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