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Cepheid variable stars can be used to tell the distance to the galaxies in which they are

found because

• They emit gravity waves that are correlated with their absolute peak brightness

• They explode in a supernova with a known absolute peak brightness

• They send out radio signals at regular intervals, like a lighthouse beacon

• Their brightness fluctuates, and the period of this fluctuation tells you their absolute peak brightness

A key piece of evidence for the Big Bang is

• Type 1a supernova are observed in distant galaxies

• The number of neutron stars that are observed

• Galaxies are observed to rotate faster than expected

• Almost all galaxies are observed to be moving away from us

The time that most helium was formed in the universe was

• 3 to 10 minutes after the Big Bang

• 9 billion years after the Big Bang

• 1 second after the Big Bang

• 380,000 years after the Big Bang

Answer explanation

JUST REMEMBER!

- 1 second is too short

- 380,000: universe used to be pitch black

A white dwarf star can be roughly characterized as having

• The size of the moon and the mass of Jupiter

• The size of the Earth and the mass of 10 Jupiters

• The size of the moon and the mass of the Earth

• The size of the Earth and the mass of the sun

Answer explanation

JUST REMEMBER!

Size: Earth

Mass: Sun

The Principle of Relativity states that

• An object will accelerate at a constant rate if the force on it is constant

• An object will move with constant speed and direction if there are no forces on it

• The laws of physics are the same in all reference frames, accelerating or not

• An object in an inertial reference frame will be at rest in that frame

• The laws of physics are the same in all inertial reference frames

Answer explanation

DEFINITION! REMEMBER!

PoR-s(h)ia irf

One of the key observational differences between the merger of two black holes and the merger of two neutron stars is that

• Black hole mergers are sources of cosmic rays, whereas neutron star mergers are not

• Black hole mergers are much rarer than neutron star mergers

• Black hole mergers emit only gravity waves, whereas neutron star mergers emit both gravity and electromagnetic waves

• Black hole mergers emit neutrinos whereas neutron star mergers do not

Answer explanation

- black holes: not sources of cosmic rays

- rarity: not a difference

- emission: nothing escapes a black hole

According to general relativity, the reason objects released in free fall all fall at the same rate is because

• Their gravitational masses are the same

• They all fall towards the same object, namely the Earth

• They are at rest and are not falling

• Their inertial masses are the same

Answer explanation

- gravitational AND inertial masses are equal

- this principle is true ANYWHERE

- freely falling frames are INERTIAL

According to general relativity, the reason galaxies can appear to move away from us at faster than the speed of light "c" is that

• Galaxies are mostly dark matter, which in some cases can travel at speeds larger than c

• They are not in fact moving; it is the space between the galaxy and us that is expanding at a speed faster than c

• If a mass exceeds a certain threshold, it can sometimes travel at speeds faster than c

• Because of their large rotational speeds, galaxies can sometimes travel at speeds faster than c

Answer explanation

- DARK MATTER: not related; as long as something has mass, it can't go faster than c

- mass can NOT travel faster than c

- even when an object is rotational, it can't break the speed of light if it has mass

Dark energy is best described as

• As-yet undiscovered elementary particles

• The energy carried by gravity waves

• The energy of empty space

• The energy present inside a black hole

Answer explanation

- PARTICLES: not related to particles plus this says “undiscovered”

- GRAVITATIONAL energy, not DARK energy

- not what we would call energy in a BLACK HOLE

Suppose you and your friend Thanh travel to the vicinity of a black hole. You and

Thanh have the same age (born at the same time on the same day). You stay far away from the black hole while Thanh descends very close to the event horizon (but not inside) to explore. After returning to the spaceship, which one of you is older relative to the other?

• You are older than Thanh

• Thanh is older than you

• You and Thanh still have the same age as each other

Answer explanation

KNOW THIS!