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In the 1920s Edwin Hubble showed that, on the largest scales, all the galaxies were moving away from each other. It is simple to deduce from this that at some time in the past they must have been much closer together than they are now. Cosmology is the study of the origin and development of the Universe and the currently most popular theory is that of the Big Bang.
This theorises that at about 13,000,000,000 years ago all the matter and space that make up the Universe were concentrated into a very small volume. The theory states that the Universe came into being as an extremely small volume full of energy, which gave the Universe a very high temperature. As the Universe expanded so the fundamental atomic particles were formed as a mixture dominated by hydrogen with some helium and almost nothing else.
Some of the greatest current problems in astrophysics arise from consideration of how the galaxies formed, and what is the nature of the mass of the Universe (we can only identify 10 percent of what must be there!).
The study of the early Universe is possible due to the finite speed of light. As we look at galaxies many millions of light years away we see them as they were when the light left them -- many millions of years ago. These remote objects are, of course, faint and hence astronomers always want to use larger telescopes and more efficient detectors so that they can measure further back in time.
Evidence for the Big Bang
Until well into this century astronomers did not know that the Milky Way was a galaxy and that the ‘island universes’ seen through large telescopes were galaxies, systems of many, many stars grouped together like the Milky Way.
The fundamental discovery which demonstrated this was made by Hubble. He showed, from spectra of the galaxies, that there was an increase in the velocity of recession with distance. The deduction from this is that space is expanding and it was soon appreciated that the Milky Way was one of a very great number of galaxies and that it, like the Sun, had no special place in the system of galaxies.
This theory received a boost in 1965 when radiation at 3 degrees K, the microwave background radiation, was discovered coming from all directions in space. This radiation was predicted to be a remnant from the very early time in the age of the Universe, before matter had been formed, when the Universe was still filled with hot radiation. The radiation was isotropic and it corresponded to a temperature that was consistent with red-shifted radiation from the Big Bang.
From the observation of galaxies using optical wavelengths it was not possible to find evolutionary effects and so the hypothesis that the Universe was in a steady state was a plausible one. In 1997 the Hubble Space Telescope (HST) obtained very deep exposures of galaxies at enormous distances from the Milky Way. Objects at this distance were seen as they were when the Universe was only 10% of its current age. The ‘Hubble Deep Field’ images demonstrated a clear difference in the appearance of galaxies in the early Universe from those seen today – definitive evidence for evolutionary change.
Predictions from the Big Bang
The theoretical analysis of the Big Bang has had various successes in predicting properties of the resulting universe. The biggest of these is the prediction of the relative abundances of the elements and their isotopic ratios. When the oldest stars, whose material has been altered least by the accumulation of material processed in the centres of earlier generations of stars, are investigated it has been shown that their abundance ratios are in excellent agreement with those predicted.
The expansion of the Universe from the Big Bang is strongly dependent on the mass of the Universe. There is one critical value which would mean that the Universe will expand for a long time, gradually slowing down and then reach a steady state. A mass less than this value will mean that the Universe will go on expanding for ever while a greater value will mean that the Universe will expand to a maximum size and then will start to contract – eventually returning to a very small volume. Astronomers think that the mass of the Universe is equal to this critical value but can only `see' one tenth of the matter necessary to reach this value. The same discrepancy is seen in the gravitational pull of individual galaxies and in clusters of galaxies. The mass appears to be there but we cannot identify it. This is called the `missing mass problem'.
Results from the COBE satellite and the BOOMERANG experiment
NASA’s Cosmic Background Explorer (COBE) satellite was developed by the Goddard Space Flight Centre to measure the infrared and microwave radiation from the early Universe that is difficult to detect from the surface of the Earth. It was launched on a Delta rocket from the Vandenburg Air Force Base at Pt. Arguello, California on November 18th, 1989.
COBE carried three primary instruments that could detect any electromagnetic radiation with wavelengths between 1 and 10,000 microns (1 micron="1 millionth" of 1 m). The COBE science team began by using the Diffuse Infrared Background Experiment (DIRBE) to scan half the entire sky once a week, over a 10-month period from December 1989 to September 1990. It found a key prediction of the Big Bang theory – detecting minute irregularities in the CMBR for the first time. Astronomers then modelled and subtracted the infrared glow from the foreground objects in our Solar System, stars in our Galaxy and the vast clouds of cold dust in interstellar space.
