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Archaeological remains show that the first group of modern people to live in the British Isles were hunter-gatherers after the last ice age ended. The date is not known: perhaps as early as 8000 but certainly by 5000. They built mesolithic wood and stone monuments. Stonehenge was built between 3000 and 1600. Celtic tribes arrived from mainland Europe. Britain was a changing collection of tribal areas, with no overall leader. Julius Caesar tried to invade (take over) the island in 55 but was not able to do so. The Romans successfully invaded in 43.
Written history began in Britain when writing was brought to Britain by the Romans. Rome ruled in Britain from 44 to 410. They ruled the southern two-thirds of Great Britain. The Romans never took over Ireland and never fully controlled Caledonia, the land north of the valleys of the River Forth and River Clyde. Their northern border varied from time to time, and was marked sometimes at Hadrian's Wall (in modern England), sometimes at the Antonine Wall (in modern Scotland).
After the Romans, waves of immigrants came to Britain. Some were German tribes: the Angles, Saxons and Jutes. Others were Celts, like the Scoti, who came to Great Britain from Ireland. English and Scots are Germanic languages. They developed from Old English, the language spoken by the Anglo-Saxons of Anglo-Saxon England, an area stretching from the River Forth to the River Tamar.
A later wave of immigration was that of the Vikings, during the Early Middle Ages' Viking Age. During the Viking invasion of Britain, they set up their own kingdom in north-western England, which the Anglo-Saxons named the "Danelaw", after the Danes who lived there and controlled the land. Vikings from Scandinavia also controlled most of the islands which are now part of Scotland, including the Outer Hebrides, the Inner Hebrides, and the Northern Isles (the Shetland Islands and the Orkney Islands).After a long period when Anglo-Saxon England was split into various kingdoms, it was made into one kingdom by Æthelstan (Athelstan) in 945. In the 13th century, the lands of Wales were unified by force with England by the wars of Edward I of England ("Edward Longshanks").
There were hundreds of years of fighting between both kingdoms of Great Britain. In 1603, when Queen Elizabeth I of England died, her closest relative was King James VI of Scotland. He became king of England and Ireland as well as king of Scotland. The kingdoms of England, Ireland, and Scotland had the same monarch ever since. James VI and I was the first to be named "King of Great Britain", and he ordered the design of the Union Jack. The Union Jack has been the British national flag ever since.
In 1707, the Parliaments of England and Scotland agreed the Treaty of Union, which joined the two countries into one country called the "United Kingdom of Great Britain" under Queen Anne with the Acts of Union 1707. This union merged Scotland and England into one kingdom. England and Scotland kept their own laws, with English law in England and Wales and Scots law in Scotland. The division between the Church of Scotland and the Church of England continued. Ireland and Great Britain continued to have the same king, but Ireland did not become part of the new kingdom in 1707.
Scotland and England had already independently had much influence over Ireland since 1200. In 1800 laws were passed in the parliaments of Great Britain and Ireland to merge the two kingdoms and their two parliaments. The country was then called the "United Kingdom of Great Britain and Ireland". The Union Jack was changed so that the flag of Saint Patrick (a red saltire) shows Ireland to be a part of the country.
In 1922 much of Ireland became independent from the United Kingdom as the Irish Free State (now called Ireland). However, six northern counties (called Northern Ireland) are part of the United Kingdom. The country was renamed the "United Kingdom of Great Britain and Northern Ireland" in 1927.
The new Parliament of Northern Ireland set up in the 1920s stopped working in the 1970s, because of The Troubles. However, devolution started again with the Northern Ireland Assembly after the Belfast Agreement (the "Good Friday Agreement") in 1998. Devolution in Scotland and Wales started the Scottish Parliament and the Welsh Parliament the same year.
The UK is north-west off the coast of mainland Europe. Around the UK are the North Sea, the English Channel and the Atlantic Ocean. The UK also rules, usually indirectly, a number of smaller places (mostly islands) around the world, which are known as British Overseas Territories. They were once part of the British Empire. Examples are Gibraltar (on the Iberian Peninsula next to the Strait of Gibraltar) and the Falkland Islands (in the south Atlantic Ocean).
