question: What causes the universe to expand?
comment: Scientists aren't 100% sure, but our best guess is that **it's the momentum from the Big Bang,** plus the effects of dark energy (which we're even less 100% sure about.)
passages: Section::::Cosmic inflation.
   Section::::Understanding the expansion of the universe.:Scale factor.
   Until the theoretical developments in the 1980s no one had an explanation for why this seemed to be the case, but with the development of models of cosmic inflation, the expansion of the universe became a general feature resulting from vacuum decay. Accordingly, the question "why is the universe expanding?" is now answered by understanding the details of the inflation decay process which occurred in the first 10 seconds of the existence of our universe. During inflation, the metric changed exponentially, causing any volume of space that was smaller than an atom to grow to around 100 million light years across in a time scale similar to the time when inflation occurred (10 seconds).
   Independent lines of evidence from Type Ia supernovae and the CMB imply that the universe today is dominated by a mysterious form of energy known as dark energy, which apparently permeates all of space. The observations suggest 73% of the total energy density of today's universe is in this form. When the universe was very young, it was likely infused with dark energy, but with less space and everything closer together, gravity predominated, and it was slowly braking the expansion. But eventually, after numerous billion years of expansion, the growing abundance of dark energy caused the expansion of the universe to slowly begin to accelerate.
   Section::::Understanding the expansion of the universe.:Other conceptual models of expansion.
  
  The expansion of space is often illustrated with conceptual models which show only the size of space at a particular time, leaving the dimension of time implicit.
   During the inflationary epoch about 10 of a second after the Big Bang, the universe suddenly expanded, and its volume increased by a factor of at least 10 (an expansion of distance by a factor of at least 10 in each of the three dimensions), equivalent to expanding an object 1 nanometer (10 m, about half the width of a molecule of DNA) in length to one approximately 10.6 light years (about 10 m or 62 trillion miles) long. A much slower and gradual expansion of space continued after this, until at around 9.8 billion years after the Big Bang (4 billion years ago) it began to gradually expand more quickly, and is still doing so.
   Section::::Theoretical basis and first evidence.:Inflation as an explanation for the expansion.
   Hubble's law has two possible explanations. Either we are at the center of an explosion of galaxies—which is untenable given the Copernican principle—or the universe is uniformly expanding everywhere. This universal expansion was predicted from general relativity by Alexander Friedmann in 1922 and Georges Lemaître in 1927, well before Hubble made his 1929 analysis and observations, and it remains the cornerstone of the Big Bang theory as developed by Friedmann, Lemaître, Robertson, and Walker.
   Section::::Evidence for acceleration.:Baryon acoustic oscillations.
   The expansion of space is sometimes described as a force which acts to push objects apart. Though this is an accurate description of the effect of the cosmological constant, it is not an accurate picture of the phenomenon of expansion in general. For much of the universe's history the expansion has been due mainly to inertia. The matter in the very early universe was flying apart for unknown reasons (most likely as a result of cosmic inflation) and has simply continued to do so, though at an ever-decreasing rate due to the attractive effect of gravity.
   The metric expansion of space is of a kind completely different from the expansions and explosions seen in daily life. It also seems to be a property of the universe as a whole rather than a phenomenon that applies just to one part of the universe or can be observed from "outside" it.
   In the earliest stages of the universe, tiny fluctuations within the universe's density led to concentrations of dark matter gradually forming. Ordinary matter, attracted to these by gravity, formed large gas clouds and eventually, stars and galaxies, where the dark matter was most dense, and voids where it was least dense. After around 100 - 300 million years, the first stars formed, known as Population III stars. These were probably very massive, luminous, non metallic and short-lived. They were responsible for the gradual reionization of the Universe between about 200-500 million years and 1 billion years, and also for seeding the universe with elements heavier than helium, through stellar nucleosynthesis. The Universe also contains a mysterious energy - possibly a scalar field - called dark energy, the density of which does not change over time. After about 9.8 billion years, the Universe had expanded sufficiently so that the density of matter was less than the density of dark energy, marking the beginning of the present dark-energy-dominated era. In this era, the expansion of the Universe is accelerating due to dark energy.
   Section::::Understanding the expansion of the universe.:Metric expansion and speed of light.
  
