DARK ENERGY

In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe.^[1]Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding at an accelerating rate. In the standard model of cosmology, dark energy currently accounts for 73% of the total mass-energy of the universe.^[2]

Two proposed forms for dark energy are the cosmological constant, a constant energy density filling space homogeneously,^[3] and scalar fields such as quintessence or moduli, dynamicquantities whose energy density can vary in time and space. Contributions from scalar fields that are constant in space are usually also included in the cosmological constant. The cosmological constant is physically equivalent to vacuum energy. Scalar fields which do change in space can be difficult to distinguish from a cosmological constant because the change may be extremely slow.

High-precision measurements of the expansion of the universe are required to understand how the expansion rate changes over time. In general relativity, the evolution of the expansion rate is parameterized by the cosmological equation of state (the relationship between temperature, pressure, and combined matter, energy, and vacuum energy density for any region of space). Measuring the equation of state for dark energy is one of the biggest efforts in observational cosmology today.

Adding the cosmological constant to cosmology's standard FLRW metric leads to the Lambda-CDM model, which has been referred to as the "standard model" of cosmology because of its precise agreement with observations. Dark energy has been used as a crucial ingredient in a recent attempt to formulate a cyclic model for the universe.^[4]

A 2011 survey of more than 200,000 galaxies appears to confirm the existence of dark energy, although the exact physics behind it remains unknown.^[5][6]

UFO

A term originally coined by the military, an unidentified flying object (usually abbreviated toUFO or U.F.O.) is an unusual apparent anomaly in the sky that is not readily identifiable to the observer as any known object. While a small percentage remain unexplained, the majority of UFO sightings are often later identified as any number of various natural phenomenon or man-made objects.^{[citation needed]}
While technically a UFO refers to any unidentified flying object, in modern popular culture the term UFO has generally become synonymous with alien spacecraft. Proponents argue that because these objects appear to be technological and not natural phenomenon, and are alleged to display flight characteristics or have shapes seemingly unknown to conventional technology, the conclusion is then that they must not be from Earth.^[1][2][3][4] Though UFO sightings have occurred throughout recorded history, modern interest in them dates from World War II (seefoo fighter), further fueled in the late 1940s by Kenneth Arnold's coining of the term flying saucer and the Roswell UFO Incident. Since then governments have investigated UFO reports, often from a military perspective- and UFO researchers have investigated, written about, and created organizations devoted to the subject. One such investigation, The UK's Project Condign report, notes that Russian, Former Soviet Republics, and Chinese authorities have made a co-ordinated effort to understand the UFO topic and that State military organizations, particularly in Russia, have done "considerably more work (than is evident from open sources)" on military applications which have stemmed from their UFO research. The report also noted that "several aircraft have been destroyed and at least four pilots have been killed 'chasing UFOs'."^[5]

Studies have established that the majority of UFO observations are misidentified conventional objects or natural phenomenon— most commonly aircraft, balloons, noctilucent clouds, nacreous clouds, or astronomical objects such as meteors or bright planets with a small percentage even being hoaxes.^[8] After excluding incorrect reports, however, it is acknowledged that between 5% and 20% of reported sightings remain unexplained, and as such can be classified as unidentified in the strictest sense. Many reports have been made by trained observers such as pilots, police, and the military; some involve radar traces, so not all reports are visual.^[9] Proponents of the extraterrestrial hypothesis believe that these unidentified reports are of alien spacecraft, though various other hypotheses have been proposed.

While UFOs have been the subject of extensive investigation by various governments, and some scientists support the extraterrestrial hypothesis, few scientific papers about UFOs have been published in peer-reviewed journals.^[10] There has been some debate in the scientific community about whether any scientific investigation into UFO sightings is warranted.^{[11][12][13][14][15][16][17]}

The void left by the lack of institutional scientific study has given rise to independent researchers and groups, most notably MUFON (Mutual UFO Network) ^[18] and CUFOS (Center for UFO Studies).^[19] The term "Ufology" is used to describe the collective efforts of those who study reports and associated evidence of unidentified flying objects. According to MUFON, as of 2011 the number of UFO reports to their worldwide offices has increased by 67% from the previous 3 years, which now average around 500 reported sightings per month.^[20]