In the British Isles, the UK is made up of four different countries: Wales, England and Scotland and Northern Ireland. The capital city of Wales is Cardiff. The capital city of England is London. The capital city of Scotland is Edinburgh and the capital city of Northern Ireland is Belfast. Other large cities in the UK are Birmingham, Bristol, Manchester, Liverpool, Newcastle upon Tyne, Leeds, Sheffield, Glasgow, Southampton, Leicester, Coventry, Bradford and Nottingham.
The physical geography of the UK varies greatly. England consists of mostly lowland terrain, with upland or mountainous terrain only found north-west of the River Tees-River Exe line. The upland areas include the Lake District, the Pennines, the North York Moors, Exmoor, and Dartmoor. The lowland areas are typically traversed by ranges of low hills, frequently composed of chalk, and flat plains. Scotland is the most mountainous country in the UK and its physical geography is distinguished by the Highland Boundary Fault which goes across the Scottish mainland from Helensburgh to Stonehaven. The Royal Observatory, Greenwich is the is the defining point of the Prime Meridian.
The weather of the United Kingdom is changeable and unpredictable. Summers are moderately warm, winters are cool to cold. Rain falls throughout the year, and more on the west than the east because of its northerly latitude and the warm water from the Atlantic Ocean's Gulf Stream. The usually moderate prevailing winds from the Atlantic may be interrupted by Arctic air from the north-east or hot air from the Sahara.
The United Kingdom is reducing greenhouse gas emissions. It has met some Kyoto Protocol targets. It has signed the Paris Agreement. The British government want the UK to be carbon neutral by the year 2050.
The United Kingdom is a parliamentary democracy based on a constitutional monarchy. The people of the United Kingdom vote for a members of Parliament to speak for them and to make laws for them. Queen Elizabeth II is the Queen of the United Kingdom of Great Britain and Northern Ireland and is the head of state. The government, led by the Prime Minister, governs the country and appoints cabinet ministers. Today, the Prime Minister is Boris Johnson, who is the leader of the centre-right Conservative Party.
Parliament is where laws are made. It has three parts: the House of Commons, the House of Lords, and the Queen. The House of Commons is the most powerful part. It is where Members of Parliament sit. The Prime Minister sits here as well, because they are a Member of Parliament.
Scotland has its own devolved Parliament with power to make laws on things like education, health and Scottish law. Northern Ireland and Wales have their own devolved legislatures which have some powers but less than the Scottish parliament. The Parliament of the United Kingdom is sovereign and it could end the devolved administrations at any time. The UK is a unitary state and not a federation of states.
The Parliament of the United Kingdom is the legislature, the political assembly that makes laws and decides tax. The British people are represented by members of parliament (MPs) in the House of Commons of the United Kingdom. MPs are chosen in elections. The MPs in the House of Commons decide who will be the Prime Minister of the United Kingdom. The prime minister decides who will be in the British Government (Her Majesty's Government). The government is not controlled by the king or queen, but by Parliament. In Britain, Parliament is made up of the House of Commons and the House of Lords.
Unlike the House of Commons, the people in the House of Lords are not elected. The people who sit in the House of Lords are called peers. Most peers are appointed by the government. There are some who are hereditary peers (who inherit their peerages from ancestors or other family members). Certain bishops in the established Church of England also attend the House of Lords. (The Church of England is the national church in England. The Church of Scotland does not have bishops, and neither Wales nor Northern Ireland have an established national church.) Together, the two houses make a bicameral legislature, in which the House of Commons has more power. In the past, the House of Lords had more power. Before the 20th century, the prime minister was often a member of the House of Lords. As the House of Lords lost its powers, as political reforms tried to improve democracy, the House of Commons became more powerful and the prime minister started always to be a member of the House of Commons.
