The stability of the Solar System is a subject of much inquiry in astronomy. Though the planets have historically been stable as observed, and will be in the "short" term, their weak gravitational effects on one another can add up in ways that are not predictable by any simple means.
For this reason (among others), the Solar System is chaotic in the technical sense defined by mathematical chaos theory, and that chaotic behavior degrades even the most precise long-term
numerical or analytic models for the orbital motion in the Solar System,
so they cannot be valid beyond more than a few tens of millions of
years into the past or future – about 1% its present age.
The Solar System is stable on the time-scale of the existence of humans,
and far beyond, given that it is unlikely any of the planets will
collide with each other or be ejected from the system in the next few
billion years, and that Earth's orbit will be relatively stable.
Since Newton's law of gravitation (1687), mathematicians and astronomers (such as Laplace, Lagrange, Gauss, Poincaré, Kolmogorov, V. Arnold, and J. Moser)
have searched for evidence for the stability of the planetary motions,
and this quest has led to many mathematical developments and several
successive "proofs" of stability of the Solar System.
The orbits of the planets are open to long-term variations. Modeling the Solar System is a case of the n-body problem
of physics, which is generally unsolvable except by numerical
simulation. Because of the chaotic behavior embedded in the mathematics,
long-term predictions can only be statistical, rather than certain.
Resonance
Graph showing the numbers of Kuiper belt objects for a given distance (in AU; i.e., the distance from the Sun to Earth) from the Sun
An orbital resonance
happens when the periods of any two objects have a simple numerical
ratio. The most fundamental period for an object in the Solar System is
its orbital period, and orbital resonances pervade the Solar System. In 1867, the American astronomer Daniel Kirkwood noticed that asteroids in the main belt are not randomly distributed. There were distinct gaps in the belt at locations that corresponded to resonances with Jupiter.
For example, there were no asteroids at the 3:1 resonance — a distance
of 2.5 AU (370 million km; 230 million mi) — or at the 2:1 resonance, at
3.3 AU (490 million km; 310 million mi). These are now known as the Kirkwood gaps.
Some asteroids were later discovered to orbit in these gaps, but when
closely analyzed their orbits were determined to be unstable and they
will eventually break out of the resonance due to close encounters with a
major planet.
Another common form of resonance in the Solar System is spin–orbit resonance, where the rotation period
(the time it takes the planet or moon to rotate once about its axis)
has a simple numerical relationship with its orbital period. An example
is the Moon, which is in a 1:1 spin–orbit resonance that keeps its far side away from Earth. (This feature is also known as tidal locking.) Another example is Mercury, which is in a 3:2 spin–orbit resonance with the Sun.
Predictability
The planets' orbits are chaotic over longer time scales, in such a way that the whole Solar System possesses a Lyapunov time in the range of 2~230 million years. In all cases, this means that the positions of individual planets along
their orbits ultimately become impossible to predict with any certainty.
In some cases, the orbits themselves may change dramatically. Such
chaos manifests most strongly as changes in eccentricity, with some planets' orbits becoming significantly more – or less – elliptical.
The Neptune–Pluto system lies in a 3:2 orbital resonance. C.J. Cohen and E.C. Hubbard at the Naval Surface Warfare Center Dahlgren Division
discovered this in 1965. Although the resonance itself will remain
stable in the short term, it becomes impossible to predict the position
of Pluto with any degree of accuracy, as the uncertainty in the position
grows by a factor e with each Lyapunov time, which for Pluto is 10–20 million years. Thus, on a time scale of hundreds of millions of years Pluto's orbital
phase becomes impossible to determine, even if Pluto's orbit appears to
be perfectly stable on 10 myr time scales.
The planet Mercury is especially susceptible to Jupiter's influence because of a small celestial coincidence: Mercury's perihelion,
the point where it gets closest to the Sun, precesses at a rate of
about 1.5 degrees every 1,000 years, and Jupiter's perihelion precesses
only a little slower. At one point, the two may fall into sync, at which
time Jupiter's constant gravitational tugs could accumulate and pull
Mercury off course, with 1–2% probability, 3–4 billion years into the
future. This could eject it from the Solar System altogether or send it on a collision course with Venus, the Sun, or Earth.
Mercury's perihelion-precession rate is dominated by
planet–planet interactions, but about 7.5% of Mercury's perihelion
precession rate comes from the effects described by general relativity. The work by Laskar and Gastineau (described below) showed the importance of general relativity
(G.R.) in long-term Solar System stability. Specifically, without G.R.
the instability rate of Mercury would be 60 times higher than with G.R. By modelling the instability time of Mercury as a one-dimensional Fokker–Planckdiffusion process,
the relationship between the instability time of Mercury and the
Mercury–Jupiter 1:1 perihelion-precession resonance can be investigated
statistically. This diffusion model shows that G.R. not only distances Mercury and
Jupiter from falling into a 1:1 resonance, but also decreases the rate
at which Mercury diffuses through phase space. Thus, not only does G.R. decrease the likelihood of Mercury's
instability, but also extends the time at which it is likely to occur.
Galilean moon resonance
Jupiter's Galilean moons experience strong tidal dissipation and mutual interactions due to their size and proximity to Jupiter. Currently, Io, Europa, and Ganymede are in a 4:2:1 Laplace resonance
with each other, with each inner moon completing two orbits for every
orbit of the next moon out. In around 1.5 billion years, outward
migration of these moons will trap the fourth and outermost moon, Callisto,
into another 2:1 resonance with Ganymede. This 8:4:2:1 resonance will
cause Callisto to migrate outward, and it may remain stable with
approximately 56% probability, or become disrupted with Io usually
exiting the chain.
Chaos from geological processes
Another example is Earth's axial tilt, which, due to friction raised within Earth's mantle by tidal interactions with the Moon, will be rendered chaotic between 1.5 and 4.5 billion years from now.
Objects
coming from outside the Solar System can also affect it. Though they
are not technically part of the Solar System for the purposes of
studying the system's intrinsic stability, they nevertheless can change
it. Unfortunately, predicting the potential influences of these extrasolar objects
is even more difficult than predicting the influences of objects within
the system simply because of the sheer distances involved. Among the
known objects with a potential to significantly affect the Solar System
is the star Gliese 710, which is expected to pass near the system in approximately 1.281 million years. Though the star is not expected to substantially affect the orbits of the major planets, it could substantially disrupt the Oort cloud,
potentially causing major comet activity throughout the Solar System.
