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Underground Lab To Probe Ratio of Matter To Antimatter
Wired reports on the Enriched Xenon Observatory 200, a particle detector scientists hope will answer the question of why there is significantly more matter than antimatter in the universe. Quoting:
"The new detector will try to fill in the picture, determining basic features of [neutrinos], like their mass and whether or not they, unlike almost all other particles, are their own antiparticles. That quirk is why some scientists believe neutrinos could be the mechanism for the creation of our matter-filled universe. Almost all other particles have an antiparticle twin that, if it comes into contact with the particle, immediately annihilates it. But if neutrinos are their own antiparticles they could conceivably be knocked onto matter's 'team,' thereby causing the cascading win for matter over antimatter that we know occurred. As the Indian theoretical physicist G. Rajasekaran put it in a speech [PDF] earlier this year, neutrinos that are their own antiparticles would explain 'how, after [the] annihilation of most of the particles with antiparticles, a finite but small residue of particles was left to make up the present Universe.'"
If there had been an exactly equal amount of matter and antimatter since the birth of the universe, it would have long ago annihilated into photons. Since this has manifestly not occurred, one is in excess of the other. Which one you label as "matter" and which one "antimatter" is purely down to personal preference.
Some differences can be observed in the behaviour of matter and antimatter; for example, in a magnetic field, electrons will curve in one direction and positrons in another. Another example is the asymmetries observed in the oscillations of mesons (the classic example being the K0) which reveal a clear, fundamental difference between matter and antimatter.
If these particle detector scientists watched TNG, they'd know that there's only one ratio of matter to antimatter.
Who says there is more matter than antimatter in the universe? Has anyone ever gone to the Andromeda galaxy? So how do we now it consists of normal matter?
Gas is observed between the galaxies, but not the hard radiation that would be produced when it reached a galaxy and annihilated.
Even an easier explanation. The annihilation of antimatter with matter gives off very distinct spectral patterns. We know what these are, and we have calculated them for any observable red/blue-shift range. We've yet to observe any great quantity, or even a 'halo' around any other galaxies. In an equally distributed universe, you would see such things, as high-velocity matter from one universe collided with another, and give off pretty lights. We don't see such, hence, the universe is mostly matter.
Does antimatter attract matter or repulse it
*Charged* antimatter will attract its matter counterpart electrostatically via the Coulomb force. For example, an electron will attract a positron. This does not occur for neutral particles.
The gravitational attraction between bodies must also be considered: it is negligible between individual particles, but comes into effect for macroscopic objects. The gravitational interaction is determined by the amount of mass present, and is always attractive, regardless of whether it is between matter or antimatter.
(could a double star, one of antimatter and one of matter, i.e. where the stars revolve around each other exist?).
A star will be (on average) neutrally charged, otherwise Coulomb repulsion between like charges will break it apart. Therefore it is fair to say that both the matter and antimatter star will be electrically neutral or at least not significantly charged.
Even if there were two charged objects, the Coulomb force, like the gravitational force, follows the inverse square law, so extra electrostatic attraction will be equivalent to more mass.
In either case, a stable orbit could form.
Would it be a prerequisite that a big bang produces as much matter as antimatter
I'm moving out of my depth here, but the answer is probably no. Even if it did, future interactions between particles may cause an imbalance (as is thought to have occurred in our universe). A prerequisite for what, anyway?