When linear dimensions are doubled, the energy density in radiation falls by a factor of sixteen (see the solution of the energy density continuity equation for an ultra-relativistic fluid). In the early days of General Relativity, Albert Einstein introduced the cosmological constant to allow a static solution, which was a three-dimensional sphere with a uniform density of matter. As such, although predictions of inflation have been consistent with the results of observational tests, many open questions remain. Since Guth's early work, each of these observations has received further confirmation, most impressively by the detailed observations of the cosmic microwave background made by the Planck spacecraft. He calls 'bad inflation' a period of accelerated expansion whose outcome conflicts with observations, and 'good inflation' one compatible with them: "Not only is bad inflation more likely than good inflation, but no inflation is more likely than either [...] Roger Penrose considered all the possible configurations of the inflaton and gravitational fields. If so, we say that the wave is polarized. [95] However, in his model the inflaton field necessarily takes values larger than one Planck unit: for this reason, these are often called large field models and the competing new inflation models are called small field models. [17] Another effect remarked upon since the first cosmic microwave background satellite, the Cosmic Background Explorer is that the amplitude of the quadrupole moment of the CMB is unexpectedly low and the other low multipoles appear to be preferentially aligned with the ecliptic plane. Radiation could only be generated in collisions between bubble walls. Ekpyrotic models avoid the magnetic monopole problem as long as the temperature at the Big Crunch/Big Bang transition remains below the Grand Unified Scale, as this is the temperature required to produce magnetic monopoles in the first place. An expanding universe generally has a cosmological horizon, which, by analogy with the more familiar horizon caused by the curvature of Earth's surface, marks the boundary of the part of the Universe that an observer can see. This is necessary to ensure that the Universe appears flat, homogeneous and isotropic at the largest observable scales. [62][63][64] In the context of inflation, they were worked out independently of the work of Mukhanov and Chibisov at the three-week 1982 Nuffield Workshop on the Very Early Universe at Cambridge University. is 0.06 or lower: consistent with the null hypothesis, but still also consistent with many remaining models of inflation. There are well-known theorems that this steady state cannot continue forever into the past. The basic inflationary paradigm is accepted by most physicists, as a number of inflation model predictions have been confirmed by observation;[4] however, a substantial minority of scientists dissent from this position. = [47] It was discovered that Einstein's universe is unstable, and that small fluctuations cause it to collapse or turn into a de Sitter universe. Cosmological models employing a variable speed of light have been proposed to resolve the horizon problem of and provide an alternative to cosmic inflation. He eventually noted that gravitational effects would be significant, but he did not calculate these effects and did not apply the results to cosmology. N Is the theory of cosmological inflation correct, and if so, what are the details of this epoch? The team announced the tensor-to-scalar power ratio He found that if the universe contained a field in a positive-energy false vacuum state, then according to general relativity it would generate an exponential expansion of space. Join our Space Forums to keep talking space on the latest missions, night sky and more! Guth proposed that as the early universe cooled, it was trapped in a false vacuum with a high energy density, which is much like a cosmological constant. In the eternal inflation theory, which is a variant of the cosmic inflation theory, the multiverse or space as a whole is stretching and will continue doing so forever, but some regions of space stop stretching and form distinct bubbles (like gas pockets in a loaf of rising bread). The term inflation refers to the explosively rapid expansion of space-time that occurred a tiny fraction of a second after the Big Bang. n In the Soviet Union, Alexei Starobinsky noted that quantum corrections to general relativity should be important for the early universe. In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. [...] BICEP did a wonderful service by bringing all the Inflation-ists out of their shell, and giving them a black eye. Nevertheless, many physicists, mathematicians, and philosophers of science have voiced criticisms, claiming untestable predictions and a lack of serious empirical support. (The exact drop is model-dependent, but in the first models it was typically from 1027 K down to 1022 K.