quarta-feira, 19 de março de 2014

The Big Bang theory developed from observations of the structure of the universe and from theoretical considerations. In 1912 Vesto Slipher measured the first Doppler shift of a "spiral nebula" (spiral nebula is the obsolete term for spiral galaxies), and soon discovered that almost all such nebulae were receding from Earth. He did not grasp the cosmological implications of this fact, and indeed at the time it was highly controversial whether or not these nebulae were "island universes" outside our Milky Way.[43][44] Ten years later, Alexander Friedmann, a Russian cosmologist and mathematician, derived the Friedmann equations from Albert Einstein's equations of general relativity, showing that the universe might be expanding in contrast to the static universe model advocated by Einstein at that time.[45] In 1924 Edwin Hubble's measurement of the great distance to the nearest spiral nebulae showed that these systems were indeed other galaxies. Independently deriving Friedmann's equations in 1927, Georges Lemaître, a Belgian physicist and Roman Catholic priest, proposed that the inferred recession of the nebulae was due to the expansion of the universe.[46]
In 1931 Lemaître went further and suggested that the evident expansion of the universe, if projected back in time, meant that the further in the past the smaller the universe was, until at some finite time in the past all the mass of the universe was concentrated into a single point, a "primeval atom" where and when the fabric of time and space came into existence.[47]
Starting in 1924, Hubble painstakingly developed a series of distance indicators, the forerunner of the cosmic distance ladder, using the 100-inch (2,500 mm) Hooker telescope at Mount Wilson Observatory. This allowed him to estimate distances to galaxies whose redshifts had already been measured, mostly by Slipher. In 1929 Hubble discovered a correlation between distance and recession velocity—now known as Hubble's law.[15][48] Lemaître had already shown that this was expected, given the Cosmological Principle.[34]
In the 1920s and 1930s almost every major cosmologist preferred an eternal steady state universe, and several complained that the beginning of time implied by the Big Bang imported religious concepts into physics; this objection was later repeated by supporters of the steady state theory.[49] This perception was enhanced by the fact that the originator of the Big Bang theory, Monsignor Georges Lemaître, was a Roman Catholic priest.[50] Arthur Eddington agreed with Aristotle that the universe did not have a beginning in time, viz., that matter is eternal. A beginning in time was "repugnant" to him.[51][52] Lemaître, however, thought that
If the world has begun with a single quantum, the notions of space and time would altogether fail to have any meaning at the beginning; they would only begin to have a sensible meaning when the original quantum had been divided into a sufficient number of quanta. If this suggestion is correct, the beginning of the world happened a little before the beginning of space and time.[53]
During the 1930s other ideas were proposed as non-standard cosmologies to explain Hubble's observations, including the Milne model,[54] the oscillatory universe (originally suggested by Friedmann, but advocated by Albert Einstein and Richard Tolman)[55] and Fritz Zwicky's tired light hypothesis.[56]
After World War II, two distinct possibilities emerged. One was Fred Hoyle's steady state model, whereby new matter would be created as the universe seemed to expand. In this model the universe is roughly the same at any point in time.[57] The other was Lemaître's Big Bang theory, advocated and developed by George Gamow, who introduced big bang nucleosynthesis (BBN)[58] and whose associates, Ralph Alpher and Robert Herman, predicted the cosmic microwave background radiation (CMB).[59] Ironically, it was Hoyle who coined the phrase that came to be applied to Lemaître's theory, referring to it as "this big bang idea" during a BBC Radio broadcast in March 1949.[60][notes 4] For a while, support was split between these two theories. Eventually, the observational evidence, most notably from radio source counts, began to favor Big Bang over Steady State. The discovery and confirmation of the cosmic microwave background radiation in 1964[62] secured the Big Bang as the best theory of the origin and evolution of the cosmos. Much of the current work in cosmology includes understanding how galaxies form in the context of the Big Bang, understanding the physics of the universe at earlier and earlier times, and reconciling observations with the basic theory.
Significant progress in Big Bang cosmology have been made since the late 1990s as a result of advances in telescope technology as well as the analysis of data from satellites such as COBE,[63] the Hubble Space Telescope and WMAP.[64] Cosmologists now have fairly precise and accurate measurements of many of the parameters of the Big Bang model, and have made the unexpected discovery that the expansion of the universe appears to be accelerating.
According to Poe, the initial state of matter was a single "Primordial Particle". "Divine Volition", manifesting itself as a repulsive force, fragmented the Primordial Particle into atoms. Atoms spread evenly throughout space, until the repulsive force stops, and attraction appears as a reaction: then matter begins to clump together forming stars and star systems, while the material universe is drawn back together by gravity, finally collapsing and ending eventually returning to the Primordial Particle stage in order to begin the process of repulsion and attraction once again. This part of Eureka describes a Newtonian evolving universe which shares a number of properties with relativistic models, and for this reason Poe anticipates some themes of modern cosmology [4]

