Cosmic inflation

Cosmic inflation is a set of proposals within the framework of theoretical physics to explain the ultra-fast expansion of the universe in the initial moments and to solve the so-called horizon problem.

Introduction

It was first proposed by American physicist and cosmologist Alan Guth in 1981 and independently Andréi Linde, Andrea Albrecht together with Paul Steinhardt gave it its modern form.

Although the detailed particle physics mechanism responsible for inflation is unknown, the basic picture provides a number of predictions that have been confirmed by observational tests. Inflation is currently considered to be part of the standard hot Big Bang cosmological model (an update of the old theory, which has the same main idea anyway). The elementary particle or hypothetical field that is thought to be responsible for inflation is called an inflaton.

Inflation suggests that there was a period of exponential expansion in the very pre-primal Universe. The expansion is exponential because the distance between two fixed observers increases exponentially, due to the expansion metric of the Universe (a space-time with this property is called a de Sitter space). The physical conditions from one moment to the next are stable: the expansion rate, given by the Hubble constant, is nearly constant, leading to high levels of symmetry. Inflation is often known as a period of accelerated expansion because the distance between two fixed observers increases at an accelerating rate as they move apart. (However, this does not mean that the Hubble parameter is increasing, see deceleration parameter).

On March 17, 2014, BICEP2 astrophysicists announced the presumed detection of inflationary gravitational waves by observing B-modes in the cosmic microwave background polarization. The B modes in the cosmic microwave background could be due to Guth's inflation theory and to the Big Bang.

Motivation

Inflation solves several problems in Big Bang cosmology that were pointed out in the 1970s. These problems stem from the observation that to resemble the universe today, the universe would have to having started from "special"initial conditions; or very tuned up near the Big Bang. Inflation solves these problems by providing a dynamic mechanism that drives the universe into this special state, thus making the universe much more natural like ours in the context of the Big Bang theory.

Cosmic inflation has the important effect of resolving heterogeneities, anisotropy and the curvature of space. This puts the universe in a very simple state, in which it is completely dominated by the inflaton field and the only significant heterogeneities are the weak quantum fluctuations in the inflaton. Inflation also dilutes exotic heavy particles, such as the magnetic monopoles predicted by many extensions of the Standard Model of particle physics. If the universe were hot enough to form such pre-inflation particles, they would not be observed in nature, since they would be so rare that it is quite likely that there are none in the observable universe. Together, these effects are called the "inflationary no-hair theorem" by analogy with the no-hair theorem for black holes.

The "hairlessness theorem" it's essentially because the universe expands by a huge factor during inflation. In an expanding universe, energy densities generally drop as the volume of the universe increases. For example, the density of matter (dust) "cold" Ordinary energy is inversely proportional to volume: when the linear dimensions are doubled, the energy density falls by a factor of eight. The energy density in radiation falls even more rapidly as the universe expands: when the linear dimensions are doubled, the radiation energy density falls by a factor of sixteen. During inflation, the energy density in the inflaton field is nearly constant. However, the energy density in heterogeneities, curvature, anisotropies and exotic particles is decreasing and with sufficient inflation these become negligible. This leaves an empty, flat, symmetrical universe that is filled with radiation when inflation ends.

A key requirement is that inflation has to continue long enough to produce the current observable universe from a single small inflationary Hubble volume. This is necessary to ensure that the universe appears flat, homogeneous, and isotropic on the largest observable scales. This requirement is generally thought to be satisfied if the universe expanded by a factor of at least 10²⁶ during inflation. At the end of inflation, a process called reheating occurs. >, in which the inflaton particles decay into radiation starting the hot Big Bang. It is not known how long the inflation lasted, but it is thought to have been extremely short compared to the age of the universe. Assuming that the inflation energy scale is between 10¹⁵ and 10¹⁶ eV, as suggested by the simplest models, the period of inflation responsible for the observable Universe likely lasted about 10^-33 seconds.

