Goal: To understand the expansion of our universe. Objectives: 1)To learn about Einstein’s blunder 2)To explore Hubble’s discoveries 3)To understand How expansion works 4)To learn about What the expansion of the universe can tell us. 5)To examine some Problems and unresolved questions 6)To understand Quasars

Einstein’s blunder Long story coming here… Universe static BUT gravity goes to infinity! How is this possible?

Told you it was long Cosmological constant! There had to be some force pushing outwards which held the universe together and kept everything from collapsing in on the middle of the universe. BUT…

Hubble’s Discoveries In the early 1920’s Hubble discovered that the universe was not static! All of the galaxies except for the closest (such as LMC, SMC, and Andromeda) were moving away from us. Also, the smaller the galaxies appeared on the images (thus the further away they were) the faster they seemed to be moving away from us. The velocities seemed to be linear with distance also. Twice as small (which meant twice as far away) seemed to have twice the velocity away from us.

Einstein’s reaction Upon hearing of Hubble’s discoveries Einstein proclaimed his cosmological constant his life’s greatest blunder. If the universe was expanding there did not need to be a force pushing outwards to counteract gravity. However – turns out he was not too far off, but for a completely different reason…

Hubble’s Law V = Ho * D Basically, the velocity that a galaxy is moving away from us can be found from the above equation. Ho is Hubble’s constant (currently estimated to be about km/s / Mpc) D is of course the distance to the galaxy.

How do we do this? We find a common line (usually with Hydrogen or Carbon Monoxide) that is very bright. We compare that to the expected. V = (change in wavelength / true wavelength) * speed of light

What does it mean? Lets create a cosmological conga line! Each person starts right behind the person in front of them. However the line expands! Every song the distance between people (for some unknown reason) expands by 1 meter. Lets consider the ramifications of this…

After only 1 song The person next to you is 1 meter from you. The person in front of them is 1 meter from them which is 2 meters for you. What about the people behind you? Well they have done the same thing. So, from your point of view everyone is moving away from you. Furthermore the velocity they move away from you depends on their distance from you!

Where is the center of the conga line? A) you are the center because everyone is moving away from you B) There is a center which can be determined but it is not you. C) There is a center but it cannot be determined where it is. D) There is not necessarily a center and if there is it cannot be determined/

Wait, there is more! Lets suppose that this conga line is huge. Lets also imagine it is on the surface of a balloon that is expanding. Now the conga line is expanding because of the unseen expanding of the balloon. It is only the effects of this expansion that we can see. What are the effects?

Effects If you look a long ways down the conga line you will notice that the people seem to move away from you very quickly. You will be able to notice this at the end of the song when they all say “cha-cha-cha”. Sound moves (just like light). You will hear the words a little bit later from those further away from you (one way to tell how far away they are – sort of). Another is by the Doppler shift of their pitch.

Low tones Since everyone is moving away from you the pitch you hear will be lower than the pitch they yell out. At some point the people will be effectively moving away from you so quickly that sound can’t even get to you (they are “moving” at faster than the speed of sound. This would be one limit to the size of the “hearable” conga line. Another is such that if they are too far away than the “cha-cha-cha” from the first song has not had time to reach you – the 2 nd limit to the size of the “hearable” conga line. Finally, the distance conga people will be hard to hear because their sound has spread out and is very quiet from your position.

Distance If we use some method to find the distance do we get the correct distance? Answer is no. Might seem easy but what distance are you getting? The distance at the shout? Well, no because in the time that the sound goes from the person to your ear the distance between you and them has increased! Is it the distance now? Well, no, because that distance will be larger. Really what you get is a “look back time” to see what the conga line was like some time ago. Also, we have assumed the expansion is constant – what happens if it is not?

Even bigger picture Suppose your conga line was really big! At some point the conga people would effectively be moving away from you at FASTER than the speed of light. Wait – didn’t you say that you cannot move faster than the speed of light? What gives?

