Published on January 1st, 2011 | by Carl Mundy0
The Cosmic Microwave Background and its Formation
There is something amazing that fills the whole universe. It’s everywhere, and it is support for one of the most influential theories in cosmology, and indeed physics – the inflationary hot big bang. This amazing something is a sea of light that fills the void between stars and galaxies – the background – and it is evidence for an inflationary period in the early history of the universe. It is the cosmic microwave background (CMB) radiation.
The Early Universe
At an unimaginably small time after the big bang the universe was incredibly small and incredibly hot and bathed in a near uniform opaque glow from the hydrogen plasma that filled it. At these incredibly small scales, quantum effects came into play and so the glow was not perfectly uniform – it was not all the same.
A short time after this, the young universe inflated to be around 10 billion billion billion (~1028) times its original size in a very small amount of time. Inflation is just a period of extremely rapid expansion. This is an incredibly large number, yet large numbers are fairly common in cosmology so it is nothing out of the ordinary. Please appreciate just how large this number is, though – a one with twenty-eight zeros after it!
Formation of the CMB
After this rapid inflation, the universe was much larger and so much cooler. This means that the light that was present in the young universe now had a much lower energy associated with it. As the energy is related to the wavelength, this meant that the wavelength increased as the universe continually cooled. As this happened, the universe became less opaque and more transparent.
This occurred until the light in the universe no longer had enough energy to ionise any atoms that formed (strip the electrons from the atoms). This tipping point is calleddecoupling and is when the CMB radiation was formed. The universe was around 300,000 years old and had a temperature of about 3000 kelvin at this point. Like the static black and white ‘snow’ we see when tuning our televisions that bathes the airwaves around us, the CMB fills the universe, albeit faintly.
Over the past two decades we have managed to map the CMB radiation in unprecedented detail. Space-based telescopes, namely COBE, WMAP and Planck, have found that the CMB is uniform to around 1 part in 100,000 and peaks in the microwave part of the spectrum (hence the name).
This puts the temperature of our universe today at 2.725 kelvin – much cooler than the temperature when decoupling occurred and unimaginably insignificant compared to the temperature just before inflation occurred.
The CMB radiation we observe today can tell us many things about our universe. The curvature of our universe, and so its fate, can be constrained from the peaks in the CMB spectrum, as well as the baryon density (protons, neutrons, anti-protons, anti-neutrons etc.) and amazingly even the density of dark matter.
The discovery and measurement of the cosmic microwave background radiation should, in my opinion, be treated as one of the greatest achievements of science.
Further Your Knowledge
This has been a basic introduction to the CMB, so we suggest that if you are eager to learn more then you check out the following topics of interest:
- The Big Bang
- Evidence for the big bang
Images courtesy of WMAP/NASA and theogeo.