https://en.wikipedia.org/wiki/Dark_matter
Dark matter is a hypothetical type of matter distinct from baryonic matter (ordinary matter such as protons and neutrons), neutrinos and dark energy.
Dark matter has never been directly observed; however, its existence would explain a number of otherwise puzzling astronomical observations.[1][2]
The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum.[3] Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter,[4] gravitational lensing, its influence on the universe's large-scale structure, on galaxies, and its effects on the cosmic microwave background.
The standard model of cosmology indicates that the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[5][6][7][8]
Thus, dark matter constitutes 84.5%[note 1] of total mass, while
dark energy plus dark matter constitute 95.1% of total mass–energy content.[9][10][11][12]
The great majority of ordinary matter in the universe is also unseen, since visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass-energy density of the universe.[13]
The most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles (WIMPs) that interact only through gravity and the weak force.[14]
The dark matter hypothesis plays a central role in current modeling of cosmic structure formation, galaxy formation and evolution, and on explanations of the anisotropies observed in the cosmic microwave background (CMB).
All these lines of evidence suggest that galaxies, galaxy clusters, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals.[15]
Many experiments to detect proposed dark matter particles through non-gravitational means are under way;[16] however, no dark matter particle has been conclusively identified.
Although the existence of dark matter is generally accepted by most of the astronomical community, a minority of astronomers,[17] motivated by the lack of conclusive identification of dark matter, or by observations that don't fit the model,[18] argue for various modifications of the standard laws of general relativity, such as MOND, TeVeS, and conformal gravity[19] that attempt to account for the observations without invoking additional matter.[20]
The more massive an object, the more lensing is observed.
Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens.
It has been observed around many distant clusters including Abell 1689.[53]
By measuring the distortion geometry, the mass of the intervening cluster can be obtained.
In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters.[54]
Lensing can lead to multiple copies of an image.
By analyzing the distribution of multiple image copies, scientists have been able to deduce and map the distribution of dark matter around the MACS J0416.1-2403 galaxy cluster.[55][56]
Weak gravitational lensing investigates minute distortions of galaxies, using statistical analyses from vast galaxy surveys.
By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized.
The mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.[57]
Dark matter & dark energy.
Dark Matter Candidates
Dark matter is a hypothetical type of matter distinct from baryonic matter (ordinary matter such as protons and neutrons), neutrinos and dark energy.
Dark matter has never been directly observed; however, its existence would explain a number of otherwise puzzling astronomical observations.[1][2]
The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum.[3] Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter,[4] gravitational lensing, its influence on the universe's large-scale structure, on galaxies, and its effects on the cosmic microwave background.
The standard model of cosmology indicates that the total mass–energy of the universe contains 4.9% ordinary matter, 26.8% dark matter and 68.3% dark energy.[5][6][7][8]
Thus, dark matter constitutes 84.5%[note 1] of total mass, while
dark energy plus dark matter constitute 95.1% of total mass–energy content.[9][10][11][12]
The great majority of ordinary matter in the universe is also unseen, since visible stars and gas inside galaxies and clusters account for less than 10% of the ordinary matter contribution to the mass-energy density of the universe.[13]
The most widely accepted hypothesis on the form for dark matter is that it is composed of weakly interacting massive particles (WIMPs) that interact only through gravity and the weak force.[14]
The dark matter hypothesis plays a central role in current modeling of cosmic structure formation, galaxy formation and evolution, and on explanations of the anisotropies observed in the cosmic microwave background (CMB).
All these lines of evidence suggest that galaxies, galaxy clusters, and the universe as a whole contain far more matter than that which is observable via electromagnetic signals.[15]
Many experiments to detect proposed dark matter particles through non-gravitational means are under way;[16] however, no dark matter particle has been conclusively identified.
Although the existence of dark matter is generally accepted by most of the astronomical community, a minority of astronomers,[17] motivated by the lack of conclusive identification of dark matter, or by observations that don't fit the model,[18] argue for various modifications of the standard laws of general relativity, such as MOND, TeVeS, and conformal gravity[19] that attempt to account for the observations without invoking additional matter.[20]
Contents
- 1 History
- 2 Technical definition
- 3 Observational evidence
- 3.1 Galaxy rotation curves
- 3.2 Velocity dispersions
- 3.3 Galaxy clusters
- 3.4 Gravitational lensing
- 3.5 Cosmic microwave background
- 3.6 Sky surveys and baryon acoustic oscillations
- 3.7 Redshift-space distortions
- 3.8 Type Ia supernova distance measurements
- 3.9 Lyman-alpha forest
- 3.10 Structure formation
- 4 Composition of dark matter: baryonic vs. nonbaryonic
- 5 Classification of dark matter: cold, warm or hot
- 6 Detection of dark matter particles
- 7 Alternative theories
- 8 In philosophy of science
- 9 In popular culture
- 10 See also
- 11 Notes
- 12 References
- 13 External links
Gravitational lensing
One of the consequences of general relativity is that massive objects should act as a lens to bend the light from a more distant source (such as a quasar) around a massive object (such as a cluster of galaxies) lying between the source and the observer.The more massive an object, the more lensing is observed.
Strong lensing is the observed distortion of background galaxies into arcs when their light passes through such a gravitational lens.
