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Hubble Ultra Deep Field image of a region of the observable universe (equivalent sky area size shown in bottom left corner), near the constellation Fornax. Each spot is a galaxy, consisting of billions of stars. The light from the smallest, most redshifted galaxies originated roughly 13 billion years ago.

Physical cosmology

Universe · Big Bang Age of the universe Timeline of the Big Bang Ultimate fate of the universe Early universe Inflation · Nucleosynthesis GWB · Neutrino background Cosmic microwave background Expanding universe Redshift · Hubble's law Metric expansion of space Friedmann equations FLRW metric Structure Formation Shape of the universe Structure formation Reionization Galaxy formation Large-scale structure Galaxy filaments Components Lambda-CDM model Dark energy · Dark matter Timeline Timeline of cosmological theories Timeline of the Big Bang Future of an expanding universe Experiments Observational cosmology 2dF · SDSS COBE · BOOMERanG · WMAP · Planck Scientists Isaac Newton · Einstein · Hawking · Friedman · Lemaître · Hubble · Penzias · Wilson · Gamow · Dicke · Zel'dovich · Mather · Rubin · Smoot· others

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In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion. Assuming the Universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction—that is, the observable universe is a spherical volume (a ball) centered on the observer, regardless of the shape of the Universe as a whole. The actual shape of the Universe may or may not be spherical. However, the portion of it that we (humans, from the perspective of planet Earth) are able to observe is determined by whether or not the light and other signals originating from distant objects has had time to arrive at our point of observation (planet Earth). Therefore, the observable universe appears from our perspective to be spherical. Every location in the Universe has its own observable universe which may or may not overlap with the one centered around the Earth.

The word observable used in this sense does not depend on whether modern technology actually permits detection of radiation from an object in this region (or indeed on whether there is any radiation to detect). It simply indicates that it is possible in principle for light or other signals from the object to reach an observer on Earth. In practice, we can see objects only as far as the surface of last scattering, before which the Universe was opaque to photons. However, it may be possible in the future to observe the still older neutrino background, or even more distant events via gravitational waves (which also move at the speed of light). Sometimes a distinction is made between the visible universe, which includes only signals emitted since the last scattering time, and the observable universe, which includes signals since the beginning of the cosmological expansion (the Big Bang in traditional cosmology, the end of the inflationary epoch in modern cosmology). The radius of the observable universe is about 2% larger than the radius of the visible universe by this definition.^{[citation needed]}

The age of the Universe is about 13.7 billion years, but due to the expansion of space we are now observing objects that are now considerably farther away than a static 13.7 billion light-years distance. The edge of the observable universe is now located about 46.5 billion light-years away.^[1]

Estimates of the matter content of the observable universe indicate that it contains on the order of 10⁸⁰ atoms. The vast majority of the energy density is contributed by dark matter and dark energy.^{[citation needed]}

The Universe versus the observable universe

While special relativity constrains objects in the Universe from moving faster than the speed of light with respect to each other, there is no such constraint when space itself is expanding. This means that the size of the observable universe could be smaller than the entire universe; there are some parts of the Universe which might never be close enough for the light to overcome the speed of the expansion of space, in order to be observed on Earth. Some parts of the Universe which are currently observable may later be unobservable due to ongoing expansion.^[2][3]

Some parts of the Universe may simply be too far away for the light from there to have reached Earth, but despite the expansion of space, at a later time could be observed.

Both popular and professional research articles in cosmology often use the term "Universe" to mean "observable universe". This can be justified on the grounds that we can never know anything by direct experimentation about any part of the Universe that is causally disconnected from us, although many credible theories require a total Universe much larger than the observable universe. No evidence exists to suggest that the boundary of the observable universe corresponds precisely to the physical boundary of the universe (if such a boundary exists); this is exceedingly unlikely in that it would imply that Earth is exactly at the center of the Universe, in violation of the Copernican principle. It is likely that the galaxies within our visible universe represent only a minuscule fraction of the galaxies in the Universe. According to the theory of cosmic inflation and its founder, Alan Guth, the lower bound for the diameter of the entire Universe could be at least in the range of 10²³ to 10²⁶ times as large as the observable universe.

It is also possible that the Universe is smaller than the observable universe. In this case, what we take to be very distant galaxies may actually be duplicate images of nearby galaxies, formed by light that has circumnavigated the Universe. It is difficult to test this hypothesis experimentally because different images of a galaxy would show different eras in its history, and consequently might appear quite different. A 2004 paper^[4] claims to establish a lower bound of 24 gigaparsecs (78 billion light-years) on the diameter of the whole Universe, making it, at most, only slightly smaller than the observable universe. This value is based on matching-circle analysis of the WMAP data. However, if the recent discovery of dark flow proves to be accurate, it strongly suggests that there is matter beyond the observable universe.

