Lecture 33

Question 1

What is happening to spacetime?

Answer 1

It is expanding as the fabric of space between galaxies is stretching and carrying galaxies away from one another. It is not an explosion of galaxies through space from a centre.

Question 2

How is the universe’s expansion affected by velocity and acceleration?

Answer 2

• the size of the universe can grow with no acceleration (constant velocity expansion)
• when the initial velocity is positive the universe can expand while decelerating

Question 3

What could change the expansion rate of the universe?

Answer 3

• attractive force (gravity) = deceleration (Hubble constant gets smaller)
• repulsive force (spacetime pushed apart) = acceleration ( Hubble constant gets larger)
• balanced forces = no acceleration= coasting

Question 4

Why would it be expected for the expansion of the universe to slow?

Answer 4

As gravity is an attractive force would act to slow the initial expansion of the universe

Question 5

How does the Hubble constant show expansion?

Answer 5

The rate of expansion is represented by the Hubble constant, which is the slope of the Hubble diagram- a big Ho = fast expansion

Question 6

What can be used as standard candles when measuring universe expansion?

Answer 6

White dwarf supernova- z~1 (a=0.5)

Question 7

What is the process of observing the difference in the expansion theories of the early universe?

Answer 7

Compare redshift distance with luminosity distance. In doing this, it can be seen that the farther away the object, the bigger the difference between the different models.

Question 8

What did the 1998 measurements show about the expansion of the universe?

Answer 8

The measurements showed that the universe was actually expanding. The observation of type 1a SN appeared fainter than expected for their redshifts, therefore further away than expected. This indicated that the universe was accelerating.

Question 9

What is thought to be pushing spacetime apart?

Answer 9

Dark energy

Question 10

What are some of the reasons people thought the universe could not be accelerating?

Answer 10

• mistakes in observations/calculations
• dust in distant galaxies made the SN brighter
• ancient SN could have behaved differently to modern SN.

Question 11

Around what time did the universe stop decelerating in expansion?

Answer 11

About 5 billion years ago (around redshift 0.5)

Question 12

What is the theorised purpose and effect of dark energy?

Answer 12

It is proposed to be a counter the effects of gravity and causes the acceleration of the universe

Question 13

What proportion of the universe is dark energy?

Answer 13

It is the primary constituent of the universe accounting for around 70% of its makeup.

Question 14

What is dark energy?

Answer 14

The leading hypothesis is that dark energy is a cosmological constant- it is negative pressure of space

Question 15

What is the density of dark energy in the universe?

Answer 15

The amount of dark energy in the universe remains constant even though the universe expands meaning that the relative density of dark energy compared to matter increases over time.

Question 16

What did the Sloan survey show?

Answer 16

It used distant galaxies to show that expansion was faster in the past

Question 17

What will happen as the expansion of the universe speeds up?

Answer 17

• there will be less and less galaxies visible in the sky a more distant sources move outside the visible horizon
• fewer new galaxies and stars will be formed because the expansion will lower the density of intergalactic gas
• gravitationally bound structures (galaxies and dark matter halos) will be left unaffected

Question 18

What is a spacetime diagram?

Answer 18

• a graph of time vs space
• light travels on world lines at angles greater than or equal to 45 degrees
• nothing travels faster than the speed of light so all world lines must be equal to or steeper than 45 degrees

Question 19

What are the effects of the spacetime diagram?

Answer 19

We can only receive and transmit information to spacetime locations (events) inside our light cone

Question 20

What happens to the light cone as we move forward in time?

Answer 20

The light cone is expands and is larger at future times

Question 21

What does the horizon show us?

Answer 21

• the horizon depends on when and where we are
• the age of the universe sets the maximum distance we can see
• the actual universe may be larger or infinite but we cannot tell