A Brief History of Astronomy

Cosmology

(From 1930)


1930

Abbé Lemaître and George Gamow explain the observation of the expanding Universe by postulating that it began in a huge explosion. This is colourfully known as the Big Bang Theory.

Lemaître suggests that all the matter in the Universe was once contained in a very dense "cosmic egg". This object exploded and the matter was spread out through space. We see the effects of this explosion when we observe the galaxies moving away from each other.

Gamow predicts that the echo of the explosion should be detectable as radiation with a temperature of about 5 degrees above Absolute Zero. This radiation should permeate throughout the Universe. It would not be detected for over 30 years.

Using Hubble's Law and working backwards, they estimate that the age of the Universe is 2 thousand million years. This figure is smaller than the age of the Earth as calculated by geologists.

Alexander Friedman uses Einstein's equations of General Relativity to work out that there are two possible ends to the Big Bang Universe.

If the amount of matter in the Universe is above a certain critical level, then the expansion of the Universe would eventually slow down and stop. The Universe would then contract with all the galaxies and stars moving towards each other until they were back in a small area. This is known as the Big Crunch.

If the amount of matter in the Universe is below the critical level, then the expansion would continue forever. Eventually the Universe would expand so much that galaxies would not be visible to each other. Cold, dark, and isolated embers would be all that was left of the galaxies.

To distinguish between these two scenarios requires a knowledge of how quickly the Universe is expanding compared to how much matter it contains. This problem would not be solved for 70 years.

1932

Karl Jansky discovers radio waves from space. He finds that these signals come from the centre of the Galaxy, its position agreeing with Shapley's. Radio waves open a new window to the Universe; a window that is not affected by gas and dust.

This marks the birth of Radio Astronomy.

1935

Otto Struve proves that invisible interstellar dust and gas exists by finding a spectral line of Calcium.

He develops a new theory of planetary formation that is a normal part of stellar evolution rather than the rare stellar encounter of Jeans' model.

1938

Hans Bethe and Carl Weizsächer work out the details of how the Sun produces its energy. It is by nuclear fusion, converting Hydrogen to Helium. Every second over 3 million tonnes of the Sun's matter is converted into energy.

1942

Harold Jones measures the Astronomical Unit (the distance between the Earth and the Sun) to an accuracy of over 99.99%.

Walter Baade studies the stars in the Andromeda Galaxy. He discovers that there are two populations, each with different ages and chemical compositions. The Cepheids of each population have a slightly different Period-Luminosity Law. This discovery corrects the distances to the galaxies as measured by Hubble.

The distance to the Andromeda Galaxy is tripled to over 2 million Light Years.

These changes increase the age of the Universe to 6 thousand million years. This is longer than the geologists' estimate of the age of the Earth.

1948

Thomas Gold and Fred Hoyle suggest an alternative cosmology to explain the expanding Universe. The Steady State Theory describes a Universe essentially unchanging in space and time. As the Universe expands, new matter is created to fill in the gaps left. There was no Big Bang.

Nobody can suggest how this new matter arises. For the idea to work a few hundred atoms would need to be created per cubic kilometre every year.

1950

Martin Ryle finds radio emissions from the Andromeda Galaxy. He finds that many (but not all) galaxies give out radio waves. These radio galaxies tend to be more abundant amongst the further galaxies rather than those nearby. Looking at great distances implies looking at the past. This is the first hint that the Universe has changed with time. If so, the Steady State Theory could not be correct.

William Morgan studies the distribution of luminous hot blue stars in our galactic neighbourhood. He finds that they are arranged in parallel lines which mark out our Galaxy's spiral arms. The arm that includes the Sun is called the Local Arm. Away from the centre is the Perseus Arm. Closer to the centre is the Sagittarius Arm.

These observations are later confirmed by studying the distribution and motions of glowing nebulae. Using optical techniques, observations can only be made to a distance of about 10,000 Light Years. This is only one third of the distance to the centre of the Galaxy. The Galaxy contains dust and gas which block out light from the very distant stars.