After the English Civil War during the Wars of the Three Kingdoms, Oliver Cromwell became Lord Protector, and the monarchy ended for a time. The British Isles were a republic, which Cromwell named the "Commonwealth of England, Scotland, and Ireland". Although the monarchy was restored after his death, the Crown slowly became the secondary power, and Parliament the first. Until the early twentieth century, only men who owned property could vote to choose MPs. In the nineteenth century, more people were given suffrage. In 1928, all men and women got the vote: this is called universal suffrage.
Almost all members of Parliament belong to political parties. The biggest parties are the Conservative Party, Labour Party, the Scottish National Party and the Liberal Democrats. Members of the same party agree to work together. A party with more than half the seats (a majority) forms the government. The leader of the party becomes the prime minister, who then chooses the other ministers. Because the government has a majority in Parliament, it can normally control what laws are passed.
The British parliament is in Westminster, in London, but it has power over the whole of the United Kingdom. Wales, Scotland and Northern Ireland each have their own parliaments as well, and these have more limited powers. (England does not have a separate parliament.) Scotland has the Scottish Parliament at Holyrood in Edinburgh. Wales has the Welsh Parliament in Cardiff. Northern Ireland has the Northern Ireland Assembly at Stormont in Belfast. There are also parliaments in the Isle of Man and in Jersey and Guernsey (the Channel Islands), which are all island states for which the UK has some responsibility in international law. Man, Jersey, and Guernsey are "crown dependencies". Some British Overseas Territories have their own legislatures.
The United Kingdom has one of the most advanced militaries in the world, alongside such countries such as the USA and France, and operates a large navy (Royal Navy), a sizable army, (British Army) and an air force (Royal Air Force).
From the 18th century to the early 20th century, the United Kingdom was one of the most powerful nations in the world, with a large and powerful navy (due to the fact it was surrounded by sea, so a large navy was the most practical option). This status has faded in recent times, but it remains a member of various military groups such as the UN Security Council and NATO. It is also still seen as a great military power.
The United Kingdom is a developed country with the sixth largest economy in the world. It was a superpower during the 18th, 19th and early 20th century and was considered since the early 1800s to be the most powerful and influential nation in the world, in politics, economics (For it was the wealthiest country at the time.) and in military strength.
Britain continued to be the biggest manufacturing economy in the world until 1908 and the largest economy until the 1920s. The economic cost of two world wars and the decline of the British Empire in the 1950s and 1960s reduced its leading role in global affairs. The United Kingdom has strong economic, cultural, military and political influence and is a nuclear power. The United Kingdom holds a permanent seat on the United Nations Security Council, and is a member of the G8, NATO, World Trade Organization and the Commonwealth of Nations. The City of London, in the capital, is famous as being the largest centre of finance in the world.
William Shakespeare was an English playwright. He wrote plays in the late 16th century. Some of his plays were "Romeo and Juliet" and "Macbeth". In the 19th century, Jane Austen and Charles Dickens were novelists. Twentieth century writers include the science fiction novelist H. G. Wells and J. R. R. Tolkien. The children's fantasy "Harry Potter" series was written by J. K. Rowling. Aldous Huxley was also from the United Kingdom.
English language literature is written by authors from many countries. Eight people from the United Kingdom have won the Nobel Prize in Literature. Seamus Heaney is a writer who was born in Northern Ireland.
Arthur Conan Doyle from Scotland wrote the Sherlock Holmes detective novels. He was from Edinburgh. The poet Dylan Thomas brought Welsh culture to international attention.
The nature of education is a devolved matter in Scotland, Wales and Northern Ireland. They, and England, have separate, but similar, systems of education. They all have laws that a broad education is required from ages five to eighteen, except for in Scotland where school departure is allowed from the age of sixteen. Pupils attend state funded schools (academy schools, faith schools, grammar schools, city technology colleges, studio schools) and other children attend independent schools (known as public schools).
