Text | Xiao Peng's brilliant notes
Editor|Xiao Peng's brilliant notes
preface
The advent of radio telescopes is a giant leap forward in our exploration of the universe, these extraordinary instruments have unveiled the mysteries of the universe in unprecedented ways, revealing the hidden secrets of celestial bodies and cosmic phenomena, and the core of radio astronomy lies in understanding radio waves, which are electromagnetic radiation with wavelengths longer than visible light, and the electromagnetic spectrum encompasses a variety of wavelengths, from gamma rays and X-rays to ultraviolet, visible and infrared, and radio waves.
Radio waves vary in wavelength from millimeters to meters, making them ideal for studying celestial bodies and phenomena that emit or interact with radio frequency radiation, and radio telescopes are designed to detect and study radio radiation from a variety of celestial sources, including stars, galaxies, supernovae, pulsars, quasars, and cosmic microwave background radiation, the latter being remnants of the Big Bang.
Celestial bodies emit radio waves through a variety of mechanisms, including thermal, synchrotron and molecular line radiation, which provide valuable information about the composition, temperature, density and magnetic fields of celestial bodies, and thermal radiation, which is emitted by celestial bodies with non-zero temperatures, which can be detected by radio telescopes, allowing astronomers to measure the temperature of stars, galaxies, and interstellar gas clouds.
Synchrotron radiation, which emits synchrotron radiation from charged particles swirling in a magnetic field, is ubiquitous in a variety of cosmic phenomena, including active galactic nuclei, pulsars and supernova remnants, molecular line radiation, and radio telescopes can detect specific spectral lines associated with molecular transitions in interstellar gas clouds that reveal the presence of molecules such as hydrogen, carbon monoxide, and water vapor.
The origins of radio astronomy can be traced back to the early 20th century, when scientists such as Karl Jansky and Grote Reber made breakthrough discoveries, and Yansky detected radio waves from the galactic center in 1932, marking the birth of radio astronomy as a discipline.
In 1937, the self-taught radio engineer and amateur astronomer Grote Reiber built the first true radio telescope in his backyard in Wheaton, Illinois, and this 9-meter-diameter astronomical telescope allowed him to make the first radio sky map, revealing the radio radiation of the Milky Way.
The development of radar technology during World War II played a crucial role in the development of radio astronomy, and scientists and engineers repurposed wartime radar equipment to build more advanced radio telescopes.
Built in 2000, the Green Bank Telescope (GBT) is one of the most iconic and powerful radio telescopes in the world, boasting a huge dish antenna with a diameter of 100 meters that operates at a wide range of radio frequencies, enabling astronomers to study a variety of celestial bodies.
Very Long Baseline Interferometry (VLBI) revolutionized radio astronomy by combining data from multiple radio telescopes separated by distant distances, a technique that dramatically improved angular resolution, allowing astronomers to create high-resolution images of distant radio sources.
Radio telescopes have played an important role in mapping the universe at radio frequencies, providing detailed images of galaxies, revealing the structure of galaxies, star-forming regions, and massive black holes at the centers of galaxies, radio astronomy has led to the discovery of pulsars, which are fast-spinning neutron stars that emit radio beams, and these precise cosmic clocks have helped confirm the existence of gravitational waves and provide a key test for general relativity.
Radio telescopes have played a vital role in the discovery of cosmic microwave background radiation – remnants of the Big Bang, and observations of cosmic microwave background radiation provide strong evidence for the Big Bang theory and contribute to our understanding of the early universe.
Radio astronomy expands our knowledge of extragalactic objects, including quasars, which are extremely bright and distant active galactic nuclei, high-energy phenomena that emit strong radio waves and which have been extensively studied by radio telescopes.
Radio telescopes have enabled astronomers to study the complex chemistry of interstellar space, detecting molecular shifts in space, revealing the presence of various molecules, and providing insight into the conditions in interstellar clouds where new stars form.
Radio astronomy has enabled scientists to map the magnetic field of the universe and gain insight into the role of magnetism in the evolution of the universe, observations that have implications for the formation and behavior of galaxies and galaxy clusters, and radio telescopes have also played an important role in detecting the nature of dark matter through observations of gravitational lensing and the study of galaxy clusters, which reveal the distribution of dark matter in the universe.
