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73-475: The epoch between recombination and the emergence of stars and galaxies is termed the "cosmic Dark Ages ". In this era, neutral hydrogen predominated the universe's matter composition. While this hydrogen has not yet been directly observed, ongoing experiments aim to detect the characteristic hydrogen line from this period. The hydrogen line arises when an electron in a neutral hydrogen atom transitions between hyperfine states, either by excitation to

146-655: A Lissajous orbit about the Lagrangian point. One of the NASA missions to the Moon under study would send a sample-return lander to the South Pole–Aitken basin , the location of a major impact event that created a formation nearly 2,400 km (1,500 mi) across. The force of this impact has created a deep penetration into the lunar surface, and a sample returned from this site could be analyzed for information concerning

219-411: A redshift of approximately 1100, the cooling of primordial plasma allowed protons and electrons to merge into neutral hydrogen atoms, rendering the universe transparent as photons ceased to interact significantly with matter . These ancient photons are detectable in the present as the cosmic microwave background (CMB). The CMB reveals a universe that remained smooth and homogeneous. Following

292-570: A "coupling" occurs, rendering the hydrogen line undetectable. Observation of the hydrogen line is feasible only when there is a temperature discrepancy between the neutral hydrogen and the CMB. In the immediate aftermath of the Big Bang, the universe was characterized by intense heat, density, and near- uniformity . Its subsequent expansion and cooling created conducive conditions for nuclear and atomic formation. Around 400,000 years post- Big Bang , at

365-647: A catalog of 500 distinguished features of the landscape. In 1961, the first globe (1: 13 600 000 scale ) containing lunar features invisible from the Earth was released in the USSR , based on images from Luna 3. On 20 July 1965, another Soviet probe, Zond 3 , transmitted 25 pictures of very good quality of the lunar far side, with much better resolution than those from Luna 3. In particular, they revealed chains of craters, hundreds of kilometers in length, but, unexpectedly, no mare plains like those visible from Earth with

438-748: A differential spectral calibration technique is anticipated to mitigate intense cosmic foregrounds, thereby facilitating the detection of the faint cosmic 21-cm signal. In addition to the DARE mission, several other initiatives have been proposed to investigate this field. These include the Precision Array for Probing the Epoch of Reionization (PAPER), the Low Frequency Array (LOFAR), the Murchison Widefield Array (MWA),

511-422: A low-frequency radio spectrograph and geological research tools. The far side of the Moon provides a good environment for radio astronomy as interferences from the Earth are blocked by the Moon. In February 2020, Chinese astronomers reported, for the first time, a high-resolution image of a lunar ejecta sequence , as well as direct analysis of its internal architecture. These were based on observations made by

584-493: A month later on June 25 with humanity's first lunar samples retrieved from the far side. Astronomers have suggested installing a large radio telescope on the far side, where the Moon would shield it from possible radio interference from Earth. Tidal forces from Earth have slowed the Moon's rotation to the point where the same side is always facing the Earth—a phenomenon called tidal locking . The other face, most of which

657-454: A state with aligned spins or by de-excitation as the spins move from alignment to anti-alignment. The energy differential between these hyperfine states, 5.9 × 10 − 6 {\displaystyle 5.9\times 10^{-6}} electron volts , equates to a photon with a wavelength of 21 centimeters. When neutral hydrogen attains thermodynamic equilibrium with cosmic microwave background (CMB) photons,

730-506: A universe with different values for the baryon-to-photon ratio and matter density, recombination and photon decoupling need not have occurred at the same epoch. Far side of the Moon The far side of the Moon is the lunar hemisphere that always faces away from Earth , opposite to the near side , because of synchronous rotation in the Moon 's orbit . Compared to the near side,

803-427: Is a blackbody spectrum representing the photons present at recombination, shifted in energy by the expansion of the universe. A blackbody is completely characterized by its temperature; the shift is called the redshift denoted by z : T CMB = 2.7   K × ( 1 + z ) {\displaystyle T_{\text{CMB}}=\mathrm {2.7~K} \times (1+z)} where 2.7 K

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876-518: Is fully neutral, charge neutrality implies n e = n p , i.e. x e is also the fraction of ionized hydrogen. It is possible to find a rough estimate of the redshift of the recombination epoch assuming the recombination reaction p + e − ⟷ H + γ {\displaystyle p+e^{-}\longleftrightarrow H+\gamma } is fast enough that it proceeds near thermal equilibrium. The relative abundance of free electrons, protons and neutral hydrogen

