Published on 24/09/2012
How do astronomers observe the beginning of the universe: what wavelengths do we use, and what instruments are best for these measurements?
The early universe is of great interest to astronomers and cosmologists. It is also a popular topic amongst the general public, as we like to know where we came from. Astronomy has typically relied on light, that is electromagnetic radiation, for observing the universe. Electromagnetic radiation comes in a range of wavelengths, or equivalently frequencies, of which familiar visible light is just a small part. The spectrum goes from gamma rays at the highest energies, smallest wavelengths and highest frequencies, through X-rays, ultraviolet light, visible light and infrared to radio waves at the lowest energies, longest wavelengths and lowest frequencies.
The most useful for observing the early universe are microwaves (part of the spectrum of radio waves). We can measure the cosmic microwave background (CMB), which is the oldest light still in existence. Before this was emitted, the universe was effectively opaque: light couldn't travel freely because it was so hot and dense. Our cosmological models predict that the light that makes up the CMB was emitted about 400,000 years after the Big Bang. So we can't quite see the beginning of the universe, but it is close!
The CMB has been observed using many instruments. It was first discovered using radio receivers on the ground by American astronomers Penzias and Wilson in 1964. They would win the 1978 Nobel Prize for this. Lots of exciting science has been done more recently following a number of space-based missions. There was COBE launched in 1989 (the team won the 2006 Nobel Prize), then WMAP launched in 2001. The most recent mission is Planck, from which the data is still to be released (due January 2013). This will produce the most detailed map of the CMB sky yet.
While the space based missions are best for entire sky surveys, it is possible to measure parts of the sky from Earth. There are a number of ground-based and even balloon-based detectors. One telescope, the Arcminute Microkelvin Imager (AMI) is based here in Cambridge.
The CMB is currently our best tool, but it may be possible in the future to see further back in time. For this we would need to shift from using electromagnetic waves to gravitational waves. Gravitational waves are predicted by Einstein's theory of general relativity, they are tiny ripples in spacetime. We know they exist from careful observations of binary pulsars (for which Hulse and Taylor won the 1993 Nobel Prize). However, we have still to detect them directly.
We have ground-based detectors (LIGO in America, and Virgo in Europe), that are currently being upgraded. Japan is also in the process of building a detector, KAGRA. It is hoped will make the first direct detection in the next few years. However, these are unlikely to see gravitational waves from the early universe - just like electromagnetic radiation, gravitational radiation comes in a range of frequencies. We might be able to detect the gravitational background using pulsar timing arrays. Radio telescopes watch the signals emitted from a network of pulsars across the sky and look for correlations in deviations from their regular signals. The difficulty with these measurements, apart from having to make very careful timings, is that the frequencies are so low, you have to wait for years to see an entire wave. It is hoped that the planned Square kilometre Array will make these measurements easier.
Another possibility would be a space-based detector. This would be sensitive to frequencies between pulsar timing arrays and the ground-based detectors. Amongst many other sources, it could detect a gravitational wave background. Unfortunately, there is no funded space based detector mission at the moment. There may be eventually a detector based upon the design of the Laser Interferometer Space Antenna (LISA). The European Space Agency is launching LISA Pathfinder in 2014 to test the technology needed, and hopefully a full mission shall follow that.