Theory A Current View of the Universe

The current state of knowledge scientific in cosmological matters is based upon two theories: Einstein's theory of Relativity in terms of the explanation of the main structures of the Universe, the "Standard Model of Physics,' or the 'quantum physics,' in terms of particle physics. These are, since the 1970s, both the scientific theories reflecting the physical world and which are the bases on which studies of it continue. These two theories are based on the scientific revolution of the early 20th century, which came to exceed the Newtonian theory of Nature that was associated with the previous scientific revolution, the one of the Renaissance and Modern times. These two theories however does not yet constitute a certain and final view of the Universe, even if they are close to. That view is like follows: at a time 0, a explosion, termed the 'Big Bang,' is occurring. Time 0 is explained, to the best of current knowledge, like a 'quantum singularity.' Quantum physics explains that, in a vacuum in the scientific sense, there may be energy, which can sometimes turn into matter. It is such a transformation, with a huge energy and temperature which constitutes the Big Bang. That explosion almost immediately gave birth to all the fundamental particles of the Universe, and a continuous expansion of it. All the rest is currently mere speculation. Scientists indeed do not manage to link the theory of Relativity and quantum physics as the latter does not feature a explanation for gravity. For the theory of Relativity, gravity results from a curvature of space-time, that item which provides for the background of all of what the Universe is made. The consequence is that science does not allow to fully understand mechanisms which are taking place between the original explosion and a infinitesimal fraction of time and dimension reached by Universe, which are termed the 'Planck time' and 'Planck length'. Ahead of the Planck values, according to the laws of quantum physics, the entire Universe should behave as a global system described by a wave function (or the description a particle's quantum state and behavior, which are function of space and time and usually yields a probabilistic depiction). The equations to determine that wave function are quite complicated however. It is that lack of explanation, along with the recent discovery, on a one hand, of the dark matter, the Universe's main component, of which nothing is known however and, on a other hand, of the dark energy, a acceleration of the Universe's expansion which cosmologists cannot explain, which triggered miscellaneous attempts of explanation which, currently, for none of them, can be scientifically demonstrated. To that -which is about the beginnings of undersanding the 'how' of the Universe- the question of the 'why' of it is necessarily linked and necessarily brings to philosophical, or religious

The Big Bang Model, the Predominant Theory of the Universe

By the early 20th century, astronomers did not yet grasped completely what they could observe in the sky, especially what they called spiral nebulae -with today we call galaxies. That time, on a other hand, came to be the one of the Einsteinian revolution, when Einstein along with other physicists set new fundations for the explanation of Nature in replacement of the old Newtonian physics. With the idea of the speed of light like a constant and spacetime curved through the interaction of objects moving, Einstein eventually enounced his 'field equations,' or a series of maths formulae which, when solved or detailed, come to hint, prove or describe some specific physical features or objects of Nature. Cosmology, by Einstein himself included, swiftly came of the favorite fields of application of such new concepts. As cosmological ideas of the time mostly advocated a static Universe, Einstein added a 'cosmological constant' inasmuch as his views came to a dynamic Universe either contracting or expanding due to that spacetime is bent by mass, producing motion. Relativity's intern logics was that the Universe was contracting. By 1922, Russian cosmologist and mathematician Alexander Friedmann derived indeed from the Einstein equations that the Universe might be expanding, or contracting as, in any case dynamic. De Sitter, another cosmologist mathematician, as far as he is concerned, was a proponent of the Universe being static by a lack of matter. By 1924, American Edwin Hubble showed that spiral nebulae were 'island universes' akin to how astronomers then viewed our own Milky Way Galaxy. By 1931, Belgian cleric and University of Louvain alumni Abbot Lemaître independently had derived Friedmann's equations and suggested that the evident expansion in time of the Universe likely hinted to that one could think that when moving backwards in time, it came to contract into some 'primeval atom' at the origin of all what existed. In 1929, basing upon a idea by Lemaître already had stated, Hubble eventually discovered at Mount Wilson Observatory a correlation between distance and recession velocity of galaxies —now known as Hubble's law, each of those faraway galaxies getting distant from each other at tremendous speed, which dramatically expanded the Universe's dimensions. Einstein, the idea of the eternity of the Universe of whom was taking place in a long tradition since Aristotle (who himself was contradicting Platon and Pythagoricians who thought that the Universe had had a beginning) eventually conceded about 1931, that the Universe was expanding. Such moves however did not come to a comprehensive theory at that time with even concurrent cosmologies proposed

->How Scientists Worked Upon Einstein's Equations!
Belgian cosmologist Georges Lemaître and others pointed out in the 1920s that a Universe that satisfied Einstein’s theory should either expand or contract, which U.S. astronomer Edwin Hubble and others showed that it was expanding indeed. Solving Einstein’s equations on a cosmic scale however was impossible without making assumptions to simplify the calculations. To reach their conclusions, Lemaître and the other early relativists assumed that matter was uniformly distributed as a continuum across space, rather than being concentrated in stars and galaxies. Full relativistic calculations remained difficult even for supercomputers. It might that a necessary trick was to assume regions of slight overdensity developping into lumpy structures under the pull of gravity, and assuming also a uneven spread of matter only for the relatively small area studied while maintaining a uniform distribution on the largest scales

The Earth, or galaxies, are bending spacetime around them, determining gravity. picture site 'Amateur Astronomy'

