A molecular cloud , sometimes called a stellar nursery (if star formation is occurring within), is a type of interstellar cloud , the density and size of which permit absorption nebulae , the formation of molecules (most commonly molecular hydrogen , H 2 ), and the formation of H II regions . This is in contrast to other areas of the interstellar medium that contain predominantly ionized gas .
150-585: Current observations suggest that the expansion of the universe will continue forever. The prevailing theory is that the universe will cool as it expands, eventually becoming too cold to sustain life. For this reason, this future scenario once popularly called " Heat Death " is now known as the "Big Chill" or "Big Freeze". If dark energy —represented by the cosmological constant , a constant energy density filling space homogeneously, or scalar fields , such as quintessence or moduli , dynamic quantities whose energy density can vary in time and space—accelerates
300-414: A − 3 {\displaystyle \rho \propto a^{-3}} , where a {\displaystyle a} is the scale factor . For ultrarelativistic particles ("radiation"), the energy density drops more sharply, as ρ ∝ a − 4 {\displaystyle \rho \propto a^{-4}} . This is because in addition to the volume dilution of
450-476: A galaxy exchange kinetic energy in a process called dynamical relaxation , making their velocity distribution approach the Maxwell–Boltzmann distribution . Dynamical relaxation can proceed either by close encounters of two stars or by less violent but more frequent distant encounters. In the case of a close encounter, two brown dwarfs or stellar remnants will pass close to each other. When this happens,
600-548: A linear relationship between distance to galaxies and their recessional velocity . Edwin Hubble observationally confirmed Lundmark's and Lemaître's findings in 1929. Assuming the cosmological principle , these findings would imply that all galaxies are moving away from each other. Astronomer Walter Baade recalculated the size of the known universe in the 1940s, doubling the previous calculation made by Hubble in 1929. He announced this finding to considerable astonishment at
750-407: A quasar , as long as enough matter is present there. In an expanding universe with decreasing density and non-zero cosmological constant , matter density would reach zero, resulting in most matter except black dwarfs , neutron stars , black holes , and planets ionizing and dissipating at thermal equilibrium . The following timeline assumes that protons do decay. The subsequent evolution of
900-576: A spectral line at a frequency of 1420.405 MHz . This frequency is generally known as the 21 cm line , referring to its wavelength in the radio band . The 21 cm line is the signature of HI and makes the gas detectable to astronomers back on earth. The discovery of the 21 cm line was the first step towards the technology that would allow astronomers to detect compounds and molecules in interstellar space. In 1951, two research groups nearly simultaneously discovered radio emission from interstellar neutral hydrogen. Ewen and Purcell reported
1050-485: A GMC, the volume of a GMC is so great that it contains much more mass than the Sun. The substructure of a GMC is a complex pattern of filaments, sheets, bubbles, and irregular clumps. Filaments are truly ubiquitous in the molecular cloud. Dense molecular filaments will fragment into gravitationally bound cores, most of which will evolve into stars. Continuous accretion of gas, geometrical bending, and magnetic fields may control
1200-630: A combined mass of more than the Chandrasekhar limit of about 1.4 solar masses happen to merge. The resulting object will then undergo runaway thermonuclear fusion, producing a Type Ia supernova and dispelling the darkness of the Degenerate Era for a few weeks. Neutron stars could also collide , forming even brighter supernovae and dispelling up to 6 solar masses of degenerate gas into the interstellar medium. The resulting matter from these supernovae could potentially create new stars. If
1350-459: A crucial role in the initial conditions of star formation and the origin of the stellar IMF. The densest parts of the filaments and clumps are called molecular cores, while the densest molecular cores are called dense molecular cores and have densities in excess of 10 to 10 particles per cubic centimeter. Typical molecular cores are traced with CO and dense molecular cores are traced with ammonia . The concentration of dust within molecular cores
1500-485: A description in which space does not expand and objects simply move apart while under the influence of their mutual gravity. Although cosmic expansion is often framed as a consequence of general relativity , it is also predicted by Newtonian gravity . According to inflation theory , the universe suddenly expanded during the inflationary epoch (about 10 of a second after the Big Bang), and its volume increased by
1650-564: A detectable radio signal . This discovery was an important step towards the research that would eventually lead to the detection of molecular clouds. Once the war ended, and aware of the pioneering radio astronomical observations performed by Jansky and Reber in the US, the Dutch astronomers repurposed the dish-shaped antennas running along the Dutch coastline that were once used by the Germans as
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#17327974585671800-464: A distance ct in a time t , as the red worldline illustrates. While it always moves locally at c , its time in transit (about 13 billion years) is not related to the distance traveled in any simple way, since the universe expands as the light beam traverses space and time. The distance traveled is thus inherently ambiguous because of the changing scale of the universe. Nevertheless, there are two distances that appear to be physically meaningful:
1950-455: A factor of at least 10 (an expansion of distance by a factor of at least 10 in each of the three dimensions). This would be equivalent to expanding an object 1 nanometer across ( 10 m , about half the width of a molecule of DNA ) to one approximately 10.6 light-years across (about 10 m , or 62 trillion miles). Cosmic expansion subsequently decelerated to much slower rates, until around 9.8 billion years after
2100-496: A false vacuum ; 95% confidence interval is 10 to 10 years due in part to uncertainty about the top quark mass. In 10 years, cold fusion occurring via quantum tunneling should make the light nuclei in stellar-mass objects fuse into iron-56 nuclei (see isotopes of iron ). Fission and alpha particle emission should make heavy nuclei also decay to iron, leaving stellar-mass objects as cold spheres of iron, called iron stars . Before this happens, however, in some black dwarfs
2250-424: A fast transition, forming "envelopes" of mass, giving the impression of an edge to the cloud structure. The structure itself is generally irregular and filamentary. Cosmic dust and ultraviolet radiation emitted by stars are key factors that determine not only gas and column density, but also the molecular composition of a cloud. The dust provides shielding to the molecular gas inside, preventing dissociation by
2400-482: A final heat death of the universe. Infinite expansion does not constrain the overall spatial curvature of the universe . It can be open (with negative spatial curvature), flat, or closed (positive spatial curvature), although if it is closed, sufficient dark energy must be present to counteract the gravitational forces or else the universe will end in a Big Crunch . Observations of the Cosmic microwave background by
2550-404: A finite distance. The comoving distance that such particles can have covered over the age of the universe is known as the particle horizon , and the region of the universe that lies within our particle horizon is known as the observable universe . If the dark energy that is inferred to dominate the universe today is a cosmological constant, then the particle horizon converges to a finite value in
2700-435: A finite scale factor. If the current vacuum state is a false vacuum , the vacuum may decay into an even lower-energy state. Presumably, extreme low- energy states imply that localized quantum events become major macroscopic phenomena rather than negligible microscopic events because even the smallest perturbations make the biggest difference in this era, so there is no telling what will or might happen to space or time. It