Early 20th century scientific developments

Monseigneur Georges Lemaître, a Belgian Catholic Priest, was the originator of what would become known as the "Big Bang Theory".
Observationally, in the 1910s, Vesto Slipher and later, Carl Wilhelm Wirtz, determined that most spiral nebulae (now correctly called spiral galaxies) were receding from Earth. Slipher used spectroscopy to investigate the rotation periods of planets, the composition of planetary atmospheres, and was the first to observe the radial velocities of galaxies. Wirtz observed a systematic redshift of nebulae, which was difficult to interpret in terms of a cosmology in which the Universe is filled more or less uniformly with stars and nebulae. They weren't aware of the cosmological implications, nor that the supposed nebulae were actually galaxies outside our own Milky Way.[citation needed]
Also in that decade, Albert Einstein's theory of general relativity was found to admit no static cosmological solutions, given the basic assumptions of cosmology described in the Big Bang's theoretical underpinnings. The universe (i.e., the space-time metric) was described by a metric tensor that was either expanding or shrinking (i.e., was not constant or invariant). This result, coming from an evaluation of the field equations of the general theory, at first led Einstein himself to consider that his formulation of the field equations of the general theory may be in error, and he tried to correct it by adding a cosmological constant. This constant would restore to the general theory's description of space-time an invariant metric tensor for the fabric of space/existence. The first person to seriously apply general relativity to cosmology without the stabilizing cosmological constant was Alexander Friedmann. Friedmann derived the expanding-universe solution to general relativity field equations in 1922. Friedmann's 1924 papers included "Über die Möglichkeit einer Welt mit konstanter negativer Krümmung des Raumes" (About the possibility of a world with constant negative curvature) which was published by the Berlin Academy of Sciences on 7 January 1924.[5] Friedmann's equations describe the Friedmann–Lemaître–Robertson–Walker universe.
In 1927, the Belgian Catholic priest Georges Lemaître proposed an expanding model for the universe to explain the observed redshifts of spiral nebulae, and forecast the Hubble law. He based his theory on the work of Einstein and De Sitter, and independently derived Friedmann's equations for an expanding universe. Also, the red shifts themselves were not constant, but varied in such manner as to lead to the conclusion that there was a definite relationship between amount of red-shift of nebulae, and their distance from observers.[citation needed]
In 1929, Edwin Hubble provided a comprehensive observational foundation for Lemaître's theory. Hubble's experimental observations discovered that, relative to the Earth and all other observed bodies, galaxies are receding in every direction at velocities (calculated from their observed red-shifts) directly proportional to their distance from the Earth and each other. In 1929, Hubble and Milton Humason formulated the empirical Redshift Distance Law of galaxies, nowadays known as Hubble's law, which, once the redshift is interpreted as a measure of recession speed, is consistent with the solutions of Einstein's General Relativity Equations for a homogeneous, isotropic expanding space. The isotropic nature of the expansion was direct proof that it was the space (the fabric of existence) itself that was expanding, not the bodies in space that were simply moving further outward and apart into an infinitely larger preexisting empty void. It was this interpretation that led to the concept of the expanding universe. The law states that the greater the distance between any two galaxies, the greater their relative speed of separation. This discovery later resulted in the formulation of the Big Bang model.[citation needed]
In 1931, Lemaître proposed in his "hypothèse de l'atome primitif" (hypothesis of the primeval atom) that the universe began with the "explosion" of the "primeval atom" — what was later called the Big Bang. Lemaître first took cosmic rays to be the remnants of the event, although it is now known that they originate within the local galaxy. Lemaître had to wait until shortly before his death to learn of the discovery of cosmic microwave background radiation, the remnant radiation of a dense and hot phase in the early Universe.[6]