949-427: Is generally described in terms of the free electron fraction x e as a function of redshift . It is the ratio of the abundance of free electrons to the total abundance of hydrogen (both neutral and ionized). Denoting by n e the number density of free electrons, n H that of atomic hydrogen and n p that of ionized hydrogen (i.e. protons), x e is defined as Since hydrogen only recombines once helium

1022-461: Is misleading, since the Big Bang theory does not posit that protons and electrons had been combined before, but the name exists for historical reasons since it was named before the Big Bang hypothesis became the primary theory of the birth of the universe. Immediately after the Big Bang , the universe was a hot, dense plasma of photons , leptons , and quarks : the quark epoch . At 10 seconds,

1095-469: Is never visible from the Earth, is therefore called the "far side of the Moon". Over time, some crescent-shaped edges of the far side can be seen due to libration . In total, 59 percent of the Moon's surface is visible from Earth at one time or another. Useful observation of the parts of the far side of the Moon occasionally visible from Earth is difficult because of the low viewing angle from Earth (they cannot be observed "full on"). A common misconception

1168-484: Is related to a higher concentration of heat-producing elements on the near-side hemisphere, as has been demonstrated by geochemical maps obtained from the Lunar Prospector gamma-ray spectrometer . While other factors, such as surface elevation and crustal thickness, could also affect where basalts erupt, these do not explain why the far side South Pole–Aitken basin (which contains the lowest elevations of

1241-473: Is situated near the center of the far side, and the 3-kilometer-high (2 mi) rim would help to block stray communications from orbiting satellites. Another potential candidate for a radio telescope is the Saha crater . Before deploying radio telescopes to the far side, several problems must be overcome. The fine lunar dust can contaminate equipment, vehicles, and space suits. The conducting materials used for

1314-469: Is slower than what would be predicted from Saha equilibrium and takes place around redshift z ≈ 2000. The details of helium recombination are less critical than those of hydrogen recombination for the prediction of cosmic microwave background anisotropies, since the universe is still very optically thick after helium has recombined and before hydrogen has started its recombination. Prior to recombination, photons were not able to freely travel through

1387-428: Is that the Moon does not rotate on its axis. If that were so, the whole of the Moon would be visible to Earth over the course of its orbit. Instead, its rotation period matches its orbital period, meaning it turns around once for every orbit it makes: in Earth terms, it could be said that its day and its year have the same length (i.e., ~29.5 earth days). The phrase "dark side of the Moon" does not refer to "dark" as in

1460-543: Is the Mare Orientale , which is a prominent impact basin spanning almost 1,000 km (600 miles), yet this was not even named as a feature until 1906, by Julius Franz in Der Mond . The true nature of the basin was discovered in the 1960s when rectified images were projected onto a globe. The basin was photographed in fine detail by Lunar Orbiter 4 in 1967. Before space exploration began, astronomers expected that

1533-527: Is then given by the Saha equation : where m e is the mass of the electron , k B is the Boltzmann constant , T is the temperature, ħ is the reduced Planck constant , and E I = 13.6 eV is the ionization energy of hydrogen. Charge neutrality requires n e  =  n p , and the Saha equation can be rewritten in terms of the free electron fraction x e : All quantities in

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1606-401: Is today's temperature. The thermal energy at the peak of the blackbody spectrum is the Boltzmann constant , k B , times the temperature, k B T CMB ( z ) {\displaystyle k_{B}T_{\text{CMB}}(z)} but simply comparing this to the ionization energy of hydrogen atoms will not consider the spectrum of energies. A better estimate evaluates

1679-684: The Giant Metrewave Radio Telescope (GMRT), and the Large Aperture Experiment to Detect the Dark Ages (LEDA). Recombination (cosmology) In cosmology , recombination refers to the epoch during which charged electrons and protons first became bound to form electrically neutral hydrogen atoms . Recombination occurred about 378 000 years after the Big Bang (at a redshift of z  =  1100 ). The word "recombination"

1752-564: The Lunar Penetrating Radar (LPR) on board the Yutu-2 rover. CNSA launched Chang'e 6 on 3 May 2024, which conducted the first lunar sample return from Apollo Basin on the far side of the Moon. It was CNSA's second lunar sample return mission, the first achieved by Chang'e 5 from the lunar near side four years earlier. It also carried a mini "Jinchan" rover to conduct infrared spectroscopy of lunar surface and imaged

