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Starts With A Bang

The galaxy cluster that broke modified gravity

19 years ago, the Bullet Cluster provided an empirical proof for dark matter. Even today, modified gravity still can’t explain it.
Bullet Cluster separation mass gravity x-ray lensing
The Bullet Cluster, the aftermath of a galaxy cluster collision that occurred 3.8 billion years ago in a region of space located ~3.7 billion light-years away, represents very strong evidence for the existence of dark matter. The separation of the gravitational effects (blue, reconstructed through gravitational lensing) from the location of the majority of the normal matter (pink, revealed by Chandra's X-ray capabilities) is very difficult to explain without dark matter's presence.
Credit: NASA/CXC/M. Weiss
Key Takeaways
  • We can measure the amount of matter in the Universe and also the effects of gravity, and these two methods, with normal matter alone, just don’t add up.
  • One can imagine either adding a new ingredient, like dark matter, or changing the laws of gravity, modifying them from Einstein’s original form.
  • But one class of systems, that of colliding galaxy clusters, gives us a way to tell the two ideas apart. Unless modified gravity is a near-perfect dark matter mimic, the idea falls apart in the face of this evidence.

For 90 years now, the Universe hasn’t added up.

spiral galaxy comparison dark matter
A galaxy that was governed by normal matter alone (left) would display much lower rotational speeds in the outskirts than toward the center, similar to how planets in the Solar System move. However, observations indicate that rotational speeds are largely independent of radius (right) from the galactic center, leading to the inference that a large amount of invisible, or dark, matter must be present. These types of observations were revolutionary in helping astronomers understand the necessity for dark matter in the Universe, and also explain the shapes and behavior of matter located within a galaxy’s spiral arms.
Credit: Ingo Berg/Wikimedia Commons; Acknowledgement: E. Siegel

From matter’s behavior, measuring stars and galaxies reveals their normal matter contents.

cigar galaxy messier 82
This close-up view of Messier 82, the Cigar Galaxy, shows not only stars and gas, but also the superheated galactic winds and the distended shape induced by its interactions with its larger, more massive neighbor: M81. (M81 is located off-screen, to the upper right.) When star-formation actively occurs across an entire galaxy, it becomes what’s known as a starburst galaxy, characterized by violent, gas-expelling winds. The lower in mass the wind-driven starburst galaxy is, the more of its normal matter it loses during an event like this.
Credit: R. Gendler, R. Croman, R. Colombari; Acknowledgement: R. Jay GaBany; VLA Data: E. de Block (ASTRON)

From gravitational effects, we recover the “total mass” of such objects.

Kraken messier 63 sunflower
Whether we examine satellites orbiting around planets, planets orbiting around stars, stars moving around a galaxy, or galaxies moving within a galaxy cluster, the effects of gravity are what keep these objects moving in bound, stable orbits. Measuring the properties of the orbiting objects helps reveal the mass, and total gravitational effects, of all of these large-scale systems.
(Credit: Tony and Daphne Hallas/

Since the 1930s, we’ve known these numbers don’t match.

coma cluster zwicky dark matter
The Coma Cluster of galaxies, as seen with a composite of modern space and ground-based telescopes. The infrared data comes from the Spitzer Space telescope, while ground-based data comes from the Sloan Digital Sky Survey. The Coma Cluster is dominated by two giant elliptical galaxies, with over 1000 other spirals and ellipticals inside. Gas-free, red-and-dead elliptical galaxies are very common, especially toward the cluster center, in large galaxy clusters such as this one. The speed of galaxies within the cluster can be used to help determine the cluster’s total mass.
Credit: NASA / JPL-Caltech / L. Jenkins (GSFC)

Possible solutions include either unseen matter or modifying Einstein’s gravity.

dark matter rotation curve modified gravity
The extended rotation curve of M33, the Triangulum galaxy. These rotation curves of spiral galaxies ushered in the modern astrophysics concept of dark matter to the general field. The dashed curve would correspond to a galaxy without dark matter, which represents less than 1% of galaxies. Vera Rubin’s work throughout the 1970s was essential in demonstrating that galaxies practically universally require an explanation for this unexpected but robustly observed behavior.
Credit: Mario de Leo/Wikimedia Commons

Colliding galaxy clusters can conceivably tell those scenarios apart.

