THE COSMIC MICROWAVE BACKGROUND: The beginning

  INTRODUCTION

Cosmic Microwave Background is a collective entity constituting the photons/electromagnetic radiation which were emitted randomly and/or isotropically in space during the termination period of recombination epoch. As per the currently accepted data the recombination occurred about 378,000 years after The big bang. These photons flew in all directions and have been flowing since then, also known as the relic radiation because their structure and arrangement contain immense information about the primordial universe.

The universe has been expanded since its beginning, due to this the dominant part of the observable cosmic microwave background radiation shifted from the infrared spectrum to the microwave spectrum and so the name suggests.

The wavelength of cosmic microwave background radiation during the present time corresponds to a perfect isolated black body at an approximate temperature of 2.726 kelvin, the wavelength corresponded to a perfect isolated black body at an approximate temperature of 3000 kelvin during the onset of CMB radiation. We have also observed relatively tiny fluctuation negating the theoretically acceptable isotropic nature of the cosmic microwave background known as the "CMB ANISOTROPIES". a vague idea about the same can be attained by observing the given CMB temperature map.

Note : A common description of cosmic microwave background is that its temperature is 2.726 kelvin. what we mean by this is that if a perfect isolated blackbody radiated electromagnetic radiation in the same intensity-frequency fashion as observed of cosmic microwave background itself then the temperature would be 2.726 kelvin.

 Satellites like Cosmic microwave background explorer have helped us to understand the nature of the CMB to a considerable extent spawning further the understanding of many aspects of the universe. 

Cosmic Microwave Background Explorer(COBE) was launched by NASA on November 18,1989 carrying three Instruments -

1.                Cosmic microwave background explorer - Far Infrared Absolute Spectrophotometer(FIRAS)
2.                Cosmic microwave background explorer - Differential Microwave Radiometer(DMR)
3.                Cosmic microwave background explorer - Diffused Infrared Background Experiment(DIRBE)

DISCOVERY OF CMB AND ITS RELATION TO THE BIG BANG -

In the year 1964, two scientists Robert Woodrow Wilson and Arno Allon Penzias in Bell Labs, New Jersey were working with a horn antenna specially created by them to detect the radio wave signals of Echo Balloon satellites. During doing so they detected an unrecognizable noise, A noise which was regular and steady in all directions of space and was present at all times. after several more attempts, this noise was finalized to be a characteristic of the whole universe itself , A noise that would in future become the reason for the two to win The 1978 Nobel prize for Physics. Coincidentally 3 Astrophysicists David Wilkinson, Robert H. Dicke and Jim Peebles and their team in Princeton University were working on THE BIG BANG THEORY. The theory predicted that the big bang must have also released a huge amount of electromagnetic radiation isotropically in all directions , this radiation theoretically was described by them to be in the microwave region during the present time and to be of the same nature everywhere. Penzias and Wilson after knowing what Dicke, Wilkinson and Peebles were working at contacted them. after several conversation Penzias and Wilkinson invited the 3 scientists to the Bell Labs .

This Interaction between the two groups led to the publishing of a joint paper of their results in the Astrophysical Journal Letters which got worldwide applause.

"The noise became the biggest evidence for The Big Bang"

HOW DID COSMIC MICROWAVE BACKGROUND CAME INTO EXISTENCE -

During the early stage of the universe, as per the Inflationary big bang model, The Universe was intensely hot, hot enough to keep electrons and protons disengaged from each other. during this period the radiation continuously got scattered off free electrons due to which the universe was rather a dense opaque fluid like entity containing plasma and photons.it was opaque because of scattering as well as the fact that the mean free path of each photon between two arbitrary electron encounters was negligible. a more concise way to speak of this is to say that system of matter and system of radiation/system of subatomic particles and system of photons were coupled together forming one fused system.

"A good analogy is to think about a street light surrounded by winter smog. the light keeps bouncing away from the particulate matter and water vapors causing larger part of light to stay isolated about the street light" 

NOTE - Analogy implies the system to be translucent, the universe though was opaque

As the space kept on expanding in time, The temperatures inevitably kept on decreasing. After about 380,000 years the temperature was about 3000 kelvin, cool enough for protons and electrons to unite with each other due to their attractive tendencies. this phenomenon of unification went on for a long period of time known as 'recombination'.

This also meant that the photons got decoupled from electrons, once this happened ,the photons traveled freely through space independently. this set of photons travelling since then is what constitutes the CMB. 

NOTE - We do not have any satisfactory proof that what happened was re-combination. more rational and theoretically acceptable idea suggests that what happened was combination, yet we call it recombination for historical reasons.

CMB although is isotropic to a huge extent still possesses anisotropies. different forms of irregularities which will be talked about soon.

Image Credits: Nature.com, Cobe firas data , Testing gravity on cosmological Scales, Tien Bui , NASA

-Sanatan (MS22145)