There are at least a dozen other stars that have a potential to make a
close approach in the next few million years. In 2022, Garett Brown and Hanno Rein of the University of Toronto
published a study exploring the long-term stability of the Solar System
in the presence of weak perturbations from stellar flybys. They
determined that if a passing star altered the semi-major axis of Neptune by at least 0.03 AU
(4.49 million km; 2.79 million miles) it would increase the chance of
instability by 10 times over the subsequent 5 billion years. They also estimated that a flyby of this magnitude is not likely to occur for 100 billion years.
Recent studies
LonGStOP, 1982
Project
LonGStOP (LOng-term Gravitational Study of the Outer Planets) was a
1982 international consortium of Solar System dynamicists led by A.E. Roy.
It involved creation of a model on a supercomputer, integrating the
orbits of (only) the outer planets. Its results revealed several curious
exchanges of energy between the outer planets, but no signs of gross
instability.
Digital Orrery, 1988
Another project involved constructing the Digital Orrery by G. Sussman
and his MIT group in 1988. The group used a special-purpose computer
whose multiprocessor architecture was optimized for integrating the
orbits of the outer planets. It was used to integrate out to 845 million
years – some 20% of the age of the Solar System. In 1988, Sussman and Wisdom found data using the Orrery that revealed that Pluto's orbit shows signs of chaos, due in part to its peculiar resonance with Neptune.
If Pluto's orbit is chaotic, then technically the whole Solar
System is chaotic. This might be more than a technicality, since even a
Solar System body as small as Pluto might affect the others to a
perceptible extent through cumulative gravitational perturbations.
Laskar, 1989
In 1989, Jacques Laskar of the Bureau des Longitudes
in Paris published the results of his numerical integration of the
Solar System over 200 million years. These were not the full equations
of motion, but rather averaged equations along the lines of those used
by Laplace. Laskar's work showed that the Earth's orbit is chaotic (as are the orbits of all the inner planets)
and that an error as small as 15 metres in measuring the position of
the Earth today would make it impossible to predict where the Earth
would be in its orbit in just over 100 million years' time.
Laskar and Gastineau, 2009
Jacques Laskar
and his colleague Mickaël Gastineau in 2008 took a more thorough
approach by directly simulating 2,501 possible futures. Each of the
2,501 cases has slightly different initial conditions: Mercury's
position varies by about 1 metre (3.3 feet) between one simulation and the next. In 20 cases, Mercury goes into a dangerous orbit and often ends up
colliding with Venus or plunging into the Sun. Moving in such a warped
orbit, Mercury's gravity is more likely to shake other planets out of
their settled paths: In one simulated case, Mercury's perturbations sent
Mars heading toward Earth.
Batygin and Laughlin, 2008
Independently of Laskar and Gastineau, Batygin and Laughlin were also directly simulating the Solar System 20 billion years into the future. Their results reached the same basic conclusions as did Laskar and Gastineau, while additionally providing a lower bound of a billion years on the dynamical lifespan of the Solar System.
Brown and Rein, 2020
In 2020, Garett Brown and Hanno Rein of the University of Toronto published the results of their numerical integration of the Solar System over 5 billion years. Their work showed that Mercury's orbit is highly chaotic and that an error as small as 0.38 millimeters (0.015 inches)
in measuring the position of Mercury today would make it impossible to
predict the eccentricity of its orbit in just over 200 million years'
time.
Footnotes
The dynamical modelling of the Solar System beyond approximately 4 billion years into the future is greatly complicated by the transition of the Sun into its old-age giant phase:
The Sun will lose mass at an uncertain rate, heat up, and greatly
expand, all of which will change the dynamics of planetary orbits.
Solar mass-loss will slow all planetary orbits, uniformly slowing the
time scale of change in the Solar System. The mass-loss will also reduce
Solar perturbations on planets and in relative terms increase
perturbations by planets on the Sun and on each other. The gas ejected
by the aged Sun may slightly perturb planetary orbits, either by drag
(unlikely) or adding to planetary masses (only slightly more likely).
Heating up and expansion of the Sun will severely affect some of the inner planets:
It will at least ablate their atmospheres and possibly some of their
surfaces (reducing their mass and hence diminishing their perturbations
on other planets and the Sun). The only planet certain to be drastically
affected is Mercury,
which will be enclosed inside the Sun, and presumably slowly dissolved
(hence smearing out and removing its perturbations entirely), if it has
not previously been ejected from its close Solar orbit.
The effect of orbital eccentricity
oscillation on the shape of the orbit is analogous to the shape change
of the rim of a ringing bell, neglecting the side-to-side displacement
of the orbit's geometric center. The analogy fails to represent the
entire orbital change, because while the gravitational center of the
orbit remains nearly fixed on the Sun, its geometric center swings from
side to side at the same rate as the eccentricity oscillation; a ringing
bell's geometric center remains fixed, or can only swing several orders of magnitude more slowly than its edge vibrates.
Research published in 2015 estimates the earliest stages of the universe's existence as taking place 13.8 billion years ago, with an uncertainty of around 21 million years at the 68% confidence level.
The current accepted model of the history of the universe is based on the concept of the Big Bang:
the universe started hot and dense then expanded and cooled.
Different particles interact during each major stage in the expansion;
as the universe expands the density falls and some particle interactions
cease to be important. The character of the universe changes. Moreover,
the rate of the expansion itself depends upon the nature of the
existing particles, creating an interplay between cosmology and particle physics.
Time
The lookback time of extragalactic observations by their cosmological redshift up to z=20.
In cosmology, time and space are connected: space expands as time
increases. Time at each point in space (for example a galaxy) can be
uniquely defined in terms of an imaginary clock at that point. These
clocks move with the point in space as the universe expands; they are
synchronized to a single point in the distance past. Light from distant
galaxies is emitted in the past then travels at the speed of light:
knowledge about a distant galaxy is limited to one point in time called
the lookback time.
During the journey from a distant point, the universe continues to
expand, stretching the wavelength of the light along the way, an effect
called cosmological redshift. The redshift can be measured by comparing incoming light to known
spectroscopic lines and the resulting value can be related to the comoving distance to the emitter. Consequently, experimental knowledge about the chronology of the universe is derived by observing distant light.
