What Is The Use Of Cosmic Background Monitoring? How Does The CMB Reveal The Secrets Of The Birth Of The Universe?

The scientific field of cosmic background monitoring is to carry out long-term and systematic observations of the cosmic microwave background radiation (CMB) and its anisotropy. It is not a one-time detection mission, but continuously tracks the subtle changes in the CMB. The purpose is to reveal the very early history of the universe, verify cosmological models, and detect new physical signals such as possible primitive gravitational waves. This work is like listening to the remaining sounds of "baby crying" 380,000 years after the birth of the universe, and trying to distinguish the fainter "heartbeat" rhythm from them.

What is cosmic background radiation and how is it produced?

The cosmic microwave background radiation is the kind of electromagnetic radiation that fills the entire universe. Its temperature is about 2.725 Kelvin and corresponds to the microwave band. It was produced about 380,000 years after the Big Bang. Before that, the universe was high-temperature and dense. In the ion soup state, photons and charged particles collide frequently, so they cannot propagate freely. As the universe continues to expand and cool down, protons and electrons combine to form neutral hydrogen atoms, and the photons are decoupled, and then begin to travel freely through space.

Those decoupled photons went through a long journey of more than 13.7 billion years, and also encountered the red shift effect caused by the expansion of the universe. Finally, their energy was reduced to the microwave band, thus forming the CMB we observe today. Therefore, CMB is the earliest light that appeared in the universe. It provides us with a "snapshot" of the infancy of the universe. It is imprinted with the original imprint of the density fluctuations of the early universe. It is one of the most critical observational evidences for studying the origin of the universe.

Why continuous monitoring of the cosmic background is needed

Carrying out a single precision survey and mapping of the CMB, like the missions performed by WMAP and Planck satellites, has been a great success, but continuous monitoring is also of great significance. Parameters related to cosmology, such as the Hubble constant, may show signs of evolution over time, which requires long-term data to confirm or rule out. In addition, there are extremely weak secondary effects in the CMB, such as gravitational lensing from large-scale structures and imprints of reionization scattering.

The signal strength of these secondary effects is extremely low, and their characteristics are likely to change slightly depending on time or observation area. Long-term monitoring can accumulate a huge amount of data, and through more sophisticated statistical analysis and cross-validation, these weak signals can be extracted from the noise. This will help us to build a more accurate model of the universe, and it may be possible to discover physical phenomena that are outside the standard model.

What are the main scientific goals of cosmic background monitoring?

The most important goal is to accurately measure the polarization signal of the CMB, especially the B-mode polarization. The theory predicts that the original gravitational waves generated by the inflation process in the very early universe will leave a unique B-mode polarization imprint in the CMB. Detecting it will directly confirm the inflation theory and open a window to extremely high-energy scale physics (such as quantum gravity). Currently, this is still one of the Holy Grails in the field of cosmology.

It is a precise measurement of the anisotropy of the temperature and polarization power spectrum of the CMB. The results it brings can help us determine the basic structure of the universe with high precision, that is, understand the ratio between ordinary matter, dark matter and dark energy. At the same time, the data obtained by monitoring can be used to limit the mass of neutrinos and the geometry of the universe (whether it is flat or not), and can also be used to test the correctness of general relativity in the scope of the universe.

What are the major cosmic background monitoring projects currently available?

The main force is the ground observation project. It is located in the Atacama Desert of Chile and is called the Simons Observatory, which is a next-generation flagship project. This project is about to deploy tens of thousands of highly sensitive detectors for measuring the temperature and polarization of the CMB. The precision and resolution relied on for mapping have never been achieved before. The core search The target is originally the original gravitational wave signal. Another project that occupies an important position is the BICEP/Keck series of telescope arrays located in the Antarctic. It relies on the extremely good atmospheric conditions of the Antarctic and monitors a small area of ​​the sky at specific microwave frequencies for many years. The purpose is to eliminate interference caused by foreground radiation from the Milky Way.

In addition to these telescopes dedicated to the CMB, there are some large-scale comprehensive sky survey projects, such as the deep optical observation data of the LSST (Large Synoptic Survey Telescope), which can be cross-correlated with the gravitational lensing measurements of the CMB, so that the properties of dark matter and dark energy can be studied more deeply.

What are the technical challenges faced by cosmic background monitoring?

The biggest challenge comes from removing foreground noise, which is not easy. The sky signals we see are a mixture of the CMB and many different radiations in the Milky Way, such as synchrotron radiation, dust radiation, etc. Foreground radiation is particularly strong in the microwave band, and its spectral characteristics are not entirely clear. It requires simultaneous observations at multiple different frequencies and complex foreground separation algorithms to "strip" the pure CMB signal.

Another core challenge lies in the sensitivity of the detectors, as well as the number of detectors. If you want to detect tiny temperature fluctuations on the nanokelvin scale, as well as weaker polarization signals, you need to develop superconducting detectors with extremely low noise, such as microwave dynamic inductance detectors MKIDs. At the same time, in order to cover a large area of ​​the sky within a reasonable time frame and achieve sufficient angular resolution, such detector arrays must be deployed on a large scale, which in turn brings great engineering challenges and cooling challenges.

In what direction will future cosmic background monitoring develop?

Future monitoring will develop towards collaborative observations of multiple bands and multi-messengers. The next-generation CMB experiment will work together with optical, near-infrared, radio and future gravitational wave observatories. For example, the gravitational lensing map of the CMB will be combined with the weak gravitational lensing map of the optical galaxy survey, which can more rigorously impose constraints on the growth history of the universe's structure and the equation of state of dark energy.

In the long term, specialized types of space monitoring missions may be proposed. Even though the Planck satellite mission has ended, given that the space environment can completely avoid atmospheric interference, more advanced space CMB polarization monitors are still on the agenda in the future. At the same time, the so-called "field of view" of monitoring is also likely to be expanded. It is not limited to the microwave band. For example, it will try to detect the cosmic neutrino background or the primordial gravitational wave background that may exist at the theoretical level. This will take us towards an earlier moment in the universe.

In your opinion, when exploring the ultimate origin of the universe, continuous monitoring of cosmic background radiation is a way of listening to ancient echoes; compared with other new windows such as direct detection of primordial gravitational waves in the future, which one has the greater possibility of bringing about disruptive breakthroughs first? You are welcome to share your personal opinions in the comment area. If you think this article can help you gain something, please feel free to like it and forward it.