Language
Country/Area
No result found
The Transparent Age of the Universe: What is the Secret of the Recombination Period?
In the observable universe, the cosmic microwave background (CMB) is the ubiquitous microwave radiation that fills the entire observable space. The background space between galaxies and stars observed by ordinary optical telescopes is almost completely dark, but if we use a sensitive enough radio telescope, we can detect a faint background glow that is not associated with any stars, galaxies or other objects. This faint light is most intense in the microwave region of the electromagnetic spectrum.
In 1965, the accidental discovery by American radio astronomers Arno Penzias and Robert Wilson was of great significance, marking the end of the work of scientists in the 1940s. The emergence of cosmic microwave background radiation became a milestone evidence for the Big Bang theory. In the Big Bang model of the universe, the early universe was filled with an opaque, dense, hot plasma of subatomic particles. As the universe expanded, these plasmas cooled and protons and electrons merged to form neutral atoms, mostly hydrogen. These atoms are unable to scatter thermal radiation via Thomson scattering, rendering the universe transparent.
Combined with this decoupling event of the epoch, photons were freed to travel freely through space. However, as the Universe expands, the energy of these photons decreases due to the redshift caused by the expansion of the universe.
This is called the "surface of last scattering" and is the correct distance range at which photons can be received that were originally emitted during decoupling. Although the CMB is roughly uniform, it is not completely smooth and exhibits slight anisotropies. Ground-based and space experiments such as COBE, WMAP, and Planck have been used to measure these temperature inhomogeneities.
The anisotropic structure is determined by the various interactions between matter and photons at the decoupling point, forming a characteristic pattern of bumps and bumps that varies with angular scale.
The anisotropic distribution of the CMB has grid frequency components that can be represented by a power spectrum showing a series of peaks and valleys. The peaks of this spectrum carry key information about the physical properties of the early universe: the first peak determines the overall curvature of the universe, while the second and third peaks detail the densities of normal matter and so-called dark matter.
It can be challenging to extract detail from CMB data because the radiation is modified by foreground features such as galaxy clusters.
Characteristics of the Cosmic Microwave Background
The cosmic microwave background radiation is a uniform emission of blackbody thermal energy from all directions, with an intensity measured in Kelvin (K). The CMB's hot blackbody spectrum is most clearly defined at a temperature of 2.72548±0.00057 K. Changes in intensity are expressed as changes in temperature, and the blackbody temperature can uniquely describe the radiation intensity at all wavelengths. The measured brightness temperature at any wavelength can be converted to the blackbody temperature.
The CMB's radiation is very uniform across the sky, with little structure compared to the clumps of matter in stars or galaxies. Its radiation is isotropic in all directions to about 1 part in 25,000.
Although the anisotropy of the CMB is extremely small, many aspects of it can be measured with high precision, and these measurements are crucial for cosmological theories. In addition to temperature anisotropy, the CMB should have angular variations in polarization. The polarization direction in each direction of the sky is described by E-mode and B-mode polarization. The intensity of the E-mode signal is 10 times smaller than the temperature anisotropy. It serves as a complement to the temperature data and is correlated with them.
The B-mode signal is weaker but may contain additional cosmological data, and the origin of the anisotropy is also related to the physics of polarization.
The CMB is also expected to show tiny spectral distortions in the spectrum that depart from the blackbody law. This is also one of the current active research focuses, and researchers hope to measure them for the first time in the next few decades because they contain rich information about the primordial universe and the formation of later structures.
According to Chuck in Hubble's V4 Given a size ratio of 400 to 1, the CMB contains the majority of photons in the Universe, with a number density a billion times that of matter in the Universe. This means that without the expansion of the Universe to cool the CMB, the night sky would be as bright as the Sun.
From a historical perspective
The existence of the cosmic microwave background was predicted and explored by early scholars. In 1931, Georges Lemaître speculated that the remnants of the early universe could be observed in the form of radiation; and in 1948, Ralph Alph and Robert Hermann further predicted the existence of the cosmic microwave background and estimated its temperature to be about 5 Kelvin. Although there was a slight deviation, the theoretical foundation had been formed.
The first positive detection of the cosmic microwave background occurred in 1964, when scientists from Princeton University began building instruments to measure the cosmic microwave background, and then in 1964, Arno Penzias and Robert Wilson accidentally discovered the existence of the microwave background at Bell Labs.
In 1965, this discovery not only demonstrated the existence of the microwave background, but also became a major breakthrough in the field of cosmology, confirming the Big Bang model.
With the development of technology, detectors such as COBE, WMAP and Planck have continued to conduct in-depth research on the cosmic microwave background, providing solid evidence and theoretical guidance for our understanding of the formation and evolution of the universe.
Today, research on the cosmic microwave background is still ongoing, and scientists are still enthusiastic about exploring information about the early universe. So, what unsolved mysteries do you think the cosmic microwave background hides?