2850-420: A half-life comparable to that of protons. Planets (substellar objects) would decay in a simple cascade process from heavier elements to hydrogen and finally to photons and leptons while radiating energy. If the proton does not decay at all, then stellar objects would still disappear, but more slowly. See § Future without proton decay below. Shorter or longer proton half-lives will accelerate or decelerate
3000-441: A molecular cloud assembles enough mass, the densest regions of the structure will start to collapse under gravity, creating star-forming clusters. This process is highly destructive to the cloud itself. Once stars are formed, they begin to ionize portions of the cloud around it due to their heat. The ionized gas then evaporates and is dispersed in formations called ‘ champagne flows ’. This process begins when approximately 2% of
3150-435: A non-zero Riemann curvature tensor in curvature of Riemannian manifolds . Euclidean "geometrically flat" space has a Riemann curvature tensor of zero. "Geometrically flat" space has three dimensions and is consistent with Euclidean space. However, spacetime has four dimensions; it is not flat according to Einstein's general theory of relativity. Einstein's theory postulates that "matter and energy curve spacetime, and there
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#17327974585673300-433: A priori constraints) on how the space in which we live is connected or whether it wraps around on itself as a compact space . Though certain cosmological models such as Gödel's universe even permit bizarre worldlines that intersect with themselves, ultimately the question as to whether we are in something like a " Pac-Man universe", where if traveling far enough in one direction would allow one to simply end up back in
3450-499: A process called 'stellar ignition' occurs, and its lifetime as a star will properly begin. Stars of very low mass will eventually exhaust all their fusible hydrogen and then become helium white dwarfs . Stars of low to medium mass, such as our own sun , will expel some of their mass as a planetary nebula and eventually become white dwarfs ; more massive stars will explode in a core-collapse supernova , leaving behind neutron stars or black holes . In any case, although some of
3600-403: A smaller, denser galaxy. Since encounters are more frequent in this denser galaxy, the process then accelerates. The result is that most objects (90% to 99%) are ejected from the galaxy, leaving a small fraction (maybe 1% to 10%) which fall into the central supermassive black hole . It has been suggested that the matter of the fallen remnants will form an accretion disk around it that will create
3750-441: A timescale shorter than 10 million years—the time it takes for material to pass through the arm region. Perpendicularly to the plane of the galaxy, the molecular gas inhabits the narrow midplane of the galactic disc with a characteristic scale height , Z , of approximately 50 to 75 parsecs, much thinner than the warm atomic ( Z from 130 to 400 parsecs) and warm ionized ( Z around 1000 parsecs) gaseous components of
3900-427: A warning radar system and modified into radio telescopes , initiating the search for the hydrogen signature in the depths of space. The neutral hydrogen atom consists of a proton with an electron in its orbit. Both the proton and the electron have a spin property. When the spin state flips from a parallel condition to antiparallel, which contains less energy, the atom gets rid of the excess energy by radiating
4050-466: A weak rotational and vibrational modes, making it virtually invisible to direct observation. The solution to this problem came when Arno Penzias , Keith Jefferts, and Robert Wilson identified CO in the star-forming region in the Omega Nebula . Carbon monoxide is a lot easier to detect than H 2 because of its rotational energy and asymmetrical structure. CO soon became the primary tracer of
4200-458: Is a disagreement between this measurement and the supernova-based measurements, known as the Hubble tension . A third option proposed recently is to use information from gravitational wave events (especially those involving the merger of neutron stars , like GW170817 ), to measure the expansion rate. Such measurements do not yet have the precision to resolve the Hubble tension. In principle,
4350-540: Is accelerating in the present epoch. By assuming a cosmological model, e.g. the Lambda-CDM model , another possibility is to infer the present-day expansion rate from the sizes of the largest fluctuations seen in the cosmic microwave background . A higher expansion rate would imply a smaller characteristic size of CMB fluctuations, and vice versa. The Planck collaboration measured the expansion rate this way and determined H 0 = 67.4 ± 0.5 (km/s)/Mpc . There
4500-459: Is difficult to detect by infrared and radio observations, so the molecule most often used to determine the presence of H 2 is carbon monoxide (CO). The ratio between CO luminosity and H 2 mass is thought to be constant, although there are reasons to doubt this assumption in observations of some other galaxies. Within molecular clouds are regions with higher density, where much dust and many gas cores reside, called clumps. These clumps are
4650-409: Is dispersed after this time. The lack of large amounts of frozen molecules inside the clouds also suggest a short-lived structure. Some astronomers propose the molecules never froze in very large quantities due to turbulence and the fast transition between atomic and molecular gas. Due to their short lifespan, it follows that molecular clouds are constantly being assembled and destroyed. By calculating
Future of an expanding universe - Misplaced Pages Continue
4800-443: Is enough matter and energy to provide for curvature." In part to accommodate such different geometries, the expansion of the universe is inherently general-relativistic. It cannot be modeled with special relativity alone: Though such models exist, they may be at fundamental odds with the observed interaction between matter and spacetime seen in the universe. The images to the right show two views of spacetime diagrams that show
4950-442: Is essentially pressureless, with | p | ≪ ρ c 2 {\displaystyle |p|\ll \rho c^{2}} , while a gas of ultrarelativistic particles (such as a photon gas ) has positive pressure p = ρ c 2 / 3 {\displaystyle p=\rho c^{2}/3} . Negative-pressure fluids, like dark energy, are not experimentally confirmed, but
5100-424: Is expanding. The words ' space ' and ' universe ', sometimes used interchangeably, have distinct meanings in this context. Here 'space' is a mathematical concept that stands for the three-dimensional manifold into which our respective positions are embedded, while 'universe' refers to everything that exists, including the matter and energy in space, the extra dimensions that may be wrapped up in various strings , and
5250-505: Is in the form of a cosmological constant , the expansion will eventually become exponential, with the size of the universe doubling at a constant rate. If the theory of inflation is correct, the universe went through an episode dominated by a different form of dark energy in the first moments of the Big Bang; but inflation ended, indicating an equation of state much more complicated than those assumed so far for present-day dark energy. It
5400-534: Is known. The object's distance can then be inferred from the observed apparent brightness . Meanwhile, the recession speed is measured through the redshift. Hubble used this approach for his original measurement of the expansion rate, by measuring the brightness of Cepheid variable stars and the redshifts of their host galaxies. More recently, using Type Ia supernovae , the expansion rate was measured to be H 0 = 73.24 ± 1.74 (km/s)/Mpc . This means that for every million parsecs of distance from
5550-414: Is likely to be the main mechanism. Those regions with more gas will exert a greater gravitational force on their neighboring regions, and draw surrounding material. This extra material increases the density, increasing their gravitational attraction. Mathematical models of gravitational instability in the gas layer predict a formation time within the timescale for the estimated cloud formation time. Once
