Lecture 24: The Epochs of the Universe Astronomy 1143 – Spring 2014

Key Ideas As a result of the Hot Big Bang, the whole Universe was extremely hot & dense Blackbody photons very energetic Matter in very different states Starts out as fundamental particles and builds up Matter/anti-matter pairs created, but slight preference for matter Neutrinos originally coupled to matter, later froze out Forces unified First gravity, then strong, then weak forces decouple from the electromagnetic force

Key Ideas We cannot predict the state of the Universe all the way back to the highest densities/temperatures Before the Planck time – Universe must be described by quantum gravity High energies of the Big Bang also seen Stellar explosions Cosmic ray interactions in Earth’s atmosphere Particle accelerators, such as the Large Hadron Collider

Overview

Planck Length The world of the small in the present Universe is governed by the equations of quantum mechanics. Universe is ruled by uncertainty and probability Example: Heisenberg uncertainty principle states that we can only know the position or velocity of a particle precisely “Precisely” means < meters

Quantum mechanical particles

Before the Planck Time When the Universe was younger than ~ seconds, the size of the observable Universe was about the same size as the uncertainty in quantum mechanical positions Gravity NOT smooth – spacetime is a “quantum mechanical froth” Physics has nothing to say right now about what happens before the Planck time.

TOEs (Theories of Everything) To understand what happens at the Planck time and before, we need a theory of everything which will unite gravity and quantum mechanics Physicists have had success in understanding how the 3 other forces are related using particle physics They hope to use the same approach to unify gravity

The Particle Physics View of World The modern view of forces is that they are carried by particles Choice in the way you think about things – what matters is what you can predict Very successful at predicting the interaction between particles and light

The Four Fundamental Forces

Strength of Force depends on energy/separation of particles At large energies, the force of gravity may also be a lot stronger – microscopic black holes?

The Universe Cuts Off Its TOE Before t= s Quantum foam means spacetime is not static TOE is needed After t= s End of the Planck Epoch=Planck time Universe has expanded to bigger than the Planck length Gravity becomes distinct from other forces This is still speculative; not yet tested

The Universe Eliminates Its GUT Before t= s Strong, weak, & electromagnetic forces united Such a theory has not been conclusively defined, but that class of theories is called Grand Unified Theories or GUTs After t= s Strong forces becomes distinct from the electroweak force Triggers Inflation – Universe goes from size m to m

The Universe Favors Matter over Antimatter At t= s Universe is hot enough for photon collisions to produce particle/anti-particle pairs Usually processes produce a particle/anti- particle pair. But that can’t always be true, otherwise there would be no matter for the world Baryogenesis occurs at this point – some process that ~1 out of a billion times favors matter over anti-matter Creation of the material world

The Universe Abandons the Weak Before t= s Weak & electromagnetic forces are unified Same processes can happen with both forces After t= s Temperature drops below K, particles lack enough energy to keep weak force in balance Weak & electromagnetic forces separate Leads to difficulty in detecting neutrinos, for example

The Universe Confines Its Quarks Before t=10 -6 s Temperatures high enough that neutrons & protons can’t stick together – quark soup After t=10 -6 s Temperatures are low enough that the Universe undergoes the quark-hadron transition Quarks can now be confined to baryons and mesons (though mesons are unstable, so that’s not as exciting)

The Universe Annihilates Most of the Material World After t=10 -5 s Universe has cooled enough that photon-photon collisions no longer have enough energy to create baryon/anti-baryon pairs Now baryons/anti-baryons annihilate, but no new pairs are created Left behind with 1 baryon (yay!) for every 2 billion photons Electron-positron annihilation happens later because they have lower masses = photons need less energy

The Universe Imprisons Baryons but Liberates Neutrinos Before t=1s Universe hot & dense enough that neutrinos interact regularly This allows baryons to change types Briefly helps the Universe make free neutrons Anti-neutrinos also participate in reactions Neutrinos & anti-neutrinos interact via weak interactions, so particle/anti-particle annihilation less of a problem (same is true for WIMPs)

The Universe Imprisons Baryons but Liberates Neutrinos After t=1s Neutrinos (& anti-neutrinos) decouple (or “freeze out”) Cease frequent interactions Free-stream throughout the Universe Creates cosmic neutrino background So far, this background has not been detected e - and e + annihilate about now, as photons have less energy than 2m e c 2

The Universe Imprisons Baryons but Liberates Neutrinos Between t=3 minutes and 15 minutes Universe has cooled enough that heavier nuclei can form and not be disintegrated by photons Big Bang Nucleosynthesis occurs Imprisons baryons in nuclei (good news for neutrons!) Universe is now about 25% He Electrons are still not bound to atoms and the Universe is still opaque

The Universe Sees Matter Overthrow Radiation At t=72,000 years Energy density in photons drops below energy density in matter This is because the density of different components (photons, matter, dark energy) varied at different rates as space expanded Energy density of photons decrease more rapidly than energy density of matter because photons get redshifted

photons How does energy density of photons evolve with time? L Consider a cube filled with photons As the Universe expands, the number of photons per cubic meter declines, but so does their energy

constant As the cube expands, the number of photons inside the cube is roughly constant L(t) not However, the energy of each photon is not constant.

Photons Lose Out As space expands, the wavelength of light expands. LongerLonger wavelength → lowerlower frequency → smallersmaller photon energy. Density of matter decreases less rapidly as it is not stretched by the expansion of space

matter How does density of matter evolve with time? L Consider a cube filled with matter. Energy density comes from the amount of matter.

As the cube expands, the number of particles is constant. The mass per particle is constant. L(t) constant Thus, the total mass M within the cube is constant.

As the universe expands, energy density of photons declines more rapidly than the density of matter Density Scale factor dark energy matter photons

The Universe Imprisons Electrons but Liberates Photons At t=400,000 years Temperature drops to 3000K Electrons combine with nuclei to make atoms Photons start streaming through space Origin of the Cosmic Microwave Background

The Universe Builds Objects and Changes Its Mind about Electrons Over the next few hundred million years, gravity succeeds in pulling gas into the first stars and galaxies The new stars (and accretion disk about black holes) give off high-energy photons These photons re-ionize most of the hydrogen in the Universe, but density of free electrons small enough to keep the Universe transparent.

The Universe Sees Dark Energy Overthrow Dark Matter At t=9.5 billion years The Universe become dark-energy dominated The density of dark energy is very small (example: spread the calorie content of a candy bar over a cube 70 miles on a side…) But it doesn’t change as the Universe expands Benefit of acting like you arise from the vacuum of space – it keeps up with the expansion

Density Scale factor ↓now ↓transparency ←nucleosynthesis dark energy matter Dark Matter for the Win!

Testing physics of the 1 st few seconds How do we know we are on the right track with our description of the physics of the early Universe? We can recreate the high energies of particles in particle accelerators and explore the kind of particles and interactions that theory predicts For example, GUTs predict the existence of the Higgs boson, a very massive particle