There have been universities in Britain since the Middle Ages. The "ancient universities" started in this time and in the Renaissance. They are: the University of Oxford, the University of Cambridge, the University of St Andrews, the University of Glasgow, the University of Aberdeen, and the University of Edinburgh. These are the oldest universities in the English-speaking world.
The University of Cambridge, the University of Oxford, and London universities (University College London, the London School of Economics, King's College London and Imperial College London) collectively form the Golden Triangle of universities in the south east of England. A broader group of twenty universities form the Russell Group of research universities.
The BBC is an organisation in the United Kingdom. It broadcasts in the United Kingdom and other countries on television, radio and the Internet. The BBC also sells its programmes to other broadcasting companies around world. The organisation is run by a group of twelve governors who have been given the job by the Queen, on the advice of government ministers.
Road traffic in the United Kingdom drives on the left hand side of the road (unlike the Americas and most of Europe), and the driver steers from the right hand side of the vehicle. The road network on the island of Great Britain is extensive, with most local and rural roads having evolved from Roman and Medieval times. Major routes developed in the mid 20th Century were made to the needs of the motor car. The high speed motorway (freeway) network was mostly constructed in the 1960s and 1970s and links together major towns and cities.
The system of rail transport was invented in England and Wales, so the United Kingdom has the oldest railway network in the world. It was built mostly during the Victorian era. At the heart of the network are five long distance main lines which radiate from London to the major cities and secondary population centres with dense commuter networks within the regions. The newest part of the network connects London to the Channel Tunnel from St Pancras station. The British Rail network is part privatised, with privately owned train operating companies providing service along particular lines or regions, whilst the tracks, signals and stations are owned by a Government controlled company called Network Rail. In Northern Ireland the NI Railways is the national railway. The system of underground railways in London, known as the Tube, has been copied by many other cities.
Most domestic air travel in the United Kingdom is between London and the major cities in Scotland and the North of England and Belfast. London-Heathrow is the nation’s largest airport and is one of the most important international hubs in the world. Other major airports with principal international service include London-Gatwick, Birmingham, Manchester and Glasgow. An extensive system of ferry networks operates.
Major languages spoken in the United Kingdom other than English include Polish (500,000 approximate number of speakers in the United Kingdom), Eastern Panjabi or Punjabi (471,000), Bengali (400,000), Urdu (400,000), Cantonese (300,000), Greek (200,000), Southwestern Caribbean Creole English (170,000).
Native languages include:
Universe
The Universe is all of time and space and its contents. It is made of many millions of billions of stars and planets and enormous clouds of gas separated by a big space.
Astronomers can use telescopes to look at very distant galaxies. This is how they see what the Universe looked like a long time ago. This is because the light from distant parts of the Universe takes a very long time to reach us. From these observations, it seems the physical laws and constants of the Universe have not changed.
Physicists are currently unsure if anything existed before the Big Bang. They are also unsure whether the size of the Universe is infinite, meaning it’s size never ends as it has been expanding since the Big Bang.
People have long had ideas to explain the Universe. Most early models had the Earth at the centre of the Universe. Some ancient Greeks thought that the Universe has infinite space and has existed forever. They thought it had a set of celestial spheres which corresponded to the fixed stars, the Sun and various planets. The spheres circled about a round but unmoving Earth.
Over hundreds of years, better observations led to Copernicus's Sun-centred model. This was very controversial at the time, and was fought by religious authorities, most famously by the Christian church (see Giordano Bruno and Galileo).
The invention of the telescope in the Netherlands, 1608, was a very important moment in astronomy. By the middle of the 1800s, telescopes were good enough for other galaxies to be seen. The modern optical (uses visible light) telescope is still more advanced. Meanwhile, Isaac Newton improved the ideas of gravity and dynamics (equations) and showed how the Solar System worked.