->Hubble Advances In the 1920's, on the other hand, Hubble was able to understood, through his observations with a telescope, that galaxies were island-universes similiar to our own Milky Way Galaxy as, using the light of the variable Cepheid stars, he demonstrated that those galaxies were getting distant from each other. Astronomers at the time considered the Milky Way a single 'island universe' of stars, with nothing observable beyond its boundaries as the Andromeda Galaxy for example was cataloged as just one of many faint, fuzzy patches of light astronomers called 'spiral nebulae'. Astronomers did not know for sure whether these spiral nebulae were part of the Milky Way or independent island universes. That's what Edwin Hubble found when he observed a variable, Cepheid star in the Andromeda Galaxy as that Hubble variable number one, or V1, still observable today in the galaxy, became the most important star in the history of cosmology. That view still is extant also on a 4-inch-by-5-inch photographic glass plate. Hubble had spent several months in 1923 scanning Andromeda with the 100-inch Hooker telescope, the most powerful telescope of that era, at Mount Wilson Observatory in California. On the night of Oct. 5, 1923, Hubble began an observing run that lasted until the early hours of Oct. 6. Under poor viewing conditions, the astronomer made a 45-minute exposure that yielded three suspected novae, a class of exploding star astronomers were using at the time like distance candles (which distance process was only very approximate). He wrote the letter "N," for nova, next to each of the three objects. Later, however, Hubble made a startling discovery when he compared the Oct. 5-6 plate with previous exposures of the novae. One of the so-called novae dimmed and brightened over a much shorter time period than seen in a typical nova. Hubble obtained enough observations of the star to plot its light curve, determining a period of 31.4 days, indicating the object really was a Cepheid variable. The period yielded the star's intrinsic brightness, which Hubble then used to calculate its distance. The star turned out to be 1 million light-years from Earth! Taking out his marking pen, Hubble crossed out the 'N' next to the newfound Cepheid variable and wrote 'VAR,' for variable, followed by an exclamation point. Hubble eliminated any doubt that Andromeda was extragalactic as prior to that discovery, by 1920 for example, a public debate had been held between astronomers by which the views still were that either our Galaxy had to be small to leave room for other island universes beyond, or spiral nebulae to be much smaller that the Milky Way and part of it. One of both astronomers and Henry Norris Russell urged Hubble to write a paper for a joint meeting of the American Astronomical Society and American Association for the Advancement of Science at the end of December 1924 even if since Hubble already had informed the leading astronomers of his result months earlier. Hubble's paper was entitled 'Extragalactic Nature of Spiral Nebulae.' Edwin Hubble's observations of V1 became the critical first step in uncovering a larger, grander universe. He went on to find many galaxies beyond the Milky Way. Those galaxies, in turn, allowed him to determine that the Universe is expanding. Hubble's observations thus were the evidence for that the concept of a expanding Universe was the right one

By 1945, two only theories remained, one was Fred Hoyle's Steady State model as new matter would be created as the Universe seemed to expand and Lemaître's Big Bang theory advocated and developed by George Gamow as that Big Bang theory was turning largely known. It is Fred Hoyle who ironically coined the terms 'Big Bang' during a BBC Radio broadcast in March 1949. Lemaître himself used the terms 'big noise' to qualify the primordial explosion. Only observations could favor one theory, most notably from radio source counts as the Big Bang model eventually was secured by the discovery and confirmation of the cosmic microwave background radiation (CMB) in 1964, a concept predicted by Gamow associates Ralph Alpher and Robert Herman. The CMB was accidentally discovered by Arno Penzias and Robert Wilson while using a new microwave receiver in the purpose of tuning the coms with satellites as they were working for the U.S. Bell corporate. They were hearing a background frequency which was uniform as they even suspected that a pigeons' nest was the culprit. On a train which took Penzias to Princeton to find a explanation, the latter met with a physician who presented him to two collegues who were searching for that sound since 2 years. That was the sound of the Big Bang, micowaves under the form of a electromagnetic imprint that is. A other evidence to the existence of the Big Bang, generally, it that the sky at night is black. This is because no a infinity of stars are shining since a infinity of time! Better evidence still came by the end of the century with space telescopes and dedicated missions like COBE, the Hubble Space Telescope and the WMAP mission which brought precise and accurate measurements of many of the parameters of the Big Bang model. The Hubble Space Telescope was instrumental into furthering studies at such distant times as ground observatories, by the 1990's were reaching at about 7 billion years ago and the Hubble reached 12 billion in 1995, or 800 million years after the Big Bang in 2004 and 480 in 2010. A distance of 200 million years after the Big Bang should be reached in the futur by the next, James Webb Space Telescope. Advances in the matter of physics of particles and the completion of the Standard Model of Physics in the 1970's and further research in that field of quantum physics on a other hand, came to complement the Big Bang theory with a better understanding of the world of particles hence the physics of the early Universe. NASA's Wilkinson Microwave Anisotropy Probe (WMAP) mission, which had launched by 2001 and last results were published by late 2012, has confirmed the Big Bang model of the Universe as the study of the microwave patterns allowed to defintively turn cosmology from speculation to precision science, and likely inaugurating what might be called the 'Standard Model of Cosmology.' It determined values of the Big Bang cosmology with about 68,000 times more accuracy than before. The afterglow of the Universe at a time when it was only 375,000 years old allowed to constrained what could have possibly happened earlier, and after and what happened in the billions of year since, confirming also the simplest version of the theory called 'inflation,' when the Universe grew by more than a trillion trillion-fold in less than a trillionth of a trillionth of a second. Using the 'Sunyaev-Zel'dovich (SZ) effect,' which was first observed about the 1980's, astronomers can observe the CMB constituent microwave photons which, on their journey to us, possibly passed through galaxy clusters containing high-energy electrons, giving CMB photons a tiny boost of energy. That helps about the location and distribution of dense galaxy clusters in the Universe. Until in 1966, the largest telescopes on Earth could only see about halfway across the Universe. 4 billion years ago, the Universe was not that big, objects of it were closer together

The COBE, WMAP and Planck missions successively improved the accuracy of the CMB image! picture site 'Amateur Astronomy'