5700-513: Is normally sufficient to block light from background stars so that they appear in silhouette as dark nebulae . GMCs are so large that local ones can cover a significant fraction of a constellation; thus they are often referred to by the name of that constellation, e.g. the Orion molecular cloud (OMC) or the Taurus molecular cloud (TMC). These local GMCs are arrayed in a ring in the neighborhood of
5850-432: Is now an almost pure vacuum (possibly accompanied with the presence of a false vacuum ). The expansion of the universe slowly causes itself to cool down to absolute zero . The universe now reaches an even lower energy state than the earlier one mentioned. Whatever event happens beyond this era is highly speculative. It is possible that a Big Rip event may occur far off into the future. This singularity would take place at
6000-399: Is perceived that the laws of "macro-physics" will break down, and the laws of quantum physics will prevail. The universe could possibly avoid eternal heat death through random quantum tunneling and quantum fluctuations , given the non-zero probability of producing a new Big Bang creating a new universe in roughly 10 years. Expansion of the universe The expansion of the universe
6150-492: Is possible that the dark energy equation of state could change again resulting in an event that would have consequences which are extremely difficult to parametrize or predict. In the 1970s, the future of an expanding universe was studied by the astrophysicist Jamal Islam and the physicist Freeman Dyson . Then, in their 1999 book The Five Ages of the Universe , the astrophysicists Fred Adams and Gregory Laughlin divided
Future of an expanding universe - Misplaced Pages Continue
6300-400: Is smaller in the past and larger in the future. Extrapolating back in time with certain cosmological models will yield a moment when the scale factor was zero; our current understanding of cosmology sets this time at 13.787 ± 0.020 billion years ago . If the universe continues to expand forever, the scale factor will approach infinity in the future. It is also possible in principle for
6450-584: Is the equation of state parameter . The energy density of such a fluid drops as Nonrelativistic matter has w = 0 {\displaystyle w=0} while radiation has w = 1 / 3 {\displaystyle w=1/3} . For an exotic fluid with negative pressure, like dark energy, the energy density drops more slowly; if w = − 1 {\displaystyle w=-1} it remains constant in time. If w < − 1 {\displaystyle w<-1} , corresponding to phantom energy ,
6600-417: Is the gravitational constant , ρ {\displaystyle \rho } is the energy density within the universe, p {\displaystyle p} is the pressure , c {\displaystyle c} is the speed of light , and Λ {\displaystyle \Lambda } is the cosmological constant. A positive energy density leads to deceleration of
6750-428: Is the increase in distance between gravitationally unbound parts of the observable universe with time. It is an intrinsic expansion, so it does not mean that the universe expands "into" anything or that space exists "outside" it. To any observer in the universe, it appears that all but the nearest galaxies (which are bound to each other by gravity) move away at speeds that are proportional to their distance from
6900-464: The Adler–Bell–Jackiw anomaly , virtual black holes , or higher-dimension supersymmetry possibly with a half-life of under 10 years. 2018 estimate of Standard Model lifetime before collapse of a false vacuum ; 95% confidence interval is 10 to 10 years due in part to uncertainty about the top quark mass. Although protons are stable in standard model physics, a quantum anomaly may exist on
7050-678: The Big Bang , the Milky Way and the Andromeda galaxy will collide with one another and merge into one large galaxy based on current evidence. Up until 2012, there was no way to confirm whether the possible collision was going to happen or not. In 2012, researchers came to the conclusion that the collision is definite after using the Hubble Space Telescope between 2002 and 2010 to track the motion of Andromeda. This results in
7200-414: The Big Bang , the first star formed. Since then, stars have formed by the collapse of small, dense core regions in large, cold molecular clouds of hydrogen gas. At first, this produces a protostar , which is hot and bright because of energy generated by gravitational contraction . After the protostar contracts for a while, its core could become hot enough to fuse hydrogen, if it exceeds critical mass,
7350-457: The Big Bang . Due to their pivotal role, research about these structures have only increased over time. A paper published in 2022 reports over 10,000 molecular clouds detected since the discovery of Sagittarius B2. Within the Milky Way , molecular gas clouds account for less than one percent of the volume of the interstellar medium (ISM), yet it is also the densest part of it. The bulk of
7500-651: The Local Supercluster will be redshifted to such an extent that even gamma rays they emit will have wavelengths longer than the size of the observable universe of the time. Therefore, these galaxies will no longer be detectable in any way. By 10 (100 trillion) years from now, star formation will end, leaving all stellar objects in the form of degenerate remnants . If protons do not decay , stellar-mass objects will disappear more slowly, making this era last longer . By 10 (100 trillion) years from now, star formation will end. This period, known as
7650-545: The Local Supercluster will pass behind the cosmological horizon . It will then be impossible for events in the Local Supercluster to affect other galaxies. Similarly, it will be impossible for events after 150 billion years, as seen by observers in distant galaxies, to affect events in the Local Supercluster. However, an observer in the Local Supercluster will continue to see distant galaxies, but events they observe will become exponentially more redshifted as
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#17327974585677800-469: The Milky Way per year. Two possible mechanisms for molecular cloud formation have been suggested by astronomers. Cloud growth by collision and gravitational instability in the gas layer spread throughout the galaxy. Models for the collision theory have shown it cannot be the main mechanism for cloud formation due to the very long timescale it would take to form a molecular cloud, beyond the average lifespan of such structures. Gravitational instability
7950-594: The Wilkinson Microwave Anisotropy Probe and the Planck mission suggest that the universe is spatially flat and has a significant amount of dark energy . In this case, the universe might continue to expand at an accelerating rate. The acceleration of the universe's expansion has also been confirmed by observations of distant supernovae . If, as in the concordance model of physical cosmology (Lambda-cold dark matter or ΛCDM), dark energy
8100-510: The electroweak level, which can cause groups of baryons (protons and neutrons) to annihilate into antileptons via the sphaleron transition. Such baryon/lepton violations have a number of 3 and can only occur in multiples or groups of three baryons, which can restrict or prohibit such events. No experimental evidence of sphalerons has yet been observed at low energy levels, though they are believed to occur regularly at high energies and temperatures. After 10 years, black holes will dominate
8250-440: The equivalence principle of general relativity, the rules of special relativity are locally valid in small regions of spacetime that are approximately flat. In particular, light always travels locally at the speed c ; in the diagram, this means, according to the convention of constructing spacetime diagrams, that light beams always make an angle of 45° with the local grid lines. It does not follow, however, that light travels
8400-418: The large-scale structure of the universe . Around 3 billion years ago, at a time of about 11 billion years, dark energy is believed to have begun to dominate the energy density of the universe. This transition came about because dark energy does not dilute as the universe expands, instead maintaining a constant energy density. Similarly to inflation, dark energy drives accelerated expansion, such that
8550-410: The "Degenerate Era", will last until the degenerate remnants finally decay. The least-massive stars take the longest to exhaust their hydrogen fuel (see stellar evolution ). Thus, the longest living stars in the universe are low-mass red dwarfs , with a mass of about 0.08 solar masses ( M ☉ ), which have a lifetime of over 10 (10 trillion) years. Coincidentally, this is comparable to
