Observational cosmology
Encyclopedia
Observational cosmology is the study of the structure, the evolution and the origin of the universe
through observation
, using instruments such as telescope
s and cosmic ray
detectors.
as it is practiced today had its subject material defined in the years following the Shapley-Curtis debate when it was determined that the universe
had a larger scale than the Milky Way galaxy. This was precipitated by observations that established the size and the dynamics of the cosmos that could be explained by Einstein's General Theory of Relativity. In its infancy, cosmology was a speculative science based on a very limited number of observations and characterized by a dispute between steady state
theorists and promoters of Big Bang
cosmology. It was not until the 1990s and beyond that the astronomical observations would be able to eliminate competing theories and drive the science to the "Golden Age of Cosmology" which was heralded by David Schramm at a National Academy of Sciences
colloquium in 1992.
can be used to measure the distance to nearby stars, the observational limits imposed by the difficulty in measuring the minuscule parallaxes associated with objects beyond our galaxy meant that astronomers had to look for alternative ways to measure cosmic distances. To this end, a standard candle measurement for Cepheid variable
s was discovered by Henrietta Swan Leavitt
in 1908 which would provide Edwin Hubble
with the rung on the cosmic distance ladder
he would need to determine the distance to spiral nebula. When Hubble trained the 100-inch Hooker Telescope at Mount Wilson Observatory
, he was able to identify individual star
s in those galaxies
, and determine the distance to the galaxies by isolating individual Cepheids. This firmly established the spiral nebula as being objects well outside the Milky Way galaxy. Determining the distance to "island universes", as they were dubbed in the popular media, established the scale of the universe and settled the Shapley-Curtis debate once and for all.
By combining his distance measurements with Vesto Slipher
's determinations of redshift
s for these objects, Hubble was able to determine a correlation between the distance to galaxies and what was termed their "recessional velocities". This correlation would come to be known as Hubble's Law
and would serve as the observational foundation for the expanding universe theories
on which cosmology is still based. The publication of Hubble's data and the acceptance by the theorists of its theoretical implications in light of Einstein's General theory of relativity is considered the beginning of the modern science of cosmology.
measurements of light from astronomical objects and the identification of emission and absorption lines which corresponded to particular electronic transitions in chemical element
s identified on Earth. For example, the element Helium
was first identified through its spectroscopic signature in the Sun
before it was isolated as a gas on Earth.
Computing relative abundances was achieved through corresponding spectroscopic observations to measurements of the elemental composition of meteorite
s.
and Ralph Alpher, and by Alpher and Robert Herman
as due to the hot big bang
model. Moreover, Alpher and Herman were able to estimate the temperature, but their results were not widely discussed in the community. Their prediction was rediscovered by Robert Dicke and Yakov Zel'dovich in the early 1960s with the first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by Soviet
astrophysicists A. G. Doroshkevich
and Igor Novikov
, in the spring of 1964. In 1964, David Todd Wilkinson
and Peter Roll, Dicke's colleagues at Princeton University
, began constructing a Dicke radiometer to measure the cosmic microwave background. In 1965, Arno Penzias and Robert Woodrow Wilson
at the Crawford Hill
location of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey
had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. Their instrument had an excess 3.5 K antenna temperature which they could not account for. After receiving a telephone call from Crawford Hill, Dicke famously quipped: "Boys, we've been scooped." A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background. Penzias and Wilson received the 1978 Nobel Prize in Physics
for their discovery.
has heavily influenced theoretical modeling of structure
and galaxy formation. When trying to calibrate the Hubble diagram with accurate supernova
standard candles, observational evidence for dark energy
was obtained in the late 1990s. These observations have been incorporated into a six-parameter framework known as the Lambda-CDM model
which explains the evolution of the universe in terms of its constituent material. This model has subsequently been verified by detailed observations of the cosmic microwave background, especially through the WMAP experiment.
Included here are the modern observational efforts that have directly influenced cosmology.
s and improvements in spectroscopes
, a number of collaborations have been made to map the universe in redshift
space. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure properties of the large-scale structure of the universe. The Great Wall
, a vast supercluster
of galaxies over 500 million light-year
s wide, provides a dramatic example of a large-scale structure that redshift surveys can detect.
