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This article comes from the official account of Wechat: SF Chinese (ID:kexuejiaodian), author: SF

New research shows that the universe experienced a second big bang within a month after the Big Bang, which may have brought invisible dark matter to the universe.

The Big Bang theory is the most successful theory about the origin and evolution of the universe. This theory holds that our universe originated from a big bang. About 13.8 billion years ago, the universe expanded from a state of extremely high temperature and density, and dispersed the first batch of matter throughout space. As time went on, the universe became colder and thinner, and matter and energy began to gather to form celestial bodies such as stars and galaxies.

The Big Bang theory can explain how ordinary matter is produced, but it does not explain how dark matter is produced. Dark matter is a kind of matter that does not emit light or interact with electromagnetic forces. it accounts for most of the matter in the universe, but we know very little about its nature and origin.

In order to solve these problems, some scientists have recently proposed a new theory, called the "Dark Big Bang Theory", which believes that there was a "Dark Big Bang" after the initial Big Bang, which produced dark matter.

Dark matter was born in the Dark Big Bang. The Dark Big Bang theory was proposed by University of Texas physicist Katherine Fritz (Katherine Freese) and Swedish Royal Institute of Technology physicist Martin Winkler (Martin Winkler). They believe that there was a dark region of dark vacuum energy in the early universe. At some point after the initial Big Bang, the dark region experienced a phase transition (the physical process of changing from one state to another, such as ice turning into water). This phase transition causes dark vacuum energy to be converted into dark matter particles, which is called the Dark Big Bang.

Fritz and Winkler calculated that the Dark Big Bang could produce at least three types of dark matter particles. If the phase transition process is gradual, then the Dark Big Bang will produce light-mass dark matter particles. One of them is similar to the WIMP (massive weakly interacting particles) found in traditional dark matter experiments, but they interact with "dark forces" in nature, such as "dark photons" when interacting with "dark electromagnetic forces".

If these "dark forces" do not exist, then dark matter particles can no longer balance their energy by absorbing and emitting "dark photons". Instead, you get a particle that can devour each other. In other words, when dark matter particles collide with each other, they annihilate and generate energy.

If the phase transition process is abrupt, the Dark Big Bang will produce many expanding bubbles, bringing the system from one state to another. It's like thousands of small bubbles when water starts to boil. These bubbles have so much energy that when they collide, they release so much energy that it is possible to produce dark matter particles 10 trillion times the mass of protons. To reflect the astonishing mass of these dark matter particles, Fritz and Winkler call them "darkzillas," named after Godzilla, a famous monster in film history.

They also found that if the Big Bang happened too early, there would be too much dark matter in the universe, and if it happened too late, there would be too little dark matter in the universe. But if the Dark Big Bang occurred when the universe was less than a month old, it would be consistent with all known observations.

Using gravitational waves to verify the dark big bang theory is a breakthrough theory, which provides a reasonable explanation for the origin of dark matter. So, how to test this theory?

Fritz and Winkler believe that the Dark Big Bang could produce gravitational waves that can pervade the universe, which are still propagating in the universe. Fritz even thinks that we may have detected this gravitational wave.

In September 2015, the Laser interference gravitational Wave Observatory (LIGO) detected gravitational waves for the first time. Since then, gravitational wave detectors on Earth have captured nearly 100 confirmed gravitational wave events. These events are caused by the merging of neutron stars or small and medium-sized black holes.

However, there is another kind of gravitational wave in the universe, which is a gravitational wave background signal throughout the universe, which is thought to be caused by the merger of supermassive black holes or the phase transition process in the early universe. However, the frequency of such gravitational waves is so low that gravitational wave detectors on Earth cannot detect them directly. However, astronomers believe that gravitational wave background signals can interfere with pulses emitted by pulsars. If you use a radio telescope to observe a group of pulsars for a long time and find that the signals generated by multiple pulsars change regularly at some time, it means that gravitational waves have been detected.

In June 2023, astronomers successfully detected gravitational wave background signals using pulsars. However, the signal is too weak to determine its source. Although the most likely source is the merger of supermassive black holes, Fritz believes that the Dark Big Bang is also a credible alternative explanation.

At present, multinational research teams around the world are working together to use pulsars to obtain more accurate gravitational wave data. The European Space Agency's Laser interference Space Antenna (LISA) will also be launched in 2037. The LISA is made up of three spacecraft that form equilateral triangles in space and accurately measure gravitational waves by sending laser beams between them. Fritz believes that in the near future, we will be able to determine whether the gravitational wave background signal is generated by the Dark Big Bang, or even whether the dark matter particle is a "dark zilla" or a lighter particle.

References:

Https://arxiv.org/abs/2302.11579

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