Problem of flatness and fine adjustment

There is evidence that on a large scale our universe seems very close to the latitude, and therefore its global curvature is about zero K≈ ≈ 0{displaystyle Kapprox 0} (obviously locally this is not fulfilled, especially near supermassive stars or black holes). Since the Ω density parameter is related to the curvature KHubble's constant H and scale factor a by relationship:

Ω Ω (t)− − 1=Ka2(t)H2(t){displaystyle Omega (t)-1={frac {K}{a^{2}(t)H^{2}{2}}}}}}

only if initially K = 0, the value of Ω remains constant (in that case, Ω = 1). But if the curvature is not exactly zero, then Ω(t) moves away from 1 as the universe expands, indeed for a matter-dominated universe like ours:

日本語Ω Ω (t)− − 1日本語 t2/3{displaystyle ΔOmega (t)-1UDpropto t^{2/3}}

So, if the value of Ω is currently close to 1, that implies that at the beginning of the universe it was still much closer to 1, that is, close to ideal flatness. Estimates suggest that at the time nucleosynthesis began there must have been:

日本語Ω Ω (tnuc)− − 1日本語≤ ≤ 10− − 16{displaystyle ΔOmega (t_{nuc})-1ёleq 10^{-16}}}

Since it seems unlikely that by simple chance Ω would have gotten so close to the value 1, inflation is a mechanism that could explain why the universe became so finely tuned around the value Ω = 1, since many cosmologists consider that it is not chance that among many possible values the universe has a value so close precisely to the value implied by flatness.

Horizon problem

The horizon problem is the problem of determining why the universe appears statistically homogeneous and isotropic according to the cosmological principle. The gas molecules in a gas canister are homogeneously and isotropically distributed because they are in thermal equilibrium: the gas throughout the canister has had enough time to interact, to dissipate heterogeneities and anisotropies. The situation is quite different in the Big Bang model without inflation, because the gravitational expansion does not give the early Universe enough time to equilibrate. In a Big Bang containing only the matter and radiation known in the Standard Model, two widely separated regions of the observable universe cannot have equilibrated because they have never come into causal contact: in the history of the universe, going back to the earliest times, it has not been possible to send a light signal between the two regions. Since they have no interaction, it is impossible for them to balance. This is because the Hubble radius in a Universe dominated by matter or radiation expands much faster than physical lengths and such points that are cut off communicate. Historically, two proposed solutions were Georges Lemaître's Phoenix universe, Richard Tolman's related oscillating universe, and Charles Misner's Mixmaster universe. Lemaître and Tolman proposed that a universe undergoing various cycles of contraction and expansion could become a thermal balance. Their models failed, however, because of the accumulation of entropy through several cycles. Misner made the (ultimately incorrect) conjecture that the Mixmaster mechanism, which made the universe more chaotic, could lead to statistical homogeneity and isotropy.

Problem of monotony

Another problem is the monotonicity problem (sometimes known as one of Dicke's coincidences, with the other being the cosmological constant problem). In the 1960s it was known that the density of matter in the universe was comparable to the critical density needed for a flat universe (that is, a universe whose large geometric scale is the usual Euclidean geometry, rather than a hyperbolic or spherical non-Euclidean geometry). Thus, despite the shape of the universe, the contribution of space curvature to the expansion of the universe could not be much greater than the contribution of matter, but as the universe expands, the redshift curvature is slower than matter and radiation. Extrapolating back in time, a fine-tuning problem presents itself because the curvature contribution to the Universe would have to be exponentially small (sixteen orders of magnitude less than the radiation density at the nu Big Bang cleosynthesis, for example). This problem is exacerbated by recent observations of the microwave background radiation that have shown that the Universe is flat to the precision of a few percent.

Magnetic monopole problem

The magnetic monopole problem (sometimes called the alien relics problem) is a problem that suggests that if the early Universe was very hot, a large number of very heavy and stable magnetic monopoles would be produced. This problem, along with the grand unification theory, were popular in the 1970s and 1980s, which proposed that at high temperatures (as in the early universe) the electromagnetic force and the strong and weak nuclear forces are not really fundamental forces., rather they appear due to spontaneous electroweak symmetry breaking of a gauge theory. These theories predict several stable heavy particles that have not yet been observed in nature. Most notorious is the magnetic monopole, a type of stable and heavy magnetic field. Monopoles are expected to be produced copiously in Grand Unification Theory at high temperatures and should have persisted to the present day. For very high precisions high, magnetic monopoles appear to not exist in nature, while according to the Big Bang theory (without cosmic inflation) they should have been copiously produced in the hot and dense early universe, as it became the primary constituent. of the universe.

