Observable universe

Physical cosmology

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Related topics

Astrophysics General relativity Particle physics Quantum gravity

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See Universe for a general discussion of the universe.

The observable universe is a term used in cosmology to describe a ball-shaped region of space surrounding the Earth that is close enough that we might observe objects in it, i.e. there has been sufficient time for light emitted by an object to arrive at Earth. Every position has its own observable universe which may or may not overlap with the one centred around the Earth.

Both popular and professional research articles in cosmology often use the term "universe" when they really mean "observable universe". The reason for this is that unobservable physical phenomena are scientifically irrelevant; that is, they cannot affect any events that we can perceive, and therefore causally do not exist. They also cannot be measured, and therefore hypotheses about parts of the universe that are not observable may be ignored.

Size

In the sense of a comoving distance scaled to the current conditions, the universe is 13.7 billion light years in radius because the universe is 13.7 billion years old. However, space itself may expand faster than the speed of light making the physical size associated with this much larger. This occurs when space expands while a photon is in transit, hence the photon must traverse a proper distance which is greater than the Hubble distance, or the traditionally defined edge of the observable universe^[1].

There is some disagreement as to exactly how large the observable universe in proper distance is: a study of the cosmic microwave background radiation by WMAP in May 2004 states the universe is at least 78 billion light years in radius^[2], yet the March 2005 issue of Scientific American cites a figure of 46 billion light years in every direction. The ambiguity in size is dependent on the detailed models of the Hubble Law, especially the nonlinear nature of dark energy component of the universe which is causing the expansion of the universe to accelerate.

In practice we can only observe objects as far as the surface of last scattering 300,000 years after the big bang when the universe had cooled sufficiently to permit electrons to bind to atomic nuclei, which brought a halt to the Compton scattering of ambient photons, meaning that the photons can survive long enough to reach Earth. We can however infer information before this time through the detection of gravitational waves.

Contents

The observable universe contains about 5 × 10²² stars, organized in about 140 billion galaxies, which themselves form clusters and superclusters.

Two back-of-envelope calculations give the number of atoms in the observable universe to be between 1.7x10⁷⁷(1) and 4x10⁷⁹ (2).

1) The horizon size of our universe is about 14 billion light years. If we neglect space curvature effects, the volume of visible space represents 4/3 pi R³ = 8.8 x 10⁸³ cubic centimeters. The critical density of the universe for this value of the Hubble constant is 3 H^2/8 pi G, which works out to be 1.0 x 10^-29 grams/cubic centimeter or about 0.000005 atoms of hydrogen/cc. It is believed that only 4 percent of the critical density is in the form of normal atoms, so this leaves 0.000005 x 0.04 = 0.0000002 hydrogen atoms/cc. Multiplying this by the volume of the visible universe, you get about 1.7 x 10⁷⁷ hydrogen atoms [3].

2) A typical star weighs about 2x10³³ Grams, which is about 1x10⁵⁷ atoms of hydrogen per star... That is a 1 followed by 57 zeros. A typical galaxy has about 400 billion stars so that means each galaxy has 1x10⁵⁷ X 400,000,000,000 = 5x10⁶⁸ hydrogen atoms in a galaxy There are possibly 80 billion galaxies in the Universe, so that means that there are about: 5x10⁶⁸ X 80,000,000,000 = 4x10⁷⁹ hydrogen atoms in the Universe. But this is definitely a lower limit calculation, and ignores many possible atom sources [4].

See also

• Causality (physics)
• Cosmology
• Observation
• Universe

References

1. ↑  Lineweaver, Charles H., Davis, Tamara M. (March 2005). "Misconceptions about the Big Bang". Scientific American. pg 36-45.
2. ↑  Whitehouse, Dr David (May, 2004). BBC News. Astronomers size up the Universe. Retrieved January 8, 2006.fr:Horizon cosmique