Exploring the Universe with Cosmic Microwave Background (CMB):

Author Information

Name: Jishnu Nandi

School: La Martiniere for Boys, Kolkata

Country: India

Contact email: jishnunandi40@gmail.com

Abstract

This article delves into the microscopic temperature fluctuations in the Cosmic Microwave Background (CMB) radiation and how they provide evidence for the mechanism of the early Universe. Although the CMB seems to be more or less uniform, high-precision observations indicate the presence of very small temperature variations. These microscopic ‘anisotropies’, as they are called, are very important to understand the density fluctuations present in the early Universe, which later evolved into the stelliferous galaxies and clusters that we know today.

This article examines the overall properties of these anisotropies and how they are identified by satellite missions, specifically COBE, WMAP, and the Planck satellite. Further, it gives an insight into how these anisotropies are sources of useful information to scientists and cosmologists for decoding the ways of the vast and seemingly unmeasurable entity that we know to be as the Universe.

Introduction

Although there are several theories regarding the creation of the vast cosmos, the prevalent theory widely accepted by the global scientific diaspora is, of course, the Big Bang theory. This theorizes that the Universe started off as a point infinitesimally small, and later expanded into the vast collection of stars and galaxies and even more, that we see today.

However, immediately after the Big Bang, the Universe was a hot, dense plasma of protons, electrons, and photons. These photons were constantly interacting with charged particles, making the Universe ‘opaque’. As the Universe expanded and cooled, it reached a temperature low enough such that the protons and electrons could condense into neutral atoms (forming light elements, such as hydrogen and helium). Due to reduction of free electrons in this process (the process being termed as ‘recombination’), photons were free to travel through space, which made the Universe ‘transparent’. This phenomenon occurred when the young Universe was around 380,000 years old.

The light emitted due to this travelling of photons is what we define Cosmic Microwave Background (CMB) as. However, over billions of years, due to more expansion and cooling of the Universe, the wavelength of CMB increased. What was once in the range of visible and ultraviolet (UV) radiation 13.8 billion years ago is now microwave radiation, making CMB extremely hard to detect.

Image 1: Cosmic Microwave Background radiation in a simplified timeline of the Universe. Image Credit: https://bigthink.com/wp-content/uploads/2025/02/mapping-timeline.jpg?w=1400

In fact, right now, the average wavelength of the CMB is 1.9 mm and average temperature is around 2.725 K, with minute variations known as ‘anisotropies’, which, as mentioned before, hold valuable importance in the field of cosmology. For the last thirty to forty years, efforts have been made to detect CMB with variable success — the most notable attempts include COBE, WMAP, and the Planck satellite.

Methodology

Before moving on into the various satellites used to map CMB, it must be mentioned that CMB radiation was accidentally discovered in 1965 by Arno Penzias and Robert Wilson, who were using a radio antenna to study microwave signals. Apparently, they noticed a persistent and uniform sound with no clear source, and it was found out that this was the afterglow of the Big Bang. This discovery gave impetus to the Big Bang theory and also won them the Nobel Prize in Physics in 1978.

The first satellite which measured the CMB’s spectrum was COBE (Cosmic Background Explorer), built and launched by NASA; which operated from 1989 to 1993. The COBE carried three instruments — Diffuse Infrared Background Experiment (DIRBE) to search for cosmic infrared background radiation, Differential Microwave Radiometer (DMR) to map the cosmic radiation sensitively, and Far Infrared Absolute Spectrophotometer (FIRAS) to compare the spectrum of CMB radiation with precise blackbody radiation.

Image 2: Labelled diagram of COBE satellite.

Following this, NASA’s WMAP (Wilkinson Microwave Anisotropy Probe) operated from 2001 to 2010. The WMAP mission was designated to gain information concerning the geometry, content, and evolution of the Universe through a 13 arcminute FWHM (Full Width at Half Maximum) resolution full sky map concerning the temperature fluctuations of CMB. It was much more precise and effective than COBE DMR, specifically having 45 times its sensitivity and 33 times its angular resolution. WMAP used differential polarization sensitive receivers for its radiometric system, HEMT amplifiers for detection, and a 2.5 kHz phase switch for radiometer modulation.

Image 3: Labelled diagram of WMAP satellite.

The ESA (European Space Agency) launched the Planck spacecraft in 2009, which was operational until 2013. Designed to capture the CMB’s temperature and polarization anisotropies, it focused on more precision than previous missions. The model payload consisted of a 1.5 meter off-axis telescope with two focal plane arrays of detectors sharing the focal plane. It had two instruments — High Frequency Instrument (HFI) and Low Frequency Instrument (LFI), for capturing frequencies of different ranges.

Image 4: Labelled diagram of Planck satellite.

Results

As mentioned before, COBE FIRAS was responsible for comparing the spectrum of CMB radiation with precise blackbody radiation. Its finding is represented below pictorially:

Image 5: Comparing COBE FIRAS data with blackbody spectrum.

In the above representation, the COBE FIRAS data is plotted in red on the graph. Another blue line is plotted on the graph, representing precise blackbody radiation. The result is that the error margin between the two is very small, showing that the COBE data was in accordance with blackbody radiation of a thermodynamic field at 2.725 K.

