Observational cosmology is the study of the
structure, the evolution and the origin of
the universe through observation, using instruments
such as telescopes and cosmic ray detectors.
== Early observations ==
The science of physical cosmology 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.
=== Hubble's law 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 parallax 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 variables 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. Hubble used
the 100-inch Hooker Telescope at Mount Wilson
Observatory to identify individual stars 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.In 1927, by combining various
measurements, including Hubble's distance
measurements and Vesto Slipher's determinations
of redshifts for these objects, Georges Lemaître
was the first to estimate a constant of proportionality
between galaxies' distances and what was termed
their "recessional velocities", finding a
value of about 600 km/s/Mpc. He showed that
this was theoretically expected in a universe
model based on general relativity. Two years
later, Hubble showed that the relation between
the distances and velocities was a positive
correlation and had a slope of about 500 km/s/Mpc.
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 the observations by Slipher, Wirtz, Hubble
and their colleagues and the acceptance by
the theorists of their 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 spectroscopic
measurements of light from astronomical objects
and the identification of emission and absorption
lines which corresponded to particular electronic
transitions in chemical elements 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 meteorites.
=== Detection of the cosmic microwave background
===
A cosmic microwave background was predicted
in 1948 by George Gamow 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.
== 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 matter 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.
=== Redshift surveys ===
With the advent of automated telescopes 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-years 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
as of 2011 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 quasars
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 microwave background experiments
===
Subsequent to the discovery of the CMB, hundreds
of cosmic microwave background experiments
had been conducted to measure and characterize
the signatures of the radiation. The most
famous experiment is probably the NASA 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-based and balloon-based
experiments quantified CMB anisotropies on
smaller angular scales over the next decade.
The primary goal of those experiments was
to measure the angular scale of the first
acoustic peak, for which COBE did not have
sufficient resolution. The 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 approximately
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
and 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
the European Space Agency (ESA). 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.
=== Telescope observations ===
==== 
Radio ====
The brightest sources of low-frequency radio
emission (10 MHz and 100 GHz) are radio galaxies
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 from 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 infrared 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.
An additional infrared survey, the Two-Micron
All Sky Survey, has also been very useful
in revealing the distribution of galaxies,
similar to other optical surveys described
below.
==== Optical rays (visible to human eyes)
====
Optical light is still the primary means by
which astronomy occurs, and in the context
of cosmology, this means observing distant
galaxies and galaxy clusters in order to learn
about the large scale structure of the Universe
as well as galaxy evolution. Redshift surveys
have been a common means by which this has
been accomplished with some of the most famous
including the 2dF Galaxy Redshift Survey,
the Sloan Digital Sky Survey, and the upcoming
Large Synoptic Survey Telescope. These optical
observations generally use either photometry
or spectroscopy to measure the redshift of
a galaxy and then, via Hubble's Law, determine
its distance modulo redshift distortions due
to peculiar velocities. Additionally, the
position of the galaxies as seen on the sky
in celestial coordinates can be used to gain
information about the other two spatial dimensions.
Very deep observations (which is to say sensitive
to dim sources) are also useful tools in cosmology.
The Hubble Deep Field, Hubble Ultra Deep Field,
Hubble Extreme Deep Field, and Hubble Deep
Field South are all examples of this.
==== Ultraviolent ====
==== 
X-rays ====
See X-ray telescope.
==== Gamma-rays ====
=== 
Cosmic ray observations ===
== 
Future observations ==
=== 
Cosmic neutrinos ===
It is a prediction of the Big Bang 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.
=== Gravitational waves ===
== 
See also ==
Big Bang
Cosmic background radiation
