This Set of Slides

• This set of slides deals with the early universe, the CMB, inflation, Dark Energy and the fate of the universe.

• Units covered: 80, 81 and 82.

• Some Review Questions at end if time allows.

Student Evaluations of Instructor

• (Most of) The physics department agreed to be one of the test departments for online evaluations of instructors.

• You should have received an email about this.

• Please take some time to go to the site and fill out the eval.

• These are taken seriously by me and the department and the university.

• I have made changes to this course (and others) based on comments made by students (for example.)

• If you want to make comments to the department (about me or this class), send a letter to the physics department.

• Thanks!

Final Exam

• Next Monday, June 7, 2010 is the final exam.• 12:00 pm (noon) till 1:50 pm.• Format is multiple choice (guess?!) and true/false.• There will be (approx) 90-110 questions.• Covers all material presented in class and labs since the

beginning of the term until the end.• Bring pencils, photo ID - STUDENT ID is best.• Locations:

A – K Weniger 151

L – O Weniger 116

P – Z Weniger 153

Light from the Big Bang

• Every time we look at the night sky, we are looking back in time.

– It takes more than 4 years for light from the closest star to arrive.

– It takes billions of years for light from the most distant galaxies to arrive.

• Can we see light from the Big Bang?

– Almost…

The Last Scattering Epoch

• Minutes after the Big Bang, the Universe was opaque.– High temperatures kept all

matter ionized.– Photons could only travel a short

distance before being absorbed.

• After 400,000 years, the Universe cooled enough for electrons and ions to recombine, allowing light to pass.

• The Universe was transparent.

Light from the Early Universe

• So what should light from 400,000 years after the Big Bang look like?– It should have a spectrum that

corresponds to the temperature of the Universe at that time, 3000 K.

– Expansion of space will stretch this light, however.

• The Universe has expanded by a factor of 1000 since this time, so the wavelength will have stretched by the same amount.

– Spectrum will correspond to a temperature of 3K.

• This light from the early Universe has been found, and is called the Cosmic Microwave Background .

Light from the Early Universe

• Originally discovered by chance. In the 1960’s, scientists noticed microwave background “noise” in telephone satellite links.

• No matter the direction a detector is pointed, this background energy is constant (nearly so.)

• Spectral peak is at about 1 millimeter.

• Wien’s Law gives 3 K.

• Once again, the data came after

the theory but supports the theory.

Era of Galaxy Formation

• Using the Hubble Space Telescope, we can image galaxies as they appeared nearly 13 billion years ago.– These galaxies must have formed

“shortly” after the Big Bang.

• Galaxies must have been able to form relatively quickly.– Gas clouds collapsed into disks.– Gravity of visible matter doesn’t appear

to be strong enough.– Dark matter was necessary to pull

everything together.

• Dark matter could form clumps in the early universe, before the last scattering epoch.

• These clumps should show up in the CMB…

Hubble deep-field view. 10,000 galaxies; less than 1 degree across!

Clumpiness in the CMB

…and they do!

Red are hotter, more dense regions; blue, cooler, less dense.

A Timeline of the Universe

• Immediately after the Big Bang, only protons and electrons existed.

• Shortly after the BB, temperature and density was high enough for deuterium to form by fusion.

• After 100 seconds or so, temperature cooled enough so that deuterium could fuse into helium nuclei.

• The temperature continued to cool, and fusion stopped after a few minutes.

• Big Bang theory predicts that around 24% of the matter in the universe is helium, which matches what we measure today.

The Origin of Helium

• Modern technology allows us to test theories back to a time 10-33 seconds after the Big Bang.

• Physics as we know it ceases to function at 10-43 seconds after the BB, a time called the Plank Time.

• Using particle colliders, scientists have uncovered a number of clues about what happened in the early universe, after the Plank time.

• The early universe underwent a period of very rapid expansion, far greater than c!

• By 10-33 seconds, the universe expanded from less than the size of a proton to the size of a basketball.

• This expansion is called inflation.

The Epoch of Inflation

Inflation

• Inflation may have created multi-universes, “bubbles” in the early mix that inflated separately, split-off, and are entirely separate from our own universe – nothing can pass between them, including light!

• Inflation explains the uniformity of the universe (the uniform CMB), but also explains the irregularities (the clumpiness of the CMB.)

• Inflation explains why the universe appears to be a flat universe (even though any geometry is possible.)

• Inflation may explain why there are 4 dimensions (3 space, 1 time) instead of the 10 or l1 of string theory – perhaps these are the only ones that expanded during inflation.

Expansion Forever? Or Collapse?

• The fate of the universe is ultimately controlled by its total amount of energy.– Energy of expansion (positive).

– Gravitational energy that can slow the expansion (negative)

• If the total energy is positive or zero, the expansion continues forever.

• If the total energy is negative, the expansion will halt, and the universe will contract and eventually collapse.

Supernova Type Ia Findings

• New attention was given to the universe expansion.

• We can measure the recession velocity of distant galaxies using Type Ia supernovae as standard candles.

• Result: It appears that the expansion rate when the universe was half its current size was less than it is today.

• This shows that the expansion rate is increasing with time!

Dark Energy

• Dark energy may provide the solution to this mystery.

• Dark energy remains constant everywhere.

• As gravity weakens (with distance) the outward push of dark energy would accelerate the expansion.

• In order for dark energy to make this occur, it must make up around 70% of all of the energy in the universe.

• As with dark matter, dark energy is not yet detectable.

If So, So What?

• The future universe will not recollapse in a Big Crunch.

• Galaxies will recede at ever-increasing speeds, faster than the speed of light. Light from them will redshift until they fade out of sight.

• The visible universe will get increasingly more sparse.

• The stars of the visible galaxies will run out of fuel, and fade into darkness. Close galaxies will merge and split…

• Matter that doesn’t disappear into black holes will be cold and dark. After long enough, decays into radiation only.

• Even black holes may evaporate into radiation only.

• An incredibly vast dark, ever-expanding nothingness.