The first redshift survey was the CfA Redshift Survey
, started in 1977 with the initial data collection completed in 1982. More recently, the 2dF Galaxy Redshift Survey
determined the large-scale structure of one section of the Universe, measuring z-values for over 220,000 galaxies; data collection was completed in 2002, and the final data set
was released 30 June 2003. (In addition to mapping large-scale patterns of galaxies, 2dF established an upper limit on neutrino
mass.) Another notable investigation, the Sloan Digital Sky Survey
(SDSS), is ongoing and aims to obtain measurements on around 100 million objects. SDSS has recorded redshifts for galaxies as high as 0.4, and has been involved in the detection of quasar
s beyond z = 6. The DEEP2 Redshift Survey
uses the Keck telescopes
with the new "DEIMOS" spectrograph
; a follow-up to the pilot program DEEP1, DEEP2 is designed to measure faint galaxies with redshifts 0.7 and above, and it is therefore planned to provide a complement to SDSS and 2dF.
Cosmic Background Explorer (COBE
) satellite that orbited in 1989–1996 and which detected and quantified the large scale anisotropies at the limit of its detection capabilities. Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation
was the right theory. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment
reported that the highest power fluctuations occur at scales of apporoximately one degree. Together with other cosmological data, these results implied that the geometry of the Universe is flat. A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array
,
Degree Angular Scale Interferometer
(DASI) and the Cosmic Background Imager
(CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode spectrum with compelling evidence that it is out of phase with the T-mode spectrum.
In June 2001, NASA
launched a second CMB space mission, WMAP, to make much more precise measurements of the large scale anisotropies over the full sky. The first results from this mission, disclosed in 2003, were detailed measurements of the angular power spectrum to below degree scales, tightly constraining various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation
as well as various other competing theories, and are available in detail at NASA's data center for Cosmic Microwave Background (CMB) (see links below). Although WMAP provided very accurate measurements of the large angular-scale fluctuations in the CMB (structures about as large in the sky as the moon), it did not have the angular resolution to measure the smaller scale fluctuations which had been observed using previous ground-based interferometers.
A third space mission, Planck, was launched in May 2009. Planck employs both HEMT
radiometers as well as bolometer
technology and measures the CMB anisotropies at a higher resolution than WMAP. Unlike the previous two space missions, Planck is a collaboration between NASA and ESA
(the European Space Agency). Its detectors got a trial run at the Antarctic Viper telescope
as ACBAR (Arcminute Cosmology Bolometer Array Receiver
) experiment – which has produced the most precise measurements at small angular scales to date – and at the Archeops
balloon telescope.
Additional ground-based instruments such as the South Pole Telescope
in Antarctica and the proposed Clover
Project, Atacama Cosmology Telescope
and the QUIET telescope in Chile
will provide additional data not available from satellite observations, possibly including the B-mode polarization.
which can be observed out to extremely high redshifts. These are subsets of the active galaxies that have extended features known as lobes and jets which extend away form the galactic nucleus distances on the order of megaparsecs. Because radio galaxies are so bright, astronomers have used them to probe extreme distances and early times in the evolution of the universe.
observations including submillimeter astronomy have revealed a number of sources at cosmological distances. With the exception of a few atmospheric windows, most of infrared light is blocked by the atmosphere, the observations generally take place from balloon or space-based instruments. Current observational experiments in the infrared include NICMOS, the Cosmic Origins Spectrograph
, the Spitzer Space Telescope
, the Keck Interferometer, the Stratospheric Observatory For Infrared Astronomy
, and the Herschel Space Observatory
. The next large space telescope planned by NASA, the James Webb Space Telescope
will also explore in the infrared.
model that the universe is filled with a neutrino background radiation
, analogous to the cosmic microwave background radiation
. The microwave background is a relic from when the universe was about 380,000 years old, but the neutrino background is a relic from when the universe was about two seconds old.
If this neutrino radiation could be observed, it would be a window into very early stages of the universe. Unfortunately, these neutrinos would now be very cold, and so they are effectively impossible to observe directly.
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
through observation
Observation
Observation is either an activity of a living being, such as a human, consisting of receiving knowledge of the outside world through the senses, or the recording of data using scientific instruments. The term may also refer to any data collected during this activity...
, using instruments such as telescope
Telescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s and cosmic ray
Cosmic ray
Cosmic rays are energetic charged subatomic particles, originating from outer space. They may produce secondary particles that penetrate the Earth's atmosphere and surface. The term ray is historical as cosmic rays were thought to be electromagnetic radiation...
detectors.