Size

The comoving distance from Earth to the edge of the visible universe (also called the particle horizon) is about 14 billion parsecs (46.5 billion light-years) in any direction.^[5] This defines a lower limit on the comoving radius of the observable universe, although as noted in the introduction, it is expected that the visible universe is somewhat smaller than the observable universe since we see only light from the cosmic microwave background radiation that was emitted after the time of recombination, giving us the spherical surface of last scattering (gravitational waves could theoretically allow us to observe events that occurred earlier than the time of recombination, from regions of space outside this sphere). The visible universe is thus a sphere with a diameter of about 28 billion parsecs (about 93 billion light-years).

Assuming that space is roughly flat, this size corresponds to a comoving volume of about 3×10⁸⁰ cubic meters. This is equivalent to a volume of about 41 decillion cubic light-years short scale (4.1 X 10³⁴ cubic light years).

The figures quoted above are distances now (in cosmological time), not distances at the time the light was emitted. For example, the cosmic microwave background radiation that we see right now was emitted at the time of recombination, 379,000^[6] years after the Big Bang, which occurred around 13.7 billion years ago. This radiation was emitted by matter that has, in the intervening time, mostly condensed into galaxies, and those galaxies are now calculated to be about 46 billion light-years from us. To estimate the distance to that matter at the time the light was emitted, a mathematical model of the expansion must be chosen and the scale factor, a(t), calculated for the selected time since the Big Bang, t. For the observationally-favoured Lambda-CDM model, using data from the WMAP spacecraft, such a calculation yields a scale factor change of approximately 1292. This means the Universe has expanded to 1292 times the size it was when the CMBR photons were released. Hence, the most distant matter that is observable at present, 46 billion light-years away, was only 36 million light-years away from the matter that would eventually become Earth when the microwaves we are currently receiving were emitted.

Misconceptions

Many secondary sources have reported a wide variety of incorrect figures for the size of the visible universe. Some of these figures are listed below, with brief descriptions of possible reasons for misconceptions about them.

• 13.7 billion light-years. The age of the Universe is about 13.7 billion years. While it is commonly understood that nothing travels faster than light, it is a common misconception that the radius of the observable universe must therefore amount to only 13.7 billion light-years. This reasoning makes sense only if the Universe is the flat spacetime of special relativity; in the real Universe, spacetime is highly curved on cosmological scales, which means that 3-space (which is roughly flat) is expanding, as evidenced by Hubble's law. Distances obtained as the speed of light multiplied by a cosmological time interval have no direct physical significance.^[7]
• 15.8 billion light-years. This is obtained in the same way as the 13.7 billion light year figure, but starting from an incorrect age of the Universe which was reported in the popular press in mid-2006.^[8][9][10] For an analysis of this claim and the paper that prompted it, see.^[11]
• 27.4 billion light-years. This is a diameter obtained from the (incorrect) radius of 13.7 billion light-years.
• 78 billion light-years. This is a lower bound for the diameter of the whole Universe, based on the estimated current distance between points that we can see on opposite sides of the cosmic microwave background radiation (CMBR). If the whole Universe is smaller than this sphere, then light has had time to circumnavigate it since the big bang, producing multiple images of distant points in the CMBR, which would show up as patterns of repeating circles.^[12] Cornish et al. looked for such an effect at scales of up to 24 gigaparsecs (78 billion light years) and failed to find it, and suggested that if they could extend their search to all possible orientations, they would then "be able to exclude the possibility that we live in a Universe smaller than 24 Gpc in diameter". The authors also estimated that with "lower noise and higher resolution CMB maps (from WMAP's extended mission and from Planck), we will be able to search for smaller circles and extend the limit to ~28 Gpc."^[4] This estimate of the maximum diameter of the CMBR sphere that will be visible in planned experiments corresponds to a radius of 14 gigaparsecs, the same number given in the previous section.
• 156 billion light-years. This figure was obtained by doubling 78 billion light-years on the assumption that it is a radius. Since 78 billion light-years is already a diameter, the doubled figure is incorrect. This figure was very widely reported.^[13][14][15]
• 180 billion light-years. This estimate accompanied the age estimate of 15.8 billion years in some sources;^[16] it was obtained by adding 15% to the figure of 156 billion light years.

Even prominent physicists have made errors here. See Tamara Davis's 2004 Ph.D. for details at http://www.physics.uq.edu.au/download/tamarad/papers/thesis_complete.pdf

Matter content

The observable universe contains about 3 to 7 × 10²² stars (30 to 70 sextillion stars),^[17] organized in more than 80 billion galaxies, which themselves form clusters and superclusters.^[18]

Two approximate calculations give the number of atoms in the observable universe to be a minimum of 10⁸⁰.