1825-483: The Moon’s far side —a region considered devoid of human-made radio frequency interference and substantial ionospheric activity. The mission’s scientific apparatus, affixed to an RF-quiet spacecraft bus, comprises a three-part radiometer system featuring an electrically short, tapered, biconical dipole antenna , along with a receiver and a digital spectrometer. DARE’s utilization of the antenna’s smooth frequency response and

1898-412: The ground state (lowest energy) of hydrogen are very inefficient, these hydrogen atoms generally form with the electrons in a high energy state, and the electrons quickly transition to their low energy state by emitting photons . Two main pathways exist: from the 2p state by emitting a Lyman-a photon – these photons will almost always be reabsorbed by another hydrogen atom in its ground state – or from

1971-456: The 2s state by emitting two photons, which is very slow. This production of photons is known as decoupling , which leads to recombination sometimes being called photon decoupling , but recombination and photon decoupling are distinct events. Once photons decoupled from matter, they traveled freely through the universe without interacting with matter and constitute what is observed today as cosmic microwave background radiation (in that sense,

2044-486: The Chang'e 6's lander on the lunar surface. The lander-ascender-rover combination was separated with the orbiter and returner before landing on 1 June 2024 at 22:23 UTC. It landed on the Moon's surface on 1 June 2024. The ascender was launched back to lunar orbit on 3 June 2024 at 23:38 UTC, carrying samples collected by the lander, and later completed another robotic rendezvous and docking in lunar orbit. The sample container

2117-404: The Earth. L2 is proposed to be "an ideal location" for a propellant depot as part of the proposed depot-based space transportation architecture. The China National Space Administration (CNSA) 's Chang'e 4 achieved humanity's first ever soft landing on the lunar far side on 3 January 2019 and deployed the Yutu-2 lunar rover onto the lunar surface. The craft included a lander equipped with

2190-515: The Moon and possesses a thin crust) was not as volcanically active as Oceanus Procellarum on the near side. It has also been proposed that the differences between the two hemispheres may have been caused by a collision with a smaller companion moon that also originated from the Theia collision . In this model, the impact led to an accretionary pile rather than a crater, contributing a hemispheric layer of extent and thickness that may be consistent with

2263-497: The Moon experiences two weeks of sunlight while the opposite location experiences night. About 18 percent of the far side is occasionally visible from Earth due to oscillation and to libration . The remaining 82 percent remained unobserved until 1959, when it was photographed by the Soviet Luna 3 space probe. The Soviet Academy of Sciences published the first atlas of the far side in 1960. The Apollo 8 astronauts were

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2336-446: The Moon have dramatically different appearances, with the near side covered in multiple, large maria (Latin for 'seas', since the earliest astronomers incorrectly thought that these plains were seas of lunar water ). The far side has a battered, densely cratered appearance with few maria. Only 1% of the surface of the far side is covered by maria, compared to 31.2% on the near side. One commonly accepted explanation for this difference

2409-449: The Moon was formed is the reason the near side has fewer impact craters. The lunar crust consists primarily of plagioclases formed when aluminium and calcium condensed and combined with silicates in the mantle. The cooler far side experienced condensation of these elements sooner and so formed a thicker crust; meteoroid impacts on the near side would sometimes penetrate the thinner crust here and release basaltic lava that created

2482-639: The Moon were out of direct radio communication with the Earth, and had to wait until the orbit allowed transmission. During the Apollo missions , the main engine of the Service Module was fired when the vessel was behind the Moon, producing some tense moments in Mission Control before the craft reappeared. Geologist-astronaut Harrison Schmitt , who became the last to step onto the Moon, had aggressively lobbied for Apollo 17's landing site to be on

2555-575: The Soviet Union. As many prominent landscape features of the far side were discovered by Soviet space probes, Soviet scientists selected names for them. This caused some controversy, though the Soviet Academy of Sciences selected many non-Soviet names, including Jules Verne , Marie Curie and Thomas Edison . The International Astronomical Union later accepted many of the names. On 26 April 1962, NASA 's Ranger 4 space probe became

2628-617: The US and Yakov Borisovich Zel'dovich and collaborators in the USSR independently computed the non-equilibrium recombination history of hydrogen. The basic elements of the model are the following. This model is usually described as an "effective three-level atom" as it requires keeping track of hydrogen under three forms: in its ground state, in its first excited state (assuming all the higher excited states are in Boltzmann equilibrium with it), and in its ionized state. Accounting for these processes,

