Isotropy is the observation that, in its general properties, the Universe looks different in every ...

Answer to q. 1

FALSE

Answer to q. 2

When astronomers measure the masses of galaxies, they often find that the measured masses are much larger than expected from the luminosities of the galaxies. Measured masses of galaxies amount to 10 to 100 times more mass than you can see. Dark matter is difficult to detect, and it is even harder to explain. Some astronomers have suggested that dark matter consists of low-luminosity white dwarfs and brown dwarfs scattered through the halos of galaxies. Searches for white dwarfs and brown dwarfs in the halo of our Galaxy have found a few but not enough to make up most of the dark matter. The dark matter can't be hidden in vast numbers of black holes and neutron stars, because astronomers don't see the X-rays these objects would emit. The evidence indicates there is 10 to 100 times more dark matter than visible matter in galaxies, and if there were that many black holes they would produce X-rays that would be easy to detect. Because observations imply that the dark matter can't be composed of familiar objects or material, astronomers are forced to conclude that the dark matter is made up of unexpected forms of matter.

Answer to q. 3

The most precise method for measuring the mass of a galaxy is called the rotation curve method. It requires knowing the following: (1 ) the true sizes of the orbits of stars or gas clouds within a galaxy, which in turn requires knowing the distance of that galaxy; and (2 ) the orbital speeds of the stars or gas clouds, measured from the Doppler shifts of their spectral lines. That is enough information to use Kepler's third law and find the mass of the part of the galaxy contained within the star orbits with measured sizes and velocities. The rotation curve method works only for galaxies near enough to be well resolved. More distant galaxies appear so small that astronomers cannot measure the radial velocity at different points across the galaxy and must use other, less precise, methods to estimate masses.

Answer to q. 4

Quasars are small and powerful sources of energy in the active cores of very distant galaxies. Some quasars are more than 10 billion light-years away, and because of their large look-back times, they appear as they were when the Universe was only 10 percent of its present age. The first clouds of gas that formed galaxies would have also made supermassive black holes at the centers of those galaxies' central bulges. The abundance of matter flooding into those early black holes could have triggered outbursts that are seen as quasars. The galaxies were closer together when the Universe was young and had not expanded very much. Because they were closer together, the forming galaxies collided more often, and you have seen how collisions between galaxies could throw matter into central supermassive black holes and trigger eruptions. Quasars are often located in host galaxies that are distorted as if they were interacting with other galaxies. Astronomers have discovered that nearly all galaxies contain supermassive black holes, and those black holes may have suffered quasar eruptions when the Universe was younger, galaxies were closer together, and infalling gas and dust were more plentiful. Quasar eruptions became less common as galaxies became more stable and as the abundance of gas and dust in the centers of galaxies was exhausted.