In the 1900s, even better telescopes led astronomers to realize that the Solar System is in a galaxy made of billions of stars, which we call the Milky Way. They also realized that other galaxies exist outside it, as far as we can see. This started a new kind of astronomy called cosmology, in which astronomers study what these galaxies are made of and how they are spread out through so they can learn more about the history of the Universe and how it works. By measuring the redshift of galaxies, cosmologists soon discovered that the Universe is expanding (see: Hubble).
The most used scientific model of the Universe is known as the Big Bang theory, which says the Universe expanded from a single point that held all the matter and energy of the Universe. There are many kinds of scientific evidence that support the Big Bang idea. Astronomers think that the Big Bang happened about 13.73 billion years ago, making the Universe 13.73 billion years old. Since then, the universe has expanded to be at least 93 billion light years, or 8.80 metres, in diameter. It is still expanding right now, and the expansion is getting faster.
However, astronomers are still not sure what is causing the universe to expand. Because of this, astronomers call the mysterious energy causing the expansion dark energy. By studying the expansion of the Universe, astronomers have also realized most of the matter in the Universe may be in a form which cannot be observed by any scientific equipment we have. This matter has been named dark matter. Just to be clear, dark matter and energy have not been observed directly (that is why they are called 'dark'). However, many astronomers think they must exist, because many astronomical observations would be hard to explain if they didn't.
Some parts of the universe are expanding even faster than the speed of light. This means the light will never be able to reach us here on Earth, so we will never be able to see these parts of the universe. We call the part of the universe we can see the observable universe.
The word "Universe" comes from the Old French word "Univers", which comes from the Latin word "universum". The Latin word was used by Cicero and later Latin authors in many of the same senses as the modern English word is used.
A different interpretation (way to interpret) of "unvorsum" is "everything rotated as one" or "everything rotated by one". This refers to an early Greek model of the Universe. In that model, all matter was in rotating spheres centered on the Earth; according to Aristotle, the rotation of the outermost sphere was responsible for the motion and change of everything within. It was natural for the Greeks to assume that the Earth was stationary and that the heavens rotated about the Earth, because careful astronomical and physical measurements (such as the Foucault pendulum) are required to prove otherwise.
The most common term for "Universe" among the ancient Greek philosophers from Pythagoras onwards was "το παν" (The All), defined as all matter ("το ολον") and all space ("το κενον").
The broadest word meaning of the Universe is found in "De divisione naturae" by the medieval philosopher Johannes Scotus Eriugena, who defined it as simply everything: everything that exists and everything that does not exist.
Time is not considered in Eriugena's definition; thus, his definition includes everything that exists, has existed and will exist, as well as everything that does not exist, has never existed and will never exist. This all-embracing definition was not adopted by most later philosophers, but something similar is in quantum physics.
Usually the Universe is thought to be everything that exists, has existed, and will exist. This definition says that the Universe is made of two elements: space and time, together known as space-time or the vacuum; and matter and different forms of energy and momentum occupying space-time. The two kinds of elements behave according to physical laws, in which we describe how the elements interact.
A similar definition of the term "universe" is everything that exists at a single moment of time, such as the present or the beginning of time, as in the sentence "The Universe was of size 0".
In Aristotle's book "The Physics", Aristotle divided το "παν" (everything) into three roughly analogous elements: "matter" (the stuff of which the Universe is made), "form" (the arrangement of that matter in space) and "change" (how matter is created, destroyed or altered in its properties, and similarly, how form is altered). Physical laws were the rules governing the properties of matter, form and their changes. Later philosophers such as Lucretius, Averroes, Avicenna and Baruch Spinoza altered or refined these divisions. For example, Averroes and Spinoza have "active" principles governing the Universe which act on "passive" elements.
It is possible to form space-times, each existing but not able to touch, move, or change (interact with each other. An easy way to think of this is a group of separate soap bubbles, in which people living on one soap bubble cannot interact with those on other soap bubbles. According to one common terminology, each "soap bubble" of space-time is denoted as a universe, whereas our particular space-time is denoted as "the Universe", just as we call our moon "the Moon". The entire collection of these separate space-times is denoted as the multiverse. In principle, the other unconnected universes may have different dimensionalities and topologies of space-time, different forms of matter and energy, and different physical laws and physical constants, although such possibilities are speculations.