Overview
The NASA diagram shows the history of the universe from inflation until the present.
The chronology of the universe can be divided into five parts:
Inflation, the first era supported by experimental
evidence, a period of exponential expansion that ends with the
conversion of energy into particles,
Quark soup, the initial particles cool and coalesce, dark matter forms,
Big bang nucleosynthesis, combining nucleons create the cores of the first atoms,
Gravity builds cosmic structure, reduced density allows matter to dominate over radiation for control of expansion, photons decouple to form the cosmic background radiation, and gravitational attraction builds stars, galaxies, and clusters of galaxies.
Cosmic acceleration, continued expansion allows dark energy to overcome gravitational force, inhibiting larger structures.
With these large subsections are many interesting events and
transitions. Older models divided the chronology differently, using
different terminology or emphasis.
Tabular summary
Modern cosmological chronologies begin with inflation,
the earliest time period supported by solid observational evidence.
Anything earlier is considered non-standard cosmology, the subject of a
great deal of as-yet-unconfirmed research.
Matter and antimatter are created with one extra particle of matter for every 1010 pairs. The pairs annihilate producing photons and leaving the matter particles. Many mechanisms have been proposed but no observations select one.
As the temperature falls, photons no longer have sufficient energy
to produce electron/positron pairs. Electrons and positrons annihilate,
leaving photons.
Protons and neutrons are bound into primordial atomic nuclei: hydrogen and helium-4. Trace amounts of deuterium, helium-3, and lithium-7
also form. At the end of this epoch, the spherical volume of space
which will become the observable universe is about 300 light-years in
radius, baryonic matter density is on the order of 4 grams per m3 (about 0.3% of sea level air density)—however, most energy at this time is in electromagnetic radiation.
Electrons and atomic nuclei first become bound to form neutral atoms.
Photons are no longer in thermal equilibrium with matter and the
universe first becomes transparent. Recombination lasts for about 100
ka, during which the universe is becoming more and more transparent to
photons. The photons of the cosmic microwave background
radiation originate at this time. The spherical volume of space that
will become the observable universe is 42 million light-years in radius
at this time. The baryonic matter density at this time is about 500
million hydrogen and helium atoms per cubic metre, approximately a billion times higher than today. This density corresponds to pressure on the order of 10−17 atm.
The time between recombination and the formation of the first stars. During this time, the only source of photons was hydrogen emitting radio waves at hydrogen line. Freely propagating CMB photons quickly (within about 3 million years) red-shifted to infrared, and the universe was devoid of visible light.
The most distant astronomical objects observable with telescopes date to this period; as of June 2025, the most remote galaxy observed is MoM-z14, at a redshift of 14.44. The earliest "modern" Population I stars are formed in this period.
Farthest observable photons at this moment are CMB photons. They
arrive from a sphere with a radius of 46 billion light-years. The
spherical volume inside it is commonly referred to as the observable
universe.
Alternative subdivisions of the chronology (overlapping several of the above periods)
The time between the first formation of Population III stars until the cessation of star formation, leaving all stars in the form of degenerate remnants.
The stelliferous era will end as stars eventually die and fewer are
born to replace them, leading to a darkening universe. Various theories
suggest a number of subsequent possibilities. Assuming proton decay, the matter may eventually evaporate into a Dark Era (heat death). Alternatively, the universe may collapse in a Big Crunch. Other suggested ends include a false vacuum catastrophe or a Big Rip as possible ends to the universe.
Hot Big Bang
The
physical model for the chronology of the universe with strong
observational and theoretical support is called the hot Big Bang model. The concept includes an early state of extreme temperature and density followed by expansion of the universe continuing to this day. A high-precision version of the Big Bang model using conventional physics, known as Lambda-CDM, agrees with a wide array of astrophysical observations. The concept is not extrapolated back to zero time. Within the standard model of cosmology the initial state is set by a
process called inflation. The relative timeline for the earliest
phenomena is unclear. Speculation on processes occurring before
inflation involves physics considered outside of standard cosmology.
At this point of the very early universe, the universe is thought to have expanded by at least a factor of 1026 in time on the order of 10−36 seconds. All of the mass-energy in all of the galaxies currently visible started in a sphere with a radius around 4×10−29 m, then grew to a sphere with a radius around 0.09m by the end of inflation. This phase of the cosmic expansion history is known as inflation or sometimes as the inflationary epoch.
Inflation explains how today's universe has concentrations of
matter, like galaxies and clusters of galaxies, rather than having matter spatially uniform through the universe.[4]: 324
Tiny quantum fluctuations in the universe, amplified by inflation, are
believed to be the basis of large-scale structures that formed much
later.
The rapid expansion meant that any potential particles (or other
"unwanted" artifacts, such as topological defects) remaining from the
time before inflation were now distributed very thinly across the
universe.
It is not known exactly when the inflationary epoch ended, but it is thought to have been between 10−33 and 10−32 seconds after the Big Bang. The rapid expansion of space meant that the temperature was relatively cool and any elementary particles
remaining from the grand unification epoch were now distributed very
thinly across the universe. However, the large potential energy of the
inflaton field was released at the end of the inflationary epoch, as the
inflaton field decayed into other particles, known as "reheating". This
heating effect led to the universe being repopulated with a dense, hot
mixture of quarks, anti-quarks, and gluons.
After inflation ended, the universe continued to expand. A region
the size of a melon at that time has since grown to be our entire observable universe.
The mechanism that drove inflation remains unknown, although many
models have been put forward. In several of the more prominent models,
it is thought to have been triggered by the separation
of the strong and electroweak interactions which ended the grand
unification epoch. One of the theoretical products of this phase
transition was a scalar field called the inflaton field. As this field settled into its lowest-energy state throughout the
universe, it generated an enormous repulsive force that led to a rapid
expansion of the universe.
Baryons are subatomic particles such as protons and neutrons, that are composed of three quarks. It would be expected that both baryons, and particles known as antibaryons
would have formed in equal numbers. However, this does not seem to be
what happened—as far as we know, the universe was left with far more
baryons than antibaryons. In fact, almost no antibaryons are observed in
nature. It is not clear how this came about. Any explanation for this
phenomenon must allow the Sakharov conditions related to baryogenesis to have been satisfied at some time after the end of cosmological inflation.