Other issues

The cascade mechanism of division and elongation of photons (CDEF) that precedes the formation of matter was proposed to explain the elongation of cosmic background radiation (Cosmic Microwave Background: CMB) by Alfredo Bennun, Rutgers University. This model was subjected to a simulation where it is proposed that the primordial energy can be described as a radiation, which allows to characterize the same depending on its wavelength even if of a physical nature it is not established. Thus this ultra-fast frequency (v) and ultra-small wavelength (λ) radiation could be evaluated as very high energy photons limited by Planck constant (1022{displaystyle 10^{22}MeV). These would initially be confined within a three-dimensional space of the order of a Fermi radio 10− − 13{displaystyle 10^{-13}cm) avoiding the punctual and therefore non-physical nature of a timeless singularity. The waterfall was considered a repeated sequence of 66 times, (1x2)66{displaystyle (1x2)^{66}} divisions of the initial photons but the initial increase in the radius of the universe is expressed in base 4 and exponential 66 or (1x2x2)66{displaystyle (1x2x2)^{66} because in each division or partition of the photons simultaneously bends its number and the width of wavelength. Both processes are not limited by the speed of the propagation of light in space because they involve transitions from the amplitude of time space. This expansive and antagonistic mechanism to gravitational attraction is therefore assimilable to Einstein's cosmological constant and is totally different from that proposed by Alan Guth although they are obtained values similar to those that are standard to characterize the end of the inflation scenario.

History

Inflation was proposed in 1981 by Alan Guth as a mechanism to solve these problems. Various precursors, most importantly the work of Willem de Sitter who demonstrated the existence of a highly symmetric inflationary Universe, called de Sitter space. De Sitter, however, did not apply it to any cosmological problem that Guth was interested in. A contemporary of Guth, Alexei Starobinsky argued that quantum corrections to gravity would replace the initial singularity of the Universe with a state of exponential expansion. Demosthenes Kazanas anticipated part of Guth's work suggesting that exponential expansion could remove the particle horizon and perhaps solve the horizon problem, and Sato suggested that exponential expansion could remove domain walls (another type of exotic relic). However, Guth was the first to assemble a complete drawing of how all these initial conditions could be solved by using an exponentially expanding state.

Guth proposed that as the early Universe cooled, it was trapped in a false vacuum with a high energy density, which resembles a cosmological constant. As the early Universe cooled it was trapped in a metastable state (it was supercooled) that could only decay through the process of bubble nucleation via quantum tunneling. Bubbles of the true vacuum form spontaneously in the sea of false vacuum and it rapidly begins to expand at the speed of light. Guth recognized that this model was problematic because the model did not reheat properly: when the bubbles nucleated, they did not generate any radiation. Radiation could only be generated in collisions between bubble walls. But if inflation lasted long enough to fix the initial condition problems, bubble collisions became exceedingly rare. (Even though the bubbles are expanding at the speed of light, the bubbles are far from the expansion of space causing the distance between them to expand much faster.)

This problem was solved by Andrei Linde and independently by Andreas Albrecht and Paul Steinhardt in a model called new inflation or slow-turnover inflation (the Guth model). became known thereafter as old inflation). In this model, instead of tunneling from a false vacuum state, inflation occurred by a scalar field rotating down a mountain of potential energy. When the field rotates very slowly compared to the expansion of the Universe, inflation occurs. However, when the mountain gets steeper, inflation stops and overheating can occur.

