Journey to the Edge of the Universe
The Particle Horizon



Journey to the Edge of the Universe
The Particle Horizon

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.

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 Universe and the End of Greatness

Sky surveys and mappings of the various wavelength bands of electromagnetic radiation have yielded much information on the content and character of the universe's structure.

The organization of structure appears to follow as a hierarchical model with organization up to the scale of superclusters and filaments.


Larger than this, there seems to be no continued structure, a phenomenon which has been referred to as the End of Greatness.

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 years after the Big Bang, which occurred around 13.7 billion (13.7×109) 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.

The visible universe is a sphere with a diameter of about 28 billion parsecs (about 93 billion light-years).

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.


The universe is very large and possibly infinite in volume. The region visible from Earth (the observable universe) is about 92 billion light years across, based on where the expansion of space has taken the most distant objects observed.

For comparison, the diameter of a typical galaxy is only 30,000 light-years, and the typical distance between two neighboring galaxies is only 3 million light-years.

As an example, our Milky Way Galaxy is roughly 100,000 light years in diameter, and our nearest sister galaxy, the Andromeda Galaxy, is located roughly 2.5 million light years away. There are probably more than 100 billion galaxies in the observable universe. Typical galaxies range from dwarfs with as few as ten million stars up to giants with one trillion stars, all orbiting the galaxy's center of mass.

Thus, a very rough estimate from these numbers would suggest there are around one sextillion stars in the observable universe; though a 2003 study by Australian National University astronomers resulted in a figure of 70 sextillion.


Lost Horizons: The Big Bang


Aired: September 4th, 2008 on BBC 4

To coincide with the switch-on on Wednesday of the Large Hadron Collider, the world's largest particle accelerator complex, Professor Jim Al Khalili delves into over 50 years of the BBC science archive to tell the story behind the emergence of one of the greatest theories of modern science, the Big Bang.

The remarkable idea that our universe simply began from nothing has not always been accepted with the conviction it is today and, from fiercely disputed leftfield beginnings, took the best part of the 20th century to emerge as the triumphant explanation of how the universe began.

Using curious horn-shaped antennas, U-2 spy planes, satellites and particle accelerators, scientists have slowly pieced together the cosmological jigsaw, and this documentary charts the overwhelming evidence for a universe created by a Big Bang.


Sky surveys and mappings of the various wavelength bands of electromagnetic radiation (in particular 21-cm emission) have yielded much information on the content and character of the universe's structure.

The organization of structure appears to follow as a hierarchical model with organization up to the scale of superclusters and filaments. Larger than this, there seems to be no continued structure, a phenomenon which has been referred to as the End of Greatness.


The organization of structure arguably begins at the stellar level, though most cosmologists rarely address astrophysics on that scale. Stars are organized into galaxies, which in turn form clusters and superclusters that are separated by immense voids, creating a vast foam-like structure sometimes called the "cosmic web".

Prior to 1989, it was commonly assumed that virialized galaxy clusters were the largest structures in existence, and that they were distributed more or less uniformly throughout the universe in every direction.

However, based on redshift survey data, in 1989 Margaret Geller and John Huchra discovered the "Great Wall", a sheet of galaxies more than 500 million light-years long and 200 million wide, but only 15 million light-years thick.

The existence of this structure escaped notice for so long because it requires locating the position of galaxies in three dimensions, which involves combining location information about the galaxies with distance information from redshifts. In April 2003, another large-scale structure was discovered, the Sloan Great Wall.

The Sloan Great Wall is a giant wall of galaxies (a galactic filament) and to the present day is the largest known structure in the universe. Its discovery was announced on October 20th, 2003 by J. Richard Gott III of Princeton University and Mario Jurić and their colleagues, based on data from the Sloan Digital Sky Survey.

The wall measures 1.37 billion light years in length, which is approximately 1/60 of the diameter of the observable universe, and is located approximately one billion light-years from Earth.

In August 2007, a possible supervoid was detected in the constellation Eridanus. It coincides with the 'WMAP Cold Spot', a cold region in the microwave sky that is highly improbable under the currently favored cosmological model.

This supervoid could cause the cold spot, but to do so it would have to be improbably big, possibly a billion light-years across. In more recent studies the universe appears as a collection of giant bubble-like voids separated by sheets and filaments of galaxies, with the superclusters appearing as occasional relatively dense nodes.

In astronomy, the 2dF Galaxy Redshift Survey (Two-degree-Field Galaxy Redshift Survey), 2dF or 2dFGRS is a redshift survey conducted by the Anglo-Australian Observatory (AAO) with the 3.9m Anglo-Australian Telescope between 1997 and 11 April 2002. The data from this survey were made public on 30 June 2003.

The survey determined the large-scale structure in one section of the local Universe. As of July 2009, it is the second largest redshift survey next to the Sloan Digital Sky Survey which began in 2000. Matthew Colless, Steve Maddox and John Peacock were in charge of the project.


This network is clearly visible in the 2dF Galaxy Redshift Survey. In the figure a 3-D reconstruction of the inner parts of the survey is shown, revealing an impressive view on the cosmic structures in the nearby universe.

Several superclusters stand out, such as the Sloan Great Wall, the largest structure in the universe known to date.


The End of Greatness is an observational scale discovered at roughly 100 Mpc (roughly 300 million lightyears) where the lumpiness seen in the large-scale structure of the universe is homogenized and isotropized as per the Cosmological Principle.

The superclusters and filaments seen in smaller surveys are randomized to the extent that the smooth distribution of the universe is visually apparent. It was not until the redshift surveys of the 1990s were completed that this scale could accurately be observed.