[24]) This relatively low temperature is maintained during the inflationary phase. In this situation, the predictions of effective field theory are thought to be invalid, as renormalization should cause large corrections that could prevent inflation. Loop quantum gravity assumes a quantized spacetime. This is not usually considered to be a critical problem, however, because the scale of inflation corresponds naturally to the scale of gauge unification. Brandenberger commented on fine-tuning in another situation. This problem is exacerbated by recent observations of the cosmic microwave background that have demonstrated that the Universe is flat to within a few percent. Some physicists believe this paradox can be resolved by weighting observers by their pre-inflationary volume. Therefore, there ought to be corrections from Planck-scale physics, in particular the unknown quantum theory of gravity. This requirement is generally thought to be satisfied if the Universe expanded by a factor of at least 1026 during inflation. Although nearly uniform, the CMB contains ripples. was between 0.15 and 0.27 (rejecting the null hypothesis; In physical cosmology, cosmic inflation, cosmological inflation, or just inflation, is a theory of exponential expansion of space in the early universe. The acceleration of this expansion due to dark energybegan after the universe was already over 9 billion years old (~4 billion yea… The modern explanation for the metric expansion of space was proposed by physicist Alan Guth in 1979, while investigating the problem of why no magnetic monopoles are seen today. This structure for the perturbations has been confirmed by the Planck spacecraft, WMAP spacecraft and other cosmic microwave background (CMB) experiments, and galaxy surveys, especially the ongoing Sloan Digital Sky Survey. It appears that special conditions are necessary since inflation occurs in tunneling between two vacua in the string landscape. Inflation theory Cosmic inflation, cosmological inflation, or simply inflation, is a hypothesis of accelerated space expansion in the early universe of physical cosmology. The term inflation refers to the explosively rapid expansion of space-time … The ekpyrotic and cyclic models are also considered adjuncts to inflation. [51] It is now believed by some that the inflaton cannot be the Higgs field[90] although the recent discovery of the Higgs boson has increased the number of works considering the Higgs field as inflaton. Cosmic inflation theory, first proposed by Alan Guth in 1981, modified the Big Bang theory (the Standard Model) by proposing that the universe, instead of unfolding at a steady pace, expanded rapidly shortly after it was created, which could account for apparent fine-tuning. Much of the historical context is explained in chapters 15–17 of Peebles (1993). There is evidence for a slight deviation from scale invariance. The discovery of flux compactifications opened the way for reconciling inflation and string theory. If you take a spherical object and suddenly inflate it into a gigantic sphere, then our perception of it appears to become flatter & flatter (ex: ant on balloon). This is a problem with Grand Unified Theories, which propose that at high temperatures (such as in the early universe) the electromagnetic force, strong, and weak nuclear forces are not actually fundamental forces but arise due to spontaneous symmetry breaking from a single gauge theory. [43][44] Not only is that not the case, but all searches for them have failed, placing stringent limits on the density of relic magnetic monopoles in the Universe. Current work on this model centers on whether it can succeed in stabilizing the size of the compactified dimensions and produce the correct spectrum of primordial density perturbations. These generically lead to curvature-squared corrections to the Einstein–Hilbert action and a form of f(R) modified gravity. [52] At the same time, Starobinsky argued that quantum corrections to gravity would replace the initial singularity of the Universe with an exponentially expanding de Sitter phase. Recent observations of the cosmic microwave background radiation in the post-inflationary universe support the inflation theory. Inflation is not the only naturalist theory on offer. [127] This model focuses on the dynamics of the early universe considered as a hot gas of strings. When investigating the effects the theory of loop quantum gravity would have on cosmology, a loop quantum cosmology model has evolved that provides a possible mechanism for cosmological inflation. [124] The flatness and horizon problems are naturally solved in the Einstein-Cartan-Sciama-Kibble theory of gravity, without needing an exotic form of matter or free parameters. Don Page was an outspoken critic of inflation because of this anomaly. Therefore, hybrid inflation is not eternal. This was quickly interpreted as meaning galaxies were receding from Earth. One of these