The Universe

Motions of galaxies

The expanding Universe

Recap

• Milky Way– Distances in the Milky Way: our Galaxy is very large

• Other galaxies– Galaxy properties

• Morphology: spiral and elliptical galaxies, spirals can appear different depending on viewing angle

• Masses and sizes of galaxies• Composition of galaxies: not all galaxies have same type

of stars; not all galaxies have same type of interstellar matter

– Motions of galaxies: galaxies move in space!

What about motions of galaxies?

• Despite large distances, we can get some information about motions of galaxies

• Motion of objects can be split into components:– Sideways, or transverse, motion– Radial motion: part of motion towards or away from us

• Transverse motion very hard to measure for distant objects, impossible for galaxies

• Radial motion can be measured using a phenomenon called the Doppler shift: need to understand properties of light to do this and use an instrument that splits light into its constituent colors

Expansion of the Universe

• Using the Doppler shift, we discovered that essentially all galaxies appear to be moving away from us

• The apparent speed of recession is proportional to the distance of the galaxy– A galaxy twice as far away appears to be moving twice as

fast; three times as far away is moving three times as fast; ten times as far away, ten times as fast, etc.

– This pattern of galaxy expansion is often called Hubble’s law

What does this mean?

• Let’s set up a simulation, with 5 galaxies (ABCDE) spaced at 3 feet apart: VOLUNTEERS?

• Imagine we live in Galaxy A • Let’s look at Galaxy B and say it is moving away from

us at 1 foot/second• Galaxy C is twice as far as galaxy A: how fast is it

moving?• Galaxy D is three times as far: how fast?• Galaxy E is four times as far: how fast?• Let’s do it! For 3 seconds.

What does this look like?

• Consider the view from our galaxy (A)

Galaxy Distance (ft)

Speed (ft/sec)

A 0 to 0 0 (0/3)

B 3 to 6 1 (3/3)

C 6 to 12 2 (6/3)

D 9 to 18 3 (9/3)

E 12 to 24 4(12/3)

What about from other galaxies?• Make the same table from the point of view of another

galaxy: choose B, C, D, or E

Galaxy Distance (ft)

Speed (ft/sec)

A 0 to 0 0 (0/3)B 3 to 6 1 (3/3)C 6 to 12 2 (6/3)D 9 to 18 3 (9/3)E 12 to

244(12/3)

Galaxy Distance (ft)

Speed (ft/sec)

A

B

C 3 to 6 1 (3/3)

D 0 to 0 0 (0/3)

E

Example: from galaxy D

From any galaxy: Hubble’s Law!

What does this mean?• Galaxies all appear to be flying away from us!!• Are we at the center of the Universe? …………NO!!!!!!…………

– No matter what galaxy you live on, all other galaxies appear to be flying away from you!

– Galaxies appear to be moving apart from each other --- NO CENTER!!

– What’s the key observation? --> speed is proportional to distance

• Alternative way of thinking about things: all galaxies stay still, but space between them is expanding– This is a more accurate picture: note that we see distant

galaxies that appear to be moving away faster than the speed of light, but that’s because they’re not really moving: space is expanding!

Recap

When we observe galaxies with scientific instrumentation and an understanding of light, we

A. Can’t measure anything about how galaxies are moving

B. Can measure sideways (transverse) motion, but not radial motion

C. Can measure apparent radial motion, but not sideways motion

D. Can measure everything about the motion of a galaxy

RecapWhen we observe galaxies, we find

A. All galaxies appear to be moving towards us

B. About half the galaxies appear to be moving towards us and half away from us

C. Almost all galaxies appear to be moving away from us, all at about the same speed

D. Almost all galaxies appear to be moving away from us, with nearer galaxies moving faster than more distant ones

E. Almost all galaxies appear to be moving away from us, with more distant galaxies moving faster than nearer ones

Recap

If we observe two galaxies, A and B, where B is 5 times farther away as A, we find that

A. Galaxy A appears to be moving 5 times as fast as galaxy B

B. Galaxy A appears to be moving at the same speed as galaxy B

C. Galaxy A appears to be moving 1/5th as fast as galaxy B

D. Galaxy A appears to be moving 1/25th as fast as galaxy B

E. No clue

RecapWith the observed pattern (speed proportional to

distance), if we lived in another galaxy

A. We would see all other galaxies appear to be moving towards us

B. We would see all other galaxies appear to be moving away from us at the same speed

C. We would see all other galaxies appear to be moving away from us, but with a different pattern than we observe from our galaxy

D. We would see all other galaxies appear to be moving away from us, with the exact same pattern that we see from our galaxy

E. No clue

Recap• While from our perspective, it appears that all

galaxies are moving away from US, the exact same thing would be observed from ANY galaxy

• Implication: the Universe is expanding, everything appears to be moving away from everything else

• No center!

What’s expanding and what isn’t?

• Although over very large distances everything is getting farther apart from everything else, this isn’t the case over shorter distances

• Why not? – Gravity is a force that acts to pull things towards one another– The strength of gravity depends, in part, on the distance between

objects– For objects relatively nearby to each other, gravity is easily strong

enough to pull objects towards or around each other• Only on large distances, between galaxies that are far apart, does the

expansion overpower gravity– The Solar System is not expanding!– The Milky Way is not expanding!– Even groups and clusters of galaxies are not expanding!

Model: Expanding Universe• We observe that the Universe is expanding now• What about in the past? What about in the future?• Model: Universe has always been expanding at same speed

and will continue to do so– Prediction: in the past everything was closer together– Let’s do it!– Implication: everything comes together at the SAME TIME– Call this the BIG BANG

• Is this the only model we can think of?

Big Bang Model

• Does Big Bang model make predictions?– Work out physics, find that when all matter is much

closer together, it would have a lot more interactions and be much hotter

– Hotter matter in the Universe would glow– Prediction: Universe was glowing a long time ago– Can this be confirmed by observations? – Telescopes are time machines!– Prediction: glow will appear today all around us in a

kind of light called microwave light

Big Bang test• Observation: is the prediction observed?

– Yes! The cosmic microwave background• Do other models make this same prediction? All that’s

required is that Universe has always been expanding, but not that the rate has always been the same

• There are other predictions of the Big Bang theory that are also verified by observation, so it is very widely accepted (although, of course, not by every single person!)– Abundances of light elements– Large scale structure as observed

The Cosmic Microwave Background !!

To Do