The Bullet Cluster (1E 0657-56) consists of two colliding clusters of galaxies. Strictly speaking, the name ''Bullet Cluster'' refers to the smaller subcluster, moving away from the larger one. It is at a comoving radial distance of . The object is of a particular note for astrophysicists, because

gravitational lens A gravitational lens is matter, such as a galaxy cluster, cluster of galaxies or a point particle, that bends light from a distant source as it travels toward an observer. The amount of gravitational lensing is described by Albert Einstein's Ge ...

ing studies of the Bullet Cluster are claimed to provide strong evidence for the existence of

dark matter In astronomy, dark matter is an invisible and hypothetical form of matter that does not interact with light or other electromagnetic radiation. Dark matter is implied by gravity, gravitational effects that cannot be explained by general relat ...

. Observations of other galaxy cluster collisions, such as MACS J0025.4-1222, similarly support the existence of dark matter.

Overview

The major components of the cluster pair—

star A star is a luminous spheroid of plasma (physics), plasma held together by Self-gravitation, self-gravity. The List of nearest stars and brown dwarfs, nearest star to Earth is the Sun. Many other stars are visible to the naked eye at night sk ...

gas Gas is a state of matter that has neither a fixed volume nor a fixed shape and is a compressible fluid. A ''pure gas'' is made up of individual atoms (e.g. a noble gas like neon) or molecules of either a single type of atom ( elements such as ...

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, visible light, or visible radiation is electromagnetic radiation that can be visual perception, perceived by the human eye. Visible light spans the visible spectrum and is usually defined as having wavelengths in the range of 400– ...

, 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 An X-ray (also known in many languages as Röntgen radiation) is a form of high-energy electromagnetic radiation with a wavelength shorter than those of ultraviolet rays and longer than those of gamma rays. Roughly, X-rays have a wavelength ran ...

s, represents most of the

baryon In particle physics, a baryon is a type of composite particle, composite subatomic particle that contains an odd number of valence quarks, conventionally three. proton, Protons and neutron, neutrons are examples of baryons; because baryons are ...

ic, or "ordinary", matter in the cluster pair. The gases of the

intracluster medium In astronomy, the intracluster medium (ICM) is the superheated plasma (physics), plasma that permeates a galaxy cluster. The gas consists mainly of ionized hydrogen and helium and accounts for most of the baryonic material in galaxy clusters. The ...

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

ing of background objects, as calculated using the best available theory of gravity

general relativity General relativity, also known as the general theory of relativity, and as Einstein's theory of gravity, is the differential geometry, geometric theory of gravitation published by Albert Einstein in 1915 and is the current description of grav ...

. This provides support for the idea that most of the gravitation in the cluster pair is in the form of two regions of collisionless dark matter, which bypassed the gas regions during the collision. 1e0657 scale

The Bullet Cluster is one of the hottest-known clusters of galaxies. It provides an observable constraint for cosmological models, which may diverge at temperatures beyond their predicted critical cluster temperature. Observed from

Earth Earth is the third planet from the Sun and the only astronomical object known to Planetary habitability, harbor life. This is enabled by Earth being an ocean world, the only one in the Solar System sustaining liquid surface water. Almost all ...

, the subcluster passed through the cluster center 150 million years ago, creating a "bow-shaped

shock wave In physics, a shock wave (also spelled shockwave), or shock, is a type of propagating disturbance that moves faster than the local speed of sound in the medium. Like an ordinary wave, a shock wave carries energy and can propagate through a me ...

located near the right side of the cluster" formed as "70 million kelvin gas in the sub-cluster plowed through 100 million kelvin gas in the main cluster at a speed of about nearly 10 million km/h (6 million miles per hour)". The bow shock radiation output is equivalent to the energy of 10 typical

quasar A quasar ( ) is an extremely Luminosity, luminous active galactic nucleus (AGN). It is sometimes known as a quasi-stellar object, abbreviated QSO. The emission from an AGN is powered by accretion onto a supermassive black hole with a mass rangi ...