problems is thehorizon problem. [121] Brane inflation suggests that inflation arises from the motion of D-branes[122] in the compactified geometry, usually towards a stack of anti-D-branes. Vilenkin put forth one such scenario. : New Theory on the Universe's Birth", "String Gas Cosmology and Structure Formation", International Journal of Modern Physics A, "Creating spatial flatness by combining string gas cosmology and power law inflation", Annals of the New York Academy of Sciences. Inflation will occur in virtually any universe that begins in a chaotic, high energy state that has a scalar field with unbounded potential energy. Other models of inflation relied on the properties of Grand Unified Theories. See Guth (1997) for a popular description of the workshop, or, This is known as a "red" spectrum, in analogy to, Technically, these conditions are that the, Technically, this is because the inflaton potential is expressed as a Taylor series in φ/, Breakthrough Prize in Fundamental Physics, the solution of the energy density continuity equation for an ultra-relativistic fluid, Learn how and when to remove this template message, "The Medallists: A list of past Dirac Medallists", "Laureates of the Breakthrough Prize in Fundamental Physics in 2012", "NASA Technology Views Birth of the Universe", "Space Ripples Reveal Big Bang's Smoking Gun", Using Tiny Particles To Answer Giant Questions, "Three-year Wilkinson Microwave Anisotropy Probe (WMAP) observations: Implications for cosmology", The Astrophysical Journal Supplement Series, "Our Baby Universe Likely Expanded Rapidly, Study Suggests", Monthly Notices of the Royal Astronomical Society, International Journal of Modern Physics D, "Magnetic monopoles in Unified Gauge Theories", "Phase Transitions and Magnetic Monopole Production in the Very Early Universe", "Cosmological production of superheavy magnetic monopoles", "Einstein's theory of gravitation and its astronomical consequences. Paul Steinhardt, who first introduced the eternal inflationary model,[99] later became one of its most vocal critics for this reason.[101][102][103]. The inflationary epoch lasted from 10−36 seconds after the conjectured Big Bang singularity to some time between 10−33 and 10−32 seconds after the singularity. Now imagine a photon was released very early in the Universe and travelled freely until it hits the North Pole of t… r Inflationary cosmology is in trouble because of eternal inflation and quantum runaway leading to a multiverse of possible outcomes and no definite predictions [citation needed] In the Soviet Union, this and other considerations led Belinski and Khalatnikov to analyze the chaotic BKL singularity in General Relativity. "[132] The problem of specific or "fine-tuned" initial conditions would not have been solved; it would have gotten worse. [36]:61, Therefore, regardless of the shape of the universe the contribution of spatial curvature to the expansion of the Universe could not be much greater than the contribution of matter. Such an interaction averts the unphysical Big Bang singularity, replacing it with a cusp-like bounce at a finite minimum scale factor, before which the Universe was contracting. These regions in which the inflaton fluctuates upwards expand much faster than regions in which the inflaton has a lower potential energy, and tend to dominate in terms of physical volume. The bubble collision problem was solved by Linde[59] and independently by Andreas Albrecht and Paul Steinhardt[60] in a model named new inflation or slow-roll inflation (Guth's model then became known as old inflation). The multiverse theory has created significant dissension in the scientific community about the viability of the inflationary model. [9] In 2012 Alan Guth and Andrei Linde were awarded the Breakthrough Prize in Fundamental Physics for their invention and development of inflationary cosmology.[10]. {\displaystyle r} Therefore, the most likely number of non-compact (large) spatial dimensions is three. As the very early universe cooled it was trapped in a metastable state (it was supercooled), which it could only decay out of through the process of bubble nucleation via quantum tunneling. Inflation attempts to resolve these problems by providing a dynamical mechanism that drives the Universe to this special state, thus making a universe like ours much more likely in the context of the Big Bang theory. During inflation, the energy density in the inflaton field is roughly constant. If the energy density is larger than can be held by the quantized spacetime, it is thought to bounce back.