# 4/29/16 How can the darkness of the sky lead you to conclude that the universe had a beginning?

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4/29/16 How can the darkness of the sky lead you to conclude that the universe had a beginning?

Learnin g Goals: I will: understand the properties of different types of galaxies. understand how the universe came to be what we observe today. understand how astronomers use astronomical objects (standard candles) as a distance ladder to estimate the size of the universe and to measure large distances in the universe. understand how astronomers determine the age and size of the universe?. 4. Complex Knowledge: demonstrations of learning that go aboveand above and beyond what was explicitly taught. 3. Knowledge: meeting the learning goals and expectations. 2. Foundational knowledge: simpler procedures, isolated details, vocabulary. 1. Limited knowledge: know very little details but working toward a higher level.

Review Question From Yesterday What is this? And how would you explain to a non-astronomy student why it is important?

Back to the story of the big bang: Redshifts  expansion + dark night sky + CMBR  Big bang As yet, we can’t trace the universe to time t = 0, The physics is not understood well enough yet… But we can come pretty darn close! (10 millionths of a second old!) HubbleOlbers Penzias & Wilson

Disclaimer Your brains will hurt today!!

Big Bang Theory: Timeline of Universe Hubble’s Law shows that the universe has been expanding for billions of years - the universe is denser the further back in time you look. At some point, you reach an infinitely dense point at which T age of universe = 0  Big Bang

T = 0 seconds to 10 -43 seconds BIG BANG occurs. Something causes infinitely dense point to expand (into Nothing). Density of universe is so high that time and space are curled up and the laws of physics that we know today do not apply. –NO REAL CLUE WHAT’S HAPPENING!! All four forces in nature were unified. This is time is called the Planck Time.

Separation of Forces After the Planck time, the temperature had decreased to 10 32 K and gravity was the first force to separate. The remaining three forces were still united - these are the conditions that particle physicists today try to replicate.

T = 10 -35 to 10 -32 seconds Inflation caused the size to the universe to increase exponentially by a factor of 10 50. This time is called the inflationary epoch.

Inflation Theory: an answer to two big problems The horizon problem -- the puzzle that the Universe looks the same on opposite sides of the sky (opposite horizons) even though there has not been time since the Big Bang for light (or anything else) to travel across the Universe and back. –So how do the opposite horizons "know" how to keep in step with each other? The second puzzle is called the flatness problem This is the puzzle that the spacetime of the Universe is very nearly flat (more on this next week!) –the Universe sits just on the dividing line between eternal expansion and eventual recollapse.

Inflation Theory: The universe looks the same everywhere on a large scale (cosmological principle), but There are fluctuations (stars and galaxies) on a small scale:

Inflation Theory: Alan Guth (1970s) had a solution: –The universe must have expanded exponentially very early for a short period of time. Go from ridiculously small to ridiculously big –This explains why different parts of the universe look similar on a large scale –This would also account for the clumping of matter that we observe

Evidence for Inflation Theory Guth predicted that the average density of the universe should be equal to the critical density (6 protons/m 3 ) –Flatness problem –This was confirmed by powerful telescopes. Evidence from WMAP shows that the clumping of matter is consistent with the amount of accelerated expansion during inflation. –Horizon problem

Extent of Inflation Today, evidence and theory show that: At T = 10 -35 sec, universe d = 10 -24 cm Between T = 10 -35 sec and T = 10 -32 sec, the universe expanded exponentially by a factor of 10 50.. –Much smaller than an electron to the size of the Virgo supercluster! For the briefest moment, the universe expanded faster than the speed of light. –This means physics as we know them don’t work correctly… –we don’t know exactly what happened before this

After Inflation Stops Matter is created: d 10 -32 to 10 -11 –Photons collide and produce particle-antiparticle pairs such as electrons and positrons, and quarks and antiquarks.

