When the first starlight breaks through the darkness

Just like detectives, astronomers often can’t get the photos they need when they break the mystery. Most of the time, they have to piece together subtle evidence by carefully searching for clues in the sky. Among them, a cosmology case that has plagued astronomers for many years is when the first stars in the universe were born.

Recently, a research team led by Judd Bowman of Arizona State University announced that they had finally detected the case. Using a table-sized radio antenna set in the Australian desert, they discovered the weak “fingerprints” left by the first stars in the universe and calculated the approximate time of the birth of the stars – 180 million years after the big bang .

So how did they solve the case?

Background radiation absorbed by hydrogen
Shortly after the big bang, the universe was a very dark and cold place before the first stars were ignited. There are no galaxies, no supernovae, no quasars. The universe at this time is basically composed of a large amount of neutral hydrogen, which floats in the residual heat left by the big bang – the cosmic microwave background radiation. Over time, gravity slowly converges the densest regions of hydrogen into dense nebulae. Eventually, these dense nebulae collapsed to form the first stars and began to glow. This moment is called the dawn of the universe.

The light from the first stars has now become very dim, and no astronomical telescope built by humans can be observed. However, when they begin to glow in the dark space, the ultraviolet rays that affect the properties of hydrogen in the interstellar space give the hydrogen a new skill: they can absorb frequencies from the cosmic microwave background radiation to about 1420 MHz. Photon. These absorbed photons correspond to a wavelength of approximately 21 cm. As long as the absorption signal left by hydrogen can be found, it can detect that the intensity of the cosmic microwave background radiation is significantly reduced at this frequency, then the “fingerprint” left by the first stars is found.

However, as the universe expands, the wavelength of photons in background radiation will lengthen over time and its frequency will decrease. Similarly, the absorption signal left by hydrogen on the background radiation will also be elongated over time and the frequency will decrease. Astronomers can estimate the time when the signal was born by how much the frequency is reduced, and thus the time when the first stars were born.

To find this signal, Bowman’s team set up a radio antenna in a desert in Australia. Unlike other large astronomical devices, their radio antennas have only one table size. Because our Milky Way and humans produce a lot of radio waves with the same signal frequency, which means they must carefully filter out these powerful interferences. After more than a decade of observation and analysis, the research team announced in February 2018 that they found the absorption signal of hydrogen on the background radiation, which has a frequency of about 78 MHz and a corresponding wavelength of about 385 cm. Estimated the approximate time of the birth of the first stars – 180 million years after the big bang, which is much earlier than the astronomers estimated.

Dark matter is making a ghost?
However, their findings have led to a big problem. The cosmic microwave background radiation they detected was only 0.1% weaker at the corresponding frequency, but it was more than twice as much as expected. To this end, the research team spent two years testing whether the observation was caused by the antenna itself or by external interference. They even built a second antenna and pointed the antenna to different areas of the sky at different times. They excluded all the interference factors and the results remained unchanged. They believe that this observation should be correct.

Why is the decline so large? Israeli dark matter expert Lenan Bakana speculates that this is because the hydrogen around the first stars is cooler than expected, they can absorb more background radiation. Barkana believes that the only thing that makes hydrogen colder is dark matter.

Everything we can see, including stars, trees, buildings and ourselves, is a regular substance. According to astronomers, in the composition of the entire universe, conventional materials accounted for only 4.9%, while dark matter accounted for 26.8%, and the remaining 68.3% were dark energy. Dark matter neither absorbs light nor emits light, and we cannot see them directly. At present, we can only find them indirectly through their gravitational effects on conventional materials. Barkana believes that dark matter absorbs some heat from hydrogen, leaving hydrogen at a lower temperature. If Barkana’s point of view is correct, then this will be the first evidence of the existence of dark matter not found by gravity.

However, if dark matter can really cool down hydrogen, then this will challenge our previous understanding of dark matter. Previously, particle physicists speculated that dark matter might be composed of a mass of particles without any charge, which caused its interaction with conventional materials to be too weak to be directly observed. Among them, the so-called “mass weak interaction particles” has become the most popular candidate, it is an imaginary massive particle, which only works by weak interaction and gravity. In recent years, researchers have been sparing no effort to find such dark matter particles. The researchers conducted a number of experiments under the ice of the Antarctic, deep in abandoned mines, and on the International Space Station, and used particle accelerators to search, but all found nothing.

But according to Bakana’s point of view, dark matter must be able to interact with conventional substances sufficiently to absorb some heat, which means that dark matter should be composed of lighter particles with a small charge. Some astronomers believe that this discovery may bring two Nobel Prizes, one for the discovery of the first stars and one for a new understanding of dark matter.

But it may be too early to completely remove large amounts of dark matter particles from the candidates. Some astronomers believe that if dark matter is entirely composed of light particles, or they carry a charge, then we can observe their interaction with conventional matter long ago, which is in contradiction with our current observations. One way to resolve contradictions is to assume that about 2% to 30% of dark matter is made up of light particles, the remaining dark matter is still made up of massive particles, and that any dark matter light particles should carry no more than One hundred thousandth of an electronic charge.

Maybe it is caused by gravitational theory or black holes?

American astronomer Stacy McGowan believes that excessive absorption of signals has nothing to do with dark matter. McGonagall has been studying the theory of replacing dark matter, and believes that there is no dark matter in the universe, but that our theory of gravity has a problem. McGowan believes that without dark matter, the universe will expand faster, and the space between the first stars and stars will be larger, which means that there will be more hydrogen between the stars, so the background radiation absorbed by hydrogen will be more than expected. want more.

There is another way to explain the strong absorption signal without the need to quote new dark matter particles or modify the gravitational theory. American astronomer Gil Holder believes that when the first stars were born, there was extra radiation in the universe, and hydrogen also absorbed 21 centimeters of photons from the extra radiation, and the current test results did not Excess radiation is excluded, which may result in an increase in the absorption signal. But this point of view has to explain the source of additional radiation. Holder believes that the extra radiation may be generated when the original black hole swallowed the surrounding material. The original black holes were an imaginary type of black hole, which was formed by the collapse of ultra-high-density objects at the moment after the big bang.

In any case, extraordinary claims require strong evidence. After all, in order to detect this absorption signal, the research team needs to carefully remove a lot of strong interference. For example, in the 78 MHz band, the radio waves emitted by our Milky Way are more than 10,000 times stronger than the cosmic microwave background radiation to be detected. Perhaps, errors occurred during the process of eliminating interference, resulting in abnormal results. Therefore, the primary task of astronomers now is to further verify this observation.

Then, while the first starlight breaks through the darkness, can it bring us a new cosmology? Let us wait and see.