Early observations
The science of physical cosmologyPhysical cosmology
Physical cosmology, as a branch of astronomy, is the study of the largest-scale structures and dynamics of the universe and is concerned with fundamental questions about its formation and evolution. For most of human history, it was a branch of metaphysics and religion...
as it is practiced today had its subject material defined in the years following the Shapley-Curtis debate when it was determined that the universe
Universe
The Universe is commonly defined as the totality of everything that exists, including all matter and energy, the planets, stars, galaxies, and the contents of intergalactic space. Definitions and usage vary and similar terms include the cosmos, the world and nature...
had a larger scale than the Milky Way galaxy. This was precipitated by observations that established the size and the dynamics of the cosmos that could be explained by Einstein's General Theory of Relativity. In its infancy, cosmology was a speculative science based on a very limited number of observations and characterized by a dispute between steady state
Steady state
A system in a steady state has numerous properties that are unchanging in time. This implies that for any property p of the system, the partial derivative with respect to time is zero:...
theorists and promoters of Big Bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
cosmology. It was not until the 1990s and beyond that the astronomical observations would be able to eliminate competing theories and drive the science to the "Golden Age of Cosmology" which was heralded by David Schramm at a National Academy of Sciences
United States National Academy of Sciences
The National Academy of Sciences is a corporation in the United States whose members serve pro bono as "advisers to the nation on science, engineering, and medicine." As a national academy, new members of the organization are elected annually by current members, based on their distinguished and...
colloquium in 1992.
Edwin Hubble and the cosmic distance ladder
Distance measurements in astronomy have historically been and continue to be confounded by considerable measurement uncertainty. In particular, while stellar parallaxStellar parallax
Stellar parallax is the effect of parallax on distant stars in astronomy. It is parallax on an interstellar scale, and it can be used to determine the distance of Earth to another star directly with accurate astrometry...
can be used to measure the distance to nearby stars, the observational limits imposed by the difficulty in measuring the minuscule parallaxes associated with objects beyond our galaxy meant that astronomers had to look for alternative ways to measure cosmic distances. To this end, a standard candle measurement for Cepheid variable
Cepheid variable
A Cepheid is a member of a class of very luminous variable stars. The strong direct relationship between a Cepheid variable's luminosity and pulsation period, secures for Cepheids their status as important standard candles for establishing the Galactic and extragalactic distance scales.Cepheid...
s was discovered by Henrietta Swan Leavitt
Henrietta Swan Leavitt
Henrietta Swan Leavitt was an American astronomer. A graduate of Radcliffe College, Leavitt went to work in 1893 at the Harvard College Observatory in a menial capacity as a "computer", assigned to count images on photographic plates...
in 1908 which would provide Edwin Hubble
Edwin Hubble
Edwin Powell Hubble was an American astronomer who profoundly changed the understanding of the universe by confirming the existence of galaxies other than the Milky Way - our own galaxy...
with the rung on the cosmic distance ladder
Cosmic distance ladder
The cosmic distance ladder is the succession of methods by which astronomers determine the distances to celestial objects. A real direct distance measurement of an astronomical object is possible only for those objects that are "close enough" to Earth...
he would need to determine the distance to spiral nebula. When Hubble trained the 100-inch Hooker Telescope at Mount Wilson Observatory
Mount Wilson Observatory
The Mount Wilson Observatory is an astronomical observatory in Los Angeles County, California, United States. The MWO is located on Mount Wilson, a 5,715 foot peak in the San Gabriel Mountains near Pasadena, northeast of Los Angeles...
, he was able to identify individual star
Star
A star is a massive, luminous sphere of plasma held together by gravity. At the end of its lifetime, a star can also contain a proportion of degenerate matter. The nearest star to Earth is the Sun, which is the source of most of the energy on Earth...
s in those galaxies
Galaxy
A galaxy is a massive, gravitationally bound system that consists of stars and stellar remnants, an interstellar medium of gas and dust, and an important but poorly understood component tentatively dubbed dark matter. The word galaxy is derived from the Greek galaxias , literally "milky", a...
, and determine the distance to the galaxies by isolating individual Cepheids. This firmly established the spiral nebula as being objects well outside the Milky Way galaxy. Determining the distance to "island universes", as they were dubbed in the popular media, established the scale of the universe and settled the Shapley-Curtis debate once and for all.
By combining his distance measurements with Vesto Slipher
Vesto Slipher
Vesto Melvin Slipher was an American astronomer. His brother Earl C. Slipher was also an astronomer and a director at the Lowell Observatory....
