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The **Big Bounce** is a hypothesized cosmological model for the origin of the known universe. It was originally suggested as a phase of the * cyclic model * or *oscillatory universe* interpretation of the Big Bang, where the first cosmological event was the result of the collapse of a previous universe. It receded from serious consideration in the early 1980s after inflation theory emerged as a solution to the horizon problem, which had arisen from advances in observations revealing the large-scale structure of the universe. In the early 2000s, inflation was found by some theorists to be problematic and unfalsifiable in that its various parameters could be adjusted to fit any observations, so that the properties of the observable universe are a matter of chance. Alternative pictures including a Big Bounce may provide a predictive and falsifiable possible solution to the horizon problem, and are under active investigation as of 2017.^{ [1] }

The concept of the Big Bounce envisions the Big Bang as the beginning of a period of expansion that followed a period of contraction. In this view, one could talk of a * Big Crunch * followed by a *Big Bang*, or more simply, a *Big Bounce*. This suggests that we could be living at any point in an infinite sequence of universes, or conversely the current universe could be the very first iteration. However, if the condition of the interval phase "between bounces", considered the 'hypothesis of the primeval atom', is taken into full contingency such enumeration may be meaningless because that condition could represent a singularity in time at each instance, if such perpetual return was absolute and undifferentiated.^{[ citation needed ]}

The main idea behind the quantum theory of a Big Bounce is that, as density approaches infinity, the behavior of the * quantum foam * changes. All the so-called fundamental physical constants, including the speed of light in a vacuum, need not remain constant during a Big Crunch, especially in the time interval smaller than that in which measurement may never be possible (one unit of Planck time, roughly 10^{−43} seconds) spanning or bracketing the point of inflection.^{[ citation needed ]}

Big bounce models have a venerable history^{[ further explanation needed ]} and were endorsed on largely aesthetic grounds^{[ which? ]}^{[ when? ]} by cosmologists including Willem de Sitter, Carl Friedrich von Weizsäcker, George McVittie and George Gamow (who stressed that "from the physical point of view we must forget entirely about the precollapse period").^{ [2] }

By the early 1980s, the advancing precision and scope of observational cosmology had revealed that the large-scale structure of the universe is flat, homogenous and isotropic, a finding later accepted as the Cosmological Principle to apply at scales beyond roughly 300 million light-years. It was recognized that it was necessary to find an explanation for how distant regions of the universe could have essentially identical properties without ever having been in light-like communication. A solution was proposed to be a period of exponential expansion of space in the early universe, as a basis for what became known as Inflation theory. Following the brief inflationary period, the universe continues to expand, but at a less rapid rate.

Various formulations of inflation theory and their detailed implications became the subject of intense theoretical study. In the absence of a compelling alternative, inflation became the leading solution to the horizon problem. In the early 2000s, inflation was found by some theorists to be problematic and unfalsifiable in that its various parameters could be adjusted to fit any observations, a situation known as a fine-tuning problem. Furthermore, inflation was found to be inevitably eternal, creating an infinity of different universes with typically different properties, so that the properties of the observable universe are a matter of chance.^{ [3] } An alternative concept including a Big Bounce was conceived as a predictive and falsifiable possible solution to the horizon problem,^{ [4] } and is under active investigation as of 2017.^{ [5] }^{ [1] }

The phrase "Big Bounce" appeared in the scientific literature in 1987, when it was first used in the title of a pair of articles (in German) in *Stern und Weltraum* by Wolfgang Priester and Hans-Joachim Blome.^{ [6] } It reappeared in 1988 in Iosif Rozental's *Big Bang, Big Bounce*, a revised English-language translation of a Russian-language book (by a different title), and in a 1991 article (in English) by Priester and Blome in *Astronomy and Astrophysics*. (The phrase apparently originated as the title of a novel by Elmore Leonard in 1969, shortly after increased public awareness of the Big Bang model with of the discovery of the cosmic microwave background by Penzias and Wilson in 1965.)

Martin Bojowald, an assistant professor of physics at Pennsylvania State University, published a study in July 2007 detailing work somewhat related to loop quantum gravity that claimed to mathematically solve the time before the Big Bang, which would give new weight to the oscillatory universe and Big Bounce theories.^{ [7] }

One of the main problems with the Big Bang theory is that at the moment of the Big Bang, there is a singularity of zero volume and infinite energy. This is normally interpreted as the end of the physics as we know it; in this case, of the theory of general relativity. This is why one expects quantum effects to become important and avoid the singularity.