1. Observations of the cosmic microwave background from the Wilkinson Microwave Anisotropy Probe suggest that the spatial curvature of the Universe is very close to zero, which in current cosmological models implies that the value of the density parameter must be very close to a certain critical value. This works out to 9.9×10⁻²⁷ kg/m³,^[19] which would be equal to about 5.9 hydrogen atoms per cubic meter. Analysis of the WMAP results suggests that only about 4.6% of the critical density is in the form of normal atoms, while 23% is thought to be made of cold dark matter and 72% is thought to be dark energy,^[19] so this leaves 0.27 hydrogen atoms/m³. Multiplying this by the volume of the visible universe, you get about 8×10⁷⁹ hydrogen atoms.
2. A typical star has a mass of about 2×10³⁰ kg, which is about 1×10⁵⁷ atoms of hydrogen per star. A typical galaxy has about 400 billion stars so that means each galaxy has 1×10⁵⁷ × 4×10¹¹ = 4×10⁶⁸ hydrogen atoms. There are possibly 80 billion galaxies in the Universe, so that means that there are about 4×10⁶⁸ × 8×10¹⁰ = 3×10⁷⁹ hydrogen atoms in the observable universe. But this is definitely a lower limit calculation, and it ignores many possible atom sources such as intergalactic gas.^[20]

Mass

The mass of the matter in the observable universe can be estimated based on density and size.^[21]

Estimation based on the measured stellar density

One way to calculate the mass of the visible matter which makes up the observable universe is to assume a mean stellar mass and to multiply that by an estimate of the number of stars in the observable universe. The estimate of the number of stars in the Universe is derived from the volume of the observable universe

and a stellar density calculated from observations by the Hubble Space Telescope

yielding an estimate of the number of stars in the observable universe of 9 × 10²¹ stars (9 billion trillion stars).

Taking the mass of Sol (2 × 10³⁰ kg) as the mean stellar mass (on the basis that the large population of dwarf stars balances out the population of stars whose mass is greater than Sol) and rounding the estimate of the number of stars up to 10²² yields a total mass for all the stars in the observable universe of 3 × 10⁵² kg.^[22] However, as noted in the "matter content" section, the WMAP results in combination with the Lambda-CDM model predict that less than 5% of the total mass of the observable universe is made up of visible matter such as stars, the rest being made up of dark matter and dark energy.

Sir Fred Hoyle calculated the mass of an observable steady-state universe using the formula:^[23]

which can also be stated as

or approximately 8 × 10⁵² kg.

Here H = Hubble constant, ρ = Hoyle's value for the density, G = gravitational constant and c = speed of light.

Most distant objects

The most distant astronomical object observed as of 2009 is a gamma ray burst, most likely caused by a star which collapsed when the universe was approximately 600 million years old.^[24]

Diameter of entire universe

As noted above, according to the theory of cosmic inflation and its founder, Alan Guth, the entire cosmos could be at least 10²³ to 10²⁶ times as large as the observable cosmos. Rounding off the 9.3 X 10¹⁰ light year diameter of the visible cosmos given above to 10¹¹ light years, multiplying this by 10²³ gives 10³⁴ and multiplying this by 10²⁶gives 10³⁷. Thus a ballpark estimate of a lower bound for the diameter of the entire cosmos (assuming the theory of cosmic inflation to be valid) would be between 10³⁴ light years and 10³⁷ light years, that is, somewhere between 10 decillion and 10 undecillion light years (short scale).

This is equivalent to (assuming the inflation theory is valid) a volume of between 439 duotrigintillion cubic light-years and 439 quinquatrigintillion cubic light-years (short scale) (Between 4.39 X 10¹⁰¹ cubic light years and 4.39 X 10¹¹⁰ cubic light years)(again, this is a lower bound).

Using the rounded off figure of 10¹¹ light years for the diameter of the observable cosmos and the lower figure of 10³⁴ light years for the diameter of the entire cosmos means that there is a difference of at least 23 orders of magnitude between the size of the observable cosmos and the size of the entire cosmos, which is equivalent to the difference in size between a proton and the planet Earth.^[25]

See also

References

External links

• Cosmology FAQ
• Hubble, VLT and Spitzer Capture Galaxy Formation in the Early Universe
• Star Survey reaches 70 sextillion
• Inflation and the Cosmic Microwave Background, Lineweaver 2003
• Animation of the cosmic light horizon
• Logarithmic Maps of the Universe