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Starts With A Bang

This Is How Galaxy Cluster Collisions Prove The Existence Of Dark Matter

A merging galaxy cluster in MACS J0416.1–2403 exhibits a small separation of X-ray gas from the gravitational signal, consistent with the notion that dark matter and not normal matter is responsible for the majority of gravitational effects seen in the Universe. Although the offset is small, this is expected, as this particular cluster is in a later stage of its merger than most, and still displays an offset between where the normal matter (in X-rays) and the total mass (from lensing; in blue) is located. (X-RAY: NASA/CXC/SAO/G.OGREAN ET AL.; OPTICAL: NASA/STSCI; RADIO: NRAO/AUI/NSF)

If dark matter is fundamentally different from the normal matter we know, there should be a way to test it. Here are the results.


Dark matter — despite the enormous indirect evidence for it — sounds like a colossal misunderstanding.

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. The time-delay observations of the Refsdal supernova cannot be explained without dark matter in this galaxy cluster. (A. E. EVRARD. NATURE 394, 122–123 (09 JULY 1998))

It’s clear that data from

  • gravitational lensing,
  • galaxy clustering,
The way galaxies cluster together is impossible to achieve in a Universe without dark matter. The clustering patterns seen due to baryon acoustic oscillations, imprinted in the Universe’s power spectrum, and on the largest scales of the cosmic web are all consistent with dark matter, but have never been explicable via any attempted modification of gravity. (NASA, ESA, CFHT, AND M.J. JEE (UNIVERSITY OF CALIFORNIA, DAVIS))
  • individual galaxies,
  • and the cosmic microwave background,

all require masses that don’t interact electromagnetically.

The final results from the Planck collaboration show an extraordinary agreement between the predictions of a dark energy/dark matter-rich cosmology (blue line) with the data (red points, black error bars) from the Planck team. All 7 acoustic peaks fit the data extraordinarily well, but about half of those peaks would not be present if there were no dark matter. (PLANCK 2018 RESULTS. VI. COSMOLOGICAL PARAMETERS; PLANCK COLLABORATION (2018))

However, a longstanding alternative suggests modifying gravity could explain them without dark matter.

The internal rotational motions of individual galaxies could, in principle, be explained by either dark matter or a modification to gravity. Observations on larger scales cannot be explained by the same modification of gravity that have been found to work on the scales of individual galaxies (while adding dark matter is successful), but that is not sufficient to disprove the idea of modified gravity on its own. (STEFANIA.DELUCA OF WIKIMEDIA COMMONS)

In 2005, a team of astronomers devised a clever test to investigate dark matter’s existence.

When two galaxy clusters collide — a cosmically rare but important event — its internal components behave differently.

The Bullet cluster, the first classic example of two colliding galaxy clusters where the key effect was observed. In the optical, the presence of two nearby clusters (left and right) can be clearly discerned. (NASA/STSCI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.)

The intergalactic gas must collide, slow, and heat up, creating shocks and emitting X-rays.

The X-ray observations of the Bullet Cluster, as taken by the Chandra X-ray observatory. Note the white portions of the image, which show gas that’s heated sufficiently that it requires a shock wave to explain. (NASA/CXC/CFA/M.MARKEVITCH ET AL., FROM MAXIM MARKEVITCH (SAO))

If there were no dark matter, this gas, comprising the majority of normal matter, should be the primary source of gravitational lensing.

The gravitational lensing map (blue), overlayed over the optical and X-ray (pink) data of the Bullet cluster. The mismatch of the locations of the X-rays and the inferred mass is undeniable. (X-RAY: NASA/CXC/CFA/M.MARKEVITCH ET AL.; LENSING MAP: NASA/STSCI; ESO WFI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.; OPTICAL: NASA/STSCI; MAGELLAN/U.ARIZONA/D.CLOWE ET AL.)

Instead, gravitational lensing maps indicate that most of the mass is displaced from the normal matter.