The Big Bang Chronologically

Based on measurements of the expansion of the Universe, the most recent data bring to that the Universe originated 13.75 billion years ago. The earliest epoch of our Universe is unfolding from zero to approximately 10-43 seconds, which is called the 'Planck time.' The only science experiments allowed into the Planck epoch are space probes sent to study the cosmic microwave background radiation, or neutrinos detectors. Data from particle accelerators also provides insight into that epoch as well. As physicists, for example, are crushing particles into smaller pieces, they have determined that the so-called 'quark–gluon plasma,' that early phase of matter, behaved more like a liquid than a gas, or the CERN Large Hadron Collider (LHC) in Geneva, Switzerland should be able to probe still earlier phases of matter, albeit no accelerator current or planned will be capable of probing the Planck period directly. In the primordial plasma, electromagnetic radiation is so much linked with particles that ordinary laws of electrodynamics, including those of quantum electrodynamics, no longer apply. From that comes that all ideas concerning the beginnings of the Universe are speculative mostly, physics at those temperatures are ill-understood, and proposed scenarios differ radically. A its starting point, the Universe has a dimension 1 billion of billions of billions times smaller than the hydrogen atom as it is very energetic and at a temperature of 100,000 billion of billions of billions of degrees. The main hurdle into the Planck epoch is that the Big Bang is mainly both understood through Einstein's General Relativity, which is a theory of gravity, and the physics of particles, which was standardized in the 1970's under the quantum, Standard Model of Physics. But gravity and the three main, atomic forces of the Standard Model are, until now, unreconciliable albeit physicists are variedly looking for a unification between Einstein's Relativity and quantum physics. Because causality, in quantum theory, is about the way objects influence each other through time and space, it could help uniting the Standard Model with Einstein's General Relativity, in which the causal structure plays a central role). Relativity, on the one hand, predicts a gravitational singularity like the origin of the Universe as that singularity should possesses a infinite density by itself. Quantum physics, on the other hand, allows for that singularity to depart from that state. Also the exact manner in which the fundamental forces -and gravity- were, or not unified, and how they came to be separate entities, is still poorly understood and incorporating gravity under the form of quantum gravity would thus be of use as quantum effects of gravity likely were important at that moment especially to have the singularity to pass, from a infinite density, into the different constituants of the Universe. A consistent and mathematically rigorous quantum field theory of gravitation, or quantum gravity, is thus under construction but still not completed. Quantum gravity theory, simply, would allow at the Planck time, to a unified view, or a 'theory of All,' of the four forces of the Standard Model of physics, that synthesis appeared in the 1970's. The Planck time is when the immensely large meets the immensely small. As far as what could have existed before the initial moment of the Big Bang is concerned, a idea, which refers to Pythagoricians, is that it could have existed a form of a 'cosmological code' (like the DNA is at the origin of any human being). It could be about a kind of a 'fog of numbers,' a numeric reality which was not yet materialized. The Universe, during that early phase, reached a maximal dimension of the Planck length, the distance light travels in one Planck time, or about 1.616 × 10-35 meters! Unconceivably hot and dense, the state of the Universe during the Planck epoch was unstable or transitory, tending to evolve. At the smallest scales of distance and duration we can mesure by the beginnings of the Universe, three dimensions of space plus time appears to be smooth and structureless. However, certain aspects of quantum mechanics predict that spacetime would not be smooth but rather have a foamy, jittery nature and would consist of many small, ever-changing, regions for which space and time are no longer definite, but fluctuating . Predicted scale of spacetime 'foam' is about ten times a billionth of the diameter of a hydrogen atom’s nucleus. Below the Planck length no instrument is able to measure anything smaller because, due to the quantum uncertainty principle, the Universe is probabilistic and indeterminate. This scale is also thought to be the demarcating line between general relativity and quantum mechanics. The gravitational field, under the Planck length is so strong that, for example, it can produce a black hole out of the energy of the field. Should the Big Bang singularity prove to be a quantum one, or a 'quantum singularity,' or 'quantum fluctuation,' it likely was filled homogeneously and isotropically with an incredibly high energy density, huge temperatures and pressures and forces of physics of a primitive nature. Quantum gravity only could describe the space-time at that period. In any case, it is from that unknown, most powerful state that the Universe had been born like a tremendous release of energy, explaining why the history of the Universe since that time is a continual expansion and cooling. Of note that a better view than to state that matter and energy in the Universe had been born from a single point in space and time, is to that space be created with a certain, fixed amount of energy and matter as what followed mostly was in terms of expansion. A view associating the Higgs boson with the Big Bang -a particle supposed to give their masses to particles of the Standard Model of Physics- is that, before the original state of the Universe's disruption of equilibrium, all particles are at a 0-energy, a 0-mass and in equilibrium and symetry comprised inside the Planck length, or a very minute field. It is when that field's equilibrium is disrupted that the Universe is having its beginning and that the Higgs boson, with the Higgs field, are giving particles their masses as symetry is also broken. Thence, the Univers, on a time length of 3 minutes, passed to a temperature of 1 billion degrees only as the main of the Universe's mass was created!

A chronology of the history of the Universe! picture site 'Amateur Astronomy'

A fundamental second phase is the emergence of the current Standard Model of physics in terms of force and characteristics of fundamental particles. By 10-32 second, strong atomic force splits from the weak and the electromagnetism which are remaining united in the 'electroweak force' as at 10-11 seconds, weak and electromagnetic force split in turn. The Universe consists of a quark–gluon plasma, as well as all other elementary particles. Temperatures are so high that the random motion of particles occur at relativistic speeds. That time also is the one of the particle–antiparticle pairs which are continuously created and destroyed. At some point in time, a still unknown reaction called 'baryogenesis' violated the 'conservation of baryon number principle' and led to that only a tenuous excess of quarks and leptons survived to their antiparticles, of the order of one part in 30 million, which in other terms realized the predominance of matter over antimatter in the Universe. As matter and antimatter annihilated each other, that produced photons. By that time, the Universe kept expanding in size and temperatures falling with the energy of each particle decreasing. A process called 'symmetry breaking phase transitions' then now definitively stabilized the Standard Model into its current forms in terms of forces and parameters of elementary particles

By 10-11 second, begins a period of transition when those fundamental forces and particles bring to the creation of the fundamental bricks of the objects in the Universe, mostly because particle energies now droped to values that can be attained in particle accelerators. At 10-6 seconds, quarks and force carried by gluons form baryons, which are protons and neutrons as electrons and positrons form after that. Of note is that the dominance of matter over antimatter is perpetuated during such creations with the temperature no longer high enough to create new pairs and mass annihilation immediately following, leaving fewer basic bricks and none of their antiparticles. Between 10-2 seconds and 3 mn after the Big Bang, atoms nuclei form as basic particles are no longer moving at relativistic speeds. The energy budget of the Universe mostly was then composed of photons with a minor contribution from neutrinos as the temperature had droped to 1 billion degree K and the density the one of our air. In a process called 'Big Bang nucleosynthesis' or BBN (which had been predicted by Gamow), most protons remained uncombined and formed hydrogen nuclei, as some formed deuterium and helium nuclei by merging with neutrons. The heat of the early Universe was such that all matter existed as a dense plasma, absorbing light and making the Universe opaque. The Universe from a extremely smooth state, with matter evenly distributed in its infancy nearly 14 billion years ago, was to turned clumpy since with the appearance of galaxies, gas clouds and other structures which constituted the web Universe

->The Matter-Antimatter Asymmetry Still Ill-explained The asymmetry between matter and antimatter in the Universe theoretically remains ill-explained. Observations suggest that the Universe in its most distant parts alreday was made almost entirely of matter. A unknown process called 'baryogenesis' created the asymmetry. For baryogenesis to occur, the 'Sakharov conditions' must be satisfied requiring that baryon number is not conserved, C and CP-symmetry be violated and that the Universe depart from a thermodynamic equilibrium. All these conditions have been observed in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry. Several theories try to account for, like matter and antimatter do not have a same mass with opposite charges and spins or that some more matter existed at the time of the Big Bang. A promising way to study that question resides into spectroscopic measurements of the 'hyperfine structure,' which is shifts in energy levels, in antiatoms

