Ever since the dawn of consciousness, human beings have pondered over the origin of the universe. Presently, the Big Bang theory is the leading explanation we have about the beginning of the universe. The cosmic microwave background radiation (CMB) provides key proof for the Big Bang Theory.
According to the Big Bang cosmological model, nearly 14 billion years ago, the universe exploded into existence in an extremely hot plasma state consisting of particles such as electrons, neutrons, protons and photons. As the universe expanded, its temperature gradually dropped, which resulted in the cosmic microwave background.
The Origin of CMB
To understand the origin of the CMB, we must peer into the early stages of the universe. When the universe was just moments old, it had temperatures unimaginably hot — around 273 million degrees above absolute zero. There were no atoms at this point because their constituents, namely, protons and electrons, had disassembled during that time. The continuous scattering of photons by the free electrons made the universe opaque.
But as the universe kept on expanding, its temperature went down slowly. Approximately 380,000 years since the Big Bang, the universe had cooled to about 3000 kelvin, allowing the electrons to combine with the protons to form neutral hydrogen atoms in a process known as “recombination.” This allowed photons to move freely, making the universe transparent.
As space expanded, the wavelength of the photons stretched up to 1mm, thereby reducing their effective temperature to a mere 2.7 kelvin. This is the cosmic microwave background radiation that we now observe across the universe. As the photons are in the microwave range of the spectrum, they are not visible to the naked eye, but they can be detected by far-infrared and radio telescopes.
The Accidental Discovery
American cosmologist Ralph Apher, along with Robert Herman and George Gamow, first suggested the presence of the cosmic microwave background (CMB) in 1948 when embarking on studies concerning Big Bang nucleosynthesis. But the CMB was discovered by chance in 1965 when Arno Penzias and Robert Wilson, on a telephone laboratory radio, noticed an unexpected noise from all directions across the sky.
Coincidentally, Robert Dicke and his team, who were actively searching for the cosmic microwave background, came to know of Penzias’s and Wilson’s findings, prompting both groups to publish their results swiftly in the Astrophysical Journal. Penzias and Wilson were subsequently honored with the Nobel Prize in Physics in 1978 for their groundbreaking discovery.
Unveiling the Mysteries of the Universe
Originating from all directions, the CMB is a treasure trove of information about the early universe. Regardless of the direction observed, the cosmic microwave background seems almost identical — a condition referred to as isotropy in cosmology. However, on closer examination, scientists noticed minor deviations from this uniformity, known as anisotropies. These anisotropies have been instrumental in providing insights into the structure and composition of the universe.
Before the CMB originated, there was an era of inflation, when the universe expanded at exponential rates within a fraction of a second after the Big Bang. This made the universe grow by an astonishing factor of 1⁰³⁰. Inflation thus magnified primordial quantum fluctuations in the space-time over vast scales during the superluminal expansion. The regions with slightly higher densities of matter began to draw in more material through gravitational attraction, laying the groundwork for the formation of cosmic structures — stars, galaxies, and galaxy clusters.
These fluctuations manifest as temperature variations observed in the CMB. As photons journeyed through the cosmos, they carried with them a memory of the density distribution of the universe at the time before “recombination.” A photon situated in a region of somewhat higher density needed to expend energy to move away from the gravitational pull of that denser area, causing it to become slightly cooler when compared to the average temperature of photons. Conversely, photons in less dense regions lost less energy as they departed, making them a little hotter than average. This is how temperature variations in the cosmic microwave background (CMB) reflect the underlying structure of matter present in the early universe at the time of the CMB release.
The cosmic microwave background also stands as one of the most compelling pieces of evidence for the theory of the Big Bang. According to the theory, photons scattered by the hot plasma in the early universe should exhibit a blackbody spectrum of radiation. The Far Infrared Absolute Spectrophotometer (FIRAS) experiment aboard NASA’s Cosmic Background Explorer (COBE) satellite confirmed this prediction with extraordinary precision. The FIRAS experiment measured the energy spectrum of the CMB radiation and found it to match the predicted blackbody curve almost flawlessly. The accuracy of this measurement serves as a powerful validation of the Big Bang theory, as no alternative theory has yet been proposed that can account for this energy spectrum.
Missions to Study the CMB
In the years following its initial discovery, various efforts were made to map portions of the cosmic microwave background. One of them is NASA’s Cosmic Background Explorer (COBE) mission, which was launched in 1989. It provided the first full-sky map of the CMB. This is often referred to as the “baby picture” of the universe. One of the significant findings of the Cosmic Background Explorer (COBE) was that the cosmic microwave background (CMB) exhibits a spectrum closely resembling that of a “black body.”
In 2003, the Wilkinson Microwave Anisotropy Probe (WMAP) was launched, to study small-scale fluctuations observed by COBE in more detail. In 2009, ESA launched the Planck mission. This was unlike the previous ones since Planck has a broader frequency range, more bands and higher sensitivity. By offering an extensive view of the CMB with fine variations, Planck expects higher precision in confirming the standard model of cosmology. The final data of Planck, released in 2018, proved the existence of dark matter and dark energy.
Conclusion
The cosmic microwave background (CMB) stands as a window into the early universe, offering invaluable insights into its origin and evolution. As we gaze into the cosmic microwave background, we peer back in time to the earliest moments of the universe. Its discovery and subsequent study have revolutionized our understanding of cosmology, providing profound insights into the birth, evolution, and fate of the cosmos. As researchers continue to delve deeper into its mysteries, the CMB remains a cornerstone of cosmology, guiding our quest to unlock the secrets of the cosmos and our place within it.