2701-570: The United States Department of Energy . Because the far side of the Moon is shielded from radio transmissions from the Earth, it is considered a good location for placing radio telescopes for use by astronomers . Small, bowl-shaped craters provide a natural formation for a stationary telescope similar to Arecibo in Puerto Rico . For much larger-scale telescopes, the 100-kilometer-diameter (60 mi) crater Daedalus

2774-533: The Universe had expanded and cooled sufficiently to allow for the formation of protons : the hadron epoch . This plasma was effectively opaque to electromagnetic radiation due to Thomson scattering by free electrons, as the mean free path each photon could travel before encountering an electron was very short. This is the current state of the interior of the Sun. As the universe expanded , it also cooled. Eventually,

2847-408: The absence of light, but rather "dark" as in unknown: until humans were able to send spacecraft around the Moon, this area had never been seen. In reality, both the near and far sides receive (on average) almost equal amounts of light directly from the Sun. This symmetry is complicated by sunlight reflected from the Earth onto the near side ( earthshine ), and by lunar eclipses, which occur only when

2920-466: The concluding phase of Reionization, the initial stars and galaxies from the Dark Ages and Cosmic Dawn remain beyond the observational reach of contemporary instruments. The envisioned DARE mission aims to conduct pioneering measurements of the inception of the first stars and black holes, as well as ascertain the characteristics of hitherto undetectable stellar populations. These observations would contextualize existing data and enhance our comprehension of

2993-406: The cosmic background radiation is infrared and some red black-body radiation emitted when the universe was at a temperature of some 3000 K, redshifted by a factor of 1100 from the visible spectrum to the microwave spectrum). The time frame for recombination can be estimated from the time dependence of the temperature of the cosmic microwave background (CMB). The microwave background

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3066-442: The developmental processes of the first galaxies from antecedent cosmic structures. The DARE mission aims to analyze the spectral profile of the sky-averaged, redshifted 21-cm signal within a 40–120 MHz radio bandpass, targeting neutral hydrogen at redshifts between 11-35, corresponding to a period 420-80 million years subsequent to the Big Bang. DARE’s tentative schedule involves a 3-year lunar orbit, focusing on data collection above

3139-408: The dimensions of the far side highlands. The chemical composition of the far side is inconsistent with this model. The far side has more visible craters. This is thought to be a result of the effects of lunar lava flows, which cover and obscure craters, rather than a shielding effect from the Earth. NASA calculates that the Earth obscures only about 4 square degrees out of 41,000 square degrees of

3212-401: The emergence of the first stars—a milestone known as Cosmic Dawn. The formation of additional stars and the assembly of the earliest galaxies inundated the universe with ultraviolet photons , which had the potential to ionize hydrogen gas. Several hundred million years post-Cosmic Dawn, the initial stars emitted sufficient ultraviolet photons to reionize the vast majority of hydrogen atoms in

3285-418: The far side is already dark. Lunar eclipses mean that the side facing earth receives fractionally less sunlight than the far side when considered over a long period of time. At night under a "full Earth" the near side of the Moon receives on the order of 10 lux of illumination (about what a city sidewalk under streetlights gets; this is 34 times more light than is received on Earth under a full Moon ) whereas

3358-407: The far side of the Moon during the lunar night receives only about 0.001 lux of starlight. Only during a full Moon (as viewed from Earth) is the whole far side of the Moon dark. The word dark has expanded to refer also to the fact that communication with spacecraft can be blocked while the spacecraft is on the far side of the Moon, during Apollo space missions for example. The two hemispheres of

3431-474: The far side of the Moon, targeting the lava-filled crater Tsiolkovskiy . Schmitt's ambitious proposal included a special communications satellite based on the existing TIROS satellites to be launched into a Farquhar–Lissajous halo orbit around the L2 point so as to maintain line-of-sight contact with the astronauts during their powered descent and lunar surface operations. NASA administrators rejected these plans on

3504-520: The far side would be similar to the side visible to Earth. On 7 October 1959, the Soviet probe Luna 3 took the first photographs of the lunar far side, eighteen of them resolvable, covering one-third of the surface invisible from the Earth. The images were analysed, and the first atlas of the far side of the Moon was published by the USSR Academy of Sciences on 6 November 1960. It included

3577-594: The far side's terrain is rugged, with a multitude of impact craters and relatively few flat and dark lunar maria ("seas"), giving it an appearance closer to other barren places in the Solar System such as Mercury and Callisto . It has one of the largest craters in the Solar System , the South Pole–Aitken basin . The hemisphere has sometimes been called the " Dark side of the Moon ", where "dark" means "unknown" instead of "lacking sunlight" – each location on

