Astronomy · Rung 11 of 12

Cosmology

Almost every galaxy is moving away from us, and the further away the faster: that is the Hubble-Lemaître law, and it does not mean we are at the centre but that space itself is stretching. It is measured by redshift, the stretching of the wavelengths of arriving light. If the universe is expanding, it was once closer together and hotter, and a remnant of that era can still be measured today: the cosmic microwave background, the light from when the universe was 380,000 years old and turned transparent. From it comes the age, 13.8 billion years, and the uncomfortable sum: the matter and energy we know about are 5 % of the total.

Where this sits

Comes after
The Milky Way and the galaxies

Exercises from this rung

Generated by the app, from easiest to hardest

  1. The universe is 13.8 billion years old. Where does that number come from?

    • From the microwave background and the expansion rate
    • From the size of the most distant galaxy
    • From the oldest known star
    • From dating meteorites

    Why Mostly from the cosmic microwave background: the tiny temperature variations in that map, measured by WMAP and Planck, give the model's parameters, and with the expansion rate the age follows. It agrees with two independent checks: the oldest globular clusters are about 13 billion years old, and meteorites date the Solar System at 4.57 billion.

    easier

  2. If the universe were infinite, eternal and full of stars, the night sky would blaze. Why doesn't it?

    • Because stars are very far apart
    • Because dust hides the stars
    • Because night is Earth's shadow
    • Because the universe has a finite age

    Why Olbers' paradox. In an infinite, eternal universe every line of sight would end on a star and the whole sky would blaze like the Sun. The way out is not dust, which would heat up and glow too: it is that the universe is 13.8 billion years old, so light has reached us from a finite volume only. The darkness of night is a cosmological measurement.

    in between

  3. What fraction of the universe's content is matter and energy of the kind we know?

    • About 5 %
    • Practically all of it
    • About 50 %
    • About 95 %

    Why 5 % ordinary matter, 27 % dark matter and 68 % dark energy. Dark matter was inferred because galaxies rotate too fast at their edges for the mass we can see; dark energy, because the expansion is accelerating instead of slowing. We do not know what either is made of, and that is the biggest discomfort in current physics.

    harder