The Universe Just Got A Bit Older!

by Nicholas Mee on March 25, 2013

The universe just got a little bit older – a mere 80 million years older. More precisely, a much improved estimate of the age of the universe has just been published. Astrophysicists at the European Space Agency (ESA) have released the most accurate map of the cosmic microwave background radiation yet compiled. This is the fruit of data collected by ESA’s probe Planck. From this information it is possible to deduce that 13.82 billion years have elapsed since the origin of the universe in the Big Bang.

The Cosmic Microwave Background

The cosmic microwave background was first detected in 1964 by Arno Penzias and Robert Wilson working at Bell Labs in Holmdel, New Jersey. Penzias and Wilson were testing a sensitive new horn antenna that had been built to receive signals from Echo balloon satellites, but were plagued by interference. They were unable to eradicate or explain this background noise until a friend suggested that they look at some papers by the physicists George Gamow, Ralph Alpher and Robert Herman. In  the late 1940s Gamow and his colleagues had predicted that if the universe really had begun in the Big Bang there would be a tell-tale signal that could be detected – the universe would be filled with leftover microwave radiation that was produced shortly after the Big Bang, and this is what Penzias and Wilson had discovered.

Penzias and Wilson in front of their horn antenna.

Where Did It Come From?

The universe was created as a very hot soup of particles. As the universe expanded the temperature fell, but for the first few hundred thousand years the matter consisted mainly of a plasma of protons and electrons – the temperature was too high for atoms to form. Any electron that found itself bound to a proton in a hydrogen atom would have immediately been knocked out of the atom by violent collisions with other particles. The early universe was also filled with radiation made up of vast quantities of photons. These photons were continually scattering off the protons and electrons.

After around 380,000 years the matter had cooled to a temperature of about 3,000 degrees above absolute zero. It was now cool enough for protons and electrons to combine into hydrogen atoms. During this era the universe became transparent as the photons could no longer interact with the matter. Electrons can only occupy a discrete set of energy levels in an atom. This means that a photon can only interact with an electron in an atom if it has just the right energy to promote the electron into a higher energy level or sufficient energy to knock it out of the atom entirely. With the temperature now below 3,000 degrees the photons swarming through the universe no longer had enough energy to do this. From here on the universe has been filled with vast numbers of photons that are unable to interact with the matter that it contains.

Hot Bodies

We can tell how hot an object is by the radiation it emits. All objects give off photons and as they cool down the energy of these photons is reduced and their wavelengths becomes longer. For instance, at room temperature infra-red radiation is emitted. Infra-red cameras can pick out humans and other living creatures against background objects because they are warmer than the background and therefore emit radiation of a slightly shorter wavelength.

Our Expanding Universe

The background radiation that fills the universe last interacted with matter when the ambient temperature of the universe was around 3,000 degrees, so the radiation had the energy and wavelengths characteristic of a body of this temperature. Since then the universe has continued to expand. But it is not expanding into a pre-existing empty box. Strange as it might seem, space itself has been stretching and this has had a dramatic effect on the radiation that it contains. It means that the source of any radiation is racing away from us and the further away it is the faster it is receding.

Each cosmic microwave background photon that the Planck probe has detected last interacted with matter 13.8 billion years ago. Since this final fling with a proton or an electron the photon has sped across the universe, but throughout its epic journey the universe has grown dramatically until it is now over 1,000 times the size that it was when it was emitted. The result is an enormous stretching of the wavelength of the photon corresponding to a huge Doppler red shift. The photon is detected with a wavelength in the microwave range that is over 1,000 times as long as when it was emitted. For this reason the radiation detected by Planck has the characteristics of radiation that would be emitted by a body whose temperature is around 2.7 degrees above absolute zero, which is less than 1,000th of the original temperature.

The Planck Probe as it was being prepared for tests prior to launch.
Copyright European Space Agency.

Lumpy Cosmic Porridge

The cosmic background radiation contains a lot of information about the early universe and it is this information that cosmologists are now teasing out. For instance, some regions of the universe were slightly warmer than average and these can be detected. These regions are the places where matter had already begun to clump and would later evolve into clusters of galaxies.

The image at the top of this article shows a map of the microwave radiation detected by the Planck probe. The map shows the entire sky projected onto an ellipse. It includes radiation emitted from within our galaxy – the plane of the galaxy can be seen across the centre of the map. This radiation must be subtracted out to leave the full sky map of the cosmic microwave background. The result is the illustration shown below.

The colours in the map indicate the temperature of the cosmic microwave background across the whole sky as indicated by the wavelengths of the radiation detected by Planck. Red corresponds to slightly higher temperature. (Note: the colouring in this map is different to the colouring of the image at the top of the article.) The hotter regions are those that 13.8 billion years ago were in the process of condensing into galactic clusters.

Galactic Clusters

The Hubble Space Telescope has produced some incredible deep sky images of galactic clusters. These clusters are incredibly distant, but still much closer than the clumps detected in the microwave background. Nevertheless they give a good idea of what the clumps would eventually evolve into. In the Hubble image below each dot and smudge is a galaxy around the size of our Milky Way galaxy, home to several hundred thousand million stars and their associated planetary systems.

Distant galaxy cluster as imaged by the Hubble Space Telescope. Copyright NASA.

More Information

More detailed information about the Planck probe and what it is revealing about the universe is available on the European Space Agency website at:
http://sci.esa.int/science-e/www/area/index.cfm?fareaid=17

Previous post:

Next post: