T3: 19-21

Question 1

Contrast Newton’s and Einstein’s ideas about space and time:

Answer 1

Newton - most gravity laws made by him at this point. Explained forces, balance, and momentum.
Make a space that says where and when - gives coordinates.
Nothing can change time
Made these laws from an intuitive sense of how the world would work
Crystalized and cleared up these older ideas.
Cartesian coordinates - grid doesn’t change, only the points on it (calculus - imaginary numbers)

Albert Einstein: 1915 - General Relativity - theory of gravity, that is not a force.
Gravity is bend and curved through matter in it. (John wheeler)
Built on species relativity - set by distribution of matter

Question 2

Describe the historical context where general relativity was discovered:

Answer 2

By Einstein (a jewish physicist) in Germany before the first WW1. While this friend Hilbert was being sucked into Nazi scientists to make chemical-gas to kill for them.

Question 3

What does it mean for spacetime to be dynamical in General Relativity?

Answer 3

General relativity - space time is dynamical, properties change as matter moves in it (intricate set of ideas and concepts)
Light and objects on curved surfaces move in a straight line, but they are on a curved surface, which makes them then move in a curved line in spacetime.

Question 4

General relativity is key to understanding what?

Answer 4

expansion of the Universe - uses 3 Classical Tests (Perihelion precession, time dilation, and bending of starlight)

Question 5

What 3 pieces of evidence make General Relativity be seen as an accurate theory of gravity?

Answer 5

1. Mercury’s most eccentric orbit - perihelion movement
2. Bending of starlight
3. time dilation

Question 6

Explain how mercury’s perihelion helped prove General Relativity:

Answer 6

Mercury’s most eccentric orbit (egg-shaped) of all planets and it’s closest toh te sun.

Perihelion moves through orbit - is not stationary like Newton’s laws would predict. Or that the change in perihelion would be because of gravity from other objects around the sun.

Observed move in perihelion : 5600 arcsec/century
Excess: 43 arcsec/ century

Einstein’s GR was able to explain this excess arcsec movement through retrodiction (1915)

Question 7

Explain how the Bending of Starlight helped prove General Relativity:

Answer 7

Newton predicted the pending of “particles” moving at speed of light
Einstein predicted more bending compared to Newton - this theory was able to be tested in 1919 through a solar eclipse to see if star positions would move between time of solar eclipse and a few days after.

When moon is blocking sun, photos were taken. Few days after star positions were taken again, and it showed a change in positioning of stars similar to what Einstein predicted.

Question 8

Explain how time dilation helps prove General Relativity

Answer 8

Special relativity - moving clock ticks slower
General relativity - clock in stronger gravitational field ticks slowly than one in a weeker field.

Question 9

What other two pieces of evidence were proved by einstein’s General Relativity?

Answer 9

Black Holes - existence predicted by einstein
Gravitational Waves - (1916-2016) ??

Question 10

Whatare the key ideas of an expanding universe in General Relativity?

Answer 10

Relativity and Cosmology of the expanding universe

Question 11

Who is de Sitter and what did he do?

Answer 11

1917 - De Sitter made first paper on cosmology that universe is expanding

Question 12

What did einstein do in 1920?

Answer 12

Einstein looked for static universe in General Relativity, couldn’t explain the fact that i isn’t static and infinite as a universe so made a cosmological principle

Question 13

What do we assume about the universe? That it is… and ….

Answer 13

homogenous and isotropic

Question 14

Explain homogenous and isotropic:

Answer 14

Assume universe is homogeneous (looks same in every place), and isotropic (looks same in every direction)

Question 15

How are Alexander Freidmann and George Lameitre and what did they do?

Answer 15

Alexander Freidmann - developed cosmological solutions in GR. Homogenous and isotropic universe
George Lemaitre - a catholic priest that taught about a rimeral atom or cosmig egg. Independent of Friedmann, but still came to the same conclusion, and work was known to Hubble.

Becasue each was able to come to the same conclusions about general realtivtiy, this was able to prove that the universe really was homogenous and isotropic

Question 16

What is the universe expanding into?

Answer 16

Universe is not actually “expanding” so much as making “new space”

From any galaxy it looks like they’re not moving (or they are in the center of the galaxy) while everything else is moving away from them - homogenous

Question 17

How big is the universe?

Answer 17

Universe is infinitely big as it continually expands.
No Center of the Universe as its assumed through homogenous - no special place
Everyone in expanding universe things they are at center of universe
Light travels 14 billion light years - as that is about how old the Universe is - but its stretched by new space added making it larger.

Question 18

Where did the Big Bang Happen?

Answer 18

No specific point in which it happened - nothing in itself if particulalry special as the universe is expanding everywhere

Question 19

Are some parts moving faster than the speed of light?

Answer 19

More space between galaxies, and it looks like it moves faster than speed of light

You can’t move through space faster than speed of light

Anything moving faster than speed of light cannot be seen

Question 20

Waht is the Hubble Tension? Explain how one method differs to the other:

Answer 20

Hubble’s Tension - the discrepancy between two different methods for measuring the expansion rate of the universe (hubble’s constant). Suggests that there is an error in our current understanding of cosmology or unknown physics at play (e.g. new particles or interactions)

One method uses the cosmic distance ladder to measure expansion rate directly

The other method uses cosmic microwave background (CMB) which predicts the expansion rate based on the universe’s initial conditions.

Question 21

What is the history of measurement of Hubble’s constant?

