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Neutrino oscillations may also explain the low number of observed solar neutrinos. Detectors for observing solar neutrinos are specifically designed to detect electron neutrinos ν e size 12{ν rSub { size 8{e} } } {} produced in huge numbers by fusion in the Sun. A large fraction of electron neutrinos ν e size 12{ν rSub { size 8{e} } } {} may be changing flavor to muon neutrinos v μ size 12{v rSub { size 8{μ} } } {} on their way out of the Sun, possibly enhanced by specific interactions, reducing the flux of electron neutrinos to observed levels. There is also a discrepancy in observations of neutrinos produced in cosmic ray showers. While these showers of radiation produced by extremely energetic cosmic rays should contain twice as many v μ size 12{v rSub { size 8{μ} } } {} s as ν e size 12{ν rSub { size 8{e} } } {} s, their numbers are nearly equal. This may be explained by neutrino oscillations from muon flavor to electron flavor. Massive neutrinos are a particularly appealing possibility for explaining dark matter, since their existence is consistent with a large body of known information and explains more than dark matter. The question is not settled at this writing.

The most radical proposal to explain dark matter is that it consists of previously unknown leptons (sometimes obtusely referred to as non-baryonic matter). These are called weakly interacting massive particles , or WIMPs    , and would also be chargeless, thus interacting negligibly with normal matter, except through gravitation. One proposed group of WIMPs would have masses several orders of magnitude greater than nucleons and are sometimes called neutralinos    . Others are called axions     and would have masses about 10 10 size 12{"10" rSup { size 8{ - "10"} } } {} that of an electron mass. Both neutralinos and axions would be gravitationally attached to galaxies, but because they are chargeless and only feel the weak force, they would be in a halo rather than interact and coalesce into spirals, and so on, like normal matter (see [link] ).

The figure shows a satellite in orbit above the glowing atmosphere of the Earth. The satellite is tube shaped and has a cover that is open at one end. It has two solar panels and is covered in what looks like aluminum foil.
The Hubble Space Telescope is producing exciting data with its corrected optics and with the absence of atmospheric distortion. It has observed some MACHOs, disks of material around stars thought to precede planet formation, black hole candidates, and collisions of comets with Jupiter. (credit: NASA (crew of STS-125))
A close-up photograph of a slow-moving stream carrying leaves.
Dark matter may shepherd normal matter gravitationally in space, as this stream moves the leaves. Dark matter may be invisible and even move through the normal matter, as neutrinos penetrate us without small-scale effect. (credit: Shinichi Sugiyama)

Some particle theorists have built WIMPs into their unified force theories and into the inflationary scenario of the evolution of the universe so popular today. These particles would have been produced in just the correct numbers to make the universe flat, shortly after the Big Bang. The proposal is radical in the sense that it invokes entirely new forms of matter, in fact two entirely new forms, in order to explain dark matter and other phenomena. WIMPs have the extra burden of automatically being very difficult to observe directly. This is somewhat analogous to quark confinement, which guarantees that quarks are there, but they can never be seen directly. One of the primary goals of the LHC at CERN, however, is to produce and detect WIMPs. At any rate, before WIMPs are accepted as the best explanation, all other possibilities utilizing known phenomena will have to be shown inferior. Should that occur, we will be in the unanticipated position of admitting that, to date, all we know is only 10% of what exists. A far cry from the days when people firmly believed themselves to be not only the center of the universe, but also the reason for its existence.

Section summary

  • Dark matter is non-luminous matter detected in and around galaxies and galactic clusters.
  • It may be 10 times the mass of the luminous matter in the universe, and its amount may determine whether the universe is open or closed (expands forever or eventually stops).
  • The determining factor is the critical density of the universe and the cosmological constant, a theoretical construct intimately related to the expansion and closure of the universe.
  • The critical density ρ c is the density needed to just halt universal expansion. It is estimated to be approximately 10 –26 kg/m 3 .
  • An open universe is negatively curved, a closed universe is positively curved, whereas a universe with exactly the critical density is flat.
  • Dark matter’s composition is a major mystery, but it may be due to the suspected mass of neutrinos or a completely unknown type of leptonic matter.
  • If neutrinos have mass, they will change families, a process known as neutrino oscillations, for which there is growing evidence.

Conceptual questions

Discuss the possibility that star velocities at the edges of galaxies being greater than expected is due to unknown properties of gravity rather than to the existence of dark matter. Would this mean, for example, that gravity is greater or smaller than expected at large distances? Are there other tests that could be made of gravity at large distances, such as observing the motions of neighboring galaxies?

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How does relativistic time dilation prohibit neutrino oscillations if they are massless?

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If neutrino oscillations do occur, will they violate conservation of the various lepton family numbers ( L e size 12{L rSub { size 8{e} } } {} , L μ size 12{L rSub { size 8{μ} } } {} , and L τ size 12{L rSub { size 8{τ} } } {} )? Will neutrino oscillations violate conservation of the total number of leptons?

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Lacking direct evidence of WIMPs as dark matter, why must we eliminate all other possible explanations based on the known forms of matter before we invoke their existence?