's determinations of redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
s for these objects, Hubble was able to determine a correlation between the distance to galaxies and what was termed their "recessional velocities". This correlation would come to be known as Hubble's Law
Hubble's law
Hubble's law is the name for the astronomical observation in physical cosmology that: all objects observed in deep space are found to have a doppler shift observable relative velocity to Earth, and to each other; and that this doppler-shift-measured velocity, of various galaxies receding from...
and would serve as the observational foundation for the expanding universe theories
Metric expansion of space
The metric expansion of space is the increase of distance between distant parts of the universe with time. It is an intrinsic expansion—that is, it is defined by the relative separation of parts of the universe and not by motion "outward" into preexisting space...
on which cosmology is still based. The publication of Hubble's data and the acceptance by the theorists of its theoretical implications in light of Einstein's General theory of relativity is considered the beginning of the modern science of cosmology.
Nuclide abundances
Determination of the cosmic abundance of elements has a history dating back to early spectroscopicSpectroscopy
Spectroscopy is the study of the interaction between matter and radiated energy. Historically, spectroscopy originated through the study of visible light dispersed according to its wavelength, e.g., by a prism. Later the concept was expanded greatly to comprise any interaction with radiative...
measurements of light from astronomical objects and the identification of emission and absorption lines which corresponded to particular electronic transitions in chemical element
Chemical element
A chemical element is a pure chemical substance consisting of one type of atom distinguished by its atomic number, which is the number of protons in its nucleus. Familiar examples of elements include carbon, oxygen, aluminum, iron, copper, gold, mercury, and lead.As of November 2011, 118 elements...
s identified on Earth. For example, the element Helium
Helium
Helium is the chemical element with atomic number 2 and an atomic weight of 4.002602, which is represented by the symbol He. It is a colorless, odorless, tasteless, non-toxic, inert, monatomic gas that heads the noble gas group in the periodic table...
was first identified through its spectroscopic signature in the Sun
Sun
The Sun is the star at the center of the Solar System. It is almost perfectly spherical and consists of hot plasma interwoven with magnetic fields...
before it was isolated as a gas on Earth.
Computing relative abundances was achieved through corresponding spectroscopic observations to measurements of the elemental composition of meteorite
Meteorite
A meteorite is a natural object originating in outer space that survives impact with the Earth's surface. Meteorites can be big or small. Most meteorites derive from small astronomical objects called meteoroids, but they are also sometimes produced by impacts of asteroids...
s.
Detection of the cosmic microwave background
A cosmic microwave background was predicted in 1948 by George GamowGeorge Gamow
George Gamow , born Georgiy Antonovich Gamov , was a Russian-born theoretical physicist and cosmologist. He discovered alpha decay via quantum tunneling and worked on radioactive decay of the atomic nucleus, star formation, stellar nucleosynthesis, Big Bang nucleosynthesis, cosmic microwave...
and Ralph Alpher, and by Alpher and Robert Herman
Robert Herman
Robert Herman was a United States scientist, best known for his work with Ralph Alpher in 1948-50, on estimating the temperature of cosmic microwave background radiation from the Big Bang explosion....
as due to the hot big bang
Big Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
model. Moreover, Alpher and Herman were able to estimate the temperature, but their results were not widely discussed in the community. Their prediction was rediscovered by Robert Dicke and Yakov Zel'dovich in the early 1960s with the first published recognition of the CMB radiation as a detectable phenomenon appeared in a brief paper by Soviet
Soviet Union
The Soviet Union , officially the Union of Soviet Socialist Republics , was a constitutionally socialist state that existed in Eurasia between 1922 and 1991....
astrophysicists A. G. Doroshkevich
A. G. Doroshkevich
A. G. Doroshkevich is a Russian theoretical astrophysicist and cosmologist.He is best known for his work with Igor Novikov on the recognition of Cosmic microwave background radiation as a detectable phenomenon in the spring of 1964....
and Igor Novikov
Igor Dmitriyevich Novikov
Igor Dmitriyevich Novikov is a Russian theoretical astrophysicist and cosmologist.Novikov formulated the Novikov self-consistency principle in the mid-1980s, an important contribution to the theory of time travel.Novikov gained his Ph.D. degree in astrophysics in 1965 and the Russian D.Sc....
, in the spring of 1964. In 1964, David Todd Wilkinson
David Todd Wilkinson
David Todd Wilkinson was a world-renowned pioneer in the field of cosmology, specializing in the study of the cosmic microwave background radiation left over from the Big Bang. He was born in Hillsdale, Michigan, and earned his Ph.D. in physics at the University of Michigan under the supervision...
and Peter Roll, Dicke's colleagues at Princeton University
Princeton University
Princeton University is a private research university located in Princeton, New Jersey, United States. The school is one of the eight universities of the Ivy League, and is one of the nine Colonial Colleges founded before the American Revolution....