Finally, it was shown that the new inflation does not produce a perfectly symmetrical Universe, but rather weak quantum fluctuations are generated in the inflaton. These faint fluctuations formed the primordial seeds for all structures created in the later Universe. These fluctuations were first calculated by Viatcheslav Mukhanov and G. V. Chibisov in the Soviet Union analyzing the similar Starobinsky model. In the context of inflation, they obtained the results independently of the work of Mukhanov and Chibisov at the 1982 Nuffield Workshop. on the Early Universe at the University of Cambridge. The fluctuations were calculated by four groups working separately during the working group's career: Stephen Hawking, Starobinsky, Guth and So-Young Pi; and James M. Bardeen, Paul Steinhardt and Michael Turner.

Observational state

Inflation is a concrete mechanism for realizing the cosmological principle that is the basis of our model of physical cosmology: it is responsible for the homogeneity and isotropy of the observable Universe. It further accounts for the observed monotony and the absence of magnetic monopoles. Like Guth's early work, each of these observations has received further confirmation, impressively by detailed observations of the microwave background radiation made by the WMAP satellite. This analysis shows that the Universe is flat to a precision of at least a few percent and is homogeneous and isotropic to one part in 10,000.

In addition, inflation predicts that the structures visible in the Universe today formed through the gravitational collapse of disturbances that were generated as quantum mechanical fluctuations in the inflationary epoch. The detailed form of the disturbance spectrum called a nearly invariant random Gaussian Field (or Harrison-Zel'dovich spectrum) is very specific and has only two free parameters, the amplitude of the spectrum and the spectral index which measures slight deviations from the scale invariance predicted by inflation (the scale with perfect invariance corresponds to de Sitter's idealized Universe). Inflation predicts that the observed disturbances should be in thermal equilibrium with each other (these are called adiabatic or isentropic disturbances). This perturbation structure has been confirmed by the WMAP satellite and other microwave radiation background experiments, and the measurement of galaxies, especially the current Sloan Digital Sky Survey. These experiments have shown that one part in 10,000 of the heterogeneities observed have exactly the shape predicted by the theory. In addition, the slight deviation from scale invariance has been measured. The spectral index, n_s is equal to one for a scale-invariant spectrum. The simplest models of inflation predict this quantity to be between 0.92 and 0.98. The WMAP satellite has measured n_s = 0.95 and shows that it is different from one to two levels of the standard deviation (2σ). This is considered an important confirmation of the inflation theory.

Various theories of inflation have been proposed that make radically different predictions, but generally have much more fine-tuning than necessary. As a physical model, however, inflation is most valuable in robustly predicting the initial conditions of inflation. Universe based on only two adjustable parameters: the spectral index (which can only change in a small range) and the amplitude of the disturbances. Except in artificial models, this is true regardless of how inflation is done in particle physics.

Occasionally, effects are observed that seem to contradict the simplest models of inflation. This first year of data from WMAP suggests that the spectrum need not be nearly invariant, but may have a slight curvature. However, the third year of data revealed that the effect was a statistical anomaly. Another effect that has been remarked since the first satellite on the microwave background radiation, the Cosmic Background Explorer (COBE): the amplitude of the moment of the quadrupole of the microwave radiation background is unexpectedly low and the other low multipoles seem be preferably aligned with the ecliptic plane. It has been said that this is a non-Gaussian signature and contradicts the simplest models of inflation. Others suggest that the effect may be due to other new physical effects, background contamination, or even publication deviation.

An experimental program is in the process of testing deeper inflation tests with more precise measurements of the background microwave radiation. In particular, the high-precision measurements of the so-called "B-modes" From the polarization of the microwave background radiation, the gravitational radiation produced by inflation would be evident and it would be demonstrated if the inflation energy scale predicted by the simplest models (10¹⁵-10 ¹⁶ eV) is correct. These measurements were expected to be made by Planck, although it is unclear whether the signal is visible or whether contamination from background sources will interfere with these measurements. Other measurements to come, such as those from the 21-centimeter radiation (radiation emitted and absorbed from neutral hydrogen before the first stars ignited), can measure the power spectrum with even higher resolution than background microwave radiation and galaxy measurements, although it is not known if these measurements will be possible or if the interference with radiation sources on Earth and in the galaxy will be too great.

