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The Importance of Cosmic Background Radiation in Modern Cosmology

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Cosmology Series
  1. What is Cosmology?
  2. The Speed of Light
  3. Redshift and Blueshift
  4. What is the Cosmic Microwave Background Radiation?
  5. Cosmic Scale Factor
  6. The Physics Governing the Universe
  7. The Nature of Light
  8. The Four Forces in the Universe
  9. The Expansion of the Universe
  10. The Importance of Cosmic Background Radiation in Modern Cosmology

The Cosmic Background Radiation is the afterglow from the early universe and provides strong evidence for the theory of a Hot Big Bang. We take a look at what the CBR is, how it was detected and why it is important for cosmology.

What is the Cosmic Background Radiation?

The Cosmic Microwave Background Radiation (CBR) is the afterglow from the early universe and provides strong evidence for the theory of a hot Big Bang. The Cosmic Background Radiation was predicted by the Big Bang theory in 1948 by George Gamow.

The Cosmic Background Radiation was discovered by accident at the Bell Labs Horn Antenna by Penzias and Wilson in 1965. While working with microwave communication technology Penzias and Wilson discovered a background noise, uniform in all directions, which they could not account for. Dicke and Peebles of Princeton University were at the time working on the hot Big Bang theory and realised that this "noise" was the radiation left over from the Big Bang.

The horn antenna at bell labs, crucial to discovering cosmic background radiation.
The horn antenna at bell labs, crucial to discovering cosmic background radiation.

First observations of the Cosmic Background Radiation showed that it was uniform in all directions; however recent observations show that there are tiny variations (±0.003 K) across the visible universe. These variations from the average have been interpreted as evidence of the lumpy universe.

The temperature of the Cosmic Background Radiation was first accurately measured by the COBE satellite, which also showed that the Cosmic Background Radiation was a black body (also predicted by Big Bang theory). Subsequent analysis has produced more and more detailed maps of the CBR which show evidence of the conditions in the early universe including the origin of galaxies and large structures in the universe. The Cosmic Background Radiation is also very strong evidence for the hot Big Bang theory as it shows that the early universe was once very hot. COBE microwave measurements also show redshift and blueshift, indicating that our solar system is travelling through space.

Combined sky images of microwave radiation commected by the WMAP satellite.
Combined sky images of microwave radiation commected by the WMAP satellite.

The image 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. The first slide shows the uniformity of the CBR, while the second slide shows the results from the COBE dipole subtraction. This result is important because it shows redshift and blueshift caused by our motion (and that of our local group) relative to the CBR. This has been estimated at 600 kms towards the Virgo cluster (bottom left of the image).

The final slide shows tiny variations in the Cosmic Background Radiation which are thought to be lumps, or density ripples, in the early universe. As the early universe expanded these clumps of matter went on to form galaxies and other large structures.

One of the main reasons that the microwave radiation detected is believed to have originated from the Big Bang is its uniformity. No other source of microwave radiation could possibly create such a uniform source across the entire sky, no matter how far away it is.

Although important data has been recorded from COBE and WMAP, there are many other sources of data including ground based observations and high altitude balloons (BOOMERanG), all of which have an important role.

Where did it come from?

As previously discussed, the Cosmic 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. As the universe expanded the radiation cooled and the matter density became dominant. The stage is known as the matter era. The transitional phase in between was the age of recombination where the universe cooled to around 3000K. During this time photons could exist without ionising atoms and they began travelling across the universe.

The speed of light is finite and takes a time to reach us, for example when we look at the Sun, we look at it as it was 8 minutes ago. In the same way, when we look at the Cosmic Background Radiation, we see it as it was just after recombination, revealing early structures in the construction of the universe. The Cosmic Background Radiation is observed in the Microwave spectrum because the original light emitted has been redshifted due to the expansion of the universe.

The Cosmic Background Radiation is the remnant of the end of the radiation era and as such, it can tell us a great deal about the initial parameters of the early universe which can verify or refine existing models of the big bang.

What does it mean?

The temperature of the Cosmic Background Radiation 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

Temperature scale factor
Equation 32 - Temperature scale factor

This can also be simplified to become a factor of redshift:

Temperature redshift.
Equation 33 - Temperature redshift.

Where T0 represents the current temperature of the universe.

This is a more useful function because we do not always know time, but we can measure redshift from observations of spectra. We can say that for redshift (z = 1) the temperature of the universe was 5.45K.

COBE results also showed that there is a pattern to the polarisation of the Cosmic Background Radiation . This polarisation was an important discovery because it is 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 verifies results recording tiny lumps in the Cosmic Background Radiation , but also validates our current understanding and theories of the universe.

The WMAP (Wilkinson Microwave Anisotropy Probe) probe, successor to COBE has extended the measurements with an even greater definition which allows us to see greater detail in the variations in the temperature of the Cosmic Background Radiation 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 Cosmic Background Radiation by WMAP has enabled a greater determination of the content of the universe when the Cosmic Background Radiation was emitted, new evidence for the inflation theory and refining values for the age of the universe and the Hubble constant.

Future of Cosmic Background Radiation Analysis

Scheduled for launch in April 2009 the Planck Surveyor will take off from where WMAP left off. It will record and analyse the Cosmic Background Radiation in a higher resolution and will investigate the polarisation of the Cosmic Background Radiation, gravitational lensing, the shape of the universe and cataloguing galaxy clusters through the Sunyaev-Zel'dovich effect (distortions caused by high energy electrons).

Last updated on: Monday 22nd January 2018

 

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