Bullet cluster
Encyclopedia
The Bullet cluster consists of two colliding clusters of galaxies
. Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter
. At a statistical significance
of 8σ, it was found that the spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law. Observations of other galaxy cluster collisions, such as MACS J0025.4-1222
, also show significant displacement between their center of visible matter and their gravitational potential.
s, gas
and the putative dark matter, behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible light
, were not greatly affected by the collision, and most passed right through, gravitationally slowed but not otherwise altered. The hot gas of the two colliding components, seen in X-ray
s, represents most of the mass
of the ordinary (baryon
ic) matter in the cluster pair. The gases interact electromagnetically, causing the gases of both clusters to slow much more than the stars. The third component, the dark matter, was detected indirectly by the gravitational lens
ing of background objects. In theories
without dark matter, such as Modified Newtonian Dynamics
, the lensing would be expected to follow the baryonic matter; i.e. the X-ray gas. However, the lensing is strongest in two separated regions near the visible galaxies. This provides support for the idea that most of the mass in the cluster pair is in the form of collisionless dark matter.
The Bullet cluster is one of the hottest
known clusters of galaxies
. Observed from Earth, the subcluster passed through the cluster center 150 million years ago creating a "bow-shaped shock wave
located near the right side of the cluster" formed as "70 million degree Celsius gas in the sub-cluster plowed through 100 million degree Celsius gas in the main cluster at a speed of about 6 million miles per hour". Strictly speaking, the name Bullet cluster refers to the smaller subcluster, moving away from the larger one.
and provides "evidence against some of the more popular versions of Modified Newtonian Dynamics
(MOND)" as applied to large galactic clusters.
"Particularly compelling results were inferred from the Chandra
observations of the 'bullet cluster' (1E0657-56; Fig. 2) by Markevitch et al. (2004) and Clowe et al. (2004). Those authors report that the cluster is undergoing a high-velocity (around 4500 km/s) merger, evident from the spatial distribution of the hot, X-ray
emitting gas, but this gas lags behind the subcluster galaxies. Furthermore, the dark matter clump, revealed by the weak-lensing
map, is coincident with the collisionless galaxies
, but lies ahead of the collisional gas. This—and other similar observations—allow good limits on the cross-section of the self-interaction of dark matter
."
In an independent confirmation of results from the Bullet Cluster, more recent observations of the cluster MACS J0025.4-1222
indicate that a titanic collision has separated the dark from ordinary matter.
However, while the Bullet Cluster phenomenon may provide direct evidence for dark matter on large cluster scales, it offers no specific insight into the original galaxy rotation problem. In fact, the observed ratio of visible matter to dark matter in a typical rich galaxy cluster is much lower than predicted. This may indicate that the prevailing cosmological model
is insufficient to describe the mass discrepancy on galaxy scales, or that its predictions about the shape of the universe
are incorrect.
of dark matter have cautioned that astronomers expect sizable quantities of non-luminous baryonic matter
to reside in large galactic clusters, positing that the Bullet Cluster phenomenon can be explained without requiring non-baryonic dark matter. However, this explanation requires that baryonic dark matter be of the same amount as the luminous baryonic matter in the Bullet Cluster. This means that ~6 times the visible galactic mass exists at the gravitational centroids, possibly in the galaxies as MACHOs
, brown dwarves, or cold gas clouds.
Galaxy groups and clusters
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe...
. Studies of the Bullet cluster, announced in August 2006, provide the best evidence to date for the existence of dark matter
Dark 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...
. At a statistical significance
Statistical significance
In statistics, a result is called statistically significant if it is unlikely to have occurred by chance. The phrase test of significance was coined by Ronald Fisher....
of 8σ, it was found that the spatial offset of the center of the total mass from the center of the baryonic mass peaks cannot be explained with an alteration of the gravitational force law. Observations of other galaxy cluster collisions, such as MACS J0025.4-1222
MACS J0025.4-1222
MACS J0025.4-1222 is a cluster created by the collision of two galaxy clusters. Like the earlier discovered Bullet Cluster, this cluster shows a clear separation between the centroid of the intergalactic gas , shown in pink and the mass centroids of the colliding clusters...
, also show significant displacement between their center of visible matter and their gravitational potential.
Overview
The major components of the cluster pair, starStar
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, gas
Gas
Gas is one of the three classical states of matter . Near absolute zero, a substance exists as a solid. As heat is added to this substance it melts into a liquid at its melting point , boils into a gas at its boiling point, and if heated high enough would enter a plasma state in which the electrons...
and the putative dark matter, behave differently during collision, allowing them to be studied separately. The stars of the galaxies, observable in visible light
Light
Light or visible light is electromagnetic radiation that is visible to the human eye, and is responsible for the sense of sight. Visible light has wavelength in a range from about 380 nanometres to about 740 nm, with a frequency range of about 405 THz to 790 THz...