According to a still-more-restrictive definition, the Universe is everything within our connected space-time that could have a chance to interact with us and vice versa.
According to the general idea of relativity, some regions of space may never interact with ours even in the lifetime of the Universe, due to the finite speed of light and the ongoing expansion of space. For example, radio messages sent from Earth may never reach some regions of space, even if the Universe would exist forever; space may expand faster than light can traverse it.
It is worth emphasizing that those distant regions of space are taken to exist and be part of reality as much as we are; yet we can never interact with them, even in principle. The spatial region within which we can affect and be affected is denoted as the "observable universe".
Strictly speaking, the observable Universe depends on the location of the observer. By travelling, an observer can come into contact with a greater region of space-time than an observer who remains still, so that the observable Universe for the former is larger than for the latter. Nevertheless, even the most rapid traveler may not be able to interact with all of space. Typically, the 'observable Universe' means the Universe seen from our vantage point in the Milky Way Galaxy.
The Universe is huge and possibly infinite in volume. The matter which can be seen is spread over a space at least 93 billion light years across. For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years. As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away. The observable Universe contains more than 2 trillion (10) galaxies and, overall, as many as an estimated stars (more stars than all the grains of sand on planet Earth).
Typical galaxies range from dwarf galaxies with as few as ten million (10) stars up to giants with one trillion (10) stars, all orbiting the galaxy's center of mass. Thus, a very rough estimate from these numbers would suggest there are around one sextillion (10) stars in the observable Universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion (7 x 10).
The matter that can be seen is spread throughout the Universe when averaged over distances longer than 300 million light-years. However, on smaller length-scales, matter is observed to form 'clumps', many atoms are condensed into stars, most stars into galaxies, most galaxies into galaxy groups and clusters and, lastly, the largest-scale structures such as the Great Wall of galaxies.
The present overall density of the Universe is very low, roughly 9.9 × 10 grams per cubic centimetre. This mass-energy appears to consist of 73% dark energy, 23% cold dark matter and 4% ordinary matter. The density of atoms is about a single hydrogen atom for every four cubic meters of volume. The properties of dark energy and dark matter are not known. Dark matter slows the expansion of the Universe. Dark energy makes its expansion faster.
The Universe is old, and changing. The best good guess of the Universe's age is 13.798±0.037 billion years old, based on what was seen of the cosmic microwave background radiation. Independent estimates (based on measurements such as radioactive dating) agree, although they are less precise, ranging from 11–20 billion years.
to 13–15 billion years.
The Universe has not been the same at all times in its history. This getting bigger accounts for how Earth-bound people can see the light from a galaxy 30 billion light-years away, even if that light has traveled for only 13 billion years; the very space between them has expanded. This expansion is consistent with the observation that the light from distant galaxies has been redshifted; the photons emitted have been stretched to longer wavelengths and lower frequency during their journey. The rate of this spatial expansion is accelerating, based on studies of Type Ia supernovae and other data.
The relative amounts of different chemical elements — especially the lightest atoms such as hydrogen, deuterium and helium — seem to be identical in all of the Universe and throughout all of the history of it that we know of. The Universe seems to have much more matter than antimatter. The Universe appears to have no net electric charge. Gravity is the dominant interaction at cosmological distances. The Universe also seems to have no net momentum or angular momentum. The absence of net charge and momentum is expected if the Universe is finite.
The Universe appears to have a smooth space-time continuum made of three spatial dimensions and one temporal (time) dimension. On the average, space is very nearly flat (close to zero curvature), meaning that Euclidean geometry is experimentally true with high accuracy throughout most of the Universe. However, the Universe may have more dimensions, and its spacetime may have a multiply connected global topology.