Helium hydride was the first molecule to form after the Big Bang, 100,000 years after that as it participated to the cooling of the Universe, which allowed star formation -- a deed observationally confirmed in 2019. After a much longer time than elapsed until 3 minute after the original explosion of the Big Bang, the Universe, by 400,000 years of age entered a new, major step. It has cooled and expanded further as the density of matter now dominated that of light (or photon) radiation. Nuclei could capture electrons into mostly hydrogen atoms with deuterium, helium or lithium also appearing. That process, at that time, determined the 'recombination epoch' and the light of the Universe observable because hydrogen became not ionized anymore, or neutral. The Universe at that moment is having the form of a heated plasma bound to cool. Until then Universe had remained foggy because light, or photons could not travel far as photons were colliding and scattering against free-ranging electrons and nuclei -or the 'Thomson scattering.' That early, or relic light of the Universe journeying unimpeded is just the Cosmic Background Radiation or Microwave Background Radiation and a direct picture of the Universe then. The MBR lost energy as it is now seen like microwaves only. After the Big Bang, the Universe expanded so swiftly that matter itself resonated to create a deep bass noise as the sound waves resonated inside the early Universe which had closed back on itself. Such a sound left its imprint on the cosmic microwave background. To be hearable such sound waves have to be boosted 100 septillion times in frequency. When the CMB interacts with the hot gas permeating huge cosmic structures like galaxies and galaxy clusters, its energy distribution is modified in a characteristic way, a phenomenon known as the Sunyaev–Zel’dovich (SZ) effect. Neutrinos by those earlier times, existed in such huge numbers they affected the Universe’s early development and that they are found back now, forming a ominous neutrino background

The Chandra Deep Field South (CDFS) is showing portions of the Universe like it was extant 600 millions years after the Big Bang. picture site 'Amateur Astronomy'

The next period in the history of the Universe is that it then staid empty and dark during 500 million years, a period called the 'dark ages' and a other scale in time. Universe at that time looks empty because, on the one hand, it is really empty and, on a other hand, because light of first stars and galaxies which formed about 200 million years after the Big Bang can not escape. Indeed, that light of those first large-scale structures became trapped that time by neutral hydrogen which permeated the Universe since the recombination epoch. Neutral hydrogen just trapped the ultraviolet light with shortwavelength photons immediately grabbed by hydrogen atoms. The neutral hydrogen signal, also known as the '21-centimetre signal' is weak and has not yet been really detected. For much of the Universe's first 500 million years, normal matter remained too hot to coalesce into the first stars. Those first objects of the Universe formed from the original hydrogen clouds, the 'Population III' stars, with a mass 20-100 solar mass, or about 500, solar masses and galaxies. Population III stars came to be the first to explode as supernovae and releasing first heavy elements, which in turn came to be the basis for the second generation of stars. The ultraviolet light from all such objects and galaxies progressively was able to have its photons to tear apart electrons from the neutral hydrogen. Only extreme ultraviolet photons pack enough energy to rip apart hydrogen atoms. It is likely that starburst galaxies, which pack hot stars into small regions and which possess few light-blocking dust, accounted for that extreme ultraviolet. Like the recombination epoch had allowed the light of the Universe to journey unimpeded at 400,000 years after the Big Bang, the reionization epoch allowed the Universe to turn transparent to ultraviolet light. A study by late 2015 found that it was small galaxies of the time which allowed to disperse the fog then as the reionization should have come to a end by about 700 million years after the beginnings of the Universe. A study by 2016 has shown that first stars in the Universe started forming later than previous observations of the cosmic microwave background indicated, as the study also shows that stars were the only sources needed to clear the opaque fog of dark ages. The mid-epoch of reionization thus occurred when the Universe reached a age of 700 million years. A significant number of black holes accompanied the first stars in the Universe as one of every five sources is a black hole. The Universe thus became filled with ionized hydrogen allowing light to travel again. That constituted the 'reionization epoch' during about 1 billion years. First objects in the Universe which shone in visible or the ultraviolet light 500 million years after the Big Bang have since seen their light stretched out to the longer, infrared wavelengths, forming what is called the 'cosmic infrared background,' (CMB) as observations confirmed the first objects were numerous in quantity and furiously burned cosmic fuel. The cosmic microwave background (CMB) contains slight temperature fluctuations that correspond to regions of slightly different densities, representing the seeds of all cosmic structure we see around us today. Astronomers think the CMB arose from clusters of massive suns in the Universe's first stellar generations, as well as black holes, which produce vast amounts of energy as they accumulate gas. The CMB comes with the cosmic infrared background (CIB), which is more irregular than can be explained by distant unresolved galaxies, and this excess structure is thought to be light emitted when the Universe was less than a billion years old (scientists say it likely originated from the first luminous objects to form in the Universe, which includes first stars and black holes), and with the faint, diffuse X-ray glow that constitutes the cosmic X-ray background (CXB). Isolated stars beyond the edges of galaxies might also provide for that mysterious glow of infrared light seen across the entire sky. Such stars were probably stripped from their parent-galaxy into empty space due to violent galactic mergers. Hydrogen nuclei and their electrons that time remained scattered in such a 'soup of particles,' or that ionized plasma which nowadays still constitutes the main part of our Universe, which diluted itself due to the expansion of the Universe and allowed light to journey through. From there a further episode of reionization was extant as between 11.7 to 11.3 billion years ago, the ultraviolet light emitted by quasars stripped electrons off helium atoms, a process known like the 'helium-reionization', heating the intergalactic helium from 18,000 degrees Fahrenheit to nearly 40,000 degrees (9800-22000 degrees Celsius) and inhibiting the gas from gravitationally collapsing to form new generations of stars in some small galaxies. It thus tooks another two billion years before the Universe produced sources of ultraviolet radiation with enough energy to reionize that primordial helium

