Small density perturbations have collapsed into galaxies and clusters, because after recombination, the coupling between individual baryons has become too weak in order to keep these in an equilibrium state. The freeze-out of Dark Matter is a non-equilibrium process as well. Furthermore, a deviation from thermal equilibrium is a necessary condition for the generation of the asymmetry between matter and antimatter in the Universe.
A viable explanation for the generation of primordial density perturbations that have eventually grown into galaxies and clusters is the amplification of quantum fluctuations in the expanding space-time, which can be described by Quantum Field Theory in curved space times. Therefore, light travelling through ionized hydrogen and neutral hydrogen shows different absorption lines. In addition, the light will have travelled for billions of years to reach us, so any absorption by neutral hydrogen will have been redshifted by varied amounts, rather than by one specific amount, indicating when it happened.
These features make it possible to study the state of ionization at many different times in the past. They show that reionization began as "bubbles" of ionized hydrogen which became larger over time. These observations have narrowed down the period of time during which reionization took place, but the source of the photons that caused reionization is still not completely certain. To ionize neutral hydrogen, an energy larger than Protons and electrons will recombine if energy is not continuously provided to keep them apart, which also sets limits on how numerous the sources were and their longevity.
However, by this time, matter had become far more spread out due to the ongoing expansion of the universe. Although the neutral hydrogen atoms were again ionized, the plasma was much more thin and diffuse, and photons were much less likely to be scattered. Despite being reionized, the universe remained largely transparent during reionization. As the universe continued to cool and expand, reionization gradually ended.
Matter continues to draw together under the influence of gravity , to form galaxies. The stars from this time period, known as Population II stars, are formed early on in this process, with more recent Population I stars formed later. Gravitational attraction also gradually pulls galaxies towards each other to form groups, clusters and superclusters. The Hubble Ultra Deep Field observatory has identified a number of small galaxies merging to form larger ones, at million years of cosmic time 13 billion years ago  this age estimate is now believed to be slightly overstated.
The universe has appeared much the same as it does now, for many billions of years. It will continue to look similar for many more billions of years into the future. Based upon the emerging science of nucleocosmochronology , the Galactic thin disk of the Milky Way is estimated to have been formed 8. Its behavior had originally been dominated by radiation relativistic constituents such as photons and neutrinos for the first 47, years, and since about , years of cosmic time, its behavior had been dominated by matter. During its matter-dominated era, the expansion of the universe had begun to slow down, as gravity reigned in the initial outward expansion.
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But from about 9. While the precise cause is not known, the observation is accepted as correct by the cosmologist community. By far the most accepted understanding is that this is due to an unknown form of energy which has been given the name " dark energy ". Research is ongoing to understand this dark energy. Dark energy is now believed to be the single largest component of the universe, as it constitutes about Dark energy is believed to act like a cosmological constant - a scalar field that exists throughout space. Unlike gravity, the effects of such a field do not diminish or only diminish slowly as the universe grows.
While matter and gravity have a greater effect initially, their effect quickly diminishes as the universe continues to expand. Objects in the universe, which are initially seen to be moving apart as the universe expands, continue to move apart, but their outward motion gradually slows down. This slowing effect becomes smaller as the universe becomes more spread out. Eventually, the outward and repulsive effect of dark energy begins to dominate over the inward pull of gravity.
Instead of slowing down and perhaps beginning to move inward under the influence of gravity, from about 9. The universe has existed for around Beyond that, we need to better understand the universe to make any accurate predictions. Therefore, the universe could follow a variety of different paths beyond this time. There are several competing scenarios for the possible long-term evolution of the universe. Which of them will happen, if any, depends on the precise values of physical constants such as the cosmological constant , the possibility of proton decay , the energy of the vacuum meaning, the energy of "empty" space itself , and the natural laws beyond the Standard Model.
If the expansion of the universe continues and it stays in its present form, eventually all but the nearest galaxies will be carried away from us by the expansion of space at such a velocity that our observable universe will be limited to our own gravitationally bound local galactic cluster. In the very long term after many trillions — thousands of billions — of years, cosmic time , the Stelliferous Era will end, as stars cease to be born and even the longest-lived stars gradually die.
Beyond this, all objects in the universe will cool and with the possible exception of protons gradually decompose back to their constituent particles and then into subatomic particles and very low level photons and other fundamental particles , by a variety of possible processes.
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Ultimately, in the extreme future, the following scenarios have been proposed for the ultimate fate of the universe. The effect would be that the quantum fields that underpin all forces, particles and structures, would undergo a transition to a more stable form. New forces and particles would replace the present ones we know of, with the side effect that all current particles, forces and structures would be destroyed and subsequently if able reform into different particles, forces and structures. In this kind of extreme timescale, extremely rare quantum phenomena may also occur that are extremely unlikely to be seen on a timescale smaller than trillions of years.
These may also lead to unpredictable changes to the state of the universe which would not be likely to be significant on any smaller timescale. For example, on a timescale of millions of trillions of years, black holes might appear to evaporate almost instantly, uncommon quantum tunneling phenomena would appear to be common, and quantum or other phenomena so unlikely that they might occur just once in a trillion years may occur many times. From Wikipedia, the free encyclopedia. Events since the Big Bang, Early universe. Subject history.
Discovery of cosmic microwave background radiation. Religious interpretations of the Big Bang theory. Universe events since the Big Bang Nature timeline. This box: view talk edit. Matter-dominated era.
A DIY take on the early universe may reveal cosmic secrets
Single-celled life. Multicellular life. Dark Ages. Earliest stars. Earliest galaxy. Omega Centauri. Andromeda Galaxy. Milky Way spirals.
The Physics of the Early Universe
Alpha Centauri. Earliest life. Earliest oxygen. Atmospheric oxygen. Sexual reproduction. Earliest plants.
Cambrian explosion. Earliest mammals. Earliest apes.
See also: Human timeline , and Life timeline. It includes the Planck epoch , during which currently understood laws of physics may not apply; the emergence in stages of the four known fundamental interactions or forces — first gravity , and later the strong , weak and electromagnetic interactions; and the expansion of space itself and supercooling of the still immensely hot universe due to cosmic inflation , which is believed to have been triggered by the separation of the strong and electroweak interaction.
The early universe , lasting around , years. Initially, various kinds of subatomic particles are formed in stages.
PHYS Particle Physics and the Early Universe | School of Physics
These particles include almost equal amounts of matter and antimatter , so most of it quickly annihilates, leaving a small excess of matter in the universe. Much later, hydrogen and helium hydride react to form molecular hydrogen, the fuel needed for the first stars At about , years, [ citation needed ] the universe finally becomes cool enough for neutral atoms to form " recombination " , and as a result it also became transparent for the first time.
Dark Ages and large-scale structure emergence , from , years until about 1 billion years. After recombination and decoupling , the universe was transparent but the clouds of hydrogen only collapsed very slowly to form stars and galaxies , so there were no new sources of light.
The only photons electromagnetic radiation, or "light" in the universe were those released during decoupling visible today as the cosmic microwave background and 21 cm radio emissions occasionally emitted by hydrogen atoms. The decoupled photons would have filled the universe with a brilliant pale orange glow at first, gradually redshifting to non-visible wavelengths after about 3 million years, leaving it without visible light.