Reconstructing the past

The laws of physics can be used to predict how a system will develop in the future. Equally, they can be used to reconstruct the past. By observing the universe as it is now, one can form models of what it was like in the distant past. This is a fascinating exercise, an epic detective story based on clues we observe around us.

In the middle of the 20th century, there were two conflicting theories on the origin of the universe.

5.4.1 The steady state cosmological model

Originally put forward by James Jeans (1877-1946) in about 1920, and revised in 1948 by Thomas Gold (1920-) and Hermann Bondi (1919-) and later by Fred Hoyle (1915-2001), this model assumed that the universe is and has always been effectively homogeneous in space and time. There was no beginning, there will be no end, the universe remains in equilibrium, always essentially the same. One of the predictions of this model is that matter is being continuously created out of the vacuum. The creation is extremely slow, of the order of one hydrogen atom per cubic metre every 1010 years; nevertheless we are continuously getting something out of nothing, and such a model must violate the principle of conservation of energy.

5.4.2 The big bang' theory

This cosmological model, somewhat controversial at the time, incorporated the expansion of the universe into its framework. If the universe is expanding now, there is no reason why it should not have been expanding in the past. Its expansion might have been slower before, because of the gravitational attraction of matter in the then smaller and denser universe, but the expansion was always there.

George Gamow (1904-1968) and his colleagues reversed the expansion mathematically and found that, projecting back about 15 billion years, the universe started as a point infinitely dense, and infinitely hot.

Presumably, at that first instant the universe was born, and exploded as a 'big bang'. This implies an instant of creation, beyond the laws of physics, and is one of the reasons why the theory was controversial. At the same time, it is difficult to see why continuous creation, as depicted in the steady state model, should be any easier to accept.

Assuming that the laws of physics immediately after the big bang were the same as they are now, the first moments can be reconstructed, right back to a fraction of a second. Thus, in the first one hundredth of a second, the temperature was about 1011 °C (one hundred thousand million degrees Celsius). Over the first 3 minutes the universe cooled down rapidly to one thousand million degrees (109 °C). Fundamental particles such as electrons, positrons, neutrinos, and later protons and neutrons, were in violent motion. Pairs of each were continually being created out of energy, and quickly annihilated. Nuclear forces produced pairs of particles of opposite charge, and electric charges in motion gave rise to electromagnetic waves. Eventually the universe was filled with light.

The study of what happened during the very early history of the universe is a most exciting intellectual adventure. Mathematical models based on the laws of physics can be developed, and to some extent tested at high energy particle accelerators. Another approach is to search for evidence of 'cosmological relics' of these early moments.

5.4.3 A blast from the past

In 1960, the Bell Telephone laboratory built a 'horn' antenna on a hill in Holmdel New Jersey, to be used for communication with the Echo satellite. As it turned out, within a couple of years the Telstar satellite was launched and the Echo system became obsolete. Arno Penzias (1933- ) and Robert Wilson (1936- ), who had joined Bell Labs as radio astronomers, had had their eye on the horn and jumped at the opportunity to use it as a radio telescope when it became free to use for pure research.

As soon as they began to use the antenna, they noticed a troublesome background of 'microwave noise' which, no matter how they tried, they were unable to eliminate. The background had a wavelength of 7.35 cm, far shorter than the communication band, and had therefore not interfered with the Echo

system. They pointed the horn in the direction of New York City — it was not urban interference. They even removed the pigeons which were nesting in the horn, but the noise persisted. There was no seasonal variation. The only conclusion they could come Echo horn antenna. Courtesy of NASA. to was that it was a genuine electromagnetic signal, which appeared to be coming from every direction.

Unaware of these 'technical' problems in New Jersey, P.J. Peebles (1935-), a Princeton theorist, gave a talk at Johns Hopkins University in the spring of 1965. In his lecture, he presented work by Robert Dicke (1916-1997) and himself, which predicted the existence of electromagnetic radiation left over from the early universe, still present as a 'cosmological relic' of the big bang. Such radiation was necessary to carry off energy from nuclear interactions during the formation of matter in the first few minutes. The theory even predicted that as the universe expanded, the wavelength of the radiation would become longer and would by now be in the microwave region.