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Problems exercises

If the dark matter in the Milky Way were composed entirely of MACHOs (evidence shows it is not), approximately how many would there have to be? Assume the average mass of a MACHO is 1/1000 that of the Sun, and that dark matter has a mass 10 times that of the luminous Milky Way galaxy with its 10 11 size 12{"10" rSup { size 8{"11"} } } {} stars of average mass 1.5 times the Sun’s mass.

1 . 5 × 10 15 size 12{1 "." 5 times "10" rSup { size 8{"15"} } } {}
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The critical mass density needed to just halt the expansion of the universe is approximately 10 26 kg / m 3 size 12{"10" rSup { size 8{ - "26"} } `"kg"/m rSup { size 8{3} } } {} .

(a) Convert this to eV / c 2 m 3 size 12{"eV"/c rSup { size 8{2} } cdot m rSup { size 8{3} } } {} .

(b) Find the number of neutrinos per cubic meter needed to close the universe if their average mass is 7 eV / c 2 size 12{7`"eV"/c rSup { size 8{2} } } {} and they have negligible kinetic energies.

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Assume the average density of the universe is 0.1 of the critical density needed for closure. What is the average number of protons per cubic meter, assuming the universe is composed mostly of hydrogen?

0 . 6 m 3 size 12{0 "." 6`m rSup { size 8{ - 3} } } {}
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To get an idea of how empty deep space is on the average, perform the following calculations:

(a) Find the volume our Sun would occupy if it had an average density equal to the critical density of 10 26 kg / m 3 size 12{"10" rSup { size 8{ - "26"} } `"kg"/m rSup { size 8{3} } } {} thought necessary to halt the expansion of the universe.

(b) Find the radius of a sphere of this volume in light years.

(c) What would this radius be if the density were that of luminous matter, which is approximately 5% size 12{5%} {} that of the critical density?

(d) Compare the radius found in part (c) with the 4-ly average separation of stars in the arms of the Milky Way.

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Questions & Answers

explain how a light wave can be propagated in accordance with the principal of reversibility
Chibuzo Reply
what is the meaning of musical instruments
Chibuzo
the definition of photon
Bright Reply
8kg of a hot liquid initial T is 90°© is missed with another liquid 3kg at 20° calculate e équilibrium T
Balki Reply
8kg of a hot liquid initial T is 90°© is missed with another liquid 3kg at 20° calculate e équilibrium T
Balki
answer plz
Bright
what are the products when acid and base mixed?
Austin
salt and water
Gift
What's this place about?
michael
@Austin your answer should be SALT and WATER
michael
salts and water
Roben
i guess salt and water
Chibuzo
what work done
Dennis Reply
work done is the product of force and distance moved in the direction of force
Gift
Work done = force (F) * distance (D)
abdulsalam
what is resounance
Abdul
y
Tracy
explain the three laws of isaac Newton with the reference
glory Reply
1st law ; a body will continue to stay at a state of rest or continue to move at a uniform motion on a straight line unless an external force is been acted upon
Austine
3rd law; in every action there is an equal or opposite reaction
Austine
2nd law: F=ma
Austine
why am i not having access to the Link in your exemples /figures ?
Augustine Reply
what is circut
hasiya Reply
newtons law of motion
hasiya
First law:In an inertial frame of reference, an object either remains at rest or continues to move at a constant velocity, unless acted upon by a force.
Manan
a circuit is an electric part Wich is linked by a wire
Chibuzo
is the ability to do work
Adjah Reply
Energy
Nwany
u from
Hejreen
any body online hain
Hejreen
ability to do work is energy
Irshad
energy
Chibuzo
what is energy
Mercy Reply
energy is ability of the capacity to doing work
shafiu
what is vector
mosco Reply
A quantity that has both magnitude and direction
Donaldo
can a body with out mass float in space
mosco
Is the quantity that has both magnitude and direction
Amoah
Yes it can float in space,e.g.polyethene has no mass that's why it can float in space
Amoah
that's my suggestion,any other explanation can be given also,thanks
Amoah
A charge of 1.6*10^-6C is placed in a uniform electric field in a density 2*5^10Nc^-1, what is the magnitude of the electric force exerted on the charge?
Omotosho Reply
what's phenomena
Enoch Reply
Phenomena is an observable fact or event.
Love
Prove that 1/d+1/v=1/f
James Reply
What interference
Moyinoluwa Reply
What is a polarized light called?
Moyinoluwa
what is a half life
Mama Reply
the time taken for a radioactive element to decay by half of its original mass
ken
what is radioactive element
mohammed
Half of the total time required by a radioactive nuclear atom to totally disintegrate
Justice
radioactive elements are those with unstable nuclei(ie have protons more than neutrons, or neutrons more than protons
Justice
in other words, the radioactive atom or elements have unequal number of protons to neutrons.
Justice
state the laws of refraction
Fabian
state laws of reflection
Fabian
Why does a bicycle rider bends towards the corner when is turning?
Mac
When do we say that the stone thrown vertically up wards accelerate negatively?
Mac
Give two importance of insulator placed between plates of a capacitor.
Mac
Macho had a shoe with a big sole moving in mudy Road, shanitah had a shoe with a small sole. Give reasons for those two cases.
Mac

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Source:  OpenStax, College physics. OpenStax CNX. Jul 27, 2015 Download for free at http://legacy.cnx.org/content/col11406/1.9
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