Particle Production in Early Universe As the size of the universe increases and the temperature decreases, the particles produced are of decreasing energy. The fundamental forces and parameters of elementary particles at the time that symmetry was broken are the same as they are today. The time between the birth of the universe and t = 10 -11 is rather unknown, but we can speculate what is happening based on other observations; beyond this time is less speculative as these are conditions that particle physicist try to replicate.

After Inflation Stops Matter is created: –As the universe expanded, it cooled. The photons lost some of that energy The energy of the photons wasn’t high enough to create particle-antiparticle pairs –The remaining quarks and anti-quarks annihilated back into photons, but… –For some reason, there were more particles than antiparticles For every billion pairs, one regular particle survived This is what makes up all the matter in the universe –result: symmetry breaking. Pair Production occurred until T = 6,000,000,000K, but pair annihilation happens independent of temperature.

T = 10 -11 to 10 -6 seconds Temperature has cooled enough for baryons (Protons, Neutrons) to form from quarks. Like the leptons (electrons, anti-electrons), baryons formed in pair production (proton, anti-proton). Once the temperature has decreased past the point at which baryons can no longer be produced, pair annihilation occurs again, leaving a slight excess of baryons over antibaryons. Reason for slight excess of matter over antimatter is because of an unknown reaction known as baryogenesis, in which conservation of baryon number is violated. All the neutrons, protons and electrons were created in the first second of the universe –Also, at this temperature, all particles are no longer moving relativistically, so the universe becomes dominated by the higher energy photons (radiation-dominated universe).

STILL –LESS THAN A SECOND HAS GONE BY

T = 3 minutes: Before this time, photons still had enough energy to break apart atomic nuclei No nuclei before t = 3 minutes Just a bunch of protons, neutrons and electrons running around like crazy Sort of like a hot subatomic particle soup

T = >3 minutes Temperature ~ 10,000K, density ~ that of air. Neutrons combine with protons making deuterium and helium nuclei, most protons remain independent (hydrogen nuclei). Called Big Bang nucleosynthesis. Temperature is still too high to form atoms as they would be ionized immediately. (no electrons) The universe would appear opaque during all this time because photons and matter would be interacting due to high temperatures. No light  known as the “Opaque Epoch”

T = 20 to 30 minutes: Temp cooled enough to where nuclear reactions stopped no more nucleosynthesis ~ 25% of the universe was He nuclei and ~ 75% was still H nuclei (protons) (This matches the abundances seen in the oldest stars…) Another bit of observational evidence that corroborates our theory of the Big Bang

The timeline of the big bang: T < 50,000 years: The universe was dominated by radiation (opaque)  Photons couldn’t travel far without hitting matter  Radiation and matter were locked together in a “dance”  Nuclei couldn’t capture electrons to form atoms (and hence, couldn’t emit light…)  The gas was ionized…

T = 380,000 years Universe is now cool enough (3000K) that matter energy is greater than radiative energy, thus allowing atoms to form. –Protons could finally capture electrons to form hydrogen –Before this, matter couldn’t clump together because the photons kept everything “smoothed out” Radiation is decoupled from matter and photons are able to travel without being scattered (and kept that energy of 3000K), and they started streaming throughout space - origin of CMB radiation. –Let there be light! This time is known as the epoch of recombination. –Universe is now matter-dominated.

Time ~ 300 million - 400 million years Since epoch of recombination, slightly denser regions slowly attracted matter nearby and the first stars begin to form. Regions continue to acquire matter and other objects like galaxies, and gas clouds form. Universe begins to look like how we know it today (still expanding and still cooling). Before this: NO STARS – Known as the “Dark Era” –380,000 years to 400 million years

400 million years – ~10 billion years old Galaxies, Galaxy Clusters, Galaxy Super Clusters, Gas Clouds, Galaxy Mergers, Star Clusters, Black Holes, Planets, Red Dwarfs, Etc. are all happy coming into existence Gravity Dominated Universe

Time~ 10 Billion Years - Present Anti-Gravity starts to win –Dark Energy is increasing –What is Dark Energy? The Expansion of the Universe starts accelerating –Between inflation and this time, it had been slowing down due to gravity