After 2006, it is not clear that the relationship of any period of cosmic inflation has to do with dark energy. Dark energy is broadly similar to inflation and is thought to be the cause of the acceleration in the expansion of the current Universe. However, the energy scale of dark energy is much lower, 10^-12 eV, about 27 orders of magnitude less than the scale of inflation.

State of the theory

Unresolved problems of physics: Is the theory of cosmic inflation correct, and if so, what are the details of this time?What is the hypothetical inflation field that gives rise to inflation?

In Guth's first proposal, the inflaton was thought to be the Higgs field, the field that explains the mass of elementary particles. It is now believed that the inflaton cannot be the Higgs field (although the recent discovery at CERN of the Higgs boson is leading to the emergence of new models that do use the Higgs field). Other models of inflation rely on the properties of grand unified theories. As the simpler Grand Unified Theory models have failed, many physicists think that inflation will be included in a supersymmetric theory such as string theory or a supersymmetric grand unification theory. One promising suggestion is brane inflation. So far, however, inflation is mainly understood for its detailed predictions of initial conditions for the hot early Universe and particle physics is largely modeled ad hoc. As such, despite the stringent observational tests that inflation has passed, there are many open questions about the theory.

Fine adjustment problem

One of the biggest challenges for inflation arises from the need to fine-tune inflationary theories. In new inflation, slow turnover conditions must be in place for inflation to occur. The conditions for slow rotation say that the potential has to be uniform (compared to the high energy of a vacuum) and that the inflaton particles have to have a small mass. For the new inflation theory of Linde, Albrecht and Steinhardt to be Therefore, it seems that the Universe has a scalar field with a particularly flat potential and special initial conditions.

Andrei Linde proposed a theory known as chaotic inflation in which he suggested that the conditions for inflation are indeed generically satisfied and inflation will occur in any Universe that virtually starts out in a chaotic and have a scalar field with unbounded potential energy. However, in his model the inflaton field necessarily takes values greater than one Planck unit: for this reason, they are often called large field models and the new inflation models are called small field models. In this situation, the predictions of the faulty field theory are thought to be invalid and the renormalization should cause large corrections that would prevent inflation. This problem has not yet been resolved and some cosmologists argue that small field models, in which inflation can occur at much smaller energy scales are better models of inflation. While inflation depends on quantum field theory (and the semiclassical approach to quantum gravity) in important ways, it has not been fully reconciled. with these theories.

Robert Brandenberger has commented on fine tuning in another situation. The amplitude of the primal heterogeneities produced by inflation is directly related to the energy scale of inflation. There are strong assumptions that this scale is about 10¹⁶ eV or 10⁻³ times the Planck energy. The natural scale is naively like the Planck scale in such a way that this small value could be seen as another form of fine tuning (called the hierarchy problem): the energy density given by the scalar potential is below 10⁻¹² compared to the Planck density. This is not normally considered a critical problem, however, because the scale of inflation naturally corresponds to the scale of the unification gauge.

Eternal inflation

Cosmic inflation appears to be eternal the way it is theorized. Although the new inflation is classically the downward rotation of the potential, quantum fluctuations can sometimes cause it to return to previous levels. These regions where the inflaton fluctuates upwards expand much faster than regions where the inflaton has lower potential energy and tends to dominate in terms of physical volume. This steady state, first developed by Vilenkin, is called 'eternal inflation'. Any inflationary theory with an unbounded potential has been shown to be eternal. It is a popular belief among physicists that the steady state cannot continue forever into the past. Inflationary spacetime, which is similar to space de Sitter, is incomplete without a region of contraction. However, despite the de Sitter space, fluctuations in a contracting inflationary space will collapse to form a gravitational singularity, a point where the densities will become infinite. Therefore, it is necessary to have a theory for the initial conditions of the Universe. This interpretation was disputed by Linde.

Initial conditions

Some physicists have attempted to avoid this problem by proposing models for an eternally inflationary Universe with no origin. These models propose an "initial" especially when the Universe has a minimum size and in which time begins.