The other two instruments were the Far Infrared Absolute Spectrophotometer (FIRAS) which compared the cosmic background radiation to an accurate blackbody source and observed the dust and light emission from the Galaxy; and the Differential Microwave Radiometer (DMR), which looked for differences in temperature between regions of the CMBR.
COBE first discovered the predicted small variations in the temperature of the microwave radiation indicating inhomogeneity at a very early time in the age of the Universe. These inhomogeneities eventually developed into the stars and galaxies we see today.
BOOMERANG (Balloon Observations Of Milli-metric Extragalactic Radiation And Geophysics) is an instrument designed to measure anisotropies in the Cosmic Microwave Background Radiation (CMBR). To do this Boomerang contains an array of detectors cooled to 0.28 degrees above absolute zero (0.28 K). This is mounted at the focus of a 1.3m aperture telescope attached to a gondola hanging from a NASA high altitude balloon. This flies at an altitude of 35 km above the Antarctic. UK astronomers from the Universities of Oxford and Cardiff as well as Queen Mary and Westfield College are involved in this project.
The BOOMERANG images are the first to bring the CMBR into sharp focus. The images reveal hundreds of complex regions that are visible as tiny variations - typically only 100-millionths of a degree (0.0001°C) - in the temperature of the CMBR. The complex patterns visible in the images confirm predictions of the patterns that would result from sound waves racing through the early Universe, creating the structures that have now evolved into giant clusters and super-clusters of galaxies.
This experiment has also allowed the geometry of the Universe to be determined for the first time. Computer simulations predict the angular size of the hotter and colder spots in the CMBR. If the Universe has a flat geometry then the spots should be around 1 degree across whereas the bending of light resulting from a curvature in space would distort this size. A closed Universe would cause parallel rays of light to converge, magnifying the spots. Conversely an open Universe would cause the rays of light to diverge, making the spots appear smaller. Results from BOOMERANG indicate that the Universe is flat.
Future missions: the Microwave Anisotropy Probe (MAP) and Planck missions
The MAP satellite is designed to probe conditions in the early Universe. MAP will measure temperature differences (anisotropy) in the Cosmic Microwave Background Radiation (CMBR) in an effort to explore how and when structures of galaxies formed in the Universe.
To explore this MAP will produce an accurate full sky map of the cosmic microwave background temperature with high sensitivity and angular resolution.
The temperature fluctuations in the CMBR are minute: one part of the sky may have a temperature of 2.7281 Kelvin (K or degrees above absolute zero), while another part of the sky has a temperature of 2.7279 K. NASA’s Cosmic Background Explorer (COBE) satellite detected these tiny fluctuations on large angular scales. MAP will measure anisotropy with much higher resolution and sensitivity than COBE did.
These measurements should reveal details of the primordial structures that grew to form galaxies and will test ideas about their origins.
MAP is due to be launched in April 2001 and will eventually operate at a stable point in space, the L2 Lagrangian point. This is about 1.5 million km away from the Earth, heading directly away from the Sun. At this position MAP will not be significantly affected by the radiation emitted by the Earth itself.
Planck is a European Space Agency (ESA) mission due to be launched in 2007. It will extend the study of the CMBR temperature fluctuations.
The 'Shape' of the Universe
One of the hardest concepts to accept is that the Universe is everything that is. Not only the matter and energy but all the dimensions as well. There is no 'outside' to the Universe and it has no 'edge'.
When we think of the Big Bang we instinctively think of the small Universe expanding like a sphere into an empty void. Unfortunately this is incorrect. The dimensions that we commonly use, three spatial and one time, are all mixed up when the early Universe is concerned and our normal concepts of space and time are not valid.
The only way that it can be partly understood is to consider the two-dimensional analogue of the surface of a balloon, which is being inflated. The surface is everywhere continuous, has no edge and yet is expanding. The three-dimensional analogue (whose understanding defeats the writer!) will represent the Universe.
The COBE datasets were developed by the NASA Goddard Space Flight Center under the guidance of the COBE Science Working Group and were provided by the NSSDC.