UFOs have become a relevant theme in modern culture,^[21] and the social phenomena have been the subject of academic research in sociology and psychology.^[10]

BIG BANG

The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe.^[1] According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in its present continuously expanding state. According to the most recent measurements and observations, this original state existed approximately 13.7 billion years ago,^[2][3] which is considered the age of the Universe and the time the Big Bang occurred.^[4][5]After its initial expansion from a singularity, the Universe cooled sufficiently to allow energy to be converted into various subatomic particles. It would take thousands of years for some of these particles (protons, neutrons, and electrons) to combine and form atoms, the building blocks of matter. The first element produced was hydrogen, along with traces of helium andlithium. Eventually, clouds of hydrogen would coalesce through gravity to form stars, and theheavier elements would be synthesized either within stars or during supernovae.

The Big Bang is a well-tested scientific theory which is widely accepted within the scientific community because it is the most accurate and comprehensive explanation for the full range of phenomena astronomers observe. Since its conception, abundant evidence has arisen to further validate the model.^[6][7] Georges Lemaître first proposed what would become the Big Bang theory in what he called his "hypothesis of the primeval atom." Over time, scientists would build on his initial ideas to form the modern synthesis. The framework for the Big Bang model relies on Albert Einstein's general relativity and on simplifying assumptions (such ashomogeneity and isotropy of space). The governing equations had been formulated byAlexander Friedmann. In 1929, Edwin Hubble discovered that the distances to far away galaxieswere generally proportional to their redshifts—an idea originally suggested by Lemaître in 1927. Hubble's observation was taken to indicate that all very distant galaxies and clusters have an apparent velocity directly away from our vantage point: the farther away, the higher the apparent velocity.^[8]

If the distance between galaxy clusters is increasing today, everything must have been closer together in the past. This idea has been considered in detail back in time to extreme densitiesand temperatures,^[9][10][11] and large particle accelerators have been built to experiment on and test such conditions, resulting in significant confirmation of this model. On the other hand, these accelerators have limited capabilities to probe into such high energy regimes. There is little evidence regarding the absolute earliest instant of the expansion. Thus, the Big Bang theorycannot and does not provide any explanation for such an initial condition; rather, it describesand explains the general evolution of the universe going forward from that point on. The observed abundances of the light elements throughout the cosmos closely match the calculated predictions for the formation of these elements from nuclear processes in the rapidly expanding and cooling first minutes of the universe, as logically and quantitatively detailed according to Big Bang nucleosynthesis.

Fred Hoyle is credited with coining the term Big Bang during a 1949 radio broadcast. It is popularly reported that Hoyle, who favored an alternative "steady state" cosmological model, intended this to be pejorative, but Hoyle explicitly denied this and said it was just a striking image meant to highlight the difference between the two models.^[12][13][14] After the discovery of the cosmic microwave background radiation in 1964, and especially when its spectrum (i.e., the amount of radiation measured at each wavelength) was found to match that of thermal radiationfrom a black body, most scientists were fairly convinced by the evidence that some version of the Big Bang scenario must have occurred.

BUTTERFLY EFFECT

In chaos theory, the butterfly effect is the sensitive dependence on initial conditions; where a small change at one place in a nonlinear system can result in large differences to a later state. The name of the effect, coined by Edward Lorenz, is derived from the theoretical example of a hurricane's formation being contingent on whether or not a distant butterfly had flapped its wings several weeks before.

Although the butterfly effect may appear to be an esoteric and unusual behavior, it is exhibited by very simple systems: for example, a ball placed at the crest of a hill might roll into any of several valleys depending on slight differences in initial position.

The butterfly effect is a common trope in fiction when presenting scenarios involvingtime travel and with "what if" cases where one storyline diverges at the moment of a seemingly minor event resulting in two significantly different outcomes.

The term "butterfly effect" itself is related to the work of Edward Lorenz, and it is based in chaos theory and sensitive dependence on initial conditions, already described in the literature in a particular case of the three-body problem by Henri Poincaré in 1890.^[1] He later proposed that such phenomena could be common, say in meteorology.

In 1898,^[1] Jacques Hadamard noted general divergence of trajectories in spaces of negative curvature, and Pierre Duhem discussed the possible general significance of this in 1908.^[1] The idea that one butterfly could eventually have a far-reaching ripple effect on subsequent historic events first appears in "A Sound of Thunder", a 1952 short story by Ray Bradbury about time travel (see Literature and print here) although Lorenz made the term popular.