  At the end of the early universe's inflationary period, all the matter and energy in the universe was set on an inertial trajectory consistent with the equivalence principle and Einstein's general theory of relativity and this is when the precise and regular form of the universe's expansion had its origin (that is, matter in the universe is separating because it was separating in the past due to the inflaton field).
   Based on large quantities of experimental observation and theoretical work, the scientific consensus is that "space itself is expanding", and that it expanded very rapidly within the first fraction of a second after the Big Bang. This kind of expansion is known as "metric expansion". In mathematics and physics, a "metric" means a measure of distance, and the term implies that "the sense of distance within the universe is itself changing".
   Section::::Observational evidence.:Hubble's law and the expansion of space.
   Based on a huge amount of experimental observation and theoretical work, it is now believed that the reason for the observation is that "space itself is expanding", and that it expanded very rapidly within the first fraction of a second after the Big Bang. This kind of expansion is known as a ""metric"" expansion. In the terminology of mathematics and physics, a "metric" is a measure of distance that satisfies a specific list of properties, and the term implies that "the sense of distance within the universe is itself changing", although at this time it is far too small an effect to see on less than an intergalactic scale.
   One of the biggest puzzles in contemporary cosmology is why the expansion of the Universe is accelerating. About seventy percent of the Universe today consists of so-called dark energy that counteracts gravity's attractive force and causes this acceleration. Very little is known about what dark energy is. CHIME is in the process of making precise measurements of the acceleration of the Universe to improve the knowledge of how dark energy behaves. The experiment is designed to observe the period in the Universe's history during which the standard ΛCDM model predicts that dark energy began to dominate the energy density of the Universe and when decelerated expansion transitioned to acceleration.
   Section::::Understanding the expansion of the universe.:Topology of expanding space.
   Section::::Inflationary epoch.
  
  The expansion is thought to have been triggered by the phase transition that marked the end of the preceding grand unification epoch at approximately 10 seconds after the Big Bang. 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 a repulsive force that led to a rapid expansion of space. This expansion explains various properties of the current universe that are difficult to account for without such an inflationary epoch.
   BULLET::::- In studies of large-scale structure of the cosmos taken from redshift surveys a so-called "End of Greatness" was discovered at the largest scales of the universe. Until these scales were surveyed, the universe appeared "lumpy" with clumps of galaxy clusters, superclusters and filaments which were anything but isotropic and homogeneous. This lumpiness disappears into a smooth distribution of galaxies at the largest scales.
  
  BULLET::::- The isotropic distribution across the sky of distant gamma-ray bursts and supernovae is another confirmation of the Cosmological Principle.
   The more matter there is in the Universe, the stronger the mutual gravitational pull of the matter. If the Universe were "too" dense then it would re-collapse into a gravitational singularity. However, if the Universe contained too "little" matter then the self-gravity would be too weak for astronomical structures, like galaxies or planets, to form. Since the Big Bang, the universe has expanded monotonically. Perhaps unsurprisingly, our universe has just the right mass-energy density, equivalent to about 5 protons per cubic meter, which has allowed it to expand for the last 13.8 billion years, giving time to form the universe as observed today.
   Following this, in the early universe, the evolution of the universe proceeded according to known high energy physics. This is when the first protons, electrons and neutrons formed, then nuclei and finally atoms. With the formation of neutral hydrogen, the cosmic microwave background was emitted. Finally, the epoch of structure formation began, when matter started to aggregate into the first stars and quasars, and ultimately galaxies, clusters of galaxies and superclusters formed. The future of the universe is not yet firmly known, but according to the ΛCDM model it will continue expanding forever.
  
  Section::::Areas of study.
   The proposed field and its quanta (the subatomic particles related to it) have been named "inflaton". If this field did not exist, scientists would have to propose a different explanation for all the observations that strongly suggest a metric expansion of space has occurred, and is still occurring much more slowly today.
  
  Section::::Overview of metrics and comoving coordinates.
  
  To understand the metric expansion of the universe, it is helpful to discuss briefly what a metric is, and how metric expansion works.
   BULLET::::- the Copernican Principle which demands that no place in the universe is preferred (that is, the universe has no "starting point").
  
  Scientists have tested carefully whether these assumptions are valid and borne out by observation. Observational cosmologists have discovered evidence — very strong in some cases — that supports these assumptions, and as a result, metric expansion of space is considered by cosmologists to be an observed feature on the basis that although we cannot see it directly, scientists have tested the properties of the universe and observation provides compelling confirmation. Sources of this confidence and confirmation include:
   Section::::Problems and related issues in physics.:Dark energy.
  
  Measurements of the redshift–magnitude relation for type Ia supernovae indicate that the expansion of the universe has been accelerating since the universe was about half its present age. To explain this acceleration, general relativity requires that much of the energy in the universe consists of a component with large negative pressure, dubbed "dark energy".
  