[123]. This presents a mystery: how did these new regions know what temperature and curvature they were supposed to have? Though, as cosmologist Martin Rees has written, "Skeptics about exotic physics might not be hugely impressed by a theoretical argument to explain the absence of particles that are themselves only hypothetical. [39][40] Monopoles are predicted to be copiously produced following Grand Unified Theories at high temperature,[41][42] and they should have persisted to the present day, to such an extent that they would become the primary constituent of the Universe. It was very quickly realised that such an expansion would resolve many other long-standing problems. r [29][33], The flatness problem is sometimes called one of the Dicke coincidences (along with the cosmological constant problem). Brandenberger and Vafa show that a dimension of spacetime can only expand if the strings that wind around it can efficiently annihilate each other. [125][126] This theory extends general relativity by removing a constraint of the symmetry of the affine connection and regarding its antisymmetric part, the torsion tensor, as a dynamical variable. [86][87] By 2018, additional data suggested, with 95% confidence, that In the early 1970s Zeldovich noticed the flatness and horizon problems of Big Bang cosmology; before his work, cosmology was presumed to be symmetrical on purely philosophical grounds. Others have suggested that the effect may be due to other new physics, foreground contamination, or even publication bias. [78] However, the third-year data revealed that the effect was a statistical anomaly. [5][114]:223–225 However, Albrecht and Lorenzo Sorbo argued that the probability of an inflationary cosmos, consistent with today's observations, emerging by a random fluctuation from some pre-existent state is much higher than that of a non-inflationary cosmos. [79], An experimental program is underway to further test inflation with more precise CMB measurements. Linde proposed a theory known as chaotic inflation in which he suggested that the conditions for inflation were actually satisfied quite generically. According to the theory of inflation, the early Universe expanded exponentially fast for a fraction of a second after the Big Bang. However, for many years it was not clear why or how the universe might be expanding, or what it might signify. Coleman found the most likely decay pathway for vacuum decay and calculated the inverse lifetime per unit volume. For example, classically scale invariant field theories, where scale invariance is broken by quantum effects, provide an explanation of the flatness of inflationary potentials, as long as the theory can be studied through perturbation theory.[94]. The Universe passes through the Big Crunch and emerges in a hot Big Bang phase. When three physicists published “Cosmic inflation theory faces challenges” earlier this year in Scientific American, the article itself constituted a challenge. Extrapolated into the past, this presents a fine-tuning problem because the contribution of curvature to the Universe must be exponentially small (sixteen orders of magnitude less than the density of radiation at Big Bang nucleosynthesis, for example). Cosmic inflation is a faster-than-light expansion of the universe that spawned many others. [133][134] Counter-arguments were presented by Alan Guth, David Kaiser, and Yasunori Nomura[135] and by Andrei Linde,[136] saying that "cosmic inflation is on a stronger footing than ever before".[135]. Once the other object has fallen through this horizon it can never return, and even light signals it sends will never reach the first object (at least so long as the space continues to expand exponentially). What is the hypothetical inflaton field giving rise to inflation? Inflation theory largely resolves these problems as well, thus making a universe like ours much more likely in the context of Big Bang theory. Except in contrived models, this is true regardless of how inflation is realized in particle physics. Some disagreement remains about the magnitude of this effect: about whether it is just on the threshold of detectability or completely undetectable. For the case of exactly exponential expansion, the vacuum energy has a negative pressure p equal in magnitude to its energy density ρ; the equation of state is p=−ρ. [8], In 2002 three of the original architects of the theory were recognized for their major contributions; physicists Alan Guth of M.I.T., Andrei Linde of Stanford, and Paul Steinhardt of Princeton shared the prestigious Dirac Prize "for development of the concept of inflation in cosmology". = The rapid expansion immediately after the Big Bounce explains why the present Universe at largest scales appears spatially flat, homogeneous and isotropic. [80], Other potentially corroborating measurements are expected from the Planck spacecraft, although it is unclear if the signal will be visible, or if contamination from foreground sources will interfere. Unless the rate of decay to the non-inflating phase is sufficiently fast, new inflating regions are produced more rapidly than non-inflating regions. The basic idea is that at high energies matter is better described by fields than by classical means. Obtaining a flat universe is unlikely overall. All models of