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Cosmic microwave background

Article curated by Grace Mason-Jarrett

In 1965 a soft hiss was detected by the radio telescopes of two astronomers in the Bell Laboratories, a hiss so faint that previous technologies had been unable to detect it, despite it seemingly coming from every point in the sky. This enigmatic hum is the Cosmic Microwave Background (CMB) which we now know to be very low energy electromagnetic radiation (microwaves) which fills every part of the Universe, commonly believed to be left over electromagnetic radiation from the Big Bang. While the CMB may be omnipresent, the faint signal we are able to detect makes it very difficult to accurately observe it. Now, thanks to highly sensitive instruments placed on satellites orbiting our planet, we have an extensive map covering the entire sky showing properties and characteristics of the CMB. From analysing this data scientists can see patterns emerging, as well as a number of unexpected phenomena. Scientists hope that from observations of the CMB we can find clues about the early Universe, and how the modern Universe formed. However for every discovery about the CMB, more questions are left unanswered.

NASA/ WMAP Science Team
13.77 billion year old temperature fluctuations shown as color differences on this all-sky map of the Cosmic Microwave Background, compiled using 9 years of data collected by the WMAP mission. Image credit: Public domain

Problems with looking at the CMB

You may be familiar with pictures of the CMB portraying a relatively even, blobby, surface coloured in greens and blues. This map, pieced together from data collected by NASA’s WMAP mission,is the first map ever made to cover every inch of the sky. Now, ESA’s (European Space Agency’s) Planck satellite has added even more detail to this image, revealing an unusual haze surrounding the centre of our galaxy. At first glance, this blurry bubble appeared to be very similar to the type of energy (known as synchrotron emission) astronomers usually associate with supernova events. The difference is, this cloud detected by Planck is brighter at different frequencies, and so it cannot be said that supernovae are to blame.

Other guesses include galactic winds or even the elusive dark matter particles interacting with each other. Once the source of this mysterious fog is solved, astronomers can focus on looking at the CMB without the galactic haze interfering with results.


What could we learn from the CMB?

Astronomers have now noticed swirls and arches tracing out intricate patterns in the polarization of the CMB. If you imagine light travelling as a wave along a string, then that string can vibrate in any direction, i.e. up and down, side to side etc. Polarized light is only oscillating in one direction, whereas unpolarized light travels along oscillating all over the place. Radiation does not start out polarized - polarization occurs when the electric and magnetic fields of the radiation align, by any cause, in the same direction. This property can tell us a lot about the material through which the radiation is travelling, and therefore could potentially reveal more about the source of this radiation. In this case some theories say that the fingerprint-esque pattern could be a result of Einstein’s hypothesised gravitational waves - ripples transporting gravitational energy throughout space-time. If this is found to be true it could be evidence for Inflation Theory - the theory in which the universe expanded at an explosive speed in an infinitesimally small amount of time shortly after the Big Bang. Finding and measuring gravitational waves would also reveal much about the nature of Gravity. However, measurements of the polarization were taken by a ground based telescope known as BICEP2[1], while assuming that the galactic haze and other foreground radiation such as large clouds of carbon monoxide in our galaxy is negligible - a fairly large assumption to make when it isn’t yet clear what the causes of these are. Furthermore, the polarization may not have been caused by gravitational waves at all, and may simply be a result of the radiation interacting with space dust[2].


We are able to see fluctuations in the density of the CMB, and from this determine a snap-shot of what the early Universe may have been like at the time the CMB was formed. This picture depicts a universe made up solely of individual hydrogen atoms whizzing around too fast and hot to combine with each other. But currently we have no way of telling for sure because there are no detectable signals from after the CMB formed. Any light emitted at this time was snapped up by the hydrogen smog leaving nothing for us to pick up now. Telescopes simply cannot ‘see’ through the mist and so this era is known as ‘The Dark Ages’.

Theories suggest that over time the hydrogen must have slowed down enough to collect together and form nebulae from which the very first stars will have formed. It is thought that the thermal energy from these stars caused atoms to lose electrons, or pick up extra ones – a process known as ionisation, and the Universe became almost completely ionised. This is known as the Epoch of Re-ionisation and as soon as it was over, light from every part of the spectrum was no longer being absorbed and the Dark Ages were over. We can see the end of the Dark Ages from looking at red-shifted light and we can see the Universe as it was before from looking at the CMB. The gap in-between gives very little away. As it stands, astronomers and scientists are looking out for light from the very first stars formed to see if there are any snippets of information there, and for signals of hydrogen atoms absorbing and emitting light during the Dark Ages. The signals will be incredibly difficult to detect, but if found could unveil invaluable information about how our Universe came to be the way it is now.


This article was written by the Things We Don’t Know editorial team, with contributions from Jon Cheyne, Cait Percy, and Grace Mason-Jarrett.

why don’t all references have links?

[1] Planck takes Magnetic Fingerprint of our Galaxy ESA Science & Technology
[2] Flauger, R., Hill, J. C., & Spergel, D. N. (2014). Toward an understanding of foreground emission in the BICEP2 region. Journal of Cosmology and Astroparticle Physics, 2014(08), 039.

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