It has been observed around many distant clusters including Abell 1689.[53]
By measuring the distortion geometry, the mass of the intervening cluster can be obtained.
In the dozens of cases where this has been done, the mass-to-light ratios obtained correspond to the dynamical dark matter measurements of clusters.[54]
Lensing can lead to multiple copies of an image.
By analyzing the distribution of multiple image copies, scientists have been able to deduce and map the distribution of dark matter around the MACS J0416.1-2403 galaxy cluster.[55][56]
Weak gravitational lensing investigates minute distortions of galaxies, using statistical analyses from vast galaxy surveys.
By examining the apparent shear deformation of the adjacent background galaxies, the mean distribution of dark matter can be characterized.
The mass-to-light ratios correspond to dark matter densities predicted by other large-scale structure measurements.[57]
Dark matter & dark energy.
Ordinary
Matter
vs
Dark
Matter
•
Matter occupies26% and dark energy occupies74% ofthe energy of the universe at present.
•
What do we mean by“matter”?
–
Ordinary
Matter atoms (baryons)
•
Atoms exist mostly in the formof gas
–
Extra-ordinary Matter: Dark Matter Dark matter exists in the form of
freely-moving particles
•
Observations of deuterium abundance in the cold gas
clouds as well as anisotropy of the cosmic microwave
background suggest that the atoms contribute only to 4%
of the energy of the universe.
Therefore, dark matter must contribute to 22% of the energy of the universe.
Therefore, dark matter must contribute to 22% of the energy of the universe.
What Is Dark Matter?
•
We do not know the precise nature of dark matter.
particles, but we do know some of their properties.
•
Dark matter interacts with the ordinary matter only
weakly and gravitationally.
–
Why? We must have detected dark matter particles already
otherwise.
–
Dark matter does not emit light or absorb light, which means
that dark matter does not interact via the electro-magnetic force.
–
Dark matter does not have any charges.
•
Dark matter particles should move slowly, much more
slowly than the speed of light.
–
Why? Galaxies would not be formed otherwise.
–
Dark matter particles that move slowly are called “cold” dark
matter.
–
Neutrinos are “hot” dark matter, as they move at nearly the speed of light.
Dark Matter Candidates
•
It is most likely that dark matter particles are
something that we have not seen in the laboratory yet.
What could they be?
•
Particles of
“supersymmetry
”
?
–
All the particles may be divided into two classes: bosons and fermions
, depending on their spins.
•
Bosons have integer spins:photons,pions,gravitons,
...
•
Fermions have half-integer spins:electrons,neutrinos,quarks,
...
–
Theory of “Super symmetry ” requires that each fermion have the corresponding boson, and vice versa.
For example,
For example,
photon’s fermionic superpartner is called photino, and
electron’s bosonic superpartner is called selectron
.
–
One of the popular supersymmetric dark matter candidates
is gravitino, a fermionic superpartner of graviton.
•
But, these are still theoretical possibilities
...
How Do We Detect
Dark Matter?
•
Very exciting possibility
:
direct detection of super-particles may actually be possible
in 2007- LHC (Large Hadron Collider) @ CERN (Geneva)
–
Collide two proton beams to create lots of particles:
LHC will reach the energy that is equivalent of 7x10^16
K!!
–
Physicists are expecting to detect supersymmetric
particles in LHC, finding evidence for Supersymmetry.
•What if they found nothing?
–
They would need to build a yet larger accelerator
...
What
Is
Dark
Energy
?
•
The present universe is accelerating, which implies that
we have a positive cosmological constant in the universe.
But, what is cosmological constant anyway?
–
Could it be something else?
Dark Energy.
•
We know very little about dark energy.
–
We may have two dark energy components.
•
Early dark energy which caused inflation in the very early universe.
•Late dark energy which is causing the universe to accelerate now.
–
Dark energy influences visible or dark matter via gravity only.
–
Dark energy is very smooth: it cannot cluster.
–
Dark energy has a large,negative pressure.
Dark Energy Candidates
•
Energy of vacuum
(energy of empty space)
•
Quintessense
•
Modification
to
Einstein
’s
General
Theory
of
Gravity
•
None
of
the
above
How
Do
We
Detect
Dark
Energy?
•
There are no
“particles
” of dark energy
–
So, there is no way to detect dark energy directly.
•
To constrain the nature of dark energy, one has to
determine how exactly the expansion of the universe is
accelerating. Therefore, the cosmological observations
are the only methods that we can use to constrain dark
energy properties.
–
Brightness-redshift relation (luminosity distance)
–
Size-redshift relation (angular diameter distance)
•
The key parameter is
w
. (
w
=pressure/density)
–
Density of dark energy
evolves as 1/
R
3(1+
w
)
–
w=-1: density does not change
!
cosmological constant
–
The current observational data: w<-0 .="" font="">
8
•
Determination of
w
is
the
most important subject in
modern cosmology now.
26 august 2017
26 august 2017
The name refers to the fact that it does not emit or interact with electromagnetic radiation, such as light, and is thus invisible to the entire electromagnetic spectrum.[3] Although dark matter has not been directly observed, its existence and properties are inferred from its gravitational effects such as the motions of visible matter,[4] gravitational lensing, its influence on the universe's large-scale structure, on galaxies, and its effects on the cosmic microwave background.
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