Since 1965, the 'troublesome' radiation which Penzias and

Wilson had been trying to eliminate has been studied and documented by radio astronomers. Little doubt remains that it is precisely the radiation predicted by Peebles, a genuine relic from the very distant past. Penzias and Wilson had found gold — a signal coming from the first moments of the universe. They were rewarded with the Nobel Prize for Physics in 1978.

5.5 The life and death of a star

The central core of a star is its 'boiler-house'. In a typical star, such as our sun, hydrogen nuclei are driven together to form helium in thermonuclear reactions which liberate energy. As a result the core exerts pressure outwards, and there is a delicate balance between that pressure and gravitational forces which tend to collapse the star into itself.

5.5.1 White dwarfs

Subrahmanyan Chandrasekhar (1910-1995), an Indian astrophysicist born in Lahore, was one of the first to combine the laws of quantum mechanics with the classical laws of gravitation and thermodynamics, in a physical model of the evolution of a star. During a voyage from India to England he developed the basics of a theory of the 'death' of a star, and in particular the dependence of the process on the original size of the star.

As the hydrogen 'fuel' becomes used up, the pressure within the 'boiler-house' drops, and the balance is upset. The centre, now mostly helium, is compressed further by gravitational forces until its density becomes far greater than the density of the heaviest material found on the earth.

Further nuclear fusion of helium into carbon and heavier elements takes place, producing more energy and more outward pressure, but eventually the forces of gravitation win and the star collapses. During the collapse, gravitational potential energy is converted into radiation and heat. Material outside the core is blasted off. If the mass of the star is below 1.44 times the mass of the sun (the Chandrasekhar limit), the star dies relatively gently. It becomes a white dwarf and fades away over billions of years. Such a fate eventually awaits our sun, which will finally end up as a cold dark sphere.

A sugar cube of white dwarf material, if brought to the earth, would weigh about 5 tons.

5.5.2 Supernovae

If the mass of the star is greater than the Chandrasekhar limit, a much more violent death awaits it. The end, when it comes, comes dramatically. The collapse becomes sudden and the core shrinks until the electrons are forced to combine with the protons present in the nuclei of the atoms. In the core of the star gravitation scores a victory over both electric and nuclear forces. The rest of the star is ejected at speed into space. The star becomes a supernova, expelling as much energy in a matter of weeks as it had previously radiated in its lifetime.

Only the core of the star remains, made up of matter entirely composed of neutrons: a neutron star. Its density is equal to the density of an atomic nucleus, about 1017 kg/m3. A sugar cube of neutron star material would weigh about 100 million tons. To put this into perspective, the heaviest tank to see action in World War II was the German King Tiger 2, weighing 70 tons. The weight of the sugar cube of concentrated neutrons would equal the weight of over a million

Tiger tanks — in fact, more than the combined armoured vehicles of all armies participating in wars in the 20th century!

Five such supernova explosions have been recorded in our galaxy in the last 1000 years. The first dates back to 4 July 1054:

'In the first year of the period of Chih-ho, the fifth moon, the day Chi-ch'ou, a guest star, appeared south-east of T'ien-kuan. After more than a year it gradually became invisible.' (Chinese manuscript of the Sung Dynasty.)

We now know that what the Chinese saw was the death and not the birth of a star.

Supernovae in other galaxies

With the aid of modern telescopes, several hundred supernova explosions are now being observed in distant galaxies every

Table 5.2 Supernovae in our galaxy.

Year Supernova

1054 Chinese 1151

1172 Tycho

1604 Kepler

1667 Cassiopei (deduced from its remnant)

year. Most of these are so faint that they can be studied only with difficulty.

In 1987, a supernova was observed in the Large Magellanic Cloud. Named SN1987a, it marked the death of the star Sanduleak-69 202, with a mass approximately 20 times that of the sun. The first observation was made on the night of 23 February 1987 in Chile and, as the earth rotated, there were further observations in New Zealand, Australia and southern Africa.

The picture on the right was taken by the Hubble Space Telescope in 1991. It shows a gaseous ring around SN1987, which became the progenitor star several thousand years before the supernova explosion. The light from the far edge of the ring arrived at the earth nearly one year after the arrival of light from the forward edge. This gives a very accurate value of the physical diameter of the ring, which, when compared to its angular diameter of 1.66 arc seconds, enabled astronomers to calculate the distance from the earth to the Large Magellanic Cloud as 169,000 light years (to within 5%). As the explosive expansion continues over the next few years, the envelope will become more transparent, enabling astronomers to carry out the most detailed study to date of such a remnant.