Big Bang theory vs. Steady State theory

Hubble's Law suggested that the universe was expanding, contradicting the cosmological principle whereby the universe, when viewed on sufficiently large distance scales, has no preferred directions or preferred places. Hubble's idea allowed for two opposing hypotheses to be suggested. One was Lemaître's Big Bang, advocated and developed by George Gamow. The other model was Fred Hoyle's Steady State theory, in which new matter would be created as the galaxies moved away from each other. In this model, the universe is roughly the same at any point in time. It was actually Hoyle who coined the name of Lemaître's theory, referring to it as "this 'big bang' idea" during a radio broadcast on 28 March 1949, on the BBC Third Programme. 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.[7] Hoyle repeated the term in further broadcasts in early 1950, as part of a series of five lectures entitled The Nature of The Universe. The text of each lecture was published in The Listener a week after the broadcast, the first time that the term "big bang" appeared in print.[8] As evidence in favour of the Big Bang model mounted, and the consensus became widespread, Hoyle himself, albeit somewhat reluctantly, admitted to it by formulating a new cosmological model that other scientists later referred to as the "Steady Bang".[9]

1950 to 1980s

Comparison of the predictions of the standard Big Bang model with experimental measurements. The power spectrum of the cosmic microwave background radiation anisotropy is plotted in terms of the angular scale (or multipole moment) (top).
From around 1950 to 1965, the support for these theories was evenly divided, with a slight imbalance arising from the fact that the Big Bang theory could explain both the formation and the observed abundances of hydrogen and helium, whereas the Steady State could explain how they were formed, but not why they should have the observed abundances. However, the observational evidence began to support the idea that the universe evolved from a hot dense state. Objects such as quasars and radio galaxies were observed to be much more common at large distances (therefore in the distant past) than in the nearby universe, whereas the Steady State predicted that the average properties of the universe should be unchanging with time. In addition, the discovery of the cosmic microwave background radiation in 1965 was considered the death knell of the Steady State, although this prediction was only qualitative, and failed to predict the exact temperature of the CMB. (The key big bang prediction is the black-body spectrum of the CMB, which was not measured with high accuracy until COBE in 1990). After some reformulation, the Big Bang has been regarded as the best theory of the origin and evolution of the cosmos. Before the late 1960s, many cosmologists thought the infinitely dense and physically paradoxical singularity at the starting time of Friedmann's cosmological model could be avoided by allowing for a universe which was contracting before entering the hot dense state, and starting to expand again. This was formalized as Richard Tolman's oscillating universe. In the sixties, Stephen Hawking and others demonstrated that this idea was unworkable,[citation needed] and the singularity is an essential feature of the physics described by Einstein's gravity. This led the majority of cosmologists to accept the notion that the universe as currently described by the physics of general relativity has a finite age. However, due to a lack of a theory of quantum gravity, there is no way to say whether the singularity is an actual origin point for the universe, or whether the physical processes that govern the regime cause the universe to be effectively eternal in character. Wk

terça-feira, 18 de março de 2014

A bet about detecting the origins of the universe is at the centre of a good-natured disagreement between Stephen Hawking and Canadian physicist Neil Turok

via Guardian Science
Stephen Hawking claims victory in gravitational wave bet
Cosmologist says he has won the wager with a Canadian physicist about what happened in first moments after big bangAccording to the Big Bang theory, the expansion of the observable universe began with the explosion of a single particle at a definite point in time. This startling idea first appeared in scientific form in 1931, in a paper by Georges Lemaître, a Belgian cosmologist and Catholic priest. The theory, accepted by nearly all astronomers today, was a radical departure from scientific orthodoxy in the 1930s. Many astronomers at the time were still uncomfortable with the idea that the universe is expanding. That the entire observable universe of galaxies began with a bang seemed preposterous.