8700-586: The 1952 meeting of the International Astronomical Union in Rome. For most of the second half of the 20th century, the value of the Hubble constant was estimated to be between 50 and 90 km⋅s ⋅ Mpc . On 13 January 1994, NASA formally announced a completion of its repairs related to the main mirror of the Hubble Space Telescope , allowing for sharper images and, consequently, more accurate analyses of its observations. Shortly after
8850-439: The Big Bang (4 billion years ago) it began to gradually expand more quickly , and is still doing so. Physicists have postulated the existence of dark energy , appearing as a cosmological constant in the simplest gravitational models, as a way to explain this late-time acceleration. According to the simplest extrapolation of the currently favored cosmological model, the Lambda-CDM model , this acceleration becomes dominant in
9000-419: The Big Bang. During the matter-dominated epoch, cosmic expansion also decelerated, with the scale factor growing as the 2/3 power of the time ( a ∝ t 2 / 3 {\displaystyle a\propto t^{2/3}} ). Also, gravitational structure formation is most efficient when nonrelativistic matter dominates, and this epoch is responsible for the formation of galaxies and
9150-401: The Hubble horizon are not dynamical, because gravitational influences do not have time to propagate across them, while perturbations much smaller than the Hubble horizon are straightforwardly governed by Newtonian gravitational dynamics . An object's peculiar velocity is its velocity with respect to the comoving coordinate grid, i.e., with respect to the average expansion-associated motion of
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#17327974585679300-418: The Hubble rate H {\displaystyle H} quantifies the rate of expansion. H {\displaystyle H} is a function of cosmic time . The expansion of the universe can be understood as a consequence of an initial impulse (possibly due to inflation ), which sent the contents of the universe flying apart. The mutual gravitational attraction of the matter and radiation within
9450-457: The ISM . The exceptions to the ionized-gas distribution are H II regions , which are bubbles of hot ionized gas created in molecular clouds by the intense radiation given off by young massive stars ; and as such they have approximately the same vertical distribution as the molecular gas. This distribution of molecular gas is averaged out over large distances; however, the small scale distribution of
9600-708: The Leiden-Sydney map of neutral hydrogen in the galactic disk in 1958 on the Monthly Notices of the Royal Astronomical Society . This was the first neutral hydrogen map of the galactic disc and also the first map showing the spiral arm structure within it. Following the work on atomic hydrogen detection by van de Hulst, Oort and others, astronomers began to regularly use radio telescopes, this time looking for interstellar molecules . In 1963 Alan Barrett and Sander Weinred at MIT found
9750-422: The Local Supercluster becomes causally impossible. 8 × 10 (800 billion) years from now, the luminosities of the different galaxies, approximately similar until then to the current ones thanks to the increasing luminosity of the remaining stars as they age, will start to decrease, as the less massive red dwarf stars begin to die as white dwarfs . 2 × 10 (2 trillion) years from now, all galaxies outside
9900-469: The Milky Way and the Andromeda Galaxy, are gravitationally bound to each other. It is expected that between 10 (100 billion) and 10 (1 trillion) years from now, their orbits will decay and the entire Local Group will merge into one large galaxy. Assuming that dark energy continues to make the universe expand at an accelerating rate, in about 150 billion years all galaxies outside
10050-489: The NASA/IPAC Extragalactic Database of Galaxy Distances, "Lundmark's extragalactic distance estimates were far more accurate than Hubble's, consistent with an expansion rate (Hubble constant) that was within 1% of the best measurements today." In 1927, Georges Lemaître independently reached a similar conclusion to Friedmann on a theoretical basis, and also presented observational evidence for
10200-593: The Sun coinciding with the Gould Belt . The most massive collection of molecular clouds in the galaxy forms an asymmetrical ring about the galactic center at a radius of 120 parsecs; the largest component of this ring is the Sagittarius B2 complex. The Sagittarius region is chemically rich and is often used as an exemplar by astronomers searching for new molecules in interstellar space. Isolated gravitationally-bound small molecular clouds with masses less than
10350-409: The beginning of star formation if gravitational forces are sufficient to cause the dust and gas to collapse. The history pertaining to the discovery of molecular clouds is closely related to the development of radio astronomy and astrochemistry . During World War II , at a small gathering of scientists, Henk van de Hulst first reported he had calculated the neutral hydrogen atom should transmit
10500-573: The black hole's mass decreases, its temperature increases, becoming comparable to the Sun 's by the time the black hole mass has decreased to 10 kilograms. The hole then provides a temporary source of light during the general darkness of the Black Hole Era. During the last stages of its evaporation, a black hole will emit not only massless particles, but also heavier particles, such as electrons , positrons , protons , and antiprotons . After all
10650-488: The black holes have evaporated (and after all the ordinary matter made of protons has disintegrated, if protons are unstable), the universe will be nearly empty. Photons , leptons , baryons , neutrinos , electrons , and positrons will fly from place to place, hardly ever encountering each other. Gravitationally , the universe will be dominated by dark matter , electrons , and positrons (not protons ). By this era, with only very diffuse matter remaining, activity in
10800-477: The clouds where star-formation occurs. In 1970, Penzias and his team quickly detected CO in other locations close to the galactic center , including the giant molecular cloud identified as Sagittarius B2 , 390 light years from the galactic center, making it the first detection of a molecular cloud in history. This team later would receive the Nobel prize of physics for their discovery of microwave emission from
10950-671: The combined mass is not above the Chandrasekhar limit but is larger than the minimum mass to fuse carbon (about 0.9 M ☉ ), a carbon star could be produced, with a lifetime of around 10 (1 million) years. Also, if two helium white dwarfs with a combined mass of at least 0.3 M ☉ collide, a helium star may be produced, with a lifetime of a few hundred million years. Finally, brown dwarfs could form new stars by colliding with each other to form red dwarf stars, which can survive for 10 (10 trillion) years, or by accreting gas at very slow rates from
11100-457: The cosmic scale factor grew exponentially in time. In order to solve the horizon and flatness problems, inflation must have lasted long enough that the scale factor grew by at least a factor of e (about 10 ). The history of the universe after inflation but before a time of about 1 second is largely unknown. However, the universe is known to have been dominated by ultrarelativistic Standard Model particles, conventionally called radiation , by
11250-513: The cosmic expansion history can also be measured by studying how redshifts, distances, fluxes, angular positions, and angular sizes of astronomical objects change over the course of the time that they are being observed. These effects are too small to have yet been detected. However, changes in redshift or flux could be observed by the Square Kilometre Array or Extremely Large Telescope in the mid-2030s. At cosmological scales,
11400-456: The decay of particles' peculiar momenta, as discussed above. It can also be understood as adiabatic cooling . The temperature of ultrarelativistic fluids, often called "radiation" and including the cosmic microwave background , scales inversely with the scale factor (i.e. T ∝ a − 1 {\displaystyle T\propto a^{-1}} ). The temperature of nonrelativistic matter drops more sharply, scaling as