, were not greatly affected by the collision, and most passed right through, gravitationally slowed but not otherwise altered. The hot gas of the two colliding components, seen in X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
s, represents most of the mass
Mass
Mass can be defined as a quantitive measure of the resistance an object has to change in its velocity.In physics, mass commonly refers to any of the following three properties of matter, which have been shown experimentally to be equivalent:...
of the ordinary (baryon
Baryon
A baryon is a composite particle made up of three quarks . Baryons and mesons belong to the hadron family, which are the quark-based particles...
ic) matter in the cluster pair. The gases interact electromagnetically, causing the gases of both clusters to slow much more than the stars. The third component, the dark matter, was detected indirectly by the gravitational lens
Gravitational lens
A gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...
ing of background objects. In theories
Scientific theory
A scientific theory comprises a collection of concepts, including abstractions of observable phenomena expressed as quantifiable properties, together with rules that express relationships between observations of such concepts...
without dark matter, such as Modified Newtonian Dynamics
Modified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
, the lensing would be expected to follow the baryonic matter; i.e. the X-ray gas. However, the lensing is strongest in two separated regions near the visible galaxies. This provides support for the idea that most of the mass in the cluster pair is in the form of collisionless dark matter.
The Bullet cluster is one of the hottest
Temperature
Temperature is a physical property of matter that quantitatively expresses the common notions of hot and cold. Objects of low temperature are cold, while various degrees of higher temperatures are referred to as warm or hot...
known clusters of galaxies
Galaxy groups and clusters
Galaxy groups and clusters are the largest known gravitationally bound objects to have arisen thus far in the process of cosmic structure formation. They form the densest part of the large scale structure of the universe...
. Observed from Earth, the subcluster passed through the cluster center 150 million years ago creating a "bow-shaped shock wave
Shock wave
A shock wave is a type of propagating disturbance. Like an ordinary wave, it carries energy and can propagate through a medium or in some cases in the absence of a material medium, through a field such as the electromagnetic field...
located near the right side of the cluster" formed as "70 million degree Celsius gas in the sub-cluster plowed through 100 million degree Celsius gas in the main cluster at a speed of about 6 million miles per hour". Strictly speaking, the name Bullet cluster refers to the smaller subcluster, moving away from the larger one.
Significance to dark matter
The Bullet Cluster provides the best current evidence for the nature of 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...
and provides "evidence against some of the more popular versions of Modified Newtonian Dynamics
Modified Newtonian dynamics
In physics, Modified Newtonian dynamics is a hypothesis that proposes a modification of Newton's law of gravity to explain the galaxy rotation problem. When the uniform velocity of rotation of galaxies was first observed, it was unexpected because Newtonian theory of gravity predicts that objects...
(MOND)" as applied to large galactic clusters.
Chandra X-ray Observatory
The Chandra X-ray Observatory is a satellite launched on STS-93 by NASA on July 23, 1999. It was named in honor of Indian-American physicist Subrahmanyan Chandrasekhar who is known for determining the maximum mass for white dwarfs. "Chandra" also means "moon" or "luminous" in Sanskrit.Chandra...
observations of the 'bullet cluster' (1E0657-56; Fig. 2) by Markevitch et al. (2004) and Clowe et al. (2004). Those authors report that the cluster is undergoing a high-velocity (around 4500 km/s) merger, evident from the spatial distribution of the hot, X-ray
X-ray
X-radiation is a form of electromagnetic radiation. X-rays have a wavelength in the range of 0.01 to 10 nanometers, corresponding to frequencies in the range 30 petahertz to 30 exahertz and energies in the range 120 eV to 120 keV. They are shorter in wavelength than UV rays and longer than gamma...
emitting gas, but this gas lags behind the subcluster galaxies. Furthermore, the dark matter clump, revealed by the weak-lensing
Gravitational lens
A gravitational lens refers to a distribution of matter between a distant source and an observer, that is capable of bending the light from the source, as it travels towards the observer...
map, is coincident with the collisionless 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...
, but lies ahead of the collisional gas. This—and other similar observations—allow good limits on the cross-section of the self-interaction of dark matter
Dark 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...
."