Answer 21

Hubble initially got 500 km/sec/mpc for hubble’s constant as he mistook a cepheid variable for having a different pulsation period time
1960-1980 = astronomers through Hubble’s Constant was between 50-100. Didn’t allow for much progress
1990s - Hubble’s constant mainly around mid 70s
Today - between mid-60s to early 70s. Difference is bigger than apparent error bars with lots of variables that impact the constant.

Question 22

Contrast the theory and observation of galaxy speed changing in the expanding universe:

Answer 22

In theory - depends on what the universe contains
In Observation - measure how the shape of the “line” changes with distance

Question 23

Explain in observation, how galaxy speed changes in the expanding universe:

Answer 23

In Observation - measure how the shape of the “line” changes with distance
Gravity of everything in universe pulls on everything else
General relativity predicts the change in Hubble’s Constant through the way it changes. Once you know H’sC evolution we can figure out its age, speed, contents, and dynamics.

Question 24

The further away a galaxy is,… however, if the speed of the galaxy stays the same….

Answer 24

the faster it travels away from us.
However, if the speed of the galaxy stays the same, then Hubble’s Constant changes

Question 25

how is Hubble's constant evolving?

Answer 25

The evolution of Hubble’s Constant is how Hubble’s constant changes over time. - expansion rates are generally considered to accelerate due to dark energy. As stated in Hubble’s Law, the velocity of a galaxy’s recession is proportional to its distance. So the further away a galaxy is, the faster it appears to be moving away from us due to the expansion of the universe. Hubble’s constant is affected by other factors like dark energy.

Question 26

hwo can we quantify how much Hubble's constant changes?

Answer 26

In order to quantify how much Hubble’s constant changes, we must know the density of the universe. Density decreases as the universe expands as its spread out - we can figure this out through hubble’s constant

Question 27

waht is accelerated expansion?

Answer 27

A distant galaxy moving away from us, and the universe expands. If hubble’s constant is constant (or decreases slowly) the galaxy speeds up - this is accelerated expansion

Question 28

Waht causes the expansion of the universe to slow down?

Answer 28

We also know the universe is full of matter that exerts a gravitational force on all other matter in the universe - this causes the expansion of the universe to slow down This causes Hubble’s constant to decrease and the expansion decelerates.

Question 29

waht is critical density? What if our universe's density is less than or mroe than this critical density?

Answer 29

Critical Density - amount of matter and energy in the universe needed to eventually halt its expansion, and send Hubble’s constant to zero. If the universe’s density is less than critical density it will expand forever (Open universe) If the density is greater than critical density, the expansion will eventually slow down, and reverse, leading to the Great Crunch. (closed universe)

Question 30

how much of the critical density is our universe's matter making up?

Answer 30

Only about 30% of matter in the universe (regular matter and dark matter) of critical density - meaning our universe is open or underdense.

Question 31

What does the density parameter mean?

Answer 31

Density parameter - the ratio of the actual universe’s density to the critical density. A value of 1 indicates the universe has critical density, anything less than 1 means it’s less than critical density, and anything more than 1 means..

Question 32

What does the redshift graph of Type 1a SN indicate?

Answer 32

Shows supernova redshift seem to best fit either the line on the graph for a flat accelerating vacuum-dominated model of the universe OR the closed dark energy dominated fit to the supernova data. Data suggests that the universe initially slowed down, then sped up again - we think this is due to dark energy. The curve also suggests that it’s not consistent with any amount of matter by itself, but instead consistent with a cosmological constant (Dark energy and matter) - an idea made in 1920 by einstein. - this was the discovery of Dark energy.

Question 33

Waht is dark energy?

Answer 33

Dark energy - a theoretical form of energy thought to act in opposition to gravity and occupy the entire universe, accounting for most energy in it and causing the universe’s expansion to accelerate. Dark energy density is consistent with energy conservation in association with negative pressure (like a stretched spring)

Question 34

How does Dark energy drive our universe's accelerating expansion?

Answer 34

Initially, matter is spread out and decreases in density. Dark matter has a constant density, and is able to push outwards and accelerate universe expansion. Dark energy becomes the dominant component, drives acceleration, and seems to ensure the overall universe is at critical density.

Question 35

Waht is the evidence that dark energy exists?

Answer 35

Evidence of Dark energy: supernova, microwave backgrounds, evolution of galaxy clusters, and Baryon Acoustic oscillations.

Question 36

Waht si the cosmic pie?

Answer 36

Cosmic Pie - changes as a function of time (matter density decreases by factor of 8 everytime universe doubles in size) 70% Dark energy 5% atoms (4% hot gas, and 1% stars) 25% Dark matter

Question 37

What do we know about hubble's constant, distant galaxies, universe's ingredients, and General relativity today?

Answer 37

Eventually Hubble’s constant will become constant as its changing very slowly in a dark energy dominated universe Distant galaxies appear to speed up, and eventually becomes invisible. Basic ingredients of the universe - dark matter, dark energy, radiation, atoms (baryons), neutrino background Universe now “dark energy dominated” - and will remain dark energy dominated as it expands forever General Relativity predicts: Hubble’s constant always decreasing (slowly enough that distant objects appear to accelerate). And cosmological constant

Question 38

what si the coincidence problem?

Answer 38

Coincidence problem - the fact that dark energy is about to take over the universe. And the fact that we were able to make the correct machines at the same time as figuring out these specific ideas.