Expansion of the universe
The expansion of the universe 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 move away at speeds that are proportional to their distance from 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.
The expansion of the universe was discovered by separate theoretical and observational work in the 1920s. Since then, the expansion has become a core aspect of the astrophysical field of cosmology. Many major scientific projects have sought to characterize the expansion and understand its effects.
Cosmic expansion is a key feature of Big Bang cosmology. Within the theory of general relativity, it is modeled mathematically with the Friedmann–Lemaître–Robertson–Walker metric. The consensus or "standard" model of cosmology, the Lambda-CDM model, hypothesizes different expansion rates during different times, depending on the physical properties of the contents of spacetime. The very earliest expansion, called inflation saw the universe suddenly expand by a factor of at least 1026 in every direction about 10−32 of a second after the Big Bang. Cosmic expansion subsequently decelerated to much slower rates, until around 9.8 billion years after the Big Bang 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 which is predicted to be dominant in the future.
The concept of the expansion of the universe is difficult to explain, leading to several misconceptions about its nature, origin, and effects.
History of the concept
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.The key astronomical demonstration of the expansion of the universe is known as Hubble's law or sometimes as the Hubble–Lemaître law. The name and thus the honor of the discovery has been debated. In 1924, Swedish astronomer Knut Lundmark found observational evidence for expansion. While his results were reasonably accurate even by today's standards, they relied upon galaxy diameter measurements and the distance to the Andromeda Galaxy which were unproven at the time.
In 1927, Georges Lemaître derived solutions for Einstein's equations of general relativity and applied them to data published by Slipher and Hubble to propose a linear relationship between distance to galaxies and their recessional velocity. The linear relationship was firmly established by Edwin Hubble in 1929 using multiple methods and cross-checked with proven techniques.
Hubble himself did not associate the relationship now called Hubble's law to the expansion of the universe and his estimate of the proportionality constant was too large by a factor of 7, but his publications sparked interest in the earlier theoretical work and initiated increasingly sophisticated efforts to measure the constant. Astronomer Walter Baade recalculated the size of the known universe in the 1940s, doubling the previous value. For most of the second half of the 20th century, the value of the Hubble constant was estimated to be between.
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 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. 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 the systematic measurement errors of the Hubble constant, to. 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 supernovae, which estimates a Hubble constant of. In 2003, David Spergel's analysis of the cosmic microwave background during the first year observations of the Wilkinson Microwave Anisotropy Probe satellite further agreed with the estimated expansion rates for local galaxies,.
Structure of cosmic expansion
Distant galaxies in all directions are observed to move away from Earth and their velocity is proportional to their distance from Earth. This observation, known as Hubble's law, combined with the observation that the universe at the largest scales is homogeneous and isotropic, means that the universe is expanding uniformly at the present time. This means the distance between any two galaxies increases over time by the same factor. Uniform expansion is equivalent to the observed linear relationship between the recession velocities of a galaxy and its positions :where the Hubble constant quantifies the rate of expansion today.
While the expansion in space is uniform, it is not uniform across long time intervals: the rate of expansion varies with time and this variation is a central object of study in cosmology. Using cosmic time with indicating the present, the Hubble constant,, is the present day value of the Hubble parameter,, describing the dynamics of expansion.
Dynamics of cosmic expansion
The expansion of the universe can be understood as resulting from an initial condition in which the contents of the universe are flying apart. The mutual gravitational attraction of the matter and radiation within the universe gradually slows this expansion over time, but their density is too low to prevent continued expansion. In addition, recent observational evidence suggests that dark energy is now accelerating the expansion.Mathematically, the expansion of the universe is quantified by the scale factor,, 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 at the present time. Because the universe is expanding, 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 the universe to stop expanding and begin to contract, which corresponds to the scale factor decreasing in time.
The scale factor 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, is the gravitational constant, is the energy density within the universe, is the pressure, is the speed of light, and is the cosmological constant. A positive energy density leads to deceleration of the expansion,, and a positive pressure further decelerates expansion. On the other hand, sufficiently negative pressure with leads to accelerated expansion, and the cosmological constant also accelerates expansion. Nonrelativistic matter is essentially pressureless, with, while a gas of ultrarelativistic particles has positive pressure. Negative-pressure fluids, like dark energy, are not experimentally confirmed, but the existence of dark energy is inferred from astronomical observations.
Distances in the expanding universe
Comoving coordinates
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 the spatial coordinates in the FLRW metric.Shape of the universe
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 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.Cosmological horizons
An expanding universe typically has a finite age. Light, and other particles, can have propagated only 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 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 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.