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LUMINOUS MATTER

LUMINOUS MATTER. W luminous = 0.004 The matter that astronomers see in the Universe (stars, dust clouds, etc.) makes up less than 1/2 of one percent of the amount required to ‘close’ it , i.e. , to make it a ‘flat’ universe ( W =1). BARYONIC DARK MATTER. Galactic Rotation Curves

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LUMINOUS MATTER

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  1. LUMINOUS MATTER • Wluminous = 0.004 • The matter that astronomers see in the Universe (stars, dust clouds, etc.) makes up less than 1/2 of one percent of the amount required to ‘close’ it, i.e., to make it a ‘flat’ universe (W=1).

  2. BARYONIC DARK MATTER • Galactic Rotation Curves • Spiral galaxies do not rotate the way they should if their mass were distributed according to the distribution of luminous matter. Since the majority of the luminous matter is concentrated in the galactic center, the material in the spiral arms should rotate more slowly as the distance from the galactic center increases…just as the outer planets revolve around the Sun slower than does the Earth. Instead, the observed rotational velocity remains constant with distance from the galactic center. • This observation can be understood if the spiral galaxies are embedded in a huge sphere of ‘dark matter’. The total mass of a galaxy then turns out to be about 5 times larger than the luminous mass, so Wgalactic= 0.02. This is less than WBBNS= 0.04, which implies that all this matter could be ‘baryonic’ dark matter, i.e., made of normal neutrons and protons.

  3. BARYONIC DARK MATTER • Massive Compact Halo Objects (MACHOs) • What is this dark matter in the galactic ‘halo’? Possible candidates: primordial black holes, brown dwarves, etc. Recently, objects of this kind have been located via gravitationalmicrolensing, the effect that they have on light coming from distant galaxies. Such objects have been called MACHOs…an acronym for Massive Compact Halo Objects.

  4. NON-BARYONIC DARK MATTER? • Great Attractors, etc. • The motion of galaxies within clusters indicates that there is a lot more missing mass than even the galactic rotation curves would indicate. An example is the Great Attractor in Centaurus that is mainly responsible for the dipole component of the CMB. When this is taken into account, its seems that the ‘best bet’ for the amount of mass in the Universe gives Wmatter=0.3. Since this is greater than the limit on W from BBNS, it appears that most of the matter in the Universe is ‘exotic’ non-baryonic dark matter.

  5. NON-BARYONIC DARK MATTER? • Dark Matter Types • Hot Dark Matter: Massive Neutrinos • There are many neutrinos in the Universe (Cosmic Neutrino Background)---about 109 times the number of nucleons. [At any time, twenty million cosmic neutrinos are zipping through your body]. • They are now known to have a small mass, but probably too small to close the Universe. The best estimates give Wn = 0.001. • Another problem: with such a small mass, their velocity during galaxy formation would have been quite high (hence, HOT dark matter). Calculations indicate that this would completely disrupt galaxy formation, so massive neutrinos do not seem to be the answer. • Cold Dark Matter • Weakly Interacting Massive Particles (WIMPs)? • Axions?

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