Current particle physics suggests asymmetries under which these
conditions would be met, but these asymmetries appear to be too small to
account for the observed baryon-antibaryon asymmetry of the universe.
Theory predicts that about 1 neutron remained for every 6
protons, with the ratio falling to 1:7 over time due to neutron decay.
This is believed to be correct because, at a later stage, the neutrons
and some of the protons fused, leaving hydrogen, a hydrogen isotope
called deuterium, helium and other elements, which can be measured. A
1:7 ratio of hadrons would indeed produce the observed element ratios in
the early and current universe.
As the universe's temperature continued to fall below 159.5±1.5 GeV/kB, electroweak symmetry breaking happened. So far as we know, it was the penultimate symmetry breaking event in the formation of the universe, the final one being chiral symmetry breaking in the quark sector. This has two related effects:
Via the Higgs mechanism, all elementary particles interacting with the Higgs field became massive, having been massless at higher energy levels.
As a side-effect, the weak nuclear force and electromagnetic force, and their respective bosons (the W and Z bosons
and photon) began to manifest differently in the present universe.
Before electroweak symmetry breaking, these bosons were all massless
particles and interacted over long distances, but at this point the W
and Z bosons abruptly became massive particles only interacting over
distances smaller than the size of an atom, while the photon remained
massless and remained a long-distance interaction.
After electroweak symmetry breaking, the fundamental interactions we
know of—gravitation, electromagnetic, weak and strong interactions—all
took their present forms, and fundamental particles had their expected
masses, but the temperature of the universe was still too high to allow
the stable formation of many of the particles we now see in the
universe, so there were no protons or neutrons, and therefore no atoms, atomic nuclei,
or molecules. (More precisely, any composite particles that formed by
chance almost immediately broke up again due to the extreme energies.)
Quantum chromodynamics phase transition
Between 10−12 seconds and 10−5 seconds after the Big Bang
After cosmic inflation ended, the universe was filled with a hot quark–gluon plasma,
the remains of reheating. From this point onwards the physics of the
early universe is much better understood, and the energies involved in
the quark epoch are directly accessible in particle physics experiments and other detectors.
The quark epoch began approximately 10−12
seconds after the Big Bang. This was the period in the evolution of the
early universe immediately after electroweak symmetry breaking when the
fundamental interactions of gravitation, electromagnetism, the strong
interaction and the weak interaction had taken their present forms, but
the temperature of the universe was still too high to allow quarks to bind together to form hadrons. The quark epoch ended when the universe was about 10−5 seconds old; two non-equilibrium events must have occurred next, formation of baryons and of dark matter.
Neutrino decoupling and cosmic neutrino background (CνB)
At approximately 1 second after the Big Bang neutrinos decouple and
begin travelling freely through space. As neutrinos rarely interact with
matter, these neutrinos still exist today, analogous to the much later
cosmic microwave background emitted during recombination, around 370,000
years after the Big Bang. The neutrinos from this event have a very low
energy, around 10−10 times the amount of those observable with present-day direct detection. Even high-energy neutrinos are notoriously difficult to detect, so this cosmic neutrino background (CνB) may not be directly observed in detail for many years, if at all.
However, Big Bang cosmology makes many predictions about the CνB,
and there is very strong indirect evidence that the CνB exists, both
from Big Bang nucleosynthesis
predictions of the helium abundance, and from anisotropies in the
cosmic microwave background (CMB). One of these predictions is that
neutrinos will have left a subtle imprint on the CMB. It is well known
that the CMB has irregularities. Some of the CMB fluctuations were
roughly regularly spaced, because of the effect of baryonic acoustic oscillations. In theory, the decoupled neutrinos should have had a very slight effect on the phase of the various CMB fluctuations.
In 2015, it was reported that such shifts had been detected in
the CMB. Moreover, the fluctuations corresponded to neutrinos of almost
exactly the temperature predicted by Big Bang theory (1.96±0.02 K compared to a prediction of 1.95 K), and exactly three types of neutrino, the same number of neutrino flavors predicted by the Standard Model.
Cosmological models of this early time remain unsettled.
The Standard Model of particle physics is only tested up to temperatures of order 1017K (10 TeV) in particle colliders, such as the Large Hadron Collider. Moreover, new physical phenomena not yet covered by the Standard Model could have been important before the time of neutrino decoupling, when the temperature of the universe was about 1010K (1 MeV).
Electron-positron annihilation
Between 1 second and 10 seconds after the Big Bang
The majority of hadrons and anti-hadrons annihilate each other leaving leptons (such as the electron, muons
and certain neutrinos) and antileptons, dominating the mass of the
universe.
Initially leptons and antileptons are produced in pairs. About 10
seconds after the Big Bang the temperature of the universe falls to the
point at which new lepton–antilepton pairs are no longer created and
most remaining leptons and antileptons quickly annihilated each other,
giving rise to pairs of high-energy photons, and leaving a small residue
of non-annihilated leptons. After most leptons and antileptons are annihilated, most of the mass–energy in the universe is left in the form of photons.
Nucleosynthesis of light elements
Between 2 minutes and 20 minutes after the Big Bang
Between about 2 and 20 minutes after the Big Bang nuclear fusion
reactions convert a 1:7 mixture of neutrons and protons in to a mix of
protons, deuterium (a proton fused with a neutron), 3He, 4He, with trace amounts of 7Li and 7Be.
These reactions end when the temperature falls below the 0.07MeV needed
for nuclear fusion. The final mixture depends upon the reaction rates,
the temperature, and the density of the components. The reaction rates
can be measured in nuclear physics laboratories while the temperature and densities can be calculated from models of the expansion of the universe.
About 25% of the protons, and all the neutrons fuse to form deuterium, a hydrogen isotope, and almost all of the deuterium quickly fuses to form helium-4. Helium-4 has much higher binding energy than nuclei with 5 to 8 nucleons so only trace amounts of those nuclei are created.
Heavier nuclei produced in stars do not appear because they require the combination of three Helium-4 nuclei
and the density of Helium-4 is too low for many three way collisions to
occur before the expansion cools the universe below the fusion
temperature.
Small amounts of tritium (another hydrogen isotope) and beryllium-7 and -8 are formed, but these are unstable and are quickly decay. A small amount of deuterium is left unfused.