->The Reionization Epoch Detailed From place to place, spheres of reionized hydrogen grew into larger ones and overlapped. The whole Universe eventually lets pass ultraviolet light and it becomes transparent for the second time. The complete process was a progressive one, which took about one billion years as galaxies slowly built up their stars and chemical elements. Traces of neutral hydrogen are still observed until about redshift 6.4 (zone of ancient quasars), 10 percent is still seen when the Universe is 850 million years old. Eventually, no more neutral hydrogen is see about redshift 5, i.e. when Universe is 1.2 billion years old (debate is still pending about the chronology of this process. Recentest works are showing that the first wave of massive, ultrabright, stars should have formed as soon as 200 million years after the Big Bang (redshift 20) and that although they begun to reionize hydrogen, the Universe fell back into a colder, denser state before any other, smaller, stars could form further. Reionization is still seen weak at 700 million years after the Big Bang (redshift 7-8) as numerous dwarf galaxies, with heavy elements are providing the bulk of the reionization at 900 million years (redshift 6). The reionization might continue after that as a zone of higher density in the number of galaxies is seen at 1.2 billion years (redshift 5.9). This might point to a inhomogeneous Universe where reionization occurs at different times according to density. All this might show a reionization process lasting beyond 1.2 billion years after the Big Bang). The first stars, on the other hand, during about 500 million years, since 400 million years after the Big Bang, created a 'cosmic fog' of electrons around them. The cosmic infrared background, on a other hand, is made up of light from stars evolving in the early Universe and of very distant, massive star-forming galaxies as detailed maps of it were allowed by the ESA-USA Herschel space mission. The X-ray background of the Universe is due to heavily dust-shrouded -or not- black holes at the centers of active galaxies when the universe was 7 billion years old. The total sum of starlight in the cosmos is known to astronomers as the extragalactic background light (EBL) composed of a fog in the visible and the ultraviolet. Star formation reached a peak when the Universe was about 3 billion years old and has been declining ever since. From the EBL one may deduce that the average stellar density in the cosmos is about 1.4 stars per 100 billion cubic light-years, which means the average distance between stars in the Universe is about 4,150 light-years

Black holes by those times furiously converted some of the gravitational energy of the current matter to powerful ultraviolet radiation blazing out of galaxies as quasars likely resulting from colliding galaxies. After the helium was reionized, intergalactic gas again cooled down as existing dwarf galaxies continued to form and merge, leading to larger galaxies. The question of whether the helium reionization occurred uniformely in all directions of the Universe is still unanswered. Thence between 2 and 6 billion years after the Big Bang (between 12 and 8 billion years ago), the Universe eventually entered a steady phase during which other galaxies formed. Among those our Milky Way Galaxy. Our Sun and the Earth formed about 4.6 billion years ago. Thence we close to the present history, with life appearing on Earth 3.5 billion years ago, the geological ages unfolding on enormous amounts of time and mankind evolving since a apparition 7 million years ago, spreading over the continents and developing technology to eventually create farming about 11,000 years ago. The Big Bang model, generally, derives from two major assumptions, the universality of laws of Nature and the 'cosmological principle' (which states that Universe on large scales is homogeneous and isotropic) both of which started like postulates as they have been tested, with the fine structure constant, one of laws of physics which determines the existence of atoms and molecules, for example is constant by 10-5 and General Relativity passed stringent tests. Homogeneity and the isotropic character has has been confirmed to a level of 10-5 through observations of the CMB and the Universe measured to be homogeneous on the largest scales at the 10% level. From the cosmological principle, the Univers is best described by the 'Friedmann–Lemaître–Robertson–Walker metric' or FLRW metric, a determination of distances in the Universe which, through its scale factor, stays valid like a expanding coordinate system matching the expansion of the Universe. The Big Bang at last is not a explosion and expansion filling a preexisting empty void but a perpetual creation of space. The expansion seen in the redshifts of galaxies, detailed measurements of the CMB, the abundance of light elements or the large scale distribution and evolution of galaxies are sometimes called 'the four pillars of the Big Bang theory'

->More About the FLRW Metric The shape of the Universe is the same everywhere and everywhere the shape is composed of matter bringing to that, according to General Relativity, the Universe is necessarily curved by matter. On that base, only three geometries as discovered by Friedmann and Lemaître in the 1920s and complemented by Robertson and Walker in 1935, are possible. Such a concept is called the FLRW, for 'Friedmann-Lemaître-Robertson-Walker' metric. Should the Universe's curvature be negative, Universe is open (or hyperbolic) and its volume infinite. The curvature positive, Universe be closed (or spherical) and its volume is finite. Curvature null, the Universe be flat, and its volume infinite. A open Universe alike with a a flat Universe are universes always expanding and everlasting. A closed universe begins with an expanding phase then eventually falls back on itself, towards a Big Crunch. Bending of the Universe is dependent on its density, or all what it contains (visible matter, radiations, etc) in there. That is expressed by the value W, which expresses the relation between the density of the Universe and a critical density of 3 atoms of hydrogen per cubic-meter. If Universe's density is large (if W is greater than 1), expansion is checked and Universe is closed. If Universe's density is small (if W is smaller than 1), expansion keeps on for ever and Universe is open. At last a flat universe has a density just equal to the critical density. Mount Palomar telescope was instrumental in this reflexion. The COBE mission (or Cosmic Background Explorer), launched in November 1989, was further able to precisely measured and mapped the cosmic microwave background, showing that the radiation's spectrum agrees with the predictions based on the Big Bang theory The cosmic microwave background radiation that permeates the universe (and is the remnant energy of the theoretical Big Bang) has a temperature of 2.725 degrees Kelvin – that's minus 455 degrees Fahrenheit (minus 260 degrees Celsius). Universe is 13.73 billion years, geometrically flat to within 1 percent of flat (the Universe obey the rules of Euclidean geometry so the sum of the interior angles of a triangle add to 180 degrees), and expanding forever. In the FLRW models, the Universe is presumed to be perfectly smooth, or homogeneous, on the largest scales. Some researches are focusing upon the idea that the Universe is not spatially homogeneous as they go beyond a FLRW interpretation of cosmological data. The idea comes from that quantum physics needs that spacetime at the Planck era, when the Universe was no larger than one atom, was inherently 'grainy.' It is that grainy nature which resulted in the quantum fluctuations of the very earliest moments of the Big Bang; in a expansion controled by gravity, on a other hand, density lumpiness necessarily always increases with time. The question thus moves to why our current Universe eventually turned homogeneous. Usually the very early inflationary episode is hypothetized to have homogenized the universe as inflation itself however requires a small perfectly smooth patch from which it started. Proponents of a non-homogeneous hypothesis are obliged to admit that initial conditions involving viscous shear and pressure gradients can produce homogeneous regions from inhomogeneous ones rather quickly, hence a homogeneous patch from which the inflationary episode could produce a homogeneous Universe at largest scales. A answer, which also questions the currently admitted views of the Universe, is that to ask whether the observed smoothness of the Universe extends beyond our 'light cone' or that region of space, that part of the Universe close enough that light can have reached us within a Universe aged 14 billion years. Data at last which allow for determining the structures of the Universe from observational data remain imprecise, with mostly estimates of galaxy masses along with galaxy number counts together with angular-diameter distances only