3650-487: The first humans to see the far side in person when they orbited the Moon in 1968. All crewed and uncrewed soft landings had taken place on the near side of the Moon , until January 3, 2019, when the Chang'e 4 spacecraft made the first landing on the far side. The Chang'e 6 sample-return mission was launched on May 3, 2024, landed in the Apollo basin in the southern hemisphere of the lunar far side and returned to Earth

3723-471: The first spacecraft to impact the far side of the Moon, although it failed to return any scientific data before impact. The first truly comprehensive and detailed mapping survey of the far side was undertaken by the American uncrewed Lunar Orbiter program launched by NASA from 1966 to 1967. Most of the coverage of the far side was provided by the final probe in the series, Lunar Orbiter 5 . The far side

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3796-438: The formation of the initial hydrogen atoms, the universe was composed of an almost entirely neutral, uniformly distributed intergalactic medium (IGM), predominantly made up of hydrogen gas. This epoch, devoid of luminous bodies, is referred to as the cosmic Dark Ages. Theoretical models forecast that, over subsequent hundreds of millions of years, gravitational forces gradually compressed the gas into denser regions, culminating in

3869-432: The free electron fraction much slower than what one would obtain from the Saha equilibrium calculation. With modern values of cosmological parameters, one finds that the universe is 90% neutral at z ≈ 1070 . The simple effective three-level atom model described above accounts for the most important physical processes. However it does rely on approximations that lead to errors on the predicted recombination history at

3942-434: The grounds of added risk and lack of funding. The idea of utilizing Earth–Moon L 2 for communications satellite covering the Moon's far side has been realized, as China National Space Administration launched Queqiao relay satellite in 2018. It has since been used for communications between the Chang'e 4 lander and Yutu 2 rover that have successfully landed in early 2019 on the lunar far side and ground stations on

4015-402: The interior of the Moon. Because the near side is partly shielded from the solar wind by the Earth, the far side maria are expected to have the highest concentration of helium-3 on the surface of the Moon. This isotope is relatively rare on the Earth, but has good potential for use as a fuel in fusion reactors. Proponents of lunar settlement have cited the presence of this material as

4088-538: The level of 10% or so. Due to the importance of recombination for the precise prediction of cosmic microwave background anisotropies, several research groups have revisited the details of this picture over the last two decades. The refinements to the theory can be divided into two categories: Modern recombination theory is believed to be accurate at the level of 0.1%, and is implemented in publicly available fast recombination codes. Helium nuclei are produced during Big Bang nucleosynthesis , and make up about 24% of

4161-495: The lower number of free electrons. Shortly after recombination, the photon mean free path became larger than the Hubble length , and photons traveled freely without interacting with matter. For this reason, recombination is closely associated with the last scattering surface, which is the name for the last time at which the photons in the cosmic microwave background interacted with matter. However, these two events are distinct, and in

4234-416: The lunar limb on the far side, but only up to 59% of the total surface of the Moon. These features were seen from a low angle, hindering useful observation (it proved difficult to distinguish a crater from a mountain range). The remaining 82% of the surface on the far side remained unknown, and its properties were subject to much speculation. An example of a far side feature that can be seen through libration

4307-412: The maria, but would rarely do so on the far side. The far side exhibits more extreme variations in terrain elevation than the near side. The Moon's highest and lowest points, along with its tallest mountains measured from base to peak, are all located on the far side. Until the late 1950s, little was known about the far side of the Moon. Librations periodically allowed limited glimpses of features near

4380-476: The matter (baryon) density. The ratio of photons to baryons, η {\displaystyle \eta } , is known from several sources including measurements by the Planck satellite to be around 10 . Solving for z rec {\displaystyle z_{\text{rec}}} gives value around 1100, which converts to a cosmic time value around 400,000 years. The cosmic ionization history

4453-589: The naked eye. In 1967, the second part of the Atlas of the Far Side of the Moon was published in Moscow , based on data from Zond 3, with the catalog now including 4,000 newly discovered features of the lunar far side landscape. In the same year, the first Complete Map of the Moon (1: 5 000 000 scale ) and updated complete globe (1: 10 000 000 scale), featuring 95 percent of the lunar surface, were released in

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4526-492: The previous section would be recovered by setting the left-hand side to zero, i.e. assuming that the net rates of recombination and photoionization are large in comparison to the Hubble expansion rate, which sets the overall evolution timescale for the temperature and density. However, C α B n p is comparable to the Hubble expansion rate, and even gets significantly lower at low redshifts, leading to an evolution of