, began constructing a Dicke radiometer to measure the cosmic microwave background. In 1965, Arno Penzias and Robert Woodrow Wilson
Robert Woodrow Wilson
For the American President, see Woodrow Wilson.Robert Woodrow Wilson is an American astronomer, 1978 Nobel laureate in physics, who with Arno Allan Penzias discovered in 1964 the cosmic microwave background radiation...
at the Crawford Hill
Crawford Hill
Crawford Hill is located in Holmdel Township, New Jersey, United States. It is Monmouth County's highest point, standing at least 380 feet above sea level...
location of Bell Telephone Laboratories in nearby Holmdel Township, New Jersey
Holmdel Township, New Jersey
Holmdel Township is a township in Monmouth County, New Jersey, United States. As of the 2010 United States Census, the township population was 16,773. Holmdel Township was formed by an Act of the New Jersey Legislature on February 23, 1857, from portions of Raritan Township .Holmdel is a suburb of...
had built a Dicke radiometer that they intended to use for radio astronomy and satellite communication experiments. Their instrument had an excess 3.5 K antenna temperature which they could not account for. After receiving a telephone call from Crawford Hill, Dicke famously quipped: "Boys, we've been scooped." A meeting between the Princeton and Crawford Hill groups determined that the antenna temperature was indeed due to the microwave background. Penzias and Wilson received the 1978 Nobel Prize in Physics
Nobel Prize in Physics
The Nobel Prize in Physics is awarded once a year by the Royal Swedish Academy of Sciences. It is one of the five Nobel Prizes established by the will of Alfred Nobel in 1895 and awarded since 1901; the others are the Nobel Prize in Chemistry, Nobel Prize in Literature, Nobel Peace Prize, and...
for their discovery.
Modern observations
Today, observational cosmology continues to test the predictions of theoretical cosmology and has led to the refinement of cosmological models. For example, the observational evidence for dark matterDark matter
In astronomy and cosmology, dark matter is matter that neither emits nor scatters light or other electromagnetic radiation, and so cannot be directly detected via optical or radio astronomy...
has heavily influenced theoretical modeling of structure
Structure formation
Structure formation refers to a fundamental problem in physical cosmology. The universe, as is now known from observations of the cosmic microwave background radiation, began in a hot, dense, nearly uniform state approximately 13.7 Gyr ago...
and galaxy formation. When trying to calibrate the Hubble diagram with accurate supernova
Supernova
A supernova is a stellar explosion that is more energetic than a nova. It is pronounced with the plural supernovae or supernovas. Supernovae are extremely luminous and cause a burst of radiation that often briefly outshines an entire galaxy, before fading from view over several weeks or months...
standard candles, observational evidence for dark energy
Dark energy
In physical cosmology, astronomy and celestial mechanics, dark energy is a hypothetical form of energy that permeates all of space and tends to accelerate the expansion of the universe. Dark energy is the most accepted theory to explain recent observations that the universe appears to be expanding...
was obtained in the late 1990s. These observations have been incorporated into a six-parameter framework known as the Lambda-CDM model
Lambda-CDM model
ΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...
which explains the evolution of the universe in terms of its constituent material. This model has subsequently been verified by detailed observations of the cosmic microwave background, especially through the WMAP experiment.
Included here are the modern observational efforts that have directly influenced cosmology.
Redshift surveys
With the advent of automated telescopeTelescope
A telescope is an instrument that aids in the observation of remote objects by collecting electromagnetic radiation . The first known practical telescopes were invented in the Netherlands at the beginning of the 1600s , using glass lenses...
s and improvements in spectroscopes
Astronomical spectroscopy
Astronomical spectroscopy is the technique of spectroscopy used in astronomy. The object of study is the spectrum of electromagnetic radiation, including visible light, which radiates from stars and other celestial objects...
, a number of collaborations have been made to map the universe in redshift
Redshift
In physics , redshift happens when light seen coming from an object is proportionally increased in wavelength, or shifted to the red end of the spectrum...
space. By combining redshift with angular position data, a redshift survey maps the 3D distribution of matter within a field of the sky. These observations are used to measure properties of the large-scale structure of the universe. The Great Wall
Great Wall (astronomy)
The Great Wall , sometimes specifically referred to as the CfA2 Great Wall, is one of the largest known super-structures in the Universe...
, a vast supercluster
Supercluster
Superclusters are large groups of smaller galaxy groups and clusters and are among the largest known structures of the cosmos. They are so large that they are not gravitationally bound and, consequently, partake in the Hubble expansion.-Existence:...
of galaxies over 500 million light-year
Light-year
A light-year, also light year or lightyear is a unit of length, equal to just under 10 trillion kilometres...
s wide, provides a dramatic example of a large-scale structure that redshift surveys can detect.