Hendrik van de Hulst uses radio telescopes to map the positions of clouds of Hydrogen. This allows the Galaxy to be mapped over a larger area. He finds another spiral arm outside the Perseus. Radio waves travel through gas and dust better than light does.

1958

Allan Sandage calculates the age of the Universe by studying distances to nearby galaxies. His age is 13 thousand million years. This is older than the Earth and the Sun. It is not as old as the oldest Globular Clusters.

1961

Yuri Gagarin becomes the first human being to orbit the Earth.

1963

Maarten Schmidt studies a group of radio objects that appear to be stars. These "stars" are shown to have very large Red Shifts. This indicates that they are further than most galaxies. They are labelled as "quasi-stellar objects" (or, more commonly, Quasars).

Quasars are mysterious objects: highly luminous and very small. The nearest Quasar (called 3C273) is at a distance of 2 thousand million Light Years. This is over 800 times further than the Andromeda Galaxy. It shines with the luminosity of 100 normal galaxies! Its brightness varies in periods of about a month so it must be small compared to a galaxy. 3C273 has been estimated to have a diameter of over 750,000 million kilometres. This is a million times smaller than our Galaxy or 4800 times the distance between the Sun and the Earth.

No Quasars are found in the regions of space near our Galaxy. They are now considered to be very young and active galaxies.

Because light takes time to travel across space, Quasars show that the early Universe was different in the past. The Universe is therefore changing in time; it is an evolving Universe. This contradicts the Steady State Theory.

Arno Penzias and Robert Wilson discover a Universal Background Radiation coming from all directions equally. This is an effect of the Big Bang predicted by Gamow. The heat produced during the explosion should have cooled down to a temperature of a few degrees above Absolute Zero.

The new radiation indicates a temperature around 3 degrees above Absolute Zero. This phenomenon cannot be explained by the Steady State Theory.

The Big Bang Theory is now accepted by most scientists. Speculation begins about how the Universe will end. Will it expand forever or will it eventually contract back to nothingness? This depends on the amount of matter in the Universe.

1965

Roger Penrose shows that very massive stars could collapse in on themselves. In theory, they could form an object with a gravity so high that even light could not escape from them. These objects are called Black Holes and are dismissed by most scientists. Black Holes have the peculiar property of absorbing matter but never allowing any to escape. As the matter approaches the Black Holes, it would radiate huge amounts of energy as it becomes compressed by the gravitational forces.

At the centre of a Black Hole, there would be an object with an infinite density and zero size. This is called a Singularity. As bizarre as they sound, Singularities are not precluded by the General Theory of Relativity.

Stephen Hawking shows that if the Theory of Relativity is correct, then the Universe would have begun as a Singularity rather than as Lemaître's "cosmic egg". At the time of the Big Bang, the Singularity would have exploded and the Universe would have come into being. Space, time, and energy would have been created and would expanded together. The original state would have had an extremely high temperature. As the temperature dropped, matter would form out of the energy and eventually, stars and galaxies would have formed out of the matter.

1969

Neil Armstrong and Edwin Aldrin become the first human beings to step on another world, the Moon.

1973

Paul Richards accurately measures the spectrum of the Universal Background Radiation. He finds that it agrees with theoretical predictions for the Big Bang.

The abundances of various isotopes of certain elements within galaxies also agree with theoretical predictions for the Big Bang.

1975

Gustav Tammann refines the age of the Universe from Sandage's work. His figure of 18 thousand million years is older than the oldest known objects in the Universe. It would later be shown to be an over-estimate.

1977

R Brent Tully, J Richard Fisher and others develop several new distance yardsticks with which to measure the size (and age) of the Universe. These are described briefly below.

The luminosity of a spiral galaxy is related to the properties of a particular radio emission in its spectrum.

The apparent light smoothness of elliptical galaxies is related to their distance.

Distant galaxies that give off X-rays affect the Universal Background Radiation lying between them and us in a way dependent on the distance.

Distant Quasars passing close to a large galactic mass may have their light bent. This produces double or multiple images of the Quasar. There is a relation between the angle of the bending, the time between light variation of the Quasar to be repeated in the duplicate images, and the distance to the Quasar.

