Cosmic Microwave Background - Looking at the Distant Past
.. By Geoff Burling
A unique feature of Astronomy, when compared to other sciences, is the ability to look directly back into the past. When we observe any "body", whether it be the Sun, another star, a galaxy, a planet or the Moon, we see it as it was when the light or radio waves, with which we make the observation, left that body. In the case of the Sun, Moon and planets, this time difference is insignificant in comparison to the age of the Universe. Travelling at 186,000 miles second, it takes light, or any other form of electromagnetic radiation, less than a second and a half to reach us from the Moon and the journey time from the Sun is still only 8 minutes.
With distant galaxies, however, the story is very different. These are so far away, that it can take light millions of years to reach us. Indeed, in recent research projects, observations have been made of galaxies and clusters of galaxies at distances from Earth of billions of light-years (a light-year is a unit of distance equivalent to that travelled by light in one year). In effect, therefore, the observer is looking back in time to when the Universe was billions of years younger than it is today. It is possible to see galaxies, clusters of galaxies, gas clouds and dust in their distant past and, it is observed, behaving rather differently, often more violently, than equivalent objects which are closer to us and therefore of a more recent time frame.
Observing objects at greater and greater distances and, by inference, further and further back in time, we build a picture of how the Universe has expanded and developed over the billions of years (currently estimated between 12 and 15) since the Big Bang. As research continues, the picture gradually builds up and cosmologists are trying to fill in the missing pieces with the ultimate goal of understanding the mechanism of the Big Bang and how this resulted in the Universe that we now observe.
Much of the picture has come from decades of thoroughly prepared, organised and funded research with specific objectives based upon theories but perhaps the most fortunate and, at the time, most significant of discoveries was chanced upon, by accident, in the course of an unrelated project.
In 1964, at Bell Labs in New Jersey, U.S.A., two researchers, named Arno Penzias and Robert Wilson, were using a large horn antenna to investigate possible sources of electromagnetic "noise" or interference that might affect communications with artificial satellites. They were surprised to find that, regardless of the part of the sky at which the antenna was pointed, they detected the same level of signal. This was a "microwave" signal which is a form of radio wave with very short wavelength.
Initially, they thought this may be due to equipment malfunction and even suspected the influence of pigeon droppings within the horn antenna but, with the help of a team from nearby Princeton, they became convinced that it was a true phenomena of the Universe.
The radiation being received was at a constant intensity or "temperature" of approximately 3.5 degrees Kelvin. The concept of "temperature", when measuring the intensity of radiation, is important because it is a measure of the energy or violence of its source. It had previously been predicted that the violence, or temperature, of the Big Bang would have caused intense radiation and that, after a period of 12 to 15 billion years, this would gradually have reduced from billions of degrees K to less than 10 degrees K. The radiation that Penzias and Wilson had detected was, indeed, the remnants of the energy of the Big Bang, otherwise known as the Cosmic Microwave Background Radiation.
This microwave background possessed two very significant properties. Firstly, it was isotropic – the same in all directions – as has already been mentioned but, as important, it was found to be "black-body" in nature. A "black-body" is a theoretical object which is considered to be a perfect absorber and emitter of radiation and one in which the wavelength of the radiation depends only on its temperature. At any given temperature, there will be a relationship between wavelength and energy with a peak at one characteristic wavelength. The higher the temperature, the shorter will be that peak wavelength.
Experiments subsequent to the Penzias and Wilson discovery have shown a very close correlation to the theoretical "black-body" characteristic, better in fact than has ever been reproduced in artificial laboratory conditions. This close fit has been most significant in convincing scientists that this is truly the result of a natural phenomena in an untainted environment and our first direct measurement of the Big Bang.
The detection and measurement of the microwave background has been a very significant advance in our ability to look back in time towards the origins of the Universe and has whetted the appetite of astronomers, cosmologists and physicicts anxious to understand the detailed processes of the Big Bang and the way in which the Universe took on its present form. It can be shown mathematically that the structures that we now observe, the clusters and super clusters of galaxies, must have been determined at a very early stage in the evolutionary process, in fact within the first few fractions of a second of the Big Bang.