However, research in loop quantum cosmology purported to show that a previously existing universe collapsed, not to the point of singularity, but to a point before that where the quantum effects of gravity become so strongly repulsive that the universe rebounds back out, forming a new branch. Throughout this collapse and bounce, the evolution is unitary.

Bojowald also claims that some properties of the universe that collapsed to form ours can also be determined. Some properties of the prior universe are not determinable however due to some kind of uncertainty principle.

This work is still in its early stages and very speculative. Some extensions by further scientists have been published in *Physical Review* Letters.^{ [8] }

In 2003, Peter Lynds has put forward a new cosmology model in which time is cyclic. In his theory our Universe will eventually stop expanding and then contract. Before becoming a singularity, as one would expect from Hawking's black hole theory, the universe would bounce. Lynds claims that a singularity would violate the second law of thermodynamics and this stops the universe from being bounded by singularities. The Big Crunch would be avoided with a new Big Bang. Lynds suggests the exact history of the universe would be repeated in each cycle in an eternal recurrence. Some critics argue that while the universe may be cyclic, the histories would all be variants.^{[ citation needed ]} Lynds' theory has been dismissed by mainstream physicists for the lack of a mathematical model behind its philosophical considerations.^{ [9] }

In 2006, it was proposed that the application of loop quantum gravity techniques to Big Bang cosmology can lead to a bounce that need not be cyclic.^{ [10] }

In 2011, Nikodem Popławski showed that a nonsingular Big Bounce appears naturally in the Einstein-Cartan-Sciama-Kibble theory of gravity.^{ [11] } 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. The minimal coupling between torsion and Dirac spinors generates a spin-spin interaction which is significant in fermionic matter at extremely high densities. 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. This scenario also explains why the present Universe at largest scales appears spatially flat, homogeneous and isotropic, providing a physical alternative to cosmic inflation.

In 2012, a new theory of nonsingular big bounce was successfully constructed within the frame of standard Einstein gravity.^{ [12] } This theory combines the benefits of matter bounce and Ekpyrotic cosmology. Particularly, the famous BKL instability, that the homogeneous and isotropic background cosmological solution is unstable to the growth of anisotropic stress, is resolved in this theory. Moreover, curvature perturbations seeded in matter contraction are able to form a nearly scale-invariant primordial power spectrum and thus provides a consistent mechanism to explain the cosmic microwave background (CMB) observations.

A few sources argue that distant supermassive black holes whose large size is hard to explain so soon after the Big Bang, such as ULAS J1342+0928,^{ [13] } may be evidence for a Big Bounce, with these supermassive black holes being formed before the Big Bounce.^{ [14] }^{ [15] }

- Abhay Ashtekar – Indian theoretical physicist
- Anthropic principle – Philosophical premise that all scientific observations presuppose a universe compatible with the emergence of sentient organisms which make those observations
- Big Crunch – Theoretical scenario for the ultimate fate of the universe
- Big Rip – cosmological model based on an exponentially increasing rate of expansion
- Big Freeze
- False vacuum – A hypothetical vacuum, less stable than true vacuum
- Eternal return – A theory that the universe and all existence is perpetually recurring
- John Archibald Wheeler – American theoretical physicist
- Loop quantum cosmology – A finite, symmetry-reduced model of loop quantum gravity
- Loop quantum gravity – Theory of quantum gravity, merging quantum mechanics and general relativity
- Supernova – Star exploding at the end of its stellar evolution

**Physical cosmology** is a branch of cosmology concerned with the study of cosmological models. A **cosmological model**, or simply **cosmology**, provides a description of the largest-scale structures and dynamics of the universe and allows study of fundamental questions about its origin, structure, evolution, and ultimate fate. Cosmology as a science originated with the Copernican principle, which implies that celestial bodies obey identical physical laws to those on Earth, and Newtonian mechanics, which first allowed those physical laws to be understood. Physical cosmology, as it is now understood, began with the development in 1915 of Albert Einstein's general theory of relativity, followed by major observational discoveries in the 1920s: first, Edwin Hubble discovered that the universe contains a huge number of external galaxies beyond the Milky Way; then, work by Vesto Slipher and others showed that the universe is expanding. These advances made it possible to speculate about the origin of the universe, and allowed the establishment of the Big Bang theory, by Georges Lemaître, as the leading cosmological model. A few researchers still advocate a handful of alternative cosmologies; however, most cosmologists agree that the Big Bang theory best explains the observations.

