Richard 2.3

Question 1

What is gravity according to Einstein’s General Relativity?

Answer 1

Mass curves space-time; gravity is curved space-time, not a force

Question 2

What is the “magic carpet” analogy?

Answer 2

Space-time is like a flexible carpet that bends with mass.

Question 3

When was General Relativity developed?

Answer 3

1915, during WWI.

Question 4

What were the 3 classical tests of General Relativity?

Answer 4

Mercury’s perihelion precession, gravitational time dilation, bending of starlight.

Question 5

How does General Relativity explain Mercury’s orbit?

Answer 5

Precession of 43 arcseconds/century explained by curved spacetime.

Question 6

What is gravitational time dilation?

Answer 6

Clocks in stronger gravitational fields tick slower than those in weaker fields

Question 7

How does light behave near a massive object?

Answer 7

Light bends as it follows straight lines in curved space-time

Question 8

What experiment confirmed light bending?

Answer 8

1919 solar eclipse: starlight bent 1.8 arcseconds near the Sun

Question 9

What major objects and phenomena does General Relativity predict?

Answer 9

Black holes and gravitational waves.

Question 10

What did the cosmological principle state?

Answer 10

Universe is homogeneous and isotropic on large scales

Question 11

Who predicted an expanding universe in the 1920s?

Answer 11

Alexander Friedmann and Georges Lemaître.

Question 12

What is the Big Bang in General Relativity?

Answer 12

Space itself expands; there is no central point or edge.

Question 13

Can parts of the universe expand faster than light?

Answer 13

Yes, due to space expansion.

Question 14

How large is the observable universe?

Answer 14

About 40 billion light-years across (due to expansion).

Question 15

What does Hubble’s Constant (HC) measure?

Answer 15

The expansion rate of the universe (km/s per Mpc)

Question 16

Why was Hubble’s original HC wrong?

Answer 16

Misidentified Cepheid variable stars.

Question 17

What is today’s debated range of HC values?

Answer 17

Mid-60s to early 70s km/s/Mpc.

Question 18

Why does Hubble’s Constant change over time?

Answer 18

Universe’s gravity affects expansion rate
- universe expansion was decelerating, now accelerating

Question 19

What is critical density?

Answer 19

The density needed to halt expansion and cause collapse.

Question 20

What is the current matter density relative to critical?

Answer 20

About 30% of critical density → underdense universe.

Question 21

What is accelerated expansion?

Answer 21

Universe expansion speeding up rather than slowing.

Question 22

What causes accelerated expansion?

Answer 22

Dark energy (consistent with cosmological constant).

Question 23

What is dark energy?

Answer 23

Unknown form of energy with negative pressure, constant density across space.

Question 24

How does dark energy behave as universe expands?

Answer 24

Density stays constant, unlike matter which thins out.

Question 25

When did universe transition from decelerating to accelerating?

Answer 25

About 4 billion years ago.

Question 26

What fraction of the universe is dark matter?

Answer 26

About 25%.

Question 27

What happens to Hubble’s Constant in the future?

Answer 27

Will slowly approach a fixed value.

Question 28

Will the universe expand forever?

Answer 28

Yes, dark energy dominance causes eternal expansion.

Question 29

What is the coincidence problem?

Answer 29

Why do we live at the time when dark energy starts to dominate?

Question 30

What happens to distant galaxies in the future?

Answer 30

They will move beyond our observable horizon.

Question 31

Could Hubble’s Constant reach zero?

Answer 31

Only if universe were to collapse (current data says it won’t).

Question 32

What is phantom energy?

Answer 32

Hypothetical energy that increases with expansion (likely sci-fi).

Question 33

What does current data suggest about dark energy?

Answer 33

Consistent with cosmological constant (Einstein’s "blunder").