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Universe History


Universe Explanation ::-- Universe Facts || Universe History ||Universe geography || Bing Bang ||


Throughout recorded history, several cosmologies and cosmogonies have been proposed to account for observations of the universe. The earliest quantitative geocentric models were developed by the ancient Greek philosophers. Over the centuries, more precise observations and improved theories of gravity led to Copernicus's heliocentric model and the Newtonian model of the Solar System, respectively. Further improvements in astronomy led to the realization that the Solar System is embedded in a galaxy composed of billions of stars, the Milky Way, and that other galaxies exist outside it, as far as astronomical instruments can reach. Careful studies of the distribution of these galaxies and their spectral lines have led to much of modern cosmology. Discovery of the red shift and cosmic microwave background radiation revealed that the universe is expanding and apparently had a beginning.

According to the prevailing scientific model of the universe, known as the Big Bang, the universe expanded from an extremely hot, dense phase called the Planck epoch, in which all the matter and energy of the observable universe was concentrated. Since the Planck epoch, the universe has been expanding to its present form, possibly with a brief period (less than 10−32 seconds) of cosmic inflation. Several independent experimental measurements support this theoretical expansion and, more generally, the Big Bang theory. Recent observations indicate that this expansion is accelerating because of dark energy, and that most of the matter in the universe may be in a form which cannot be detected by present instruments, and so is not accounted for in the present models of the universe; this has been named dark matter. The imprecision of current observations has hindered predictions of the ultimate fate of the universe.

Current interpretations of astronomical observations indicate that the age of the universe is 13.75 ± 0.17 billion years, and that the diameter of the observable universe is at least 93 billion light years or 8.80×1026 metres. According to general relativity, space can expand faster than the speed of light, although we can view only a small portion of the universe due to the limitation imposed by light speed. Since we cannot observe space beyond the limitations of light (or any electromagnetic radiation), it is uncertain whether the size of the universe is finite or infinite.



How Old is the Universe?

Until recently, astronomers estimated that the Big Bang occurred between 12 and 14 billion years ago. To put this in perspective, the Solar System is thought to be 4.5 billion years old and humans have existed as a genus for only a few million years. Astronomers estimate the age of the universe in two ways: 1) by looking for the oldest stars; and 2) by measuring the rate of expansion of the universe and extrapolating back to the Big Bang; just as crime detectives can trace the origin of a bullet from the holes in a wall.



In the beginning, the Earth was flat. At least it appeared so to its first observers, hunters and gatherers, and members of early civilisations. Not totally unreasonable, one would think, because the curvature of our planet's surface is not immediately apparent. Yet we know, and it must have been not totally inconceivable even to the archaic tribesmen, that our senses occasionally deceive us. The Earth being flat brings about the problem that it must end somewhere, unless we imagine it to extend infinitely. Infinity is a rather unfathomable conception and, hence, right down to the Middle Ages people were afraid of the possibility of falling off the Earth's boundaries.

Early cosmogonies.

What lies beyond these boundaries was largely unknown and open to speculation. The starry heavens were a source of endless wonder and inspiration. Peoples from all parts of the world created their own myths, inspired by the skies and the celestial bodies. Their cosmogonies can be seen as an attempt to explain their own place in the universe. Six thousand years ago, the Sumerians believed that the Earth is at the centre of the cosmos. This belief was later carried into the Babylonian and Greek civilisations.

According to the history books, it was the Greeks who first put forward the idea that our planet is a sphere. Around 340 BC, the Greek philosopher Aristotle made a few good points in favour of this theory in On the Heavens. First, he argued that one always sees the sails of a ship coming over the horizon first and only later its hull, which suggests that the surface of the ocean is curved. Second, he realised that the eclipses of the Moon were caused by the Earth casting its shadow on the moon. Obviously, the shadow would not always appear round, if the Earth was a flat disk, unless the Sun was directly under the centre of the disk. Third, from their travels to foreign countries, the Greeks knew that the North Star appears higher on the northern firmament and lower in the south. Aristotle explained this correctly with the parallactic shift that occurs when moving between two observation points on a spherical object. Among the Greeks, the heliocentric system was proposed by the Pythagoreans and by Aristarchus of Samos (ca. 270 BC). However, Aristotle dismissed the case for heliocentrism.