Modern radio telescopes conduct large-scale astronomical surveys, mapping the radio sky with unprecedented detail, and these observations have uncovered new radio sources, transient phenomena, and rare cosmic events.
The Square Kilometre Array (SKA), a pioneering international project to build the world's most powerful radio telescope, will consist of thousands of antennas spread across continents, providing unprecedented sensitivity and resolution, and SKA will explore a wide range of scientific questions, from the nature of dark energy to the search for extraterrestrial intelligent life.
Continued advances in ultra-long baseline interferometry (VLBI) technology will continue to improve the angular resolution of radio telescopes, enabling more detailed observations of distant objects, fast radio bursts are a mysterious and ephemeral cosmic phenomenon, the study of which will continue to be the focus of radio astronomy, and understanding the origin of fast radio bursts remains an open question.
Radio telescopes will play a key role in studying the high-redshift universe, enabling astronomers to detect the early stages of galaxy formation and shortly after the Big Bang, a fascinating fundamental branch of astronomy that gives us a glimpse into the early days of the universe.
The Doppler effect refers to the phenomenon that the frequency (or wavelength) of the observed wave changes when the wave source moves relative to the observer, and in astronomy, this effect shifts the light from distant objects in space in a direction with a longer wavelength, resulting in a "redshift", when an object is far away from the observer, its light will appear "redshifted" because the wavelength of light is elongated and moves towards the red end of the electromagnetic spectrum.
At the beginning of the 20th century, Edwin-Hubble discovered the redshift of galaxies, which is a key moment in astronomy, Hubble's law describes the relationship between the redshift of galaxies and their regression speed, indicating that galaxies are receding from us, the universe is expanding, Hubble's law is expressed by the formula v = H0d, where v represents the velocity, H0 is the Hubble constant (a measure of the current expansion rate of the universe), and d is the distance from the celestial body.
In the context of cosmological redshift, "high redshift" refers to galaxies, quasars, and other celestial bodies whose light is significantly redshifted due to the expansion of the universe, which are usually far from Earth and therefore observed the same as those of the universe when it was younger.
High redshift observations allow astronomers to go back in time and witness the prototype of the universe, by studying galaxies and quasars under high redshift, scientists can explore the early stages of the evolution of the universe, including the formation of galaxies and the growth of supermassive black holes, high redshift observations provide further evidence for the expansion of the universe, and the relationship between redshift and distance described by Hubble's law reconfirms our understanding of the dynamic nature of the universe.
The high redshift observations are an important test of various cosmological models and theories including the Big Bang theory, the consistency of the observed redshift and the predicted values supports the framework of cosmic expansion, and the study of high-redshift objects is inseparable from the study of the cosmic microwave background radiation (CMB), the afterglow of the Big Bang and provides a snapshot of the state of the universe when it was only about 380,000 years old.
High redshift observations help constrain the properties of the early universe, such as its temperature and density fluctuations, and provide valuable information for understanding the initial conditions of the evolution of the universe.
One of the main methods for determining celestial redshift is spectral analysis, where astronomers observe the spectra of celestial bodies – how light at different wavelengths is distributed, and the movement of spectral lines (such as absorption or emission lines of elements or molecules) indicates the redshift of the object.
For objects that may not have clear spectral signatures, or when spectral analysis is challenging, photometric redshift techniques can be employed, which involves measuring the brightness of an object at multiple filters or wavelengths and using these measurements to estimate its redshift.
Gravitational lensing occurs when the gravitational pull of massive objects such as galaxies or clusters of galaxies bends and amplifies light from more distant high-redshift objects, a phenomenon that allows us to observe galaxies that would otherwise be too faint to be observed.
The Lehmann-alpha line, which is associated with the transition of hydrogen atom electrons from the second to the first energy level, is often used to detect high redshift galaxies, and observing the Lehmann-alpha emission can detect the presence of young star-forming galaxies in the early universe.
conclusion
Radio telescopes illuminate the universe, uncover the secrets of the universe, expand our understanding of the universe, from mapping distant galaxies to discovering pulsars, quasars and cosmic microwave background radiation, radio astronomy has enriched our understanding of the universe in many ways, looking ahead, with the development of projects such as SKA and the next generation VLBI, the impact of radio telescopes on science and mankind will continue to expand, and it is expected to further reveal the mysteries of the universe and our place in it
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