gravitational lensing map cluster abell 1689
This Hubble Space Telescope image of galaxy cluster Abell 1689 has had its mass distribution reconstructed via the effects of gravitational lensing, and that map is overlaid atop the optical image in blue. If a major interaction can separate the gas in the intracluster medium from the position of the galaxies, the existence of dark matter can be put to the test. Differences between pre-collisional and post-collisional clusters is key evidence in the conclusion that dark matter is the leading explanation for what we observe in our Universe.
Credit: NASA, ESA, E. Jullo (Jet Propulsion Laboratory), P. Natarajan (Yale University), and J.-P. Kneib (Laboratoire d’Astrophysique de Marseille, CNRS, France); Acknowledgment: H. Ford and N. Benetiz (Johns Hopkins University), and T. Broadhurst (Tel Aviv University)

Gravitational lensing shows how foreground masses are distributed.

strong gravitational lensing horseshoe
This object isn’t a single ring galaxy, but rather two galaxies at very different distances from one another: a nearby red galaxy and a more distant blue galaxy that’s gravitationally lensed by the foreground galaxy’s mass. These objects are simply along the same line of sight, with the background galaxy’s light gravitationally distorted, stretched, and magnified by the foreground galaxy. The result is a near-perfect ring, which would be known as an Einstein ring if it made a full 360 degree circle. While lensing is more commonly seen from galaxy clusters, individual galaxies can do it if they’re compact enough and if the alignment is right.
Credit: ESA/Hubble & NASA

For galaxy clusters, most mass appears between the galaxies: in the intracluster medium.

evrard dark matter map nature 1998 galaxy cluster
A galaxy cluster can have its mass reconstructed from the gravitational lensing data available. Most of the mass is found not inside the individual galaxies, shown as peaks here, but from the intergalactic medium within the cluster, where dark matter appears to reside. More granular simulations and observations can reveal dark matter substructure as well, with the data strongly agreeing with cold dark matter’s predictions. Without the gravitational effects of dark matter, most galaxies would tear themselves apart during episodes of major star-formation.
Credit: A. E. Evrard, Nature, 1998

When clusters collide, the intracluster gas interacts.

Galaxy cluster bridge gas Abell 399 401
The full-scale image of the colliding galaxy clusters Abell 399 and Abell 401 shows X-ray data (red), Planck microwave data (yellow), and LOFAR radio data (blue) all together. The individual galaxy clusters are clearly identifiable, but the radio bridge of relativistic electrons connected by a magnetic field 10 million light-years long is incredibly illuminating. One important lesson is that the predominant population of gas within a galaxy cluster is in the intracluster medium, rather than the galaxies themselves: just like the overall mass within the cluster.
(Credit: DSS and Pan-STARRS1 (optical), XMM-Newton (X-rays), PLANCK satellite (yparameter), F. Govoni, M. Murgia, INAF)

The speeding gas heats up and slows down, attaining temperatures approaching ~100 million K.

x-ray phoenix cluster hot gas
This optical/radio composite of the Phoenix Cluster shows the enormous, bright galaxy at its core, as well as other X-ray sources nearby, from black hole emissions and the heated gas within the cluster. Spanning 2.2 million light-years across for its stellar extent, the central galaxy is even larger when measured by its radio emissions. Also, not shown, are copious levels of X-rays, including filaments and cavities, created by the powerful jets of high-energy particles originating from supermassive black holes within the cluster.
(Credit: Optical: NASA/STScI; Radio: TIFR/GMRT)

The ensuing X-ray emission allows us to map the gas’s location exquisitely.

x-ray emission 3c 295 cluster
Galaxy 3C 295, at the center of the galaxy cluster ClG J1411+5211, is shown with a composite X-ray/optical view in purple, with the X-rays blown up to reveal the central radio and X-ray loud core. At 5.6 billion light-years away, this was the most distant object known in the Universe from 1960-1964. Only with X-ray or radio telescopes, or with an enormous optical telescope, would the first extragalactic objects from MCG+01-02-015’s perspective be detectable.
Credit: X-ray: NASA/CXC/Cambridge/S.Allen et al; Optical: NASA/STScI

However, gravitational lensing reveals where the mass is.

strong and weak gravitational lensing map with shape noise
Any configuration of background points of light, whether they be stars, galaxies, or galaxy clusters, will be distorted due to the effects of foreground mass via weak gravitational lensing. Even with random shape noise, the signature is unmistakable. The El Gordo galaxy cluster shows this effect in a remarkably strong fashion, and both Hubble and JWST images help us reconstruct the mass map of the foreground galaxy cluster.
Credit: TallJimbo/Wikimedia Commons

In 2004, the Bullet cluster showed how colliding clusters behave.

optical bullet cluster HST magellan
This view of the Bullet Cluster shows optical data from the Hubble Space Telescope and the Magellan telescope in Chile, revealing the presence of the stars and galaxies inside it, as well as a series of faint, more distant background galaxies behind the main cluster.
(Credit: NASA/STScI; Magellan/U.Arizona/D.Clowe et al. )(Credit: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.)