Other proposals attempt to describe quantum cosmology's nihilistic creation of the Universe and the consequent inflation. Vilenkin proposed such a scenario. Hartle and Hawking proposed the Hartle-Hawking state for the initial creation of the Universe in which inflation occurs naturally.

Alan Guth has described the inflationary Universe as the "last free meal": new Universes, similar to our own, are continually being produced on a vast inflationary background. Gravitational interactions, in this case, circumvent (but do not violate) neither the first law of thermodynamics or conservation of energy nor the second law of thermodynamics or the arrow of time problem. However, while there is a consensus that this solves the initial conditions problem, this has been disputed by some, since it is much more likely that the Universe came from a quantum fluctuation. Donald Page has been a staunch critic of inflation for this anomaly.He stressed that the thermodynamic arrow of time needed low-entropy initial conditions, which could be highly probable. According to them, rather than solving this problem, inflation theory aggravates it—warming at the end of the inflation era increases entropy, making it necessary for the initial state of the Universe to be even more orderly than in other times. Big Bang theories without inflation phase.

Hawking and Page later found ambiguous results when they attempted to compute the probability of inflation in the Hartle-Hawking initial state. Other authors have disputed this, since inflation is eternal, the probability that it will never occur is not At precisely zero, once it starts, inflation is self-perpetuating and rapidly dominates the Universe. Recently, Lisa Dyson, Matthew Kleban, and Leonard Susskind discussed using the Holographic Principle that spontaneous inflation is exceedingly improbable. Albrecht and Lorenzo Sorbo have argued that the probability of an inflationary cosmos, consistent with current observations, emerging from a random fluctuation of some pre-existing state, compared to a non-inflationary cosmos overwhelmingly favors the inflationary scenario, simply because the "seed" Non-gravitational energy sums required for the inflationary cosmos is much less than any required for a non-inflationary alternative, which outweighs any entropic considerations.

Another problem that has occasionally been mentioned is the trans-Planckian problem or trans-Planckian effects. As the inflation energy scale and the Planck scale are relatively close, some of the quantum fluctuations that have built the structure of our Universe were smaller than the Planck length before inflation. Therefore, there could be corrections to Planck's physics, particularly in the unknown quantum theory of gravity. There has been some disagreement about the magnitude of this effect: about whether it is just on the threshold of detectability, or whether it is completely undetectable.

Overheating

The end of inflation is known as reheating or thermalization because the large potential energy breaks down into particles and fills the Universe with radiation. As the nature of the inflaton is not known, this process remains poorly understood, although it is believed to take place via parametric resonance.

Non-eternal inflation

Another type of inflation, called hybrid inflation, is an extension of new inflation. Inflation introduces additional scalar fields, such that one of those fields is responsible for normal slow-rotating inflation, another triggers the end of inflation: when inflation has lasted long enough, it becomes favorable for the second. field to decay to a much lower energy state. Unlike other inflation models, many versions of hybrid inflation are not everlasting. In hybrid inflation, one of the scalar fields is responsible for much of the density of energy (thus determining the rate of expansion), while the others are responsible for slow rotation (thus determining the period of inflation and its termination). These fluctuations in the old inflaton would not affect the end of inflation, while later fluctuations would not affect the rate of expansion. Therefore, hybrid inflation is not eternal. When the second (slowly rotating) inflaton is at the bottom of its potential, the location of the minimum of the first inflaton potentials changes, leading to a rapid rotation of this inflaton so that its potential decreases, leading to the end of inflation.

Inflation and the Cosmology of Strings

The discovery of flow compactions has opened the way to reconcile inflation and string theory. A new theory, called brane inflation, suggests that inflation arises from a D-brane falling into a deep Klebanov-Strassler gorge. This is a very different theory from ordinary inflation (it is governed by the Dirac-Born-Infeld action which is very different from the other) and the dynamics remain poorly understood. It seems that very special conditions are necessary for inflation to occur in the tunnel between two voids in the sea of strings (the process of tunneling between two voids is a form of old inflation, but the new inflation then has to occur by some other mechanism)..