Ring of gas around SN187a. Courtesy of NASA/ESA.

5.5.3 Pulsars

Neutron stars do not 'shine' like other stars, emitting a steady stream of light. They emit electromagnetic waves, which come in characteristic pulses, and hence neutron stars are often called 'pulsating radio stars', or pulsars.

All spinning objects have angular momentum and stars are no exception.

angular momentum remains the same.

A pulsar acts like a rapidly spinning magnet emitting a rotating beam of electromagnetic radiation like a 'lighthouse in the sky'. Its mass is made from the core of the original star and is greater than 1.44 times the mass of the Sun, but its radius has shrunk to about 10 km. If the earth happens to be in the line of the light beam, we will see a source pulsating every time the beam sweeps past.

The first evidence of pulsars came in 1967 at the 4.5-acre radio telescope array in Cambridge, England. Jocelyn Jocelyn Bell Burnell Bell Burnell (1943- ), a postgraduate

c

----

---A/

/ electromagnetic

x / beam

/ mf /

Sfcv^ magnetic axis

___axis of rotation

Figure 5.7 Lighthouse in the sky.

Figure 5.7 Lighthouse in the sky.

student born in Belfast, Northern Ireland, and working under the supervision of Antony Hewish (1924- ), discovered a source of radio waves which came in bursts 1.3 seconds apart. The pulses were regular, like a signal — perhaps from another planet?

The source was given a provisional name, LGM1 ('Little Green Man 1'), and kept under quiet surveillance. Within 1 month, a second source was discovered, by which time it had become 100% clear that it was not extraterrestrial intelligence but a neutron star behaving like a rapidly spinning magnet.

An extract from Jocelyn Bell's original chart records is reproduced as shown on the right. The bottom trace marks out one-second time pips and so LGM1 is pulsing every one and a third seconds. The signal is weak and often drops below the detection threshold but it is still in phase (on-beat) when it reappears.

The 1974, Nobel Prize for Physics was awarded to Anthony Hewish for the discovery of pulsars', and Martin Ryle (1918-1984) for his pioneering work in radio-astronomy'. Due credit must also be given to Jocelyn Bell, who built the radio telescope and was the first to notice the signals.

The remnant of the Chinese supernova was discovered in 1963. It is situated in the Crab Nebula 5500 light years away. Calculations tracing back its expansion showed that it should have been observable from the earth in the year 1054. The actual explosion occurred 5500 years earlier, about 4500 BC.

Courtesy of Jocelyn Bell Burnell.

5.5.4 Black holes

Game, set and match to gravity!

A neutron star is not necessarily 'the end of the line' in the life and death of a star. Gravitational forces are still at work, compressing matter more and more tightly. The more concentrated the mass, the greater the inward pull. If the original star was big enough, the resulting neutron star is merely an intermediate stage, itself unstable under gravity. Chandrasekhar calculated that if the original star had a mass in excess of about 3 solar masses, the resulting 'electron degenerate matter' (neutron star) can no longer exist. It will collapse even further into itself — the ultimate sacrifice to gravity. With so much mass concentrated in such a small volume, the force of gravity dominates, and nothing can stop it. The further the collapse proceeds, the greater the gravitational force and the density becomes infinite. The known laws of physics no longer apply. We reach a window in our universe to another world beyond our own, with which we cannot communicate. Matter can be sucked in but nothing can get out, not even light. We have a black hole.

Robert Oppenheimer (1904-1967), perhaps better known for his later work on the Manhattan Project, was the first to apply the theory of general relativity to what happens when a massive star collapses. The end result is much the same as predicted by classical Newtonian mechanics, but the mechanism is more difficult to visualise. The curvature of space-time (which has four dimensions and is described in Chapter 15) increases dramatically. This is not easy to visualise, but a useful two-dimensional analogy might be to push a knitting needle against a flat membrane. The membrane becomes more and more distorted and, in the end, it is punctured. The two-Robert Oppenheimer dimensional space ceases to exist.

