Astrophysics (Index)About

Big Bang

(theoretical event when the universe expanded from a point)

The Big Bang is a theoretical event when the expansion of the universe began from a single point, i.e., began assuming spatial dimensions. Observation shows the observable universe expanding, and if that expansion has "always" happened, projecting back sufficient time, there would be an instant when the universe expanded from a single point. Current observation places the moment at about 13.8 billion years ago, a precise determination requiring knowledge of the expansion rate over the course of this expansion, i.e., knowing the behavior of the Hubble parameter over time.

Though the finite speed of EMR allows us to observe past times by looking long distances, our ability to observe detail all the way to this theoretical Big Bang is limited, limiting what we can say about the very beginning. However, the notion that expansion has persisted since the general time indicated by observed expansion (the Big Bang theory) is well-established by observation and generally accepted, and no alternative scenario to a Big-Bang instant has any more evidence. Cosmology does explore scenarios all the way back to the apparent instant of the Big Bang, applying the established laws of physics. Some observed characteristics of the universe today are consistent with events that experimentally-established physical laws suggest would happen in that era, all the way back to a small fraction of a second after such a Big Bang.

Before the Big Bang was postulated, the tacit assumption was that the universe remains the same, and when a notion of expansion was seriously introduced (which was based upon observation and was something of a shock), the older assumption came to be termed a steady-state universe. It was this older assumption that had led Einstein to include a feature in his general relativity (GR) theory to keep the universe the same size (i.e., the cosmological constant in GR's field equation). Then when the universe's apparent expansion become evident, Einstein set aside the feature. However, for some time, serious theories of how the universe might be "steady state" continued to be developed. Eventually, features of the universe were observed that could be explained by the Big Bang theory but not by competing theories.

The term standard Big Bang (SBB) refers to a "working model" version of the Big Bang theory specifically assuming that physics and physical constants at the time were the same as now. (Other theories might speculate that some constants have varied according to the size/spread of the universe.)


Note there have been further theoretical developments beyond the presumption of a Big Bang, and now the term Big Bang theory often means the current such theory, including the later developments that are well-accepted, such as inflation, dark matter, etc.


(event,cosmology,early universe)
Further reading:
https://en.wikipedia.org/wiki/Big_Bang
https://astronomy.swin.edu.au/cosmos/b/big+bang
http://hyperphysics.phy-astr.gsu.edu/hbase/Astro/bbang.html
https://phys.org/news/2015-12-big-theory.html
http://spiff.rit.edu/classes/phys240/lectures/bb/bb.html
RedshiftParsecs
/Distance
Lightyears
/Lookback Years
  
~inf4.29Gpc13.98GlyBig Bang

Referenced by pages:
abundances
alternative cosmologies
AREPO
AzTEC-3
Big Bang nucleosynthesis (BBN)
Big Crunch
black hole (BH)
cosmic microwave background (CMB)
cosmic neutrino background (CNB)
cosmic time
cosmological model
cosmology
dark age
dark matter annihilation
decoupling
early universe
epoch of reionization (EOR)
false vacuum
first galaxies
freeze-out
helium (He)
Illustris Project
inflation
initial fluctuations
isotropy
Lambda-CDM model (ΛCDM)
light cone
lithium (Li)
mass fraction
nucleosynthesis
observable universe
Population III (Pop III)
radiation era
recombination
relic
solar neutrino
star formation rate (SFR)
wCDM

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