The amounts of each light element in the early universe can be
estimated from old galaxies, and is strong evidence for the Big Bang. For example, the Big Bang should produce about 1 neutron for every 7
protons, allowing for 25% of all nucleons to be fused into helium-4 (2
protons and 2 neutrons out of every 16 nucleons), and this is the amount
we find today, and far more than can be explained by production in
stars.
Similarly, deuterium fuses extremely easily; any alternative
explanation must also explain how conditions existed for deuterium to
form, but also left some of that deuterium unfused and not immediately
fused again into helium. Any alternative must also explain the proportions of the various light
elements and their isotopes. A few isotopes, such as lithium-7, were
found to be present in amounts that differed from theory.
Until now, the universe's large-scale dynamics and behavior have been
determined mainly by radiation—meaning, those constituents that move
relativistically (at or near the speed of light), such as photons and
neutrinos. As the universe cools, from around 47,000 years (redshift z = 3600), the universe's large-scale behavior becomes dominated by matter instead.
This occurs because the energy density of matter begins to exceed both
the energy density of radiation and the vacuum energy density. Around or shortly after 47,000 years, the densities of non-relativistic
matter (atomic nuclei) and relativistic radiation (photons) become
equal, the Jeans length,
which determines the smallest structures that can form (due to
competition between gravitational attraction and pressure effects),
begins to fall and perturbations, instead of being wiped out by free streamingradiation, can begin to grow in amplitude.
According to the Lambda-CDM model, by this stage, the matter in the universe is around 84.5% cold dark matter and 15.5% "ordinary" matter. There is overwhelming evidence that dark matter
exists and dominates the universe, but since the exact nature of dark
matter is still not understood, the Big Bang theory does not presently
cover any stages in its formation.
From this point on, and for several billion years to come, the presence of dark matter accelerates the formation of structure
in the universe. In the early universe, dark matter gradually gathers
in huge filaments under the effects of gravity, collapsing faster than
ordinary (baryonic) matter because its collapse is not slowed by radiation pressure.
This amplifies the tiny inhomogeneities (irregularities) in the density
of the universe which were left by cosmic inflation. Over time,
slightly denser regions become denser and slightly rarefied (emptier)
regions become more rarefied. Ordinary matter eventually gathers
together faster than it would otherwise do, because of the presence of
these concentrations of dark matter.
The properties of dark matter that allow it to collapse quickly without radiation pressure also mean that it cannot lose
energy by radiation. Losing energy is necessary for particles to
collapse into dense structures beyond a certain point. Therefore, dark
matter collapses into huge but diffuse filaments and haloes, and not
into stars or planets. Ordinary matter, which can lose energy by radiation, forms dense objects and also gas clouds when it collapses.
Recombination, photon decoupling, and the cosmic microwave background (CMB)
About 370,000 years after the Big Bang, two connected events occurred: the ending of recombination and photon decoupling.
Recombination describes the ionized particles combining to form the
first neutral atoms, and decoupling refers to the photons released
("decoupled") as the newly formed atoms settle into more stable energy
states.
Just before recombination, the baryonic matter
in the universe was at a temperature where it formed a hot ionized
plasma. Most of the photons in the universe interacted with electrons
and protons, and could not travel significant distances without
interacting with ionized particles. As a result, the universe was opaque
or "foggy". Although there was light, it was not possible to see, nor
can we observe that light through telescopes.
Starting around 18,000 years, the universe has cooled to a point where free electrons can combine with helium nuclei to form He+ atoms. After around 50,000 years, as the universe cools, its behavior begins to be dominated by matter rather than radiation. At around 100,000 years, after the neutral helium atoms form, helium hydride is the first molecule. Much later, hydrogen and helium hydride react to form molecular hydrogen (H2), the fuel needed for the first stars. At about 370,000 years, neutral hydrogen atoms finish forming ("recombination" of hydrogen ions and electrons), greatly reducing the Thomson scattering of photons. No longer scattered by free electrons, the photons were "decoupled" from the earlier plasma and propagated freely. The majority of these photons still exist as the cosmic microwave background (CMB). This is the oldest era of the universe that we can directly observe today.
Directly combining in a low energy state (ground state) is less
efficient, so these hydrogen atoms generally form with the electrons
still in a high-energy state, and once combined, the electrons quickly
release energy in the form of one or more photons as they transition to a
low energy state. This release of photons is known as photon
decoupling. Some of these decoupled photons are captured by other
hydrogen atoms, the remainder remain free. By the end of recombination,
most of the protons in the universe have formed neutral atoms. This
change from charged to neutral particles means that the mean free path
photons can travel before capture in effect becomes infinite, so any
decoupled photons that have not been captured can travel freely over
long distances (see Thomson scattering). The universe has become transparent to visible light, radio waves and other electromagnetic radiation for the first time in its history.
The background of this box approximates the original 4000 K color of the photons released during decoupling, before they became redshifted to form the cosmic microwave background.
The entire universe would have appeared as a brilliantly glowing fog of
a color similar to this and a temperature of 4000 K, at the time.
The photons released by these newly formed hydrogen atoms initially had a temperature/energy of around ~ 4000 K. This would have been visible to the eye as a pale yellow/orange tinted, or "soft", white color. Over billions of years since decoupling, as the universe has expanded, the photons have been red-shifted
from visible light to radio waves (microwave radiation corresponding to
a temperature of about 2.7 K). Red shifting describes the photons
acquiring longer wavelengths and lower frequencies
as the universe expanded over billions of years, so that they gradually
changed from visible light to radio waves. These same photons can still
be detected as radio waves today. They form the cosmic microwave
background, and they provide crucial evidence of the early universe and
how it developed.
Around the same time as recombination, existing pressure waves within the electron-baryon plasma—known as baryon acoustic oscillations—became
embedded in the distribution of matter as it condensed, giving rise to a
very slight preference in distribution of large-scale objects.
Therefore, the cosmic microwave background is a picture of the universe
at the end of this epoch including the tiny fluctuations generated
during inflation (see 9-year WMAP image),
and the spread of objects such as galaxies in the universe is an
indication of the scale and size of the universe as it developed over
time.
Gravity builds cosmic structure
370 thousand to about 1 billion years after the Big Bang
Even before recombination and decoupling, matter began to accumulate around clumps of dark matter. Clouds of hydrogen collapsed very slowly to form stars and galaxies.