Further, the COBE DMR found some of the irregular anisotropies, as given below:

Image 6: Mapping of Cosmic Microwave Background radiation by COBE DMR (1992).

These anisotropic spots were further enhanced in CMB maps of WMAP. As mentioned before, WMAP’s instruments were 45 times more sensitive and 33 times more angularly resolute than those of COBE. The result of WMAP’s findings is given below:

Image 7: Mapping of Cosmic Microwave Background radiation by WMAP.

As we can see, the scale and location of the anisotropies is much more precise, showing that WMAP’s data was way more effective than COBE’s.

Following this, the Planck satellite was more effective and took the research of CMB anisotropies one more step ahead. A map of Planck’s data is as follows:

Image 8: Mapping of Cosmic Microwave Background radiation by Planck spacecraft.

It must also be noted that the precision of the Planck spacecraft was such that it could detect very minute variations of even around 0.0001 K (0.1 millikelvin or 100 microkelvin).

These maps and data have indeed helped scientists and cosmologists over the past three decades or so in their quest to discover more about the characteristics, geometry, and other aspects of the vast Universe.

Discussion

Now, we come to perhaps the most important portion of this article — why is all this data useful? It provides a scientific platform for explaining and supporting many theories concerning the origin of the Universe, including the Big Bang theory. In fact, CMB maps are effectively the ‘snapshot’ of the early Universe at an age of 380,000 years. Further, if the theories of the origin of the Universe can be explained with this, it will further allow us to determine and predict many characteristics of the Universe at present and even its future.

Firstly, COBE FIRAS’ data comparison with blackbody radiation shows that the early Universe was once in a state of thermal equilibrium and also proves some of the hypothesis in the Big Bang theory. The nearly perfect blackbody spectrum shows that matter and radiation in the early Universe were in a state of equilibrium. Frequent interactions between subatomic particles seemed to redistribute energy throughout the Universe. Further, little or no deviation in COBE FIRAS’ data from blackbody spectrum at 2.725 K shows that very few high-energy events occurred after the formation of CMB. These strongly proved the Big Bang model of the Universe and gave scientists an insight into the history of the early Universe.

Secondly, the very much polarized radiation from the CMB (i.e., radiation from CMB had uniformity in direction and other such properties) shows that collisions of photons with electrons led to a uniform scattering of light just before the ‘recombination’ era. This was useful for scientists and cosmologists to analyze a quantum history of the early Universe.

Image 9: Thomson scattering of photons by electrons in the early Universe.

Thirdly, CMB also expounds on the theory of cosmic inflation, i.e., the continuous expansion of the Universe. This is assumed to be that the quantum fluctuations during the Big Bang later evolved into the anisotropies of the CMB (fluctuations in density and temperature) which further evolved into the galaxies and clusters that exist today — and this entire process is only possible if the Universe was continuously expanding.

Image 10: Cosmic inflation as a better explanation for the Big Bang theory than previous standard models. Both cosmic inflation and the Big Bang theory have been practically proven by the study of Cosmic Microwave Background.

Fourth, the CMB also shows that the Universe’s geometry is apparently flat. This is an additional proof of cosmic inflation since continuous expansion would naturally have a flattening effect.

Fifth, it provides a context for further research, especially concerning fields relatively unknown to us yet such as dark matter and dark energy. There are pressure waves present in the CMB known as baryon acoustic oscillations. These waves seem to be a result of interactions of plasma with baryonic matter (matter made of protons, neutrons, electrons, and other such baryons made of three quarks) in the early Universe. The characteristics of baryon acoustic oscillations can be used to determine the density of baryonic matter in the Universe. Also, fluctuations in the CMB are considered to be caused by the gravitational influence of dark matter, and hence the amount of dark matter can be estimated via the fluctuation distribution. Finally, the presence of dark energy can be verified since only matter cannot result in a flat Universe — dark energy would provide additional energy density to drive the expansion of the Universe.

Measurements from the Planck spacecraft’s data show that the Universe roughly consists of 5% baryonic matter, 27% dark matter, and 68% dark energy.

All these applications and extended research of the CMB show how truly a snapshot of the early Universe can drive our scientific quests to unravel the secrets of the Universe and maybe beyond.

References and Further Reading

1. Helmenstine, A. (2024, August 25). Cosmic Microwave Background Radiation (CMB or CMBR). Science Notes. https://sciencenotes.org/cosmic-microwave-background-radiation cmb-or-cmbr/

2. Greason, M. R. LAMBDA — Cosmic Background Explorer. Goddard Space Flight Center, NASA. https://lambda.gsfc.nasa.gov/product/cobe/

3. Greason, M. R. LAMBDA — Wilkinson Microwave Anisotropy Probe. Goddard Space Flight Center, NASA. https://lambda.gsfc.nasa.gov/product/wmap/current/

4. Greason, M. R. LAMBDA — The Planck Mission. Goddard Space Flight Center, NASA. https://lambda.gsfc.nasa.gov/product/planck/curr/

5. COSMOS Home — Planck — Cosmos. European Space Agency.

https://www.cosmos.esa.int/web/planck#

6. Cosmic Microwave Background Radiation. Goddard Space Flight Center, NASA. https://lambda.gsfc.nasa.gov/product/suborbit/POLAR/cmb.physics.wisc.edu/polar/ezexp.ht ml