In physical cosmology, **cosmic inflation**, **cosmological inflation**, or just **inflation**, is a theory of exponential expansion of space in the early universe. 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. Following the inflationary period, the universe continued to expand, but at a slower rate. The acceleration of this expansion due to dark energy began after the universe was already over 9 billion years old.

**General relativity** (**GR**), also known as the **general theory of relativity** (**GTR**), is the geometric theory of gravitation published by Albert Einstein in 1915 and the current description of gravitation in modern physics. General relativity generalizes special relativity and refines Newton's law of universal gravitation, providing a unified description of gravity as a geometric property of space and time, or spacetime. In particular, the *curvature of spacetime* is directly related to the energy and momentum of whatever matter and radiation are present. The relation is specified by the Einstein field equations, a system of partial differential equations.

**Quantum gravity** (**QG**) is a field of theoretical physics that seeks to describe gravity according to the principles of quantum mechanics, and where quantum effects cannot be ignored, such as in the vicinity of black holes or similar compact astrophysical objects where the effects of gravity are strong.

A **gravitational singularity**, **spacetime singularity** or simply **singularity** is a location in spacetime where the gravitational field of a celestial body is predicted to become infinite by general relativity in a way that does not depend on the coordinate system. The quantities used to measure gravitational field strength are the scalar invariant curvatures of spacetime, which includes a measure of the density of matter. Since such quantities become infinite at the singularity, the laws of normal spacetime break down.

**Loop quantum gravity** (**LQG**) is a theory of quantum gravity attempting to merge quantum mechanics and general relativity, including the incorporation of the matter of the standard model into the framework established for the pure quantum gravity case. LQG competes with string theory as a candidate for quantum gravity.

The **ekpyrotic universe** is a cosmological model of the early universe that explains the origin of the large-scale structure of the cosmos. The model has also been incorporated in the **cyclic universe** theory, which proposes a complete cosmological history, both the past and future.

The **ultimate fate of the universe** is a topic in physical cosmology, whose theoretical restrictions allow possible scenarios for the evolution and ultimate fate of the universe to be described and evaluated. Based on available observational evidence, deciding the fate and evolution of the universe have now become valid cosmological questions, being beyond the mostly untestable constraints of mythological or theological beliefs. Many possible futures have been predicted by different scientific hypotheses, including that the universe might have existed for a finite and infinite duration, or towards explaining the manner and circumstances of its beginning.

The **Big Crunch** is a hypothetical scenario for the ultimate fate of the universe, in which the expansion of the universe eventually reverses and the universe recollapses, ultimately causing the cosmic scale factor to reach zero, an event potentially followed by a reformation of the universe starting with another Big Bang. The vast majority of evidence indicates that this theory is not correct. Instead, astronomical observations show that the expansion of the universe is accelerating, rather than being slowed by gravity, suggesting that a Big Chill or Big Rip are far more likely to occur.

In general relativity, a **white hole** is a hypothetical region of spacetime and singularity which cannot be entered from the outside, although energy-matter and light can escape from it. In this sense, it is the reverse of a black hole, which can be entered only from the outside and from which energy-matter and light cannot escape. White holes appear in the theory of eternal black holes. In addition to a black hole region in the future, such a solution of the Einstein field equations has a white hole region in its past. However, some believe this region does not exist for black holes that have formed through gravitational collapse, nor are there any known physical processes through which a white hole could be formed. Although information and evidence regarding white holes remains inconclusive, the 2006 GRB 060614 has been proposed as the first documented observance of a white hole.

**Brane cosmology** refers to several theories in particle physics and cosmology related to string theory, superstring theory and M-theory.

In theoretical physics, the **Einstein–Cartan theory**, also known as the **Einstein–Cartan–Sciama–Kibble theory**, is a classical theory of gravitation similar to general relativity. The theory was first proposed by Élie Cartan in 1922. Einstein–Cartan theory is the simplest Poincaré gauge theory.

A **cyclic model** is any of several cosmological models in which the universe follows infinite, or indefinite, self-sustaining cycles. For example, the oscillating universe theory briefly considered by Albert Einstein in 1930 theorized a universe following an eternal series of oscillations, each beginning with a Big Bang and ending with a Big Crunch; in the interim, the universe would expand for a period of time before the gravitational attraction of matter causes it to collapse back in and undergo a bounce.