11550-461: The detailed fragmentation manner of the filaments. In supercritical filaments, observations have revealed quasi-periodic chains of dense cores with spacing of 0.15 parsec comparable to the filament inner width. A substantial fraction of filaments contained prestellar and protostellar cores, supporting the important role of filaments in gravitationally bound core formation. Recent studies have suggested that filamentary structures in molecular clouds play
11700-666: The detection of the 21-cm line in March, 1951. Using the radio telescope at the Kootwijk Observatory, Muller and Oort reported the detection of the hydrogen emission line in May of that same year. Once the 21-cm emission line was detected, radio astronomers began mapping the neutral hydrogen distribution of the Milky Way Galaxy. Van de Hulst, Muller, and Oort, aided by a team of astronomers from Australia, published
11850-528: The distance between Earth and the quasar when the light was emitted, and the distance between them in the present era (taking a slice of the cone along the dimension defined as the spatial dimension). The former distance is about 4 billion light-years, much smaller than ct , whereas the latter distance (shown by the orange line) is about 28 billion light-years, much larger than ct . In other words, if space were not expanding today, it would take 28 billion years for light to travel between Earth and
12000-665: The emission line of OH in the supernova remnant Cassiopeia A . This was the first radio detection of an interstellar molecule at radio wavelengths. More interstellar OH detections quickly followed and in 1965, Harold Weaver and his team of radio astronomers at Berkeley , identified OH emissions lines coming from the direction of the Orion Nebula and in the constellation of Cassiopeia . In 1968, Cheung, Rank, Townes, Thornton and Welch detected NH₃ inversion line radiation in interstellar space. A year later, Lewis Snyder and his colleagues found interstellar formaldehyde . Also in
12150-416: The energy density grows as the universe expands. Inflation is a period of accelerated expansion hypothesized to have occurred at a time of around 10 seconds. It would have been driven by the inflaton , a field that has a positive-energy false vacuum state. Inflation was originally proposed to explain the absence of exotic relics predicted by grand unified theories , such as magnetic monopoles , because
12300-405: The evidence that leads to the inflationary model of the early universe also implies that the "total universe" is much larger than the observable universe. Thus any edges or exotic geometries or topologies would not be directly observable, since light has not reached scales on which such aspects of the universe, if they exist, are still allowed. For all intents and purposes, it is safe to assume that
12450-506: The existence of dark energy is inferred from astronomical observations. In an expanding universe, it is often useful to study the evolution of structure with the expansion of the universe factored out. This motivates the use of comoving coordinates , which are defined to grow proportionally with the scale factor. If an object is moving only with the Hubble flow of the expanding universe, with no other motion, then it remains stationary in comoving coordinates. The comoving coordinates are
12600-445: The expansion of the universe, then the space between clusters of galaxies will grow at an increasing rate. Redshift will stretch ancient ambient photons (including gamma rays) to undetectably long wavelengths and low energies. Stars are expected to form normally for 10 to 10 (1–100 trillion) years, but eventually the supply of gas needed for star formation will be exhausted. As existing stars run out of fuel and cease to shine,
12750-469: The expansion, a ¨ < 0 {\displaystyle {\ddot {a}}<0} , and a positive pressure further decelerates expansion. On the other hand, sufficiently negative pressure with p < − ρ c 2 / 3 {\displaystyle p<-\rho c^{2}/3} leads to accelerated expansion, and the cosmological constant also accelerates expansion. Nonrelativistic matter
12900-401: The first few billion years of its travel time, also indicating that the expansion of space between Earth and the quasar at the early time was faster than the speed of light. None of this behavior originates from a special property of metric expansion, but rather from local principles of special relativity integrated over a curved surface. Over time, the space that makes up the universe
13050-510: The first year observations of the Wilkinson Microwave Anisotropy Probe satellite (WMAP) further agreed with the estimated expansion rates for local galaxies, 72 ± 5 km⋅s ⋅Mpc . The universe at the largest scales is observed to be homogeneous (the same everywhere) and isotropic (the same in all directions), consistent with the cosmological principle . These constraints demand that any expansion of
13200-566: The formation of Milkdromeda (also known as Milkomeda ). 22 billion years in the future is the earliest possible end of the Universe in the Big Rip scenario, assuming a model of dark energy with w = −1.5 . False vacuum decay may occur in 20 to 30 billion years if the Higgs field is metastable. The galaxies in the Local Group , the cluster of galaxies which includes
13350-420: The future" over long distances. However, within general relativity , the shape of these comoving synchronous spatial surfaces is affected by gravity. Current observations are consistent with these spatial surfaces being geometrically flat (so that, for example, the angles of a triangle add up to 180 degrees). An expanding universe typically has a finite age. Light, and other particles, can have propagated only
13500-554: The future. In 1912–1914, Vesto Slipher discovered that light from remote galaxies was redshifted , a phenomenon later interpreted as galaxies receding from the Earth. In 1922, Alexander Friedmann used the Einstein field equations to provide theoretical evidence that the universe is expanding. Swedish astronomer Knut Lundmark was the first person to find observational evidence for expansion, in 1924. According to Ian Steer of
13650-405: The galaxy approaches the horizon until time in the distant galaxy seems to stop. The observer in the Local Supercluster never observes events after 150 billion years in their local time, and eventually all light and background radiation lying outside the Local Supercluster will appear to blink out as light becomes so redshifted that its wavelength has become longer than the physical diameter of
13800-444: The gas is highly irregular, with most of it concentrated in discrete clouds and cloud complexes. Molecular clouds typically have interstellar medium densities of 10 to 30 cm , and constitute approximately 50% of the total interstellar gas in a galaxy . Most of the gas is found in a molecular state . The visual boundaries of a molecular cloud is not where the cloud effectively ends, but where molecular gas changes to atomic gas in
13950-446: The horizon. Technically, it will take an infinitely long time for all causal interaction between the Local Supercluster and this light to cease. However, due to the redshifting explained above, the light will not necessarily be observed for an infinite amount of time, and after 150 billion years, no new causal interaction will be observed. Therefore, after 150 billion years, intergalactic transportation and communication beyond
14100-418: The infinite extent of the expanse. All that is certain is that the manifold of space in which we live simply has the property that the distances between objects are getting larger as time goes on. This only implies the simple observational consequences associated with the metric expansion explored below. No "outside" or embedding in hyperspace is required for an expansion to occur. The visualizations often seen of
14250-432: The infinite future. This implies that the amount of the universe that we will ever be able to observe is limited. Many systems exist whose light can never reach us, because there is a cosmic event horizon induced by the repulsive gravity of the dark energy. Within the study of the evolution of structure within the universe, a natural scale emerges, known as the Hubble horizon . Cosmological perturbations much larger than