A black hole is a singularity in space-time. Space and time, as we know them, cease to exist. The rules of physics no longer apply.

Oppenheimer's work has become a rich source for theoretical research by many physicists and mathematicians. Stephen Hawking, Roger Penrose, John A. Wheeler and, of course, Chandrasekhar are some of the more prominent names associated with the subject.

More than 200 years ago, Pierre Simon Laplace (1749-1827) postulated:

A luminous star of the same density as the Earth, whose diameter be 250 times larger than that of the Sun, would not, in consequence of its gravitational attraction, allow any of its rays to arrive at us. It is therefore possible that the largest luminous bodies in the Universe may, through this cause, be invisible.'

5.5.5 Escape velocities

How to get away from a large mass

The first obstacle faced by prospective space travellers is the gravitational attraction of the earth. The energy to overcome the gravitational force can be provided in the form of combustible rocket fuel. Alternatively, one could imagine building up speed

Table 5.3 Some escape velocities.

To escape from:

Speed (km/s)

Speed (mph)

Earth

11.2

25,000

Moon

2.4

5,400

Jupiter

58

130,000

Sun

620

1.4 million

Neutron star

- 150,000 - 0.5c

3.4 x 108

Black hole

> c

horizontally and then flipping the rocket upwards and away from the earth, turning off the motors and using kinetic energy to overcome gravitation. The speed required to do that is called the escape velocity. Table 5.3 gives some relevant values of escape velocities.

(Appendix 5.2 shows how to calculate the escape velocity for planets such as the earth.)

5.5.6 How to 'see9 the invisible

Black holes are no longer in the realm of science fiction. Even though light cannot escape from inside a black hole, we can detect its existence from what is happening around it. Matter close to a black hole gets sucked in and spirals around it like water in a plug hole. As electric charges spiral, they emit electromagnetic waves — a 'last gasp' signal back to the universe. X-rays and other radiation are emitted, not from the black hole but from the accretion disc of matter spiralling into it.

Data reported in February 2000 from the Chandra X-ray observatory show strong evidence of the existence of a supermassive massive black hole, Sagittarius A*, near the centre of the

Artist's impression of a massive black hole accreting matter. Courtesy of NASA/JPL-Caltech.

Milky Way. According to recent estimates, this black hole is about 3.7 million times more massive than the sun and is very compact, with a radius of about 45 AU, at most. It is generally believed that most galaxies harbour supermassive black holes at their centres.

5.5.7 A strange event in the Milky Way

In 2002, V838, a star in the Monoceros constellation, 20,000 light years from the sun, suddenly brightened. Its maximum luminosity was about 1 million times the luminosity of the sun and it was one of the brightest stars in the Milky Way at that time. Figure 5.8 shows two images of Monoceros, the first taken

. *

"V •.

DSS2 ■ May 1989. Anglo-Australian Observatory

V838 Mon - March 2002 US Naval Observatory

Figure 5.8 V838 explodes. Courtesy of NASA, USNO, AAO and Z. Levy (STScI).

Figure 5.8 V838 explodes. Courtesy of NASA, USNO, AAO and Z. Levy (STScI).

Figure 5.9 Evolution of an explosion. Courtesy of NASA, Hubble Heritage Team (AURA/STScI) and ESA.

in 1989, when the star appeared quite 'normal', and the second in 2002, during the outburst.

The sequence of events was recorded by the Hubble Space Telescope and is illustrated in Figure 5.9. An interesting result of the examination of the spectrum of the emitted radiation is that it shows a strong enrichment of Li, Al, Mg and other elements. This leads to the hypothesis that these elements came from planets which had been orbiting the star and were vaporised by the explosion. In about 5 billion years our own solar system may meet a similar fate!

5.5.8 Time stands still

There are other extraordinary properties predicted for black holes. According to the general theory of relativity, time slows down in a strong gravitational field. In the extreme environment of a black hole, we must conclude that time will stand still relative to us. In an 'experiment of the mind', let us imagine a space traveller approaching a black hole. To us, his last movements will appear to be very slow. His pulse rate slows down, and his heart beats once every hundred years. Eventually he 'freezes in time'. His final image appears for ever, as he was, just before being sucked in!

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