[History of Astronomy]
[Astro History: Asian and Greek]
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A Brief History of Astronomy
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.
This marks the birth of Radio Astronomy.
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.
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.
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.
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.
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.
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.
The abundances of various isotopes of certain elements within galaxies also agree with theoretical predictions for the Big Bang.
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.
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.
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.
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.
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.
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.
New ideas, called M Theory, may explain the origin of the Big Bang as the collision of 11 dimensional spaces.
History of Science A large collection of resources looking at the history of astronomy, physics, chemistry and mathematics.