 | Big Bang: Encyclopedia II - Big Bang - Features issues and problems
Big Bang - Features issues and problems
A number of problems have arisen within the Big Bang theory throughout its history. Some of them are mainly of historical interest today, and have been avoided either through modifications to the theory or as the result of better observations. Other issues, such as the cuspy halo problem and the dwarf galaxy problem of cold dark matter, are not considered to be fatal as they can be addressed through refinements of the theory.
There are a small number of proponents of non-standard cosmologies who doubt that there was a Big Bang at all. They claim that solutions to standard problems in the Big Bang theory involve ad hoc modifications and addenda to the theory. Most often attacked are the parts of standard cosmology that include dark matter, dark energy, and cosmic inflation. However, while explanations for these features remain at the frontiers of inquiry in physics, together they are suggested by independent observations of big bang nucleosynthesis, the cosmic microwave background, large scale structure and Type Ia supernovae. The gravitational effects of these features are understood observationally and theoretically but they have not yet been successfully incorporated into the Standard Model of particle physics. Though some aspects of the theory remain inadequately explained by fundamental physics, almost all astronomers and physicists accept that the close agreement between Big Bang theory and observation have firmly established all the basic parts of the theory.
The following is a short list of Big Bang "problems" and puzzles:
Big Bang - Horizon problem
The horizon problem results from the premise that information cannot travel faster than light, and hence two regions of space which are separated by a greater distance than the speed of light multiplied by the age of the Universe cannot be in causal contact. The observed isotropy of the cosmic microwave background (CMB) is problematic in this regard, because the horizon size at that time corresponds to a size that is about 2 degrees on the sky. If the Universe has had the same expansion history since the Planck epoch, there is no mechanism to cause these regions to have the same temperature.
A resolution to this apparent inconsistency is offered by inflationary theory in which a homogeneous and isotropic scalar energy field dominates the Universe at a time 10-35 seconds after the Planck epoch. During inflation, the Universe undergoes exponential expansion, and regions in causal contact expand so as to be beyond each other's horizons. Heisenberg's uncertainty principle predicts that during the inflationary phase there would be quantum thermal fluctuations, which would be magnified to cosmic scale. These fluctuations serve as the seeds of all current structure in the Universe. After inflation, the Universe expands according to Hubble's law, and regions that were out of causal contact come back into the horizon. This explains the observed isotropy of the CMB. Inflation predicts that the primordial fluctuations are nearly scale invariant and Gaussian which has been accurately confirmed by measurements of the CMB.
Big Bang - Flatness problem
The flatness problem is an observational problem that results from considerations of the geometry associated with a Friedmann-Lemaître-Robertson-Walker metric. In general, the Universe can have three different kinds of geometries: hyperbolic geometry, Euclidean geometry, or elliptic geometry. The geometry is determined by the total energy density of the Universe (as measured by means of the stress-energy tensor): hyperbolic results from a density less than the critical density, elliptic from a density greater than the critical density, and Euclidean from exactly the critical density. The Universe is required to be within one part in 1015 of the critical density in its earliest stages. Any greater deviation would have caused either a Heat Death or a Big Crunch, and the Universe would not exist as it does today.
A possible resolution to this problem is again offered by inflationary theory. During the inflationary period, spacetime expanded to such an extent that any residual curvature associated with it would have been smoothed out to a high degree of precision. Thus, it is believed that inflation drove the Universe to be very nearly spatially flat.
Big Bang - Magnetic monopoles
The magnetic monopole objection was raised in the late 1970s. Grand unification theories predicted point defects in space that would manifest as magnetic monopoles with a density much higher than was consistent with observations, given that searches have never found any monopoles. This problem is also resolvable by cosmic inflation, which removes all point defects from the observable Universe in the same way that it drives the geometry to flatness.
Big Bang - Baryon asymmetry
It is not yet understood why the Universe has more matter than antimatter. It is generally assumed that when the Universe was young and very hot, it was in statistical equilibrium and contained equal numbers of baryons and anti-baryons. However, observations suggest that the Universe, including its most distant parts, is made almost entirely of matter. An unknown process called baryogenesis created the asymmetry. For baryogenesis to occur, the Sakharov conditions, which were laid out by Andrei Sakharov, must be satisfied. They require that baryon number be not conserved, that C-symmetry and CP-symmetry be violated, and that the Universe depart from thermodynamic equilibrium. All these conditions occur in the Standard Model, but the effect is not strong enough to explain the present baryon asymmetry. Experiments taking place at CERN near Geneva seek to trap enough anti-hydrogen to compare its spectrum with hydrogen. Any difference would be evidence of a CPT symmetry violation and therefore a Lorentz violation.
Big Bang - Globular cluster age
In the mid-1990s, observations of globular clusters appeared to be inconsistent with the Big Bang. Computer simulations that matched the observations of the stellar populations of globular clusters suggested that they were about 15 billion years old, which conflicted with the 13.7-billion-year age of the Universe. This issue was generally resolved in the late 1990s when new computer simulations, which included the effects of mass loss due to stellar winds, indicated a much younger age for globular clusters. There still remain some questions as to how accurately the ages of the clusters are measured, but it is clear that these objects are some of the oldest in the Universe.
Big Bang - Dark matter
During the 1970s and 1980s various observations (notably of galactic rotation curves) showed that there was not sufficient visible matter in the Universe to account for the apparent strength of gravitational forces within and between galaxies. This led to the idea that up to 90% of the matter in the Universe is not normal or baryonic matter but rather dark matter. In addition, assuming that the Universe was mostly normal matter led to predictions that were strongly inconsistent with observations. In particular, the Universe is far less lumpy and contains far less deuterium than can be accounted for without dark matter. While dark matter was initially controversial, it is now a widely accepted part of standard cosmology due to observations of the anisotropies in the CMB, galaxy cluster velocity dispersions, large-scale structure distributions, gravitational lensing studies, and x-ray measurements from galaxy clusters. Dark matter has only been detected through its gravitational signature; no particles that might make it up have yet been observed in laboratories. However, there are many particle physics candidates for dark matter, and several projects to detect them are underway.
Big Bang - Dark energy
In the 1990s, detailed measurements of the mass density of the Universe revealed a value that was 30% that of the critical density. Since the Universe is very nearly spatially flat, as is indicated by measurements of the cosmic microwave background, about 70% of the energy density of the Universe was left unaccounted for. This mystery now appears to be connected to another one: Independent measurements of Type Ia supernovae have revealed that the expansion of the Universe is undergoing a non-linear acceleration rather than following strictly Hubble's law. To explain this acceleration, general relativity requires that much of the Universe consist of an energy component with large negative pressure. This dark energy is now thought to make up the missing 70%. Its nature remains one of the great mysteries of the Big Bang. Possible candidates include a scalar cosmological constant and quintessence. Observations to help understand this are ongoing.
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