"The velocityVelocityIn physics, velocity is speed in a given direction. Speed describes only how fast an object is moving, whereas velocity gives both the speed and direction of the object's motion. To have a constant velocity, an object must have a constant speed and motion in a constant direction. Constant ...
of the bullet subcluster is not exceptionally high for a cluster substructure, and can be accommodated within the currently favoured Lambda-CDM modelLambda-CDM modelΛCDM or Lambda-CDM is an abbreviation for Lambda-Cold Dark Matter, which is also known as the cold dark matter model with dark energy...
cosmogonyCosmogonyCosmogony, or cosmogeny, is any scientific theory concerning the coming into existence or origin of the universe, or about how reality came to be. The word comes from the Greek κοσμογονία , from κόσμος "cosmos, the world", and the root of γίνομαι / γέγονα "to be born, come about"...
."
In an independent confirmation of results from the Bullet Cluster, more recent observations of the cluster MACS J0025.4-1222
MACS J0025.4-1222
MACS J0025.4-1222 is a cluster created by the collision of two galaxy clusters. Like the earlier discovered Bullet Cluster, this cluster shows a clear separation between the centroid of the intergalactic gas , shown in pink and the mass centroids of the colliding clusters...
indicate that a titanic collision has separated the dark from ordinary matter.
However, while the Bullet Cluster phenomenon may provide direct evidence for dark matter on large cluster scales, it offers no specific insight into the original galaxy rotation problem. In fact, the observed ratio of visible matter to dark matter in a typical rich galaxy cluster is much lower than predicted. This may indicate that the prevailing cosmological 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...
is insufficient to describe the mass discrepancy on galaxy scales, or that its predictions about the shape of the universe
Shape of the Universe
The shape of the universe is a matter of debate in physical cosmology over the local and global geometry of the universe which considers both curvature and topology, though, strictly speaking, it goes beyond both...
are incorrect.
Alternative interpretations
CriticsMordehai Milgrom
Mordehai Milgrom is an Israeli physicist and professor in the department of Condensed Matter Physics at the Weizmann Institute in Rehovot, Israel. He is most famous for his proposal of Modified Newtonian dynamics as an alternative to the dark matter and galaxy rotation curve problems, in 1981...
of dark matter have cautioned that astronomers expect sizable quantities of non-luminous baryonic matter
Baryonic dark matter
In astronomy and cosmology, baryonic dark matter is dark matter composed of baryons, i.e. protons and neutrons and combinations of these, such as non-emitting ordinary atoms...
to reside in large galactic clusters, positing that the Bullet Cluster phenomenon can be explained without requiring non-baryonic dark matter. However, this explanation requires that baryonic dark matter be of the same amount as the luminous baryonic matter in the Bullet Cluster. This means that ~6 times the visible galactic mass exists at the gravitational centroids, possibly in the galaxies as MACHOs
Massive compact halo object
Massive astrophysical compact halo object, or MACHO, is a general name for any kind of astronomical body that might explain the apparent presence of dark matter in galaxy halos. A MACHO is a body composed of normal baryonic matter, which emits little or no radiation and drifts through interstellar...
, brown dwarves, or cold gas clouds.
Further reading
- arXivArXivThe arXiv |Chi]], χ) is an archive for electronic preprints of scientific papers in the fields of mathematics, physics, astronomy, computer science, quantitative biology, statistics, and quantitative finance which can be accessed online. In many fields of mathematics and physics, almost all...
: A direct empirical proof of the existence of dark matter - arXiv: Strong and weak lensing united III: Measuring the mass distribution of the merging galaxy cluster 1E0657-56 (Marusa Bradac) Fri, 18 Aug 2006 20:06:48 GMT
- arXiv: Can MOND take a bullet? Analytical comparisons of three versions of MOND beyond spherical symmetry
- arXiv: On the Law of Gravity, the Mass of Neutrinos, and the Proof of Dark Matter
- arXiv: The Bullet Cluster 1E0657-558 evidence shows Modified Gravity in the absence of Dark Matter Brownstein and Moffat
- CXO: Bedeviling Devil's Advocate Cosmology (The Chandra Chronicles) August 21, 2006
- CXO: 1E 0657-56: NASA Finds Direct Proof of Dark Matter
- Harvard animation of the collision showing how the dark matter and normal matter become separated.
- Harvard Harvard Symposium: Markevitch PDF 36 color images and text pages proving the existence of Dark Matter from Bullet cluster data
- NASA: NASA Finds Direct Proof of Dark Matter (NASA Press Release 06-096) Aug. 21, 2006
- Scientific American Scientific American article SCIENCE NEWS August 22, 2006 Colliding Clusters Shed Light on Dark Matter that includes a movie of a simulation of the collision