14400-413: The inverse square of the scale factor (i.e. T ∝ a − 2 {\displaystyle T\propto a^{-2}} ). The contents of the universe dilute as it expands. The number of particles within a comoving volume remains fixed (on average), while the volume expands. For nonrelativistic matter, this implies that the energy density drops as ρ ∝
14550-409: The large-scale geometry of the universe according to the ΛCDM cosmological model. Two of the dimensions of space are omitted, leaving one dimension of space (the dimension that grows as the cone gets larger) and one of time (the dimension that proceeds "up" the cone's surface). The narrow circular end of the diagram corresponds to a cosmological time of 700 million years after the Big Bang, while
14700-570: The larger substructure of the cloud, having the average size of 1 pc . Clumps are the precursors of star clusters , though not every clump will eventually form stars. Cores are much smaller (by a factor of 10) and have higher densities. Cores are gravitationally bound and go through a collapse during star formation . In astronomical terms, molecular clouds are short-lived structures that are either destroyed or go through major structural and chemical changes approximately 10 million years into their existence. Their short life span can be inferred from
14850-593: The length of time over which star formation takes place. Once star formation ends and the least-massive red dwarfs exhaust their fuel, nuclear fusion will cease. The low-mass red dwarfs will cool and become black dwarfs . The only objects remaining with more than planetary mass will be brown dwarfs , with mass less than 0.08 M ☉ , and degenerate remnants ; white dwarfs , produced by stars with initial masses between about 0.08 and 8 solar masses; and neutron stars and black holes , produced by stars with initial masses over 8 M ☉ . Most of
15000-401: The mass of the Sun is called a giant molecular cloud ( GMC ). GMCs are around 15 to 600 light-years (5 to 200 parsecs) in diameter, with typical masses of 10 thousand to 10 million solar masses. Whereas the average density in the solar vicinity is one particle per cubic centimetre, the average volume density of a GMC is about ten to a thousand times higher. Although the Sun is much denser than
15150-544: The mass of the cloud has been converted into stars. Stellar winds are also known to contribute to cloud dispersal. The cycle of cloud formation and destruction is closed when the gas dispersed by stars cools again and is pulled into new clouds by gravitational instability. Star formation involves the collapse of the densest part of the molecular cloud, fragmenting the collapsed region in smaller clumps. These clumps aggregate more interstellar material, increasing in density by gravitational contraction. This process continues until
15300-416: The mass of this collection, approximately 90%, will be in the form of white dwarfs. In the absence of any energy source, all of these formerly luminous bodies will cool and become faint. The universe will become extremely dark after the last stars burn out. Even so, there can still be occasional light in the universe. One of the ways the universe can be illuminated is if two carbon – oxygen white dwarfs with
15450-429: The molecular gas is contained in a ring between 3.5 and 7.5 kiloparsecs (11,000 and 24,000 light-years ) from the center of the Milky Way (the Sun is about 8.5 kiloparsecs from the center). Large scale CO maps of the galaxy show that the position of this gas correlates with the spiral arms of the galaxy. That molecular gas occurs predominantly in the spiral arms suggests that molecular clouds must form and dissociate on
15600-491: The observer , on average. While objects cannot move faster than light , this limitation applies only with respect to local reference frames and does not limit the recession rates of cosmologically distant objects. Cosmic expansion is a key feature of Big Bang cosmology. It can be modeled mathematically with the Friedmann–Lemaître–Robertson–Walker metric (FLRW), where it corresponds to an increase in
15750-550: The observer, recessional velocity of objects at that distance increases by about 73 kilometres per second (160,000 mph). Supernovae are observable at such great distances that the light travel time therefrom can approach the age of the universe. Consequently, they can be used to measure not only the present-day expansion rate but also the expansion history. In work that was awarded the 2011 Nobel Prize in Physics , supernova observations were used to determine that cosmic expansion
15900-423: The particle count, the energy of each particle (including the rest mass energy ) also drops significantly due to the decay of peculiar momenta. In general, we can consider a perfect fluid with pressure p = w ρ {\displaystyle p=w\rho } , where ρ {\displaystyle \rho } is the energy density. The parameter w {\displaystyle w}
16050-528: The past and future history of an expanding universe into five eras. The first, the Primordial Era , is the time in the past just after the Big Bang when stars had not yet formed. The second, the Stelliferous Era , includes the present day and all of the stars and galaxies now seen. It is the time during which stars form from collapsing clouds of gas . In the subsequent Degenerate Era ,
16200-435: The present era (less in the past and more in the future). The circular curling of the surface is an artifact of the embedding with no physical significance and is done for illustrative purposes; a flat universe does not curl back onto itself. (A similar effect can be seen in the tubular shape of the pseudosphere .) The brown line on the diagram is the worldline of Earth (or more precisely its location in space, even before it
16350-416: The present universe conforms to Euclidean space , what cosmologists describe as geometrically flat , to within experimental error. Consequently, the rules of Euclidean geometry associated with Euclid's fifth postulate hold in the present universe in 3D space. It is, however, possible that the geometry of past 3D space could have been highly curved. The curvature of space is often modeled using
16500-719: The process is expected to lower their Chandrasekhar limit resulting in a supernova in 10 years. Non-degenerate silicon has been calculated to tunnel to iron in approximately 10 years. Quantum tunneling should also turn large objects into black holes , which (on these timescales) will instantaneously evaporate into subatomic particles. Depending on the assumptions made, the time this takes to happen can be calculated as from 10 years to 10 years. Quantum tunneling may also make iron stars collapse into neutron stars in around 10 years. With black holes having evaporated, nearly all baryonic matter will have now decayed into subatomic particles (electrons, neutrons, protons, and quarks). The universe
16650-532: The process. This means that after 10 years (the maximum proton half-life used by Adams & Laughlin (1997)), one-half of all baryonic matter will have been converted into gamma ray photons and leptons through proton decay. Given our assumed half-life of the proton, nucleons (protons and bound neutrons) will have undergone roughly 1,000 half-lives by the time the universe is 10 years old. This means that there will be roughly 0.5 (approximately 10) as many nucleons; as there are an estimated 10 protons currently in
16800-418: The quasar, while if the expansion had stopped at the earlier time, it would have taken only 4 billion years. The light took much longer than 4 billion years to reach us though it was emitted from only 4 billion light-years away. In fact, the light emitted towards Earth was actually moving away from Earth when it was first emitted; the metric distance to Earth increased with cosmological time for
16950-401: The range in age of young stars associated with them, of 10 to 20 million years, matching molecular clouds’ internal timescales. Direct observation of T Tauri stars inside dark clouds and OB stars in star-forming regions match this predicted age span. The fact OB stars older than 10 million years don’t have a significant amount of cloud material about them, seems to suggest most of the cloud
17100-416: The rapid expansion would have diluted such relics. It was subsequently realized that the accelerated expansion would also solve the horizon problem and the flatness problem . Additionally, quantum fluctuations during inflation would have created initial variations in the density of the universe, which gravity later amplified to yield the observed spectrum of matter density variations . During inflation,