eternal inflation produce an infinite, hypothetical multiverse, typically a fractal. Space is part of Future US Inc, an international media group and leading digital publisher. According to inflation theory, the universe was created in an unstable energy state, which forced a … [73] These experiments have shown that the one part in 100,000 inhomogeneities observed have exactly the form predicted by theory. The inflationary epoch lasted from 10 seconds after the conjectured Big Bang singularity to some time between 10 and 10 seconds after the singularity. [119][120] When the second (slow-rolling) inflaton reaches the bottom of its potential, it changes the location of the minimum of the first inflaton's potential, which leads to a fast roll of the inflaton down its potential, leading to termination of inflation. These models solve the horizon problem through an expanding epoch well before the Big Bang, and then generate the required spectrum of primordial density perturbations during a contracting phase leading to a Big Crunch. [71] The other free parameter is the tensor to scalar ratio. [23], Inflation is a period of supercooled expansion, when the temperature drops by a factor of 100,000 or so. The big bounce hypothesis attempts to replace the cosmic singularity with a cosmic contraction and bounce, thereby explaining the initial conditions that led to the big bang. This raised the contingent question of why four space-time dimensions became large and the rest became unobservably small. Most waves vibrate in all directions, but sometimes waves can vibrate more in one direction than in others. Scientists disagree about how to assign a probability distribution to this hypothetical anthropic landscape. The name "inflation" was given by Guth (1981). [45] A period of inflation that occurs below the temperature where magnetic monopoles can be produced would offer a possible resolution of this problem: monopoles would be separated from each other as the Universe around them expands, potentially lowering their observed density by many orders of magnitude. Later that same year, Alexander Vilenkin showed that eternal inflation is generic.[100]. As the inflationary field slowly relaxes to the vacuum, the cosmological constant goes to zero and space begins to expand normally. "[5][114] Together with Anna Ijjas and Abraham Loeb, he wrote articles claiming that the inflationary paradigm is in trouble in view of the data from the Planck satellite. [128] The original model did not "solve the entropy and flatness problems of standard cosmology",[129] although Brandenburger and coauthors later argued that these problems can be eliminated by implementing string gas cosmology in the context of a bouncing-universe scenario.[130][131]. Although new inflation is classically rolling down the potential, quantum fluctuations can sometimes lift it to previous levels. [53] In October 1980, Demosthenes Kazanas suggested that exponential expansion could eliminate the particle horizon and perhaps solve the horizon problem,[54][55] while Sato suggested that an exponential expansion could eliminate domain walls (another kind of exotic relic). [38] These theories predict a number of heavy, stable particles that have not been observed in nature. − At present, while inflation is understood principally by its detailed predictions of the initial conditions for the hot early universe, the particle physics is largely ad hoc modelling. Misner's Mixmaster universe attempted to use this chaotic behavior to solve the cosmological problems, with limited success. Other models have been advanced that are claimed to explain some or all of the observations addressed by inflation. This work is based upon a certain kind of polarization in the CMB. In the approximation that the expansion is exactly exponential, the horizon is static and remains a fixed physical distance away. One of the most severe challenges for inflation arises from the need for fine tuning. [72], Inflation predicts that the observed perturbations should be in thermal equilibrium with each other (these are called adiabatic or isentropic perturbations). Like a metastable phase in statistical mechanics—water below the freezing temperature or above the boiling point—a quantum field would need to nucleate a large enough bubble of the new vacuum, the new phase, in order to make a transition. However such a field would be scalar and the first relativistic scalar field proven to exist, the Higgs field, was only discovered in 2012–2013 and is still being researched. It’s often seen as powerful support of the Big Bang Theory… This is the inability of the standard Big Bang theory to explain why the cosmic microwave background (CMB) radiation is so incredibly uniform throughout our universe. Each string is a one-dimensional object, and the largest number of dimensions in which two strings will generically intersect (and, presumably, annihilate) is three. This theory, governed