Atomic nuclei were finally able to escape the plasma of the early universe.

Magnetic fields had weakened to the point that they no longer interfered with light.

Fusion of hydrogen to make helium ceased.

The universe had expanded and cooled enough for electrons to combine with nuclei to form neutral atoms.

Atoms started to combine to make the first molecules.

spin the rocket clockwise.

spin the rocket anticlockwise.

fire an engine that shoots out gas to the left.

fire an engine that shoots out gas to the right.

half as strong.

one quarter as strong.

twice as strong.

the darkness of the night sky

the observed helium abundance in the Universe

the relative motions of galaxies in the Local Group

the expansion of the Universe

Gamma rays

Radio

Visible

Atoms that have the same number of protons in the nucleus but a different number of neutrons

Atoms that have the same number of neutrons in the nucleus but a different number of protons

Atoms that have the same number of protons and neutrons in the nucleus but a different number of electrons

weight would be the same, but your mass would be different.

mass would be the same, but your weight would be different.

mass and weight would both be the same.

The universe will keep expanding forever.

The universe will end its life as a black hole.

A massive explosion about 14 billion years ago created all matter and energy that rushed out to fill the surrounding space.

All matter and energy in the universe were in an incredibly hot, dense state and space itself began expanding.

The universe formed from the collapse of a previous universe.

The universe as we see it formed from the singularity in a black hole.