Most Recent Advances and Questions About the Universe

Despite that the Big Bang model of the Universe became the mainstream theory by the 1960's and 1970's like a cosmological model, new considerations have been raised since the 1970's which may or not question fundamentally the theory or even the Standard Model of physics. A debate pending is a one about whether such discrepancies of the Big Bang model herald, or not, a same scientific revolution than when, by the end of the 19th century, Lord Kelvin was stating that there was no more than some clouds left over the great Newtonian building, clouds which turned to develop into the Theories of Relativity or quantum physics. Dr. Peebles, of Princeton, for example does not think that today science's state is similar to then as he does not argue they won't neither. Sandage, the protégé of Hubble, thinks that all these elements are going to a much greater synthesis that one could have imagine 100 years ago

Question of dark matter, by the end of 1970's arose from the fact that gravity and visible matter were not enough to account for the Universe as observed. Galaxies, for example, rotate at such speeds and in such a way that ordinary matter alone would not be able to hold them together, a question known like the 'galaxy rotation problem.' Something else had to exist leading to the idea that 90 percent of matter needs to be some form different from the observable one, a form cosmologists named 'dark matter.' Such particles do not respond to electromagnetism (thus dark matter does not emit, absorb, or radiate light or other forms of electromagnetic energy, hence the term 'dark'); they move relatively slowly (hence dark matter would mainly exist under the species "cold dark matter"; they do not interact with themselves nor with normal matter otherwise than trough gravity. Initially controversial, dark matter is now hinted too by observations like anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, or gravitational lensing studies. What dark matter precisely is however is still unknown. Albeit not incorporated into the Standard Model of physics, solutions to problems posed by its hypothesis exist which involve only further refinements of the theory. Dark matter acts like a invisible glue, holding galaxies and galaxy clusters together gravitationally. Clumping into structures such as galaxies might have happened in the Universe more slowly than indicated by earlier estimates. The question of the dark matter is part of that of the following dark energy concept as dark matter participates into the expansion of the Universe through its mass

A fortuitous observation by 1998 led to the concept of 'dark energy.' Two teams -one led by Riess and Brian Schmidt of Mount Stromlo Observatory, and the other by Saul Perlmutter of Lawrence Berkeley National Laboratory- discovered that the expansion of the Universe was accelerating since 7 billion years ago as they had observed that candlesticks Ia supernovae used to measure large distances were found farther from us than they should have been should the expansion pace of the Universe remain steady. The Big Bang model came with a vision of the Universe expansion progressively decreasing as far as large-scale structures, like stars and galaxies had formed. Accounting for that acceleration theoretically has cosmologists perplexed either taking back to the old, Einstein cosmological constant a ill-defined notion which may be included in the 1916 field equations. It would need however to be raised by 10 to the power of 120 times. The cosmological constant indeed would be a component, or 'dark energy,' with large negative pressure as that, on a other hand is a property described by the quantum-minded Standard Model of physics, or quantum void, or vacuum energy. Some cosmologists also are heading to explanations alternate to General Relativity with theories involving hidden dimensions, a modification of the theory, or a more general physical field. Paradoxically, the accelerated expansion of the Universe is braking its formation, as dark energy is braking the formation of galaxy clusters. By November 2006 the Hubble Space Telescope brought the proof that dark energy is seen existing 9 billion years ago or that it likely existed as soon as the Universe was created, albeit weak. In any case, due that the energy density in the matter of the Universe decreased with time, the one of dark energy remained constant. Thus, as matter made up the larger fraction of the Universe by the beginnings and then declined, the force of dark energy became more dominant. As the mass density of the Universe can also be infered from its 'gravitational clustering' it is found in such a case to have only about 30 percent of the needed density and dark energy would not cluster in the usual way. Galaxies are still small and irregular, without great spiral arms when the accelerated pace of expansion is beginning 7 billion years ago. One of the best evidence for dark energy likely is the 'Integrated Sachs Wolfe effect' with light from the cosmic microwave background (CMB) radiation gaining energy and turning slightly bluer as it passes through the gravitational fields of lumps of matter, which is considered proven since mid-2012. In terms of how the Universe will end, the concept of dark energy, is bringing to either a 'Big Rip,' with all objects eventually disassembling, down to the ultimate components of matter included, or to that all galaxies will get distant from each other and the Universe turning definitively dark. Dark energy, like a conclusion, may be seen like the 'energy density of the vacuum,' and one of the basic components of the Universe. A explanation to the dark energy is impeded due to the absence of a quantum theory of gravitation

