The Early Universe II AST 112

Review: Observable Universe There is a distance from us at which there is so much expanding space that an object at this distance is carried away from us at greater than the speed of light. We cannot ever observe this object. This defines the observable universe.

What’s the Universe expanding into?

Expansion of the Universe The observable universe is expanding into the unobservable universe. – So we know at least some of the universe that we are familiar with lies outside of the observable universe. We cannot observe anything outside of the observable universe.

But what about the entire universe? And does the universe have an edge?

The Entire Universe The observable universe contains matter all the way up to the boundary with the unobservable universe – And just beyond that boundary, presumably we find more galaxies

The Entire Universe Hop from one observable universe to the next. 3 obvious possibilities: – The Universe has an edge. – You get back to where you started. – The universe is infinite and you just keep seeing new universe. Some theories do try to make statements about the entire universe based on what we can see in the observable universe. But how can we ever test those theories?

Where is the center of the Universe?

The Center of the Universe We can’t talk about the “center” of the Universe. Picture the raisin bread: – The bread has a geometrical center – but we’re in a raisin that’s stuck inside the bread. – We need an outside perspective to determine the center and we’re not going to get one! And if the Universe is infinite, there is no center.

The Cosmic Microwave Background This is a view of the Universe just before galaxies were forming A hot, dense, opaque fog of glowing hydrogen redshifted into radio waves by expansion of the Universe It is very uniform but not *perfectly* uniform – This must be the case if structures are to form

CMB: Step by Step As the Universe expanded and cooled, the temperature dropped The hydrogen fog (plasma) consisted of charged particles that don’t let photons travel very far All at once, the electrons combined with the protons

CMB: Step by Step Neutral hydrogen does not scatter photons very well – They could now travel for very long distances – The Universe became transparent Picture a 1000m field of people yelling. They all stop at once. The speed of sound is 330 m/s. – What do you hear? – You can define a “surface of last yelling”!

Cosmic Microwave Background (CMB) The CMB is called the “surface of last scattering” Snapshot of the Universe at 380,000 years

CMB Emission Lines Given that the electrons combined with the protons, we should see hydrogen emission lines None have yet been detected

Just how far back can we see? We can see back to Re-combination Era – We can’t see light past here because the earlier Universe was opaque to light. We might be able to make a neutrino map – Neutrinos are “ghostly” – hard to measure We might be able to make a gravitational wave map – We’re not even sure if those exist!

The Early Universe Simulations show that the Universe began with a smooth but very slightly irregular distribution of matter It formed the filament/void structure over billions of years What should the CMB look like?

Wilkinson Microwave Anisotropy Probe (WMAP) Temperature variations are o F

The Early Universe So any simulation must begin with this smooth distribution of matter and must end up with a filament/void structure.

Observed: Simulated:

All the way to the CMB? We can’t see past the CMB. But we can see everything up to it! The earliest galaxies are thought to be greater than 30 billion LY away

Can Hubble see the earliest galaxies?

All the way to the CMB? The most distant galaxies that Hubble can see are about 30 billion LY away The hydrogen fog is seen at about 45 billion LY away

The Earliest Galaxies So there is a 15 billion LY gap between how far we can observe and how far it is possible to observe That’s a lot of the Universe!

Why are galaxies billion LY away difficult to observe?

James Webb Space Telescope Visible, UV and x-ray light at that distance is red-shifted to infrared wavelengths And these objects are very dim We need a large infrared telescope * 6.5 m mirror (Hubble’s is 1 m) * The foils are the size of a tennis court!

Dark Ages The Universe became transparent – and there were no other light sources Other than the distant hydrogen fog glow, there was nothing to see

Era of Reionization (Galaxy Formation) When galaxies first turned on, they re- ionized the transparent hydrogen fog The Universe had expanded – Fog wasn’t as dense – Still relatively transparent

The Oldest Galaxy

This galaxy is seen during the Era of Re- Ionization Other nearby galaxies (that we can’t see) cleared out enough re- formed hydrogen fog for us to see this

The Oldest Galaxy 480 million years after Big Bang It is a bright blue starburst galaxy – … why is it not blue?

The Oldest Galaxy This galaxy is seen in very far infrared – Just within Hubble’s grasp We may be able to see other galaxies, but their light is too redshifted for Hubble – JWST

Open, Closed, or Flat Universe? Space can warp locally – Like near a black hole or a large galaxy cluster Does the Universe as a whole have an average warp to it? – This refers to the “geometry of the Universe”

Open, Closed, or Flat Universe? One way to test: – Light travels in straight lines through space Closed Universe: – Parallel light rays converge Open Universe: – Parallel light rays diverge Flat Universe: – Parallel light rays stay parallel

Open, Closed, or Flat Universe? If space is curved: – Angles of a triangle will not add to 180 degrees We need a BIG triangle!

Open, Closed, or Flat Universe? Features more than 1 degree apart cannot influence each other A closed universe would spread these features out to > 1 degree An open universe would contract them to < 1 degree A flat universe makes them exactly 1 degree

Open, Closed, or Flat Universe? The brightest spots on the CMB are separated by 1 degree– to 0.5% The observable universe is flat. – Cosmologists suggest that if the rest of the universe is flat, then it is infinite. – So if the observable universe is just like the rest of the universe, it might just be infinite.