After recombination and decoupling, the universe was transparent and
had cooled enough to allow light to travel long distances, but there
were no light-producing structures such as stars and galaxies. Stars and
galaxies are formed when dense regions of gas form due to the action of
gravity, and this takes a long time within a near-uniform density of
gas and on the scale required, so it is estimated that stars did not
exist for perhaps hundreds of millions of years after recombination.
This period, known as the Dark Ages, began at photon decoupling
around 370,000 years after the Big Bang and ends over a long period of
time called reionization. During the Dark Ages, the temperature of the universe cooled from some
4000 K to about 60 K (3727 °C to about −213 °C), and only two sources of
photons existed: the photons released during recombination/decoupling
(as neutral hydrogen atoms formed), which we can still detect today as
the cosmic microwave background (CMB), and photons occasionally released
by neutral hydrogen atoms, known as the 21 cm spin line of neutral hydrogen. The hydrogen spin line is in the microwave range of frequencies, and within 3 million years,the CMB photons had redshifted out of visible light to infrared;
from that time until the first stars, there were no visible light
photons. Other than perhaps some rare statistical anomalies, the
universe was truly dark.
The first generation of stars, known as Population III stars, formed within a few hundred million years after the Big Bang. These stars were the first source of visible light in the universe
after recombination. Structures may have begun to emerge from around 150
million years, and early galaxies emerged from around 180 to 700
million years. As they emerged, the Dark Ages gradually ended. Because this process
was gradual, the Dark Ages only ended fully at around 1 billion years,
as the universe took on its present appearance.
Artist's impression of the first stars, 400 million years after the Big Bang
At present, the oldest observations of stars and galaxies are from shortly after the start of reionization, with galaxies such as GN-z11 (Hubble Space Telescope, 2016) at about z≈11.1 (about 400 million years cosmic time). Hubble's successor, the James Webb Space Telescope,
launched December 2021, is designed to detect objects up to 100 times
fainter than Hubble, and much earlier in the history of the universe,
back to redshift z≈20 (about 180 million years cosmic time). This is believed to be earlier than the first galaxies, and around the era of the first stars.
There is also an observational effort
underway to detect the faint 21 cm spin line radiation, as it is in
principle an even more powerful tool than the cosmic microwave
background for studying the early universe.
Earliest structures and stars emerge
Around 150 million to 1 billion years after the Big Bang
The Hubble Ultra Deep Fields often feature galaxies that are examples of what the early Stelliferous Era was like.Another
Hubble image shows an infant galaxy forming nearby, which means this
happened very recently on the cosmological timescale. This shows that
new galaxy formation in the universe is still occurring.
The matter in the universe is around 84.5% cold dark matter and 15.5%
"ordinary" matter. Since the start of the matter-dominated era, dark
matter has gradually been gathering in huge spread-out (diffuse)
filaments under the effects of gravity. Ordinary matter eventually
gathers together faster than it would otherwise, because of the presence
of these concentrations of dark matter. It is also slightly more dense
at regular distances due to early baryon acoustic oscillations
(BAO) which became embedded into the distribution of matter when
photons decoupled. Unlike dark matter, ordinary matter can lose energy
by many routes, which means that as it collapses, it can lose the energy
which would otherwise hold it apart, and collapse more quickly, and
into denser forms. Ordinary matter gathers where dark matter is denser,
and in those places it collapses into clouds of mainly hydrogen gas. The
first stars and galaxies form from these clouds. Where numerous
galaxies have formed, galaxy clusters and superclusters will eventually
arise. Large voids with few stars will develop between them, marking where dark matter became less common.
The exact timings of the first stars, galaxies, supermassive black holes, and quasars, and the start and end timings and progression of the period known as reionization, are still being actively researched, with new findings published periodically. As of 2019: the earliest confirmed galaxies (for example GN-z11)
date from around 380–400 million years, suggesting surprisingly fast
gas cloud condensation and stellar birth rates; and observations of the Lyman-alpha forest,
and of other changes to the light from ancient objects, allow the
timing for reionization and its eventual end to be narrowed down.
Structure formation in the Big Bang model proceeds
hierarchically, due to gravitational collapse, with smaller structures
forming before larger ones. The earliest structures to form are the
first stars (known as Population III stars), dwarf galaxies, and quasars (which are thought to be bright, early active galaxies containing a supermassive black hole surrounded by an inward-spiraling accretion disk of gas). Before this epoch, the evolution of the universe could be understood through linear cosmological perturbation theory:
that is, all structures could be understood as small deviations from a
perfect homogeneous universe. This is computationally relatively easy to
study. At this point non-linear structures begin to form, and the computational problem becomes much more difficult, involving, for example, N-body simulations with billions of particles. The Bolshoi cosmological simulation is a high precision simulation of this era.
These Population III stars are also responsible for turning the
few light elements that were formed in the Big Bang (hydrogen, helium
and small amounts of lithium) into many heavier elements. They can be
huge as well as perhaps small—and non-metallic (no elements except
hydrogen and helium). The larger stars have very short lifetimes
compared to most Main Sequence stars we see today, so they commonly
finish burning their hydrogen fuel and explode as supernovae
after mere millions of years, seeding the universe with heavier
elements over repeated generations. They mark the start of the
Stelliferous Era.
As yet, no Population III stars have been found, so the understanding of them is based on computational models
of their formation and evolution. Fortunately, observations of the
cosmic microwave background radiation can be used to date when star
formation began in earnest. Analysis of such observations made by the Planck
microwave space telescope in 2016 concluded that the first generation
of stars may have formed from around 300 million years after the Big
Bang.
Quasars provide some additional evidence of early structure formation. Their light shows evidence of elements such as carbon, magnesium, iron
and oxygen. This is evidence that by the time quasars formed, a massive
phase of star formation had already taken place, including sufficient
generations of Population III stars to give rise to these elements.
As the first stars, dwarf galaxies and quasars gradually form, the
intense radiation they emit reionizes much of the surrounding universe;
splitting the neutral hydrogen atoms back into a plasma of free
electrons and protons for the first time since recombination and
decoupling.
Reionization is evidenced from observations of quasars. Quasars
are a form of active galaxy, and the most luminous objects observed in
the universe. Electrons in neutral hydrogen have specific patterns of
absorbing ultraviolet photons, related to electron energy levels and
called the Lyman series.