In 1961, Lorenz was using a numerical computer model to rerun a weather prediction, when, as a shortcut on a number in the sequence, he entered the decimal .506 instead of entering the full .506127. The result was a completely different weather scenario.^[2] Lorenz published his findings in a 1963 paper^[3] for the New York Academy of Sciences noting^{[citation needed]} that "One meteorologist remarked that if the theory were correct, one flap of a seagull's wings could change the course of weather forever." Following suggestions from colleagues, in later speeches and papers Lorenz used the more poetic butterfly. According to Lorenz, when Lorenz failed to provide a title for a talk he was to present at the 139th meeting of the American Association for the Advancement of Science in 1972, Philip Merilees concocted Does the flap of a butterfly’s wings in Brazil set off a tornado in Texas? as a title. Although a butterfly flapping its wings has remained constant in the expression of this concept, the location of the butterfly, the consequences, and the location of the consequences have varied widely.^[4]

The phrase refers to the idea that a butterfly's wings might create tiny changes in the atmosphere that may ultimately alter the path of atornado or delay, accelerate or even prevent the occurrence of a tornado in another location. The flapping wing represents a small change in the initial condition of the system, which causes a chain of events leading to large-scale alterations of events (compare: domino effect). Had the butterfly not flapped its wings, the trajectory of the system might have been vastly different. While the butterfly does not "cause" the tornado in the sense of providing the energy for the tornado, it does "cause" it in the sense that the flap of its wings is an essential part of the initial conditions resulting in a tornado, and without that flap that particular tornado would not have existed.

NEBULA

A nebula (from Latin: "cloud";^[1] pl. nebulae or nebulæ, with ligature or nebulas) is aninterstellar cloud of dust, hydrogen gas, helium gas and other ionized gases. Originally,nebula was a general name for any extended astronomical object, including galaxies beyond the Milky Way (some examples of the older usage survive; for example, the Andromeda Galaxy was referred to as the Andromeda Nebula before galaxies were discovered by Edwin Hubble). Nebulae are often star-forming regions, such as in the Eagle Nebula. This nebula is depicted in one of NASA's most famous images, the "Pillars of Creation". In these regions the formations of gas, dust, and other materials "clump" together to form larger masses, which attract further matter, and eventually will become massive enough to form stars. The remaining materials are then believed to form planets, and other planetary system objects.

Many nebulae or stars form from the gravitational collapse of gas in the interstellar medium or ISM. As the material collapses under its own weight, massive stars may form in the center, and theirultraviolet radiation ionises the surrounding gas, making it visible at optical wavelengths. Examples of these types of nebulae are the Rosette Nebula and the Pelican Nebula. The size of these nebulae, known as HII regions, varies depending on the size of the original cloud of gas. New stars are formed in the nebulas. The formed stars are sometimes known as a young, loose cluster.

Some nebulae are formed as the result of supernova explosions, the death throes of massive, short-lived stars. The materials thrown off from the supernova explosion are ionized by the energy and the compact object that it can produce. One of the best examples of this is the Crab Nebula, in Taurus. The supernova event was recorded in the year 1054 and is labelled SN 1054. The compact object that was created after the explosion lies in the center of the Crab Nebula and is a neutron star.

Other nebulae may form as planetary nebulae. This is the final stage of a low-mass star's life, likeEarth's Sun. Stars with a mass up to 8-10 solar masses evolve into red giants and slowly lose their outer layers during pulsations in their atmospheres. When a star has lost enough material, its temperature increases and the ultraviolet radiation it emits can ionize the surrounding nebula that it has thrown off. The nebula is 97%Hydrogen and 3% Helium with trace materials.

Nebulae are classified in four major groups(stars). Galaxies and globular clusters were previously thought to be other types of nebulae. Spiral nebula were used to explain the spiral structures of galaxies.

• H II regions, which encompass diffuse nebulae, bright nebulae, and reflection nebulae.
• Planetary nebulae
• Supernova remnant
• Dark nebula

This classification does not encompass all known cloud-like structures. An example is a Herbig–Haro object.