Big Bang - the birth of our universe.

Fast forward: Despite Kant's doubts thereto, it appears that modern cosmology has answered the above question. The universe we can observe is finite. It has a beginning in space and time, before which the concept of space and time has no meaning, because spacetime itself is a property of the universe. According to the Big Bang theory, the universe began about twelve to fifteen billion years ago in a violent explosion. For an incomprehensibly small fraction of a second, the universe was an infinitely dense and infinitely hot fireball. A peculiar form of energy that we don't know yet, suddenly pushed out the fabric of spacetime in a process called "inflation", which lasted for only one millionth of a second. Thereafter, the universe continued to expand but not nearly as quickly. The process of phase transition formed out the most basic forces in nature: first gravity, then the strong nuclear force, followed by the weak nuclear and electromagnetic forces. After the first second, the universe was made up of fundamental energy and particles like quarks, electrons, photons, neutrinos and other less familiar particles.

About 3 seconds after the Big Bang, nucleosynthesis set in with protons and neutrons beginning to form the nuclei of simple elements, predominantly hydrogen and helium, yet for the first 100,000 years after the initial hot explosion there was no matter of the form we know today. Instead, radiation (light, X rays, and radio waves) dominated the early universe. Following the radiation era, atoms were formed by nuclei linking up with free electrons and thus matter slowly became dominant over energy. It took 200 million years until irregularities in the primordial gas began to form galaxies and early stars out of pockets of gas condensing by virtue of gravity. The Sun of our solar system was formed out of such a pocket of gas in a spiral arm of the Milky Way galaxy roughly five billion years ago. A vast disk of gas and debris swirling around the early Sun gave birth to the planets, including Earth, which is between 4.6 and 4.5 billion years old. This is -in short- the history of our universe according to the Big Bang theory, which constitutes today's most widely accepted cosmological viewpoint.

Will the universe expand forever?

On basis of our understanding of the past and present universe, we can speculate about its future. The prime question is whether gravitational attraction between galaxies will one day slow the expansion and ultimately force the universe into contraction, or whether it will continue to expand and cool forever. The current rate of expansion (Hubble Constant) and the average density of the universe determine whether the gravitational force is strong enough to halt expansion. The density required to halt expansion (=critical density) is 1.1 * 10^-26 kg per cubic meter, or six hydrogen atoms per cubic meter; the relation "actual density" / "critical density" is called Omega. With Omega less than 1, the universe is called "open", i.e. forever expanding. If Omega is greater than 1 the universe is called "closed", which means that it will contract and eventually collapse in a Big Crunch. In the unlikely event that Omega = 1, the expansion of the universe will asymptotically slow down until it becomes virtually imperceptible, but it won't collapse.

Big Rip!

Today, most cosmologists believe there is not enough matter in the universe to halt and revert expansion. Robert Caldwell of Dartmouth University has recently suggested a third alternative for the fate of the universe. His Big Rip scenario is based on astronomical observations made in the late 1990s according to which a mysterious force, labelled dark energy, is responsible for the expansion of the universe. Dark energy makes up 73% of the universe. If the rate of acceleration increases, there will be a point in time at which the repulsive force becomes so strong that it overwhelms gravity and the other fundamental forces. According to Caldwell, this will happen in 20 billion years. "The expansion becomes so fast that it literally rips apart all bound objects," Caldwell explains. "It rips apart clusters of galaxies. It rips apart stars. It rips apart planets and solar systems. And it eventually rips apart all matter." Even atoms would be torn apart in the last 10-19 seconds before the end of time. –Whether or not this scenario will become true is to be decided by future research. Until then, the field is open to speculation.

 

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