1979

Alan Guth studies the early history of the Universe in terms of particle physics. He suggests a reason why the Universal Background Radiation appears to be so uniform. This leads to the development of Inflationary Big Bang theories.

The idea is that the early expansion of the Universe was very rapid for a short while before settling down to the rate seen today. These theories explain several points in the Big Bang Theory. However, there is no observational evidence for them.

1980

Margaret Geller and others discover structure in the Universe. The galaxies are arranged in groups, clusters, clouds and superclusters.

Our galaxy is a member of a group (the Local Group) consisting of about 20 galaxies in a region that is 5 million Light Years in diameter. Our galaxy, The Andomeda Galaxy and a third (called M33) are all large spirals. The Andromeda galaxy is the dominant member of the group with 400 thousand million stars. Our galaxy and M33 contain about 100 thousand million stars.

The spirals have a number of satellite galaxies. The Andromeda Galaxy has two elliptical companions. Our Galaxy has five companions. Two are irregular galaxies called the Magellanic Clouds. These are visible in the Southern Hemisphere and resemble detached portions of the Milky Way. Three are small almost-spherical ellipticals hidden behind the Galactic centre. The rest of the galaxies of the group are small.

The Local Group is on the edge of a cloud of galaxies called the Coma-Sculptor Cloud. This is about 25 million Light Years across. This cloud is part of the Virgo Supercluster. This supercluster contains over 1000 galaxies that are mainly elliptical. The centre of the Virgo Supercluster is 60 million Light Years away from our Galaxy. Our Galaxy appears to be moving towards the centre of the Virgo Supercluster at a speed of 600 kilometers per second.

1983

Andrei Linde suggests that an Inflationary Universe would be perfectly balanced between its rate of expansion and the amount of matter it contains. Such a Universe would carry on expanding forever.

1992

From satellite observations, George Smoot finds temperature variations (of the order of 10-5 degrees) in the Universal Background Radiation. These "wrinkles" could explain why the Universe is clumpy with groups of galaxies rather than being perfectly smooth.

1995

The Hubble Space Telescope surveys the distant parts of the Universe. By doing so, it is looking into the past.

It is found that spiral and elliptical galaxies are generally stable and unchanging. Irregular galaxies are active and changing. Even when the Universe was only 30% of its current age, galaxies had already formed. It appears that star formation was more active when the Universe was only 50% of its current age.

1996

Bruno Leibundgut observes a time delay in the way distant supernovas decay. This is another verification that the Universe is expanding.

Carlos Frenk simulates the early history of the Universe on a supercomputer to try and reproduce the wrinkled structure of the Universe discovered by Smoot. The results only work if the expansion of the Universe increases with time.

1998

J Richard Gott finds that Clusters and Superclusters of galaxies are linked to form filaments. They form "walls" or "sheets" up to 1,000 million Light Years long and enclosing enormous voids. The Universe on the large scale has the appearance of a sponge.

The Universe resembles fractals produced by mathematical Chaos Theory. This has led to speculation that this structure may have been caused by random quantum fluctuations during the very early phase of the Universe.

Saul Perlmutter and his team complete a study of Supernovae (exploding stars) in other galaxies. The luminosity of these stars can be calculated by studying the way their brightness fades. The study looks at stars out to a distance of 7 thousand million Light Years. The results indicate that the expansion of the Universe is increasing.

Brian Schmidt confirms that the expansion of the Universe was 15% greater when the Universe was half its current age. There is speculation of a repulsive force present on the large scale. This leads to ideas about the existance of dark energy.

2000

Observations of the variation of temperature in the Universe indicate that it will expand for ever (i.e it is flat and open).

2001

A type of sub-atomic particle, the Nutrino, is shown to oscilate between three types and to have mass. This mass, although small, could have an effect on the large scale structure and evolution of the Universe.

New ideas, called M Theory, may explain the origin of the Big Bang as the collision of 11 dimensional spaces.


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External Links

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