Alternatives to inflation

String theory requires that, in addition to the three dimensions we observe, there are additional dimensions that are atrophied (see also Kaluza-Klein theory). Extra dimensions appear as frequent components of supergravity models and other alternatives to quantum gravity. This raises the question: why do all four dimensions of space-time get big and the rest get unobservably small? An attempt to address this question, called gaseous string cosmology, was proposed by Robert Brandenberger and Cumrun Vafa. This model focuses on the dynamics of the early Universe considered as a hot string gas. Brandenberger and Vafa showed that a spacetime dimension could only expand if the coiled strings could efficiently annihilate each other. Each string is a one-dimensional object and the largest number of dimensions at which two strings will generically intersect (and presumably annihilate) is three. Therefore, it is argued that the most probable number of large non-compact spatial dimensions is three. Current work on this model is focused on whether it can succeed in stabilizing the size of the stunted dimensions and producing the correct spectrum of primordial perturbation density. From a burst point of view.

Ecpyrotic and cyclical models are also considered competitors of inflation. These models solve the problem of the horizon through a time of expansion predating the Big Bang and then generate the required spectrum of primordial perturbation density during a contraction phase leading to a Big Crunch. . The Universe passes through the Big Crunch and emerges in a hot phase of the Big Bang. In this sense there are reminiscences of the "oscillating universe" proposed by Richard Chace Tolman: however, in Tolman's model the total age of the Universe is necessarily finite, while in these models it is not so necessary. Whether a correct fluctuation density spectrum can be produced and whether the Universe can navigate satisfactorily from a Big Bang to a Big Crunch remains a matter of controversy and current research.

Criticism of inflation

Since its introduction by Alan Guth in 1981, the inflationary paradigm has been continuously in vogue among cosmologists. Hailed as the culmination of the standard model of the big bang, it has been presented in the popular science literature and even in cosmology textbooks as an established and proven result of research. However, a growing number of physicists, mathematicians, and philosophers of science have cast doubt on it, pointing to its flaws, gaps, and broken promises, and its lack of empirical support. In 1999, John Earman and Jesús Mosterín published a thorough critical analysis of inflationary cosmology, concluding that "we still lack valid reasons for admitting any of the inflation models into the standard core of cosmology."

The question has been raised as to whether the alleged problems that inflation would be called upon to solve (from the absence of magnetic monopoles to the uniformity and flatness of the observable universe) could not be pseudo-problems, since magnetic monopoles have nothing to do with them. do with the big bang and that the acceptance of initial or binding conditions in mathematical models of physics is a common and well-established practice. In any case, and as Roger Penrose has been pointing out since 1986, in order to function, inflation requires extremely specific initial conditions, so the initial conditions problem (or pseudo-problem) is by no means solved.: “There is something fundamentally wrong with the attempt to explain the smoothness of the early universe as a result of a thermalization process. [...] Indeed, if thermalization does something [...], then it represents a definite increase in entropy. Therefore, the universe would have had to be even more special before thermalization than after it." The problem of finely tuned or specific initial conditions would not only have been unsolved, but would have been aggravated.

The inflationary paradigm predicts and explains inflation by invoking the inflaton field, which is neither coincident nor related to any known physical field. A recurring criticism concerns the arbitrariness of the inflaton's potential energy curve, which appears to be merely an ad hoc gimmick to accommodate whatever data we can find. It is significant that Paul J. Steinhardt, one of the founders of inflationary cosmology, has recently become one of its harshest critics. He calls a period of accelerated expansion that leads to a result that contradicts the observations 'bad inflation', and one that is compatible with them 'good inflation': “Not only is bad inflation more likely than good inflation, but it also the absence of inflation is more likely than both. […] Roger Penrose has considered all possible configurations of the inflaton and gravitational fields. Some of these configurations lead to inflation. Other configurations lead directly to a smooth, flat universe—without inflation. The outcome of a flat universe is unlikely in general. But Penrose's shocking conclusion, however, is that a flat universe without inflation is much more likely than with inflation—by a factor of 10 to the power of googol (10 to the power of 100)—".