Cosmic Microwave Background Radiation
The Cosmic Microwave Background Radiation (CMBR) or Cosmic Background Radiation (CBR) is the afterglow from the early universe and provides strong evidence for the theory of a hot Big Bang. This article looks at what the CBR is, how it was detected and why it is important for cosmology.
The temperature of the CBR was first accurately measured by the Cosmic Background Explorer (COBE) satellite in 1990. This showed that the CBR was a blackbody (as predicted by Big Bang theory) and that there are very tiny variations (±0.003 K) across the visible universe. These slight variations are evidence of the conditions in the early universe and have been interpreted as evidence of a "lumpy" universe, also predicted by the Big Bang theory. These variations are thought to be the origin of galaxies and large structures in the universe. The CBR shows very strong evidence for the hot Big Bang theory as it suggests that the early universe was once very hot.
COBE microwave measurements show redshift and blueshift indicating that our galaxy is travelling through space. From these readings we can deduce that the speed of our galaxy relative to the CBR is around 600 km s-1 towards the Virgo cluster.
The slide above shows a false colour image of the entire sky projected onto an oval (similar to a map of the Earth). The Milky Way extends horizontally across the centre of the image, and has been subtracted from the results. The variations in the CBR temperature are shown as different colours. These variations are the "lumps" (technical term: density ripples), in the early universe. As the early universe expanded these clumps of matter went on to form galaxies and other large structures.
What is the Cosmic Background Radiation?
The microwave background radiation is believed to be the "visible" remnant from the Big Bang. In the very early stages of the universe the energy density was far greater than the mass density (there was more energy per unit volume than matter). This stage is called the radiation era (energy = radiation). During the radiation era, photons were flying around with so much energy that they ionised any matter that tried to form.
As the universe expanded, the radiation cooled and matter was able to form without ionisation, causing the matter density to increase and became dominant. The stage is known as the matter era. The transitional phase between the two era is called the age of recombination in which the universe expanded and cooled to around 3000K. During this time photons could exist without ionising atoms and instead began travelling across the universe.
The speed of light is finite and takes time to reach us, for example when we look at the Sun, we see it as it was 8 minutes ago. When we look at the Andromeda galaxy, the light has taken about 2.2 million years to get to our galaxy, so we see the Andromeda galaxy as it was 2.2 million years ago. In exactly the same way, when we look at the CBR we see the universe as it was 13.7 billion years ago just after recombination. Due to the extremely rapid expansion of the universe, the CBR is observed in the microwave spectrum because the original light emitted has been redshifted into the microwave wavelengths.
Why is the Cosmic Background Radiation Important
The temperature of the CBR today has been measured at 2.725K. Because the universe is expanding, we can express the temperature of the universe through time as a function of scale factor.
Equation 32 - Temperature scale factor
This can also be simplified to become a factor of redshift like so:
Equation 33 - Temperature redshift.
Where T0 represents the current temperature of the universe. This is a far more useful function because we do not always know a time, but we can measure redshift from observations of spectra. We can say that for a time equivalent to redshift (z = 1) the temperature of the universe was 5.45K.
We can estimate the redshift of the CBR to be approximately z = 1500, so substituting this value into Equation 33 will give the temperature of the universe at the time the CBR was formed. This gives:
Equation 34 - Temperature of the CBR
Importance of the CBR to Modern Cosmology
Results from COBE showed that there is a pattern to the polarisation of the CBR. This polarisation was an important discovery because it was initially predicted by the gravitational instability theory. This theory predicts that slight variations in density of the early universe will, over time, form larger structures. This not only verified results recording tiny lumps in the CBR, but it also validates our current understanding and theories of the universe.
The Wilkinson Microwave Anisotropy Probe (WMAP) probe, a successor to COBE, has extended the original measurements with an even greater resolution, which allow us to see greater detail in the variations in the temperature of the CBR and provide accurate data for models of the shape, content and evolution of the universe. This data can then be used to test the Big Bang theory, inflation theory and any other theory of the formation of the universe.
Analysis of the CBR by WMAP has revealed that there are distinct peaks in both its power spectrum. These peaks have been attributed to dark matter and has given evidence to support the inflationary theory of the universe.
The existence of the CBR is not only confirmation that our models for the evolution of the early universe are valid, but also through it's analysis we are able to refine values for cosmological constants and ultimately move one step closer to understanding the universe in which we live.
Future of CBR analysis
Scheduled for launch in April 2009 the Planck Surveyor will take off from where WMAP left off. It will record and analyse the CBR in a higher resolution and will investigate the polarisation of the CBR, gravitational lensing, the geometry of the universe and cataloguing galaxy clusters through the Sunyaev-Zel'dovich effect (distortions caused by high energy electrons).
We eagerly await the first results from Planck Surveyor and the implications for Cosmology!