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The evolving Universe
The evolving Universe

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1.9 Structure in the Universe

Time: 1010s to 4.2 × 1017 s (300 years to 14 billion years)

Temperature: 105 K to 2.73 K

Energy: 30 eV to 7 × 10−4eV

As the Universe cooled still further, nothing much happened for a few hundred years (between 1000 s and 1010 s). As the mean energy per particle fell below a few tens of electronvolts, so electrons began to combine with nuclei to form neutral atoms.

Gradually, as this electrically neutral matter accumulated, gravity began to take over as the dominant force operating in the Universe. Slight variations in the amount of matter and radiation in different regions meant that matter began to gather together into slightly denser clumps. These clumps provided the seeds from which galaxies later grew.

By the time the Universe had cooled to a temperature of 3000 K, about 300 000 years after the Big Bang, the mean energy of the photons had fallen to about 1 eV, and most of the matter in the Universe was in the form of neutral atoms. This was the trigger for another significant change in the behaviour of the Universe. The background radiation photons — those 109 photons for every particle left over from the annihilation epoch (matter and antimatter reactions) — interacted for the last time with matter in the Universe. When hydrogen atoms are in their ground state, photons with an energy of at least 10 eV are required in order to excite them to even the next energy level. So from this point on in the history of the Universe, photons were no longer absorbed by matter. After this time, the cosmic background radiation simply expanded freely with the Universe, cooling as it did so.

When the cosmic microwave background radiation is observed today, very slight irregularities are observed in its temperature and intensity. These reflect slight differences in the matter distribution of the Universe at the time of the last interaction between the background photons and atoms. At the time of the discovery of these irregularities by the COBE satellite, they were described as 'wrinkles in the fabric of space-time' (Figure 6).

Figure 6
Figure 6

The colour-coded map in Figure 6 (a) shows departures from uniformity in the cosmic microwave background radiation over the whole sky. The two panels correspond to two 'halves' of the sky, projected onto a flat picture. The scale represents the temperature either side of the mean temperature of 2.73 K. Violet regions are slightly cooler than the mean value of 2.73 K, red regions are slightly hotter, by about 100 microkelvin (μK). Most of the variations seen are believed to represent localized variations in the density of matter at a time 300 000 years after the Big Bang when this radiation interacted with matter for the last time. This map is the final result after four years operation of the COBE satellite. The map in Figure 6 (b), produced by the Wilkinson Microwave Anisotropy Probe (WMAP) satellite, was released in 2003. It shows the whole sky. Ripples in the temperature of the microwave background are seen here on much finer scales than was possible with COBE.

Some time after the last interaction of matter and radiation, but probably before the Universe was a billion years old, the first galaxies formed. The exact time for this event is uncertain, but within these early galaxies, stars condensed out of the gas to become dense enough for nuclear reactions to start within their cores. Deep within these stars, hydrogen was converted into helium, releasing energy as electromagnetic radiation into the Universe. As stars age, so their cores contract and grow hotter, allowing helium fusion to occur. These further reactions produce heavier nuclei, such as carbon, oxygen and silicon. Low-mass stars, like the Sun, will end their cosmic chemistry here. They will eventually simply run out of nuclear fuel, and their cores will collapse to form dense, compact objects called white dwarfs.

The more massive the star though, the hotter its interior, and the more massive the elements that can be produced by nuclear fusion reactions. But there is a limit to how far nuclear fusion can go. When four protons are converted into a nucleus of helium-4, the products have a lower mass than the reactants. This mass difference is liberated as energy. Similar mass reductions apply for reactions to produce all the elements up to those with mass numbers in the range of about 56 to 62, such as iron, cobalt and nickel. However, for nuclear fusion reactions beyond this, more energy must be put into the reactions than is released from them, so these are not viable.

As the most massive stars approach the ends of their lives, when their cores are composed of nuclei that undergo no more fusion, some nuclear reactions within their interiors release free neutrons. These neutrons can then add, slowly one at a time, to the iron, cobalt or nickel nuclei to make even more massive elements. As more and more neutrons are added, some transform into protons, via beta-minus decay, and in this way massive (stable) nuclei up to lead and bismuth can be created. These are the most massive, non-radioactive nuclei that exist in the Universe.

But what happens to these massive stars? When the core is largely composed of iron, they have no further source of energy available. The outer layers fall inwards, squeezing the centre of the star down until it has a density comparable to that of an atomic nucleus. The collapse halts — suddenly — and the material rebounds, setting off a shock wave back through the outer layers of the star. The result is a supernova explosion, in which 90% of the star's mass is thrown violently out into space (see Figure 7). The star's core left behind will be revealed as a neutron star, or the ultimate compact object, a black hole.

In the final moments of its life, the star has one final surprise left. The immense temperatures and pressures created during the explosion cause electrons and protons to react to form huge numbers of free neutrons. These neutrons enable elements to be built beyond the lead and bismuth limit. All naturally radioactive elements in the Universe (apart from those which are the decay products of even more massive radioactive nuclei) were formed in such supernovae explosions, and a large proportion of the others between nickel and bismuth were also created in these violent events.

From here the star cycle repeats - but this time with a slight difference. Stars that formed after the first generation had lived and died had a richer source of raw material. A star like the Sun was formed in a galaxy that had already seen at least one generation of massive stars born, live and die in supernovae explosions. The gas and dust from which the Sun formed, about 5 billion years ago, had therefore been enriched by heavier elements produced inside these earlier stars. This leads to the possibility of the formation of planets from the rubble left behind.

Figure 7
Figure 7 The Crab nebula, a supernova remnant in the constellation of Taurus. This expanding cloud of gas was thrown off in a supernova explosion when a massive star reached the end of its life. The cloud seen here is about three parsecs across. The exploding star was seen by Chinese astronomers on 4 July 1054, and was so bright that it remained visible in full daylight for 23 days.

The Earth itself formed from such debris. Every nucleus of carbon, oxygen, nitrogen and silicon found on the Earth and within living creatures was created inside the heart of an ancient star. Every nucleus of precious metal such as silver, gold and platinum was formed either from slow neutron capture in ageing stars, or by rapid neutron capture during the supernova explosions that mark their death. And so we come full circle back to the present day, about 14 billion years after the Big Bang, when the Universe has cooled to only 2.73 K.

Homing in on a fairly average spiral galaxy, we find a fairly average star somewhere out in one its spiral arms. Orbiting this star is a small rocky planet, two-thirds covered with water, and with an atmosphere rich in oxygen. On the surface of the planet are many living creatures, including members of one species who are so interested in the origin and complexity of the Universe that they build telescopes and particle accelerators with which to study it. They observe the expansion of the Universe by the redshift of distant galaxies, and the cooling of the Universe by the spectrum of its background radiation. Using particle accelerators they recreate extreme temperatures and examine particle reactions that have not occurred in the Universe for billions of years. The revelations of such experiments confirm that no epoch or location in the Universe is subject to any special dispensation. That at all times and all places the same physical principles hold, yet manifest themselves in a gloriously evolving diversity.