4599-462: The radio dishes must also be carefully shielded against the effects of solar flares . Finally, the area around the telescopes must be protected against contamination by other radio sources. The L 2 Lagrange point of the Earth–Moon system is located about 62,800 km (39,000 mi) above the far side, which has also been proposed as a location for a future radio telescope which would perform

4672-448: The recombination history is then described by the differential equation where α B is the "case B" recombination coefficient to the excited states of hydrogen, β B is the corresponding photoionization rate and E 21 = 10.2 eV is the energy of the first excited state. Note that the second term in the right-hand side of the above equation can be obtained by a detailed balance argument. The equilibrium result given in

4745-446: The right-hand side are known functions of z, the redshift : the temperature is given by T = (1 + z ) × 2.728 K , and the total density of hydrogen (neutral and ionized) is given by n p + n H = (1 + z ) × 1.6 m . Solving this equation for a 50 percent ionization fraction yields a recombination temperature of roughly 4000  K , corresponding to redshift z  =  1500 . In 1968, physicists Jim Peebles in

4818-401: The sky as seen from the Moon. "This makes the Earth negligible as a shield for the Moon [and] it is likely that each side of the Moon has received equal numbers of impacts, but the resurfacing by lava results in fewer craters visible on the near side than the far side, even though both sides have received the same number of impacts." Newer research suggests that heat from Earth at the time when

4891-789: The thermal equilibrium between matter (atoms) and radiation. The density of photons, η γ ( E > Q H ) {\displaystyle \eta _{\gamma }(E>Q_{H})} with energy E sufficient to ionize hydrogen is the total density times a factor from the equilibrium Boltzmann distribution : η γ ( E > Q H ) = η γ exp ⁡ ( − Q H k B T CMB ( z ) ) {\displaystyle \eta _{\gamma }(E>Q_{H})=\eta _{\gamma }\exp \left({\frac {-Q_{H}}{k_{B}T_{\text{CMB}}(z)}}\right)} At equilibrium this will approximately equal

4964-819: The total mass of baryonic matter . The ionization energy of helium is larger than that of hydrogen and it therefore recombines earlier. Because neutral helium carries two electrons, its recombination proceeds in two steps. The first recombination, H e 2 + + e − ⟶ H e + + γ {\displaystyle \mathrm {He} ^{2+}+\mathrm {e} ^{-}\longrightarrow \mathrm {He} ^{+}+\gamma } proceeds near Saha equilibrium and takes place around redshift z ≈ 6000. The second recombination, H e + + e − ⟶ H e + γ {\displaystyle \mathrm {He} ^{+}+\mathrm {e} ^{-}\longrightarrow \mathrm {He} +\gamma } ,

5037-401: The universe cooled to the point that the radiation field could not immediately ionize neutral hydrogen, and atoms became energetically favored. The fraction of free electrons and protons as compared to neutral hydrogen decreased to a few parts in 10 000 . Recombination involves electrons binding to protons (hydrogen nuclei) to form neutral hydrogen atoms . Because direct recombinations to

5110-401: The universe, as they constantly scattered off the free electrons and protons. This scattering causes a loss of information, and "there is therefore a photon barrier at a redshift" near that of recombination that prevents us from using photons directly to learn about the universe at larger redshifts. Once recombination had occurred, however, the mean free path of photons greatly increased due to

5183-542: The universe. This reionization epoch signifies the IGM’s transition back to a state of near-complete ionization. Observational studies have not yet explored the universe’s emerging structural complexity. Studying the universe’s earliest structures necessitates a telescope surpassing the capabilities of the Hubble Space Telescope . While theoretical models indicate that current measurements are starting to examine

5256-492: Was first seen directly by human eyes during the Apollo 8 mission in December, 1968. Astronaut William Anders described the view: "The backside looks like a sand pile my kids have played in for some time. It's all beat up, no definition, just a lot of bumps and holes." It has been seen by all 24 men who flew on Apollo 8 and Apollo 10 through Apollo 17 , and photographed by multiple lunar probes. Spacecraft passing behind

5329-468: Was then transferred to the returner, which landed in Inner Mongolia on 25 June 2024, completing China's far side sample return mission. The Lunar Surface Electromagnetics Experiment (LuSEE-Night) lander, a mission to soft land as early as 2026 a robotic observatory on the far side designed to measure electromagnetic waves from the early history of the universe is being developed by NASA and

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