Expansion of the universe

Contrary to common misconception, it is equally valid to adopt a description in which space does not expand and objects simply move apart while under the influence of their mutual gravity.

This would be equivalent to expanding an object 1 nanometer across (10−9 m, about half the width of a molecule of DNA) to one approximately 10.6 light-years across (about 1017 m, or 62 trillion miles).

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.

In 1922, Alexander Friedmann used the Einstein field equations to provide theoretical evidence that the universe is expanding.

"[10] In 1927, Georges Lemaître independently reached a similar conclusion to Friedmann on a theoretical basis, and also presented observational evidence for a linear relationship between distance to galaxies and their recessional velocity.

Astronomer Walter Baade recalculated the size of the known universe in the 1940s, doubling the previous calculation made by Hubble in 1929.

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.

[16] 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 80±17 km⋅s−1⋅Mpc−1.

[17] 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.

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−1⋅Mpc−1.

[19] 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.

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.

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.

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).

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.

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.

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 peculiar velocities of nonrelativistic particles decay as the universe expands, in inverse proportion with the cosmic scale factor.

More generally, the peculiar momenta of both relativistic and nonrelativistic particles decay in inverse proportion with the scale factor.

Because of the high rate of expansion, it is also possible for a distance between two objects to be greater than the value calculated by multiplying the speed of light by the age of the universe.

[20] Due to the non-intuitive nature of the subject and what has been described by some as "careless" choices of wording, certain descriptions of the metric expansion of space and the misconceptions to which such descriptions can lead are an ongoing subject of discussion within the fields of education and communication of scientific concepts.

[26] However, the universe is known to have been dominated by ultrarelativistic Standard Model particles, conventionally called radiation, by the time of neutrino decoupling at about 1 second.

[27] During radiation domination, cosmic expansion decelerated, with the scale factor growing proportionally with the square root of the time.

During the matter-dominated epoch, cosmic expansion also decelerated, with the scale factor growing as the 2/3 power of the time (

Similarly to inflation, dark energy drives accelerated expansion, such that the scale factor grows exponentially in time.

Supernovae are observable at such great distances that the light travel time therefrom can approach the age of the universe.

In work that was awarded the 2011 Nobel Prize in Physics, supernova observations were used to determine that cosmic expansion is accelerating in the present epoch.

A higher expansion rate would imply a smaller characteristic size of CMB fluctuations, and vice versa.

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.

However, changes in redshift or flux could be observed by the Square Kilometre Array or Extremely Large Telescope in the mid-2030s.