Thinking about the Big Bang Pat Hall Department of Physics and Astronomy York University, Toronto, Ontario What are the basic concepts of the Big Bang? What is the past and future evolution of the Universe?

Outline Hubble’s Law 3-D universe: like a raisin cake 2-D universe: like the surface of a balloon Much hotter in past: should see redshifted heat WMAP shows universe was uniform temperature Inflation and our Observable Universe Our Past: Structure Formation Our Future: Dark Energy Olber’s Paradox

Steps to the Big Bang Here in the Milky Way, we see almost all galaxies moving away from us (Edwin Hubble)

By measuring distances to galaxies, Hubble found that redshift and distance are related in a special way

Steps to the Big Bang Here in the Milky Way, we see almost all galaxies moving away from us (Edwin Hubble) Either we’re at the center of the Universe…

Either we’re at the center of the universe...or galaxies are all moving away from each other, and the Milky Way is just another one of those galaxies.

Steps to the Big Bang Here in the Milky Way, we see almost all galaxies moving away from us (Edwin Hubble) Either we’re at the center of the Universe… …or galaxies are all moving away from each other, and the Milky Way is just another one of those galaxies.

Every galaxy is moving away from every other galaxy. (Except for galaxies so close to each other that their mutual gravity can resist the expansion of the universe.)

Surface of a balloon expands but has no center or edge

Steps to the Big Bang Here in the Milky Way, we see almost all galaxies moving away from us (Edwin Hubble) Either we’re at the center of the Universe… …or galaxies are all moving away from each other, and the Milky Way is just another one of those galaxies. Exceptions: galaxies very near to each other can orbit each other or crash into each other due to gravity. Galaxies all moving away from each other, so…

…the universe must have been much denser (and hotter) early in its history

What is matter made of? Water (H2O) is a molecule, like many everyday substances. A water molecule is made up of components called atoms: 2 hydrogen atoms (H) and 1 oxygen atom (O). Every atom consists of two parts:Every atom consists of two parts –One or more electrons surrounding... –the nucleus: ≈ 100,000 times smaller than the electron cloud, but 10,000 to 100,000 times more massive. If you increase the density and temperature and density enough, you can break apart molecules into atoms, and eventually atoms into electrons and nuclei. Hydrogen & helium atoms are transparent, but become opaque when split into nuclei and electrons.

Background radiation from Big Bang has been freely streaming across universe since atoms formed at temperature ~ 3,000 K: visible/IR

Expansion stretches photon wavelengths causing a cosmological redshift directly related to lookback time

Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves Background has a nearly perfect thermal spectrum at a temperature 2.73 degrees above absolute zero (-270 degrees Celsius)

The cosmic microwave background – the radiation left over from the Big Bang – was detected by Penzias & Wilson in 1965

Expansion of universe has redshifted thermal radiation from that time to ~1000 times longer wavelength: microwaves Background has a nearly perfect thermal spectrum at a temperature 2.73 degrees above absolute zero (-270 degrees Celsius)

Wilkinson Microwave Anisotropy Probe (WMAP) The WMAP satellite has shown that when our universe was about 380,000 years old, it was uniform in temperature to within 1 part in 5,000 How flat would the Earth be if it was smooth to within 1 part in 5,000? The highest mountain would be 640 meters high, and the deepest ocean 640 meters deep.

Remember that every point on this map of the microwave sky represents light that traveled 13.7 billion light years to reach us…

How can microwave temperature be nearly identical on opposite sides of the sky?

Inflation: regions now on opposite side of the sky were close together before inflation pushed them far apart

Inflation can make all the structure in the microwave background by stretching tiny quantum ripples to enormous size These ripples in density then become the seeds for all structures in today’s universe

Inflation of universe flattens overall geometry like the inflation of a balloon – however the universe started out, it will end up almost exactly flat. (Could there be island universes in a long, thin balloon?)

WMAP gives us detailed baby pictures of structure in the universe

Observed patterns of structure in universe – from WMAP etc. - agree (so far) with what inflation should produce

Models show that gravity of dark matter pulls mass into denser regions – universe grows lumpier with time Time in billions of years Size of expanding box in millions of lt-yrs

Will the universe continue expanding forever?

Does the universe have enough kinetic energy to escape its own gravitational pull?

Fate of universe depends on amount of dark matter Lots of dark matter Not enough dark matter

Critical density of matter Not enough dark matter Fate of universe depends on amount of dark matter Lots of dark matter

Amount of dark matter is ~25% of the critical density suggesting fate is eternal expansion Not enough dark matter

But expansion appears to be speeding up! Not enough dark matter Dark Energy?

