1. Dark matter and dark energy
Ch – 11.7
Most of the universe is invisible.
: Attraction
: Repulsion

2. How do we know about dark matter ?
Dark matter shows up by its gravitation . The speed of stars orbiting our galaxy is higher than what we would expect from the observed mass of the galaxy (stars, gas dust). There must be additional, invisible matter whose gravitational force holds the stars in their orbit.
So far we can only speculate that dark matter consists of WIMP s (weakly-interacting massive particles  like neutrinos, but massive). We are still looking for them.
Other contenders are MACHO s (massive, compact halo objects), such as burnt-out stars and black holes. But MACHO s are losing the contest. Telescope surveys did not find enough of these objects in our neighborhood.

3. Evidence for dark matter from gravita-tional lensing
A galaxy cluster deflects light from galaxies behind it, turning them into faint arcs. From these one obtains the total mass of the galaxy cluster. It is much larger than the mass of the galaxies that are visible.

4. How do we know about dark energy ?
Like dark matter, dark energy is inferred from its gravitational effect. But dark energy is repulsive instead of attractive. This repulsion leads to an accelerated expansion of the universe. There is some evidence for that from observations of the brightness of distant supernovae.
Dark energy can be described by an extra term in Einstein’s equations of general relativity. Einstein actually tried this to obtain a static universe, but called it his biggest scientific blunder after Hubble discovered the expansion of the universe. When theorists try to calculate this term, it comes out too big by a factor of

5. The evolution of the universe
Cold, empty space
The big Crunch
space
time

6. Can we look farther back than the microwave image ?
Not with our telescopes. All electromagnetic radiation (microwaves, visible, -rays) was absorbed by the free protons and electrons that prevailed at earlier times.
However, one could try neutrinos, which interact very little with matter. That is both a blessing and a curse: Neutrinos are extremely hard to detect.
One can draw indirect conclusions using the Standard Model of particle physics to calculate what happened. Such calculations can be tested against the abundance of light elements in the Universe.

7. Cosmic inflation
Cosmic inflation was proposed by Alan Guth in This looked like a crazy idea at first, but it has gained credibility. Guth proposed that our universe expanded incredibly fast at the beginning, faster than the speed of light (by a factor 1025 in seconds). Think of the balloon with the black dots being inflated explosively, like an air bag. The rapid expansion stretched out the wrinkles and bends of space, creating a flat universe. That matches our current picture (Lect. 17, Slide 17).
There is no contradiction to relativity, since Einstein’s speed limit applies only to masses moving in space, not to space itself. Empty space does not have mass.

8. Cosmic inflation, questions
What triggered inflation ?
A ‘phase transition’, analogous to the condensation of water vapor to liquid water (compare Lect. 1, Slide 8). This occurred as the universe cooled from the Big Bang.
Where did the energy for the expansion come from ?
From the condensation (like the energy of a hurricane, which comes from the condensation of water vapor).
Can we observe inflation directly ?
One would need to go back to seconds after the Big Bang. Our earliest picture is years later.