**Quantum cosmology** is the attempt in theoretical physics to develop a quantum theory of the Universe. This approach attempts to answer open questions of classical physical cosmology, particularly those related to the first phases of the universe.

**Paul Joseph Steinhardt** is an American theoretical physicist whose principal research is in cosmology and condensed matter physics. He is currently the Albert Einstein Professor in Science at Princeton University where he is on the faculty of both the Departments of Physics and of Astrophysical Sciences.

**Eternal inflation** is a hypothetical inflationary universe model, which is itself an outgrowth or extension of the Big Bang theory.

**Loop quantum cosmology** (LQC) is a finite, symmetry-reduced model of loop quantum gravity (LQG) that predicts a "quantum bridge" between contracting and expanding cosmological branches.

**Martin Bojowald** is a German physicist who now works on the faculty of the Penn State Physics Department, where he is a member of the Institute for Gravitation and the Cosmos. Prior to joining Penn State he spent several years at the Max Planck Institute for Gravitational Physics in Potsdam, Germany. He works on loop quantum gravity and physical cosmology and is credited with establishing the sub-field of loop quantum cosmology.

**Nikodem Janusz Popławski** is a Polish theoretical physicist, most widely noted for the hypothesis that every black hole could be a doorway to another universe and that the universe was formed within a black hole which itself exists in a larger universe. This hypothesis was listed by *National Geographic* and *Science* magazines among their top ten discoveries of 2010. Popławski appeared in an episode of the TV show *Through the Wormhole* titled "Are There Parallel Universes?" and in an episode of the Discovery Channel show *Curiosity* titled "Is There a Parallel Universe?", which were hosted by Morgan Freeman and aired in 2011. He was named by *Forbes* magazine in 2015 as one of five scientists in the world most likely to become the next Albert Einstein.

A **black hole cosmology** is a cosmological model in which the observable universe is the interior of a black hole. Such models were originally proposed by theoretical physicist Raj Pathria, and concurrently by mathematician I. J. Good.

- 1 2 Brandenberger, Robert; Peter, Patrick (2017). "Bouncing Cosmologies: Progress and Problems".
*Foundations of Physics*.**47**(6): 797–850. arXiv: 1603.05834 . Bibcode:2017FoPh...47..797B. doi:10.1007/s10701-016-0057-0. ISSN 0015-9018. - ↑ Kragh, Helge (1996).
*Cosmology*. Princeton, NJ: Princeton University Press. ISBN 978-0-691-00546-1. - ↑ McKee, Maggie (25 September 2014). "Ingenious: Paul J. Steinhardt – The Princeton physicist on what's wrong with inflation theory and his view of the Big Bang".
*Nautilus*(17). NautilusThink Inc. Retrieved 31 March 2017. - ↑ Steinhardt, Paul J.; Turok, Neil (2005). "The cyclic model simplified".
*New Astronomy Reviews*.**49**(2–6): 43–57. arXiv: astro-ph/0404480 . Bibcode:2005NewAR..49...43S. doi:10.1016/j.newar.2005.01.003. ISSN 1387-6473. - ↑ Lehners, Jean-Luc; Steinhardt, Paul J. (2013). "Planck 2013 results support the cyclic universe".
*Physical Review D*.**87**(12): 123533. arXiv: 1304.3122 . Bibcode:2013PhRvD..87l3533L. doi:10.1103/PhysRevD.87.123533. ISSN 1550-7998. - ↑ Overduin, James; Hans-Joachim Blome; Josef Hoell (June 2007). "Wolfgang Priester: from the big bounce to the Λ-dominated universe".
*Naturwissenschaften*.**94**(6): 417–429. arXiv: astro-ph/0608644 . Bibcode:2007NW.....94..417O. doi:10.1007/s00114-006-0187-x. PMID 17146687. - ↑ Bojowald, Martin (2007). "What happened before the Big Bang?".
*Nature Physics*.**3**(8): 523–525. Bibcode:2007NatPh...3..523B. doi: 10.1038/nphys654 . - ↑ Ashtekar, Abhay; Corichi, Alejandro; Singh, Parampreet (2008). "Robustness of key features of loop quantum cosmology".
*Physical Review D*.**77**(2): 024046. arXiv: 0710.3565 . Bibcode:2008PhRvD..77b4046A. doi:10.1103/PhysRevD.77.024046. - ↑ David Adam (14 August 2003). "The Strange story of Peter Lynds".
*The Guardian*. - ↑ "Penn State Researchers Look Beyond The Birth Of The Universe".
*Science Daily*. May 17, 2006. Referring to Ashtekar, Abhay; Pawlowski, Tomasz; Singh, Parmpreet (2006). "Quantum Nature of the Big Bang".*Physical Review Letters*.**96**(14): 141301. arXiv: gr-qc/0602086 . Bibcode:2006PhRvL..96n1301A. doi:10.1103/PhysRevLett.96.141301. PMID 16712061. - ↑ Poplawski, N. J. (2012). "Nonsingular, big-bounce cosmology from spinor-torsion coupling".
*Physical Review D*.**85**(10): 107502. arXiv: 1111.4595 . Bibcode:2012PhRvD..85j7502P. doi:10.1103/PhysRevD.85.107502. - ↑ Cai, Yi-Fu; Damien Easson; Robert Brandenberger (2012). "Towards a Nonsingular Bouncing Cosmology".
*Journal of Cosmology and Astroparticle Physics*.**2012**(8): 020. arXiv: 1206.2382 . Bibcode:2012JCAP...08..020C. doi:10.1088/1475-7516/2012/08/020. - ↑ Landau, Elizabeth; Bañados, Eduardo (6 December 2017). "Found: Most Distant Black Hole".
*NASA*. Retrieved 6 December 2017."This black hole grew far larger than we expected in only 690 million years after the Big Bang, which challenges our theories about how black holes form," said study co-author Daniel Stern of NASA's Jet Propulsion Laboratory in Pasadena, California.