Remarkably, the mass isn’t where the gas is.

x-ray emissions bullet cluster
This map shows the same optical data of the Bullet Cluster as the previous image, but with the X-ray data overlaid in pink. As one can see, the majority of the gas within the clusters has been stripped out of the main two clusters and into the space between the clusters, where they’ve been shocked, slowed, and heated due to gas collision. The central (larger) block has temperatures reaching ~100 million K, while the shocked (smaller) blob at right has temperatures of approximately ~70 million K.
(Credit: X-ray: NASA/CXC/CfA/M.Markevitch et al.; Optical: NASA/STScI; Magellan/U.Arizona/D.Clowe et al.)

Instead, the mass simply coasts, unperturbed by the collision.

dark matter evidence lensing bullet cluster
This map shows the reconstructed mass from gravitational lensing of the Bullet Cluster: Galaxy Cluster 1E0657-558. The contours, overlaid atop the optical data (left) and the X-ray data (right), clearly show a separation of the normal matter from the effects of gravitation, making it impossibly hard for modified gravity models to mimic this without behaving identically to dark matter.
Credit: V.A.Ryabov, V.A.Charev, A.M.Chovrebov/Wikimedia Commons, with data from D. Clowe et al., 2006

Gravitational effects appear separated from normal matter’s presence.

composite image bullet cluster
This composite image shows the optical data of the Bullet Cluster, the X-ray data that reveals the hot gas (in pink), representing most of the normal matter, and the effects of gravity as reconstructed from gravitational lensing (in blue). The fact that the lensing signal appears where most of the normal matter (pink) is not represents very strong empirical evidence favoring the existence of dark matter.
(Credit: X-ray: NASA/CXC/CfA/M.Markevitch, Optical and lensing map: NASA/STScI, Magellan/U.Arizona/D.Clowe, Lensing map: ESO WFI)

Other colliding galaxy clusters and groups show similar phenomena.

separation normal matter dark matter galaxy cluster
The X-ray (pink) and overall matter (blue) maps of various colliding galaxy clusters show a clear separation between normal matter and gravitational effects, some of the strongest evidence for dark matter. The X-rays come in two varieties, soft (lower-energy) and hard (higher-energy), where galaxy collisions can create temperatures ranging from several hundreds of thousands of degrees up to ~100 million K. Meanwhile, the fact that the gravitational effects (in blue) are displaced from the location of the mass from the normal matter (pink) shows that dark matter must be present. Without dark matter, these observations (along with many others) cannot be sufficiently explained.
Credit: NASA, ESA, D. Harvey (École Polytechnique Fédérale de Lausanne, Switzerland; University of Edinburgh, UK), R. Massey (Durham University, UK), T. Kitching (University College London, UK), and A. Taylor and E. Tittley (University of Edinburgh, UK)

Even non-local modified gravity can’t explain this.

el gordo colliding galaxy cluster
The colliding galaxy cluster “El Gordo,” the largest one known in the observable Universe, shows the same evidence of dark matter and normal matter separating when galaxy clusters collide, as seen in other colliding clusters. If normal matter alone is to explain gravity, its effects must be non-local: where gravity is found where the mass/matter isn’t.
Credit: NASA, ESA, J. Jee (Univ. of California, Davis), J. Hughes (Rutgers Univ.), F. Menanteau (Rutgers Univ. & Univ. of Illinois, Urbana-Champaign), C. Sifon (Leiden Obs.), R. Mandelbum (Carnegie Mellon Univ.), L. Barrientos (Univ. Catolica de Chile), and K. Ng (Univ. of California, Davis)

Pre-collisional clusters show matter and gravitational effects aligned; post-collisional ones show a separation.

intracluster galaxy cluster starlight
Here, galaxy cluster MACS J0416.1-2403 isn’t in the process of collision, but rather is a non-interacting, asymmetrical cluster. It also emits a soft glow of intracluster light, produced by stars that are not part of any individual galaxy, helping reveal normal matter’s locations and distribution. Gravitational lensing effects are co-located with the matter, showing that “non-local” options for modified gravity do not apply to objects like this. Clusters of galaxies contain all sorts of small-scale structures within them, from black holes to planets to star-forming gas and more.
Credit: NASA, ESA and M. Montes (University of New South Wales)

By separating normal from dark matter, the Bullet Cluster empirically demonstrates dark matter’s existence.

Mostly Mute Monday tells an astronomical story in images, visuals, and no more than 200 words. Talk less; smile more.


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