Remember that… It isn’t the people moving – it is space expanding – so therefore they are not really moving faster than the speed of light – just their distance from you. And this is how the expansion of the universe works!

Some expansion questions Before we go into the standard, lets take some questions from you students. Come up with a question about the expansion of the universe.

The most common: What does the universe expand into? Well, probably nothing. What did it expand from? Well, we will dedicate the 2 nd lecture today to this question. The rest of the questions: Does the rate of expansion remain constant? Will it continue to expand? Will all be addressed next week.

Quasars Quasars are QUASi-stellAR radio sources They were first observed in the 1950s. They looked like stars on an image. You can tell stars from galaxies because stars appear as a point (and have diffraction patterns if bright) but galaxies are more spread out. However, when they looked at the spectrum, they found something very strange!

These are Quasars (HST)

No known elements! The emission lines matched no known element! Why would this be? A) Quasars are made out of materials unknown in the 1950s. B) They made a mistake with the observations. C) The lines were Doppler shifted by a factor of a few. D) They forgot to take relativity into account.

They are far away! Once it was realized what the emission line difference was they quickly realize that Quasars are very far away! In fact some are on the other side of the observable universe! A red flag should go up now – if they are that far away and we can see them – how bright are they? You have a star which is brighter than any galaxy – how is that possible?

Variable Furthermore Quasars change in brightness! They can do so very quickly (in hours). The shortest a time span an object can fluctuate its brightness is related to its size. The time is the light speed time across the object. So, these are extremely bright objects located in a small area (size of a solar system). How is this possible?

Debate In the 1970s there was a lot of debate over this puzzling matter. Some suggested antimatter (the only way they could think of to convert that much energy). Some suggested that the redshift wasn’t due to distance but due to light coming out of a huge gravitational well. However the gas emitting the light was hot and diffuse (known by the emission lines) – so could not be.

The solution The light doesn’t even come from the “star”. The light is coming from an accretion disk. As gas and dust orbits friction causes the gas and dust to heat up, emit light, and fall slowly down towards the object in the center. If the object in the center is massive you can liberate 10% of the mass energy of the dust and gas as energy (as opposed to 0.7% from fusion).

Not the full story Still, how do we get a region of space the size of a solar system to radiate more light than a galaxy? Also, this light is radiated in all type of light from gamma rays to radio! How is this possible?

Black Holes! Quasars are super massive black holes! Quasars are billions of times the mass of our sun. Oddly enough their density is about the same as our sun. The amount of light they emit is dependant on the rate they accrete matter.

More bizarre news Most quasars are > 5 billion light years away. Very few are < 3 billion. Why do they seem to shut off? Some may have to do with them no longer having gas to eat. Also, as a black hole gets bigger, its density decreases. If the density gets too low it no longer plays with its food, it eats it whole (although black holes are NEVER vacuum cleaners).

Active galaxies The closest to quasars we get in the present is active galaxies. These are galaxies that for some reason have materials falling into the center so that the center is bright. Often times this is a result of a galactic merger. 1% of galaxies are like this.

What else we can learn from Quasars Quasars are light bright search lights shining though our universe. They shine not just through space but through time. Since the universe expands, the light from the searchlight expands with time too. This creates what is known as a Lyman-alpha forest (called this because the Hydrogen emission it follows is called the Lyman-alpha line)

How Lyman-alpha forest works Each time the light from the Quasar passes a cloud or galaxy the light from the Quasar is shifted to a different wavelength. The gases in the cloud will emit and absorbed (based on the properties of the cloud or galaxy) at a specific wavelength that is not shifted. So, each cloud adds its fingerprint or signature to the spectrum. This allows us to know what everything is like along the line of sight (and in some way how the gas in the universe has evolved with time).

spectrum

w/o normalization

Conclusion We find that the universe is expanding and that everything moves away from us due to the expansion of the universe and not actual motions of the objects. We find that the expansion velocity proportional to velocity Some of the most distant observable objects are Quasars which are super massive and super bright black holes that are eating large amounts of matter.