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Cosmology

Royal Greenwich Observatory
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Information Leaflet No. 32: 'Cosmology'

The Universe:

From a study of the motions of the galaxies, it can be deduced that they are all moving away from one another. 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 20,000 million 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, 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, very faint, and that is why astronomers are always wanting 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 in the Milky Way.

Hubble made the fundamental discovery which demonstrated this.
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.

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. With the advent of the large radio telescopes it was found that there were far more faint radio galaxies than one would expect in a steady-state universe. In fact it was shown that it was likely that all the galaxies originated in a very small volume -- the Big Bang.

This theory received a boost 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.

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 abundance 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.

There are problems, however, with the theory. One of these is that the very isotropic nature of the microwave background indicates that the early stages of the universe were completely uniform. When we look at the universe today, we see non-uniformities at every level. We ourselves are examples of aggregates of mass, as are stars, galaxies, and the groupings of galaxies into clusters and strings. The puzzle of how this non-isotropic nature could result from an isotropic early universe was, to some extent, relieved by the discovery by the COBE satellite, that there are small variations in the temperature of the micro-wave radiation, indicating some inhomogeneity at a very early time in the age of the universe.

The expansion of the universe from the Big Bang is strongly dependent on the mass of the universe. There is one critical value that would mean that the universe will expand for a long time, gradually slowing down, and then reaching 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 we 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 can not identify it. This is called the 'missing mass problem'.

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 that 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.

Produced by the Information Services Department of the Royal Greenwich Observatory.

PJA Wed Apr 17 13:14:13 GMT 1996

webman@mail.ast.cam.ac.uk

ARVAL Notes:
Currently (2010) we could estimate the age of the universe to about 1%: 13.7 ± 0.13 billion years.
See WMAP - Age of the Universe (NASA)

In 2013 we are estimating the age of the universe to be about 13.8 billion years.
See Planck Mission Brings Universe Into Sharp Focus (ESA-NASA) in ARVAL

Updated: August 23 '97, July 19 '10, June 26 '14

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