The first redshift survey was the CfA Redshift Survey
CfA Redshift Survey
The Center for Astrophysics Redshift Survey was the first attempt to map the large-scale structure of the universe. It began in 1977 with the initial data collection completed in 1982...
, started in 1977 with the initial data collection completed in 1982. More recently, the 2dF Galaxy Redshift Survey
2dF Galaxy Redshift Survey
In astronomy, the 2dF Galaxy Redshift Survey , 2dF or 2dFGRS is a redshift survey conducted by the Anglo-Australian Observatory 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...
determined the large-scale structure of one section of the Universe, measuring z-values for over 220,000 galaxies; data collection was completed in 2002, and the final data set
Data set
A data set is a collection of data, usually presented in tabular form. Each column represents a particular variable. Each row corresponds to a given member of the data set in question. Its values for each of the variables, such as height and weight of an object or values of random numbers. Each...
was released 30 June 2003. (In addition to mapping large-scale patterns of galaxies, 2dF established an upper limit on neutrino
Neutrino
A neutrino is an electrically neutral, weakly interacting elementary subatomic particle with a half-integer spin, chirality and a disputed but small non-zero mass. It is able to pass through ordinary matter almost unaffected...
mass.) Another notable investigation, the Sloan Digital Sky Survey
Sloan Digital Sky Survey
The Sloan Digital Sky Survey or SDSS is a major multi-filter imaging and spectroscopic redshift survey using a dedicated 2.5-m wide-angle optical telescope at Apache Point Observatory in New Mexico, United States. The project was named after the Alfred P...
(SDSS), is ongoing and aims to obtain measurements on around 100 million objects. SDSS has recorded redshifts for galaxies as high as 0.4, and has been involved in the detection of quasar
Quasar
A quasi-stellar radio source is a very energetic and distant active galactic nucleus. Quasars are extremely luminous and were first identified as being high redshift sources of electromagnetic energy, including radio waves and visible light, that were point-like, similar to stars, rather than...
s beyond z = 6. The DEEP2 Redshift Survey
DEEP2 Redshift Survey
The DEEP2 Survey or DEEP2 is a Redshift survey of the Redshift~1 universe. It uses the Keck telescopes to measure the spectra and hence redshifts of approximately 60,000 galaxies....
uses the Keck telescopes
Keck telescopes
The W. M. Keck Observatory is a two-telescope astronomical observatory at an elevation of near the summit of Mauna Kea in Hawai'i. The primary mirrors of each of the two telescopes are in diameter, making them the second largest optical telescopes in the world, slightly behind the Gran Telescopio...
with the new "DEIMOS" spectrograph
Spectrograph
A spectrograph is an instrument that separates an incoming wave into a frequency spectrum. There are several kinds of machines referred to as spectrographs, depending on the precise nature of the waves...
; a follow-up to the pilot program DEEP1, DEEP2 is designed to measure faint galaxies with redshifts 0.7 and above, and it is therefore planned to provide a complement to SDSS and 2dF.
Cosmic microwave background experiments
Subsequent to the discovery of the CMB, hundreds of cosmic microwave background experiments have been conducted to measure and characterize the signatures of the radiation. The most famous experiment is probably the NASANASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
Cosmic Background Explorer (COBE
COBE
The COsmic Background Explorer , also referred to as Explorer 66, was a satellite dedicated to cosmology. Its goals were to investigate the cosmic microwave background radiation of the universe and provide measurements that would help shape our understanding of the cosmos.This work provided...
) satellite that orbited in 1989–1996 and which detected and quantified the large scale anisotropies at the limit of its detection capabilities. Inspired by the initial COBE results of an extremely isotropic and homogeneous background, a series of ground- and balloon-based experiments quantified CMB anisotropies on smaller angular scales over the next decade. The primary goal of these experiments was to measure the angular scale of the first acoustic peak, for which COBE did not have sufficient resolution. These measurements were able to rule out cosmic strings as the leading theory of cosmic structure formation, and suggested cosmic inflation
Cosmic inflation
In physical cosmology, cosmic inflation, cosmological inflation or just inflation is the theorized extremely rapid exponential expansion of the early universe by a factor of at least 1078 in volume, driven by a negative-pressure vacuum energy density. The inflationary epoch comprises the first part...
was the right theory. During the 1990s, the first peak was measured with increasing sensitivity and by 2000 the BOOMERanG experiment
BOOMERanG experiment
The BOOMERanG experiment measured the cosmic microwave background radiation of a part of the sky during three sub-orbital balloon flights. It was the first experiment to make large, high fidelity images of the CMB temperature anisotropies...
reported that the highest power fluctuations occur at scales of apporoximately one degree. Together with other cosmological data, these results implied that the geometry of the Universe is flat. A number of ground-based interferometers provided measurements of the fluctuations with higher accuracy over the next three years, including the Very Small Array
Very Small Array
The Very Small Array is a 14-element interferometric radio telescope operating between 26 and 36 GHz that is used to study the cosmic microwave background radiation. It is a collaboration between the University of Cambridge, University of Manchester and the Instituto de Astrofisica de Canarias...