Stephen Hawking claims victory in gravitational wave bet

Cosmologist says he has won the wager with a fellow physicist about what happened in the first moments after the big bang
Stephen Hawking
Gravitational waves are 'another confirmation of inflation', Stephen Hawking told BBC Radio 4's Today programme. Photograph: Danita Delimont/Alamy
Stephen Hawking has claimed victory in a bet with a fellow scientist over the discovery of primordial gravitational waves, ripples in the structure of space-time from the birth of the universe.
The Cambridge cosmologist bet Neil Turok, director of the Perimeter Institute in Canada, that gravitational waves from the first fleeting moments after the big bang would be detected.
Speaking on BBC Radio 4's Today programme, Hawking said the discovery of gravitational waves, announced on Monday by researchers at the Harvard-Smithsonian Centre for Astrophysics, disproves Turok's theory that the universe cycles endlessly from one big bang to another.
If confirmed by other groups, the discovery would count as the strongest evidence yet for cosmic inflation, a theory which says that the universe went through a period of extremely rapid expansion soon after the big bang. The theory explains why the universe looks almost the same in every direction.
"It is another confirmation of inflation," Hawking told the Today programme. "It also means I win a bet with Neil Turok, director of the Perimeter Institute in Canada, for cyclic universe theory predicts no gravitational waves from the early universe."
But Turok was not ready to concede just yet. He told the programme that the bet rested on results from the European Space Agency's Planck space telescope, which last year failed to spot any signs of gravitational waves.
"In 2001, I gave a talk proposing a new theory of the big bang according to which the big bang was just the latest in an infinite series of big bangs, and the universe would be a cyclic universe," Turok said. "Stephen, in typical fashion, at the end of a talk, said 'I bet you that the Planck satellite will discover the gravitational wave signal of inflation, which would immediately disprove your theory', because our prediction from our theory was that there would be no gravitational wave signal."
"So, of course, the Planck satellite flew, and last year announced its results, and there was no gravitational wave signal, so thus far, I'm winning the bet," he added.
The idea of cosmic inflation came to Alan Guth, a physicist at MIT, by chance one evening in 1979. He was up late in his apartment, working with pen and notebook, hoping to understand why the universe was not filled with strange particles called magnetic monopoles. He worked out that the universe would have far fewer of the particles if it went through a rapid period of supercooling. As he worked through the equations, one step stood out. It suggested that the expansion of the early universe would be exponential.
Over the next three decades, scientists, including Andrei Linde at Stanford University, developed the theory into its modern form. In 1982, Hawking added to the work with a paper that suggested galaxies arose from tiny irregularities in the early universe.
"This paper aroused interest among other scientists who had been thinking on similar lines, so I invited them all to a workshop in Cambridge in June 1982 supported by the Nuffield Foundation. At the workshop we established the now accepted picture of inflation in the very early universe, although it was not confirmed by observation until 10 years later," Hawking told Today.
Turok urged caution over the latest claims. "First of all, I should say this is just a spectacular result, and right or wrong, it actually indicates we are right on the threshold of a completely new window into the big bang and what happened at the big bang, so it's tremendously exciting," he said.
But he added: "I have reasons for doubts about the new experiment and its results. It's not entirely convincing to me, but they have clearly seen what they claim to have seen. Verification is very important and it's wise to be a little bit sceptical at the moment when there is no confirmation. The experiment was extremely difficult, and they don't entirely explain why they are so convinced of what they claim … The problem with the inflationary theory is that it really doesn't explain the beginning. Stephen has postulated a way of starting the universe off, but it doesn't seem to work."
Hawking is well known for making bets with other scientists. He recently lost $100 to Gordon Kane at the University of Michigan after betting that scientists at Cern, home of the Large Hadron Collider near Geneva, would not find the Higgs boson. They discovered the particle in July 2012.
Turok said he needed to see more evidence for gravitational waves from the big bang before conceding the bet to Hawking. "The great thing about science is that it doesn't matter how many [scientists] you are up against. Ultimately the right ideas win out. Science is not a popularity contest. Galileo was right, but his ideas weren't popular at the time. The bet is still open," he said.
• This article was amended on 18 March 2014. The original described Neil Turok as a Canadian physicist. This has been corrected.