[4097] Physics 316: Extragalactic Astronomy and Cosmology (Summary of Lectures)
Lecture 33

[L.S.Structure] The Solar Neutrino Problem

Until recently there was a long-standing problem associated with the detection rate of neutrinos from the Sun (the "solar neutrino problem").
  • the detected rates were only ~35% that expected
The solar neutrino problem was first discovered in the late 1960s by Ray Davis & collaborators using a detector in the Homestake Mine in South Dakota. The deficit has since been confirmed by several other detectors.

The deficit, if really reflecting a reduced production rate in the center of the Sun, clearly has implications for out understanding of nucleosnthesis (we don't understand it as well as we thought !).

The nuclearsynthesis reactions currently underway in the core of the Sun are primarily producing electron-neutrinos. Previous neutrino detectors were primarily sensitive to electron-neutrinos.

  • so all ought to have been well....

A popular solution to the solar neutrino problem was that many of the electron-neutrinos change flavour (into muon- & tauon-neutrinos) during their passage from their site of production to the Earth. This process (known as an "oscillation") can indeed take place during the passage of an electron-neutrino through and inhomogeneous medium via the Mikheyev-Smirnov-Wolfenstein (MSW) effect.

  • thus neutrino oscillations could have changed many of the electron-neutrinos produced in the Solar core to other flavours which previous detectors were insensitive to.

[L.S.Structure] The Solar Neutrino Problem - Solved !

(Obviously Bothun was written prior to these developments).

In 2001 June results were announced from the Sudbury Nutrino Observatory (SNO). This uses a detector which is sensitive to all three types of neutrinos (electron, muon & tauon).
  • The total neutrino flux from the Sun is equal to the electron-neutrino production rate in the core due to nucleosynthesis
    • the "Solar neutrino problem" solved !

New results announced 2002 April 20
Neutrino oscillations are confirmed at a higher level of confidence by the SNO Team

(SNO Links) 		[Image Credit:SNO]

This has implications for cosmology since in order for the MSW effect to work, and neutrinos to be able to "oscillate" between their flavours, they must have a non-zero mass. Combined results from SNO and SuperKamiokande suggest a mass range 0.05 - 0.18 eV

  • This has implications for the Standard Model of Particle Physics (which predicts neutrinos are massless)
(SuperKamiokande Links) 		[Image Credit:SuperKamiokande]

As we have already discussed, neutrinos are also produced during the Big Bang.

  • Indeed the predicted number of neutrinos is approximately Equal to the energy density of CMB photons
  • few x108 m-3
If they have non-zero mass, could neutrinos be the Dark Matter ?

[L.S.Structure] DM Problem - Not Solved !

Critical Density
(OmegaTot = 1)
(for H0 = 65 km s-1 Mpc-1) 		4.5x109 eV cm-3
number density of neutrinos
(from the Big Bang) 		few x108 m-3

So for a range of
neutrino mass
0.05 - 0.18 eV
Omeganu/OmegaTot 4x10-3 - 0.14

Neutrinos appear to have insufficient mass to close the universe (on their own)

The very recent results (2002 Apr 09) from the 2DF team mentioned in class last time indicate that indeed Omeganu < 0.2 OmegaM

Some Neutrino Links

[L.S.Structure] Further Implications of DM

(Summary of Bothun [Sect4.7])
  • If
    DM is dominated by Stellar remnants, there are implications for the (heavy elements - ie the "metals") "chemical evolution" of various systems.
    • We have discussed several times how the characteristics of many astrophysical systems is dependent on Metallicity
    • Galaxies which are "metal-rich" may have more DM in the form of stellar remnants than "metal-poor" galaxies
      • This leads to zero-point calibation errors in techiques such as Tully-Fisher and hence in our distance indicators, and thus our determination of H0.
  • If
    some other kind of DM behaves such that it is "laid down" in waves during the early universe, & the galaxies form within these variations,
    • some galaxies will form containing more DM than other.
      • apparently "identical" galaxies could have different M/L due to this, resulting in systematic errors in the calibration of relative distances along different lines-of-sight when using techniques such as Tully-Fisher

As stated by Bothun [p178]
...one needs to ask seriously if the flows are peculiar or the galaxies are peculiar.

Since we do not understand the process of galaxy formation, there may well be slight differences between how much [DM] is mixed into Local Group galaxies compared to those [50+ Mpc] away.

    This would produce a zeropoint shift in the calibration of the distance indicators,
      ...which would turn produce a false peculiar velocity signal due to systematic errors in the determination of relative distance.

[L.S.Structure] Summary of Bothun Chapt 4

(Summary of Bothun [Sect4.7])

Despite not having yet identified the cause of the DM (out of the many candidates), we can see its gravitational influence. Furthermore the Inflationary paradigm, with its prediction that OmegaT =1 is theoretically very attractive (solves several problems) - and is gaining observational support from recent observations of the CMB

  • Flat rotation curves in many Spiral galaxies provides clear observational evidence for DM - the gravitational potential must extend the visible stellar population for such rotation laws to be possible.
    • whether such a DM component is present in all galaxies is currently unclear.
  • The large X-ray emitting halos seen in many Elliptical galaxies provides clear observational evidence for DM - the gravitational potential must be greater than that implied by the stellar population or else the gas would have been able to escape.
    • whether such a DM component is present in all galaxies is currently unclear.
  • The large X-ray emitting "cooling flows" seen in many Clusters of galaxies provides clear observational evidence for DM - the gravitational potential must be greater than that implied by the visible stars within the galaxies or else the gas would have been able to escape.
    • sub-clustering and the presence of cooling gas (& possibly energy injection mechanisms) make the M/L ratio difficult to measure.
  • We are still are not sure of the value of OmegaM. However most observations are pointing towards OmegaM ~0.3
    • We belive Omegab = 0.04
    Thus the search for the remaining DM
  • If indeed OmegaTot=1 and OmegaM ~0.3, then we need a non-zero Lambda
    • This "helps" us reconcile the age of the universe with the maximium ages of objects within it
      • the oldest Globular clusters
      • dust in high-z quasars
      • etc
    • There is independent evidence of acceleration from the SNe 1a studies

End of lecture

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