,
Degree Angular Scale Interferometer
Degree Angular Scale Interferometer
The Degree Angular Scale Interferometer was a telescope located in Antarctica. It was a 13-element interferometer operating between 26 and 36 GHz in ten bands. The instrument is similar in design to the Cosmic Background Imager and the Very Small Array...
(DASI) and the Cosmic Background Imager
Cosmic Background Imager
The Cosmic Background Imager was a 13-element interferometer perched at an elevation of 5,080 metres at Llano de Chajnantor Observatory in the Chilean Andes...
(CBI). DASI made the first detection of the polarization of the CMB and the CBI provided the first E-mode spectrum with compelling evidence that it is out of phase with the T-mode spectrum.
In June 2001, NASA
NASA
The National Aeronautics and Space Administration is the agency of the United States government that is responsible for the nation's civilian space program and for aeronautics and aerospace research...
launched a second CMB space mission, WMAP, to make much more precise measurements of the large scale anisotropies over the full sky. The first results from this mission, disclosed in 2003, were detailed measurements of the angular power spectrum to below degree scales, tightly constraining various cosmological parameters. The results are broadly consistent with those expected from cosmic inflation
Cosmic inflation
In physical cosmology, cosmic inflation, cosmological inflation or just inflation is the theorized extremely rapid exponential expansion of the early universe by a factor of at least 1078 in volume, driven by a negative-pressure vacuum energy density. The inflationary epoch comprises the first part...
as well as various other competing theories, and are available in detail at NASA's data center for Cosmic Microwave Background (CMB) (see links below). Although WMAP provided very accurate measurements of the large angular-scale fluctuations in the CMB (structures about as large in the sky as the moon), it did not have the angular resolution to measure the smaller scale fluctuations which had been observed using previous ground-based interferometers.
A third space mission, Planck, was launched in May 2009. Planck employs both HEMT
HEMT
High electron mobility transistor , also known as heterostructure FET or modulation-doped FET , is a field effect transistor incorporating a junction between two materials with different band gaps as the channel instead of a doped region, as is generally the case for MOSFET...
radiometers as well as bolometer
Bolometer
A bolometer is a device for measuring the power of incident electromagnetic radiation via the heating of a material with a temperature-dependent electrical resistance. It was invented in 1878 by the American astronomer Samuel Pierpont Langley...
technology and measures the CMB anisotropies at a higher resolution than WMAP. Unlike the previous two space missions, Planck is a collaboration between NASA and ESA
European Space Agency
The European Space Agency , established in 1975, is an intergovernmental organisation dedicated to the exploration of space, currently with 18 member states...
(the European Space Agency). Its detectors got a trial run at the Antarctic Viper telescope
Viper telescope
The Viper telescope is used to view mainly cosmic background radiation. Currently the telescope is helping scientists prove or disprove the Big Crunch theory. The telescope is also one of the most powerful of its kind...
as ACBAR (Arcminute Cosmology Bolometer Array Receiver
Arcminute Cosmology Bolometer Array Receiver
ACBAR is an experiment to measure the anisotropy of the Cosmic microwave background.The ACBAR 145 GHz measurements are the most precise high multipole measurements of the CMB to date.-External links:**...
) experiment – which has produced the most precise measurements at small angular scales to date – and at the Archeops
Archeops
Archeops was a balloon-borne instrument dedicated to measuring the Cosmic microwave background temperature anisotropies. The study of this radiation is essential to obtain precise information on the evolution of the Universe: density, Hubble constant, age of the Universe, etc...
balloon telescope.
Additional ground-based instruments such as the South Pole Telescope
South Pole Telescope
The South Pole Telescope is a 10 metre diameter telescope located at the Amundsen-Scott South Pole Station, Antarctica. It is a microwave/millimetre-wave telescope that observes in a frequency range between 70 and 300 GHz...
in Antarctica and the proposed Clover
Clover (telescope)
Clover would have been an experiment to measure the polarization of the Cosmic Microwave Background. It was approved for funding in late 2004, with the aim of having the full telescope operational by 2009...