Four colliding galaxy clusters, showing the separation between X-rays (pink) and gravitation (blue), indicative of dark matter. On large scales, cold dark matter is necessary, and no alternative or substitute will do. However, mapping out the X-ray light (pink) is not necessarily a very good indication of the dark matter distribution (blue). (X-RAY: NASA/CXC/UVIC./A.MAHDAVI ET AL. OPTICAL/LENSING: CFHT/UVIC./A. MAHDAVI ET AL. (TOP LEFT); X-RAY: NASA/CXC/UCDAVIS/W.DAWSON ET AL.; OPTICAL: NASA/ STSCI/UCDAVIS/ W.DAWSON ET AL. (TOP RIGHT); ESA/XMM-NEWTON/F. GASTALDELLO (INAF/ IASF, MILANO, ITALY)/CFHTLS (BOTTOM LEFT); X-RAY: NASA, ESA, CXC, M. BRADAC (UNIVERSITY OF CALIFORNIA, SANTA BARBARA), AND S. ALLEN (STANFORD UNIVERSITY) (BOTTOM RIGHT))

This remains true for every set of post-collisional X-ray clusters ever measured.

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. Although some of the simulations we perform indicate that a few clusters may be moving faster than expected, the simulations include gravitation alone, and other effects like feedback, star formation, and stellar cataclysms may also be important for the gas. Without dark matter, these observations (along with many others) cannot be sufficiently explained. (X-RAY: NASA/CXC/ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND/D.HARVEY NASA/CXC/DURHAM UNIV/R.MASSEY; OPTICAL/LENSING MAP: NASA, ESA, D. HARVEY (ECOLE POLYTECHNIQUE FEDERALE DE LAUSANNE, SWITZERLAND) AND R. MASSEY (DURHAM UNIVERSITY, UK))

Only if gravity is non-local, or gravitating where the matter isn’t, could the Universe not contain dark matter.

(a) Projected distribution of dark matter in the COSMOS field from the analysis of Massey et al. (2007a). The blue map reveals the density of dark matter as inferred from the pattern of weak distortions viewed in background galaxies by the Hubble Space Telescope. (b) Equivalent map for the baryonic matter as revealed by a combination of the stellar mass in galaxies imaged with the Hubble Space Telescope and hot gas imaged with the X-ray satellite XMM–Newton. (R. ELLIS, PHILOS TRANS A MATH PHYS ENG SCI. 2010 MAR 13; 368(1914): 967–987)

But in pre-merger clusters, we clearly see that gravity is local: matter and gravity line up.

The contours, above, show the reconstructed mass of the galaxy cluster from gravitational lensing, while the points show observed galaxies, color-coded for a variety of redshifts. Where the cluster is quiescent, there’s no separation of matter from gravitation. (H.S. HWANG ET AL., APJ, 797, 2, 106)

Colliding clusters cannot obey different gravitational rules from non-colliding ones.

The colliding galaxy cluster “El Gordo,” the largest one known in the observable Universe, showing the same evidence of dark matter and normal matter as other colliding clusters. There is practically no room for antimatter, severely constraining the possibility of its presence in our Universe, while the gravitational signal is clearly misaligned with the presence of the normal matter, which is heated and emits X-rays. (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))

Inescapably, dark matter must therefore exist.

Clumps and clusters of galaxies exhibit gravitational effects on the light-and-matter behind them due to the effects of weak gravitational lensing. This enables us to reconstruct their mass distributions, which line up with the observed matter for non-colliding clusters, but which show displacement for post-collisional clusters, an observation that has never been satisfactorily explained without dark matter. (ESA, NASA, K. SHARON (TEL AVIV UNIVERSITY) AND E. OFEK (CALTECH))

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

Ethan Siegel is the author of Beyond the Galaxy and Treknology. You can pre-order his third book, currently in development: the Encyclopaedia Cosmologica.

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