Non-standard cosmology

A non-standard cosmology is any physical cosmological model of the universe that was, or still is, proposed as an alternative to the then-current standard model of cosmology. The term non-standard is applied to any theory that does not conform to the scientific consensus. Because the term depends on the prevailing consensus, the meaning of the term changes over time. For example, hot dark matter would not have been considered non-standard in 1990, but would have been in 2010. Conversely, a non-zero cosmological constant resulting in an accelerating universe would have been considered non-standard in 1990, but is part of the standard cosmology in 2010.

Several major cosmological disputes have occurred throughout the

Theories which assert that the universe has an infinite age with no beginning have trouble accounting for the abundance of deuterium in the cosmos, because deuterium easily undergoes nuclear fusion in stars and there are no known astrophysical processes other than the Big Bang itself that can produce it in large quantities. Hence the fact that deuterium is not an extremely rare component of the universe suggests both that the universe has a finite age and that there was a process that created deuterium in the past that no longer occurs.

Theories which assert that the universe has a finite life, but that the Big Bang did not happen, have problems with the abundance of helium-4. The observed amount of ⁴He is far larger than the amount that should have been created via stars or any other known process. By contrast, the abundance of ⁴He in Big Bang models is very insensitive to assumptions about baryon density, changing only a few percent as the baryon density changes by several orders of magnitude. The observed value of ⁴He is within the range calculated.

Still, it was not until the discovery of the

Today, heterodox non-standard cosmologies are generally considered unworthy of consideration by cosmologists while many of the historically significant nonstandard cosmologies are considered to have been falsified. The essentials of the big bang theory have been confirmed by a wide range of complementary and detailed observations, and no non-standard cosmologies have reproduced the range of successes of the big bang model. Speculations about alternatives are not normally part of research or pedagogical discussions, except as object lessons or for their historical importance. An open letter started by some remaining advocates of non-standard cosmology has affirmed that: "today, virtually all financial and experimental resources in cosmology are devoted to big bang studies...."^[3]

In the 1990s, a dawning of a "golden age of cosmology" was accompanied by a startling discovery that the expansion of the universe was, in fact, accelerating. Previous to this, it had been assumed that matter either in its visible or invisible

Alternatives to Big Bang cosmologies

Before observational evidence was gathered, theorists developed frameworks based on what they understood to be the most general features of

1. the
   universality of physical laws
    – that the laws of physics do not change from one place and time to another,
2. the cosmological principle – that the universe is roughly homogeneous and isotropic in space though not necessarily in time, and
3. the Copernican principle – that we are not observing the universe from a preferred locale.

These assumptions when combined with General Relativity result in a universe that is governed by the

1. the universe emerged from a hot, dense state at a finite time in the past,
2. because the universe heats up as it contracts and cools as it expands, in the first minutes that time existed as we know it, the temperatures were high enough for Big Bang nucleosynthesis to occur, and
3. a cosmic microwave background pervading the entire universe should exist, which is a record of a phase transition that occurred when the atoms of the universe first formed.

These features were derived by numerous individuals over a period of years; indeed it was not until the middle of the twentieth century that accurate predictions of the last feature and observations confirming its existence were made. Non-standard theories developed either by starting from different assumptions or by contradicting the features predicted by the prevailing standard model of cosmology.^[5]

Steady State theories

The Steady State theory extends the homogeneity assumption of the cosmological principle to reflect a homogeneity in time as well as in space. This "perfect cosmological principle" as it would come to be called asserted that the universe looks the same everywhere (on the large scale), the same as it always has and always will. This is in contrast to Lambda-CDM, in which the universe looked very different in the past and will look very different in the future. Steady State theory was proposed in 1948 by Fred Hoyle, Thomas Gold, Hermann Bondi and others. In order to maintain the perfect cosmological principle in an expanding universe, steady state cosmology had to posit a "matter-creation field" (the so-called C-field) that would insert matter into the universe in order to maintain a constant density.^[5]

The debate between the Big Bang and the Steady State models would happen for 15 years with camps roughly evenly divided until the discovery of the cosmic microwave background radiation. This radiation is a natural feature of the Big Bang model which demands a "time of last scattering" where photons

The standard model of cosmology today, the Lambda-CDM model, has been extremely successful at providing a theoretical framework for structure formation, the anisotropies in the cosmic microwave background, and the accelerating expansion of the universe. However, it is not without its problems.^[7] There are many proposals today that challenge various aspects of the Lambda-CDM model. These proposals typically modify some of the main features of Lambda-CDM, but do not reject the Big Bang.