-> The Hubble Constant
In 1927, in the Annales de la Société Scientifique de Bruxelles, Belgian Abbot Lemaître showed that galaxies' velocities appeared to be proportional to their distance, to account for observations that showed galaxies seem to be moving away from Earth. That relationship became known as Hubble's law because American astronomer Hubble, two years later, using astronomical data collected by others, also derived a rate of expansion, today known as the Hubble constant. He used improved experimental data and derived a more accurate constant, essentially confirming the law that eventually bore his name. Lemaître's contribution remained less well known, possibly in part because the 1931 English translation of his paper missed out the derivation of the constant, which Lemaître acknowledged he had omitted the discussion about the constant from the translation because more reliable data had since been published. By late 2018, the International Astronomical Union (IAU) recommended that the law now be known as the Hubble-Lemaître. Hubble and Lemaître met at a IAU assembly in Leiden in 1928 and exchanged views
Astronomers using NASA’s Hubble Space Telescope have discovered, by June 2016, that the Universe is expanding 5 percent to 9 percent faster than expected as the find might help to understand dark energy, dark matter and 'dark radiation.' That was done by refining the Universe’s current expansion rate to a unprecedented uncertainty of 2.4 percent only through improving the precision of distance measurements to faraway galaxies, looking for galaxies containing both Cepheid stars and Type Ia supernovae. The improved Hubble constant value found amounts to 45.5 miles per second per megaparsec, which means the distance between cosmic objects will double in another 9.8 billion years, which contradicts measurements of the Big Bang afterglow both by NASA’s Wilkinson Microwave Anisotropy Probe (WMAP) and ESA’s Planck satellite mission yield which had predicted a value 5 percent and 9 percent smaller, respectively, for the Hubble constant. Such that discrepency was found also extant by mid-2018 between data gathered by NASA’s Hubble Space Telescope and the European Space Agency’s (ESA) Gaia space observatory, compared to the Planck ones. The first ones bring to that the Universe is expanding by 45.6 miles (73.5 kilometers) per second per megaparsec -- for every 3.3 million light-years farther away a galaxy is from us, it appears to be moving 45.6 miles per second faster -- as the Planck results are only 41.6 miles (67.0 kilometers) per second per megaparsec. There are a few possible explanations for the Universe’s excessive speed. One possibility is that dark energy might have a larger strength. Another idea is that the cosmos contained a new subatomic particle in its early history that traveled close to the speed of light, collectively referred to as 'dark radiation' and including previously known particles like neutrinos. The boost in acceleration at last, could also mean that dark matter possesses some unexpected characteristics, or even that Einstein’s theory of gravity is incomplete. The goal now is to reach a accuracy of 1 percent. Before Hubble was launched in 1990, the estimates of the Hubble constant varied by a factor of two as in the late 1990s the Hubble Space Telescope Key Project on the Extragalactic Distance Scale refined the value of the Hubble constant to within an error of only 10 percent, one of the telescope’s key goals. The Hubble constant had also been measured in 2012 by the NASA's infrared space telescope Spitzer through a better measurement of the Cepheids and the approximation had been decreased to 3 percent
The previous measurements of the Hubble constant -or 'H0' as expressed in miles per second per megaparsec: for every parsec a galaxy is away from us, it looks getting distant by such miles par second- made by the Hubble Space Telescope in May 2009 had deemed that the measurement was confirming that dark energy is mathematically equivalent of Albert Einstein's hypothesized cosmological constant - a maths concept Einstein introduced in its equation as he had seen that the weight of the Universe, without that, would lead it to crush down on itself- albeit the precise nature of it still has to be pinned down. Einstein then abandoned that concept when Hubble had discovered that galaxies are getting distant from each other. The question is still pending whether dark energy is a static cosmological constant similar to that of Einstein, or a dynamical field akin to the repulsive force which drove the inflation episode of the Universe. The question of the dark energy may be summarized like during the first 7 billion years of its age the Universe decreased its expansion gradually under the weight of its galaxies as dark energy, that antigravitational force, existed still and, when matter had sufficiently dispersed itself, the dark energy could exert itself. The Galaxy Evolution Explorer mission by NASA has confirmed by 2011 that the dark energy may be considered a constant force, uniformly affecting the Universe and propelling its runaway expansion, likely meaning that dark energy is a cosmological constant, as Einstein proposed. Dark energy is in a tug-of-war contest with gravity which dominated the Universe until 7 billion years after the Big Bang. A alternate theory which states that Albert Einstein's concept of gravity is wrong, and gravity becomes repulsive instead of attractive when acting at great distances as gravity, not dark energy, would be the force pushing space apart, is now mostly discarded. Dark energy has the astronomers to predict that the Universe ultimately will be a cosmic wasteland, with galaxies spread apart so far that no intelligence inside any of each will be able to see other. Dark energy too is modifying the speed at which galaxies are forming, along with galaxy clusters too. A precise measurement of the Hubble constant might help to discriminate between our current model -- dark matter, accelerating expansion -- of the Universe and the necessity of new physics. The narrowing of Hubble's measurement to a accuracy of 1 in 100,000, might yield a Universe 9 percent younger at at 12.5-13 billion years! A new theory of dark energy could account as it suggests that there was a third dark-energy episode -- beyond the inflation episode and the usual expansion -- not long after the Big Bang, or the 'early dark energy.' Another idea is that the Universe contains a new subatomic particles that travel close to the speed of light, collectively called 'dark radiation' with already known particles like neutrinos. Dark matter at last could interact more strongly with normal matter or radiation than previously assumed

'Inflation theory', the last questionment to the basic Big Bang model, has been born in 1979 when Alan Guth, a physicist linked to the research of the Theory of Everything, or a neo-darwinist proponent of superstring theories, theorecists since the 1980's adding gravity to the further achievements of the Standard Model is terms of trying to unify all the forces at work in the Universe into a single explanation, or 'Grand Unified Theories' or 'GUTs'. Those themselves are endeavours based upon the one by Einstein to find a comprehensive and definitive explanation of Nature during his last 30 years under the form of a 'theory of everything', a theory of the 'unified field' which would have been able to 'read the mind of God'. Everything from the Universe to the atoms would have been explained by a single equation. This equation was never found however. The inflation theory builds upon some inconsistencies spotted with the Big Bang model. The 'horizon problem' states that as information cannot travel faster than light, separate regions of the Universe however, which at a time where not at reach between them at that speed could thus not be in causal contact and a contradiction with the isotropy now observed. The flatness problem has the current Universe 'geometry' keeps to be very close to flat, which theoretically needs that no any small departure from a certain density ever occurred which might be contradictory with a 13-billion year history and possible fluctuations. The inflation theory also worked from the 'magnetic monopole' question, such magnetic objects produced in great numbers in the early Universe and yielding consequently a much higher density at the time than consistent with current observations. With the inflation theory, all such questions are solved through a still unproved but elegant explanation, basing upon quantum physics and the theory of the vacuum field, with the Big Bang a quantum, vacuum-powered event which had the Universe increased tremendously in size in 10-37 seconds shortly after the Big Bang and then came back to a more steadily expansion, with that abrupt, cosmic 'inflation' giving its name to the theory. Starting at a minute 10-34 centimeters, the Universe ended at 17 times the distance Sun-Pluto (or twice the Kuiper Belt, or 1/5th of light-year) after the inflation. Inflation, albeit speculative, thus is a good explanation to the questions mentioned above. A rapid expansion, in terms of the horizon problem made regions which had been causally closer together distant, spacetime, in terms of the flatness problem, expanded to such an extent that its curvature was been smoothed out and yet keeping to the density needed and monopoles, those point defects, removed by the inflating episode. Quantum Heisenberg's uncertainty principle also brings that during the inflationary phase quantum thermal fluctuations should occur as such minute quantum fluctuations tremendously also grew in size and became fault lines of the Universe's density, determining the current, supposed aspect of a net of filaments which gravitationally determined where stars and galaxies began to form. Such fluctuations or irregularities have been observed by measurements of the CMB. Remnants of original gas filaments of the web Universe may still be observed nowadays under the form of gas bridges binding two galaxy clusters, and a mixture of both of the elusive filaments of the cosmic web mixed with gas originating from the clusters. The 'cosmic web' is the structure of the Universe as understood nowadays. It is comprised of spindly filaments of primordial material (mostly hydrogen and helium gas) and dark matter which connect galaxies and span the chasms between them. The material in this web can feed along the filaments into galaxies and drive their growth and evolution. The first observational evidence of such a cosmic web was provided by 2019 in a galaxy cluster where matter filaments could really be observed. As most galaxies are embedded into the web Universe, materials and objects are falling from there unto. The CMB also contains a wealth of other information, with, for example, two polarized modes, the E, and B-mode respectively. B-mode arises in two ways like deflection along its way by massive objects, or from the inflation period, when violent collisions between miscellaneous clumps of matter and radiation likely created a sea of gravitational waves. The inflation theory was modified or improved by cosmologists like Andrei Linde, Paul Steinhardt and Andreas Albrecht as it also made inroads in the dark energy question which some analyze like a renewed inflationary episode when a vacuum field or force is back to work. Study of gravitational waves, those waves triggered in the early Universe by colossal collisions between massive celestial objects shoud be a test for the inflation theory giving a insight about strange physics which prevailed when Universe was only one trillionth of trillionth of trillionth of second. The CMB holds a distinctive pattern in its polarization known as 'B modes' which could reveal the existence of primordial gravitational waves

