# The Universe Motions of galaxies The expanding Universe.

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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.

How far in 5 seconds? Let’s do things a bit more carefully Galaxy B: 1 foot/sec, in 5 seconds: ? Galaxy C: 2 feet/sec, in 5 seconds: ? Galaxy D: 3 feet/sec, in 5 seconds: ? Galaxy E: 4 feet/sec, in 5 seconds: ?

What does this look like? Consider the view from our galaxy (A) Galaxy Distance (ft) Speed (ft/sec) A0 to 00 (0/3) B3 to 61 (3/3) C6 to 122 (6/3) D9 to 183 (9/3) E12 to 244(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) A0 to 00 (0/3) B3 to 61 (3/3) C6 to 122 (6/3) D9 to 183 (9/3) E12 to 24 4(12/3) Galaxy Distance (ft) Speed (ft/sec) A B C3 to 61 (3/3) D0 to 00 (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

Recap When 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

Recap With 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 !!

Theoretical support for Big Bang Totally apart from observations of galaxies (in fact before radial motions were observed), Einstein developed a theory of gravity called general relativity When he applied it to the Universe as a whole, he found that in the simplest form of his theory, that the Universe couldn’t be “static”, it had to be expanding or contracting He thought this was unlikely, so he added an arbitrary term to his equations to get rid of this feature. After expansion was discovered to exist, he called this his “greatest mistake” – However, it now turns out that maybe it wasn’t …. In any case, Big Bang model has theoretical support as well as observational

Past and future Microwave background strongly suggests that universe has been expanding for a long time But has it been expanding at the same speed? Gravity is a force that pulls things together. Even though it’s not strong enough over large scales to counteract expansion of Universe, it was expected that it would slow it down over time: deceleration Can we observe change of expansion rate over time? – Telescopes are time machines! – What do we observe? Acceleration! Expansion is speeding up! Dark energy!

Recap: Overview of the Universe Where do we live? – Earth – Solar System – Milky Way galaxy – Universe Perspective – Is there a center of the Solar System? Do we live there? – Is there a center of the Milky Way? Do we live there? – Is there a center of the Universe?

Perspective Some video resources – Powers of ten Powers of ten – AMNH `Simulated Universe AMNH `Simulated Universe – How large is the Universe How large is the Universe

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