Anisotropic universe

Isotropicity – the idea that the universe looks the same in all directions – is one of the core assumptions that enters into the Friedmann equations. In 2008 however, scientists working on Wilkinson Microwave Anisotropy Probe data claimed to have detected a 600–1000 km/s flow of clusters toward a 20-degree patch of sky between the constellations of Centaurus and Vela.^[8] They suggested that the motion may be a remnant of the influence of no-longer-visible regions of the universe prior to inflation. The detection is controversial, and other scientists have found that the universe is isotropic to a great degree.^[9]

Massive compact halo object (MACHO)

Solitary black holes, neutron stars, burnt-out dwarfs, and other massive objects that that are hard to detect are collectively known as MACHOs; some scientists initially hoped that baryonic MACHOs could account for and explain all the dark matter.^[10][11] However, evidence has accumulated that these objects cannot explain a large fraction of the dark matter mass.^[12]

Exotic dark matter

In Lambda-CDM, dark matter is a form of matter that that interacts with both ordinary matter and light only through gravitational effects. To produce the large-scale structure we see today, dark matter is "cold" (the 'C' in Lambda-CDM), i.e. non-relativistic. Dark matter has not been conclusively identified, and its exact nature is the subject of intense study. Hypothetical

• Self-interacting dark matter, wherein dark matter particles interact with themselves.
• Warm dark matter, which are more relativistic than cold dark matter, but less relativistic than the observationally-excluded hot dark matter.
• Fuzzy cold dark matter, which have particles much lighter than axions – in the 10⁻²² eV range.

Yet other theories attempt to explain dark matter and dark energy as different facets of the same underlying fluid (see dark fluid), or hypothesize that dark matter could decay into dark energy.

Exotic dark energy

In Lambda-CDM, dark energy is an unknown form of energy that tends to accelerate the expansion of the universe. It is less well-understood than dark matter, and similarly mysterious. The simplest explanation of dark energy is the cosmological constant (the 'Lambda' in Lambda-CDM). This is a simple constant added to the Einstein field equations to provide a repulsive force. Thus far observations are fully consistent with the cosmological constant, but leave room for a plethora of alternatives, e.g.:

• cosmic inflation
  shortly after the Big Bang. In quintessence, dark energy will usually vary over time (as opposed to the cosmological constant, which remains a constant).
• Inhomogeneous cosmology. One of the fundamental assumptions of Lambda-CDM is that the universe is homogeneous – that is, it looks broadly the same regardless of where the observer is. In the inhomogeneous universe scenario, the observed dark energy is a measurement artefact caused by us being located at an emptier-than-average region of space.
• Variable dark energy, which is similar to quintessence in that the properties of dark energy vary over time (see figure), but different in that dark energy is not due to a scalar field.

Alternatives to general relativity

General relativity, upon which the FRW metric is based, is an extremely successful theory which has met every observational test so far. However, at a fundamental level it is incompatible with quantum mechanics, and by predicting singularities, it also predicts its own breakdown. Any alternative theory of gravity would immediately imply an alternative cosmological theory since Lambda-CDM is dependent on general relativity as a framework assumption. There are many different motivations to modify general relativity, such as to eliminate the need for dark matter or dark energy, or to avoid such paradoxes as the firewall.

There are very many modified gravity theories, none of which have gained widespread acceptance, although it remains an active field of research. Some of the more notable theories are below.

Machian universe

Tensor–vector–scalar gravity (TeVeS) is a proposed relativistic theory that is equivalent to Modified Newtonian dynamics (MOND) in the non-relativistic limit, which purports to explain the galaxy rotation problem without invoking dark matter. Originated by Jacob Bekenstein in 2004, it incorporates various dynamical and non-dynamical tensor fields, vector fields and scalar fields.

The break-through of TeVeS over MOND is that it can explain the phenomenon of

f(R) gravity is a family of theories that modify general relativity by defining a different function of the

• Kaluza–Klein theory, which posits an extra spatial dimension, thereby making our universe 5D instead of the 4D of General Relativity. The DGP model is one of the models in this category, claimed to be able to explain dark energy without invoking a cosmological constant.
• Entropic gravity, which describes gravity as an entropic force with macro-scale homogeneity but which is subject to quantum-level disorder. The theory claims to be able to remove the need for dark matter, as well as provide a natural explanation for dark energy.
• The GRSI model modifies General Relativity by adding self-interaction terms similar to those in quantum chromodynamics, leading to an effect similar to
  quark confinement in gravity. It is claimed to be able to explain observations without needing dark matter or dark energy.^[21]
• Shockwave cosmology, proposed by Joel Smoller and Blake Temple in 2003, has the “big bang” as an explosion inside a black hole, producing the expanding volume of space and matter that includes the observable universe. This black hole eventually becomes a white hole as the matter density reduces with the expansion.^[22] A related theory proposes that the acceleration of the expansion of the observable universe, normally attributed to dark energy, may be caused by an effect of the shockwave.^[23]

See also

• Quantum cosmology

Notes