17250-409: The rate at which stars are forming in our galaxy, astronomers are able to suggest the amount of interstellar gas being collected into star-forming molecular clouds in our galaxy. The rate of mass being assembled into stars is approximately 3 M ☉ per year. Only 2% of the mass of a molecular cloud is assembled into stars, giving the number of 150 M ☉ of gas being assembled in molecular clouds in
17400-824: The relationship between molecular clouds and star formation. Embedded in the Taurus molecular cloud there are T Tauri stars . These are a class of variable stars in an early stage of stellar development and still gathering gas and dust from the cloud around them. Observation of star forming regions have helped astronomers develop theories about stellar evolution . Many O and B type stars have been observed in or very near molecular clouds. Since these star types belong to population I (some are less than 1 million years old), they cannot have moved far from their birth place. Many of these young stars are found embedded in cloud clusters, suggesting stars are formed inside it. A vast assemblage of molecular gas that has more than 10 thousand times
17550-425: The remaining interstellar medium until they have enough mass to start hydrogen burning as red dwarfs. This process, at least on white dwarfs, could induce Type Ia supernovae. Over time, the orbits of planets will decay due to gravitational radiation , or planets will be ejected from their local systems by gravitational perturbations caused by encounters with another stellar remnant . Over time, objects in
17700-465: The repairs were made, Wendy Freedman 's 1994 Key Project analyzed the recession velocity of M100 from the core of the Virgo Cluster , offering a Hubble constant measurement of 80 ± 17 km⋅s ⋅Mpc . Later the same year, Adam Riess et al. used an empirical method of visual-band light-curve shapes to more finely estimate the luminosity of Type Ia supernovae . This further minimized
17850-567: The same place like going all the way around the surface of a balloon (or a planet like the Earth), is an observational question that is constrained as measurable or non-measurable by the universe's global geometry . At present, observations are consistent with the universe having infinite extent and being a simply connected space , though cosmological horizons limit our ability to distinguish between simple and more complicated proposals. The universe could be infinite in extent or it could be finite; but
18000-527: The same velocity as its own. More generally, the peculiar momenta of both relativistic and nonrelativistic particles decay in inverse proportion with the scale factor. For photons, this leads to the cosmological redshift . While the cosmological redshift is often explained as the stretching of photon wavelengths due to "expansion of space", it is more naturally viewed as a consequence of the Doppler effect . The universe cools as it expands. This follows from
18150-460: The same year George Carruthers managed to identify molecular hydrogen . The numerous detections of molecules in interstellar space would help pave the way to the discovery of molecular clouds in 1970. Hydrogen is the most abundant species of atom in molecular clouds, and under the right conditions it will form the H 2 molecule. Despite its abundance, the detection of H 2 proved difficult. Due to its symmetrical molecule, H 2 molecules have
18300-424: The scale factor grows exponentially in time. The most direct way to measure the expansion rate is to independently measure the recession velocities and the distances of distant objects, such as galaxies. The ratio between these quantities gives the Hubble rate, in accordance with Hubble's law. Typically, the distance is measured using a standard candle , which is an object or event for which the intrinsic brightness
18450-399: The scale of the spatial part of the universe's spacetime metric tensor (which governs the size and geometry of spacetime). Within this framework, the separation of objects over time is associated with the expansion of space itself. However, this is not a generally covariant description but rather only a choice of coordinates . Contrary to common misconception, it is equally valid to adopt
18600-461: The second most common compound. Molecular clouds also usually contain other elements and compounds. Astronomers have observed the presence of long chain compounds such as methanol , ethanol and benzene rings and their several hydrides . Large molecules known as polycyclic aromatic hydrocarbons have also been detected. The density across a molecular cloud is fragmented and its regions can be generally categorized in clumps and cores. Clumps form
18750-620: The spatial coordinates in the FLRW metric . The universe is a four-dimensional spacetime, but within a universe that obeys the cosmological principle, there is a natural choice of three-dimensional spatial surface. These are the surfaces on which observers who are stationary in comoving coordinates agree on the age of the universe . In a universe governed by special relativity , such surfaces would be hyperboloids , because relativistic time dilation means that rapidly receding distant observers' clocks are slowed, so that spatial surfaces must bend "into
18900-536: The star's matter may be returned to the interstellar medium , a degenerate remnant will be left behind whose mass is not returned to the interstellar medium. Therefore, the supply of gas available for star formation is steadily being exhausted. The Andromeda Galaxy is approximately 2.5 million light years away from our galaxy, the Milky Way galaxy, and they are moving towards each other at approximately 300 kilometers (186 miles) per second. Approximately five billion years from now, or 19 billion years after
19050-558: The stars will have burnt out, leaving all stellar-mass objects as stellar remnants — white dwarfs , neutron stars , and black holes . In the Black Hole Era , white dwarfs, neutron stars, and other smaller astronomical objects have been destroyed by proton decay , leaving only black holes. Finally, in the Dark Era , even black holes have disappeared, leaving only a dilute gas of photons and leptons . This future history and
19200-474: The surrounding material. It is a measure of how a particle's motion deviates from the Hubble flow of the expanding universe. The peculiar velocities of nonrelativistic particles decay as the universe expands, in inverse proportion with the cosmic scale factor . This can be understood as a self-sorting effect. A particle that is moving in some direction gradually overtakes the Hubble flow of cosmic expansion in that direction, asymptotically approaching material with
19350-426: The systematic measurement errors of the Hubble constant, to 67 ± 7 km⋅s ⋅Mpc . Reiss's measurements on the recession velocity of the nearby Virgo Cluster more closely agree with subsequent and independent analyses of Cepheid variable calibrations of Type Ia supernova , which estimates a Hubble constant of 73 ± 7 km⋅s ⋅Mpc . In 2003, David Spergel 's analysis of the cosmic microwave background during
19500-494: The temperature reaches a point where the fusion of hydrogen can occur. The burning of hydrogen then generates enough heat to push against gravity, creating hydrostatic equilibrium . At this stage, a protostar is formed and it will continue to aggregate gas and dust from the cloud around it. One of the most studied star formation regions is the Taurus molecular cloud due to its close proximity to earth (140 pc or 430 ly away), making it an excellent object to collect data about
19650-444: The theories described above, then the Degenerate Era will last longer, and will overlap or surpass the Black Hole Era. On a time scale of 10 years solid matter is theorized to potentially rearrange its atoms and molecules via quantum tunneling , and may behave as liquid and become smooth spheres due to diffusion and gravity. Degenerate stellar objects can potentially still experience proton decay, for example via processes involving
19800-413: The time of neutrino decoupling at about 1 second. During radiation domination, cosmic expansion decelerated, with the scale factor growing proportionally with the square root of the time. Since radiation redshifts as the universe expands, eventually nonrelativistic matter came to dominate the energy density of the universe. This transition happened at a time of about 50 thousand years after
