Observational evidence Dark matter

1 observational evidence

1.1 galaxy rotation curves
1.2 velocity dispersions
1.3 galaxy clusters
1.4 gravitational lensing
1.5 cosmic microwave background
1.6 structure formation
1.7 type ia supernova distance measurements
1.8 sky surveys , baryon acoustic oscillations
1.9 redshift-space distortions
1.10 lyman-alpha forest

observational evidence


this artist’s impression shows expected distribution of dark matter in milky way galaxy blue halo of material surrounding galaxy.



dark matter map of kids survey region (region g12).


galaxy rotation curves


rotation curve of typical spiral galaxy: predicted (a) , observed (b). dark matter can explain flat appearance of velocity curve out large radius.




comparison of rotating disc galaxies in distant universe , present day. imaginary galaxy on left in nearby universe , stars in outer parts orbiting rapidly due presence of large amounts of dark matter around central regions. on other hand, galaxy @ right, in distant universe, , seen ten billion years ago, rotating more in outer parts dark matter more diffuse. size of difference exaggerated in schematic view make effect clearer. distribution of dark matter shown in red.

the arms of spiral galaxies rotate around galactic centre. luminous mass density of spiral galaxy decreases 1 goes centre outskirts. if luminous mass matter, can model galaxy point mass in centre , test masses orbiting around (similar solar system). kepler s second law, expect rotation velocities decrease distance centre, similar our solar system. not observed. instead, galaxy rotation curve remains flat distant centre data available.

if assume validity of kepler s laws, obvious way resolve discrepancy conclude mass distribution in spiral galaxies not similar of solar system. in particular, there lot of non-luminous matter in outskirts of galaxy ( dark matter ).

velocity dispersions

stars in bound systems must obey virial theorem. theorem, measured velocity distribution, can used measure mass distribution in bound system, such elliptical galaxies or globular clusters. exceptions, velocity dispersion estimates of elliptical galaxies not match predicted velocity dispersion observed mass distribution, assuming complicated distributions of stellar orbits.

as galaxy rotation curves, obvious way resolve discrepancy postulate existence of non-luminous matter.

galaxy clusters

strong gravitational lensing observed hubble space telescope in abell 1689 indicates presence of dark matter—enlarge image see lensing arcs.

galaxy clusters particularly important dark matter studies since masses can estimated in 3 independent ways:

from scatter in radial velocities of galaxies within clusters
from x-rays emitted hot gas in clusters. x-ray energy spectrum , flux, gas temperature , density can estimated, hence giving pressure; assuming pressure , gravity balance determines cluster s mass profile.
gravitational lensing (usually of more distant galaxies) can measure cluster masses without relying on observations of dynamics (e.g., velocity).

generally, these 3 methods in reasonable agreement dark matter outweighs visible matter approximately 5 1.

gravitational lensing

one of consequences of general relativity massive objects (such cluster of galaxies) lying between more distant source (such quasar) , observer should act lens bend light source. more massive object, more lensing observed.

strong lensing observed distortion of background galaxies arcs when light passes through such gravitational lens. has been observed around many distant clusters including abell 1689. measuring distortion geometry, mass of intervening cluster can obtained. in dozens of cases has been done, mass-to-light ratios obtained correspond dynamical dark matter measurements of clusters. lensing can lead multiple copies of image. analyzing distribution of multiple image copies, scientists have been able deduce , map distribution of dark matter around macs j0416.1-2403 galaxy cluster.

weak gravitational lensing investigates minute distortions of galaxies, using statistical analyses vast galaxy surveys. examining apparent shear deformation of adjacent background galaxies, mean distribution of dark matter can characterized. mass-to-light ratios correspond dark matter densities predicted other large-scale structure measurements.

cosmic microwave background


the cosmic microwave background wmap

although both dark matter , ordinary matter matter , not behave in same way. in particular, in universe, ordinary matter ionized , interacted radiation via thomson scattering. dark matter not interact directly radiation, affect cmb gravitational potential (mainly on large scales), , effects on density , velocity of ordinary matter. ordinary , dark matter perturbations therefore evolve differently time, , leave different imprints on cosmic microwave background (cmb).

