The Cosmic Microwave Background

The image above is one of the most beautiful I have ever seen. You may well recognise it as the iconic cosmic microwave background data released by NASA in 2010. For the previous nine years the Wilkinson Microwave Anisotropy Probe (WMAP) had been diligently measuring temperature differences across the sky in an effort to detect the radiant heat remaining from the big bang itself.

The image, then, is essentially a heat map of the sky; red areas are warmer than blue areas. To give you a sense of the scale we are talking about, the average temperature being measured is 2.7K, so just a few degrees above absolute zero, and variation between the warmest red areas and the coolest blue areas is just ± 0.00005K. Astonishing sensitivity.

Let’s take a moment to picture exactly what it is that we are detecting here. In the first 100,000 years or so in the life of the universe all that existed was a plasma of hydrogen, essentially protons and electrons, that were at very high temperatures. This plasma was so hot and dense that light was incapable of moving through it as it would be constantly bumping into the free particles. At this point the whole universe was a white-hot fog.

Eventually it cooled to such a point that the protons and electrons began to combine to form ordinary hydrogen atoms. This allowed the photons to travel freely and the universe became transparent as it is now. Some of those earliest free photons are still, today, travelling through space, though they have been heavily red shifted into the microwave range, and it is these that WMAP detected. Personally, I find that pretty extraordinary.

The data released by WMAP and NASA was important for several reasons. It provided strong evidence to back up the leading theory of how the big bang occurred. The high degree of uniformity of the background leant credence to the Lambda Cold Dark Matter Model. These real life observations all put tight parameters on any future theory that tries to explain the beginning of the universe.

For example, the universe is 13.772 ±0.059 billion years old; the Hubble Constant (the rate of expansion of the universe) is 69.32 km/megaparsec/second; other parameters include the density of matter, dark matter and dark energy and the curvature of the universe.

In 2009, the European Space Agency launched the Plank space telescope which was able to measure the CMB even more accurately than the WMAP had. It produced the image below which, as you can see, closely resembles the original CMB data but has much finer resolution.

new cmb

The fact that there is a pattern at all, that there isn’t completely perfect uniformity to the CMB, is due to imperfections and ripples present in the first nonillionth of a second of the life of the universe. It is theorised that these perturbations are what gave rise to differences in densities of matter in the early universe and, ultimately, the pattern of clusters of galaxies that we observe today.

So there you have it, that’s what we mean when we talk about the cosmic microwave background. Hopefully, this also gives you some insight into why I think this is one of the most beautiful images ever produced. It isn’t just the abstract mixture of colours coming together to form an aesthetically pleasing picture, it is much more than that. It represents a hugely impressive feat of technical engineering and, further still, a staggering level of understanding of the universe around us, right back to its very origins. It is an achievement our species can be truly proud of.


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