Ionized hydrogen does not have electron energy levels of this kind.
Therefore, light travelling through ionized hydrogen and neutral
hydrogen shows different absorption lines. Ionized hydrogen in the intergalactic medium (particularly electrons) can scatter light through Thomson scattering
as it did before recombination, but the expansion of the universe and
clumping of gas into galaxies resulted in a concentration too low to
make the universe fully opaque by the time of reionization. Because of
the immense distance travelled by light (billions of light years) to
reach Earth from structures existing during reionization, any absorption
by neutral hydrogen is redshifted by various amounts, rather than by
one specific amount, indicating when the absorption of then-ultraviolet
light happened. These features make it possible to study the state of
ionization at many different times in the past.
Reionization began as "bubbles" of ionized hydrogen which became
larger over time until the entire intergalactic medium was ionized, when
the absorption lines by neutral hydrogen become rare. The absorption was due to the general state of the universe (the
intergalactic medium) and not due to passing through galaxies or other
dense areas. Reionization might have started to happen as early as z = 16 (250 million years of cosmic time) and was mostly complete by around z = 9 or 10 (500 million years), with the remaining neutral hydrogen becoming fully ionized z = 5 or 6 (1 billion years), when Gunn-Peterson troughs
that show the presence of large amounts of neutral hydrogen disappear.
The intergalactic medium remains predominantly ionized to the present
day, the exception being some remaining neutral hydrogen clouds, which
cause Lyman-alpha forests to appear in spectra.
These observations have narrowed down the period of time during
which reionization took place, but the source of the photons that caused
reionization is still not completely certain. To ionize neutral
hydrogen, an energy larger than 13.6 eV is required, which corresponds to ultraviolet photons with a wavelength of 91.2 nm
or shorter, implying that the sources must have produced significant
amounts of ultraviolet and higher energy. Protons and electrons will
recombine if energy is not continuously provided to keep them apart,
which also sets limits on how numerous the sources were and their
longevity. With these constraints, it is expected that quasars and first generation stars and galaxies were the main sources of energy. The current leading candidates from most to least significant are
currently believed to be Population III stars (the earliest stars;
possibly 70%), dwarf galaxies (very early small high-energy galaxies; possibly 30%), and a contribution from quasars (a class of active galactic nuclei).
However, by this time, matter had become far more spread out due
to the ongoing expansion of the universe. Although the neutral hydrogen
atoms were again ionized, the plasma was much more thin and diffuse, and
photons were much less likely to be scattered. Despite being reionized,
the universe remained largely transparent during reionization due how
sparse the intergalactic medium was. Reionization gradually ended as the
intergalactic medium became virtually completely ionized, although some
regions of neutral hydrogen do exist, creating Lyman-alpha forests.
In August 2023, images of black holes and related matter in the very early universe by the James Webb Space Telescope were reported and discussed.
Galaxies, clusters and superclusters
Computer simulated view of the large-scale structure of a part of the universe about 50 million light-years across
Matter continues to draw together under the influence of gravity, to form galaxies. The stars from this time period, known as Population II stars, are formed early on in this process, with more recent Population I stars formed later. Gravitational attraction also gradually pulls galaxies towards each other to form groups, clusters and superclusters. Hubble Ultra Deep Field
observations has identified a number of small galaxies merging to form
larger ones, at 800 million years of cosmic time (13 billion years ago). (This age estimate is now believed to be slightly overstated).
From
1 billion years, and for about 12.8 billion years, the universe has
looked much as it does today and it will continue to appear very similar
for many billions of years into the future. The thin disk of the Milky Way began to form when the universe was about 5 billion years old or 9 ± 2 Gya. The Solar System formed at about 9.2 billion years (4.6 Gya); the oldest organic matter consistent with life processes dates back 4 billion years.
The thinning of matter over time reduces the ability of the
matter to gravitationally decelerate the expansion of the universe; in
contrast, dark energy is a constant factor tending to accelerate the expansion of the universe. The universe's expansion passed an inflection point
about five or six billion years ago when the universe entered the
modern "dark-energy-dominated era" where the universe's expansion is now
accelerating rather than decelerating. The present-day universe is
quite well understood, but beyond about 100 billion years of cosmic time
(about 86 billion years in the future), scientists are less sure which
path the universe will take.
From about 9.8 billion years of cosmic time,
the universe's large-scale behavior is believed to have gradually
changed for the third time in its history. Its behavior had originally
been dominated by radiation (relativistic constituents such as photons
and neutrinos) for the first 47,000 years, and since about 370,000 years
of cosmic time, its behavior had been dominated by matter. During its
matter-dominated era, the expansion of the universe had begun to slow
down, as gravity reined in the initial outward expansion. But from about
9.8 billion years of cosmic time, observations show that the expansion
of the universe slowly stops decelerating, and gradually begins to
accelerate again, instead.
While the precise cause is not known, the observation is accepted
as correct by the cosmologist community. By far the most accepted
understanding is that this is due to an unknown form of energy which has
been given the name "dark energy". "Dark" in this context means that it is not directly observed, but its
existence can be deduced by examining the gravitational effect it has on
the universe. Research is ongoing to understand this dark energy. Dark
energy is now believed to be the single largest component of the
universe, as it constitutes about 68.3% of the entire mass–energy of the physical universe.
Dark energy is believed to act like a cosmological constant—a
scalar field that exists throughout space. Unlike gravity, the effects
of such a field do not diminish (or only diminish slowly) as the
universe grows. While matter and gravity have a greater effect
initially, their effect quickly diminishes as the universe continues to
expand. Objects in the universe, which are initially seen to be moving
apart as the universe expands, continue to move apart, but their outward
motion gradually slows down. This slowing effect becomes smaller as the
universe becomes more spread out. Eventually, the outward and repulsive
effect of dark energy begins to dominate over the inward pull of
gravity. Instead of slowing down and perhaps beginning to move inward
under the influence of gravity, from about 9.8 billion years of cosmic
time, the expansion of space starts to slowly accelerate outward at a gradually increasing rate.
Cosmological models extrapolated back to 10−43 seconds combined with particle physics models both with and beyond the Standard Model allow well-informed speculation on the character and properties of the early universe.
Approaching infinite temperature, a scale factor of zero, or time at
zero is known to be outside of our physical models. Speculating about an
initial gravitational singularity is not sensible: the conditions are outside of the range of the theory.