The Universe has the same physical laws and physical constants throughout. According to the prevailing Standard Model of physics, all matter is composed of three generations of leptons and quarks, both of which are fermions. These elementary particles interact via at most three fundamental interactions: the electroweak interaction which includes electromagnetism and the weak nuclear force; the strong nuclear force described by quantum chromodynamics; and gravity, which is best described at present by general relativity.
Special relativity holds in all the Universe in local space and time. Otherwise, general relativity holds. There is no explanation for the particular values that physical constants appear to have throughout our Universe, such as Planck's constant "h" or the gravitational constant "G". Several conservation laws have been identified, such as the conservation of charge, conservation of momentum, conservation of angular momentum and conservation of energy.
Accurate predictions of the Universe's past and future require an accurate theory of gravitation. The best theory available is Albert Einstein's general theory of relativity, which has passed all experimental tests so far. However, since rigorous experiments have not been carried out on "cosmological" length scales, general relativity could conceivably be inaccurate. Nevertheless, its predictions appear to be consistent with observations, so there is no reason to adopt another theory.
General relativity provides of a set of ten nonlinear partial differential equations for the spacetime metric (Einstein's field equations) that must be solved from the distribution of mass-energy and momentum throughout the Universe. Since these are unknown in exact detail, cosmological models have been based on the cosmological principle, which states that the Universe is homogeneous and isotropic. In effect, this principle asserts that the gravitational effects of the various galaxies making up the Universe are equivalent to those of a fine dust distributed uniformly throughout the Universe with the same average density. The assumption of a uniform dust makes it easy to solve Einstein's field equations and predict the past and future of the Universe on cosmological time scales.
Einstein's field equations include a cosmological constant (Lamda: "Λ"), that is related to an energy density of empty space. Depending on its sign, the cosmological constant can either slow (negative "Λ") or accelerate (positive "Λ") the expansion of the Universe. Although many scientists, including Einstein, had speculated that "Λ" was zero, recent astronomical observations of type Ia supernovae have detected a large amount of dark energy that is accelerating the Universe's expansion. Preliminary studies suggest that this dark energy is related to a positive "Λ", although alternative theories cannot be ruled out as yet.
The prevailing Big Bang model accounts for many of the experimental observations described above, such as the correlation of distance and redshift of galaxies, the universal ratio of hydrogen:helium atoms, and the ubiquitous, isotropic microwave radiation background. As noted above, the redshift arises from the metric expansion of space; as the space itself expands, the wavelength of a photon traveling through space likewise increases, decreasing its energy. The longer a photon has been traveling, the more expansion it has undergone; hence, older photons from more distant galaxies are the most red-shifted. Determining the correlation between distance and redshift is an important problem in experimental physical cosmology.
Other experimental observations can be explained by combining the overall expansion of space with nuclear physics and atomic physics. As the Universe expands, the energy density of the electromagnetic radiation decreases more quickly than does that of matter, since the energy of a photon decreases with its wavelength. Thus, although the energy density of the Universe is now dominated by matter, it was once dominated by radiation; poetically speaking, all was light. As the Universe expanded, its energy density decreased and it became cooler; as it did so, the elementary particles of matter could associate stably into ever larger combinations. Thus, in the early part of the matter-dominated era, stable protons and neutrons formed, which then associated into atomic nuclei. At this stage, the matter in the Universe was mainly a hot, dense plasma of negative electrons, neutral neutrinos and positive nuclei. Nuclear reactions among the nuclei led to the present abundances of the lighter nuclei, particularly hydrogen, deuterium, and helium. Eventually, the electrons and nuclei combined to form stable atoms, which are transparent to most wavelengths of radiation; at this point, the radiation decoupled from the matter, forming the ubiquitous, isotropic background of microwave radiation observed today.
Other observations are not clearly answered by known physics. According to the prevailing theory, a slight imbalance of matter over antimatter was present in the Universe's creation, or developed very shortly thereafter. Although the matter and antimatter mostly annihilated one another, producing photons, a small residue of matter survived, giving the present matter-dominated Universe.
