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Universe

Our Universe is everything that exists, from the big bang - to the end of time: it is the entirety of all space and time, all forms of matter, energy and momentum, and the physical laws and physical constants that govern them. In a well-defined, mathematical sense, the universe even contains that which does not exist; according to the path-integral formulation of quantum mechanics, even unrealized possibilities contribute to the probability amplitudes of events in the universe. The universe is sometimes denoted as the cosmos or Nature, as in "cosmology" or "natural philosophy".

Big Bang Universe

The Big Bang theory developed from observations of the structure of the universe and theoretical considerations. Observationally, it was determined that most spiral nebulae were receding from Earth, but those who made the observation weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way. In 1927, Georges Lemaître, a Belgian Roman Catholic priest, independently derived the Friedmann-Lemaître-Robertson-Walker equations from Albert Einstein's equations of General relativity and proposed, on the basis of the recession of spiral nebulae, that the universe began with the "explosion" of a "primeval atom"-what was later called the Big Bang.

Galaxies in the universe

Galaxies in the universe come in three main types: ellipticals, spirals, and irregulars. A slightly more extensive description of galaxy types based on their appearance is given by the Hubble sequence. Since the Hubble sequence is entirely based upon visual morphological type, it may miss certain important characteristics of galaxies such as star formation rate (in starburst galaxies) or activity in the core (in active galaxies).

Stars

Observation of double stars in the universe gained increasing importance during the 19th century. In 1834, Friedrich Bessel observed changes in the proper motion of the star Sirius, and inferred a hidden companion. Edward Pickering discovered the first Spectroscopic binary in 1899 when he observed the periodic splitting of the spectral lines of the star Mizar in a 104 day period. Detailed observations of many binary star systems were collected by astronomers such as William Struve and S. W. Burnham, allowing the masses of stars to be determined from computation of the orbital elements. The first solution to the problem of deriving an orbit of binary stars from telescope observations was made by Felix Savary in 1827.

Pulsars

The first pulsar in the universe was discovered in 1967, by Jocelyn Bell Burnell and Antony Hewish of the University of Cambridge, UK. While using a radio array to study the scintillation of quasars, they found a very regular signal, consisting of pulses of radiation at a rate of one in every few seconds. Terrestrial origin of the signal was ruled out because the time it took the object to reappear was a Sidereal day instead of a solar day.

Quasars

Quasars exhibit many of the same properties as active galaxies in the universe: radiation is nonthermal and some are observed to have jets and lobes like those of radio galaxies. Quasars can be observed in many parts of the electromagnetic spectrum including radio, Infrared, optical, ultraviolet, X-ray and even gamma rays. Most quasars are brightest in their rest-frame near-ultraviolet (near the 1216 angstrom (121.6 nm) Lyman-alpha emission line of hydrogen), but due to the tremendous redshifts of these sources, that peak luminosity has been observed as far to the red as 9000 angstroms (900 nm or 0.9 µm), in the near infrared.

Black hole

An open question in fundamental physics is the so-called information loss paradox, or black hole unitarity paradox. Classically, the laws of physics are the same run forward or in reverse. That is, if the position and velocity of every particle in the universe were measured, we could (disregarding chaos) work backwards to discover the history of the universe arbitrarily far in the past. In quantum mechanics, this corresponds to a vital property called unitarity which has to do with the conservation of probability.

Age of the universe

Calculating the age of the universe is only accurate if the assumptions built into the models being used are also accurate. This is referred to as strong priors and essentially involves stripping the potential errors in other parts of the model to render the accuracy of actual observational data directly into the concluded result. Although this is not a totally invalid procedure in certain contexts, it should be noted that the caveat, "based on the fact we have assumed the underlying model we used is correct", then the age given is thus accurate to the specified error (since this error represents the error in the instrument used to gather the raw data input into the model).

Ultimate fate of the universe

The theoretical scientific exploration of the ultimate fate of the universe became possible with Albert Einstein's 1915 theory of General relativity. General relativity can be employed to describe the universe on the largest possible scale. There are many possible solutions to the equations of general relativity, and each solution implies a possible ultimate fate of the universe. Alexander Friedmann proposed one such solution in 1921. This solution implies that the universe has been expanding from an initial singularity; this is, essentially, the Big Bang.

General relativity

One of the defining features of general relativity is the idea that gravitational 'force' is replaced by geometry. In general relativity, phenomena that in classical mechanics are ascribed to the action of the force of gravity (such as free-fall, Orbital motion, and Spacecraft trajectories) are taken in general relativity to represent inertial motion in a curved spacetime. So what people standing on the surface of the Earth perceive as the 'force of gravity' is a result of their undergoing a continuous physical acceleration caused by the mechanical resistance of the surface on which they are standing.