We know that our universe is expanding. Galaxies are moving away from each other. From the Hubble law we can find the recessional speed of the galaxy. If we know the H (Hubble parameter) and the distance of the galaxy from us. And in the previous episode we described the Hubble parameter how to determine it. We also discussed the uncertainty in the Hubble parameter due to the vast distances and difficulties to determine these distances.
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| Expanding Universe |
Although there are other explanations for the expansions of the universe. But the one that gained favor was based on the assumption that if the galaxies are moving away from each other. Then at some point back in time all these galaxies must have been closer to one another. Not only galaxies but all the matter in the universe must have been packed to-gather at a single point. Scientists call that point a singularity. We find that after Big Bang in the early stages of the universe it was filled with matter and radiations.
As the universe expanded both the matter and the radiations cooled and the wavelength of the photons of the radiations also stretched with the expansion. As the universe continued to expand the wavelength of the photons stretched more and more. The radiations that filled the universe in its compact state and it continues to fill the universe in its expanded state. We can still find these radiations around as but the wavelength of these radiations is so stretched that these radiations lie in the microwave region of the electromagnetic spectrum. This is known as the Cosmic Microwave Background Radiations.
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| Cosmic Microwave Background Radiation |
Discovery of the Radiations:
These radiations are discovered by the Arno Penzias and Robert Wilson of the Bell lab in 1965, who was testing a microwave antenna for satellite communication.
No matter where they pointed their antenna they found the same background ‘hiss’. Later they realized that they were indeed seeing the remnant of the early universe, which was filled with the radiations and unimaginably high density of matter. They were awarded Nobel Prize in physics in the year 1978 for their discovery.
No matter where they pointed their antenna they found the same background ‘hiss’. Later they realized that they were indeed seeing the remnant of the early universe, which was filled with the radiations and unimaginably high density of matter. They were awarded Nobel Prize in physics in the year 1978 for their discovery.
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| Arno Penzias and Robert Wilson |
Measurement of the Radiations:
Measurement of the cosmic microwave background radiations reviled that the intensity of the radiations is uniform everywhere in all directions. It does not appear to come from ant particular source in the sky. It filled the entire universe uniformly indicating the expansions of the radiations with the universe. Observations ,however, shows that there are temperature fluctuations of 10-5K between different regions of the sky, this indicates that the density of the matter was not uniform in the early universe that ed to the condensation of the stars and galaxies. The early density of the radiations can be found from the Plank’s Radiation law. The number density of these background radiations is 400 per cm3, and the energy density is about 0.25 eV/cm3 . The main energy per photon is about 0.00063eV, which suggest why we are not ordinarily aware of the presence of these photons.
M. Sajeel