Estimated age depends on dark matter and dark energy old older oldest

The night sky tells us that the observable universe isn’t infinitely old and unchanging

Olbers’ Paradox If universe were 1) infinitely old, and infinite in all directions in space 2) unchanging 3) everywhere the same Then stars would cover the night sky

Olbers’ Paradox If universe were 1) infinite 2) unchanging 3) everywhere the same Then, stars would cover the night sky

Night sky is dark because the universe changes with time, and because the observable universe is not infinitely old

Cosmological Horizon Universe’s age of 13.7 billion years limits how far we can see, and 13.7 billion years ago there weren’t any stars around yet.

The Big Bang and our Universe The Big Bang was a point in time, not in space The physical universe has no center or edge

The Big Bang and our Universe The Big Bang was a point in time, not in space The physical universe has no center or edge The observable universe is centered on us: we observe light from everything that was within 13.7 billion light years of our current location when the universe became transparent. Next year, we’ll see 1 light-year farther away.

The Big Bang and our Universe The Big Bang was a point in time, not in space The physical universe has no center or edge The observable universe is centered on us: we observe light from everything that was within 13.7 billion light years of our current location when the universe became transparent. Next year, we’ll see 1 light-year farther away. The ultimate zero-sum game? The universe’s positive mass-energy plus its negative potential energy (three kinds: gravitational, electroweak, strong-force) can sum to zero.

Thinking about a simpler universe Our universe has 3 dimensions of space (for now): up/down, left/right, forward/backward. Our universe has 1 dimension of time (past/future, but we move only forward in time). It’s easier to discuss a simpler universe -- one with 2 dimensions of space and 1 of time – and then extend that picture to 3 dimensions. Flatland by Edwin Abbott

Thinking about a simpler universe A universe with 1 dimension of time but only 2 dimensions of space might have left/right and front/back, but no up/down (like a board game). Or it might have left/right and up/down, but no front/back (like some video games [Asteroids]). This two-dimensional universe could be: Flat & infinite (infinite piece of paper) Positively curved & finite (surface of a balloon) Negatively curved & infinite (infinite Pringle)

Thinking about a ‘simpler’ universe A universe with 1 dimension of time but only 2 dimensions of space might have left/right and front/back, but no up/down (like a board game). Or it might have left/right and up/down, but no front/back (like some video games [Asteroids]). This two-dimensional universe could be: Flat & infinite (infinite piece of paper) Flat & semi-finite (infinitely long cylinder) Flat & finite (surface of a donut [a torus]) Positively curved & finite (surface of a balloon) Negatively curved & infinite (infinite Pringle) Negatively curved & finite (can’t be drawn!)

Inflation of universe flattens overall geometry like the inflation of a balloon – however the universe started out, it will end up almost exactly flat. (Could there be island universes in a long, thin balloon?)

Thinking about our universe Our three-dimensional universe appears very close to flat (1 part in 50 at the moment), as well as infinite (infinite in all spatial directions) Inflation predicts our universe is very close to flat (probably at the level of 1 part in 100,000) If the universe is that flat, for all intents and purposes it is exactly flat, and we may never know whether our universe started out exactly flat or curved (positively or negatively) (2-dimensional analogies: zero curvature = infinite piece of paper; positive curvature = like a balloon; negative curvature = like a Pringle)

What is matter made of? Water (H2O) is a molecule, like many everyday substances. A water molecule is made up of components called atoms: 2 hydrogen atoms (H) and 1 oxygen atom (O). Every atom consists of two parts:Every atom consists of two parts –One or more electrons surrounding... –the nucleus: ≈ 100,000 times smaller than the electron cloud, but 10,000 to 100,000 times more massive. Every nucleus (plural: nuclei) is a combination of... protons: different elements have different numbers of protonselements neutrons: they help hold the nucleus together  Protons and neutrons are made of quarks stuck together with gluons  Quarks & other fundamental particles might be vibrating strings

Protons and neutrons combined to make long-lasting helium nuclei when universe was ~ 3 minutes old

Aside: Spherical Geometry Flat geometry means angles of a triangle in that space add up to 180 degrees (this is the normal ‘Euclidean’ geometry taught in school) Positively curved geometry means angles of a triangle add up to more than 180 degrees (From the north pole, draw two lines to the equator: one through 0 degrees longitude and another through 90 degrees longitude. Connect them along the equator to make a triangle with three 90-degree angles.) Negatively curved: angles of a triangle add up to less than 180 degrees

Overall geometry of the universe is closely related to total density of matter & energy Density = Critical Density > Critical Density < Critical