A current view of the Universe including such advances or questionments is expressed in terms that the Universe today is 73% dark energy, 23% dark matter, 4.6% regular matter and less than 1% neutrinos and when the Universe was 380,000 years old, neutrinos made up 10% of the universe, atoms 12%, dark matter 63%, photons 15%, and dark energy was negligible. Measurements of the recent Universe also agree with that model. Strings theorecists also propose 'Brane cosmology models' in which inflation is due to the movement of branes as our three-dimensional Universe is just a 'brane', an entity, floating in a higher dimensional space where other such entities would float too; any collision between two branes would trigger a Big Bang, as seen from the branes. In some such models, the Big Bang was preceded by a Big Crunch and the Universe endlessly cycles from one process to the other. The idea of inflation also brought to the neo-Darwinist concept of 'multiverses' with the vacuum energy and inflation either producing mushrooming universes or our Universe part of a much larger and older Universe. The non-existence yet of a quantum theory of gravity in physics also allows some varying hypothesis about how the Universe had been born and works. Some, for example question the original singularity in that sense that it leaves some major questions unanswered like what started the Big Bang, ended the inflation or what the dark energy is, as they come with the concept of torsion of spacetime through particles' spin. Torsion generated by spin in the early Universe could manifest itself like a repulsive force opposing to gravity which results from the curvature of spacetime. In a void of spacetime, particles' spin would concentrate other ones as that gathering would eventually rebound like a gravitational expansion. That would only be possible if the Big Bang originated from inside a black hole lying in a other universe than ours. That theory could thus better explain that matter might have turned into nowadays particles, antimatter into dark matter, and dark energy. Black holes in varied existing universes generally, would provide for the birth of numerous universes. The Big Bounce theory which is based on that an infinite density of the universe at time T0 be incompatible avec the uncertainty principle of the quantum mechanics and the Universe might be in an eternal bouncing (after the completion of the whole consequences of the Big Bang, it would 'crunch' and bang again). The Hartle–Hawking no-boundary condition in which the whole of space-time is finite and the Big Bang does also represent the limit of time, but without the need for a singularity. British physicist Stephen Hawking is adding to those by saying that whatever the observations, one cannot infer that any origin of the Universe may be discussed. The Universe, according to him, might well have no origin at all because it might be no absolute references in physics, be it the classical or quantum one and that supposing any existing 'something' before time existed since the Big Bang is a kind of contradiction as that existence was of a type separated from the causes and natural laws which brought to time zero, or multiverse. That unpossibility further is related by Hawking to a a famed theological debate, or 'God's paradox,' by which God is said to be able, or unable to create himself. Stephen Hawking considers too that philosophy has died. Should his theories prove right, he thus would have demonstrated that God is not existing. When the Big Bang idea began to be expressed by the 1920's and 1930's cosmologists tended to refuse it and prefer a eternal steady state Universe as any notion of a beginning of time eventually was introducing religious concepts into physics. The Big Bang theory was officially recognized in agreement with Bible teachings and the concept of Creation by Pope Pius XII in 1951, which was the time too when the Vatican authorized Roman Catholic scientists to use darwinism in relation to paleoanthropology and, by January 2011, pope Benedict XVI invited Christians to see the Universe not like due to mere chance like some would have them to believe, but that divine will is found behind such theories like the one of the Big Bang. Pope Francis renewed that view by 2014 stating that the Big Bang may be in agreement with the idea of a creating God. Conservative Protestant Christian denominations also welcomed the Big Bang theory as allowing for a historical approach of Creation. As far as other religions are concerned, reactions varied as to the Big Bang implications to their respective cosmologies from trying to incorporate it, accept it merely like a science evidence, or simply rejecting the Big Bang views

Conclusion

Usually the future of the Universe is seen either like it would reach a maximum size and then collapse, returning to a denser and hotter state, should the mass density be greater than the critical density. Or, most likely the case with a density equal to or lesser than the critical density, the expansion would last forever -albeit slowing and all matter eventually burnt in stars only leave white dwarfs, neutron stars, and black holes eventually merging into large black holes with a temperature averaging absolute zero. As black holes would eventually evaporate in turn, the entropy of the Universe would leave no organized form of energy. Come with the concept of 'dark energy' some other models of how the Universe could end. The Universe should first keep accelerating. Our Milky Way Galaxy, nor the Andromeda Galaxy never would merge into the Virgo Cluster as, about a hundred billion years from now, all other galaxies ultimately would disappear from the Milky Way's horizon and, eventually, the local superclusters of galaxies also would disintegrate. Other think that, with dark energy, only gravitationally bound systems like galaxies would survive in a first time but then also end into a void Universe together as some think that dark energy acceleration will stop and the Universe will turn back to a Big Crunch in only 10 to 20 billion years from now. The extreme, Big Rip theory states that ever-accelerating dark energy will progressively tear apart galaxies, stars, planets down to atoms nuclei and particles themselves in a faraway future. Some eventually think that 'mathematical modeling, in particular when dealing with the early and earliest epochs of the [U]niverse, cannot produce but the cosmological myths adequate for our time'