ASTRONOMY ( HERCHEL)

The content of the universe: Cosmology in 2001

Dr Rodney Hillier on 7 April 2001

Dr Hillier delivered one of his inimitable talks in the style well known by many who have attended astronomical courses at the BRLSI before. He reviewed our present understanding of the nature of the Universe, explaining the results obtained by the recent BOOMERang observations made using far infra-red detectors on balloons over Antarctica and the `2dF' and `Type I Supernova' projects. The notes that accompanied the talk follow:

Cosmological models of the Universe are based on General Relativity

As the Universe expands the wavelength of radiation travelling in it is stretched by the same factor. This is the cosmological red shift. The expansion of space implies that the Universe has a finite age (~14 billion years)

Observations of the distant Universe:

If the light from a distant galaxy left when the Universe was 8 billion years old, the `look-back time' is 6 billion years and the red shift tells us how much the Universe has expanded in that time. The light from the galaxy has been spreading out into space for 6 billion years. Hence, from the apparent brightness of a standard star in the galaxy, we can calculate the `look-back' time. The relationship between red shift and apparent brightness of a standard star is one test of a cosmological model.

The finite age of the Universe and the speed of light set a fundamental limit to the distance we can see - even in an infinite Universe. We can only see as far back to when the Universe was 1/3 million years old - when the first atoms formed. Before this `re-combination era', the Universe was opaque. Radiation from this period is still present in the Universe as the Cosmic Background Radiation.

The effects of mass in the Universe:

1. It curves space-time. According to General Relativity, with no mass, space-time would be negatively curved (a saddle is a 3-dimensional analogue). The gravity of a lot of mass (high density) curves space-time positively (3-D analogue is a sphere). There is a critical density where the positive curvature of gravity cancels out the tendency for space-time to be negatively curved and produces a flat space-time.

2. Its gravity decelerates the expansion of the universe. A critically dense (i.e. flat) Universe will slow to zero in an infinite time

To measure the curvature of space-time, we need to determine its geometrical properties. In negatively curved space-time, the apparent (i.e. angular) size of an object of known size at a known distance is less than what we would expect if space-time were flat. In positively curved space, it is greater

Structure at the recombination era:

Sound waves in the plasma produced variations in density and temperature. These show up as fluctuations in the cosmic background radiation when we look in different directions. Calculations show that, because of resonance effects there will be standing waves and certain wavelengths will be stronger than others and which wavelengths will be dependent on the density of matter at that time. The BOOMERang experiment, using far infra-red detectors on balloons over Antarctica last year showed that the variations are strongest on scales of approximately 1°, which implies that matter in the Universe is at the critical density and therefore the Universe is flat.

The mass content of the Universe:

1. Normal matter is composed of protons, neutrons and electrons, which form atoms at low temperatures. We observe it through its interaction with radiation and most of it is in the stars and galactic and intergalactic gas. We have determined the density of this type of matter to constitute only 4% of the critical density.

2. Radiation contributes to the mass density through E = mc². The Cosmic background Radiation, which dominated the early Universe, constitutes only 0.03% of the critical density.

3. Dark matter has been invoked to explain the stability of individual galaxies and clusters of galaxies whose internal motions are inconsistent with the amount of normal matter (which we can observe by its interaction with radiation). Dark matter can only be inferred from its gravitational effects. Current theories suspect dark matter to be mainly undiscovered fundamental particles, which are electrically neutral and not found in atoms. Such particles are called `weakly interacting massive particles' or WIMPs. More dark matter seems to be required on the scale of clusters of galaxies than on the scale of individual galaxies.

The large-scale structure of the present Universe:

On the largest scale, the universe has a sponge-like structure, galaxies and clusters of galaxies forming huge filaments (like the soapy water in foam) surrounding vast voids. The gravity associated with the mass concentration (both normal and dark matter) in the filaments give rise to local velocities of the galaxies whose red-shifts are therefore a combination of cosmological red shifts and local Doppler effects. The recent `2dF project' has measured the red shifts of 140,000 galaxies in 2°x2° fields on the sky. Results published in March 2001 show that, from an analysis of the local velocities of the galaxies, the mass in the filaments (both normal and dark matter) amounts to approximately 30% of the critical density.

The expansion rate of the Universe in the past:

Using observations of very distant galaxies with long look-back times, we can measure the expansion rate of the early Universe but to do this we need `standard' stars with very high luminosities. Type I supernova result from the explosion of a white-dwarf star in a close binary system. The white dwarf accretes material from its companion and as soon as its mass reaches the Chandrasekhar limit (~1.4 solar masses) it becomes unstable, resulting in an explosion which releases a standard amount of energy. Such supernovae can be identified from the shape of their light-curves. They occur in a typical galaxy approximately once every 100 years.

The Type I Supernova project:

Several thousand galaxies with look-back times ranging from 2 billion to 7 billion years were monitored and 42 Type I supernovae had been detected by 1999. Measurements of the apparent brightness gave the look-back time and the red shift gave the expansion factor of the Universe since the event occurred. The gravitational effect of matter in the Universe should be a slowing of the expansion rate but the measurements showed that it was speeding up! This implied that there is a repulsive `cosmological force' which is opposing the effects of gravity and the measurements showed that the energy associated with this force contributes approximately 70% of the critical density.

Richard Phillips