Project, Atacama Cosmology Telescope
Atacama Cosmology Telescope
The Atacama Cosmology Telescope is a six-metre telescope on Cerro Toco in the Atacama Desert in the north of Chile, near the Llano de Chajnantor Observatory. It is designed to make high-resolution, microwave-wavelength surveys of the sky in order to study the cosmic microwave background radiation...
and the QUIET telescope in Chile
Chile
Chile ,officially the Republic of Chile , is a country in South America occupying a long, narrow coastal strip between the Andes mountains to the east and the Pacific Ocean to the west. It borders Peru to the north, Bolivia to the northeast, Argentina to the east, and the Drake Passage in the far...
will provide additional data not available from satellite observations, possibly including the B-mode polarization.
Radio
The brightest sources of low-frequency radio emission (10 MHz and 100 GHz) are radio galaxiesRadio galaxy
Radio galaxies and their relatives, radio-loud quasars and blazars, are types of active galaxy that are very luminous at radio wavelengths, with luminosities up to 1039 W between 10 MHz and 100 GHz. The radio emission is due to the synchrotron process...
which can be observed out to extremely high redshifts. These are subsets of the active galaxies that have extended features known as lobes and jets which extend away form the galactic nucleus distances on the order of megaparsecs. Because radio galaxies are so bright, astronomers have used them to probe extreme distances and early times in the evolution of the universe.
Infrared
Far infraredInfrared
Infrared light is electromagnetic radiation with a wavelength longer than that of visible light, measured from the nominal edge of visible red light at 0.74 micrometres , and extending conventionally to 300 µm...
observations including submillimeter astronomy have revealed a number of sources at cosmological distances. With the exception of a few atmospheric windows, most of infrared light is blocked by the atmosphere, the observations generally take place from balloon or space-based instruments. Current observational experiments in the infrared include NICMOS, the Cosmic Origins Spectrograph
Cosmic Origins Spectrograph
The Cosmic Origins Spectrograph is a science instrument that was installed on the Hubble Space Telescope during Servicing Mission 4 in May 2009. It is designed for ultraviolet spectroscopy of faint point sources with a resolving power of ≈1,550–24,000...
, the Spitzer Space Telescope
Spitzer Space Telescope
The Spitzer Space Telescope , formerly the Space Infrared Telescope Facility is an infrared space observatory launched in 2003...
, the Keck Interferometer, the Stratospheric Observatory For Infrared Astronomy
Stratospheric Observatory for Infrared Astronomy
The Stratospheric Observatory for Infrared Astronomy is a joint project of NASA and the German Aerospace Center to construct and maintain an airborne observatory. NASA awarded the contract for the development of the aircraft, operation of the observatory and management of the American part of the...
, and the Herschel Space Observatory
Herschel Space Observatory
The Herschel Space Observatory is a European Space Agency space observatory sensitive to the far infrared and submillimetre wavebands. It is the largest space telescope ever launched, carrying a single mirror of in diameter....
. The next large space telescope planned by NASA, the James Webb Space Telescope
James Webb Space Telescope
The James Webb Space Telescope , previously known as Next Generation Space Telescope , is a planned next-generation space telescope, optimized for observations in the infrared. The main technical features are a large and very cold 6.5 meter diameter mirror, an observing position far from Earth,...
will also explore in the infrared.
Cosmic neutrinos
It is a prediction of the Big BangBig Bang
The Big Bang theory is the prevailing cosmological model that explains the early development of the Universe. According to the Big Bang theory, the Universe was once in an extremely hot and dense state which expanded rapidly. This rapid expansion caused the young Universe to cool and resulted in...
model that the universe is filled with a neutrino background radiation
Cosmic neutrino background
The cosmic neutrino background is the universe's background particle radiation composed of neutrinos.Like the cosmic microwave background radiation , the CνB is a relic of the big bang, and while the CMB dates from when the universe was 379,000 years old, the CνB decoupled from matter when the...
, analogous to the cosmic microwave background radiation
Cosmic microwave background radiation
In cosmology, cosmic microwave background radiation is thermal radiation filling the observable universe almost uniformly....
. The microwave background is a relic from when the universe was about 380,000 years old, but the neutrino background is a relic from when the universe was about two seconds old.
If this neutrino radiation could be observed, it would be a window into very early stages of the universe. Unfortunately, these neutrinos would now be very cold, and so they are effectively impossible to observe directly.