Since the standard model of cosmology predicts expansion of the
universe from a very hot time in the distant past, it can be followed
back to smaller and smaller scales. However, it cannot be followed back
to zero space. Below distance known as a Planck length,
the basis for the equations breaks down. The energy of particles in
this time is so large that quantum effects take over from classical
equations for gravity. The Planck time, 10−43 seconds, is therefore the beginning time for the Big Bang model of cosmology.
Between 10−43 seconds and 10−36 seconds after the Big Bang
After the Planck era, the universe could, in principle, be modeled by extensions of the Standard Model of particle physics, for example, those called grand unified theories.
Many such theories have proposed, but none have been successful in
producing quantitative agreement with modern astrophysical observations.
Nevertheless, the time between 10−43 and 10−36 seconds has been called the grand unification epoch.
Before the GUT epoch, the temperature of the universe exceeded 1015 GeV. As the universe expanded and cooled, it may have crossed a cosmological phase transition, which may have resulted in the large ratio of matter to antimatter we observe today. This phase transition is a thermodynamic effect similar to condensation of a gas or freezing
of a liquid. While the transition in the GUT epoch is speculative,
electroweak and quark-hadron transitions which happen later are
supported by theoretical models with some successful predictions.
Starting anywhere between 10−22 and 10−15 seconds after the Big Bang, until 10−12 seconds after the Big Bang
Sometime after inflation, the created particles went through thermalization, where mutual interactions lead to thermal equilibrium. Before the electroweak symmetry breaking, at a temperature of around 1015 K, approximately 10−15 seconds after the Big Bang, the electromagnetic and weak interaction had not yet separated, and the gauge bosons and fermions had not yet gained mass through the Higgs mechanism. This epoch ended with electroweak symmetry breaking, potentially through a phase transition. In some extensions of the Standard Model of particle physics, baryogenesis
also happened at this stage, creating an imbalance between matter and
antimatter (though in extensions to this model, this may have happened
earlier). Little is known about the details of these processes.
There are several competing scenarios for the long-term evolution of
the universe. Which of them will happen, if any, depends on the precise
values of physical constants such as the cosmological constant, the possibility of proton decay, the energy of the vacuum (meaning, the energy of "empty" space itself), and the natural laws beyond the Standard Model.
If the expansion of the universe continues and it stays in its
present form, eventually all but the nearest galaxies will be carried
away from us by the expansion of space at such a velocity that the
observable universe will be limited to our own gravitationally bound local galaxy cluster.
In the very long term (after many trillions—thousands of billions—of
years, cosmic time), the Stelliferous Era will end, as stars cease to be
born and even the longest-lived stars gradually die. Beyond this, all objects in the universe will cool and (with the possible exception of protons)
gradually decompose back to their constituent particles and then into
subatomic particles and very low-level photons and other fundamental particles, by a variety of possible processes.
The following scenarios have been proposed for the ultimate fate of the universe:
As expansion continues, the universe becomes larger, colder, and
more dilute; in time, all structures eventually decompose to subatomic
particles and photons.
In the case of indefinitely continuing cosmic expansion, the energy
density in the universe will decrease until, after an estimated time of
101000 years, it reaches thermodynamic equilibrium and no more structure will be possible. This will happen only after an extremely long time because first, some (less than 0.1%) matter will collapse into black holes, which will then evaporate extremely slowly via Hawking radiation. The universe in this scenario will cease to be able to support life much earlier than this, after some 1014 years or so, when star formation ceases., §IID In some Grand Unified Theories, proton decay after at least 1034
years will convert the remaining interstellar gas and stellar remnants
into leptons (such as positrons and electrons) and photons. Some
positrons and electrons will then recombine into photons., §IV, §VF In this case, the universe has reached a high-entropy state consisting of a bath of particles and low-energy radiation. It is not known, however, whether it eventually achieves thermodynamic equilibrium., §VIB, VID The hypothesis of a universal heat death stems from the 1850s ideas of William Thomson
(Lord Kelvin), who extrapolated the classical theory of heat and
irreversibility (as embodied in the first two laws of thermodynamics) to
the universe as a whole.
Expansion of space accelerates and at some point becomes so extreme that even subatomic particles and the fabric of spacetime are pulled apart and unable to exist.
For any value of the dark energy content of the universe where the
negative pressure ratio is less than −1, the expansion rate of the
universe will continue to increase without limit. Gravitationally bound
systems, such as clusters of galaxies, galaxies, and ultimately the
Solar System will be torn apart. Eventually the expansion will be so
rapid as to overcome the electromagnetic forces holding molecules and
atoms together. Even atomic nuclei will be torn apart. Finally, forces
and interactions even on the Planck scale—the
smallest size for which the notion of "space" currently has a
meaning—will no longer be able to occur as the fabric of spacetime
itself is pulled apart and the universe as we know it will end in an
unusual kind of singularity.
Expansion eventually slows and halts, then reverses as all matter
accelerates towards its common centre. Currently considered to be likely
incorrect.
In the opposite of the "Big Rip" scenario, the expansion of the
universe would at some point be reversed and the universe would contract
towards a hot, dense state. This is a required element of oscillatory
universe scenarios, such as the cyclic model,
although a Big Crunch does not necessarily imply an oscillatory
universe. Current observations suggest that this model of the universe
is unlikely to be correct, and the expansion will continue or even
accelerate.
Collapse of the quantum fields that underpin all forces, particles and structures, to a different form.
Cosmology traditionally has assumed a stable or at least metastable universe, but the possibility of a false vacuum in quantum field theory implies that the universe at any point in spacetime might spontaneously collapse into a lower-energy state (see Bubble nucleation), a more stable or "true vacuum", which would then expand outward from that point with the speed of light.
In this kind of protracted timescale, extremely rare quantum phenomena
may also occur that are unlikely to be seen on a timescale smaller than
trillions of years. These may also lead to unpredictable changes to the
state of the universe which would not be likely to be significant on
any smaller timescale. For example, on a timescale of millions of
trillions of years, black holes might appear to evaporate almost
instantly, uncommon quantum tunnelling
phenomena would appear to be common, and quantum (or other) phenomena
so unlikely that they might occur just once in a trillion years may
occur many times.
_(2023-122).jpg)