19950-413: The time through which various events take place. The expansion of space is in reference to this 3D manifold only; that is, the description involves no structures such as extra dimensions or an exterior universe. The ultimate topology of space is a posteriori – something that in principle must be observed – as there are no constraints that can simply be reasoned out (in other words there cannot be any
20100-569: The timeline below assume the continued expansion of the universe. If space in the universe begins to contract, subsequent events in the timeline may not occur because the Big Crunch , the collapse of the universe into a hot, dense state similar to that after the Big Bang, will prevail. The observable universe is currently 1.38 × 10 (13.8 billion) years old. This time lies within the Stelliferous Era. About 155 million years after
20250-422: The trajectories of the objects involved in the close encounter change slightly, in such a way that their kinetic energies are more nearly equal than before. After a large number of encounters, then, lighter objects tend to gain speed while the heavier objects lose it. Because of dynamical relaxation, some objects will gain just enough energy to reach galactic escape velocity and depart the galaxy, leaving behind
20400-419: The ultraviolet radiation. The dissociation caused by UV photons is the main mechanism for transforming molecular material back to the atomic state inside the cloud. Molecular content in a region of a molecular cloud can change rapidly due to variation in the radiation field and dust movement and disturbance. Most of the gas constituting a molecular cloud is molecular hydrogen , with carbon monoxide being
20550-414: The universe accord with Hubble's law , in which objects recede from each observer with velocities proportional to their positions with respect to that observer. That is, recession velocities v → {\displaystyle {\vec {v}}} scale with (observer-centered) positions x → {\displaystyle {\vec {x}}} according to where
20700-459: The universe are predicted to continue to grow. Larger black holes of up to 10 (100 trillion) M ☉ may form during the collapse of superclusters of galaxies. Even these would evaporate over a timescale of 10 to 10 years. Hawking radiation has a thermal spectrum . During most of a black hole's lifetime, the radiation has a low temperature and is mainly in the form of massless particles such as photons and hypothetical gravitons . As
20850-743: The universe depends on the possibility and rate of proton decay . Experimental evidence shows that if the proton is unstable, it has a half-life of at least 10 years. Some of the Grand Unified theories (GUTs) predict long-term proton instability between 10 and 10 years, with the upper bound on standard (non-supersymmetry) proton decay at 1.4 × 10 years and an overall upper limit maximum for any proton decay (including supersymmetry models) at 6 × 10 years. Recent research showing proton lifetime (if unstable) at or exceeding 10–10 year range rules out simpler GUTs and most non-supersymmetry models. Neutrons bound into nuclei are also suspected to decay with
21000-415: The universe gradually slows this expansion over time, but expansion nevertheless continues due to momentum left over from the initial impulse. Also, certain exotic relativistic fluids , such as dark energy and inflation, exert gravitational repulsion in the cosmological context, which accelerates the expansion of the universe. A cosmological constant also has this effect. Mathematically, the expansion of
21150-500: The universe growing as a bubble into nothingness are misleading in that respect. There is no reason to believe there is anything "outside" the expanding universe into which the universe expands. Even if the overall spatial extent is infinite and thus the universe cannot get any "larger", we still say that space is expanding because, locally, the characteristic distance between objects is increasing. As an infinite space grows, it remains infinite. Molecular cloud Molecular hydrogen
21300-462: The universe is infinite in spatial extent, without edge or strange connectedness. Regardless of the overall shape of the universe, the question of what the universe is expanding into is one that does not require an answer, according to the theories that describe the expansion; the way we define space in our universe in no way requires additional exterior space into which it can expand, since an expansion of an infinite expanse can happen without changing
21450-402: The universe is quantified by the scale factor , a {\displaystyle a} , which is proportional to the average separation between objects, such as galaxies. The scale factor is a function of time and is conventionally set to be a = 1 {\displaystyle a=1} at the present time. Because the universe is expanding, a {\displaystyle a}
21600-480: The universe to stop expanding and begin to contract, which corresponds to the scale factor decreasing in time. The scale factor a {\displaystyle a} is a parameter of the FLRW metric , and its time evolution is governed by the Friedmann equations . The second Friedmann equation, shows how the contents of the universe influence its expansion rate. Here, G {\displaystyle G}
21750-871: The universe will eventually tail off dramatically (compared with previous eras), with very low energy levels and very large time scales, with events taking a very long time to happen if they ever happen at all. Electrons and positrons drifting through space will encounter one another and occasionally form positronium atoms. These structures are unstable, however, and their constituent particles must eventually annihilate. However, most electrons and positrons will remain unbound. Other low-level annihilation events will also take place, albeit extremely slowly. The universe now reaches an extremely low-energy state. If protons do not decay, stellar-mass objects will still become black holes , although even more slowly. The following timeline that assumes proton decay does not take place. 2018 estimate of Standard Model lifetime before collapse of
21900-436: The universe will slowly and inexorably grow darker. According to theories that predict proton decay , the stellar remnants left behind will disappear, leaving behind only black holes , which themselves eventually disappear as they emit Hawking radiation . Ultimately, if the universe reaches thermodynamic equilibrium , a state in which the temperature approaches a uniform value, no further work will be possible, resulting in
22050-497: The universe, none will remain at the end of the Degenerate Age. Effectively, all baryonic matter will have been changed into photons and leptons . Some models predict the formation of stable positronium atoms with diameters greater than the observable universe's current diameter (roughly 6 × 10 metres) in 10 years, and that these will in turn decay to gamma radiation in 10 years. If the proton does not decay according to
22200-431: The universe. They will slowly evaporate via Hawking radiation .A black hole with a mass of around 1 M ☉ will vanish in around 2 × 10 years. As the lifetime of a black hole is proportional to the cube of its mass, more massive black holes take longer to decay. A supermassive black hole with a mass of 10 (100 billion) M ☉ will evaporate in around 2 × 10 years. The largest black holes in
22350-412: The wide end is a cosmological time of 18 billion years, where one can see the beginning of the accelerating expansion as a splaying outward of the spacetime, a feature that eventually dominates in this model. The purple grid lines mark cosmological time at intervals of one billion years from the Big Bang. The cyan grid lines mark comoving distance at intervals of one billion light-years in
22500-443: Was formed). The yellow line is the worldline of the most distant known quasar . The red line is the path of a light beam emitted by the quasar about 13 billion years ago and reaching Earth at the present day. The orange line shows the present-day distance between the quasar and Earth, about 28 billion light-years, which is a larger distance than the age of the universe multiplied by the speed of light, ct . According to
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