Although the Universe can be considered uniform or homgeneous to within a very small percentage in all directions, this small percentage does allow for the existence of clusters and super clusters of galaxies. In the Big Bang therefore, there must have been corresponding minute variations in the atomic and sub-atomic processes in which energy was converted into matter. It follows, therefore, that, if the microwave background is a remnant of the energy of the Big Bang, it might be reasonable to expect similarly minute variations of the temperature of the microwave background that we observe today.
In the twenty or so years following the Penzias and Wilson discovery, measurement of the background radiation continued with increasing sensitivity and accuracy, initially from ground based observatories and later with the use of microwave receivers carried aloft by balloons in order to reduce the effects of the Earth’s atmosphere. In this way, it was possible to arrive at a more precise figure for the actual temperature of the background radiation of just under 3 degrees Kelvin.
For more detail, however, it would be necessary to observe from outside the Earth’s atmosphere altogether and, in 1989, NASA launched the Cosmic Background Explorer satellite (COBE) carrying extremely sensitive microwave receivers. The task was to map the entire sky, making high resolution measurements of the background radiation temperature and its spectrum.
The first results, announced in 1992, certainly provided convincing evidence of the minute variations in radiation temperature that had been predicted. These variations, or "wrinkles", are of the order of one part in 100,000 and are the present day remnants of the energy which was present when the structure of the Universe was determined. These data represented the first look back in time to the birth of the clusters and super clusters of galaxies that make up the present Universe and is considered one of the most important pieces of evidence for the Big Bang.
Not only were the "wrinkles" observed but the spectrum was seen to be a perfect fit to the theory of black-body radiation. The temperature was also measured more accurately than ever before at 2.73 degrees Kelvin.
The COBE satellite continued taking data for another four years, building up more detail of its initial findings but, in the light of its fundamental revelations, other experiments have been designed to probe further into the microwave background. By choosing different wavelengths of observation and using current state-of-the-art receivers, it became possible for ground based microwave observatories to map the finer detail of the variations in background temperature.
The Cosmic Anisotropy Telescope (CAT) at Cambridge has improved the resolution of observations by an order of magnitude over COBE. Designed to measure the small amounts by which the background radiation is not isotropic, it has detected features of just 0.25 of a degree of arc whereas the earlier COBE results were much more "blurred", with a resolution of 10 degrees. The result is a vast improvement in our knowledge of the structure of the early Universe. We are therefore able to look back in time in much more detail.
The technology of the CAT has been developed further in a new telescope, designed by the same team and due to go into service at the top of a mountain in Tenerife in 2000. This is the VSA (Very Small Array) project, working at shorter wavelengths than COBE or CAT and intended to provide even finer detail of the early structures. It will map the microwave background radiation and hopefully provide observers with more insights into the formation of the Universe.
Although it is believed that the structure of the Universe was determined by irregularities in the very early atomic processes of the Big Bang, and possibly within the first 3 minutes, our observations of the cosmic microwave background do not take us back quite that far. For the first half a million years or so, it is thought that the Universe would have been opaque, rather like the Sun. Not until its temperature had cooled to around 3000 degrees would the radiation be free to travel out through space unimpeded from what is described as the "last scattering surface". It is this surface that represents the limits of how far back in time we can directly observe using the microwave background.
Half a million years is a very small time span compared to the 12-15 billion year life of the Universe and there is left a significant proportion of this history to study but physicists believe that perhaps the most fundamental relationships of energy and matter are to be understood only in the very early stages of the Big Bang, certainly within the first three minutes and possibly within the first few microseconds. There is, at present, no way to make direct observations of this period. Our ability to look back in time appears to have its limits.
Wrinkles in Time, George Smoot & Keay Davidson, Little, Brown & Co. Images of the Universe, Ed. C. Stott;The First One Second of the Universe, Paul Davies, Cambridge University Press.
Oxford Dictionary of Astronomy, I. Ridpath, Oxford University Press.