Several lines of evidence also suggest that a rapid cosmic inflation of the Universe occurred very early in its history (roughly 10 seconds after its creation). Recent observations also suggest that the cosmological constant ("Λ") is not zero, and that the net mass-energy content of the Universe is dominated by a dark energy and dark matter that have not been characterized scientifically. They differ in their gravitational effects. Dark matter gravitates as ordinary matter does, and thus slows the expansion of the Universe; by contrast, dark energy serves to accelerate the Universe's expansion.
Some people think that there is more than one universe. They think that there is a set of universes called the multiverse. By definition, there is no way for anything in one universe to affect something in another.
The multiverse is not yet a scientific idea because there is no way to test it. An idea that cannot be tested or is not based on logic is not science. So it is not known if the multiverse is a scientific idea.
The future of the Universe is a mystery. However, there has a couple of theories based on the possible shapes of the Universe:
Unit of measurement
Units of measurement give "standards" so that the numbers from our measurements refer to the same thing. Measurement is a process that uses numbers to describe a physical quantity. We can measure how big things are, how warm they are, how heavy they are, and many other features.
For example, the metre is a standard unit to measure length. Before 1982, one meter was defined as the distance between two markers on a special metal rod. During that time, saying that something had a length of two meters meant that it was exactly twice as long as the rod used to define the meter. Now scientists define the meter by using the speed of light.
In the past, different units were used in different countries. Today, most units of measure fall into one of three systems:
The older two, the British imperial system and the closely related US customary system use the foot as a measure of length, the pound as a measure for weight, and the second as a measure for time. They use other units as well. The number of smaller units that make the bigger units in these two systems varies: For example, there are 12 inches in a foot and 16 ounces in a pound.
The newest and most used of the three systems is the metric system or SI system which use 10, 100 or 1000 of a smaller unit to make a bigger one. For instance, there are 100 centimetres in one metre or 1000 grams in one kilogram. This system uses the metre for length and kilogram for mass.
The common, non-metric measurement of time does not follow this pattern. The second is the basis for time measurement, and it is based on the sexagesimal system: 60 seconds make one minute, and 60 minutes make one hour.
The property of the thing being measured is given as a number of units of measure. The number only has sense when the unit of measurement is also given. By that number it represents a measurement of something.
For example, The Eiffel Tower in Paris, France is tall. That is, the distance from the top to the bottom of the Eiffel Tower is 300 metres. The property of the Eiffel Tower being measured is a distance. The number measured is 300. 300 of what? The unit of measurement is the metre.
Standards are usually special objects used to make measurements. A metre stick is an example of a standard. When you measure something with a metre stick, you can compare that measurement to anything else that is also measured with a metre stick. This makes measurement easier and comparisons between measurements easier.
Science, medicine and engineering use smaller units of measurement to measure small things with less error. It is easy to measure large things using larger units of measurement. Astronomical measurements like the width of a galaxy use light years and parsecs.
Small measurements like the mass of an atom use special units of measurement.
There are many different standards and units used all over the world. Some became less used during the 19th and 20th centuries.
The metric system is a system of measurement used in most of the world. It is also called the International System of Units, or SI.
Units of measure in the metric system include:
Imperial units were defined in the United Kingdom in 1824. These units were based on similar units that were in use before 1824. Imperial units were used in countries that were part of the British Empire. While many of these countries, including the United Kingdom, have officially adopted SI, the older system of units are still used.
US customary units are the official units used in the US. These are similar to the British imperial units and also based on the units used in the United Kingdom from before American Independence. Some of the units are different to the British ones. For example, there are 20 imperial fluid ounces in an imperial pint, but 16 US fluid ounces in a US pint. Additionally, the US fluid ounce is slightly bigger than the imperial fluid ounce. The result is that US pints and gallons are smaller than imperial pints and gallons. In the United States, the metric system has been legal for trade since 1866 but other measurements such as the gallon, inch, and the pound are still widely used.