by the Dirac-Born-Infeld action, is different from ordinary inflation. The simplest inflation models predict that ns is between 0.92 and 0.98. [98] The amplitude of the primordial inhomogeneities produced in inflation is directly tied to the energy scale of inflation. The situation is quite different in the big bang model without inflation, because gravitational expansion does not give the early universe enough time to equilibrate. Future US, Inc. 11 West 42nd Street, 15th Floor, This results in the observables: This prediction implies that the total ordinary matter, dark matter and residual vacuum energy in the Universe have to add up to the critical density, and the evidence supports this. If Earth is not in some special, privileged, central position in the universe, then it would mean all galaxies are moving apart, and the further away, the faster they are moving away. However, explanations for these fine-tunings have been proposed. [5] In 1999, John Earman and Jesús Mosterín published a thorough critical review of inflationary cosmology, concluding, "we do not think that there are, as yet, good grounds for admitting any of the models of inflation into the standard core of cosmology. [3] Many physicists also believe that inflation explains why the universe appears to be the same in all directions (isotropic), why the cosmic microwave background radiation is distributed evenly, why the universe is flat, and why no magnetic monopoles have been observed. The theory of inflation thus explains why the temperatures and curvatures of different regions are so nearly equal. "[7], A recurrent criticism of inflation is that the invoked inflaton field does not correspond to any known physical field, and that its potential energy curve seems to be an ad hoc contrivance to accommodate almost any data obtainable. In an expanding universe, energy densities generally fall, or get diluted, as the volume of the Universe increases. Philosophical predispositions color efforts to debunk a popular theory about the evolution of the universe. [112] He stressed that the thermodynamic arrow of time necessitates low entropy initial conditions, which would be highly unlikely. , Whether the correct spectrum of density fluctuations can be produced, and whether the Universe can successfully navigate the Big Bang/Big Crunch transition, remains a topic of controversy and current research. Inflation theory was developed in the late 1970s and early 80s, with notable contributions by several theoretical physicists, including Alexei Starobinsky at Landau Institute for Theoretical Physics, Alan Guth at Cornell University, and Andrei Linde at Lebedev Physical Institute. In the VSL models, the fundamental constant c, denoting the speed of light in vacuum, is greater in the early universe than its present value, effectively increasing the particle horizon at the time of decoupling sufficiently to account for the observed isotropy of the CMB. The big idea that actually succeeded is known, today, as cosmic inflation. With exponentially expanding space, two nearby observers are separated very quickly; so much so, that the distance between them quickly exceeds the limits of communications. [74][72] As a physical model, however, inflation is most valuable in that it robustly predicts the initial conditions of the Universe based on only two adjustable parameters: the spectral index (that can only change in a small range) and the amplitude of the perturbations. But if inflation lasted long enough to solve the initial conditions problems, collisions between bubbles became exceedingly rare. [97] While inflation depends on quantum field theory (and the semiclassical approximation to quantum gravity) in an important way, it has not been completely reconciled with these theories. [48] This resolved the cosmology problems and led to specific predictions for the corrections to the microwave background radiation, corrections that were then calculated in detail. The minimal coupling between torsion and Dirac spinors generates a spin-spin interaction that is significant in fermionic matter at extremely high densities. Some physicists have tried to avoid the initial conditions problem by proposing models for an eternally inflating universe with no origin. But as the Universe expands, the curvature redshifts away more slowly than matter and radiation. ], stable magnetic monopoles would have been produced. Inflation predicts that the structures visible in the Universe today formed through the gravitational collapse of perturbations that were formed as quantum mechanical fluctuations in the inflationary epoch. Some have claimed that this is a signature of non-Gaussianity and thus contradicts the simplest models of inflation. However, unlike de Sitter space, fluctuations in a contracting inflationary space collapse to form a gravitational singularity, a point where densities become infinite. 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