the cosmic microwave background close perfect blackbody, contains small temperature anisotropies of few parts in 100,000. sky map of anisotropies can decomposed angular power spectrum, observed contain series of acoustic peaks @ near-equal spacing different heights. series of peaks can predicted assumed set of cosmological parameters modern computer codes such cmbfast , camb, , matching theory data therefore constrains cosmological parameters. first peak shows density of baryonic matter, while third peak relates density of dark matter, measuring density of matter , density of atoms.

the cmb anisotropy first discovered cobe in 1992, though had coarse resolution detect acoustic peaks. after discovery of first acoustic peak balloon-borne boomerang experiment in 2000, power spectrum precisely observed wmap in 2003-12, , more precisely planck spacecraft in 2013-15. results support lambda-cdm model.

the observed cmb angular power spectrum provides powerful evidence in support of dark matter, precise structure fitted lambda-cdm model difficult reproduce competing model such mond.

structure formation


3d map of large-scale distribution of dark matter, reconstructed measurements of weak gravitational lensing hubble space telescope.

structure formation refers period after big bang when density perturbations collapsed form stars, galaxies, , clusters. prior structure formation, friedmann solutions general relativity describe homogeneous universe. later, small anisotropies gradually grew , condensed homogeneous universe stars, galaxies , larger structures. ordinary matter affected radiation, dominant element of universe @ times. result, density perturbations washed out , unable condense structure. if there ordinary matter in universe, there not have been enough time density perturbations grow galaxies , clusters see today.

dark matter provides solution problem because unaffected radiation. therefore, density perturbations can grow first. resulting gravitational potential acts attractive potential ordinary matter collapsing later, speeding structure formation process.

type ia supernova distance measurements

type ia supernovae can used standard candles measure extragalactic distances, can in turn used measure how fast universe has expanded in past. data indicates universe expanding @ accelerating rate, cause of ascribed dark energy. since observations indicate universe flat, expect total energy density of in universe sum 1 (Ωtot ~ 1). measured dark energy density ΩΛ = ~0.690; observed ordinary matter energy density Ωm = ~0.0482 , energy density of radiation negligible. leaves missing Ωdm = ~0.258 nonetheless behaves matter (see technical definition section above) – dark matter.

sky surveys , baryon acoustic oscillations

baryon acoustic oscillations (bao) regular, periodic fluctuations in density of visible baryonic matter (normal matter) of universe. these predicted arise in lambda-cdm model due universe s acoustic oscillations in photon-baryon fluid , can observed in cosmic microwave background angular power spectrum. baos set preferred length scale baryons. dark matter , baryons clumped after recombination, effect weaker in galaxy distribution in nearby universe, detectable subtle (~ 1 percent) preference pairs of galaxies separated 147 mpc, compared separated 130 or 160 mpc. feature predicted theoretically in 1990s , discovered in 2005, in 2 large galaxy redshift surveys, sloan digital sky survey , 2df galaxy redshift survey. combining cmb observations bao measurements galaxy redshift surveys provides precise estimate of hubble constant , average matter density in universe. results support lambda-cdm model.

redshift-space distortions

large galaxy redshift surveys may used make three-dimensional map of galaxy distribution. these maps distorted because distances estimated observed redshifts; redshift contains contribution galaxy s so-called peculiar velocity in addition dominant hubble expansion term. on average, superclusters expanding more cosmic mean due gravity, while voids expanding faster average. in redshift map, galaxies in front of supercluster have excess radial velocities towards , have redshifts higher distance imply, while galaxies behind supercluster have redshifts low distance. effect causes superclusters appear squashed in radial direction, , likewise voids stretched ; angular positions unaffected. effect not detectable 1 structure since true shape not known, can measured averaging on many structures assuming not @ special location in universe.

the effect predicted quantitatively nick kaiser in 1987, , first decisively measured in 2001 2df galaxy redshift survey. results in agreement lambda-cdm model.

lyman-alpha forest

in astronomical spectroscopy, lyman-alpha forest sum of absorption lines arising lyman-alpha transition of neutral hydrogen in spectra of distant galaxies , quasars. lyman-alpha forest observations can constrain cosmological models. these constraints agree obtained wmap data.

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