s. According to Greg Madejski: According to Eric Hayashi: A 2010 study claimed that the velocities of the collision are "incompatible with the prediction of a LCDM model". However, subsequent work has found the collision to be consistent with LCDM simulations, with the previous discrepancy stemming from small simulations and the methodology of identifying pairs. Earlier work claiming the Bullet Cluster was inconsistent with standard cosmology was based on an erroneous estimate of the in-fall velocity based on the speed of the shock in the X-ray-emitting gas. Based on the analysis of the shock driven by the merger, it was recently argued that a lower merger velocity ~3,950 km/s is consistent with the

Sunyaev–Zeldovich effect The Sunyaev–Zeldovich effect (named after Rashid Sunyaev and Yakov B. Zeldovich and often abbreviated as the SZ effect) is the spectral distortion of the cosmic microwave background (CMB) through inverse Compton scattering by high-energy e ...

and X-ray data, provided that the equilibration of the electron and ion downstream temperatures is not instantaneous.

As evidence against modified gravity

Bullet_Cluster_with_gravitational_potential_in_MOND

The Bullet Cluster has been claimed as a significant challenge for all theories proposing a modified gravity solution to the missing mass problem, including

modified Newtonian dynamics Modified Newtonian dynamics (MOND) is a theory that proposes a modification of Newton's laws to account for observed properties of galaxies. Modifying Newton's law of gravity results in modified gravity, while modifying Newton's second law resul ...

(MOND). Astronomers measured the distribution of stellar and gas mass in the clusters using visible and

light, respectively, and also mapped the gravitational potential using gravitational lensing. As shown in the images on the right, the X-ray gas is in the center, while the galaxies are on the outskirts. During the collision, the X-ray gas interacted and slowed down, remaining in the center, while the galaxies largely passed by one another, as the distances between them were vast. The gravitational potential reveals two large concentrations centered on the galaxies, not on the X-ray gas, where most of the normal matter is located. In ΛCDM one would also expect the clusters to each have a dark matter halo that would pass through each other during the collision (assuming, as is conventional, that dark matter is collisionless). This expectation for the dark matter is a clear explanation for the offset between the peaks of the gravitational potential and the X-ray gas which was detected at a

statistical significance In statistical hypothesis testing, a result has statistical significance when a result at least as "extreme" would be very infrequent if the null hypothesis were true. More precisely, a study's defined significance level, denoted by \alpha, is the ...

of 8. It is this offset between the gravitational potential and normal matter that was claimed by Clowe et al. as ''"A Direct Empirical Proof of the Existence of Dark Matter"'' arguing that modified gravity theories fail to account for it. However, this study by Clowe et al. made no attempt to analyze the Bullet Cluster using MOND or any other modified gravity theory. Furthermore, in the same year, Angus et al. demonstrated that MOND does indeed reproduce the offset between the gravitational potential and the X-ray gas in this highly non-spherically symmetric system. In MOND, one would expect the "missing mass" to be centred on regions which experience accelerations lower than a₀, which, in the case of the Bullet Cluster, correspond to the areas containing the galaxies, not the X-ray gas. Nevertheless, MOND still fails to fully explain this cluster, as it does with all other galaxy clusters, due to the remaining mass residuals in several core regions of the Bullet Cluster. Mordehai Milgrom, the original proposer of MOND, has posted an online rebuttal of claims that the Bullet Cluster proves the existence of dark matter. He contends that the observed characteristics of the Bullet Cluster could just as well be caused by undetected standard matter. He has argued that all galaxy clusters could host cold dense hydrogen gas clouds of roughly equal to the mass of the visible baryons which could explain the failures of MOND in galaxy clusters. Such cold dense hydrogen clouds are unlikely to exist however due to feedback from AGNs which prevent hydrogen gas from cooling. There are other alternate theories of gravity like the MOG and Many-body gravity (MBG), which claim to be able to explain the bullet cluster's weak gravitational lensing.

Overview

As evidence against modified gravity

See also

References

Further reading