- ↑ Jamie Seidel (7 December 2017). "Black hole at the dawn of time challenges our understanding of how the universe was formed". News Corp Australia. Retrieved 9 December 2017.
It had reached its size just 690 million years after the point beyond which there is nothing. The most dominant scientific theory of recent years describes that point as the Big Bang—a spontaneous eruption of reality as we know it out of a quantum singularity. But another idea has recently been gaining weight: that the universe goes through periodic expansions and contractions—resulting in a “Big Bounce”. And the existence of early black holes has been predicted to be a key telltale as to whether or not the idea may be valid. This one is very big. To get to its size—800 million times more mass than our Sun—it must have swallowed a lot of stuff. ... As far as we understand it, the universe simply wasn’t old enough at that time to generate such a monster.

- ↑ Youmagazine staff (8 December 2017). "A Black Hole that is more ancient than the Universe" (in Greek). You Magazine (Greece). Retrieved 9 December 2017.
This new theory that accepts that the Universe is going through periodic expansions and contractions is called "Big Bounce"

- Angha, Nader (2001).
*Expansion & Contraction Within Being (Dahm).*Riverside, CA: M.T.O Shahmaghsoudi Publications. ISBN 0-910735-61-1. - Bojowald, Martin (2008). "Follow the Bouncing Universe".
*Scientific American*.**299**(October 2008): 44–51. Bibcode:2008SciAm.299d..44B. doi:10.1038/scientificamerican1008-44. PMID 18847084. - Magueijo, João (2003).
*Faster than the Speed of Light: the Story of a Scientific Speculation*. Cambridge, MA: Perseus Publishing. ISBN 978-0-7382-0525-0. - Taiebyzadeh, Payam (2017).
*String Theory; A unified theory and inner dimension of elementary particles (BazDahm).*Riverside, Iran: Shamloo Publications Center. ISBN 978-600-116-684-6.

- Overduin, James; Blome, Hans-Joachim; Hoell, Josef (2007). "Wolfgang Priester: From the big bounce to the $\Lambda$ -dominated universe".
*Naturwissenschaften*.**94**(6): 417–429. arXiv: astro-ph/0608644 . Bibcode:2007NW.....94..417O. doi:10.1007/s00114-006-0187-x. PMID 17146687. - Pitts, Trevor (1998). "Dark Matter, Antimatter and Time-Symmetry". arXiv: physics/9812021v2 .
- Penn State Researchers Look Beyond The Birth Of The Universe (Penn State) May 12, 2006
- What Happened Before the Big Bang? (Penn State) July 1, 2007
- From big bang to big bounce (Pen State) NewScientist December 13, 2008
- Nurgaliev, I. S. (2010). "Singularities are averted by vortices".
*Gravitation and Cosmology*.**16**(4): 313–315. Bibcode:2010GrCo...16..313N. doi:10.1134/S0202289310040092.

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